Application of in-situ process monitoring to optimize laser parameters during laser powder bed fusion printing of Ti-6Al-4V parts with overhang structures

Abstract

Geometrically complex features, such as overhang structures, can be a challenge in laser powder bed fusion (L-PBF), as they can be associated with print defects, such as porosity. In this study, Ti-6Al-4V alloy overhang structures were fabricated using an L-PBF system. Differences were observed in the microstructure in the region around the overhang structures, compared with that observed for the bulk alloy. These included larger grain sizes and a less homogenous microstructure in the print layers closest to overhang structures. It is hypothesized that these microstructural changes are associated with the excess heat generated in the overhang region due to the decreased thermal conductivity of the powder immediately below the print layers, compared with solid alloy. Also observed was an increased level of porosity (up to 0.08%) in the overhang print alloy, compared with the corresponding <0.02% in the alloy bulk. During printing, in-situ process monitoring of the meltpool emissions was obtained in the near-infrared range and correlated with the properties of the printed parts. This in-process data assisted in selecting optimal laser processing conditions in the first fifteen layers above the overhang to prevent meltpool overheating. By systematically controlling the laser energy during the printing of L-PBF overhang structures, the level of porosity was reduced to match that of the bulk alloy. There was also an associated reduction in the roughness of the overhang itself, with its Ra decreasing from 62.4 ± 7.3 µm to 7.5 ± 1.9 µm.