**4. Additive manufacturing technologies and properties of parts produced from metal powders**

At this moment, there are three main technologies for additive manufacturing from metal powders: powder bed fusion, directed energy deposition, and binder jetting (**Figure 4**).

**Figure 4.** Technological schemes of powder bed fusion (a), directed energy deposition (b) and binder jetting (c).

**Figure 3.** SEM images of powders obtained by different technologies. (a) gas atomized In718; (b) chemical reduction Fe; (c) gas atomized Ti–6Al–4V; (d) plasma atomized Ti–6Al–4V; and (e) mechanically alloyed Fe–18Cr–8Ni–12Mn–N.

One of such methods is described in work [14]. The method is based on measuring of dynamic properties of powder. For measuring, FT4 powder rheometer was used, which allows the measuring of shear, dynamic, and bulk properties. Dynamically determined powder proper‐ ties are particularly more helpful for defining flowability under the low stress conditions that

Another promising method for testing of powder material was named revolution powder analyzer. The revolution powder analyzer consists of rotating drum covered on the both sides with transparent glass and camera that records pictures of rotating drum (0–200 min-i) before backlight. This method allows the modelling of powder behavior during the coating in powder

apply to the most parts of AM process.

224 New Trends in 3D Printing

For powder bed fusion technology, AM‐system manufacturers usually use laser as an energy source [EOS, Concept Laser, SLM Solutions, 3D Systems (ex Phenix Systems), Renishaw, Realizer], but there is one company that offers systems with electron beam (Arcam).

The use of electron beam has some features: First of all, electron beam may effectively work only in high vacuum (laser systems work in inert gas atmosphere), and this is a good advantage in working with high‐reactive metals and alloys such as titanium; the second one is that before selective melting, whole layer of powder treated by multiple passes of low power electron beam for heating and sintering powder bed, this gives some limitation in geometry, because the sintered powder has to be removed after building.

Laser based on the powder bed fusion systems has differences among themselves. EOS, Concept Laser, and 3D Systems of AM systems feed initial powder from the neighboring to the main build platform tank, whereas SLM Solutions, Renishaw, and Realizer systems feed the initial powder from the main tank which is placed in the upper part of the system. This difference may have an influence on needed properties of powder (flowability). One more difference consists in recoating mechanism, and 3D Systems has patented a mechanism with roller, whereas the others use blades and the use of roller may expand a range of available for the process powders and give an advantage in using of fine powder (less 10 μm) with poor flowability.

Directed energy deposition is usually called cladding. Manufacturers of this type of systems use laser as an energy source, and powder is usually fed coaxial to a laser beam with inert gas. Depending on the cladding nozzle, it is possible to manage speed and accuracy (coaxial nozzle gives the highest accuracy, off axis is the fastest), but anyway, it is impossible to build very complex part such as lattice structures, closed cooling channels with this technology. Advan‐ tages of the technology consist in capability to deposit more than one material simultaneously, creating functionally graded coatings and parts. Most directed energy deposition systems use a 4‐ or 5‐axis motion system or a robotic arm to position the deposited head, so the build process is not limited to successive horizontal layers on parallel planes. This capability makes the process suitable for adding of material to an existing part, such as repairing a worn part or tool [1].

Binder jetting is a process, by which a liquid bonding agent is selectively deposited through inkjet print head nozzles to join powder materials in a powder bed. Binder jetting is similar to material jetting in its use of inkjet printing to dispense material. The difference lies in the fact that the dispensed material with binder jetting is not a build material, but rather a liquid one, which is deposited onto a bed of powder to hold the powder in the desired shape [1].

Producing of the parts with binder jetting technology includes 3D printing, debinding, sintering, and sometimes infiltration by another material. The advantage of binder jetting technology consists in lack of need to use support structures; powder bed makes this role; and the absence of high‐temperature gradients and phase transformations allow to save desired shape of a future part. Binder jetting in this moment has limited success for producing of metal parts and looks more promising in manufacturing ceramic parts because of its multi‐step process, or because the final properties of metal parts are not very high.

The authors have an experience and made researches in the field of using of laser powder bed fusion system. The results of selected researches in this field of additive manufacturing will be presented. Properties of metal part manufactured by selective laser melting (SLM) process (here and further, this name will be used for laser powder bed fusion of additive manufacturing technology) have strong dependence on parameters of process. The main parameters of SLM are layer thickness, laser speed and power, hatch distance, strategy of hatching. Layer thickness is very important parameter, because of its dependence on powder. It is possible to manage speed and accuracy of SLM process by changing of layer thickness; sometimes, it is not very important to have a high accuracy in Z‐direction. It gives an opportunity to use larger layer thickness that may increase build speed more than two times. At this moment, most powder bed fusion systems' manufacturers use 400 W laser in their systems, and "standard" layer thicknesses are 20 or 30and 40 or 50 μm. Layer thickness determines maximum of particle size that can be used in process, and particles with sizes more than layer thickness physically will not take part they will be thrown off by recoating blade or roller. Of course, it should be taken in account that there is some changing of density between apparent density of powder after recoating and density of material after melting; additionally, there is shrinkage effect that changes the real layer thickness during the process and depends on type of material, also some volume of particles with large size needed to save flowability properties of powder. That is why it is usually recommended to use a 10–63 μm, 10–45 μm powder in depends on density of alloy (in referring flowability) and layer thickness.

Use of powders with fine particles (for example 0–45 μm) has some ambiguity. The presence of fine particles increases apparent density, which may increase final density after SLM in the same time PBF system manufacturers do not recommend to use powders with fine particles because of the danger of their falling into the working mechanisms of systems. Researchers from University of Nottingham have made investigation about the effect of particle size distribution on processing parameters [16]. They have the following results: Final density after SLM is higher with using of powder 0–45 μm, but strength properties are higher with using of powder 10–45 μm. Another important result consists in the fact that parameters of selective laser melting for reaching the maximum density were different for powder with particle size in range of 0–45 and 10–45 μm. One more research about an influence of powder particle size distribution on properties of final part is presented in [17].

SLM is characterized by rapid laser treatment with melting and solidification of metal, and the process was accompanied with active spark formation. For sparks removing and fuming, it is usually used as the creation of "wind of inert gas" above powder bed which blowing out the sparkles and fume from working zone. In **Figure 5**, it is shown the SEM images of particles that were blown out by "wind of inert gas".

**Figure 5.** SEM images of In718 fume powder after SLM.

beam for heating and sintering powder bed, this gives some limitation in geometry, because

Laser based on the powder bed fusion systems has differences among themselves. EOS, Concept Laser, and 3D Systems of AM systems feed initial powder from the neighboring to the main build platform tank, whereas SLM Solutions, Renishaw, and Realizer systems feed the initial powder from the main tank which is placed in the upper part of the system. This difference may have an influence on needed properties of powder (flowability). One more difference consists in recoating mechanism, and 3D Systems has patented a mechanism with roller, whereas the others use blades and the use of roller may expand a range of available for the process powders and give an advantage in using of fine powder (less 10 μm) with poor

Directed energy deposition is usually called cladding. Manufacturers of this type of systems use laser as an energy source, and powder is usually fed coaxial to a laser beam with inert gas. Depending on the cladding nozzle, it is possible to manage speed and accuracy (coaxial nozzle gives the highest accuracy, off axis is the fastest), but anyway, it is impossible to build very complex part such as lattice structures, closed cooling channels with this technology. Advan‐ tages of the technology consist in capability to deposit more than one material simultaneously, creating functionally graded coatings and parts. Most directed energy deposition systems use a 4‐ or 5‐axis motion system or a robotic arm to position the deposited head, so the build process is not limited to successive horizontal layers on parallel planes. This capability makes the process suitable for adding of material to an existing part, such as repairing a worn part or tool

Binder jetting is a process, by which a liquid bonding agent is selectively deposited through inkjet print head nozzles to join powder materials in a powder bed. Binder jetting is similar to material jetting in its use of inkjet printing to dispense material. The difference lies in the fact that the dispensed material with binder jetting is not a build material, but rather a liquid one,

Producing of the parts with binder jetting technology includes 3D printing, debinding, sintering, and sometimes infiltration by another material. The advantage of binder jetting technology consists in lack of need to use support structures; powder bed makes this role; and the absence of high‐temperature gradients and phase transformations allow to save desired shape of a future part. Binder jetting in this moment has limited success for producing of metal parts and looks more promising in manufacturing ceramic parts because of its multi‐step

The authors have an experience and made researches in the field of using of laser powder bed fusion system. The results of selected researches in this field of additive manufacturing will be presented. Properties of metal part manufactured by selective laser melting (SLM) process (here and further, this name will be used for laser powder bed fusion of additive manufacturing technology) have strong dependence on parameters of process. The main parameters of SLM are layer thickness, laser speed and power, hatch distance, strategy of hatching. Layer thickness is very important parameter, because of its dependence on powder. It is possible to manage

which is deposited onto a bed of powder to hold the powder in the desired shape [1].

process, or because the final properties of metal parts are not very high.

the sintered powder has to be removed after building.

flowability.

226 New Trends in 3D Printing

[1].

As it seen from the figure/as the figure shows, some particles have dots, which may be some effect of oxidation or changing phase of the composition of powder (In718—gamma prime precipitation hardened nickel superalloys). Typically, the powders received by atomization technologies have a single phase (thanks to rapid solidification during atomization), and this fact makes an applying of powders in AM technologies easier, because the different phases may have different properties (physical density, coefficient of laser absorption, thermal conductivity etc.) and make influence of the process. Powders reuse with some fume content is an actual task for research at this stage of developing of AM.

Another important theme in reusing of powders in SLM process is an agglomeration of particles and loss of spherical shape. Some quantity of particles, lying near to manufactured parts, has been taken by heat effect that leads to sintering with each other. Such agglomerates may have large sizes, and they will be separated by sieving. But there also exist agglomerates from the small particles which can move through sieve (see **Figure 6**).

**Figure 6.** SEM images of In718 (a) and Ti–6Al–4V (b) powders used in SLM.

Phase composition of agglomerates may be different in comparison of virgin powder, during subsequent reusing of quantity of such agglomerates will grow and quality of final parts may decrease.

Mechanical properties of metal parts manufactured by SLM are usually higher than cast metal and sometimes comparable with wrought materials (see **Table 1**) [18–24]



As it seen from the figure/as the figure shows, some particles have dots, which may be some effect of oxidation or changing phase of the composition of powder (In718—gamma prime precipitation hardened nickel superalloys). Typically, the powders received by atomization technologies have a single phase (thanks to rapid solidification during atomization), and this fact makes an applying of powders in AM technologies easier, because the different phases may have different properties (physical density, coefficient of laser absorption, thermal conductivity etc.) and make influence of the process. Powders reuse with some fume content

Another important theme in reusing of powders in SLM process is an agglomeration of particles and loss of spherical shape. Some quantity of particles, lying near to manufactured parts, has been taken by heat effect that leads to sintering with each other. Such agglomerates may have large sizes, and they will be separated by sieving. But there also exist agglomerates

Phase composition of agglomerates may be different in comparison of virgin powder, during subsequent reusing of quantity of such agglomerates will grow and quality of final parts may

Mechanical properties of metal parts manufactured by SLM are usually higher than cast metal

**strength, MPa**

Horizontal 807 ± 15 1051 ± 18 22 ± 4 83.8 ± 3.5 Vertical 675 ± 12 957 ± 15 28 ± 3 91.3 ± 4.0

vertical 1160 1350 17.6

**Elongation δ, %**

**KCV, J/cm2**

and sometimes comparable with wrought materials (see **Table 1**) [18–24]

Inconel 718 (casting) [20] – 477 752 33.8 Inconel 718 – 1048–1116 1288–1341 21–27

**Sample Yield strength, MPa Ultimate**

is an actual task for research at this stage of developing of AM.

**Figure 6.** SEM images of In718 (a) and Ti–6Al–4V (b) powders used in SLM.

decrease.

228 New Trends in 3D Printing

Inconel 718 (SLM)

Inconel 718

(SLM + heat treatment)

from the small particles which can move through sieve (see **Figure 6**).

**Table 1.** Properties of samples from In718 and Ti–6Al–4V manufactured by SLM and traditional technologies.

There is anisotropy of mechanical properties of samples manufactured parallel (horizontal samples) and perpendicular (vertical samples) according to the main platform of SLM system. The reason of anisotropy is a layer‐based synthesis (grain microstructure is elongated in Z‐ direction). Also flat defects in X–Y plane and residual stresses influence on anisotropy of mechanical properties (**Figure 7a**). Effect of anisotropy may be decreased by heat treatment (stress relief, stable microstructure, and phase composition) and hot isostatic pressing (closing internal defects such as pores and cracks). High residual stresses during SLM is one of the limitation of this technology (see **Figure 7b**), and for solving of this problem, it should be used special strategies of hatching (for example, "chessboard hatching") and carefully prepared support structures.

**Figure 7.** Internal defects after SLM (a) and residual stresses (b) influence on building a sample.

As it was already noticed, selective laser melting is a process with high melting and cooling rates. This fact affects on the microstructure and phase composition of manufactured metal or alloy. It is common to make heat treatment after SLM, and a type of heat treatment strongly depends on a type of alloy; for example, for single phase of austenite stainless steels (such as 316 L), stress relieve annealing might be enough, but for precipitation of hardened nickel superalloy (such as Inconel 718), it needs to make multistage heat treatment (homogenization and aging). The **Table 2** shows the results of XRD analysis of Inconel 718 and Ti–6Al–4V samples [20, 21].


**Table 2.** The results of XRD analysis of Inconel 718 and Ti–6Al–4V samples.

Synthesis of initial powder material is a result of high‐speed solidification of the melt droplets in an inert gas stream, that is, crystallization takes place under non‐equilibrium conditions, which affects the completeness of the phase transition.

In Table 2, it is shown the changing of powder phase composition and compact samples after SLM and heat treatments. The powder of Inconel 718 and the compact sample after SLM have a similar high content of γ‐Ni matrix‐phase which is a result of rapid solidification, but due to the presence of heat‐affected zones, the quantity of γ″‐Ni3Nb and δ‐Ni3Nb phases in the compact sample is higher. Heat treatments of the compact samples lead to the changing of phase composition: Homogenization dissolves δ‐Ni3Nb phase, aging increases quantity of precipitates.

and aging). The **Table 2** shows the results of XRD analysis of Inconel 718 and Ti–6Al–4V

**Sample Qualitative composition Quantitative composition [vol%]**

γ'‐Ni3Al 3.5–3.9 γ″‐Ni3Nb 4.3–4.5 δ‐Ni3Nb 1.8–2.0

γ‐Ni 86.8 γ'‐Ni3Al 1.9 γ″‐Ni3Nb 8.0 δ‐Ni3Nb 3.3

γ‐Ni 90.1 γ'‐Ni3(Al,Ti) 1.9 γ″‐Ni3Nb 8.0

γ‐Ni 67.3 γ'‐Ni3 (Al,Ti) 8 γ″‐Ni3Nb 4 δ‐Ni3Nb 3.5 γ'‐Ni3Al 17.2

α'‐phase 94.49 β‐phase 5.51

α‐Ti 11.3 α'‐Ti 73.8 β‐Ti 14.9

Synthesis of initial powder material is a result of high‐speed solidification of the melt droplets in an inert gas stream, that is, crystallization takes place under non‐equilibrium conditions,

In Table 2, it is shown the changing of powder phase composition and compact samples after SLM and heat treatments. The powder of Inconel 718 and the compact sample after SLM have a similar high content of γ‐Ni matrix‐phase which is a result of rapid solidification, but due to the presence of heat‐affected zones, the quantity of γ″‐Ni3Nb and δ‐Ni3Nb phases in the compact sample is higher. Heat treatments of the compact samples lead to the changing of

Inconel 718 powder γ‐Ni 90.0

Ti–6Al–4V powder α'‐phase 100

**Table 2.** The results of XRD analysis of Inconel 718 and Ti–6Al–4V samples.

which affects the completeness of the phase transition.

samples [20, 21].

230 New Trends in 3D Printing

Inconel 718 SLM before heat treatment

>Inconel 718 SLM +  homogenization

Inconel 718 SLM +  homogenization + aging

Ti–6Al–4V SLM without heat treatment

Ti–6Al–4V SLM with heat treatment The study of the phase composition of the initial powder alloy Ti–6Al–4V showed that the powder consists of more than 99% of α'‐phase. Qualitative phase composition of the compact sample after SLM is different from powder material by the presence of β‐phase (its content is 5.51%).

Changing of the phase content also can be seen on the microstructure investigations (**Fig‐ ures 8** and **9**).

In Figure 8, microstructure of In718 samples after SLM, homogenization, and aging is pre‐ sented. Grains of γ‐Ni are elongated along building direction (Z‐axis) and have different size from 10 to 200 μm. Coagulated precipitates are uniformly distributed and have size of 4–5 μm. Some precipitates lined up in chains with length up to 10 μm; however, there is not seen full edging of γ‐Ni grains. Also some of precipitates observed inside of γ‐Ni grains.

**Figure 8.** Microstructure of cross section of Inconel 718 specimens, manufactured by SLM, before heat treatment (a), after homogenization (b) and aging (c).

**Figure 9.** Microstructure of cross section of Ti–6Al–4V specimens, manufactured by SLM, before heat treatment (a) and after annealing (b).

Before heat treatment, Ti–6Al–4V specimen has a basket type of microstructure (see **Figure 9**). After annealing, some α‐ and β‐phases stood at grain boundaries, initial martensite of needles enlarged in size, borders become more rounded compared to the sample without annealing.

The study of fractography of fracture surfaces of impact strength specimens showed some imperfections of SLM process (see **Figure 10**).

**Figure 10.** Fractography of Inconel 718 (a, c) and Ti–6Al–4V (b, d) specimen fracture surfaces before (a, b) and after heat treatment (c, d).

As shown in **Figure 10**, there are some micropores on the fracture surface, which function is stress concentration for cracks growth. Some micropores contain not melted powder particles.
