Effects of time per layer and part geometry on thermal history and microcracking in the fabrication of nickel superalloy samples by laser powder bed fusion

The fabrication of near-net-shape products using laser powder bed fusion (L-PBF, PBF-LB) is expected to bring benefits such as part consolidation and light-weighting to the aircraft industry. However, the challenge for practical application is to suppress microcracking in building parts during the process so that it is necessary to understand the effects of process parameters on the cracking behavior. Both time per layer and part geometry contribute to heat loss and thermal history through the process, resulting in changes in microstructure and material properties. The purpose of the current study is to understand their effects on microcracking behavior as well as thermal history, microstructure, and mechanical properties in Inconel 738 LC samples. A part-scale finite element thermal analysis confirmed that significant heat accumulation occurred during the process in the samples with constricted geometry with short time per layer. In the fabricated IN738LC samples with the heat accumulation, more coarse microstructure, greater hardness, and severe cracking size and density were observed. The cracking mechanism based on the RDG model with Rosenthal's analytical temperature field suggests that heat accumulation during the PBF-LB process should be avoided to fabricate crack-free IN738LC parts, and a sufficiently long time per layer is preferable.