5.1.6. Gas Flow Strategies

Most modern L-PBF equipment using gas flow removes process by-products from the process zone to enable an undisturbed process. Ladewig et al. [87] examined the influence of the protective gas flow uniformity and rate on single-laser tracks and the hatching process during the building procedure of bulk material. The efficiency of spatter removal decreased as the velocity of the protective gas flow reduced. Chien et al. [134] proposed to optimize and calibrate the inert purge airflow in an L-PBF build chamber using simulation framework methods such as coupled computational fluid dynamics (CFD) and the discrete element method (DEM). Wang et al. [126] created a full-scale geometric model to explore the interaction between the protective gas flow and the laser-induced spatter particles. The flow field was found to be steady up to a height of 30 mm above the surface of the powder bed. It was discovered that printing in this region could improve the final quality due to the consistent high-velocity flow of the protective airflow in the center of the powder bed, which removed by-products such as spatter.

#### *5.2. Equipment and Materials for L-PBF*

In addition to regulating process parameters, research on L-PBF equipment and materials has become a major focus for mitigating the effect of spatter. These two research areas will also contribute to the future commercialization of L-PBF technology. A summary of the research on L-PBF equipment and materials is shown in Table 10.


**Table 10.** A summary of the research on L-PBF equipment and materials.

#### 5.2.1. Research on L-PBF Equipment

Spatter generation can be reduced by optimizing L-PBF equipment. Koike et al. [138,139] developed a high-gravity L-PBF system that generated a strong gravitational field by centrifugal acceleration. At a high gravity acceleration of more than 10 G, the spatters were greatly suppressed. As illustrated in Figure 26, the height of the spatter trajectory was inversely related to the increased gravitational acceleration. They noted that when a suitably strong gravitational acceleration was applied, spatter generation was dramatically suppressed.

L-PBF equipment

Powder materials

316L,

**Table 10.** A summary of the research on L-PBF equipment and materials.

Aluminum Uniformity of flow field Philo et al. (2018) [135]

316L, 13-93 bioactive glass Increasing the viscosity of melt Leung et al. (2018) [140]

316L Prevent powder from blowing away Zhang et al. (2020) [137] 316L High gravity powder bed Koike et al. (2021) [138,139]

Spatter generation can be reduced by optimizing L-PBF equipment. Koike et al. [138,139] developed a high-gravity L-PBF system that generated a strong gravitational field by centrifugal acceleration. At a high gravity acceleration of more than 10 G, the spatters were greatly suppressed. As illustrated in Figure 26, the height of the spatter trajectory was inversely related to the increased gravitational acceleration. They noted that when a suitably strong gravitational acceleration was applied, spatter generation was dra-

Xiao et al. (2021) [136]

Heiden et al. (2019) [141] Fedina et al. (2020) [142] Fedina et al. (2021) [143]

 **Materials Spatter Countermeasures References** 

AISI 4130; 316L Reducing the oxygen content of powder

5.2.1. Research on L-PBF Equipment

matically suppressed.

**Figure 26.** Classification and suppression of spatter under high gravitational acceleration: ① solid spatter; ② metallic ejected spatter; ③ powder agglomeration spatter; ④ entrainment melting spatter; and ⑤ defect induced spatter. (Reprinted with permission from Ref. [139]. Copyright 2021 Elsevier B.V.). **Figure 26.** Classification and suppression of spatter under high gravitational acceleration: <sup>1</sup> solid spatter; <sup>2</sup> metallic ejected spatter; <sup>3</sup> powder agglomeration spatter; <sup>4</sup> entrainment melting spatter; and <sup>5</sup> defect induced spatter. (Reprinted with permission from Ref. [139]. Copyright 2021 Elsevier B.V.).

Philo et al. [135] used numerical simulations to investigate the interaction between the gas flow and spatter. They discovered that the parameters of the protective gas inlet and outlet in the build chamber (e.g., the radius of the inlet nozzles, the heights of the inlet and outlet) significantly affect the flow velocity, uniformity, and spatter concentration. Xiao et al. [136] simulated the flow field in an L-PBF build chamber to optimize the flowfield structure. The flow-field state was evaluated using the particle tracer method. It was shown that the flow-field distribution was made more uniform by structural optimization, which can improve the ability of the gas flow to entrain spatter. Philo et al. [135] used numerical simulations to investigate the interaction between the gas flow and spatter. They discovered that the parameters of the protective gas inlet and outlet in the build chamber (e.g., the radius of the inlet nozzles, the heights of the inlet and outlet) significantly affect the flow velocity, uniformity, and spatter concentration. Xiao et al. [136] simulated the flow field in an L-PBF build chamber to optimize the flowfield structure. The flow-field state was evaluated using the particle tracer method. It was shown that the flow-field distribution was made more uniform by structural optimization, which can improve the ability of the gas flow to entrain spatter.

To increase the capability for spatter removal, Zhang et al. [137] proposed a novel design for the gas flow system in the build chamber, as illustrated in Figure 27. The effect of the gas flow on the solid particles was obtained using the fully coupled CFD-DPM fluid–particle interaction model. The new design increased the spatter removal rate by reducing the Coanda effect, which substantially affected the spatter removal process. In addition, another row of nozzles was added directly under the primary inlet nozzles. To increase the capability for spatter removal, Zhang et al. [137] proposed a novel design for the gas flow system in the build chamber, as illustrated in Figure 27. The effect of the gas flow on the solid particles was obtained using the fully coupled CFD-DPM fluid–particle interaction model. The new design increased the spatter removal rate by reducing the Coanda effect, which substantially affected the spatter removal process. In addition, another row of nozzles was added directly under the primary inlet nozzles.

Current novel L-PBF machines generally use multi-laser beams to print simultaneously to increase efficiency, which generates more spatter. Optimizing the equipment, especially the build chamber, to remove spatter has become a major concern for many L-PBF machine manufacturers. SLM Solutions GmbH (Lubeck, Germany) has introduced adopting the building chamber to a high pressure in order to minimize the spatter activity, which hence has lowered spatter generation [144]. Through the streamlined special-shaped design of the flow channel, Bright Co. Ltd. (Xi'an, China) [145] reduced the vortex current at the outlet of the protective gas, and the steam plume and spatters are ensured to be blown away and not redeposit on the forming surface during forming, which solves the quality problem of the forming surface during printing. General Electric Co. invented a gas flow system for an additive manufacturing machine that uses a gas flow parallel to the powder bed to remove by-products (including spatter) from the L-PBF manufacturing process [146]. The MYSINT 100 3D printer from SISMA [147], Italy, has a stable and uniform flow field to ensure spatter removal efficiency.

**Figure 27.** CAD model of counter-Coanda effect in L-PBF build chamber: (a) the counter-Coanda effect design that employs another row of nozzles directly under the primary nozzles; (b) A-A' plane: transient velocity and pressure field. (Reprinted with permission from Ref. [137]. Copyright 2020 Elsevier B.V.). **Figure 27.** CAD model of counter-Coanda effect in L-PBF build chamber: (**a**) the counter-Coanda effect design that employs another row of nozzles directly under the primary nozzles; (**b**) A-A' plane: transient velocity and pressure field. (Reprinted with permission from Ref. [137]. Copyright 2020 Elsevier B.V.).

#### Current novel L-PBF machines generally use multi-laser beams to print simultane-5.2.2. Research on Powder Material

ously to increase efficiency, which generates more spatter. Optimizing the equipment, especially the build chamber, to remove spatter has become a major concern for many L-PBF machine manufacturers. SLM Solutions GmbH (Lubeck, Germany) has introduced adopting the building chamber to a high pressure in order to minimize the spatter activity, The physical properties and oxygen content of the powder can also contribute to differences in spatter behaviors, which can be reduced by a high viscosity, high thermal conductivity, and high density. Powders with a low oxygen content caused significantly less spatter in L-PBF.


the fluid in the melt pool, leading to spattering as the melt pool broke into molten droplets [148]. Fedina et al. [142] investigated L-PBF dynamics and powder behavior by comparing water-atomized and gas-atomized powders. They discovered that the water-atomized powder had more frequent spatter ejection and speculated that the higher oxygen level in the powder caused the melt pool to become unstable, resulting in an excessive number of spatters.

Manufacturers are also concentrating their efforts on developing powder materials suitable for L-PBF, offering a wide variety of powder materials such as various titanium alloys, nickel alloys, aluminum alloys, and cobalt–chromium alloy powder materials for the aerospace, automotive, and biomedical fields.
