Combining alloy and process modification for micro-crack mitigation in an additively manufactured Ni-base superalloy

Abstract

The additive manufacturing (AM) of the γ` precipitation strengthened Ni-base superalloys still remains a challenge due to their susceptibility to micro-cracking. Post-processing, such as HIPing, has been shown to heal the micro-cracks but it remains desirable to prevent the micro-cracking from even occurring. Numerous studies highlighting potential mechanisms for micro-cracking exist but few solutions have been demonstrated. The intent of this study was to identify the micro-crack mechanisms and demonstrate how process and alloy modifications can reduce the micro-cracking. The micro-crack surfaces exhibit a dendritic appearance that is indicative of solidification cracking. Additionally, Gleeble experiments, simulating the L-PBF induced Heat Affected Zone (HAZ), were conducted below the γ` solvus temperature and reveal the existence of grain boundary liquation, indicative of liquation cracking. Two cracking mechanisms are thus coexisting during Laser Powder Bed Fusion (L-PBF) of CM247LC. Based on experimental evidence, reduction in the solidification interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. The study also involved two separate and distinct processing conditions to highlight the importance of melt pool geometry on micro-crack density. Samples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with optimized processing conditions exhibit a reduction in crack density of 98 %. This study demonstrates the importance of both processing conditions and alloy chemistry on micro-cracking in L-PBF fabricated γ` hardening Ni-base superalloys.