Atmosphere Effects in Laser Powder Bed Fusion: A Review

Abstract

The use of components fabricated by laser powder bed fusion (LPBF) requires the development of processing parameters that can produce high-quality material. Manipulating the most commonly identified critical build parameters (e.g., laser power, laser scan speed, and layer thickness) on LPBF equipment can generate acceptable parts for established materials and moderately intricate part geometries. The need to fabricate increasingly complex parts from unique materials drives the limited research into LPBF process control using underutilized parameters, such as atmosphere composition and pressure. As presented in this review, manipulating atmosphere composition and pressure in laser beam welding has been shown to expand processing windows and produce higher-quality welds. The similarities between laser beam welding and laser-based AM processes suggest that this atmosphere control research could be effectively adapted for LPBF, an area that has not been widely explored. Tailoring this research for LPBF has significant potential to reveal novel processing regimes. This review presents the current state of the art in atmosphere research for laser beam welding and LPBF, with a focus on studies exploring cover gas composition and pressure, and concludes with an outlook on future LPBF atmosphere control systems.

Keywords: additive manufacturing; cover gas; laser powder bed fusion; pressure.

Figure 1

Schematic comparing the differences between conduction and keyhole mode melting.

Figure 2

Micrographs of LPBF Inconel 625 with varying parameters showing different melt pool geometries and defect types. (a) Demonstrates keyhole mode melting and resulting porosity. (b) Demonstrates a more nominal conduction mode melting with some stochastic porosity still present. (c) Demonstrates conduction mode melting with insufficient energy, resulting in a lack of fusion-type porosity. Reproduced with permission from [12]. 2022, Elsevier.

Figure 3

Micrographs displaying an example of excess spatter and the resulting porosity in LPBF AlSi10Mg. The sample shown in (a) was built with a 50 $\mathsf{\mu }$m layer and the sample in (b) was built with a 30 $\mathsf{\mu }$m layer. Reproduced with permission from [13]. 2018, Elsevier.

Figure 4

Observed results of reduction in electron temperature and plasma plume under helium cover gas compared to argon when using pulsed Nd:YAG laser-welded 304 stainless steel. Increased penetration depth is also observed as a result of helium cover gas. Reproduced with permission from [30]. 2019, Elsevier.

Figure 5

Evolving melt pool shape in gas tungsten arc welded 304 stainless steel under argon cover gas, affected by increasing levels of oxygen shown in (a–f) on the left, and increasing carbon dioxide shown in (A–F) on the right. Small amounts of oxygen and carbon dioxide promote the preferred Marangoni flow, enhancing melt pool depth (b–d,B–D). With sufficient addition, oxides begin to form, reverting the flow direction and causing shallow melt pools, as shown in (e–f,E–F). Reproduced with permission from [47]. 2004, Elsevier.

Figure 6

The effects of reducing pressure from ambient at 101 kPa to 0.01 kPa on Yb-fiber welded A5083 aluminum. As pressure is decreased, plume formation is reduced and penetration depth is increased. Reproduced with permission from [56]; 2017, Elsevier.

Figure 7

Effect of pressure on attenuation [19,56,59].

Figure 8

Transient effects of hyperbaric pressures ranging from 0.1 MPa to 1.8 MPa on plasma plume generation in Yb-fiber welding of TA1 titanium from 20 ms after laser exposure to 180 ms after laser exposure. In addition to increased plume intensity with additional pressure, the duration of the plume increases as well. Reproduced with permission from [72]; 2021, Elsevier.

Figure 9

Reduced plume and spatter in LPBF of Ti6Al4V through the use of helium cover gas compared to argon. Reproduced with permission from [87]; 2021, Elsevier.

Figure 10

Keyhole vapor depression differences between ambient pressure argon and high vacuum ($1.3\times {10}^{-3}$ pa) environments in LPBF 316L at varying laser powers. Reproduced with permission from [88]; 2020, Elsevier.

Figure 11

Demonstration of powder denuding at various laser scan parameters for LPBF 316L under 0.1 kPa pressure. Laser power and scan speeds of (a) 50 W and 0.2 m/s, (b) 100 W and 0.4 m/s, and (c) 200 W and 0.8 m/s. Reproduced with permission from [90]; 2018, Elsevier.