Patterned keyhole porosity formation in laser powder bed fusion caused by local disturbances in the shielding gas flow

Abstract

Porosity is a major challenge in laser powder bed fusion systems (PBF) and a crucial contributor to fatigue life. The current approach to remedy this challenge is thermal management, which involves optimizing process parameters to minimize porosity formation from repeated stacking of layers to produce a fully dense part. However, auxiliary mechanisms other than laser power, scan speed, and hatch spacing simultaneously affect the repeatability of the process. For example, the irregular flow of shielding gas over a powder bed disrupts the melt’s pool convective heat transfer resulting in keyhole porosity. This study aims to explain the defect formation associated with the print process caused by local disturbances of the shielding gas flow pattern and the interaction of adjacent rasters. Additively manufactured 316 stainless steel cuboids were investigated by x-ray computed tomography. Results indicated that pores were spatially aligned in a pattern and uniformly distributed. A comparison of the CT scans and the print file showed that the pore pattern aligned with the layout of print cells during the melting of odd layers. During odd layers, the position of the recoater disrupted the shielding gas flow. The disrupted flow created regions with low gas velocity, which led to 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.