**Meet the editor**

Dr. S. Sivasankaran is a faculty member in the Department of Mechanical Engineering, College of Engineering, Qassim University. He has completed his PhD degree in Production Engineering from National Institute of Technology, Tiruchirappalli, India. He has passed his bachelor degree of Mechanical Engineering in First Class with Distinction under the University of Madras, India.

He did his M. Tech in Manufacturing Technology and passed in First Class with Distinction from Department of Production Engineering, National Institute of Technology, Tiruchirappalli, India. After his doctoral degree, he has worked as an associate professor in KPR Institute of Engineering and Technology (KPRIET), Coimbatore, India. He has established advanced materials research laboratory and entrepreneurship development cell in KPRIET. He has received many research and seminar grants from the government of India. Further, he has served as an associate professor in Hawassa University, East Africa where he has developed BSc, Industrial Engineering; MSc, Industrial Engineering and Management; and MSc, Manufacturing Technology curricula. He has published more than 50 research articles in refereed international journals and is acting as a reviewer for more than ten refereed international journals. He has organized more than ten national- and international-level workshops. Finally, his research interests are nanocomposites, mechanical alloying, powder metallurgy, stir casting, advanced welding, 3D printing, metal matrix composites, polymer matrix composites, mechanical behaviour, machining behaviour, advanced characterization techniques, optimization and modelling.

Contents

**Preface VII**

**in Aluminum Alloys 1**

**Al-Based Alloys 17** Ildiko Peter and Bela Varga

Marcela Pokusová

**Al-Si Cast Alloys 59**

Natarajan

**Deposition 119**

Esther Titilayo Akinlabi

Valtierra and Fawzy H. Samuel

**Electromagnetic Levitation 39**

Williams S. Ebhota and Tien-Chien Jen

Chapter 5 **Friction Stir Welding of Aluminium Alloys 81**

Heng Li, Chao Lei and He Yang

Chapter 1 **Effects of Grain Refining on Columnar-to-Equiaxed Transition**

Chapter 2 **Some Considerations on the Structure Refinement in**

Chapter 3 **Continuous Casting of Thin Aluminum Strips Using the**

Chapter 4 **Effects of Modification Techniques on Mechanical Properties of**

Rajkumar Vijayakumar, Venkatesh Kannan and Arivazhagan

Chapter 6 **Formability and Performance of Al-Zn-Mg-Cu Alloys with**

Chapter 7 **The Effects of Rapid Cooling on the Improved Surface**

**Different Initial Tempers in Creep Aging Process 99**

**Properties of Aluminium Based Coatings by Direct Laser**

Olawale Samuel Fatoba, Mamookho Elizabeth Makhatha and

Hicham Tahiri, Serageldin S. Mohamed, Herbert W. Doty, Salvador

## Contents

#### **Preface XI**

Preface

essing routes, and its appropriate applications.

In the twenty-first century, aluminium-based lightweight materials are being used for many commercial applications, which save both the energy and cost. The major features are lighter in weight, considerable strength, high value of corrosion resistance and considerable ductili‐ ty. Most of the products made from aluminium alloys are from their wrought products that completely replace the usage of cast iron and steel as these materials would enhance the weight of the products (both cast iron and steel). Moreover, aluminium alloys can be recy‐ clable, which are available more in the world. The present book has several sections that address the real research works and review works contributed by several researchers in the world that cover the latest processing routes of aluminium alloys including composites through casting, forming, severe plastic deformation, various grain refinement techniques especially from equal channel angular pressing, friction stir welding and corrosion behav‐ iour, effect of alloying elements on the properties improvement, various characterization techniques and rapid solidifications. Further, detailed mechanical behaviour of aluminium alloys is also being incorporated in the present book. This book will help all academicians, the research community and industries for selection of aluminium-based materials, its proc‐

> **Dr. S. Sivasankaran** College of Engineering, Qassim University, Saudi Arabia

## Preface

Chapter 8 **Effect of the Processing Conditions on the Microstructural**

Chapter 9 **Gluconates as Corrosion Inhibitor of Aluminum in Various**

Omotayo Sanni and Abimbola Patricia Popoola

Chapter 10 **Influence of Rapid Solidification on the Thermophysical and Fatigue Properties of Laser Additive Manufactured**

Chapter 11 **Aluminum Sheet Metal Damage Mechanisms Application to**

Chapter 12 **Microbiologically Influenced Corrosion in Aluminium Alloys**

Chapter 13 **Effect of Alloying Element on the Integrity and Functionality of**

Chapter 14 **Equal Channel Angular Extrusion Characteristics on Mechanical**

Chapter 15 **Microstructural Characteristics and Mechanical Behaviors of New Type SIMA Processed Aluminum Alloy 279** Chia-Wei Lin, Fei-Yi Hung and Truan-Sheng Lui

Vejar V. Nelson, Orrego T. Maria, Sancy V. Mamiè and Paez C.

Ojo Sunday Isaac Fayomi, Abimbola Patricia Idowu Popoola and

Abiodun Ayodeji Abioye, Ojo Sunday Isaac Fayomi, Abimbola Patricia Idowu Popoola and Oluwabunmi Pamilerin Abioye

Tomasz Tański and Przemysław Snopiński

**Corrosive Media 157**

**VI** Contents

**Ti-6Al-4V Alloy 181**

Elizabeth Makhatha

**7075 and 2024 225**

Nduka Ekene Udoye

Maritza

**Trimming and Hemming 203** Helmut Klöcker and Christopher Yukna

**Aluminium-Based Alloy 243**

**Behavior of Aluminum Alloy 263**

**Features and Mechanical Behavior of Aluminum Alloys 137**

Olawale Samuel Fatoba, Esther Titilayo Akinlabi and Mamookho

In the twenty-first century, aluminium-based lightweight materials are being used for many commercial applications, which save both the energy and cost. The major features are lighter in weight, considerable strength, high value of corrosion resistance and considerable ductili‐ ty. Most of the products made from aluminium alloys are from their wrought products that completely replace the usage of cast iron and steel as these materials would enhance the weight of the products (both cast iron and steel). Moreover, aluminium alloys can be recy‐ clable, which are available more in the world. The present book has several sections that address the real research works and review works contributed by several researchers in the world that cover the latest processing routes of aluminium alloys including composites through casting, forming, severe plastic deformation, various grain refinement techniques especially from equal channel angular pressing, friction stir welding and corrosion behav‐ iour, effect of alloying elements on the properties improvement, various characterization techniques and rapid solidifications. Further, detailed mechanical behaviour of aluminium alloys is also being incorporated in the present book. This book will help all academicians, the research community and industries for selection of aluminium-based materials, its proc‐ essing routes, and its appropriate applications.

> **Dr. S. Sivasankaran** College of Engineering, Qassim University, Saudi Arabia

**Chapter 1**

**Provisional chapter**

which have no grain-refining

Ti

**Effects of Grain Refining on Columnar-to-Equiaxed**

The effects of grain refining in ultra-pure aluminum, commercially pure aluminum (1050), and Al-7%Si binary alloy were investigated, using different additions of Al-10%Ti, Al-5%Ti-1%B, and Al-4%B master alloys. Thermal analysis and metallography were used to assess the variations in microstructure resulting from these additions, at solidification rates of 0.8°C/s and ~10°C/s. The results revealed that addition

effect. Without grain refiner addition, the pure aluminum microstructure exhibits a mixture of columnar and equiaxed grains. Addition of 30ppm Ti is sufficient to promote equiaxed grains at ~10°C/s but requires addition of 1000 ppm B to obtain similar results at 0.8°C/s. Increasing the Si content to 7% reduces the initial grain size of pure aluminum from 2800 μm to ~1850 μm, and further to 450 μm with ddition of ~500ppm B. In commercial aluminum, the B reacts with traces of Ti forming Al3

phases which are active grain-refiners. In Al-7%Si, Ti reacts with Si forming

Ti phase, which is a poor refining agent. This phenomenon is termed poisoning. No interaction between B and Si is observed in the commercial aluminum or Al-7%Si

**Keywords:** aluminum, grain refining, poisoning, columnar-to-equiaxed transition,

**Effects of Grain Refining on Columnar-to-Equiaxed**

DOI: 10.5772/intechopen.70346

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Master alloys of the type Al-B are largely used in production of ultra-pure aluminum to react with transition elements such as V, Cr, and Zr [1]. Boron is not considered as

**Transition in Aluminum Alloys**

**Transition in Aluminum Alloys**

Hicham Tahiri, Serageldin S. Mohamed, Herbert W. Doty, Salvador Valtierra and

Hicham Tahiri, Serageldin S. Mohamed, Herbert W. Doty, Salvador Valtierra

Additional information is available at the end of the chapter

of Al-4%B to ultra-pure aluminum forms AlB12 and AlB2

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70346

Fawzy H. Samuel

**Abstract**

and TiB2

alloy when B is added.

solidification rate, macrostructure

(Al,Si)2

**1. Introduction**

and Fawzy H. Samuel

**Provisional chapter**

## **Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys Transition in Aluminum Alloys**

**Effects of Grain Refining on Columnar-to-Equiaxed**

DOI: 10.5772/intechopen.70346

Hicham Tahiri, Serageldin S. Mohamed, Herbert W. Doty, Salvador Valtierra and Fawzy H. Samuel Herbert W. Doty, Salvador Valtierra and Fawzy H. Samuel Additional information is available at the end of the chapter

Hicham Tahiri, Serageldin S. Mohamed,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70346

#### **Abstract**

The effects of grain refining in ultra-pure aluminum, commercially pure aluminum (1050), and Al-7%Si binary alloy were investigated, using different additions of Al-10%Ti, Al-5%Ti-1%B, and Al-4%B master alloys. Thermal analysis and metallography were used to assess the variations in microstructure resulting from these additions, at solidification rates of 0.8°C/s and ~10°C/s. The results revealed that addition of Al-4%B to ultra-pure aluminum forms AlB12 and AlB2 which have no grain-refining effect. Without grain refiner addition, the pure aluminum microstructure exhibits a mixture of columnar and equiaxed grains. Addition of 30ppm Ti is sufficient to promote equiaxed grains at ~10°C/s but requires addition of 1000 ppm B to obtain similar results at 0.8°C/s. Increasing the Si content to 7% reduces the initial grain size of pure aluminum from 2800 μm to ~1850 μm, and further to 450 μm with ddition of ~500ppm B. In commercial aluminum, the B reacts with traces of Ti forming Al3 Ti and TiB2 phases which are active grain-refiners. In Al-7%Si, Ti reacts with Si forming (Al,Si)2 Ti phase, which is a poor refining agent. This phenomenon is termed poisoning. No interaction between B and Si is observed in the commercial aluminum or Al-7%Si alloy when B is added.

**Keywords:** aluminum, grain refining, poisoning, columnar-to-equiaxed transition, solidification rate, macrostructure

## **1. Introduction**

Master alloys of the type Al-B are largely used in production of ultra-pure aluminum to react with transition elements such as V, Cr, and Zr [1]. Boron is not considered as

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

effective a grain refiner when added to pure aluminum [2]. Once Si is added to Al, grain refining is activated together with a change in the α-Al dendritic structure [3]. Two types of B-compounds exist in the Al-B master alloys: AlB2 and AlB12 [4]. The AlB2 compound is stable at room temperature and contains 44.5% B [5–7]. It is inferred from the Al-B binary diagram presented in **Figure 1(a)** that there is a peritectic reaction at 975°C: L + AlB12 → AlB2 . A eutectic reaction (liquid → α-Al + AlB2 ) takes place at 660°C, making the maximum solubility of B in Al about 20 ppm where the α-Al grains would precipitate on the AlB2 particles. The melting points of AlB2 and AlB12 are 1665 ± 50°C and 2163 ± 50°C, respectively, resulting in the formation of solid dispersoid particles in the molten liquid [8]. Fundamentally speaking, the addition of a grain refiner to the molten metal would result in the nucleation of new grains or reaction with other elements in the molten

metal to form nucleation sites. When Al-4%B master alloy is added to commercial alu-

Δ*G* = −73381 + 38.996T, where T is the liquid temperature [9]. The enthalpy energy associ-

In order to clarify the role of Si in activating the grain refiner, several castings were made using pure Al (99.999%), commercially pure aluminum (1050), and a binary Al-7%Si alloy. The study was conducted using thermal analysis, whereas another set of castings for grain

In this study, pure aluminum (99.999%), commercial pure aluminum (1050), and Al-7%Si were used. The compositions of the materials used are shown in **Table 1**. Each base alloy was melted in a resistance-heated clay graphite crucible and held at 750°C. The three grain refiners Al-10%Ti, Al-5%Ti-1%B, and Al-4%B were added to each melt in the amounts (Ti or B): 0.02–0.4% wt.%. Prior to casting, the liquid metal was held at 750°C for 10, 30, 60, 90, and 120 min after the grain refiner addition was made and then cast. During the holding time, the melt was continuously stirred using a graphite impeller to minimize sedimentation of Ti-containing particles and maintain uniformity. Samplings from the different melts prepared at 750°C were poured into a preheated (600°C) cylindrical graphite mold to achieve near-equilibrium solidification conditions (solidification rate ~0.8°C/s), as shown in **Figure 1(b)**. Another set of castings was made using steel cups of the reduced pressure testing machine in air. The steel cups were not preheated in order to obtain a high cooling rate of about 10°C/s, as shown in **Figure 1(c)**—solidification rate is about 10°C/s. Castings were also made using steel cups preheated at 450°C to represent an intermediate solidifica-

tion rate. Additionally, the microstructure was evaluated at both cooling rates.

The temperature-time data was obtained using a K-type (chromel-alumel) thermocouple inserted through the bottom of the graphite mold along the centerline, its tip reaching up to about one-third the height of the mold from the bottom. The cooling curves were recorded using a data acquisition system attached to the thermocouple. Chemical analysis

**Alloy Al Si Cu Mg Fe Mn Zn Ti Sr** Al pure 99.998 0 0 0 0 0 0 0 0 1050 Bal. 0.25 0.05 0.05 0.4 0.04 0.07 <0.05 0 Al-7%Si Bal. 6.78 0.02 00 0.03 0.04 0.04 <0.03 0

(s)). The free energy (ΔG) associated with this reaction can be calculated as

phase particles is fairly large – 326.41 kJ/mol—which allows

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

http://dx.doi.org/10.5772/intechopen.70346

phase

3

minum containing traces of Ti, the B atoms would react with Ti forming TiB2

refining was carried out using reduced pressure testing steel cups.

(Ti + B → TiB2

ated with the formation of TiB2

**2. Experimental procedure**

**Table 1.** Chemical composition (wt.%).

for their stability in the molten metal.

**Figure 1.** (a) Aluminum rich corner of Al-B phase diagram [7]. Broken arrow points to Al + AlB2 eutectic reaction. (b) Schematic drawing showing the graphite mold used for thermal analysis. (c) Casting made in steel cups to obtain a solidification rate of 10°C/s.

metal to form nucleation sites. When Al-4%B master alloy is added to commercial aluminum containing traces of Ti, the B atoms would react with Ti forming TiB2 phase (Ti + B → TiB2 (s)). The free energy (ΔG) associated with this reaction can be calculated as Δ*G* = −73381 + 38.996T, where T is the liquid temperature [9]. The enthalpy energy associated with the formation of TiB2 phase particles is fairly large – 326.41 kJ/mol—which allows for their stability in the molten metal.

In order to clarify the role of Si in activating the grain refiner, several castings were made using pure Al (99.999%), commercially pure aluminum (1050), and a binary Al-7%Si alloy. The study was conducted using thermal analysis, whereas another set of castings for grain refining was carried out using reduced pressure testing steel cups.

## **2. Experimental procedure**

effective a grain refiner when added to pure aluminum [2]. Once Si is added to Al, grain refining is activated together with a change in the α-Al dendritic structure [3]. Two types

is stable at room temperature and contains 44.5% B [5–7]. It is inferred from the Al-B binary diagram presented in **Figure 1(a)** that there is a peritectic reaction at 975°C: L +

the maximum solubility of B in Al about 20 ppm where the α-Al grains would precipi-

± 50°C, respectively, resulting in the formation of solid dispersoid particles in the molten liquid [8]. Fundamentally speaking, the addition of a grain refiner to the molten metal would result in the nucleation of new grains or reaction with other elements in the molten

and AlB12 [4]. The AlB2

) takes place at 660°C, making

and AlB12 are 1665 ± 50°C and 2163

compound

eutectic reaction. (b)

of B-compounds exist in the Al-B master alloys: AlB2

. A eutectic reaction (liquid → α-Al + AlB2

2 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

particles. The melting points of AlB2

**Figure 1.** (a) Aluminum rich corner of Al-B phase diagram [7]. Broken arrow points to Al + AlB2

Schematic drawing showing the graphite mold used for thermal analysis. (c) Casting made in steel cups to obtain

AlB12 → AlB2

tate on the AlB2

a solidification rate of 10°C/s.

In this study, pure aluminum (99.999%), commercial pure aluminum (1050), and Al-7%Si were used. The compositions of the materials used are shown in **Table 1**. Each base alloy was melted in a resistance-heated clay graphite crucible and held at 750°C. The three grain refiners Al-10%Ti, Al-5%Ti-1%B, and Al-4%B were added to each melt in the amounts (Ti or B): 0.02–0.4% wt.%. Prior to casting, the liquid metal was held at 750°C for 10, 30, 60, 90, and 120 min after the grain refiner addition was made and then cast. During the holding time, the melt was continuously stirred using a graphite impeller to minimize sedimentation of Ti-containing particles and maintain uniformity. Samplings from the different melts prepared at 750°C were poured into a preheated (600°C) cylindrical graphite mold to achieve near-equilibrium solidification conditions (solidification rate ~0.8°C/s), as shown in **Figure 1(b)**. Another set of castings was made using steel cups of the reduced pressure testing machine in air. The steel cups were not preheated in order to obtain a high cooling rate of about 10°C/s, as shown in **Figure 1(c)**—solidification rate is about 10°C/s. Castings were also made using steel cups preheated at 450°C to represent an intermediate solidification rate. Additionally, the microstructure was evaluated at both cooling rates.

The temperature-time data was obtained using a K-type (chromel-alumel) thermocouple inserted through the bottom of the graphite mold along the centerline, its tip reaching up to about one-third the height of the mold from the bottom. The cooling curves were recorded using a data acquisition system attached to the thermocouple. Chemical analysis

**Table 1.** Chemical composition (wt.%).

of all melts/castings was determined using a Spectrolab Jr CCD Spark Analyzer (average of three burns per sample).

The solidified castings were sectioned perpendicular to the centerline axis of the cylinder, at the level of the thermocouple tip, and polished for metallographic examination, using standard polishing procedures. The polished samples were examined using an optical microscope and an electron probe micro-analyzer (JEOL JXA-8900L operating at 20 kV), equipped with energy dispersive X-ray spectroscopic (EDS) and wavelength dispersive spectroscopic (WDS) facilities.

## **3. Results and discussion**

**Figure 2** shows the temperature-time curves of pure Al and Al-7%Si binary alloy after different melt treatments. **Figure 2(a)** shows that the addition of B to pure Al reduces the undercooling by approximately ~1°C, indicating the relative ineffectiveness of B as a grain refiner in the absence of Si. A 300 ppm level of B added to the Al-7%Si alloy eliminated the phenomenon of recalescence. Increasing the concentration of B to 800 ppm has minimal further effect on the alloy solidification behavior. As shown below, the transformation of AlB12 to a simple phase AlB2 is the principal parameter in the disappearance of the observed undercooling shown in **Figure 2(a)**.

**Figure 2(b)** demonstrates the variation in aluminum grain size as a function of Sr-B interaction in the presence of 200 ppm Sr as well as holding time. In the absence of melt treatment, the average grain size was about 2900 μm. Once the two agents (B and Sr) were simultaneously added to the melt, the grain size dropped to 420 μm at 0.1%B. The observed reduction in the grain size may be interpreted in part as due to presence of traces of Ti either in the used aluminum or in the added Al-4%B master alloy as shown in **Table 2**. At higher B concentrations, the grain size tended to increase reaching 3300 μm at holding time of 120 min. This observation may be explained in terms of agglomeration of the nucleation sites [10]. As shown in **Table 1**, a eutectic reaction takes place at about 660°C; Liquid → AlB2 + α (solid). In Si-containing aluminum alloys, i.e., 356 alloy, the melting point is less than the eutectic temperature, which facilitates the formation of AlB2 , an active nucleation agent. The melting temperature of pure Al is about 660.5°C, and hence the reported sluggishness of B as a grain refiner in pure Al [11]. **Figure 2(c)** displays the variation in the grain size of Al-7%Si alloy as a function of holding time when the alloy was grain refined using Al-4%B master alloy in the presence of 200 ppm Sr. Due to high Si content, the initial grain size dropped to approximately 1850 μm. With the addition of approximately 500 ppm B, the grain size was reduced to about 450 μm (~75%). Holding time up to 120 min seems to have a marginal effect on further decrease in the alloy grain size. These findings are in good agreement with the abovementioned discussion.

**Figure 3** reveals the possibility of the coexistence of the two B-based compounds in pure aluminum. The composition of these two phases was confirmed using the WDS technique as shown in **Table 3**. **Figure 4** shows the distribution of B- (black), and Si-phases (light gray)

in Al-7%Si alloy. As can be seen, these elements tend to precipitate away from each other (marked zones A and B in **Figure 4**). This observation is confirmed by the X-ray elemental distribution for each zone shown in **Figure 5**, indicating that there is no affinity for reaction

**Figure 2.** (a) Temperature-time solidification curves obtained from pure Al and Al-7%Si alloy following different melt treatments. (b) Effect of Sr-B interaction on the pure aluminum grain size as a function of holding time. (c) Effect of Sr-B

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

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5

interaction on the grain size of Al-7%Si alloy as a function of holding time.

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys http://dx.doi.org/10.5772/intechopen.70346 5

of all melts/castings was determined using a Spectrolab Jr CCD Spark Analyzer (average

4 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

The solidified castings were sectioned perpendicular to the centerline axis of the cylinder, at the level of the thermocouple tip, and polished for metallographic examination, using standard polishing procedures. The polished samples were examined using an optical microscope and an electron probe micro-analyzer (JEOL JXA-8900L operating at 20 kV), equipped with energy dispersive X-ray spectroscopic (EDS) and wavelength dispersive spectroscopic

**Figure 2** shows the temperature-time curves of pure Al and Al-7%Si binary alloy after different melt treatments. **Figure 2(a)** shows that the addition of B to pure Al reduces the undercooling by approximately ~1°C, indicating the relative ineffectiveness of B as a grain refiner in the absence of Si. A 300 ppm level of B added to the Al-7%Si alloy eliminated the phenomenon of recalescence. Increasing the concentration of B to 800 ppm has minimal further effect on the alloy solidification behavior. As shown below, the transformation of AlB12 to a simple

**Figure 2(b)** demonstrates the variation in aluminum grain size as a function of Sr-B interaction in the presence of 200 ppm Sr as well as holding time. In the absence of melt treatment, the average grain size was about 2900 μm. Once the two agents (B and Sr) were simultaneously added to the melt, the grain size dropped to 420 μm at 0.1%B. The observed reduction in the grain size may be interpreted in part as due to presence of traces of Ti either in the used aluminum or in the added Al-4%B master alloy as shown in **Table 2**. At higher B concentrations, the grain size tended to increase reaching 3300 μm at holding time of 120 min. This observation may be explained in terms of agglomeration of the nucleation sites [10]. As shown in **Table 1**, a eutectic reaction takes place at about 660°C; Liquid →

+ α (solid). In Si-containing aluminum alloys, i.e., 356 alloy, the melting point is less

agent. The melting temperature of pure Al is about 660.5°C, and hence the reported sluggishness of B as a grain refiner in pure Al [11]. **Figure 2(c)** displays the variation in the grain size of Al-7%Si alloy as a function of holding time when the alloy was grain refined using Al-4%B master alloy in the presence of 200 ppm Sr. Due to high Si content, the initial grain size dropped to approximately 1850 μm. With the addition of approximately 500 ppm B, the grain size was reduced to about 450 μm (~75%). Holding time up to 120 min seems to have a marginal effect on further decrease in the alloy grain size. These findings are

**Figure 3** reveals the possibility of the coexistence of the two B-based compounds in pure aluminum. The composition of these two phases was confirmed using the WDS technique as shown in **Table 3**. **Figure 4** shows the distribution of B- (black), and Si-phases (light gray)

, an active nucleation

than the eutectic temperature, which facilitates the formation of AlB2

in good agreement with the abovementioned discussion.

is the principal parameter in the disappearance of the observed undercooling

of three burns per sample).

**3. Results and discussion**

(WDS) facilities.

phase AlB2

AlB2

shown in **Figure 2(a)**.

**Figure 2.** (a) Temperature-time solidification curves obtained from pure Al and Al-7%Si alloy following different melt treatments. (b) Effect of Sr-B interaction on the pure aluminum grain size as a function of holding time. (c) Effect of Sr-B interaction on the grain size of Al-7%Si alloy as a function of holding time.

in Al-7%Si alloy. As can be seen, these elements tend to precipitate away from each other (marked zones A and B in **Figure 4**). This observation is confirmed by the X-ray elemental distribution for each zone shown in **Figure 5**, indicating that there is no affinity for reaction

**Table 2.** Chemical analysis of melt treated pure aluminum samples.

between these two elements In contrast, when Ti is added with B to the 1050 commercial aluminum, a clear interaction between Ti and B takes place leading to the formation of TiB2 , as demonstrated in **Figure 6**. Thus, B has no grain-refining capacity when added as a single element to ultra-pure aluminum, whereas when the metal contains traces of Ti, adding B leads to the possible formation of different nucleation sites that coexist, such as AlB12, AlB2 , and TiB2 in Al-Si alloys.

10°C/s [13, 14]. In the absence of a grain refiner, the macrostructure of pure aluminum is composed of three zones: (i) columnar near the bottom of the crucible due to the development of positive thermal gradient in the liquid metal leading to formation of grains with preferred orientation [14], (ii) fine equiaxed grained zone, and (iii) coarse grained zone where the grains are randomly oriented caused by fragmentation of the columnar grains driven by convection in the liquid metal, as illustrated in **Figure 7** for samples solidified at 0.8°C/s. The equiaxed

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

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7

), (b) X-ray image of B

**Figure 3.** (a) Presence of AlB12 in ultra-pure aluminum (possible decomposition of AlB12 to AlB2

distribution in AlB12 phase particles in (a)-arrowed.

It is well established that increasing solidification rate significantly enhances grain-refining effects [12–14]. In order to illustrate this, samples were poured into steel cups of the reduced pressure testing machine (about 200 g) in air corresponding to a solidification rate of about Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys http://dx.doi.org/10.5772/intechopen.70346 7

**Figure 3.** (a) Presence of AlB12 in ultra-pure aluminum (possible decomposition of AlB12 to AlB2 ), (b) X-ray image of B distribution in AlB12 phase particles in (a)-arrowed.

between these two elements In contrast, when Ti is added with B to the 1050 commercial aluminum, a clear interaction between Ti and B takes place leading to the formation of TiB2

as demonstrated in **Figure 6**. Thus, B has no grain-refining capacity when added as a single element to ultra-pure aluminum, whereas when the metal contains traces of Ti, adding B leads to the possible formation of different nucleation sites that coexist, such as AlB12, AlB2

It is well established that increasing solidification rate significantly enhances grain-refining effects [12–14]. In order to illustrate this, samples were poured into steel cups of the reduced pressure testing machine (about 200 g) in air corresponding to a solidification rate of about

and TiB2

**Pure Al, series Al-4%B**

**Trial # wt.%**

**B addition**

10 1 0.1 0.062 78 0.0403 200 145

6 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

30 1 0.1 0.062 132 0.0342 200 101

60 1 0.1 0.054 84 0.0401 200 64

90 1 0.1 0.06 48 0.0249 200 51

120 1 0.1 0.065 153 0.0367 200 58

2 0.2 0.062 114 0.0104 200 195 3 0.3 0.063 206 0.005 200 155 4 0.4 0.068 189 0.0031 200 125

2 0.2 0.066 203 0.005 200 143 3 0.3 0.064 147 <0.0013 200 145 4 0.4 0.077 225 0.0019 200 95

2 0.2 0.063 93 <0.0013 200 106 3 0.3 0.063 >360 0.0027 200 110 4 0.4 0.061 104 0.0041 200 61

2 0.2 0.064 148 <0.0013 200 74 3 0.3 0.07 165 <0.0013 200 80 4 0.4 0.057 90 <0.0013 200 49

2 0.2 0.066 49 <0.0013 200 50 3 0.3 0.0742 >360 0.0027 200 53 4 0.4 0.059 121 0.0014 200 66

**wt.% Si B (ppm) % Ti Added**

**Sr (ppm)**

**Actual Sr (ppm)**

**Holding time (min)**

in Al-Si alloys.

**Table 2.** Chemical analysis of melt treated pure aluminum samples.

5

5

,

,

10°C/s [13, 14]. In the absence of a grain refiner, the macrostructure of pure aluminum is composed of three zones: (i) columnar near the bottom of the crucible due to the development of positive thermal gradient in the liquid metal leading to formation of grains with preferred orientation [14], (ii) fine equiaxed grained zone, and (iii) coarse grained zone where the grains are randomly oriented caused by fragmentation of the columnar grains driven by convection in the liquid metal, as illustrated in **Figure 7** for samples solidified at 0.8°C/s. The equiaxed

**Table 3.** WDS analysis (at.%) of AlB2 and AlB12 phases observed in **Figure 3(a)**.

crystals have a larger size than the chilled ones, and they are the result of volumetric solidification, which proceeds when the initially high thermal gradient is reduced as solidification progresses [15]. The refinement of the primary structure is a result of the creation of phases that act as substrates of heterogeneous nucleation for the primary aluminum phase. Therefore, active centers of aluminum heterogeneous nucleation are particles that have a high melting point and close crystallographic match with aluminum, e.g., TiC, TiN, TiB, TiB2 , AlB2 , Al3 Ti, and Zn3 Ti [16–20].

When an Al-5%Ti-1%B master alloy is added to pure aluminum, it decomposes into Al3 Ti and TiB2 phase particles. Since the dissolution of Al3 Ti is rather fast in Al-Si alloys, it forms Ti(Sl1−x,Six )3 where *x* < 0.15 [21]. The latter tends to cover the TiB2 particle surface acting as a heterogeneous nucleation substrate for the α-Al through the peritectic reaction. The addition of 600 ppm Ti in the form of Al-10%Ti to pure aluminum (**Figure 7(b)**) resulted in the complete elimination of the columnar grains with marked reduction in the grain size of the two equiaxed zones observed in **Figure 7(a)**. Increasing the Ti concentration to 1000 ppm led

to formation of one zone characterized by its ultra-fine grain size (**Figure 7(c)**). Similar observations were made when the liquid aluminum was treated with Al-5%Ti-1%B master alloy

**Figure 5.** Selected X-ray for B and Si distribution of (a) B (b) Si in **Figure 4**—Al-7%Si alloy treated with 800 ppm B.

As previously mentioned, B has no grain-refining effect on ultra-pure aluminum. However, if its addition is associated with a relatively high cooling rate, i.e., 10°C/s as shown in **Figure 8**, it could result in some refining depending on B concentration as shown in **Figure 8(d)**, where at least 1000 ppm B is used to obtain more or less same level of grain size produced by the addi-

from the bottom of the crucible to the top surface. This is usually attributed to the fact that the crucible surface acts as a nucleation substrate for heterogeneous nucleation of α-Al. In addition, once solidification initiates on the relatively cold bottom of the crucible, sensible and latent heat are released to the crucible, reducing the temperature gradient and effectively slowing

(**Figure 7(e)**). These experiments always produced a grain size gradient

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

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9

(**Figure 7**(**d** and **e**)).

tion of 30 ppm TiB2

**Figure 4.** Presence of B and Si in Al-7%Si treated with 800 ppm B. Note the geometric shape of the particles in zone A.

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys http://dx.doi.org/10.5772/intechopen.70346 9

crystals have a larger size than the chilled ones, and they are the result of volumetric solidification, which proceeds when the initially high thermal gradient is reduced as solidification progresses [15]. The refinement of the primary structure is a result of the creation of phases that act as substrates of heterogeneous nucleation for the primary aluminum phase. Therefore, active centers of aluminum heterogeneous nucleation are particles that have a high melting

and AlB12 phases observed in **Figure 3(a)**.

When an Al-5%Ti-1%B master alloy is added to pure aluminum, it decomposes into Al3

erogeneous nucleation substrate for the α-Al through the peritectic reaction. The addition of 600 ppm Ti in the form of Al-10%Ti to pure aluminum (**Figure 7(b)**) resulted in the complete elimination of the columnar grains with marked reduction in the grain size of the two equiaxed zones observed in **Figure 7(a)**. Increasing the Ti concentration to 1000 ppm led

**Figure 4.** Presence of B and Si in Al-7%Si treated with 800 ppm B. Note the geometric shape of the particles in zone A.

, AlB2

Ti is rather fast in Al-Si alloys, it forms

particle surface acting as a het-

, Al3 Ti,

Ti

point and close crystallographic match with aluminum, e.g., TiC, TiN, TiB, TiB2

where *x* < 0.15 [21]. The latter tends to cover the TiB2

phase particles. Since the dissolution of Al3

**Al B Phase** 34.15 63.19 AlB2

8 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

7.32 91.56 AlB12

and Zn3

and TiB2

Ti(Sl1−x,Six

Ti [16–20].

35.40 64.47 31.47 66.64 33.62 65.51

8.52 90.78

**Table 3.** WDS analysis (at.%) of AlB2

)3

**Figure 5.** Selected X-ray for B and Si distribution of (a) B (b) Si in **Figure 4**—Al-7%Si alloy treated with 800 ppm B.

to formation of one zone characterized by its ultra-fine grain size (**Figure 7(c)**). Similar observations were made when the liquid aluminum was treated with Al-5%Ti-1%B master alloy (**Figure 7**(**d** and **e**)).

As previously mentioned, B has no grain-refining effect on ultra-pure aluminum. However, if its addition is associated with a relatively high cooling rate, i.e., 10°C/s as shown in **Figure 8**, it could result in some refining depending on B concentration as shown in **Figure 8(d)**, where at least 1000 ppm B is used to obtain more or less same level of grain size produced by the addition of 30 ppm TiB2 (**Figure 7(e)**). These experiments always produced a grain size gradient from the bottom of the crucible to the top surface. This is usually attributed to the fact that the crucible surface acts as a nucleation substrate for heterogeneous nucleation of α-Al. In addition, once solidification initiates on the relatively cold bottom of the crucible, sensible and latent heat are released to the crucible, reducing the temperature gradient and effectively slowing

**Figure 6.** (a) Decomposition of Al-5%Ti-1%B master alloy, (b) backscattered electron image and (c) element distribution of Ti-B interaction in 1050 aluminum treated with 240 ppm B. Note the attraction of Ti ring toward the cluster of B-rich particles.

the solidification rate. According to Chalmers [15], predendritic solid nuclei that formed on the mold surface during filling of the mold are driven away by circulation of the liquid metal. Depending on the degree of undercooling, some of these nuclei would survive in the liq-

**Figure 8.** Macrostructures of ultra-pure aluminum treated with progressive doses of B: (a) 30 ppm B, (b) 60 ppm B, (c) 120 ppm B, (d) 1000 ppm B. All micrographs have the same magnification. Note the progressive transition from columnar-

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

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11

Considering the Al-7%Si alloy, its structure was not affected by the phenomenon mentioned above in the case of ultra-pure aluminum. **Figure 9** shows the macrostructure of the alloy with different melt treatments consisting of fine equiaxed grains caused by the addition of a suitable dose of grain refiner associated with application of high solidification rate—note

uid until the undercooling is extracted, resulting in equiaxed grains.

to-equiaxed (CTE) with the increase in B concentration.

**Figure 7.** Macrostructures of ultra-pure aluminum treated by two types of grain refiners: (a) Al pure (no treatment), (b) Al pure + Al3 Ti (600 ppm), (c) Al pure + Al3 Ti (1000 ppm), (d) Al pure + TiB2 (15 ppm), (e) Al pure + TiB2 (30 ppm). All micrographs have same magnification-solidification time was ~200 s. Note the heterogeneity in grain size on going from bottom to top.

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys http://dx.doi.org/10.5772/intechopen.70346 11

**Figure 6.** (a) Decomposition of Al-5%Ti-1%B master alloy, (b) backscattered electron image and (c) element distribution of Ti-B interaction in 1050 aluminum treated with 240 ppm B. Note the attraction of Ti ring toward the cluster of B-rich

10 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

**Figure 7.** Macrostructures of ultra-pure aluminum treated by two types of grain refiners: (a) Al pure (no treatment), (b) Al

have same magnification-solidification time was ~200 s. Note the heterogeneity in grain size on going from bottom to top.

(15 ppm), (e) Al pure + TiB2

(30 ppm). All micrographs

Ti (1000 ppm), (d) Al pure + TiB2

particles.

pure + Al3

Ti (600 ppm), (c) Al pure + Al3

**Figure 8.** Macrostructures of ultra-pure aluminum treated with progressive doses of B: (a) 30 ppm B, (b) 60 ppm B, (c) 120 ppm B, (d) 1000 ppm B. All micrographs have the same magnification. Note the progressive transition from columnarto-equiaxed (CTE) with the increase in B concentration.

the solidification rate. According to Chalmers [15], predendritic solid nuclei that formed on the mold surface during filling of the mold are driven away by circulation of the liquid metal. Depending on the degree of undercooling, some of these nuclei would survive in the liquid until the undercooling is extracted, resulting in equiaxed grains.

Considering the Al-7%Si alloy, its structure was not affected by the phenomenon mentioned above in the case of ultra-pure aluminum. **Figure 9** shows the macrostructure of the alloy with different melt treatments consisting of fine equiaxed grains caused by the addition of a suitable dose of grain refiner associated with application of high solidification rate—note

observed when increasing the solidification rate and a progressive CTE when lowering the tem-

The results obtained show that when Al-4%B is added to ultra-pure aluminum, it decom-

no grain refiner is added to pure aluminum, the microstructure is a mixture of columnar and equiaxed grains. The addition of 30 ppm Ti to pure aluminum is sufficient to promote equiaxed grains when the metal is solidified at high rate (~10°C/s). A similar effect can be achieved with the addition of excessive amounts of B, in the order of 1000 ppm. Increasing the Si content to 7% enhances the reduction in initial grain size of pure aluminum from 2800 to 1850 μm in Al-7%Si alloy. In the case of Al-7%Si, only 500 ppm B is sufficient to reduce the initial grain size by about 75%. Due to a reduced temperature gradient as the solidification front travels from the bottom of the crucible to the top, the grain size varies accordingly. In commercial aluminum, the addition of B would react with traces of Ti leading to the forma-

is termed *poisoning*. No interaction was observed to take place between B and Si. In general, the grain size varied along the height of the crucible due to change in the liquid temperature.

The authors would like to thank Amal Samuel for enhancing the quality of the images

, Herbert W. Doty2

1 Département des Sciences Appliquées, Université du Québec à Chicoutimi, Québec,

and drawings presented in this article and Dr. E. Samuel for editing the manuscript.

phases that have no grain-refining effect in pure aluminum. When

Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

http://dx.doi.org/10.5772/intechopen.70346

13

phases that are effective grain-refining agents. In the case of Al-7%Si,

Ti phase which is a poor refining agent. This phenomenon

, Salvador Valtierra3

and

perature gradient [24].

**4. Concluding remarks**

poses into AlB12 and AlB2

Ti and TiB2

**Acknowledgements**

**Conflict of interest**

**Author details**

Hicham Tahiri1

Canada

Fawzy H. Samuel1

\*

3 Nemak, S.A., Garza Garcia, N.L., Mexico

There is no conflict of interest between the authors.

, Serageldin S. Mohamed1

2 General Motors Materials Engineering, Pontiac, MI, USA

\*Address all correspondence to<EMAIL_ADDRESS>
Ti reacts with Si forming (Al, Si)2

tion of Al3

**Figure 9.** Variation in the grain size of Al-7%Si alloy: (a) base alloy—no addition, (b) alloy treated with Al-10%Ti (300 ppm Ti), (c) alloy treated with Al-5%Ti-1%B (150 ppm of B), (d) alloy treated with Al-5%Ti-1%B (300 ppm of B), (e) alloy treated with Al-4%B (600 ppm B), (f) alloy treated with 1000 ppm B. All micrographs have the same micron bars.

the absence of columnar grains. The transition from columnar to equiaxed grains depends on several factors: (i) mold geometry, (ii) the amount of extracted heat, (iii) composition of used alloy, (iv) density of the used grain refiner, (v) fluidity of the liquid metal, (vi) interaction between grain refiner and alloy composition and (vii) sedimentation of the added grain refiner [22]. A high solidification rate, large amount of grain refiners as well as low temperature gradient would enhance fine equiaxed grains [23]. Al-7wt.% Si alloys with and without grain refiners solidified in diffusive conditions showed columnar growth in case of nonrefined alloy, and the existence of a columnar-to-equiaxed transition (CTE) in refined alloy. A sharp CTE is observed when increasing the solidification rate and a progressive CTE when lowering the temperature gradient [24].

## **4. Concluding remarks**

The results obtained show that when Al-4%B is added to ultra-pure aluminum, it decomposes into AlB12 and AlB2 phases that have no grain-refining effect in pure aluminum. When no grain refiner is added to pure aluminum, the microstructure is a mixture of columnar and equiaxed grains. The addition of 30 ppm Ti to pure aluminum is sufficient to promote equiaxed grains when the metal is solidified at high rate (~10°C/s). A similar effect can be achieved with the addition of excessive amounts of B, in the order of 1000 ppm. Increasing the Si content to 7% enhances the reduction in initial grain size of pure aluminum from 2800 to 1850 μm in Al-7%Si alloy. In the case of Al-7%Si, only 500 ppm B is sufficient to reduce the initial grain size by about 75%. Due to a reduced temperature gradient as the solidification front travels from the bottom of the crucible to the top, the grain size varies accordingly.

In commercial aluminum, the addition of B would react with traces of Ti leading to the formation of Al3 Ti and TiB2 phases that are effective grain-refining agents. In the case of Al-7%Si, Ti reacts with Si forming (Al, Si)2 Ti phase which is a poor refining agent. This phenomenon is termed *poisoning*. No interaction was observed to take place between B and Si. In general, the grain size varied along the height of the crucible due to change in the liquid temperature.

## **Acknowledgements**

The authors would like to thank Amal Samuel for enhancing the quality of the images and drawings presented in this article and Dr. E. Samuel for editing the manuscript.

## **Conflict of interest**

There is no conflict of interest between the authors.

## **Author details**

the absence of columnar grains. The transition from columnar to equiaxed grains depends on several factors: (i) mold geometry, (ii) the amount of extracted heat, (iii) composition of used alloy, (iv) density of the used grain refiner, (v) fluidity of the liquid metal, (vi) interaction between grain refiner and alloy composition and (vii) sedimentation of the added grain refiner [22]. A high solidification rate, large amount of grain refiners as well as low temperature gradient would enhance fine equiaxed grains [23]. Al-7wt.% Si alloys with and without grain refiners solidified in diffusive conditions showed columnar growth in case of nonrefined alloy, and the existence of a columnar-to-equiaxed transition (CTE) in refined alloy. A sharp CTE is

**Figure 9.** Variation in the grain size of Al-7%Si alloy: (a) base alloy—no addition, (b) alloy treated with Al-10%Ti (300 ppm Ti), (c) alloy treated with Al-5%Ti-1%B (150 ppm of B), (d) alloy treated with Al-5%Ti-1%B (300 ppm of B), (e) alloy treated with Al-4%B (600 ppm B), (f) alloy treated with 1000 ppm B. All micrographs have the same micron bars.

12 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

Hicham Tahiri1 , Serageldin S. Mohamed1 , Herbert W. Doty2 , Salvador Valtierra3 and Fawzy H. Samuel1 \*

\*Address all correspondence to<EMAIL_ADDRESS>
1 Département des Sciences Appliquées, Université du Québec à Chicoutimi, Québec, Canada

2 General Motors Materials Engineering, Pontiac, MI, USA

3 Nemak, S.A., Garza Garcia, N.L., Mexico

## **References**

[1] Stiller W, Ingenlath T. Industrial boron treatment of aluminium conductor alloys and its influence on grain refinement and electrical conductivity. Aluminium. Vol. 60; 1984. pp. E577-E580

[14] Kurz W, Bezençon C, Gäumann M. Columnar to equiaxed transition in solidification

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[15] Chalmers B. The structure of ingots. Journal of the Australian Institute of Metals.

[16] Guzowski M, Sigworth G, Sentner D. The role of boron in the grain refinement of alumi-

[17] Wróbel T. Review of inoculation methods of pure aluminum primary structure. Archives

[18] Greer. Structure and properties of grain-refined Al-20 wt.% Zn sand cast alloy.

[19] Haberl K, Krajewski WK, Schumacher P. Microstructural features of the grain-refined

[20] Wróbel T, Szajnar J. Influence of supply voltage frequency of induction coil on inoculation efficiency of pure aluminum structure. Foundry Engineering. 2010;**10**:203-208 [21] Hodaj F, Durand F. Equiaxed grains in multicomponent alloys: Effect of growth rate.

[22] Sturz L, Zimmerman G. Investigations on columnar to equiaxed transition in binary Al alloys with and without grain refiners. Materials Science Forum. 2006;**508**:419-424 [23] Sturz L, Drevermann A, Pickmann C, Zimmermann G. Influence of grain refinement on the columnar-to-equiaxed transition in binary Al alloys. Materials Science

[24] Zimmermann G, Sturz L, Billia B, Mangelinck-Noël N, Liu DR, Nguyen Thi H, Bergeon N, Gandin Ch-A, Browne DJ, Beckermann Ch, Tourret D, Karma A. Columnar-toequiaxed transition in solidification processing of AlSi7 alloys in microgravity-The

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[1] Stiller W, Ingenlath T. Industrial boron treatment of aluminium conductor alloys and its influence on grain refinement and electrical conductivity. Aluminium. Vol. 60; 1984.

[2] Cooper PS, Kearns MA. Removal of transition metal impurities in aluminium melts by boron additives. Trans Tech Publications Ltd. Aluminium Alloys: Their Physical

[3] Wang GQ, Liu SH, Li CM, Gao Q. Reaction of boron to transition metal impurities and its effect on conductivity of aluminum. Transactions of Nonferrous Metals Society

[4] Alamidari HD, Dube D, Tessier P. Behavior of boron in molten aluminum and its grain refinement mechanism. Metallurgical & Materials Transactions A. 2013;**44A**:388-394

[5] Mondolfo LF. Aluminum Alloy Structure and Properties. London: Butterworth; 1976

[6] Sigworth GK. The grain refining of aluminum and phase relationships in the aluminumtitanium-boron system. Metallurgical Transactions A: Physical Metallurgy and Materials Science. The Metallurgical Society of AIME and American Society for Metals, USA. Vol.

[7] Rogl P, Schuster JC. Phase Diagrams of Ternary Boron Nitride and Silicon Nitride

[8] Giardini AA, Kohn JA, Toman L, Eckart DW. Boron—Synthesis, Structure and Properties.

[9] Li H, Sritharan T, Lam YM, Leng NY. Effects of processing parameters on the performance of Al grain refinement master alloys Al-Ti and Al-B in small ingots. Journal

[10] Khalifa W. Rôle des inclusions dans la germination de la phase α-Aluminium et des intermétalliques contenant du fer dans le coin riche en aluminium du système ternaire Al-Si-Fe. thèse présentée à l'Université du Québec à Chicoutimi UQAC, novembre 2003. p. 340 [11] Lu HT, Wang LC, Kung SK. Grain refining in A356 alloys. Journal of Chinese

[12] Samuel AM, Samuel FH.Use ofthe reduced pressure test inthemeasurement ofthehydrogen content in the foundry of composites. In: Proceedings International Symposium on "Advances in the Production of Light Metals and Metal Matrix Composites", 31st Annual Conference of Metallurgists of CIM, Edmonton, Alberta, Canada, August 23-27,

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14 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

**Chapter 2**

**Provisional chapter**

**Some Considerations on the Structure Refinement in**

Grain size is one of the most important characteristics that affect the processing and in turn the properties of alloys. Grain refinement determines many advantages in light alloy casting: it can be achieved using different methods, based on the available technological possibilities and on the performances that one has to obtain. By using grain refinement, important benefits can be reached, for both cast and wrought aluminium alloys: among other, the most important enhancement regards the fine distribution of the second phases, improved castability, reduction of shrinkage porosity, higher mechanical properties, as well as superior fatigue life. The present chapter is not exhaustive on this argument; however, in the first part it reports some current literature data and some perspectives about the grain refinement, while in the second part which has been mostly carried out within a current PhD Thesis focalized on the improvement of the properties of Al-based alloys by physical grain refinement methods, some experimentally obtained

**Keywords:** Al-based alloys, structure refinement, grain refinement, metallurgical

**Some Considerations on the Structure Refinement in**

DOI: 10.5772/intechopen.71450

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Due to their excellent properties (high strength-to-weight ratio compared to Fe-based alloys, good corrosion resistance, etc.), light alloys (Al- or Mg-based alloys) have a key role in various engineering applications. Generally, during traditional casting process, dendritic microstructure is developed. Dendrites can be organised in columnar or equiaxed structures in agreement with the production conditions, which involve among others, the solute concentration, the heat gradient, the solidification velocity, the presence or absence of inoculating elements, etc. The control of the shape and distribution of the grains structure is made up is of

**Al-Based Alloys**

**Abstract**

**1. Introduction**

**Al-Based Alloys**

Ildiko Peter and Bela Varga

Ildiko Peter and Bela Varga

http://dx.doi.org/10.5772/intechopen.71450

Additional information is available at the end of the chapter

results have been presented and discussed.

methods, physical methods, morphology

Additional information is available at the end of the chapter

**Provisional chapter**

## **Some Considerations on the Structure Refinement in Al-Based Alloys Al-Based Alloys**

**Some Considerations on the Structure Refinement in**

DOI: 10.5772/intechopen.71450

Ildiko Peter and Bela Varga Ildiko Peter and Bela Varga Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71450

#### **Abstract**

Grain size is one of the most important characteristics that affect the processing and in turn the properties of alloys. Grain refinement determines many advantages in light alloy casting: it can be achieved using different methods, based on the available technological possibilities and on the performances that one has to obtain. By using grain refinement, important benefits can be reached, for both cast and wrought aluminium alloys: among other, the most important enhancement regards the fine distribution of the second phases, improved castability, reduction of shrinkage porosity, higher mechanical properties, as well as superior fatigue life. The present chapter is not exhaustive on this argument; however, in the first part it reports some current literature data and some perspectives about the grain refinement, while in the second part which has been mostly carried out within a current PhD Thesis focalized on the improvement of the properties of Al-based alloys by physical grain refinement methods, some experimentally obtained results have been presented and discussed.

**Keywords:** Al-based alloys, structure refinement, grain refinement, metallurgical methods, physical methods, morphology

#### **1. Introduction**

Due to their excellent properties (high strength-to-weight ratio compared to Fe-based alloys, good corrosion resistance, etc.), light alloys (Al- or Mg-based alloys) have a key role in various engineering applications. Generally, during traditional casting process, dendritic microstructure is developed. Dendrites can be organised in columnar or equiaxed structures in agreement with the production conditions, which involve among others, the solute concentration, the heat gradient, the solidification velocity, the presence or absence of inoculating elements, etc. The control of the shape and distribution of the grains structure is made up is of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

primary importance and really governs the properties of the produced alloy. In many conditions, improvement of the mechanical strength is required, especially in application where structural properties are essential. Thermal treatments and/or reducing the grain size of the constituents are commonly used methods for the improvement of some properties. Addition of grain refiners (GR) determines an increase of the mechanical properties, and in particular, it affects the strength and the fatigue resistance of the alloys, and at the same time, the quality of the cast part and the efficiency of the casting process rises [1]. According to some literature date [2–5] and some own research [6–11], one can state that the grain size is one of the most important characteristics, which affects the processing, and consequently, the properties of alloys. Presence/addition of GR determines many advantages in light alloy casting and it can be achieved using different methods, as will be presented later on. Actually at industrial level, both adding grain refiner and improving solidification conditions are two procedures for grain refinement, especially in case of Al alloys.

The reduction of the grain size has a positive effect on the microstructural homogeneity determining a fine distribution of the second phases leading to an improved castability, lower micro-porosity and shrinkage porosity as well as a reduced segregation development [4, 6].

In any case, the alloy composition has to be carefully considered in order to choose an efficient GR. Even if, Al–5Ti–B master alloy is an efficient GR for wrought Al alloys, its effect is not sufficient in case of foundry alloys: formation of Ti–Si phases occurs following the reaction between the solute Si and Ti determining the weakening of the nucleating of TiB2 particles in the master alloy, when the Si content in the alloy is higher than 3 wt.% [12, 13].

According to [14, 15], in case of master alloys containing high amount of B and higher than 4–5.5 wt% of Si, an improvement of the grain refining can be obtained. Prior to casting, an inadequate holding time negatively influences the GR efficiency and additionally their agglomeration can take place [16–18]. For Mg alloys, Zr is the ideal GR, but its use is unsuccessful when it contains Al.

Conventional GR for Al alloys are generally produced from Al–Ti–B ternary system [19–22].

On the contrary of Fe-based alloys, in case of light alloys, the primary structure after solidification is maintained and the physical and any structural transformations induced following thermal treatment and/or plastic deformations take place within the primary structure. Any outcomes coming from such technological treatments are influenced by the characteristics of the primary structure. Additionally, light alloys have higher tendency during solidification for the development of the coarse grains with respect to iron-based alloys. From these considerations, it is evident that for light alloys, the influence of the crystallisation process with the aim of the development of fine grains and the good distribution of the different phases within a homogeneous microstructure results very important and it significantly influences the properties of the final product during their use. The schematic illustration of the relation between the structure and the properties is reported in **Figure 1**. The use of the different methods for the improvement of the structure in case of light alloys significantly influences the properties related to their use and beneficially acts on the technological properties and processing by plastic deformation as well on their behaviour during thermal treatment.

Solidification of the alloys is a process, which involves the passage from the liquid state into a solid state as a result of the energy loss during the release of the heat and shows at least two

• •

•

Casting properties Processing by:

Cutting

Thermal treatment

Plastic deformation

•

Chemical composition Crystalline structure

•

•

STRUCTURE

PROPERTIES

Exploitation Technological

**Figure 1.** Schematic representation of the relation between the structure and the properties.

Metallurgical processes:

Casting

Plastic deformation Thermal treatment

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**1.** Germination: the step when development of the nuclei or crystallisation centre has been

• Homogeneous: when the centres have been developing spontaneously and has been

aspects:

Mechanical properties Physical/Chemical properties Corrosion resistance Wear resistance properties

realised. The evolution can be:

growing during the time;

**Figure 1.** Schematic representation of the relation between the structure and the properties.

primary importance and really governs the properties of the produced alloy. In many conditions, improvement of the mechanical strength is required, especially in application where structural properties are essential. Thermal treatments and/or reducing the grain size of the constituents are commonly used methods for the improvement of some properties. Addition of grain refiners (GR) determines an increase of the mechanical properties, and in particular, it affects the strength and the fatigue resistance of the alloys, and at the same time, the quality of the cast part and the efficiency of the casting process rises [1]. According to some literature date [2–5] and some own research [6–11], one can state that the grain size is one of the most important characteristics, which affects the processing, and consequently, the properties of alloys. Presence/addition of GR determines many advantages in light alloy casting and it can be achieved using different methods, as will be presented later on. Actually at industrial level, both adding grain refiner and improving solidification conditions are two procedures

18 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

The reduction of the grain size has a positive effect on the microstructural homogeneity determining a fine distribution of the second phases leading to an improved castability, lower micro-porosity and shrinkage porosity as well as a reduced segregation development [4, 6]. In any case, the alloy composition has to be carefully considered in order to choose an efficient GR. Even if, Al–5Ti–B master alloy is an efficient GR for wrought Al alloys, its effect is not sufficient in case of foundry alloys: formation of Ti–Si phases occurs following the reaction

According to [14, 15], in case of master alloys containing high amount of B and higher than 4–5.5 wt% of Si, an improvement of the grain refining can be obtained. Prior to casting, an inadequate holding time negatively influences the GR efficiency and additionally their agglomeration can take place [16–18]. For Mg alloys, Zr is the ideal GR, but its use is unsuc-

Conventional GR for Al alloys are generally produced from Al–Ti–B ternary system [19–22]. On the contrary of Fe-based alloys, in case of light alloys, the primary structure after solidification is maintained and the physical and any structural transformations induced following thermal treatment and/or plastic deformations take place within the primary structure. Any outcomes coming from such technological treatments are influenced by the characteristics of the primary structure. Additionally, light alloys have higher tendency during solidification for the development of the coarse grains with respect to iron-based alloys. From these considerations, it is evident that for light alloys, the influence of the crystallisation process with the aim of the development of fine grains and the good distribution of the different phases within a homogeneous microstructure results very important and it significantly influences the properties of the final product during their use. The schematic illustration of the relation between the structure and the properties is reported in **Figure 1**. The use of the different methods for the improvement of the structure in case of light alloys significantly influences the properties related to their use and beneficially acts on the technological properties and processing by plastic deformation as well on their behaviour during thermal treatment.

particles in

between the solute Si and Ti determining the weakening of the nucleating of TiB2

the master alloy, when the Si content in the alloy is higher than 3 wt.% [12, 13].

for grain refinement, especially in case of Al alloys.

cessful when it contains Al.

Solidification of the alloys is a process, which involves the passage from the liquid state into a solid state as a result of the energy loss during the release of the heat and shows at least two aspects:

	- Homogeneous: when the centres have been developing spontaneously and has been growing during the time;

During very slow cooling, a preferential direction of the heat release could be observed. During the development of dendrites, the dendrite arms are growing encountering during their expansion other neighbouring dendrites. When crystallisation finishes, the evolution continues on the perpendicular direction with respect to the former one and the whole crystal grows into a complex form.

Industrially, there is the need to obtain enhanced quality product. To realise this, one has to consider and satisfy some of the following conditions:

• To increase the cooling rate and the number of the crystalline germs, simultaneously with the reduction of the solidification time in some regions of casting;

Moving backward the stirring direction of the electromagnetic stirring, the oxide film on the melt surface can be protected, which is important for maintaining it undamaged. Contrarily, the layer can be fragmented by the forced melt flow losing its protective character. Some studies [23, 25, 26] reveal how the effect of a magnetic field on the crystal direction and on the phase arrangement during solidification is a valid method to increase the physical properties of the alloy.

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**Figure 2.** Schematic representation of the most important methods used for the refinement of the structure.

In [23, 27, 28], the authors have demonstrated that the rotating magnetic field can be effectively used to reduce the segregation appearance during solidification and to enhance the

In order to shortly appreciate the variations taking place during solidification, **Figure 3** reports, schematically, the enhancement of the structure using high cooling rate contemporary with the addition of some chemical element: the refinement of the structure and conse-

quently, the improvement of the general behaviour is evident.

**Figure 3.** Schematic representation of the main modifications in hypoeutectic Al-Si alloy.

structure of metallic alloys.

The solidification process can be governed by the control of the time required for the cooling and acting directly on the alloy, on the casting profile or on the final product. The control of the solidification is significantly complex especially when the alloy shows high shrinkage in liquid state and during solidification.

**Figure 2** summarises the most important methods used in order to act on the solidification. Application of a magnetic stirring field to the melt, which solidify has a grain refining effect too. There are two types of magnetic stirring:

• Heterogeneous: when the development takes place close to the existent germs on the surface of the melt (on the wall of the die, near to some hold-up in the melt or during the precipitation of some phases in mini-crystal forms) and can be favoured by the in-

• Mechanical: such effect is induced by mechanical ultrasonic vibration and/or by using

**2.** Development and growing of the crystals by the combination of the atoms from the melt on the crystal surface in form of monoatomic layers with critical size representing bi-

During very slow cooling, a preferential direction of the heat release could be observed. During the development of dendrites, the dendrite arms are growing encountering during their expansion other neighbouring dendrites. When crystallisation finishes, the evolution continues on the perpendicular direction with respect to the former one and the whole crystal

Industrially, there is the need to obtain enhanced quality product. To realise this, one has to

• To increase the cooling rate and the number of the crystalline germs, simultaneously with

• Managing the heat transfer process and the mass transfer, which directly influences the size of crystalline germs, determining the development of a fine equiaxed structure;

• Reducing the presence of internal stress avoiding in this way the crack formation [24].

The solidification process can be governed by the control of the time required for the cooling and acting directly on the alloy, on the casting profile or on the final product. The control of the solidification is significantly complex especially when the alloy shows high shrinkage in

**Figure 2** summarises the most important methods used in order to act on the solidification. Application of a magnetic stirring field to the melt, which solidify has a grain refining effect

• Electromagnetic stirring, which stimulates the development of an equiaxed crystalline zone; refinement of the solidification structure occurs with a decrease of the inclusions

• Permanent magnetic stirring, which has some benefits over the first one, because it consists

of lower cost tools, high magnetic field intensity and with no skin effect.

troduction of some particles as modifier into the melt;

20 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

turbulent convection currents.

consider and satisfy some of the following conditions:

• Homogenising as much as possible the structure;

the reduction of the solidification time in some regions of casting;

dimensional germs [23].

grows into a complex form.

• Degassing the melt;

content and

liquid state and during solidification.

too. There are two types of magnetic stirring:

**Figure 2.** Schematic representation of the most important methods used for the refinement of the structure.

Moving backward the stirring direction of the electromagnetic stirring, the oxide film on the melt surface can be protected, which is important for maintaining it undamaged. Contrarily, the layer can be fragmented by the forced melt flow losing its protective character. Some studies [23, 25, 26] reveal how the effect of a magnetic field on the crystal direction and on the phase arrangement during solidification is a valid method to increase the physical properties of the alloy.

In [23, 27, 28], the authors have demonstrated that the rotating magnetic field can be effectively used to reduce the segregation appearance during solidification and to enhance the structure of metallic alloys.

In order to shortly appreciate the variations taking place during solidification, **Figure 3** reports, schematically, the enhancement of the structure using high cooling rate contemporary with the addition of some chemical element: the refinement of the structure and consequently, the improvement of the general behaviour is evident.

**Figure 3.** Schematic representation of the main modifications in hypoeutectic Al-Si alloy.

## **2. Experimentally used devices and methods**

In the following part, some results of the current research performed within a recent PhD Thesis [10] oriented to the enhancement of the characteristics of Al-based alloys by physical grain refinement methods that have been presented and talked over, including some real case studies.

• Voltage: 110 V ~ 220 V/60 Hz ± 10%

• Acceleration:0.8–0.07 m/s2

• Dimension: 14 × 13.5 × 11.3 cm

• The device projected for the research has made possible the variation of the frequency in

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Castings have the size as reported in **Figure 5** and the K type thermocouple has been introduced in the position indicated in figure, at about ¾ height of the crucible's total length.

Castings has been obtained in a static and dynamic mode and the chemical composition has been selected to evaluate as well as the influence of the working condition on the nature of the phases developed during solidification (primary phase, solid solution and intermetallic particles, eutectic phase). Structural modification has been monitored following the dynamic solidification condition for the hypoeutectic Al-Si foundry alloy and for the deformable Al-Cu, Al-Zn, Al-Mg alloys. The effect of the dynamic condition on the areas containing columnar crystals has been assessed by observing some samples when solidified with high cooling rate, using cooling channels placed around the crucible (**Figure 6**). Directed solidification has been performed on AlSi7Mg0.3 and on AlZn10Si7 alloy to study the effect of the mechanical vibra-

The tool reported in **Figure 7** has been used for the study of the solidification in rotating electromagnetic field. The parameters adopted are: tri-phasic power supply, nominal power

Casting, industrially, has been performed using the forms reported in **Figure 8**, and they have been carried out using different cooling rates (guaranteed by the different material used for

**Figure 4.** The set-up used experimentally for the mechanical vibration of the melt (a) and command block used for the

tion on the properties of the columnar disposition of the dendrites.

• Velocity: 2.8–0.22 mm/s

the range of 4÷94 Hz.

1.5 kW, frequency 50 Hz.

variation and the registration of the frequency (b) [10].

the casting).

• Power: 100 W

• Amplitude: 0.07

The purpose of using the dynamic methods is twofold, as follows:

Both foundry and deformable alloys have been used for such investigations. The study is realised analysing:

The alloys, with the chemical composition used for the research have been casted and details on such part have been found in some previously published paper [6–12]. The vibration system, the electromagnetic stirring system and the device used as electromagnetic induction system have been projected and realised during the present research and here is shortly presented. Results on the benefits of the physical treatment performed, with some comparison related to other similar condition for the solidification have been reported and discussed integrated with some preliminary considerations related to their eventually technological transfer. Additionally, enhancement of the structure following high cooling rate using different processing methods: melt spinning, static and dynamic casting have been recalled with some consideration about the final benefits. The analysis are carried out and the results obtained can be used as guide-line for people working in this area in an economically convenient way, saving time, energy and materials.

The research has been carried out using binary and more complex alloys, belonging to the families: Al-Si, Al-Cu, Al-Zn and Al-Mg. The chemical compositions have been reported in the previously published papers [6–12].

The mechanical vibration of the melts during solidification has been performed using a laboratory device, usually employed in a dental laboratory with the details reported in **Figure 4**. The parameters adopted for the experiment are as follow:

**2. Experimentally used devices and methods**

The purpose of using the dynamic methods is twofold, as follows:

ous product, both chemically and structurally and

grains and enhancement of the mechanical properties;

realised analysing:

thermal treatment.

saving time, energy and materials.

previously published papers [6–12].

The parameters adopted for the experiment are as follow:

In the following part, some results of the current research performed within a recent PhD Thesis [10] oriented to the enhancement of the characteristics of Al-based alloys by physical grain refinement methods that have been presented and talked over, including some real case studies.

22 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

**A.** For enhancing the structure of the alloys reaching a fine microstructure and a homogene-

**B.** For reducing the scrap formation during casting with a consequent time saving possibility. Both foundry and deformable alloys have been used for such investigations. The study is

• The effect of the vibration during solidification on the development of the microstructure, lowering the porosity content and furthermore enhancing the mechanical properties;

• The effect of magnetic stirring applied on the melt, considered as a clean solution for the hating and for the treatment of the alloys; such approach allows, with no any interaction, the necessary thermal and mechanical energy for obtaining a homogeneous alloy with fine

• Solidification with high cooling rate, leading to positive results concerning the refinement of the structure, a method which allows obtaining high performance product with no any

The alloys, with the chemical composition used for the research have been casted and details on such part have been found in some previously published paper [6–12]. The vibration system, the electromagnetic stirring system and the device used as electromagnetic induction system have been projected and realised during the present research and here is shortly presented. Results on the benefits of the physical treatment performed, with some comparison related to other similar condition for the solidification have been reported and discussed integrated with some preliminary considerations related to their eventually technological transfer. Additionally, enhancement of the structure following high cooling rate using different processing methods: melt spinning, static and dynamic casting have been recalled with some consideration about the final benefits. The analysis are carried out and the results obtained can be used as guide-line for people working in this area in an economically convenient way,

The research has been carried out using binary and more complex alloys, belonging to the families: Al-Si, Al-Cu, Al-Zn and Al-Mg. The chemical compositions have been reported in the

The mechanical vibration of the melts during solidification has been performed using a laboratory device, usually employed in a dental laboratory with the details reported in **Figure 4**.

Castings have the size as reported in **Figure 5** and the K type thermocouple has been introduced in the position indicated in figure, at about ¾ height of the crucible's total length.

Castings has been obtained in a static and dynamic mode and the chemical composition has been selected to evaluate as well as the influence of the working condition on the nature of the phases developed during solidification (primary phase, solid solution and intermetallic particles, eutectic phase). Structural modification has been monitored following the dynamic solidification condition for the hypoeutectic Al-Si foundry alloy and for the deformable Al-Cu, Al-Zn, Al-Mg alloys. The effect of the dynamic condition on the areas containing columnar crystals has been assessed by observing some samples when solidified with high cooling rate, using cooling channels placed around the crucible (**Figure 6**). Directed solidification has been performed on AlSi7Mg0.3 and on AlZn10Si7 alloy to study the effect of the mechanical vibration on the properties of the columnar disposition of the dendrites.

The tool reported in **Figure 7** has been used for the study of the solidification in rotating electromagnetic field. The parameters adopted are: tri-phasic power supply, nominal power 1.5 kW, frequency 50 Hz.

Casting, industrially, has been performed using the forms reported in **Figure 8**, and they have been carried out using different cooling rates (guaranteed by the different material used for the casting).

**Figure 4.** The set-up used experimentally for the mechanical vibration of the melt (a) and command block used for the variation and the registration of the frequency (b) [10].

**Figure 5.** The shape and the size of the forms used for the casting; (a) crucible in refractory material and (b) thermocouples placed for monitoring the cooling [10].

**Figure 6.** Scheme of the casting (a), crucible with the cooler and the thermocouples before located casting (b) and the samples with no vibration and under vibration [10].

The experimental device (**Figure 9**) realised for the simulation of the solidification in different cooling rates has the benefits to associate the cooling technology of the melt jet on spinning disc with the principle of the moulds feeding from low pressure casting procedure. In case of low pressure casting, the transfer of the liquid metal from the furnace to the die cavity placed on the top of the furnace, is guaranteed by an overpressure acting on the liquid alloy enclosed in the crucible, which is hermetically sealed. As a result, the liquid metal is forced to go vertically through the connecting tube. In case of the experimental device indicated in **Figure 9**,

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**Figure 8.** Casting of Al alloys in different dies [10].

**Figure 7.** Experimental set-up (a): A-inductor coils, B-casting shape in refractory material, C-Al alloy, D-termocouples; (b): Rotating electrical field generator, (c) command block and (d) system for the registration of the frequency [8, 10].

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**Figure 8.** Casting of Al alloys in different dies [10].

**Figure 7.** Experimental set-up (a): A-inductor coils, B-casting shape in refractory material, C-Al alloy, D-termocouples; (b): Rotating electrical field generator, (c) command block and (d) system for the registration of the frequency [8, 10].

**Figure 6.** Scheme of the casting (a), crucible with the cooler and the thermocouples before located casting (b) and the

**Figure 5.** The shape and the size of the forms used for the casting; (a) crucible in refractory material and (b) thermocouples

24 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

samples with no vibration and under vibration [10].

placed for monitoring the cooling [10].

The experimental device (**Figure 9**) realised for the simulation of the solidification in different cooling rates has the benefits to associate the cooling technology of the melt jet on spinning disc with the principle of the moulds feeding from low pressure casting procedure. In case of low pressure casting, the transfer of the liquid metal from the furnace to the die cavity placed on the top of the furnace, is guaranteed by an overpressure acting on the liquid alloy enclosed in the crucible, which is hermetically sealed. As a result, the liquid metal is forced to go vertically through the connecting tube. In case of the experimental device indicated in **Figure 9**,

**Figure 9.** Experimental device for obtaining of melt-spun ribbons: (a) schematic plant, (b) melt spinning-low pressure (MS-LP) device during processing [12].

the working gas (Ar) introduced by the cover 6 acts upon the liquid metal 4 from the crucible 3 causing the fused metal raising in the feeding tube 5 followed by the melt placement on the rotating disc. The furnace and port-crucible is (1), respectively (2).

velocity = 99 mm/s2

Fe-made particles).

tom) have been analysed.

AlSi7Mg0.3 alloy (b) casted with no and under vibration [9, 10].

and amplitude = 0.7 mm. The distance from the cooler has been consid-

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ered during the microstructural analysis and the most important areas (top, middle and bot-

**Figure 10.** Cooling curves for AlCu4 casted in different mould and with no vibration and under vibration (a) and

The finest grains have been obtained for the areas situated at the bottom part, which is in direct contact with the cooler and as the distance increases, the microstructure becomes coarser as reported in **Figure 13**. Following vibration, the roughness of the sample increases (**Figure 14**). The results have the same tendency concerning the microstructure evolution (**Figure 15**) and for the roughness in case of AlZn10Si7 alloy (**Figure 16**) as well. There are some differences as their workability concerns: the second one is harder and seems to be more sensible to the impurities, which are on their surface. In this case, there are segregation of intermetallic particles made of Fe. With no vibration, trans-crystallisation is observed (columnar crystallisation),

On the upper part of the sample with vibration, some darker zones have been observed, while in the region in contact with the cooler development of finer grains have been observed. The lighter and longer appearance segregations are made of Fe, coming from the scraps re-used and this is one of the causes related to the more difficult workability. The bottom part of the sample is made of the solid solution and contains segregation of Si (darker colour than the

Industrially employed alloys have been investigated. In the case of hypoeutectic AlSi7Mg0.3 alloy is assumed that enhancement of the microstructure determines refinement of the microstructure and the modification of the eutectic phase. For the eutectic composition AlSi12CuMgNi alloy with complex composition, the vibration is of interest both for the structure of the dendrites and for the morphology of the eutectic and for the shape and distribution of the intermetallic particles segregation. For the binary AlCu4 alloy, it is important that the refinement of the dendrites of solid solution and for the development of non-equilibrium eutectic phase.

while in case of vibration, the equiaxed grain crystallisation has been observed.

The diameter of the feeding tube (5) corresponds to 1 mm and the distance to the disc results to be 5 mm. The disc rotation speed is 2600 rot/min and the working gas pressure is 1.5 bars. The aforementioned parameters have an important role, since they establish the thickness of obtained ribbons. Major details on such devise used can be found in [10].

## **3. Experimental study results**

#### **3.1. Casting in steel die, refractory material die under vibration**

AlCu4 and AlSi7Mg0.3 alloys have been casted in steel die, refractory material with no vibration and with vibration (f = 50 Hz, a = 15 m/s2 , v = 14 mm/s, A = 0.07 mm). The cooling curves are reported in **Figure 10**. Higher cooling rate can be observed when the alloy is subjected to vibration compared to the static cooling condition, confirmed by the microstructural analysis: the finest microstructure corresponds to the high cooling rate and in the presence of vibration for both alloys, as reported in **Figure 11**.

Directed solidification has been performed on AlSi7Mg0.3 and AlZn10Si7 alloy to investigate the influence of the mechanical vibration on the properties of the columnar zone. For this experiment, cylindrical casted samples positioned on Cu rings (Ø30 × 13 mm, weight 22 g) have been used. The thermocouples for the registration of the temperature variation as a function of time has been mounted on the top and on the bottom part of the refractory mould at a distance of 15 mm from the exterior part of the sample, as reported in **Figure 12**. The experiment has been carried out both with no vibration and under vibration conditions, using different parameters and varying the frequency in the range of 50÷90 Hz. The resonance frequency is 62 Hz and the other parameters adopted are: acceleration = 8.8 mm/s2 , Some Considerations on the Structure Refinement in Al-Based Alloys http://dx.doi.org/10.5772/intechopen.71450 27

**Figure 10.** Cooling curves for AlCu4 casted in different mould and with no vibration and under vibration (a) and AlSi7Mg0.3 alloy (b) casted with no and under vibration [9, 10].

the working gas (Ar) introduced by the cover 6 acts upon the liquid metal 4 from the crucible 3 causing the fused metal raising in the feeding tube 5 followed by the melt placement on the

**Figure 9.** Experimental device for obtaining of melt-spun ribbons: (a) schematic plant, (b) melt spinning-low pressure

26 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

The diameter of the feeding tube (5) corresponds to 1 mm and the distance to the disc results to be 5 mm. The disc rotation speed is 2600 rot/min and the working gas pressure is 1.5 bars. The aforementioned parameters have an important role, since they establish the thickness of

AlCu4 and AlSi7Mg0.3 alloys have been casted in steel die, refractory material with no vibra-

are reported in **Figure 10**. Higher cooling rate can be observed when the alloy is subjected to vibration compared to the static cooling condition, confirmed by the microstructural analysis: the finest microstructure corresponds to the high cooling rate and in the presence of vibration

Directed solidification has been performed on AlSi7Mg0.3 and AlZn10Si7 alloy to investigate the influence of the mechanical vibration on the properties of the columnar zone. For this experiment, cylindrical casted samples positioned on Cu rings (Ø30 × 13 mm, weight 22 g) have been used. The thermocouples for the registration of the temperature variation as a function of time has been mounted on the top and on the bottom part of the refractory mould at a distance of 15 mm from the exterior part of the sample, as reported in **Figure 12**. The experiment has been carried out both with no vibration and under vibration conditions, using different parameters and varying the frequency in the range of 50÷90 Hz. The resonance frequency is 62 Hz and the other parameters adopted are: acceleration = 8.8 mm/s2

, v = 14 mm/s, A = 0.07 mm). The cooling curves

,

rotating disc. The furnace and port-crucible is (1), respectively (2).

**3.1. Casting in steel die, refractory material die under vibration**

**3. Experimental study results**

(MS-LP) device during processing [12].

tion and with vibration (f = 50 Hz, a = 15 m/s2

for both alloys, as reported in **Figure 11**.

obtained ribbons. Major details on such devise used can be found in [10].

velocity = 99 mm/s2 and amplitude = 0.7 mm. The distance from the cooler has been considered during the microstructural analysis and the most important areas (top, middle and bottom) have been analysed.

The finest grains have been obtained for the areas situated at the bottom part, which is in direct contact with the cooler and as the distance increases, the microstructure becomes coarser as reported in **Figure 13**. Following vibration, the roughness of the sample increases (**Figure 14**).

The results have the same tendency concerning the microstructure evolution (**Figure 15**) and for the roughness in case of AlZn10Si7 alloy (**Figure 16**) as well. There are some differences as their workability concerns: the second one is harder and seems to be more sensible to the impurities, which are on their surface. In this case, there are segregation of intermetallic particles made of Fe. With no vibration, trans-crystallisation is observed (columnar crystallisation), while in case of vibration, the equiaxed grain crystallisation has been observed.

On the upper part of the sample with vibration, some darker zones have been observed, while in the region in contact with the cooler development of finer grains have been observed. The lighter and longer appearance segregations are made of Fe, coming from the scraps re-used and this is one of the causes related to the more difficult workability. The bottom part of the sample is made of the solid solution and contains segregation of Si (darker colour than the Fe-made particles).

Industrially employed alloys have been investigated. In the case of hypoeutectic AlSi7Mg0.3 alloy is assumed that enhancement of the microstructure determines refinement of the microstructure and the modification of the eutectic phase. For the eutectic composition AlSi12CuMgNi alloy with complex composition, the vibration is of interest both for the structure of the dendrites and for the morphology of the eutectic and for the shape and distribution of the intermetallic particles segregation. For the binary AlCu4 alloy, it is important that the refinement of the dendrites of solid solution and for the development of non-equilibrium eutectic phase.

**Figure 12.** Set-up of the directed solidification experiment [10].

**Figure 13.** OM microstructures of the different areas within the AlSi7Mg0.3 alloy after directed solidification [10].

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**Figure 14.** Photographs showing the roughness of the samples in AlSi7Mg0.3 alloy with and with no vibration (a), top

part of the sample under vibration (b) and with no vibration (c) [10].

**Figure 11.** OM microstructure of the AlCu4 alloy (top) and AlSi7Mg0.3 (bottom) casted in different dies and with no vibration and under vibration [10].

#### **3.2. Effect of the vibration and of the cooling velocity on the solidification condition**

Comparison of the effect of the vibration and of the cooling velocity on the solidification condition has been carried out. For the thermal analysis, samples of size 14×80×160 mm have been used, labelled as C00. **Figure 17** (left) reports the results of the thermal analysis for the AlSi7Mg0.3 alloy. The analysis of the cooling curves indicates that in case of vibration, the Some Considerations on the Structure Refinement in Al-Based Alloys http://dx.doi.org/10.5772/intechopen.71450 29

**Figure 12.** Set-up of the directed solidification experiment [10].

**3.2. Effect of the vibration and of the cooling velocity on the solidification condition**

28 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

vibration and under vibration [10].

Comparison of the effect of the vibration and of the cooling velocity on the solidification condition has been carried out. For the thermal analysis, samples of size 14×80×160 mm have been used, labelled as C00. **Figure 17** (left) reports the results of the thermal analysis for the AlSi7Mg0.3 alloy. The analysis of the cooling curves indicates that in case of vibration, the

**Figure 11.** OM microstructure of the AlCu4 alloy (top) and AlSi7Mg0.3 (bottom) casted in different dies and with no

**Figure 13.** OM microstructures of the different areas within the AlSi7Mg0.3 alloy after directed solidification [10].

**Figure 14.** Photographs showing the roughness of the samples in AlSi7Mg0.3 alloy with and with no vibration (a), top part of the sample under vibration (b) and with no vibration (c) [10].

For the AlCu4 alloy, the maximum cooling velocity in liquid state is (**Figure 18**):

**Figure 17.** Cooling curves and the transformation temperatures for the AlSi7Mg0.3 alloy [9, 10].

It can be noticed that as the vibration increases, the cooling velocity significantly increases also in the biphasic domain (liquid + solid). C0 corresponds to the liquidus temperature, the peaks labelled as C1 makes in evidence the temperature when the development of solid solution dendrites concludes, while C2 (very evident) indicates the formation of the eutectic

Cu phase. The region correspondent to the formation of the eutectic is preceded by the overcooling of the alloy, intensified by the vibration of the melt. The sample, solidified under vibration, shows higher roughness: intensification of the heat exchange between sample and mould occurs, appreciated by the value of the overall heat transfer coefficient, which in the presence of vibration has a favourable effect on the interaction between the alloy and the

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• 1.6°C/s and 2.1°C/s for the samples solidified in refractory moulds;

**Figure 18.** Cooling curves and the transformation temperatures for the AlCu4 alloy [9, 10].

• 17°C/s for the sample solidified in steel moulds (C00).

• 1.15°C/s for the sample with no vibration;

Al-Al2

**Figure 15.** OM microstructures of the different areas within the AlZn10Si7 alloy after directed solidification [10].

**Figure 16.** Photographs showing the roughness of the samples in AlZn10Si7 alloy with and with no vibration [10].

cooling rate is higher. In both cases, the liquidus temperature is 613.35°C, while the solidus temperature (eutectic) for the vibrated sample is 1.25°C lower that the sample without vibration (**Figure 17** right hand side).

It can be noticed that the eutectic transformation has a more evident appearance in the presence of vibration. The maximum cooling velocity in liquid state is 4.6°C/s and 6.7°C for the non-vibrated and vibrated sample, respectively. In case of the sample solidified under vibration, the primary solidification and the eutectic transformation take place at higher cooling velocity compared to the situation with no vibration. The period of the transformation is significantly different for the two samples: in case of vibration, a decrease of both the extent of the crystallisation and of the period of the eutectic transformation of about 50 and 31%, respectively occurs. Such reduction is determined by the increase of the cooling velocity.

**Figure 17.** Cooling curves and the transformation temperatures for the AlSi7Mg0.3 alloy [9, 10].

For the AlCu4 alloy, the maximum cooling velocity in liquid state is (**Figure 18**):

• 1.15°C/s for the sample with no vibration;

cooling rate is higher. In both cases, the liquidus temperature is 613.35°C, while the solidus temperature (eutectic) for the vibrated sample is 1.25°C lower that the sample without vibra-

**Figure 16.** Photographs showing the roughness of the samples in AlZn10Si7 alloy with and with no vibration [10].

**Figure 15.** OM microstructures of the different areas within the AlZn10Si7 alloy after directed solidification [10].

30 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

It can be noticed that the eutectic transformation has a more evident appearance in the presence of vibration. The maximum cooling velocity in liquid state is 4.6°C/s and 6.7°C for the non-vibrated and vibrated sample, respectively. In case of the sample solidified under vibration, the primary solidification and the eutectic transformation take place at higher cooling velocity compared to the situation with no vibration. The period of the transformation is significantly different for the two samples: in case of vibration, a decrease of both the extent of the crystallisation and of the period of the eutectic transformation of about 50 and 31%, respectively occurs. Such reduction is determined by the increase of the cooling velocity.

tion (**Figure 17** right hand side).

It can be noticed that as the vibration increases, the cooling velocity significantly increases also in the biphasic domain (liquid + solid). C0 corresponds to the liquidus temperature, the peaks labelled as C1 makes in evidence the temperature when the development of solid solution dendrites concludes, while C2 (very evident) indicates the formation of the eutectic Al-Al2 Cu phase. The region correspondent to the formation of the eutectic is preceded by the overcooling of the alloy, intensified by the vibration of the melt. The sample, solidified under vibration, shows higher roughness: intensification of the heat exchange between sample and mould occurs, appreciated by the value of the overall heat transfer coefficient, which in the presence of vibration has a favourable effect on the interaction between the alloy and the

**Figure 18.** Cooling curves and the transformation temperatures for the AlCu4 alloy [9, 10].

material made up the mould. [7, 10] reports more details on the calculation on the overall heat transfer coefficient, based on the cooling curves.

The results obtained indicate an increase of the overall heat transfer coefficient value. However, they do not determine the enhancement of the cooling velocity in such a level to provide a significant modification of the casting structure. The schematic interdependence of

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The effect of the rotator electromagnetic field on the structural evolution of light alloys has been further investigated. The casting of some cylindrical samples (Ø30 × 100 mm) in ZnAl4Cu1and AlSi7Mg0.3 alloy has been performed in refractory mould. Under the electromagnetic field action, the melt is subjected to the centrifugal force, like in the traditional centrifugal casting, driving the liquid metal against the die walls of the mould. In the refining procedure, such effect can be considered as a limiting factor and has to be minimised using a refractory material placed on the superior part of the mould when the electromagnetic field is applied. During the experiments, it has been found that the effect of the centrifugal electromagnetic field really has an effect when the frequency arrives to a certain value. At the same time, when the electromagnetic field is coupled, the rotation speed of the melt caused by the electromagnetic phenomenon in the system coil-melt decreases and do not permit sufficient grade of agitation. For this reason, after reaching the frequency, coupling-decoupling and the

Such device has been used for the preliminary investigations using a low melting point alloy,

In case of the hypoeutectic ZnAl4Cu1 alloy, a significant modification takes place: the transformation of the columnar dendrite into a more globular shape and the appearance of a higher level (18%) and finely dispersed eutectoid phase can be observed, as pointed out in the phase diagram. During solidification, the electromagnetic field stimulate the rearrangement of the

During solidification in the case of AlSi7Mg0.3 alloy, the electromagnetic field determines a decrease of the grain size and the α solid solution globular grains development arises. The eutectic Si does not suffer any significant modification neither in shape nor in size. In the central part of the sample, refining of the α solid solution has been observed and a higher distribution of the eutectic phase has been reached. The most relevant microstructures have

In the first step of the research, the influence of the cooling velocities (encountered in the different casting technology processes) on the structure development has been analysed.

**Figure 20.** Schematic representation of the interdependence between some features affecting the solidification of the alloys.

alloying elements. The evolution of the microstructure is reported in **Figure 21**.

such elements are summarised in **Figure 20**.

modification of the rotation senses have been carried out.

in particular ZnAl4Cu1 alloy.

been reported in **Figure 22**.

**Figure 19** reports, for comparison, the optical microstructure of the alloys investigated after solidification (no vibration and under vibration). In the case of hypoeutectic AlSi7Mg0.3 alloy (**Figure 19** top), the vibration during solidification determines, first of all, the reduction of the size of the separation of Si and of the intermetallic particles, both in length and width. The α solid solution dendrites becomes more uniform and of rounded shape, with no significant variation of their size (130 μm). For the AlSi12CuMgNi alloy (**Figure 19** middle), the secondary dendrite arm spacing under vibration is reduced (from about 70–40 μm) and the proportion of the eutectic phase is significantly increased and refinement of the Si particles occurs. In the case of the binary AlCu alloy (**Figure 19** bottom), the effect of the vibration determines the reduction (from about 245 to 200 μm) of the secondary dendrite arm spacing and more globular grains have been obtained. Additionally, the Al2 Cu particles become more fragmented.

As cooling velocity increases, the dendritic parameter decreases. Under vibration, germination of the gas bubbles is favoured and there is no promising condition for their expulsion from the melt because of the reduction of the viscosity and of the formation of the solid phase.

**Figure 19.** OM microstructure of the alloys investigated: Top: AlSi7Mg 0.3, middle AlSi12CuMgNi and bottom AlCu4; left hand side with no vibration, right hand side under vibration during solidification [9, 10].

The results obtained indicate an increase of the overall heat transfer coefficient value. However, they do not determine the enhancement of the cooling velocity in such a level to provide a significant modification of the casting structure. The schematic interdependence of such elements are summarised in **Figure 20**.

material made up the mould. [7, 10] reports more details on the calculation on the overall heat

32 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

**Figure 19** reports, for comparison, the optical microstructure of the alloys investigated after solidification (no vibration and under vibration). In the case of hypoeutectic AlSi7Mg0.3 alloy (**Figure 19** top), the vibration during solidification determines, first of all, the reduction of the size of the separation of Si and of the intermetallic particles, both in length and width. The α solid solution dendrites becomes more uniform and of rounded shape, with no significant variation of their size (130 μm). For the AlSi12CuMgNi alloy (**Figure 19** middle), the secondary dendrite arm spacing under vibration is reduced (from about 70–40 μm) and the proportion of the eutectic phase is significantly increased and refinement of the Si particles occurs. In the case of the binary AlCu alloy (**Figure 19** bottom), the effect of the vibration determines the reduction (from about 245 to 200 μm) of the secondary dendrite arm spacing and more globu-

As cooling velocity increases, the dendritic parameter decreases. Under vibration, germination of the gas bubbles is favoured and there is no promising condition for their expulsion from the melt because of the reduction of the viscosity and of the formation of the solid phase.

**Figure 19.** OM microstructure of the alloys investigated: Top: AlSi7Mg 0.3, middle AlSi12CuMgNi and bottom AlCu4;

left hand side with no vibration, right hand side under vibration during solidification [9, 10].

Cu particles become more fragmented.

transfer coefficient, based on the cooling curves.

lar grains have been obtained. Additionally, the Al2

The effect of the rotator electromagnetic field on the structural evolution of light alloys has been further investigated. The casting of some cylindrical samples (Ø30 × 100 mm) in ZnAl4Cu1and AlSi7Mg0.3 alloy has been performed in refractory mould. Under the electromagnetic field action, the melt is subjected to the centrifugal force, like in the traditional centrifugal casting, driving the liquid metal against the die walls of the mould. In the refining procedure, such effect can be considered as a limiting factor and has to be minimised using a refractory material placed on the superior part of the mould when the electromagnetic field is applied. During the experiments, it has been found that the effect of the centrifugal electromagnetic field really has an effect when the frequency arrives to a certain value. At the same time, when the electromagnetic field is coupled, the rotation speed of the melt caused by the electromagnetic phenomenon in the system coil-melt decreases and do not permit sufficient grade of agitation. For this reason, after reaching the frequency, coupling-decoupling and the modification of the rotation senses have been carried out.

Such device has been used for the preliminary investigations using a low melting point alloy, in particular ZnAl4Cu1 alloy.

In case of the hypoeutectic ZnAl4Cu1 alloy, a significant modification takes place: the transformation of the columnar dendrite into a more globular shape and the appearance of a higher level (18%) and finely dispersed eutectoid phase can be observed, as pointed out in the phase diagram. During solidification, the electromagnetic field stimulate the rearrangement of the alloying elements. The evolution of the microstructure is reported in **Figure 21**.

During solidification in the case of AlSi7Mg0.3 alloy, the electromagnetic field determines a decrease of the grain size and the α solid solution globular grains development arises. The eutectic Si does not suffer any significant modification neither in shape nor in size. In the central part of the sample, refining of the α solid solution has been observed and a higher distribution of the eutectic phase has been reached. The most relevant microstructures have been reported in **Figure 22**.

In the first step of the research, the influence of the cooling velocities (encountered in the different casting technology processes) on the structure development has been analysed.

**Figure 20.** Schematic representation of the interdependence between some features affecting the solidification of the alloys.

**Figure 21.** OM microstructure of the ZnAl4Cu1alloy: (a) with no rotator electromagnetic field and under rotator electromagnetic field, (b) top surface and (c) middle part of the sample [8, 10].

Higher the cooling rate finer becomes the structure, as reported in **Figure 23**. When the cooling rate is lower, the presence of acicular Si phase and the separation of the acicular shape particles made of Al3 Fe can be observed. In case of rapid solidification, displacement of the structure in the hypoeutectic zone can be observed with the appearance of α solid solution. At the same time, the acicular separation of small eutectic Si particles appears. Because of the high cooling rate, Fe is present as AlFeMn intermetallic particles in a polygonal shape (lighter shade than the primary Si particles). In case of casting under pressure, the morphology is close to the hypoeutectic structure, where the separation of the eutectic Si and the intermetallic particles are globular (the length/weight fraction tends to 1) and they are uniformly spread out close to the α solid solution. The chemical analysis carried out confirms that the differences between the structures observed are associated to the different solidification conditions and cooling rate.

The presence of Fe as Al9Fe2Si2 or Al12Fe3Si determines minor plasticity to the alloy, which can be balanced by high cooling rate, decreasing the negative effect especially in the case of secondary foundry alloys, where the presence of Fe is higher.

#### **3.3. Casting using melt spinning technology**

Significant structural changes, represented by the appearance of α phase, have been obtained in hypereutectic Al-Cu alloy produced by MS-LP technology. Applying a liquid quenching technique, increased solubility of Cu in the Al matrix was reached up to a value of 25.51 wt%.

After annealing treatment applied to melt-spun ribbons, the coarse separations of the interme-

**Figure 24.** Microstructure of Al10Cu alloy: (a) as-cast in metallic die (OM), (b) ribbon after TO annealing treatment

incoherent phase is uniformly distributed in the annealed ribbons, as illustrated in **Figure 24**.

Grain refinement determines many advantages in Al-based alloy casting and it can be reached by both metallurgical and physical methods. By using grain refinement method, for both cast and wrought aluminium alloys, a fine distribution of the phases, better castability, decrease of shrinkage porosity, higher mechanical properties, as well superior fatigue life were obtained.

plasticity and low content of Cu. Due to a grain refined structure of melt-spun ribbons, Al2

**Figure 23.** OM microstructure of the AlSi12CuMgNi alloy in different casting conditions [10].

Cu) was observed, which results to be embedded into the Al matrix with high

Some Considerations on the Structure Refinement in Al-Based Alloys

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35

Cu

tallic phase (Al2

(SEM) [12].

**4. Conclusions**

**Figure 22.** OM microstructure of the AlSi7Mg0.3 alloy: (a) with no rotator electromagnetic field and under rotator electromagnetic field, (b) top surface and (c) middle part of the sample [8, 10].

Some Considerations on the Structure Refinement in Al-Based Alloys http://dx.doi.org/10.5772/intechopen.71450 35

**Figure 23.** OM microstructure of the AlSi12CuMgNi alloy in different casting conditions [10].

**Figure 24.** Microstructure of Al10Cu alloy: (a) as-cast in metallic die (OM), (b) ribbon after TO annealing treatment (SEM) [12].

After annealing treatment applied to melt-spun ribbons, the coarse separations of the intermetallic phase (Al2 Cu) was observed, which results to be embedded into the Al matrix with high plasticity and low content of Cu. Due to a grain refined structure of melt-spun ribbons, Al2 Cu incoherent phase is uniformly distributed in the annealed ribbons, as illustrated in **Figure 24**.

## **4. Conclusions**

Higher the cooling rate finer becomes the structure, as reported in **Figure 23**. When the cooling rate is lower, the presence of acicular Si phase and the separation of the acicular shape

**Figure 21.** OM microstructure of the ZnAl4Cu1alloy: (a) with no rotator electromagnetic field and under rotator

34 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

structure in the hypoeutectic zone can be observed with the appearance of α solid solution. At the same time, the acicular separation of small eutectic Si particles appears. Because of the high cooling rate, Fe is present as AlFeMn intermetallic particles in a polygonal shape (lighter shade than the primary Si particles). In case of casting under pressure, the morphology is close to the hypoeutectic structure, where the separation of the eutectic Si and the intermetallic particles are globular (the length/weight fraction tends to 1) and they are uniformly spread out close to the α solid solution. The chemical analysis carried out confirms that the differences between the structures observed are associated to the different solidification conditions

The presence of Fe as Al9Fe2Si2 or Al12Fe3Si determines minor plasticity to the alloy, which can be balanced by high cooling rate, decreasing the negative effect especially in the case of

Significant structural changes, represented by the appearance of α phase, have been obtained in hypereutectic Al-Cu alloy produced by MS-LP technology. Applying a liquid quenching technique, increased solubility of Cu in the Al matrix was reached up to a value of 25.51 wt%.

**Figure 22.** OM microstructure of the AlSi7Mg0.3 alloy: (a) with no rotator electromagnetic field and under rotator

secondary foundry alloys, where the presence of Fe is higher.

electromagnetic field, (b) top surface and (c) middle part of the sample [8, 10].

electromagnetic field, (b) top surface and (c) middle part of the sample [8, 10].

**3.3. Casting using melt spinning technology**

Fe can be observed. In case of rapid solidification, displacement of the

particles made of Al3

and cooling rate.

Grain refinement determines many advantages in Al-based alloy casting and it can be reached by both metallurgical and physical methods. By using grain refinement method, for both cast and wrought aluminium alloys, a fine distribution of the phases, better castability, decrease of shrinkage porosity, higher mechanical properties, as well superior fatigue life were obtained.

However, the purpose of this chapter was twofold, it is not exhaustive on this topic. The first section was dedicated to present a short outlook about some possibilities to refine the structure of Al-based alloys based on some literature data and continues with the presentation and discussion of some experimental results obtained recently during the research performed within the PhD Thesis [10] targeted to the improvement of the properties of Al-based alloys by physical grain refinement methods.

[7] Popescu M, Bedo T, Varga B. Determination of overall heat transfer coefficient by simple thermal analyse. The Scientific Bulletin of Valahia University–Materials and Mechanics.

Some Considerations on the Structure Refinement in Al-Based Alloys

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37

[8] Popescu M, Varga B. Microstructure of aluminium alloys solidified by rotating electric field. The Scientific Bulletin of Valahia University – Materials and Mechanics. 2015;**10**:

[9] Popescu M, Varga B. Structures of Aluminum base alloys solidified by vibrations.

[10] M.Popescu, PhD Thesis. Improving of Aluminum Alloys Structure by Metallurgical and

[11] Peter I, Varga B, Rosso M. Eutectoid transformation in Zn-al alloys solidified by rapid

[12] Lichioiu I, Peter I, Varga B, Rosso M. Preparation and structural characterization of rapidly solidified Al-cu alloys. Journal of Materials Science and Technology. 2014;**30**(4):

[13] Varga B, Peter I. Structural transformations in Zn-Al22 alloy solidified by ultra-rapid

[14] Easton MA, Qian M, Prasad A, St John DH. Recent advances in grain refinement of light metals and alloys. Current Opinion in Solid State and Materials Science. 2016;**20**(1):13-24

[15] Lee YC, Dahle AK, John DHS, Hutt JEC. The effect of grain refinement and silicon content on grain formation in hypoeutectic Al-Si alloys. Materials Science and Engineering

[16] Chen Z, Wang T, Gao L, Fu H, Li T. Grain refinement and tensile properties improvement of aluminum foundry alloys by inoculation with Al-B master alloy. Materials

[17] Chen Z, Kang H, Fan G, Li J, Lu Y, Jie J, Zhang Y, Li T, Jian X, Wang T. Grain refinement

[18] Birol Y. AlB3 master alloy to grain refine AlSi10Mg and AlSi12Cu aluminium foundry

[19] Limmaneevichitr C, Eidhed W. Fading mechanism of grain refinement of aluminum– silicon alloy with Al-Ti–B grain refiners. Materials Science and Engineering A. 2003;

[20] Bunn AM, Schumacher P, Kearns MA, Boothroyd CB, Greer AL. Grain refinement by Al–Ti–B alloys in aluminium melts: A study of the mechanisms of poisoning by zirco-

[21] Birol Y. An improved practice to manufacture Al–Ti–B master alloys by reacting halide salts with molten aluminium. Journal of Alloys and Compounds. 2006;**420**(1-2):71-76

of hypoeutectic Al-Si alloys with B. Acta Materialia. 2016;**120**:168-178

nium. Journal of Materials Science and Technology. 1999;**15**:1115-1123

alloys. Journal of Alloys and Compounds. 2012;**513**:150-153

Advanced Materials Research. 2015;**1128**:88-97

cooling. Materials Science Forum. 2014;**790-791**:223-228

cooling. Metalurgia International. 2013;**18**(6):9-12

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Physical Methods; 2017. Tutor: B. Varga.

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A. 1999;**259**(1):43-52

**349**:197-206

The solidification process can be governed by the control of the time required for the cooling and acting directly on the alloy, on the casting profile or on the final product. Generally, the electromagnetic field and solidification under vibration are advantageous determining a decrease of the grain size; consequently in such conditions, a fine microstructure development is favoured. It comes out that significant microstructural changes can be obtained using physical methods, which based on the characteristics of the alloy composition and on the real use of the final component can be managed in such a way to produce a better quality Al-based alloy castings.

## **Author details**

Ildiko Peter1 \* and Bela Varga2

\*Address all correspondence to<EMAIL_ADDRESS>

## **References**

[7] Popescu M, Bedo T, Varga B. Determination of overall heat transfer coefficient by simple thermal analyse. The Scientific Bulletin of Valahia University–Materials and Mechanics. 2016;**11**:11-15

However, the purpose of this chapter was twofold, it is not exhaustive on this topic. The first section was dedicated to present a short outlook about some possibilities to refine the structure of Al-based alloys based on some literature data and continues with the presentation and discussion of some experimental results obtained recently during the research performed within the PhD Thesis [10] targeted to the improvement of the properties of Al-based alloys

36 Aluminium Alloys – Recent Trends in Processing, Characterization, Mechanical behavior and Applications

The solidification process can be governed by the control of the time required for the cooling and acting directly on the alloy, on the casting profile or on the final product. Generally, the electromagnetic field and solidification under vibration are advantageous determining a decrease of the grain size; consequently in such conditions, a fine microstructure development is favoured. It comes out that significant microstructural changes can be obtained using physical methods, which based on the characteristics of the alloy composition and on the real use of the final component can be managed in such a way to produce a better quality Al-based alloy castings.

1 Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy 2 Material Science Department, Transilvania University of Brasov, Brasov, Romania