1. The LPBF Process

The LPBF Process

The powder bed is a layer of powder characterized by a packing density (which depends on the arrangement of the particles and the unfilled areas between them), which is then subjected to the action of the laser, which melts the requested areas of the bed. The remaining unmelted powder serves, layer-upon-layer, as additional support to the part under construction, and some areas of the bed close to the laser action can be heated several times during the overall process performed to obtain a 3D object.

The interaction between the metal powder and the laser radiation during the process is associated with energy deposition following the powder coupling (absorption of radiation by metal particles, which appear as gray bodies due to their morphology) and bulk coupling (absorption of radiation by a metallic surface, related to the intrinsic properties of the considered metal) mechanisms ^[1][2]. The optical penetration is often evaluated comparing the powder bed to a bulk, but this is not completely right. The interaction with particles can, indeed, generate multiple reflections ^[3][4], which enhance the penetration depth of the laser. Since the action of the laser is very short in time and characterized by a very high energy (resulting cooling rates of about ~10

The interaction between the metal powder and the laser radiation during the process is associated with energy deposition following the powder coupling (absorption of radiation by metal particles, which appear as gray bodies due to their morphology) and bulk coupling (absorption of radiation by a metallic surface, related to the intrinsic properties of the considered metal) mechanisms [41,42]. The optical penetration is often evaluated comparing the powder bed to a bulk, but this is not completely right. The interaction with particles can, indeed, generate multiple reflections [43,44], which enhance the penetration depth of the laser. Since the action of the laser is very short in time and characterized by a very high energy (resulting cooling rates of about ~10

⁵

–10

⁶ K/s ^[5]), evaporation and melting of the exposed powder particles can easily occur ^[6][7].

K/s [45]), evaporation and melting of the exposed powder particles can easily occur [46,47].

In addition to studying the full LPBF process to realize parts and samples for tensile and fatigue tests, single-track experiments are extensively used to understand the powder–laser interaction ^{[8][9][10][11]}. These experiments allow highlighting the presence of stability zones, where the track is continuous, and instability zones, whose irregularities (i.e., distortions, balling) are highly dependent on the scanning speed values, on the laser power, on the thickness of the powder layer and the substrate material on which it is spread, and on the powder particle morphology and granulometry ^{[12][2][6][8][13][14][15]}.

In addition to studying the full LPBF process to realize parts and samples for tensile and fatigue tests, single-track experiments are extensively used to understand the powder–laser interaction [48–51]. These experiments allow highlighting the presence of stability zones, where the track is continuous, and instability zones, whose irregularities (i.e., distortions, balling) are highly dependent on the scanning speed values, on the laser power, on the thickness of the powder layer and the substrate material on which it is spread, and on the powder particle morphology and granulometry [33,42,46,48,52–54].

2. Powder Feedstock Features

Powder Feedstock Features

The quality of samples and parts built with laser powder bed fusion is strongly influenced by the properties of the metal powders particles themselves ^[16][17][18], as also outlined by official regulation agencies such as ASTM (ASTMF2924-14 ^[19]). Granulometry, flowability, particle size, density, and chemical composition are among the most crucial properties which are typically checked before processing the selected powder ^{[7][20][21][22][23][24]}.

The quality of samples and parts built with laser powder bed fusion is strongly influenced by the properties of the metal powders particles themselves [30,55,56], as also outlined by official regulation agencies such as ASTM (ASTMF2924-14 [29]). Granulometry, flowability, particle size, density, and chemical composition are among the most crucial properties which are typically checked before processing the selected powder [47,57–61].

The quality and properties of the feedstock materials depend a lot on the manufacturing process, which, in the case of metal powders for LPBF, can be quite varied ^{[25][26][27][28][29][30][31]}, ranging from rotary, water, and gas atomization ^{[30][31][32][33][34]} to the plasma rotating electrode process (PREP) ^[35][36]. Furthermore, in order to make the additive manufacturing process more cost-efficient and to reduce the price of the feedstock, the reuse of scraps and chips produced by traditional manufacturing of expensive metals and alloys (i.e., Ti-6Al-4V, aluminum) was proposed via spheroidization ^[37][38] and milling ^[39][40].

The quality and properties of the feedstock materials depend a lot on the manufacturing process, which, in the case of metal powders for LPBF, can be quite varied [18,62–67], ranging from rotary, water, and gas atomization [66–70] to the plasma rotating electrode process (PREP) [71,72]. Furthermore, in order to make the additive manufacturing process more cost-efficient and to reduce the price of the feedstock, the reuse of scraps and chips produced by traditional manufacturing of expensive metals and alloys (i.e., Ti-6Al-4V, aluminum) was proposed via spheroidization [73,74] and milling [75,76].

The literature shows that powder recycling, when referring to the LPBF process, can be done in a variety of ways ^[41][42], ranging from the reintroduction of sieved powder after each build and adding the used and sieved powder together with the virgin one with or without mixing, to the mixing of used powder with powder of the same age after each cycle (defined as the number of jobs after which the amount of feedstock in not enough to perform further jobs). According to literature, the first two procedures seem to be the most used ^{[19][43][44][45][46]}. Denti et al. ^[16] recently suggested a parameter called “average usage time” (AUT) to account for the real duration of laser–powder interaction, instead of the number of jobs performed, which could be widely adopted as a reliable method to evaluate the level of interaction. Modifications of powder properties with reuse in LPBF are highly influenced by the starting material, the process parameters, and the environment inside the build chamber. In the case of highly reactive powders (i.e., Al- and Ti-based powders), even the storage and handling conditions might affect the quality of the powder ^[47].

The literature shows that powder recycling, when referring to the LPBF process, can be done in a variety of ways [23,77], ranging from the reintroduction of sieved powder after each build and adding the used and sieved powder together with the virgin one with or without mixing, to the mixing of used powder with powder of the same age after each cycle (defined as the number of jobs after which the amount of feedstock in not enough to perform further jobs). According to literature, the first two procedures seem to be the most used [19,78–81]. Denti et al. [30] recently suggested a parameter called “average usage time” (AUT) to account for the real duration of laser–powder interaction, instead of the number of jobs performed, which could be widely adopted as a reliable method to evaluate the level of interaction. Modifications of powder properties with reuse in LPBF are highly influenced by the starting material, the process parameters, and the environment inside the build chamber. In the case of highly reactive powders (i.e., Al- and Ti-based powders), even the storage and handling conditions might affect the quality of the powder [82].

What is most striking from the results of studies performed on many different alloys (stainless steels, Ti-6Al-4V, Alsi10Mg, IN718 and IN625, Co–Cr, Scalmalloy) ^{[19][16][48][22][47][49][50][51][52][53][54]}, in terms of reused powder characterization and, most importantly, of the microstructural and mechanical properties of the final parts, is that, while the former are quite similar for the same alloy, the latter are characterized by very scattered results ^[42]. Mechanical properties could be improved, decreased, or unaffected by the reuse of feedstock material, as shown by the results reported in Table 1. This result suggests that more attention should be paid to what really happens inside the chamber during the laser–powder interaction and not only to the metal powder quality itself.

What is most striking from the results of studies performed on many different alloys (stainless steels, Ti-6Al-4V, Alsi10Mg, IN718 and IN625, Co–Cr, Scalmalloy) [19,30,37,58,82–88], in terms of reused powder characterization and, most importantly, of the microstructural and mechanical properties of the final parts, is that, while the former are quite similar for the same alloy, the latter are characterized by very scattered results [23]. Mechanical properties could be improved, decreased, or unaffected by the reuse of feedstock material, as shown by the results reported in Table 1. This result suggests that more attention should be paid to what really happens inside the chamber during the laser–powder interaction and not only to the metal powder quality itself.

Table 1.

Summary of studies on powder recycling (IED—input energy density, UTS—ultimate tensile strength, ND—not determined).

3. Heat Source

Heat Source

The interaction between metal power particles and laser radiation during LPBF is quite complex and includes a number of physical phenomena, such as chemical reactions and phase transformation, heat transfer, and a complex fluid flow within the melt pool due to the surface-tension gradient, as well as absorption and scattering of the laser radiation ^[26]. The high-energy solid-state lasers typically employed in LPBF have an axisymmetric Gaussian profile of the power density distribution, with beam diameters between 50 and 100 µm for fine resolution, and an intensity which decreases upon penetration through the powder layers deposited on top of each other ^[1][8][58], since the radiation also penetrates through the pores between the particles in the bed.

The interaction between metal power particles and laser radiation during LPBF is quite complex and includes a number of physical phenomena, such as chemical reactions and phase transformation, heat transfer, and a complex fluid flow within the melt pool due to the surface-tension gradient, as well as absorption and scattering of the laser radiation [18]. The high-energy solid-state lasers typically employed in LPBF have an axisymmetric Gaussian profile of the power density distribution, with beam diameters between 50 and 100 µm for fine resolution, and an intensity which decreases upon penetration through the powder layers deposited on top of each other [41,48,90], since the radiation also penetrates through the pores between the particles in the bed.

The connection between the shape of the laser beam profile and the melt pool was extensively studied in the literature ^[11][59][60]. Furthermore, the top-hat shape employed in laser welding ^[61] was shown to produce keyholes having a shorter depth and, for this reason, Tenbrock et al. ^[62] recently applied this profile in laser powder bed fusion of 316 L stainless steel, showing that an efficient LPBF processes can also be realized by applying diode lasers as long as a proper defined intensity threshold is exceeded (I ≈ 8–10 × 10

The connection between the shape of the laser beam profile and the melt pool was extensively studied in the literature [51,91,92]. Furthermore, the top-hat shape employed in laser welding [93] was shown to produce keyholes having a shorter depth and, for this reason, Tenbrock et al. [94] recently applied this profile in laser powder bed fusion of 316 L stainless steel, showing that an efficient LPBF processes can also be realized by applying diode lasers as long as a proper defined intensity threshold is exceeded (I ≈ 8–10 × 10

⁵

W/cm

^{2 [62]}).

[94]).

The use of ultrafast lasers (i.e., femtosecond lasers) is also under study since they could allow processing metal and alloys with high melting temperatures and thermal conductivity (i.e., rhenium) and ceramics [95,96].

Shielding Gas Flow

The use of ultrafast lasers (i.e., femtosecond lasers) is also under study since they could allow processing metal and alloys with high melting temperatures and thermal conductivity (i.e., rhenium) and ceramics ^[63][64].

The laser powder bed fusion process is always performed under inert atmosphere, in order to avoid any possible interaction between the metal particles (very reactive with a high specific surface area) and impurities such as humidity and light elements (i.e., oxygen, carbon oxides), which might affect the local chemical composition and the resulting mechanical properties of the manufactured parts [18,23,58]. Furthermore, a continuous flow of inert gas is essential in order to limit the redeposition on the powder bed of by-products during the process; this is crucial because should the removal of by-products by the shielding gas flow not be effective, there is a high risk of laser attenuation [97–99]. Owing to the formation of metal vapor plume during the laser–powder interaction, the incident laser energy could be absorbed partially. These effects were particularly studied by Grünberger and Domröse [100], who generated the so-called splashy process by changing the focal position of the laser and concluded that a proper gas flow rate is mandatory in order to avoid the occurrence of this phenomenon, since, in areas of the build chamber where the local gas flow velocity is slow, there is a higher beam scattering. The gas flow in the process chamber was found to be a crucial parameter to limit the presence of defects in parts obtained by laser powder bed fusion [97,101].

4. Shielding Gas Flow

The most used inert gases are argon and nitrogen, although, in the case of highly reactive materials prone to nitride formation, Ar is the only available option [102]. However, Pauzon et al. [103] recently studied combinations of Ar and He to process Ti-6Al-4V, and their results showed improved cooling rates and an impressively higher build rate (up to 40%).

The laser powder bed fusion process is always performed under inert atmosphere, in order to avoid any possible interaction between the metal particles (very reactive with a high specific surface area) and impurities such as humidity and light elements (i.e., oxygen, carbon oxides), which might affect the local chemical composition and the resulting mechanical properties of the manufactured parts ^[26][42][22]. Furthermore, a continuous flow of inert gas is essential in order to limit the redeposition on the powder bed of by-products during the process; this is crucial because should the removal of by-products by the shielding gas flow not be effective, there is a high risk of laser attenuation ^[65][66][67]. Owing to the formation of metal vapor plume during the laser–powder interaction, the incident laser energy could be absorbed partially. These effects were particularly studied by Grünberger and Domröse ^[68], who generated the so-called splashy process by changing the focal position of the laser and concluded that a proper gas flow rate is mandatory in order to avoid the occurrence of this phenomenon, since, in areas of the build chamber where the local gas flow velocity is slow, there is a higher beam scattering. The gas flow in the process chamber was found to be a crucial parameter to limit the presence of defects in parts obtained by laser powder bed fusion ^[65][69].

The most used inert gases are argon and nitrogen, although, in the case of highly reactive materials prone to nitride formation, Ar is the only available option ^[70]. However, Pauzon et al. ^[71] recently studied combinations of Ar and He to process Ti-6Al-4V, and their results showed improved cooling rates and an impressively higher build rate (up to 40%).

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