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 [41,42]^[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 [43^[3][4],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 ~10⁵–10⁶ K/s [45]^[5]), evaporation and melting of the exposed powder particles can easily occur [46,47]^[6][7].

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]^{[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 [33,42,46,48,52–54]^{[12][2][6][8][13][14][15]}.

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 [30^[16][17][18],55,56], as also outlined by official regulation agencies such as ASTM (ASTMF2924-14 [29]^[19]). 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]^{[7][20][21][22][23][24]}.

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^{[25][26][27][28][29][30][31]},62–67], ranging from rotary, water, and gas atomization [66–70]^{[30][31][32][33][34]} to the plasma rotating electrode process (PREP) [71,72]^[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 [73,74]^[37][38] and milling [75,76]^[39][40].

The literature shows that powder recycling, when referring to the LPBF process, can be done in a variety of ways [23^[41][42],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]^{[19][43][44][45][46]}. Denti et al. [30]^[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 [82]^[47].

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^{[19][16][48][22][47][49][50][51][52][53][54]},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]^[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.

Table 1. Summary of studies on powder recycling (IED—input energy density, UTS—ultimate tensile strength, ND—not determined).

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 [18]^[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 [41^[1][8][58],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 [51,91,92]^[11][59][60]. Furthermore, the top-hat shape employed in laser welding [93]^[61] was shown to produce keyholes having a shorter depth and, for this reason, Tenbrock et al. [94]^[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 × 10⁵ W/cm² [94]^[62]).

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]^[63][64].

Shielding Gas Flow

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]^[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 [97–99]^[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 [100]^[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 [97,101]^[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 [102]^[70]. However, Pauzon et al. [103]^[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%).

1. Fischer, P.; Romano, V.; Weber, H.P.; Karapatis, N.P.; Boillat, E.; Glardon, R. Sintering of commercially pure titanium powder with a Nd:YAG laser source. Acta Mater. 2003, 51, 1651–1662.
2. Simchi, A. Direct laser sintering of metal powders: Mechanism, kinetics and microstructural features. Sci. Eng. A 2006, 428, 148–158.
3. Von Allmen, M.F.; Blatter, A. Laser-Beam Interactions with Materials; Springer: Berlin, Germany, 1994.
4. Slavin, A.J.; Arcas, V.; Greenhalgh, C.A.; Irvine, E.R.; Marshall, D.B. Theoretical model for the thermal conductivity of apacked bed of solid spheroids in the presence of a static gas with no adjustable parameters except pressure and temperature. J. Heat Mass Transf. 2002, 45, 4151–4161.
5. Starke, E.J.; Fine, M.E. Rapidly Solidified Powder Aluminum Alloys; ASTM: West Conshohocken, PA, USA, 1986.
6. Kruth, J.P.; Froyen, L.; Van Vaerenbergh, J.; Mercelis, P.; Rombouts, M.; Lauwers, B. Selective laser melting of iron-based powder. Mater. Process. Technol. 2004, 149, 616–622.
7. Hebert, R.J. Viewpoint: Metallurgical aspects of powder bed metal additive manufacturing. Mater. Sci. 2016, 51, 1165–1175.
8. Yadroitsev, I.; Gusarov, A.; Yadroitsava, I.; Smurov, I. Single track formation in selective laser melting of metal powders. Mater. Process. Technol. 2010, 210, 1624–1631.
9. Aversa, A.; Moshiri, M.; Librera, E.; Hadi, M.; Marchese, G.; Manfredi, D.; Lorusso, M.; Calignano, F.; Biamino, S.; Lombardi, M.; et al. Single scan track analyses on aluminium based powders. Mater. Process. Technol. 2018, 255, 17–25.
10. Shrestha, S.; Chou, K. Single track scanning experiment in laser powder bed fusion process. Procedia Manuf. 2018, 26, 857–864.
11. Makoana, N.W.; Yadroitsava, I.; Möller, H.; Yadroitsev, I. Characterization of 17-4PH Single Tracks Produced at Different Parametric Conditions towards Increased Productivity of LPBF Systems—The Effect of Laser Power and Spot Size Upscaling. Metals 2018, 8, 475.
12. Simchi, A. The role of particle size on the laser sintering of iron powder. Mater. Trans. B 2004, 35, 937–948.
13. Yadroitsev, I.; Bertrand, P.; Smurov, I. Parametric analysis of selective laser melting technology. Surf. Sci. 2007, 253, 8064–8069.
14. Mumtaz, K.A.; Erasenthiran, P.; Hopkinson, N. High density selective laser melting of Waspaloy^®. Mater. Process. Technol. 2008, 195, 77–87.
15. Bricín, D.; Kříž, A. Assessment of Usability of WC-Co Powder Mixtures for SLM. Technol. 2018, 18, 719–726.
16. Zhao, X.M.; Chen, J.; Lin, X.; Huang, W.D. Study on microstructure and mechanical properties of laser rapid forming Inconel 718. Sci. Eng. A 2008, 478, 119–124.
17. Sames, W.J.; List, F.A.; Pannala, S.; Dehoff, R.R.; Babu, S.S. The metallurgy and processing science of metal additive manufacturing. Mater. Rev. 2016, 61, 315–360.
18. Slotwinski, J.A.; Garboczi, E.J.; Stutzman, P.E.; Ferraris, C.F.; Watson, S.S.; Peltz, M.A. Characterization of metal powders used for additive manufacturing. Res. Natl. Inst. Stand. 2014, 119, 460–493.
19. Santomaso, A.; Lazzaro, P.; Canu, P. Powder flowability and density ratios: The impact of granules packing. Eng. Sci. 2003, 58, 2857–2874.
20. Gatto, A.; Bassoli, E.; Denti, L. Repercussions of powder contamination on the fatigue life of additive manufactured maraging steel. Manuf. 2018, 24, 13–19.
21. Seifi, M.; Salem, A.; Beuth, J.; Harrysson, O.; Lewandowski, J.J. Overview of Materials Qualification Needs for Metal Additive Manufacturing. JOM 2016, 68, 747–764.
22. Dawes, J.; Bowerman, R.; Trepleton, R. Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain. Johnson Matthey Technol. Rev. 2015, 59, 243–256.
23. Chen, G.; Zhao, S.Y.; Tan, P.; Wang, J.; Xiang, C.S.; Tang, H.P. A comparative study of Ti-6Al-4V powders for additive manufacturing by gas atomization, plasma rotating electrode process and plasma atomization. Powder Technol. 2018, 333, 38–46.
24. Mostafaei, A.; Hilla, C.; Stevens, E.L.; Nandwana, P.; Elliott, A.M.; Chmielus, M. Comparison of characterization methods for differently atomized nickel-based alloy 625 powders. Powder Technol. 2018, 333, 180–192.
25. Goncharov, I.S.; Razumov, N.G.; Silin, A.O.; Ozerskoi, N.E.; Shamshurin, A.I.; Kim, A.; Wang, Q.S.; Popovich, A.A. Synthesis of Nb-based powder alloy by mechanical alloying and plasma spheroidization processes for additive manufacturing. Lett. 2019, 245, 188–191.
26. Si, C.; Tang, X.; Zhang, X.; Wang, J.; Wu, W. Characteristics of 7055Al alloy powders manufactured by gas-solid two-phase atomization: A comparison with gas atomization process. Des. 2017, 118, 66–74.
27. Garboczi, E.J.; Hrabe, N. Particle shape and size analysis for metal powders used for additive manufacturing: Technique description and application to two gas-atomized and plasma-atomized Ti64 powders. Manuf. 2020, 31, 100965.
28. Anderson, I.E.; Terpstra, R.L. Progress toward gas atomization processing with increased uniformity and control. Sci. Eng. A 2002, 326, 101–109.
29. Bourdeau, R.G. Rotary Atomizing Process. U.S. Patent No. 4,415,511, 15 November 1983.
30. Seki, Y.; Okamoto, S.; Takigawa, H.; Kawai, N. Effect of atomization variables on powder characteristics in the high-pressured water atomization process. Powder. Rep. 1990, 45, 38–40.
31. Champagne, B.; Angers, R. PREP (Rotating Electrode Process) atomization mechanisms. Powder Metall. Int. 1984, 16, 125–128.
32. Ozols, A.; Sirkin, H.R.; Vicente, E.E. Segregation in Stellite powders produced by the plasma rotating electrode process. Sci. Eng. A 1999, 262, 64–69.
33. Wei, W.-H.; Wang, L.-Z.; Chen, T.; Duan, X.-M.; Li, W. Study on the flow properties of Ti-6Al-4V powders prepared by radio-frequency plasma spheroidization. Powder Technol. 2017, 28, 2431–2437.
34. Ustundag, M.; Varol, R. Comparison of a commercial powder and a powder produced from Ti-6Al-4V chips and their effects on compacts sintered by the sinter-HIP method. J. Min. Met. Mater. 2019, 26, 878–888.
35. Soufiani, A.M.; Karimzadeh, F.; Enayati, M.H.; Soufiani, A.M. The effect of type of atmospheric gas on milling behavior of nanostructured Ti6Al4V alloy. Powder Technol. 2012, 23, 264–267.
36. Canakci, A.; Varol, T. A novel method for the production of metal powders without conventional atomization process. Clean. Prod. 2015, 99, 312–319.
37. Lutter-Günther, M.; Gebbe, C.; Kamps, T.; Seidel, C.; Reinhart, G. Powder recycling in laser beam melting: Strategies, consumption modeling and influence on resource efficiency. Eng. Res. Dev. 2018, 12, 377–389.
38. Strondl, A.; Lyckfeldt, O.; Brodin, H.; Ackelid, U. Characterization and control of powder properties for additive manufacturing. JOM 2015, 67, 549–554.
39. Seyda, V.; Kaufmann, N.; Emmelmann, C. Investigation of aging processes of Ti-6Al-4 V powder material in laser melting. Proc. 2012, 39, 425–431.
40. Maamoun, A.H.; Elbestawi, M.; Dosbaeva, G.K.; Veldhuis, S.C. Thermal post-processing of AlSi10Mg parts produced by selective laser melting using recycled powder. Manuf. 2018, 21, 234–247.
41. Del Re, F.; Contaldi, V.; Astarita, A.; Palumbo, B.; Squillace, A.; Corrado, P.; Di Petta, P. Statistical approach for assessing the effect of powder reuse on the final quality of AlSi10Mg parts produced by laser powder bed fusion additive manufacturing. J. Adv. Manuf. Technol. 2018, 97, 2231–2240.
42. Cordova, L.; Campos, M.; Tinga, T. Revealing the Effects of Powder Reuse for Selective Laser Melting by Powder Characterization. JOM 2019, 71, 1062–1072.
43. Asgari, H.; Baxter, C.; Hosseinkhani, K.; Mohammadi, M. On microstructure and mechanical properties of additively manufactured AlSi10Mg_200C using recycled powder. Sci. Eng. A 2017, 707, 148–158.
44. Galicki, D.; List, F.; Babu, S.S.; Plotkowski, A.; Meyer, H.M.; Seals, R.; Hayes, C. Localized Changes of Stainless Steel Powder Characteristics During Selective Laser Melting Additive Manufacturing. Mater. Trans. A 2019, 50, 1582–1605.
45. Quintana, O.A.; Alvarez, J.; Mcmillan, R.; Tong, W.; Tomonto, C. Effects of Reusing Ti-6Al-4V Powder in a Selective Laser Melting Additive System Operated in an Industrial Setting. JOM 2018, 70, 1863–1869.
46. Simonelli, M.; Tuck, C.; Aboulkhair, N.T.; Maskery, I.; Ashcroft, I.; Wildman, R.D.; Hague, R. A study on the laser spatter and the oxidation reactions during selective laser melting of 316L stainless steel, Al-Si10-Mg, and Ti-6Al-4V. Mater. Trans. A 2015, 46, 3842–3851.
47. Carrion, P.E.; Soltani-Tehrani, A.; Phan, N.; Shamsaei, N. Powder Recycling Effects on the Tensile and Fatigue Behavior of Additively Manufactured Ti-6Al-4V Parts. JOM 2019, 71, 963–973.
48. Hadadzadeh, A.; Baxter, C.; Shalchi Amirkhiz, B.; Mohammadi, M. Strengthening mechanisms in direct metal laser sintered AlSi10Mg: Comparison between virgin and recycled powders. Manuf. 2018, 23, 108–120.
49. Ali, U.; Esmaeilizadeh, R.; Ahmed, F.; Sarker, D.; Muhammad, W.; Keshavarzkermani, A.; Mahmoodkhani, Y.; Marzbanrad, E.; Toyserkani, E. Identification and characterization of spatter particles and their effect on surface roughness, density and mechanical response of 17-4 PH stainless steel laser powder-bed fusion parts. Sci. Eng. A 2019, 756, 98–107.
50. Fischer, P.; Karapatis, N.; Romano, V.; Glardon, R.; Weber, H.P. A model for the interaction of near-infrared laser pulses with metal powders in selective laser sintering. Phys. Mater. Sci. Process 2002, 74, 467–474.
51. Tapia, G.; Khairallah, S.A.; Matthews, M.J.; King, W.E.; Elwany, A. Gaussian process based surrogate modeling framework for process planning in laser powder-bed fusion additive manufacturing of 316L stainless steel. J. Adv. Manuf. Technol. 2018, 94, 3591–3603.
52. Metelkova, J.; Kinds, Y.; Kempen, K.; Formanoir, C.; Witvrouw, A.; van Hooreweder, B. On the influence of laser defocusing in selective laser melting of 316L. Manuf. 2018, 3, 161–169.
53. Kaplan, A.F.H. Modelling the primary impact of an Yb: Fibre laser beam profile on the keyhole front. Procedia 2011, 12, 627–637.
54. Tenbrock, C.; Fischer, F.G.; Wissenbach, K.; Schleifenbaum, J.H.; Wagenblast, P.; Meiners, W.; Wagner, J. Influence of keyhole and conduction mode melting for top-hat shaped beam profiles in laser powder bed fusion. Mater. Process. Technol. 2020, 278, 116514.
55. Nie, B.; Yang, L.; Huang, H.; Bai, S.; Wan, P.; Liu, J. Femtosecond laser additive manufacturing of iron and tungsten parts. Phys. A 2015, 119, 1075–1080.
56. Ngo, T.D.; Kashani, A.; Imbalzano, G.; Nguyen, K.T.Q.; Hui, D. Additive manufacturing (3D printing): A review of materials, methods, applications and challenges. Part B 2018, 143, 172–196.
57. Ladewig, A.; Schlick, G.; Fisser, M.; Schulze, V.; Glatzel, U. Influence of the shielding gas flow on the removal of process by-products in the selective laser melting process. Manuf. 2016, 10, 1–9.
58. Shcheglov, P.Y.; Gumenyuk, A.V.; Gornushkin, I.B.; Rethmeier, R.; Petrovskiy, V.N. Vapor-plasma plume investigation during high-power fiber laser welding. Laser Phys. 2012, 23, 016001.
59. Greses, J.; Hilton, P.A.; Barlow, C.Y.; Steen, W.M. Plume attenuation under high power Nd:yttrium-aluminium-garnet laser welding. Laser Appl. 2004, 16, 9.
60. Grünberger, T.; Domröse, R. Identification of process phenomena by optical in-process monitoring. Laser Technik J. 2015, 1, 45–48.
61. Sutton, A.T.; Kriewall, C.S.; Leu, M.C.; Newkirk, J.W.; Brown, B. Characterization of laser spatter and condensate generated during the selective laser melting of 304L stainless steel powder. Addit. Manuf. 2020, 31, 100904.
62. Pauzon, C.; Forêt, P.; Hryha, E.; Arunprasad, T. Effect of helium – argon mixtures as laser – powder bed fusion processing atmospheres on the properties of the built Ti-6Al-4V parts. In Proceedings of the WorldPM2018 Congress, Beijing, China, 16–20 September 2018; pp. 1633–1639.
63. Pauzon, C.; Forêt, P.; Hryha, E.; Arunprasad, T.; Nyborg, L. Argon-helium mixtures as Laser-Powder Bed Fusion atmospheres: Towards increased build rate of Ti-6Al-4V. Mater. Process. Technol. 2020, 279, 116555.