Optimization of Surface Roughness and Density of Overhang Structures Fabricated by Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) provides a rapid and versatile approach for producing parts with complex geometries. However, many parts with intricate geometries have overhang structures, which are not easily fabricated by using LPBF and are often downgraded by staircase effects, warpage, cracks, and dross formation. Thus, the present study proposes a combined numerical and experimental approach for determining the optimal settings of the laser power and scanning speed that minimize the surface roughness and maximize the density of Inconel 718 LPBF overhang structures. In the proposed approach, the heat transfer simulations are employed to determine the melt pool depth, the melt pool length, and the solid cooling rate within the feasible input space of laser power and scanning speed combinations. Notably, the simulations take account of both the difference in the material properties of the solid and powder materials, respectively, and the variation of the laser absorptivity in the depth direction of the powder layer. The simulation results are then used to train artificial neural networks for predicting the melt pool depth for 3600 combinations of the laser power and scanning speed within the input space. The resulting processing maps are screened in accordance with three quality criteria (namely the melt pool depth, the melt pool length, and the solid cooling rate) to determine the optimal processing region, which improves the surface roughness. The feasibility of the proposed approach is demonstrated by fabricating 10 × 10 and 20 × 20 mm2 horizontal overhang structures using parameter settings chosen from the optimal processing map. It shows that the optimal processing conditions result in a low surface roughness and a maximum density of 99.78%.

Air Permeability of Maraging Steel Cellular Parts Made by Selective Laser Melting.

Additive manufacturing, such as selective laser melting (SLM), can be used to manufacture cellular parts. In this study, cellular coupons of maraging steels are prepared through SLM by varying hatch distance. Air flow and permeability of porous maraging steel blocks are obtained for samples of different thickness based on the Darcy equation. By reducing hatch distance from 0.75 to 0.4 mm, the permeability decreases from 1.664 × 10-6 mm2 to 0.991 × 10-6 mm2 for 4 mm thick coupons. In addition, by increasing the thickness from 2 to 8 mm, the permeability increases from 0.741 × 10-6 mm2 to 1.345 × 10-6 mm2 at 16.2 J/mm3 energy density and 0.14 MPa inlet pressure. Simulation using ANSYS-Fluent is conducted to observe the pressure difference across the porous coupons and is compared with the experimental results. Surface artifacts and the actual morphology of scan lines can cause the simulated permeability to deviate from the experimental values. The measured permeability of maraging steel coupons is regression fit with both energy density and size of samples which provide a design guideline of porous mold inserts for industry applications such as injection molding.