**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.

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.

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.

#### **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.*

#### **Figure 5.**

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

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.


#### **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).*


#### **Table 2.**

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

*Features of the Powder Application in Direct Laser Deposition Technology DOI: http://dx.doi.org/10.5772/intechopen.108853*

#### **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.*

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.

#### **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**).

**Figure 8.**

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


#### **Table 3.**

*Chemical composition of 321 steel powders.*

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**).

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.

*Features of the Powder Application in Direct Laser Deposition Technology DOI: http://dx.doi.org/10.5772/intechopen.108853*


**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).

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).

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 10.**

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

#### **Figure 11.** *Powder particle form classification.*

#### **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.*

*Features of the Powder Application in Direct Laser Deposition Technology DOI: http://dx.doi.org/10.5772/intechopen.108853*

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:


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:

	- Particle size measurement (determination of particle size distribution).
	- Determining the shape of the particles.
	- Determination of particle defects and their description.
	- Determination of the main alloying elements.
	- Determination of the impurities 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.
