*2.1. Specimens*

Cubic specimens of 15 mm side were printed by means of an EOSINT M270 (EOS Gmbh, Krailling, Germany) Dual Mode version working under argon atmosphere. The device operates at 195 W laser power, 1200 mm/s scan speed, 0.09 mm hatching distance as well as 0.02 mm of layer thickness. The scanning strategy involves the rotation of laser beam of 67◦ between consecutive layers based on the EOS strategy. The combination of the process parameters and scanning strategy allows the production of parts with a residual porosity less than 0.1%. More in-depth information about optimization procedure and building parameters were previously published [14,35]. Specimens in as-built condition and after post-processing annealing heat treatment at 980 ◦C for 32 min were considered. Five millimeters high cylindrical specimens were obtained by cutting the hot-worked bar.

#### *2.2. Corrosion Tests*

Boiling ferric sulfate/sulfuric acid corrosion tests were carried out according to ASTM G28 standard, method A. Before testing, the surface of specimens was grinded up to 1200 grit by silicon carbides emery paper. Two faces of LPBF specimens were ground and polished up to 1 μm diamond paste in order to permit observations of corrosion morphology on main planes, i.e., XY plane and XZ plane parallel and perpendicular to the building plane, respectively. One face was polished for observing morphology on plane transversal to rolling direction on disk specimens. Afterwards, the specimens were degreased in acetone in ultrasonic bath, rinsed in water and dried before their immersion in the boiling test solution. Before and after tests, the specimens were weighed by means of analytic scale to measure the weight loss and corrosion rate.

#### **3. Results and Discussion**

Table 1 shows the results of 120 h weight loss tests in boiling ferric sulfate/sulfuric acid solutions (ASTM G28 Method A). The highest corrosion rate was observed on hot-worked alloys. The LPBF specimens showed about the half and the third the corrosion rate measured on hot-worked specimens in as-built conditions and after heat treatment at 980 ◦C for 32 min, respectively. The corrosion morphology of these specimens is flat and even. On the contrary, hot-worked alloy showed penetrating corrosion attack, more severe compared to LPBF specimens.

**Table 1.** Results of 120 h weight loss test in boiling ferric sulphate/sulfuric acid solution (ASTM G28).

 Average of 3 measurements.

\*

Figures 2 and 3 show the corrosion morphology of the specimens after 120 h exposure tests in boiling ferric sulphate/sulfuric acid solution according to ASTM G28 standard. The building direction of LPBF specimens and the rolling direction of the hot-worked bar is represented by the yellow arrows. Figure 2 shows the images of the metallographic sections of the alloys taken at low and high magnification. HW specimens showed penetrating intergranular attack mainly located at the center of the specimen (Figure 2a,d).

**Figure 2.** SEM images of Alloy 625 after intergranular corrosion tests. Metallographic section of commercial rolled alloy along the rolling direction (**a**) detail of the attack on the surface perpendicular to the rolling direction (**d**). Metallographic section of LPBF alloys along the building direction in as produced conditions (**b**,**c**) and after heat treatment (**e**,**f**).

**Figure 3.** SEM images of the polished surface of the specimens after the tests of intergranular corrosion. (**a**) hot-worked bar, (**b**) LPBF as-built, (**c**) LPBF heat treated.

The intergranular attack produced also grain dropping, as evidenced by the analysis of the surface after exposure (Figure 3a). The high value of the corrosion rate is practically due to grain dropping. The morphology of the corrosion attack on LPBF as-built is far more even (Figure 2b,e), but the attack follows di fferent paths. The morphological analysis emphasizes the unique melt pool microstructure-typical of LPBF processing—formed by overlapped laser scan traces (Figure 3b). The melt pool structure is well-defined and penetrating attacks along the borders can be clearly detected. Finally, the attack takes place in form of shallow attacks on the heat treated LPBF specimens (Figure 2c,f) and the melt pool borders are no more evident. Equiaxed grain microstructure is evidenced by the corrosion attack (Figure 3c).

The di fferences in the morphology of attack is strictly related to the microstructure of the alloy, which depends upon the manufacturing process and the heat treatments. HW alloy shows a deep intergranular attack very close to the axis of the bar, where strong segregation of several precipitates at the grain boundary occurred. This is well evidenced by the presence of certain amount of both macro and micro-precipitates (Figure 4). The EDS analysis carried out on these precipitates confirmed the presence of Nb-rich phases, mainly MC carbides.

**Figure 4.** Microstructure of HW specimens (**a**,**b**) (Kalling's attack) and EDS spectrum of macro-precipitates (**c**) taken in the point evidenced in (**a**).

Owing Kim and Lee, the formation of carbides can be a factor contributing to the degradation of corrosion resistance of Alloy 625, because intergranular areas of carbides are thermodynamically more unstable and react more readily than other intergranular ones [36]. Cortial et al. studied the effect of heat treatment on the corrosion resistance of forged and welded nickel superalloys [37,38]. They concluded that hardening between 650 and 800 ◦C results in dislocation gliding of matrix, γ" precipitation and maximum of intergranular precipitation of fine M23 C6 carbides. Tensile strength and intergranular corrosion become maxima, impact strength and reduction of area become minima in the field of M23 C6 and M6C intergranular precipitation between 700 and 950 ◦C. Furthermore, the carbides have a higher Cr concentration than that in the base metal and they deplete the surrounding matrix in chrome, thus decreasing its corrosion resistance. However, the precipitates founded in this works were mainly enriched in Nb. The role of Nb rich precipitates in the corrosion of Alloy 625 alloy was underlined by Tawancy et al. [39]. Authors made tests in boiling 10% nitric acid on Alloy 626 annealed and aged at di fferent time and temperature. They concluded that Ni3Nb phase formed during aging is preferentially corroded owing the lower chromium content compared to the surrounding matrix. The intense intergranular corrosion attack observed on the hot-worked and heat treated reference alloy can be then ascribed both to the precipitation of MC carbides and Ni3Nb phase at the grain boundary which is favored at the center of the bar due to the production process.

The intergranular attack is not so penetrating in as-built LPBF Alloy 625 alloy, and it is not detectable at all on the LPBF alloy after the heat treatment. The microstructure of the as-built samples obtained in this work shows columnar grains (CGs) developing epitaxially, thus crossing several melt pools along the building direction (z-axis) (Figure 5). The marks underline the di fferent melt pool borders (MPCs) along both the building direction (z-axis) and perpendicular to the building platform (x-y plane). The melt pools are not all placed straight along the building direction due to the scan strategy which involves laser beam rotation of 67◦ between two consecutive layers [14,40,41].

**Figure 5.** FESEM images at di fferent magnification (**a**,**b**) exhibiting melt pools with columnar and cellular primary dendrites for as-built IN625 sample; mixed acids reagent.

During the LPBF process, the solidification rate is di fferent between the upper and lower zones of the melt pool. In the lower part of the layer the cooling speed is very high, while the upper part cools more slowly.

Figure 5a reveals the FESEM (Field Emission Scanning Electron Microscope) image of as-built Alloy 625 samples along the building direction. The melt pools are made up of very fine primary dendritic structures which have both cellular and columnar shapes, as shown in Figure 5b. The cellular dendrites structure is due to the alteration of columnar dendrite structures caused by very high cooling rates, as previously reported [42].

At higher magnification (Figure 6), very fine bright precipitates along the dendritic areas indicated by arrows 1 and bright elongated phases along the inter-dendritic regions pointed out by arrows 2 can be observed. Previous TEM investigations performed on as-built Alloy 625 (produced with the same process parameters) [43] reported a relatively high concentration of Nb and C for these precipitates, suggesting the early stage formation of fine MC carbides. EDS results revealed enrichment in Nb and Mo within the inter-dendritic areas, indicated possible segregation of Nb and Mo due to their elevated tendency to segregate, as confirmed by the literature [44,45].

**Figure 6.** FESEM images of as-built IN625 sample at high magnification (**a**) columnar dendritic structures with nanoprecipitates (1) and segregation of Nb and Mo within the inter-dendritic zones (2); (**b**) cellular dendritic structures with nanoprecipitates (1) and segregation of Nb and Mo within the inter-dendritic areas (2).

These precipitates did not influence the corrosion resistance of the alloys owing to their very small sizes. On the other hand, the attack seems to be preferentially located along the border of the melt pools.

Dinda et al. hypothesized the precipitation of intermetallic γ" [Ni3Nb] and δ [Ni3Nb] phases in the heat a ffected zone at the border of melt pool [9], taking into account also the reticular parameters modifications in the Ni matrix. Anam et al. reported the presence of γ" [Ni3Nb] precipitates at the border of melt pool [46]. A similar attack morphology was observed also for aluminum alloys [29,32,34]. However, the preferential attack at the border of the melt pool does not penetrate in depth for Alloy 625.

The heat treatment at 980 ◦C for 32 min dissolved the melt pool microstructure obtained by the LPBF process (Figure 7a). The specimens presented columnar grains along the building direction, coupled with the formation of fine carbides as displayed in Figure 7b,c. The carbides had dimensions from nanometric size (around 40 nm) as well as larger size roughly around 500 nm, as can be seen in Figure 7c,b, respectively. In order to detect a chemical composition variation, the EDS analysis was performed on the largest carbides.

EDS scan line (Figure 7d) revealed that the carbides are enriched in Nb and depleted of Cr suggesting the formation of MC carbides, which is in accordance with the TTT (Time Temperature Transformation) diagram of Alloy 625 [47].

The very low susceptibility to intergranular corrosion of the heat-treated specimens could be attributed to the absence of macroscopic precipitation as the size and distribution of the precipitate are coherent with the rounded pits on the surface of the specimen at the end of the intergranular corrosion test (Figure 7a). The precipitation of second phases is not continuous. This promotes a quite even not penetrating corrosion attack that does not penetrate significantly in depth along the grain boundaries, without any dropping. Thus, very low loss of weight was measured due to this fact.

**Figure 7.** (**a**) Optical microscopy image of LPBF Alloy 625 annealed at 980 ◦C for 32 min and water quenched showing columnar grains along the building direction; (**b**,**c**) FESEM images of IN625 sample annealed at 980 ◦C for 32 min and water quenched revealing fine precipitates throughout the material, with the largest located along the grain boundaries; (**d**) EDS scan line (at higher magnification) on a carbides showing the enrichment in Nb and the depletion of Cr with respect to the austenitic matrix.


**3. Methodology**

The simulation is performed using the commercial numerical simulation software ANSYS 18.0. Within the Workbench package ANSYS CFX was one-way coupled with ANSYS Structural (for mechanical analysis). The complete rotor geometry and domain encompassing it are discretized in the approach known as the Fully-resolved Rotor Model (FRM) [22]. The flow is assumed to be periodic, so only one-third of the rotor (one blade, 120◦ section) is considered.


**3. Requirements to analysis as a geotechnical measure of karst protection**

One of the most effective geotechnical karst protection measures is the cementation of the cover soils above the karst soils. Schemes of karst protection cementation of the foundation are developed on the basis of the Russian Code (SP 22.13330.2011), which recommends cementation of the cavities and the entire thickness of the karst soils. However, in practice, these strata often reach considerable sizes (from 15 to 20 m), and their cementation to the full depth to monolithic rock soils where karst cavities do not form is not possible due to a significant rise in the cost of construction and technological problems of cementation and control its quality at great depths.

Investigations of the stress–strain state of the artificially strengthened foundation base over the karst cavity [10] made it possible to establish that the most efficient method was the cementation of the soil mass in the roof of the karst soils. In this case, the additional deformations in the foundation base are minimal when cavities occurrence in karst soils. Also, soil collapse into the karst cavity is not allowed if the height of the probable collapse area above the cavity does not exceed the thickness of the artificially strengthened soil layer.

The design forecast of the possibility of soil collapse into the karst cavity is based, as a rule, on the classical view of the distribution of stresses and the mechanism of the arches formation above the karst cavities. The arch above the cavity in the equilibrium state (up to the moment of its collapse) can be considered as an area of increased stresses and deformations, the size of which is determined by the strength and deformation characteristics of the soils. When cementing a soil layer of a given thickness over the karst soils in which the cavity growth is predicted, this area depends on the thickness and characteristics of the cemented soils, as well as on the maximum predicted cavity size for the standard operating life of the building and on the building loads.

The state of the collapse process characterizes the excess of the boundary values of the tensile and compressive stresses around the cavity that can be obtained from Mohr's circle of stress. Therefore, the boundaries of the region of increased stresses and the formation of shear strains can be determined using the strength condition according to Mohr-Coulomb failure criteria, taking it as the boundary condition of rock flow and its collapse. Thus, the boundaries of a possible collapse area are defined as the locus of points at which the Mohr-Coulomb failure criteria are met.

Determination of additional load on the bridge foundation pile under karst deformation.

During the formation of a karst hole in the base of the bridge support pile foundation the additional loads are transferred to the piles. The additional vertical loads can be taken into account as the tangential stresses on the lateral surface of the piles, directed to the pile base.


*Machining with 5 or More Axis*

5-axis machining, in addition to the X, Y, and Z axes, has two rotary axes, which greatly improves the machining efficiency and accuracy. This process is commonly used to machine blades, rotors, dies, molds and propellers, among others [78–80]. As a result, it is considered the most important piece of equipment related to cutting in the industries, which allows the production of final components with much more complex shapes, and which would be impossible to obtain in a 3- or 4-axis machining [81]. Figure 7 exemplifies a 5-axis tool with table tilt A-C and X, Y, and Z axis, showing the flexibility presented by these systems.

**Figure 7.** 5-axis tool with table tilt A-C and X, Y, and Z axis. Adapted from [78], 2021, Elsevier.

In the study by My et al. [82], a mathematical model was proposed that would be able to analyze and compare the kinematic performance of six configurations of 5-axis machines. Xu et al. [83] emphasized the great need to avoid changing the tool orientation in 5-axis machining. Thus, a smoothing method oriented to kinematic performance was proposed in the work.

Using a recent methodology called double-flank milling, Bizzarri et al. [84] investigated the fabrication of screw rotors, since the methodology was applied in 5-axis flank machining, and it was observed that for symmetrical profiles, double-flank milling is possible, and it works as a designed tool. Using this same methodology, Bo et al. [85] demonstrated a customized tool for machining narrow and curved regions with high precision, and the created algorithm was validated using commercial software.

Prabha et al. [86] used the Unigraphics NX6 CAD/CAM software, which is integrated into 5-axis CNC machines, to machine steam turbine blades, and then make the measurement through 3D coordinates, observing the great efficiency of the 5-axis machine in relation to dimensional accuracy. On the other hand, Huang et al. [87] emphasized the geometric errors occurring in a 5-axis machine, despite the precision of the machine, and defined two models: "Rotary axis component displacement" and "Rotary axis line displacement", which, analyzed through the machining of five axes, has presented discrepant results. Therefore, the accuracy of 5-axis machining is the determining factor for success in processing and manufacturing parts and products [88]. Furthermore, it has a great advantage in terms of its flexibility and efficiency [89].

In 6-axis machining, one more rotation is added. Indeed, adding one more axis expands the variety of movements and transitions, reducing cutting time. Therefore, the 6-axle machine is used for machining very complex geometries, such as turbines or engine blocks [90]. Moriya et al. [91], through 6-axis control non-rotating cutting tools, processed curved V-shaped microgrooves on a curved surface. Due to the high difficulty of machining sharp corners using 3- to 5-axis machining, Japitana et al. [92] created a method for this process through 6-axis machining with ultrasonic vibration cutting, thus being able to create sharp corners on a protruding surface. Krimpenis and Noeas [93] also referred to the advantages of the machining process associated with additive manufacturing in the microfabrication, providing an insight into how these processes can be integrated.

Yuanfei et al. [94] designed a 6-axis CNC system through open architecture based on an industrial personal computer (IPC) and digital motion controller (DMC), thus analyzing the CNC design as well as the developed software. In turn, Carpiuc-Prisacari et al. [95] used a 6-axis machine to perform an analysis on the propagation of mixed-mode cracks; for this it was necessary to initialize a crack, and its propagation through rotation, traction and shear was obtained by the 6-axis machine. Based on robotic machining with six degrees of freedom, Huynh et al. [96] modeled several flexible industrial robots with six axes to be used with milling operations, with torsion springs and dampers positioned on each axis to assist in flexibility, since the lack of joint stiffness is a possible limitation. Milling operations on aluminum and steel corners were simulated, the latter being unsatisfactory because the results did not correspond to the experimental results.

5-axis CNC machining becomes even more critical when machining difficult-tomachine materials, where the vibrations generated and the tool moving simultaneously in different axes can cause stiffness problems, causing imperfections in the machined surface. Indeed, the machinability of the material to be machined strongly impacts the process results and machine behavior [97,98].

Thus, it is observed that the machining of parts has a very broad market, but with some limitations, such as waste of material, for example. Figure 8 illustrates the SWOT analysis for this process, emphasizing that the tool and CNC market represents a large share of the global market, and this aspect is of great importance for the process.

**Figure 8.** SWOT analysis regarding machining.

#### **4. Hybrid Manufacturing Processes**

Nowadays, due to the development of new materials with better mechanical properties, less specific weight and better performance, the machining industry is faced with an enormous challenge to machine those materials, and thus, new processes had to be coupled and applied to satisfy this need. This is the case in hybrid manufacturing processes [1,99]. The concept of hybrid manufacturing is basically defined as the manufacturing process that joins two or even more manufacturing methods in a single piece of equipment (Figure 9), which has a great effect on the overall performance of the process [1]. The typical example of hybrid manufacturing is the combination of additive manufacturing and machining, using powder bed fusion (PBF) or directed energy deposition (DED) as an additive process with 5-axis machining [100]. This process is especially important when it comes to materials that are difficult to machine. However, many studies still need to be carried out, especially when dealing with complex geometries in 5-axis machining [101].

**Figure 9.** Different combinations of processes that can be performed using hybrid manufacturing, with powder bed fusion (PBF) or directed energy deposition (DED). Reproduced from [100]. MDPI Open Access Information and Policy: https://www.mdpi.com/openaccess; accessed on 21 October 2022.

Soshi et al. [102], in their work, manufactured a prototype of a mold normally produced by injection through a hybrid process involving DED technology as an additive process, followed by milling for surface finish and subtractive process. What could be observed in relation to the cooling performance of the mold is the uniformity of this cooling in relation to the traditional process and more stable temperature throughout the cycle, thus improving the cooling performance. In turn, Chen et al. [103] proposed an algorithm capable of calculating the machinability of the material and the product, determining as well the minimum number of alternations between additive and subtractive processes to form a highly complex part and guarantee an ideal cutting tool path in the subtractive operation. The algorithm was called Top-Down\_Sequential\_Maximization; however, only 3-axis machining was used by default.

Flynn et al. [104] addressed the issue of additive and subtractive hybrid machines through a careful review of the literature, considering the DED method in conjunction with CNC machining, with this being proposed in a future vision that forms a closed circuit. Li et al. [105] developed a 6-axis hybrid additive–subtractive manufacturing process, using a robotic arm with six degrees of freedom, together with a platform for manufacturing parts, and observed an improvement in the surface quality of the part, reduction of material waste and production time. In addition, this enables the need for a support, given the flexibility found in the six axes. Figure 10 illustrates the configuration used.

**Figure 10.** Configuration used in the study for the hybrid additive–subtractive manufacturing process. Reproduced from [105], 2018, Elsevier.

Following this lead, Yamazaki et al. [10] developed a hybrid manufacture system integrating the additive method LMD (laser metal deposition), with turning and milling tools, through the Mazak Hybrid multi-tasking machine and exposed application examples for this process, such as the oil industry. With this, it was possible to notice a huge advantage over the standard manufacturing process.

Chen et al. [106] implemented planning algorithms capable of combining additive technology with subtractive technology. In this case, in the first stage a subtractive technology was used to create the beginning of the product's geometry, followed by additive manufacturing, and ending with a machining process for surface finishing. The additive process used was the DED method and a 5-axis machine, more specifically the 3+2-axis machine. Newman et al. [107] worked with a structure called iAtractive, based on which a system called Re-Plan was created for process planning, and with it analyzed the capacity for the integration between additive and subtractive processes through case studies. With Re-Plan, it was possible to perceive that according to the geometry and complexity of the part, the material is added or removed, and even the material of an existing part can be reused to form a product with a new identity.

From a perspective considering the energy expenditure of the process and environmental damage, Yang et al. [108] analyzed the energy consumption performance when using a 6-axis robotic arm as a subtractive process, after manufacturing via additive manufacturing using the fused deposition modeling process. Several case studies and scenarios were used, and the result obtained was quite complex. Regarding future work, it is intended to carry out a mathematical and numerical analysis between the configuration of the robot arm and the energy consumption.

Liou et al. [109] combined the laser deposition process and a 5-axis CNC milling system through process planning and visualization, aiming at integrating all the processes. Additionally, Grzesik [110] highlighted additive–subtractive hybrid manufacturing through the techniques of LMD (laser metal deposition) and multi-axis CNC machining. Furthermore, Ren et al. [111] used the hybrid machining technique with DED and 5-axis machining to repair dies, targeting the corroded and worn surface. In addition, several other authors have based their studies on hybrid manufacturing, and Table 1 summarizes some pertinent works.

**Table 1.** Summary of studies about hybrid manufacturing, coupling addictive processes and machining.

As noted, hybrid manufacturing brings numerous advantages and benefits. However, some challenges are still encountered, as depicted in Figure 11 [100].

**Figure 11.** Schematic figure illustrating the challenges of hybrid machining. Reproduced from [100]. MDPI Open Access Information and Policy: https://www.mdpi.com/openaccess; accessed on 21 October 2022.

As a challenge to be faced, the recycling of metallic powder can be considered, since it can be harmful to human health, especially nickel or cobalt compounds, and must be removed from the machine after the addition operations are carried out, so that they do not affect machine components in order to avoid damage and breakdown [7]. In addition, the protection of the machine, safety and reliability of the process must be considered, since due to the heat generated during the additive process, it can result in the melting of some areas, which must have adequate protection [98]. Additionally, when talking about the challenges faced when implementing hybrid production, it is important to emphasize the training of qualified technicians so that they are able to understand and apply both techniques, and the correct disposal of waste, with a focus on handling the waste, dust and recycling liquid waste, such as lubricating oils and cutting fluids. Another problem faced is precisely in the use of these cutting fluids, since laser technology was developed regarding a fluid-free environment, and on the other hand, in machining this is widely used and practically essential or mandatory for some materials to be machined [7,98].

As hybrid manufacturing is a process that is still under development and expansion, many challenges are encountered and need to be resolved, both in relation to scientific and research parameters, as well as the technical parameters for its implementation on an industrial scale [124]. In this sense, some questions must be answered, such as, for example, what would be the best processing sequence, how the microstructure and properties of the part would behave after the cycles, or even how the software used would be able to detect collisions after adding more material [125].

However, according to Dilberoglu et al. [126], it is expected that in the future the use of hybrid production systems will be increased with additional improvements, as the implementation of this technology has attracted the attention of researchers in modern industries. Thus, based on the current scenario, and with the analysis carried out in this work, the following SWOT matrix for hybrid manufacturing resulted (Figure 12).

**Figure 12.** SWOT analysis corresponding to hybrid manufacturing.

Through this review work, it was possible to verify that there is a lot of work to be done in the field of hybrid manufacturing, which incorporates some developments carried out in each of the integrated processes (additive manufacturing and machining). This was evident through this study, which seeks to provide a structured, broad and integrated view of the contributions that have been made lately by several researchers in each of the areas. Additionally, it was possible to see how other researchers have taken advantage of these developments, and taking the knowledge acquired in each one of the considered processes, to use it in the integration of the two processes.

There is still a lot of work to be developed in the area of hybrid production. The interaction of the additive and subtractive processes does not have to be sequential and carried out in this order, although the additive process needs to be the first, but it can be reused later if there are problems with accessing the tools in certain areas. Indeed, the parts tend to have an increasingly complex shape, which also requires increasingly refined techniques for their manufacture. CAM software has not yet been developed efficiently for an interleaved use of processes, and this interleaved use may be absolutely necessary to realize different geometries. This is certainly a subject that will be of interest to researchers in the near future. Another issue to be aware of is the level of internal stresses left on the parts after machining. Although the volume of chips to be removed is generally small, finishing operations generally raise the temperature, which may influence the mechanical properties of the material being worked. Thus, there may be overlapping stresses in parts of the parts, which can lead to deformations that may interfere with the functionality and expected lifetime of the machined part. Thus, the induction of stresses and their level are also subjects that will certainly be investigated in the near future.

#### **5. Conclusions**

Additive manufacturing is undoubtedly an effective and emerging method for the manufacture of complex and difficult parts to be produced by other methods; however, a major limitation of the processes involved is the poor surface quality and dimensional accuracy, requiring a subtractive method to achieve the required surface quality and shape tolerance.

Thus, hybrid manufacturing has gained great prominence, bringing many benefits to modern industry. It guaranteed, through the integration between additive and subtractive processes, the production of parts that would previously be difficult to be produced with only one of the individual techniques, either due to the high hardness of the material, or because of the high complexity and dimensional tolerance.

One of the great advantages of this process is the high flexibility and versatility, as well as the reduction of waste, a problem highly faced by conventional machining. In addition, the possibility of using only one machine during the entire process eliminates transport times and process inventories. As a limitation, we can highlight the residual stresses present in the additive manufacturing process, which can result in distortions in the part, requiring further heat treatment.

In addition, in terms of challenges for this technology to be implemented, we can mention the recycling process of metallic powder, reliability and safety in the process, and training. Indeed, the proper disposal of waste, as well as the use of cutting fluids in machining remains an environmental issue, since the additive manufacturing technologyusing laser was developed applying environmentally friendly means, free of these fluids.

Based on this, this work has shown through SWOT analysis and a broad bibliographic review the studies and developments, as well as the great growth of this process, thus contributing to the study and analysis of the development of new design of parts, which previously would be impractical, and today are already a promising reality. Moreover, the combination of both technologies helps in overcoming the traditional issues related to the surface finishing of additively manufactured parts, providing the surface quality usually required in many applications. In this way, it is expected that in the future there will be a greater integration of hybrid machines in production lines, so that they can produce parts that are closer to the final product shape without the need for post-processing.

In the future, it is expected that the efforts to integrate the two technologies will evolve positively in the sense of increasing the rigidity of the equipment, the versatility of the transition from one technology to the other, and the complementarity in terms of work in both technologies. In fact, it is possible to take advantage of the knowledge acquired in terms of surface quality left by the additive manufacturing and try to minimize surface defects, thus minimizing the machining work to be carried out later. Furthermore, 5-axis machining makes it possible to take better advantage of topological optimization, making more complex shapes in a single fixture, thus increasing the accuracy obtained. The training needs are evident, as the requirements at this level start with the design of the parts, and end with obtaining the final parts in the equipment. Thus, there will have to be a greater integration of knowledge between those who develop the products and those who make them possible through the programming and operation of hybrid equipment. The improvement of these factors and a natural reduction of investment costs in the near future, will be decisive for the rapid growth of this technology in the market.

**Author Contributions:** Conceptualization, N.P.V.S. and F.J.G.S.; methodology, N.P.V.S.; validation, F.J.G.S., F.F. and V.F.C.S.; writing—original draft and designed, N.P.V.S.; writing—review and editing, F.J.G.S., F.F. and V.F.C.S.; Supervision, F.J.G.S.; Funding acquisition, F.J.G.S. and F.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** The present work was done and funded under the scope of the projects ON-SURF (ANI | P2020 | POCI-01-0247-FEDER-024521 and MCTool21 "Manufacturing of cutting tools for the 21st century: from nano-scale material design to numerical process simulation" (ref.: "POCI-01-0247- FEDER-045940") co-funded by Portugal 2020 and FEDER, through COMPETE 2020-Operational Programme for Competitiveness and Internationalisation. This work is also sponsored by FEDER National funds FCT under the project CEMMPRE ref. "UIDB/00285/2020". F.J.G. Silva also thanks INEGI-Instituto de Ciência e Inovação em Engenharia Mecânica e Engenharia Indústria due to its support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.