**1. Introduction**

Additive manufacturing (AM) is one of the fastest growing areas today [1–3]. This interest is dictated by the high demand from the part of the industry. However, now there are high requirements for additive manufacturing products, including AM technology. It should be noted that despite the prospects of introducing AM technologies into real production and substitution for some subtractive methods for the real application, a high number of parameters that affect the quality of the final product still have to be considered [4]. Laser-Direct Energy Deposition (L-DED) is one of the examples such AM technology. Many researchers at the stage of commissioning technology into real parts production have difficulties that are associated with the necessity for many parameters control, which can be divided into the following groups:

• machine specification (laser type, shielding gas, laser beam radius, alarms, and interlocks, etc.)


All of the abovementioned parameters have a high impact on the quality and performance of laser-based AM technologies [5, 6]. Errors in selected parameters or inadequate control can lead to large losses of both operating hours of expensive equipment and large volumes of powder material. This chapter discusses the basic requirements of L-DED technology for initial powder materials, as well as the basic powder control methods that can minimize the number of defective products during additive manufacturing.

**Figure 1** shows the main characteristics of metal powders that can affect the quality of the laser direct energy deposited products. The combination of these factors can significantly affect the properties of the items and its performance. The properties of products are reduced not only because of the formation of defects such us of pores and cracks, but also because of changes in the phase composition and structure of the metals and its alloys. Impurities such as hydrogen, oxygen, nitrogen as interstitial atoms change the crystal lattice parameters and lead to a change in the phase composition, as well as to a significant embrittlement of the material.

A high hydrogen content, for example, can lead to the formation of cold cracks. In titanium alloys, a high oxygen content not only increases the hardness and leads to the formation of titanium oxides, but also, as a stabilizer of the α-phase can change the phase composition of the alloy. A change in the content of niobium in the Inconel 718 alloy leads to a change in the phase composition, since niobium can form many phases (gamma, delta phase, carbides, and leaves phase). Depending on the concentrations of niobium, a different quantitative relation of phases can be formed in deposited Inconel 718 alloy. This significantly affects the properties of the final product.

All properties of powders, both physical and chemical, depend on the manufacturing process and the parameters of the corresponding process.

Among the various production methods, several technologies are usable for the manufacturing of powders suitable for L-DED technology.


Powders produced by water atomization technology are limited used for L-DED technologies. Reason is that water-atomized powders are non-spherical and are usually have an irregular shape with an average particle size of about 100 μm [8].

In gas atomization, liquid metal is dispersed with using a high-speed gas stream (air, nitrogen, argon, or helium). In gas atomization, the metal or alloy is melted in a melting chamber filled with an inert gas. Molten metal poured in a controlled mode through a sprayer. Jet of inert gas is broken the flow of liquid metal into spherical powder particles under high pressure, which solidify in flight (**Figure 2a–c**). The particles have the same chemical composition as the molten stream. Gas atomization processes can be classified by the heating method as well as by the design of the nozzle used. The most commonly encountered nozzle types in the production of metal powders for additive manufacturing are the free-fall nozzle, the close coupled nozzle, and the De Laval nozzle. The combination of parameters of gas atomization processes largely determines the shape and size of powder particles. The pressure of the spheroidizing gas determines the size of the powder and the quality of its surface. Gas atomization methods are characterized by the satellites formation on the particles surface. Satellites are formed during the collision of small particles with partially molten larger particles during gas circulation in the spray chamber. Another disadvantage of gas atomization methods is the possibility of forming internal porosity, which is formed because of the capture of the gas used for sputtering by liquid metal [9]. Satellites and

#### **Figure 2.**

*Metal powders manufacturing technologies: (a) gas atomization with inductive heating, (b) gas atomization with plasma torch, (c) electrode inert gas atomization, (d) plasma atomization, and (e) plasma rotating electrode process [7].*

internal pores during the AM process are also inherited in deposited material from gas-atomized powders that lead to reduction of the mechanical properties. In addition, some studies have shown that an increase in the number of satellites can affect the behavior of particles in the gas flow, which also leads to the formation of porosity in the metal [10].

Plasma atomization technology realizes by plasma melting of the wire. The spherical powder particles are formed when molten droplets are cooled [11]. The main parameter of plasma atomization process is the thermal power of the plasma arc, which depends on the current force and the rate of plasma gas supply. In addition, the quality of the PA metal powders depends on the cooling rate, which is determined both by the thermophysical characteristics of the sprayed material and its heat exchange with the environment surrounding the particle [12, 13]. Powders manufactured by plasma atomization (PA) are free from satellites and have a higher quality than powders got by gas atomization. The PA disadvantage is the relatively low process of productivity compared to gas atomization. This technology is most promising for the production of titanium and titanium alloys powders [14, 15]. Another minor disadvantage is only powder alloys available as wire can be made by PA.

Plasma rotating electrode process (PREP) is a centrifugal atomization method [16]. The metal melted at the end of the rod billet moves to the periphery under the action of centrifugal forces. As the metal accumulates in the rod surface, the centrifugal forces acting on the melt increase and at some point exceed the surface tension forces. The metal is sprayed. Flow rate of the melt influence on the mode of drop formation.

It is also worth mentioning that the particle size can be controlled by the electrical current applied to the plasma arc and the distance between the tip of the plasma gun and the end of the rod. In PREP, the droplets fly radially away from the metal surface in a centrifugal force; in other words, it moves in order, so the chance of collisions of droplets and particles to form satellites is very low.

**Figure 3** shows powder surface for AM depending on manufacturing technology.

#### **Figure 3.**

*Powder surface: (a) iron powders produced by water atomization; (b) 316 L powders produced by gas atomization; (c) 316 L powders produced by plasma atomization; and (d) Ti-6Al-4 V powders produced by plasma rotating electrode process.*
