**2. Technologies of metal powder production**

There are various technologies for mass metal powder production, and it should be marked that in the chapter will be shown technologies available for mass powder production and will not be considered such technologies as sol–gel, chemical vapor deposition, and physical vapor deposition that allow to receive nanosized powders with unique properties but not applicable in additive manufacturing at this moment. One of the main requirements for using of metal powder in additive manufacturing and receiving reliable and repeatable results is a spherical form of particles. Some technologies allow to produce a spherical or near to spherical powder shape directly after synthesis of powder, whereas the other technologies require a further processing to achieve the desired particles shape. Technologies for the production of metal powder conventionally are separated on base of the following methods: physical–chemical and mechanical ones. The physical–chemical methods are associated with physical and chemical transformations, chemical composition, and structure of the final product (metal powder) and significantly differ from raw materials. The mechanical methods include various types of milling processes and jet dispersion melts by high pressure of gas or liquid (also known as atomization).

#### **2.1. Mechanical methods**

Atomization is the most widespread technology for the mass production of metal powders for additive manufacturing. There are various methods; the most popular is a gas atomization process; similar to water atomization technology, another one is a plasma atomization, also known as rotating electrode atomization; and less popular is a centrifugal atomization.

The main principle of all atomization technologies is a disintegration (dispersion) of a thin stream of molten metal by subjecting it with impact of gas, high pressure of water, plasma, rotating forces etc. During this impact, molten metal is divided on small droplets, which rapidly crystallize in flight before they reach atomizer walls.

*Gas atomization*—at this moment, this is the main process for producing of metal powders for additive manufacturing. The process steps involved into the production of metal powders are melting, atomizing, and solidifying of the respective metals and alloys. Gas atomizers are usually equipped with a furnace for melting under vacuum or rarely under protective atmosphere, with feeders of liquid alloy with nozzles in atomizing chamber, where a thin flow of the melted alloy dispersed on small droplets by high pressure of inert gas, and the droplets solidify during the flight in atomizing chamber. Powders produced by gas atomization have a spherical shape, high cleanliness, fine, and homogeneous microstructure (thanks to rapid solidification).

quality by producing parts with form the closest to computer model data. All the variety of

An important part of additive manufacturing of metal parts is the initial material. There are different approaches of additive manufacturing, which use different types of initial materials, and the most popular technologies, such as selective laser or electron beam melting, laser cladding, and binder jetting, use initial material in the powdered form [2–4], but there are also

In the chapter, the state of art of metal powder based on additive manufacturing will be presented. The chapter considers three main themes—metal powders, properties of metal powders, additive technologies, and properties of metal parts. It will be shown the methods for mass production of metal powders for additive manufacturing technologies, descriptions on characterization of powder properties and microstructure and mechanical properties of

There are various technologies for mass metal powder production, and it should be marked that in the chapter will be shown technologies available for mass powder production and will not be considered such technologies as sol–gel, chemical vapor deposition, and physical vapor deposition that allow to receive nanosized powders with unique properties but not applicable in additive manufacturing at this moment. One of the main requirements for using of metal powder in additive manufacturing and receiving reliable and repeatable results is a spherical form of particles. Some technologies allow to produce a spherical or near to spherical powder shape directly after synthesis of powder, whereas the other technologies require a further processing to achieve the desired particles shape. Technologies for the production of metal powder conventionally are separated on base of the following methods: physical–chemical and mechanical ones. The physical–chemical methods are associated with physical and chemical transformations, chemical composition, and structure of the final product (metal powder) and significantly differ from raw materials. The mechanical methods include various types of milling processes and jet dispersion melts by high pressure of gas or liquid (also known

Atomization is the most widespread technology for the mass production of metal powders for additive manufacturing. There are various methods; the most popular is a gas atomization process; similar to water atomization technology, another one is a plasma atomization, also known as rotating electrode atomization; and less popular is a centrifugal atomization.

The main principle of all atomization technologies is a disintegration (dispersion) of a thin stream of molten metal by subjecting it with impact of gas, high pressure of water, plasma, rotating forces etc. During this impact, molten metal is divided on small droplets, which

rapidly crystallize in flight before they reach atomizer walls.

additive technologies are available in the annual report [1].

**2. Technologies of metal powder production**

metal samples.

216 New Trends in 3D Printing

as atomization).

**2.1. Mechanical methods**

technologies which use initial material in sheet or wire form [5, 6].

One of the European leaders in producing of equipment for gas atomization is the German company ALD vacuum technologies GmbH. The company offers different modifications of gas atomizer for producing of different alloys, which allow to produce powders and a wide range of metals and alloys. Two main modifications are VIGA and EIGA. The first one is decrypted as a vacuum induction melting combined with inert gas atomization, and this is the most popular system that allows to produce powders of nonreactive metals and their alloys. The second one is decrypted as electrode induction melting gas atomization, and this system uses the high‐reactive metals and alloys such as titanium for powder production. The other modifications are less popular and have been used in special cases:

– Plasma melting induction guiding gas atomization (PIGA) uses a plasma burner instead of melting induction and water‐cooled copper crucible. This system usually used for the pro‐ duction of ceramic‐free and reactive high‐melting alloys;

– Electroslag remelting–cold wall induction guiding (ESR‐CIG) was especially developed for the production of high performance of superalloys. It uses the so‐called "triple melt process" for reaching the highest level of cleanliness and chemical homogeneity of powder. This system uses water‐cooled copper crucible, same as in PIGA, and raw material in form of an electrode, as in EIGA;

– Vacuum induction melting based on the cold wall crucible melting technology combined with inert gas atomization (VIGA‐CC) was developed for the production of reactive alloys, which are difficult to produce in electrode form (for example, brittle intermetallic TiAl alloys) and use water‐cooled copper crucible with a bottom pouring system [7].

Powders obtained by gas atomization process usually have a spherical or near to spherical shape and have particle sizes, which mostly can be used in additive technologies. It should be noted that particle size distribution has a strong dependence on the type of atomized alloy and used system.

*Water atomization* is similar to gas atomization process, but instead of gas, it uses high pressure of water steam as atomizing medium. The water atomization is used mostly for the production of powders, unreactive materials such as steels. Due to higher cooling rates in comparing to the gas atomization, particles have irregular shapes. The main advantage of water atomization consists in the fact that it is less expensive process than the other types of atomization; disadvantage is in the limitations of purity, especially for metals and alloys inclined to oxidation.

Another relatively non‐expensive process is a compressed air atomization. This process also is used to produce unreactive materials, and particles shapes have many defects such as satellites, internal porosity etc.

*Plasma atomization* is a relatively new process, which was developed for production of high‐ purity powders of reactive metals and alloys with high melting point such as titanium, zirconium, tantalum etc. Plasma atomization allows to produce fine particle distribution powders with highly spherical particles shape and low content of oxygen. The initial material for plasma atomization process is a metal wire. Wire feedstock is fed into a plasma torches that disperse wire into droplets with subsequent solidification in powder form. Particle size distribution of powder produced by plasma atomization is 0–200 μm.

The use of a wire has advantages over the typical gas atomization process. The most significant advantage consists in the fact that the metal feedstock, and more importantly the melt, does not come into contact with cold solid surfaces. This is another approach in comparison with the use of cooling crucible to receive high‐purity powders. The first production step is a wire feeding, and the speed of the wire should be monitored in order to control and adjust the resulting particle size distribution. The low flow rate of argon is used because of using argon plasma as the atomizing medium as well as heat source, since the heated gas has a higher velocity, and thus, a stronger atomization force is applied. Additionally, the use of a hot atomizing gas instead of a cold one prevents the particles the rapidly freezing of particles together into irregular shapes. The use of plasma as a heating source enables to reach a high superheat and the result of cooling ensures to complete spheroidization. Powder collection is occurred with a typical cyclonic device, and the powder is carefully passivated to ensure the safe manipulation in the open air [8].

There is a limitation for plasma atomization technology—initial material has to be flexible enough to get it in wire feedstock, so it is impossible to atomize materials that can not to be produced in a wire form. There are two companies in Canada, which use plasma atomization as the main process for powder production: AP&C (ex Raymor) and PyroGenesis, both companies produce powders with the focus on application of additive manufacturing.

#### *2.1.1. Centrifugal atomization*

The other types of atomization processes comprise a number of centrifugal atomization processes. There exist several schemes of using centrifugal forces for dispersing of molten metal; however, two types of such processes are more popular. The first type is a rotating electrode process (REP); a metal electrode is rotated with high velocity; and the free end is melted with an arc between the metal electrode and the tungsten electrode; if a plasma arc is involved, the process is known as plasma rotating electrode process (PREP). This process is used for the production of high‐reactive powders. Melting of the electrode is carried out in an inert atmosphere. Powder particles produced by rotating electrode processes have a spherical shape with smooth and high‐quality surfaces. The particle size distribution is from 50 to 400  μm with D50 around 200 μm. In spite of all advantages, there are also disadvantages of these methods. A major of them is a limitation of rotational speed, which restricts the minimum of median particle size to about 50–150 μm. Also, the production of high‐quality metal electrode has a high cost; productivity is low; and energy consumption is high compared to other atomizing processes. In the second type of centrifugal atomization, a molten stream of metal is allowed to fall onto a rotating disc or cone, which disperses the melt on droplets under centrifugal forces [9, 10].

#### *2.1.2. Mechanical milling*

Another relatively non‐expensive process is a compressed air atomization. This process also is used to produce unreactive materials, and particles shapes have many defects such as

*Plasma atomization* is a relatively new process, which was developed for production of high‐ purity powders of reactive metals and alloys with high melting point such as titanium, zirconium, tantalum etc. Plasma atomization allows to produce fine particle distribution powders with highly spherical particles shape and low content of oxygen. The initial material for plasma atomization process is a metal wire. Wire feedstock is fed into a plasma torches that disperse wire into droplets with subsequent solidification in powder form. Particle size

The use of a wire has advantages over the typical gas atomization process. The most significant advantage consists in the fact that the metal feedstock, and more importantly the melt, does not come into contact with cold solid surfaces. This is another approach in comparison with the use of cooling crucible to receive high‐purity powders. The first production step is a wire feeding, and the speed of the wire should be monitored in order to control and adjust the resulting particle size distribution. The low flow rate of argon is used because of using argon plasma as the atomizing medium as well as heat source, since the heated gas has a higher velocity, and thus, a stronger atomization force is applied. Additionally, the use of a hot atomizing gas instead of a cold one prevents the particles the rapidly freezing of particles together into irregular shapes. The use of plasma as a heating source enables to reach a high superheat and the result of cooling ensures to complete spheroidization. Powder collection is occurred with a typical cyclonic device, and the powder is carefully passivated to ensure the

There is a limitation for plasma atomization technology—initial material has to be flexible enough to get it in wire feedstock, so it is impossible to atomize materials that can not to be produced in a wire form. There are two companies in Canada, which use plasma atomization as the main process for powder production: AP&C (ex Raymor) and PyroGenesis, both companies produce powders with the focus on application of additive manufacturing.

The other types of atomization processes comprise a number of centrifugal atomization processes. There exist several schemes of using centrifugal forces for dispersing of molten metal; however, two types of such processes are more popular. The first type is a rotating electrode process (REP); a metal electrode is rotated with high velocity; and the free end is melted with an arc between the metal electrode and the tungsten electrode; if a plasma arc is involved, the process is known as plasma rotating electrode process (PREP). This process is used for the production of high‐reactive powders. Melting of the electrode is carried out in an inert atmosphere. Powder particles produced by rotating electrode processes have a spherical shape with smooth and high‐quality surfaces. The particle size distribution is from 50 to 400  μm with D50 around 200 μm. In spite of all advantages, there are also disadvantages of these methods. A major of them is a limitation of rotational speed, which restricts the minimum of median particle size to about 50–150 μm. Also, the production of high‐quality metal electrode

distribution of powder produced by plasma atomization is 0–200 μm.

satellites, internal porosity etc.

218 New Trends in 3D Printing

safe manipulation in the open air [8].

*2.1.1. Centrifugal atomization*

Mechanical milling was long time one of the most widespread method for the production of iron powder [11]. The conversion of raw material into powder form with mechanical milling occurs in a solid or liquid state. Milling of solids is meant to reduce the primary raw size by destroying them under influence of external forces. There are three types of milling process crushing, grinding, and attrition. It is possible to combine the different types of treatment of material for reaching the purpose: compression (static), collision (dynamic), shear (incision). The first two types allow to obtain a large size of particles, and the second and third types are used for receiving of fine powders.

*Mechanical alloying* is a completely solid‐state powder processing technique. The process consists of repeated welding, fracturing (crushing), and rewelding of powder particles in a high energy mills. The process due to high intensity of impaction on the fine particles allows to receive powders with non‐equilibrium phases (metastable crystalline and quasicrystalline phases), amorphous alloys, nanosized structure. Also the process is used to produce and develop new materials and alloys such as amorphous alloys, intermetallic compound, supersaturated solid phases, and metal matrix composites. The process is used to produce a variety of materials and alloys: supersaturated solid solutions, amorphous materials, inter‐ metallic compounds, and metal‐matrix composites [10, 12].

Different types of milling equipment can be used for mechanical alloying, such as horizontal and vertical attritors, disintegrators, planetary ball mills, shaker mills [10].

The advantage of mechanical milling process consists in the possibility to use different raw materials, and it can be pure components, sponge, fibers, or ore for alloying or waste products of mechanical production: chips, shavings, flakes etc.

#### **2.2. Physical–chemical methods**

#### *2.2.1. Electrolysis*

Electrolysis is known as a physical–chemical process that consists in allocation of electrode components, which occur when the solution or electrolyte melt carry current. Raw material for electrolysis is a metal anode, and in some cases, it is possible to use pressed or sintered waste metal products, choosing needed conditions (composition and viscosity of electrolyte, current density, temperature, etc.) metals can be deposited in powder form. The limitation of electrolysis means an ability to receive pure metal, but not alloys [9].

#### *2.2.2. Chemical processes*

The leading of chemical process is a carbonyl process, which allows to produce nickel and iron powder. The crude metal reacts with gaseous carbon oxide under pressure and temperature that lead to the formation of carbonyl, which is decomposed under raising temperature and lowering pressure to metal powder.

Other chemical conversion processes include the following:

## *2.2.3. Plasma spheroidization*

One of the new technics for powder production for additive manufacturing is plasma spher‐ oidization. In fact, this is not the method for the production but method of additional treatment of non‐spherical powder, which allows to change the shape of particles to ideal spheres.

The world leader in production of plasma spheroidization equipment is a company Tekna. The company's line of products consists of four systems: from laboratory‐scale to industrial‐ scale serial production. Depending on parameters of initial powder, it is possible to make controllable process of full melting and get spherical form of particles during the flight through plasma chamber.

The process benefits do not limited by changing of the shape of particles; it also decreases internal porosity of powder, improves powder flowability, increases apparent density, and enhances powder purity. The last one is quite strong benefit for posttreatment powders after several uses in additive manufacturing [13].