Defect formation and its mitigation in selective laser melting of high γ′ Ni-base superalloys

This report focuses on the microstructural causes of cracking in CM247LC processed using Selective Laser Melting (SLM), as well as other phenomena that may increase the likelihood for cracking. Observations using high-speed imaging showed that material was lost as both vapor and discrete particles under some conditions, which may be a source of the Al-enriched particles found in recycled powder. The local chemical heterogeneities arising from such Al-rich particles may then contribute to crack and pore formation. Transmission Electron Microscopy (TEM) investigations for the as-built microstructure showed "cell-like" structures within the columnar grains, Hf-rich precipitates were found at "cell-like" structures and grain boundaries, and there were associated high dislocation densities at these boundaries acting as crack initiation points in the presence of residual stresses. Post-process Hot Isostatic Pressing (HIP) was used to heal the cracks and pores that form during processing. It was found to lead to pronounced recrystallization, as expected from the high dislocation density. Various routes for defect mitigation are discussed.