Porosity formation from disrupted gas flow in laser powder bed fusion of 316 stainless steel

Abstract

Eliminating porosity remains a major challenge in metal additive manufacturing (AM). The current approach is thermal management, which involves optimizing process parameters to minimize porosity formation from repeated stacking of layers. However, auxiliary mechanisms other than laser power, scan speed, and hatch spacing simultaneously affect the repeatability of the process. For example, the flow of shielding gas over a powder bed causes convective heat transfer that results in keyhole porosity. Although conduction is widely attributed as the main mode of heat transfer during printing, in this study, the importance of convection was highlighted when keyhole porosity was observed to be spatially aligned in a pattern and uniformly distributed based on the recoater position. The pore pattern aligned with the layout of print cells when the recoater position was on the left (odd layers). Local disturbances caused by the printing process on 316 stainless steel were investigated by x-ray computed tomography. Results indicated that porosity formation emanated from an irregular nitrogen flow across the build-plate. The nitrogen flow was periodically disrupted throughout the print as the position of the recoater during odd layers obscured gas movements close to the build-plate. These observations were validated with a simulation of the gas flow on a 2D cross-section of the build-chamber using ANSYS Fluent. The disrupted flow created regions with low gas velocity, which led to a heat retention at the boundary of the print cells. The heat retained generated a concentrated occurrence of keyhole defects since the melt pools were deeper. The effect of the shielding gas, which is usually neglected, was highlighted as an important factor to ensure part quality.