Features of the Powder Application in Direct Laser Deposition Technology

Abstract

The chapter presents the basic aspects of the use of metal powders in one of the main additive technologies—direct laser deposition (DLD). Direct laser deposition refers to a group of direct energy deposition (DED) methods and is analogous to Laser Metal Deposition (LMD) technology. The main requirements applied to DLD used metal powders are analyzed and substantiated. The influence of the basic properties of the powders on the quality of the deposited samples is demonstrated. An example of incoming quality control of powders, allowing its application in DLD technology, is presented. The results of experimental research on obtaining quality control samples for the most used metallic materials are presented. The results of structure and properties studies for the main groups of alloys based on iron, nickel, and titanium are shown. The potential for manufacturing products for various areas of industry using DLD has been demonstrated.

1. Introduction

Additive manufacturing (AM) is one of the fastest growing areas today [1, 2, 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.)

• process and environmental state (thermal history, substrate temperature distribution, laser state, chamber pressure and temperature distribution, melt pool temperature, etc.)

• Powder/material (impurity, size distribution, shape, powder type, fraction, etc.);

• Process parameters (power, powder feed rate, scanning speed, etc.);

• Motion and process control (dwell time, hatch spacing, build strategy, layer thickness, 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.

Figure 1.

Influence of metal powders characteristics on defects formation in L-DED materials.

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.

1. Gas atomization (GA). There are few melting mechanisms for gas atomization. Inductive heating (Figure 2a), plasma torch (Figure 2b), rotating rode (EIGA) (Figure 2c);

2. Plasma atomization, PA (Figure 2d);

3. Plasma Rotating Electrode Process, PREP (Figure 2e);

4. Water atomization (WA).

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].

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 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.

2. Metal powder materials for the DLD process, requirements

2.1 Size distribution

Various methods can determine the size distribution of powders, including using the laser analyzer (Figure 4a), measurement of the particle’s projection from scanning electron microscope (Figure 4b–d) or optical microscope images or by sieve analysis.

Figure 4.

Particle size distribution of nickel-based superalloy: (а) laser diffraction method (fraction of 45–100 μm), and (b–d) graphical method with using SEM images.

Predominantly, the size distribution should be normal unimodal. However, in some powders, a bimodal distribution can be observed (Figure 4c). Experimental works have shown that there is no effect of the bimodality of the powder distribution on the change in the properties and structure of the L-DED material. However, several features were found related to the influence on the structure and formation of defects of such parameters as the width of the powder fraction and particle size.

For L-DED technology, usually use following particle size distribution: 45–100 μm, 100–150 μm, 100–180 μm. Wide range of the fractional composition is possible depending on the manufacturer. However, it is not recommended to use powders smaller than 45 microns, because of a decrease in the flow rate of powder. Such powders lead to contamination of the supply system and the formation of various defects subsequently. The conducted studies also showed that the powder fraction has influenced the surface roughness [17]. Figure 5 shows the influence of the powder fraction on the surface quality.

Figure 5.

Sample surface deposited fromTi-6Al-4 V powder with (a) fraction 45–90 μm, and (b) fraction 106–180 μm.

For L-DED technology, usually use following particle size distribution: 45–100 μm, 100–150 μm, 100–180 μm. Wide range of the fractional compositions are possible depending on the manufacturer. Nevertheless, it is not recommended to use powders smaller than 45 microns, due to a decrease in flow rate of powder. Such powders lead to contamination of the supply system and the formation of various defects subsequently. The conducted studies also showed that the powder fraction has an effect on the surface roughness [17]. Figure 5 shows the influence of the powder fraction on the surface quality. Sample surface deposited fromTi-6Al-4 V powder with (a) fraction 45–90 μm (b) fraction 106–180 μm.

It can be seen from Figure 5 that the layers formed from coarse powder are less stable, while the roughness of thin walls is higher (Figure 5b). At the same time, the fine particles create a stable melt pool, resulting in a smoother surface (Figure 5a). In addition, large particles themselves introduce roughness equal to their size. An increase in roughness has negative effect the mechanical properties. That influence was shown experimentally. The effect of surface quality on mechanical properties was demonstrated on thin walls. Part of deposited walls was tested without treatment while the other part was polished mechanically. Table 1 shows the deposition mode used to obtain the experimental samples. Figure 6 presents test specimens.

Laser power, W	Speed, mm/s	Laser spot size, mm	Gas flow, l/min	Layer height, mm
1300	25	3	12.5	0.5

Table 1.

L-DED mode parameters for Ti-6Al-4 V sample production for mechanical tests.

Figure 6.

Sample with roughness removed (top) and untreated (bottom).

The samples before and after treatment were tested on uniaxial tension on a Zwick installation model Roell Z100. Test temperature was 26°С. The results of investigation are presented in Table 2. It can be seen from Table 2 that the roughness has a significant influence on the results of mechanical tests.

Before treatment		After treatment
Tensile strength, MPa	Elongation, %	Tensile strength, MPa	Elongation, %
1007	6	1050	8

Table 2.

Mechanical test results of L-DED Ti-6Al-4 V samples.

Particle size distribution also can affect the size factor of the structure, for example, the grain size. In the work of the authors present results [18] of powder size influence on the grain size for L-DED Ti-6Al-4 V titanium alloy (Figure 7). Wide powders fraction is also undesirable for an L-DED process. For example, a low laser power level can be not enough for melting particles that are too large. That led to incomplete particle melting, which can, among other things, cause effects spatter formation or powder island formation [19]. The presence of high-heat oxide films on the particle surface has the same effect. Such processes reduce the stability of the L-DED process and reduce the quality of the deposited parts as well as the process efficiency.

Figure 7.

L-DED Ti-6Al-4 V alloy: (a) microstructure of L-DED Ti-6Al-4 V obtained from 45 to 100 μm fraction powder, (b) SEM image of powder, fraction 45–100 μm, (c) size distribution of powder, fraction 45–100 μm, (d) microstructure of L-DED Ti-6Al-4 V obtained from 45 to 100 μm fraction powder, (e) SEM image of powder, fraction 180–200 μm, and (f) size distribution of powder, fraction 180–200 μm.

2.2 Chemical composition

The chemical composition of powder determines the structure and phase composition of the systems under consideration. As a rule, deviations from the required composition and content of alloying elements can lead not only to a change in the required properties, but also to a decrease in the material’s processability for the L-DED process. For example, an excess of vanadium content in the Ti-6Al-4 V titanium alloy can lead to a decrease in ductility during deposition, which in turn leads to premature failure of the product during printing.

A change in the content of the main alloying elements in 321 stainless steel leads to a change in the content of phases in the structure of the deposited material. Table 1 shows the chemical composition of steel 321 powders of different batches (labeled #1 and #2). The nickel content in powder #2 is lower than those required by the ASTM A276-98b 321 [20].

It can be seen on the X-ray diffraction pattern of the powder, the content for powder # 2 of the austenite phase was 55% and ferrite was 45% (Figure 8). An increase in the content of the ferrite leads to a change in the properties of the material, including magnetic ones. This behavior, among other things, leads to powder flow in the supply systems is deviated because of the increased magnetization of small powder particles. In this case, the stability of the deposition process and wall formation is impaired (Table 3).

Figure 8.

X-ray diffraction pattern of the deposited 321 steel sample.

Sample	Chemical composition, wt. %
	C	Si	Mn	Ni	S	P	Cr	Ti	Cu	Fe
321 #1	0,03	0,42	1,06	9,65	0,0018	0,028	18,16	0,52	0,03	bal.
321 #2	0,044	0,41	1,21	8,06	0,013	0,036	18,30
ASTM A276-98b 321*	≤0,08	≤1	≤2	9–12	≤0,03	≤0,045	17–19	≤0,8	≤0,3

Table 3.

Chemical composition of 321 steel powders.

^*

ASTM A276-06 Standard Specification for Stainless Steel Bars and Shapes.

Impurities also have a significant affect the structure during the deposition process, not only the main alloying elements content. For example, a slight increase in the iron content in a nickel superalloy leads to the crack’s formation in the deposited material. An increase in the iron content in the alloy, even a slight one, can lead to the formation of the Laves phase, which in turn leads to the formation of cracks [21, 22].

It was experimentally shown that in EP 648 nickel superalloy powders, an increase in the iron content (Table 4) led to the formation of cracks in L-DED material, as shown on Figure 9. Samples #1, #2, #3 were deposited using similar process parameters. The formation of porosity, as will be shown in the next chapter, is caused by internal porosity and the presence of satellites on the surface of the powders.

	Chemical composition, wt. %
	Al	Ti	Cr	Fe	Ni	Nb	Mo	W
1	0,93	0,76	34,50	—	55,65	0,51	2,74	4,90
2	0,85	0,98	33,99	0,55	54,50	0,78	2,86	5,49
3	0,98	1,03	33,79	0,32	53,32	0,78	2,44	4,84

Table 4.

Chemical composition of different EP 648 superalloy powders.

Figure 9.

Optical image of L-DED EP 648 superalloy microstructure obtained from powder with different Fe content.

Harmful impurities of light elements also have a significant impact on the quality of the material.

2.3 Foreign inclusions on the surface of the particles and inside

During the production of powders, various non-metallic inclusions can form, both on the surface of the particles and inside. Such inclusions can have different properties and, to varying degrees, affect the formation of the structure and the properties of the deposition material.

Figure 10 shows microstructure and chemical composition measuring results of powder and L-DED material got with using scanning electron microscopy (SEM).

Figure 10.

SEM (BSE) image and EDS analyze the results of high-entropy alloy for (a) powder surface, and (b) L-DED structure.

The results demonstrate the heredity of inclusions (presumably Cr₂O₃) that are found in the initial powders on the structure of the deposited material. Such inclusions are formed during the powder manufacturing process and can have a negative effect on the properties of the finished material. Therefore, their control is very important in the powder’s study.

2.4 Powder particle shape

The recommended and desired shape of the powder particles is spherical or subspherical. This form of particles allows to get the best fluidity in L-DED systems. Figure 11 shows the classification of particles according to their possible shape. As a rule, the particle sphericity can be expressed numerically in terms of the ratio of the maximum particle length to the minimum (lmax/lmin).

Figure 11.

Powder particle form classification.

In practice, powders with an angular shape but larger than 45 μm are also used and show good results for L-DED process of composite materials (Figure 12) [23].

Figure 12.

L-DED of metal composite material Ti-6Al-4 V/SiC SEM image (a) SiC powder, (b) cross section of deposited composite, and (c) fracture surface of deposited composite.

Powders that are spherical or subspherical and have large number of satellites or internal porosity on the surface are undesirable to use. It has been repeatedly experimentally shown that such powders lead to the formation of porosity in L-DED samples to a large extent (Figure 3). Powders consisting entirely of rod-like, acicular, laminar, and dendriform (irregular) particles are unacceptable for L-DED technology. However, particles of this shape are found in spherical powders and its content should be only 10% of such particles in the total mass of the powder is acceptable.

2.5 Incoming powder inspection

Based on experimental data incoming powder inspection methodology was developed that can use quality control. Incoming powder inspection should be carried out under the documentation developed by the technology manufacturer.

Incoming powder inspection includes several main steps:

1. Acceptance of powder and verification of regulatory documentation. Checking the condition of the package (container) for the absence of mechanical damage and traces of unauthorized opening of the package;

2. Checking the presence and legibility of the marking, the conformity of the name and marking on the product (packaging), and the information in the enclosed documents.

3. Quality control of metal powders.

Quality control of metal powder is an integral part at the initial stage of items development for L-DED process.

The main stages of powder quality control include:

• Powder Particle Surface Analysis.

  • Particle size measurement (determination of particle size distribution).

  • Determining the shape of the particles.

  • Determination of particle defects and their description.

• Analysis of the chemical composition.

  • Determination of the main alloying elements.

  • Determination of the impurities content.

• Powder Fluidity Determination.

• Determination of water content.

SEM or optical images usually using for control of powder particle surface. Particle size measurement methods were discussed in the Section 1.1. Shape of particles is determinate according to classification presented in Figure 11, Section 1.4. The number of particles of spherical and other shapes is determined by agreement between the consumer and the supplier.

Particle defects include deviations from the spherical shape of the particles, the presence of satellites, craters, and oxidation spots on the surface, the presence of pores, voids, and inclusions in the particles.

Determination of the main alloying elements is carried out using energy-dispersive spectrometry under ISO 22309:2011 «Microbeam analysis—Quantitative analysis using energy-dispersive spectrometry (EDS) for elements with an atomic number of 11 (Na) or above [24]. Quantitative analysis requires preparation of a cross section of powder particles to provide a plane-parallel surface perpendicularly placed to the electron beam. The content of the main alloying elements of the powder alloys must comply with the requirements of regulatory documents (ASTM, ISO, GOST, technical conditions, etc.).

Determination of the content of oxygen, nitrogen, hydrogen, carbon, sulfur elements is carried out under ASTM E1019-11 [25]. The content of O₂, N₂, and H₂ must comply with the requirements of ASTM, ISO, GOST, technical conditions, etc. If the content of O₂, N₂, and H₂ is not regulated for the powder alloy, the concentrations of these elements are included in the report for information. The content of sulfur and carbon is carried out for individual grades of high-alloy steels and high-temperature nickel alloys under the relevant standards (for example, ASTM-E1941 [26]).

Powder flow rate can be evaluated according to ASTM B213-20 [27]. The recommended powder flow rate for L-DED equipment and laser cladding should not exceed 30 s.

3. Laser direct energy deposition for standard construction alloys

3.1 Steels

At the moment, iron-based alloys are actively used in additive manufacturing. Steels are mainly used in the form of a spherical powder in the DED and PBF technologies [28] and also in the form of a wire in the EBM [29], WAAM [30]. The application of the austenitic stainless steels class for additive manufacturing has been the most attractive. Martensitic transformation in these steels is absent and that allows to use them almost with no problems during deposition an air atmosphere [31]. It should be noted that the requirements for the chemical composition of steels, especially for the content of light impurity, should be strictly kept. It is also should control oxygen and trying to minimize it in powders to a level of 0.02%. When the porosity high level is observed in L-DED material, it is worth checking a sulfur, phosphorus, and nitrogen content.

Other steels classes, such as high-tensile-strength carbon steels and maraging steels, require more attention before using for manufacturing by the L-DED process. Development of post-processing is also necessary for that steel class. Steels with a high carbon content, which in turn are most often limited or difficult-to-weld, require careful selection of the process mode to avoid cracking because of a high level of stress during uneven heating and cooling [2, 32, 33]. Additional heating may also be required to equalize the temperature field [34].

Steels used for welded structures are best suited for the additive process.

To select the heat treatment parameters for a particular as-deposited alloy, one should rely on data already developed for other technological processes (rolling, casting). However, standard parameters should be considered as a starting point and research should be carried out to correct the modes of annealing, hardening, and tempering. Parameters such as temperature, time, cooling rate may change up or down.

3.1.1 Input control of steel

In order to use steel powder in additive manufacturing, the first step is to ensure that the powder meets several requirements listed in the incoming inspection chapter. Powders got by the PREP and PA methods have the best quality. However, PREP is practically not used to produce steel powders because of the high cost of the process. Plasma atomization will, of course, be the best option (Figure 13).

Figure 13.

Additive manufacturing of 304 steel (a) plasma atomized powder, and (b) L-DED 304 steel.

It is worth paying attention to the content of impurities of light elements. The content of sulfur and phosphorus should be only 0.01 wt.%. The oxygen content is often not regulated by guidelines; however, for steel powders for additive manufacturing, it should be controlled, its content in steel should not exceed 0.02 wt.%. An increased amount of oxygen indicates the presence of oxide inclusions on the surface and inside the powder.

As a result, inclusions that were on the surface and inside the powders will be detected in the structure of the grown samples. Their localization may be different, but mainly at the boundary of the layers.

The high content of satellites in steel powders got by gas atomization can lead to the appearance of defects in the form of pores and non-fusion in steels. In general, gas porosity is the most common defect found in steels got with the help of L-DED. However, a small number of individual pores or an accumulation of small pores practically does not affect the mechanical characteristics.

During L-DED of non-stainless steels, it is worth using vacuum drying of powders. In the case of suspected high humidity of powders, for example, in the presence of condensate inside the can or low fluidity of powders in the absence of many satellites on the surface of the particles, it is worthwhile to vacuum dry the powders between 120 and 140°C and at least under low vacuum.

3.2 Nickel-based alloys

Nickel-based alloys are distributed the same as steel for additive manufacturing. But developed nickel alloys quantity list is the leading position for application in industry compared to the nomenclature of steels [35, 36, 37]. Main interests are heat-resistant nickel alloys [38, 39, 40], the microstructure of which is a complexly alloyed γ-solid solution of nickel and dispersed particles based on intermetallic compounds (γ′-phase Ni₃(Al,Ti) and γ′′-phase Ni₃Nb).

Multicomponent alloying of the γ-solution and γ’-phase/γ′′-phase is carried out in such a way as to ensure high phase and structural stability of the alloy [41]. Using concentrated energy sources introduces their own characteristics into the processes of structure formation and significantly affects the mechanical properties of the material. Al, Ti, Nb, and Ta are responsible for the content of the γ′-phase and γ′′-phase in nickel alloys. However, for materials used in additive manufacturing, it is recommended to use additional heating, for example, induction, if the total amount of Al, Ti in them exceeds 5 wt.%.

Strengthening of the γ-solid solution is achieved by alloying with using Co, Cr, Fe, Mo, W, Ta, Re. Strengthening of the grain boundaries is achieved by separating MC-type carbides based on Nb, Ti, Zr, as well as by selective microalloying of B, N, and rare earth metals (REM Y, La, Ce). High heat resistance of the material, grain boundary precipitates should be globular, have a size of 1 μm or less, and be dispersed along the grain boundaries, but at the same time they should not form a continuous grid. The possibility of formation of undesirable phases (σ-, μ-phases, Laves phases) should be minimized.

Despite the above difficulties, the use of additive methods for the manufacturing of products from heat-resistant nickel alloys is justified.

3.2.1 Input control of nickel-based alloys

Before using nickel-based alloy powders in additive manufacturing, the first step is to ensure that the powder corresponds a number of requirements listed in the incoming inspection chapter. Exclusively PREP and PA processes to avoid high oxide content (Figure 14) should produce nickel alloy powders with high Al and Ti content.

Figure 14.

Inconel 718 powder (a) PREP, (b) GA, (c) L-DED microstructure from PREP powder, and (d) L-DED microstructure from GA powder.

Defects in Nickel Alloys got by the L-DED technology.

For nickel alloys, as well as for steels, the content of some light elements should be strictly regulated: no more than 0.01 wt.% S and P, no more than 0.02% O. An increased amount of oxygen is usually associated with the presence of oxide inclusions on the surface and inside the powder. As a result, inclusions that were on the surface and inside the powders will be detected in the structure of the deposited samples. Their localization can be different, but mainly they are localized at the layer’s boundaries. Also, the internal porosity of the powder, which occurs in powders got by the GA method, will negatively affect (Figure 15).

Figure 15.

Nickel-based alloy EP648 powder (a) IGA, (b) EIGA, (c) L-DED microstructure from IGA powder, and (d) L-DED microstructure from EIGA powder.

3.3 Titanium alloys

Wide interest in the titanium and titanium alloys associated with a unique range of characteristics. At the same time, they are difficult to process. As a result, they are widely used in additive manufacturing.

Features of titanium alloys also appear in the production of powders. Only inert gases can be used for protection and as energy carriers in the production of powders.

The most studied and widespread titanium alloy is Ti-6Al-4 V alloy [41, 42, 43, 44]. However, there are also works by the authors on the use of other titanium powders in additive manufacturing [45, 46, 47].

Many studies have shown dependence of the L-DED process parameters on the structure and properties formation in titanium alloys [48, 49, 50, 51].

3.3.1 Input control of titanium alloys

The main attention in the input control of titanium powders is paid to the content of light impurities. Input control is carried out under the methodology presented in Section 1.6. Also, basic information on titanium powders can be found in ASTM B988–13 [51].

Opposed from steels and nickel superalloys, the low thermal conductivity of titanium causes the particle size of the powder to affect the grain size in the L-DED structure. This was shown in Section 1.1.

Since powders of titanium alloys are got by plasma atomization and PREP, they are characterized by a low content of satellites on the surface. The main problem encountered in such powders is the variation in chemical composition.

Figure 16 shows SEM images of the surface of various powders, numbered respectively A1, A2, B1, B2.

Figure 16.

Images of the powder surface from a scanning electron microscope in BSE mode: (a) A1 powder (45–100 μm), (b) A2 powder (106–180 μm), (c) B1 powder (45–100 μm), and (d) B2 powder (160–200 μm).

The results of the study of L-DED material obtained from the considered powders showed that the most significant influence is exerted by light impurities in the powder [18]. Figure 17 shows that powder B1 has an increased hydrogen content. This high hydrogen content has a significant effect on increasing hardness and reducing ductility, and porosity has also been found in the structure.

Figure 17.

Light impurities content influence on properties of L-DED Ti-6Al-4 V.

As can be seen from the L-DED figures, a sample made from a powder with a high hydrogen content has low ductility. No significant effect of the fraction on the properties was found.

3.4 Mechanical properties of L-DED material got from various powders

Practical experience in implementing of L-DED technology has demonstrated good results in terms of material properties in a wide range of nomenclature. Tables 5–7 present the chemical composition of powders that have been successfully used to produce products using the L-DED technology.

Alloy	C	S	P	Mn	Cr	Si	Ni	Cu	V	Mo	W	Fe
09CrNi2MoCu	0.08	<0.01	0.0015	0.6	0.7	0.37	2.2	0.7	0.15	0.35	0.03	base
06Cr15Ni4CuMo	0.06	0.015	0.015	0.9	15.5	0.40	4.4	1.5	—	0.28	—
316 L	0.03	0.015	0.02	0.8	17	0.6	16	—	—	3	—
08MnCuNiV	0.1	0.04	0.035	1	0.3	0,4	—	1.2	—	—	—
28Cr3SiNiMoWV	0.31	<0.01	0.01	0.8	3.2	1.2	1.2	<0.15	0.15	0.5	1.2
09Cr16Ni4Nb	0.12	0.015	0.03	0.5	16.5	0.6	4.5	0.3	0.2		0.2
15-5P	0.07	0.03	0.04	1	15.5	1	5.5	4.5	—	—	—

Table 5.

Chemical composition of steel powders.

Alloy	Al	V	Zr	V	Mo	Fe	O	Si	C	N	H	Ti
Ti-7Al-2,5Zr-2,5 V-2Mo	7	2.5	2.5	2.5	2	0.25	0.15	0.15	0.1	0.05	0.015	base
Ti-6Al-4 V	6.8	5.3	—	5.3	—	0.6	0.2	0.1	0.1	0.05	0.015
CP-Ti	—	—	—	—	—	0.25	0.2	0.1	0.07	0.04	0.01
Ti-5Al-2 V	5	2,5	0.3	2.5	—	0.25	0.15	0.12	0.1	0.04	0.006

Table 6.

Chemical composition of titanium powders.

Alloy	Cr	Mo	Fe	Nb	W	Mn	Si	Al	Ti	C	B	V	Cu	Co	Ni
EI 698	13–16	2.8–3.2	<2	1.8–2.2	—	<0.4	<0.6	1.3–1.7	2.35–2.75	<0.08	—	—	—	—	base
Inconel 718	21	3.3	base	5.5	—	—	—	0.8	1.15	0.08	0.006	—	0.3	—	55
Inconel 738	16.3	2	0.5	1.1	2.8	0.2	0.3	3.7	3.7	0.2	0.015	—	—	9	base
EK 61	18.5	5	16	5.5		0.7	0.5	1.3	1	—	—	0.6
BSh159	28	7.8	3	3.4		0.5	0.8	1.55		0.08	0.005	—	1
EP 648	35	3.3	4		5.3	0.5	0.4	1.1	1.1	0.1	0.008	1.1
Hynes 230	22	2	<3	0.5	14	0.5	0.4	0.3	0.1	0.1	0.015	—
GS32	5	5	—	5	10	—	—	8	—	0.15	0.3	—		9.3
Inconel 625	23	10	5	4,15	—	0.5	0.5	0.4	0.4	0.1	—	—	—	1

Table 7.

Chemical composition of nickel-based powders.

Most of the difficulties in deposition are solved by selecting technological parameters. The structure of L-DED materials has several features, but as a rule, heat treatment solves the problems that arise (Table 8).

Material	State	Tensile strength ${\sigma }_{В}$, MPa	Yield strength ${\sigma }_{0,2}$, MPa	Elongation δ, %	Powder
Steels
09CrNi2MoCu	L-DED	685	616	21,5	Gas atomized
	L-DED + HT	665	598	21	Gas atomized
06Cr15Ni4CuMo	L-DED	1086	888	8,35	Gas atomized
	L-DED + HT	810	515	20	Gas atomized
08MnCuNiV	L-DED	755	725	14	Gas atomized
	L-DED + HT	517	444	26,5	Gas atomized
316 L	L-DED	602	427	48,5	Gas atomized
	L-DED + HT	575	392	66	Gas atomized
316 L	L-DED	607	409	48,4	Plasma atomized
	L-DED + HT	545	245	84,6	Plasma atomized
28Cr3SiNiMoWV	L-DED	1667,2	1068,9	11,3	Gas atomized
09Cr16Ni4Nb	L-DED	1451	1167	13,5	Gas atomized
15-5P	L-DED	1123	1088	12,5	Gas atomized
Titanium alloys
Ti-7Al-2,5Zr-2,5 V-2Mo	L-DED	968,0	882,0	6,6	PREP
Ti-7Al-2,5Zr-2,5 V-2Mo	L-DED	1076,7	1056,7	1,4	Plasma atomized
	L-DED + HT	1159,2	1100,5	9,8	Plasma atomized
Ti-6Al-4 V	L-DED	1060	963	7	Plasma atomized
Ti-6Al-4 V	L-DED	1047	983	10,3	PREP
	L-DED + HT	1026	925	14,2	PREP
CP-Ti	L-DED	675	620	23,4	PREP
PT3V	L-DED	912	877	14,5	PREP
	L-DED + HT	872	834	15,3	PREP
Nickel-based alloy
EI 698	L-DED	884	565	41	PREP
	L-DED + heat treatment	1100	781	24	PREP
Inconel 718	L-DED	452,4	339	19	PREP
	L-DED + HT	1303	1155	24,6	PREP
Inconel 625	L-DED	811,9	512,8	38,5	PREP
ZhS32	L-DED	1353,0	1046,0	11,5	PREP
Hynes 230	L-DED	921,3	615,6	27,81	Inert Gas Atomizers
	L-DED + HT	922	429,2	31,92	Inert Gas Atomizers
Inconel 738	L-DED	1379	1088,9	6,44	PREP
EK61	L-DED	858	512	39,7	PREP
	L-DED HT	1152	886	29	PREP
Vsh159	L-DED	880	451	48,7	Inert Gas Atomizers
	L-DED HT	1195	921	9,6	Inert Gas Atomizers
EP 648	L-DED	781	476	38	PREP

Table 8.

Mechanical properties of L-DED material.

4. Conclusion

Additive manufacturing is of profound interest for various industries. However, a large number of parameters that affect the quality of the final product limit the application at the moment. Including the initial powders properties have a significant influence on the quality L-DED materials. For the L-DED technology, powders manufactured by gas atomization, plasma atomization, and PREP technologies are most widely used. GA powders have a high amount of satellites on the surface, which can lead to porosity in as-deposited samples. Also, a deviation from the required chemical composition has a significant impact on the properties of products. Light impurities also affect the properties of the L-DED material.

To minimize the amount of rejects due to the properties of the powder, the main requirements for powders and the input control measures that must be implemented are considered.

Considering the high influence on the structure and properties of the material of the initial quality of the powder, a method for the input control of the powder is being developed.

Input control of powder for use as feedstock in L-DED technology includes:

Particle size measurement (determination of particle size distribution).

• Determining the shape of the particles;

• Determination of particle defects and their description.

Analysis of the chemical composition:

• Determination of the main alloying elements;

• Determination of the impurities content.

Powder Fluidity Determination.

The totality of implementing measures to control the quality of the powder will reduce the number of defective products.