Crack inhibition to enhance strength-ductility of CM247LC alloy fabricated by laser powder bed fusion

Abstract

High cracking susceptibility is a critical challenge for the application of high γ′-fraction Ni-base superalloys fabricated by laser powder bed fusion (LPBF). This study systematically investigated the effect of process parameters on the cracking susceptibility and microstructure of LPBF-processed CM247LC alloy. The formation mechanisms of liquation cracks and solidification cracks were revealed. The effects of process parameters on solidification behavior, microstructure evolution and local strain level were discussed. The cracking behavior was significantly affected by the high-angle grain boundaries (HAGBs), elemental segregations (e.g., C and O) and stress concentration at cracking regions. An increase in the laser power increased the length of mushy zone and the depth/width ratio of melt pools, resulting in high solidification cracking susceptibility. The overlap between adjacent melt tracks determined by hatch spacing and layer thickness could heal partial cracks through remelting. Nearly crack-free CM247LC samples with enhanced strength-ductility (yield strength of 921 ± 38 MPa, ultimate tensile strength of 1278 ± 3 MPa, and elongation of 16.1 ± 1.3%) were obtained by optimized process parameters. The zigzag grain boundaries, refined grains and reduced grain aspect ratio in the nearly crack-free samples could relieve the stress concentration along the grain boundaries and enhance the bonding of adjacent grains and thus delay the propagation of cracks. This work may provide essential comprehension for improving the LPBF processability of high γ′-fraction Ni-base superalloys.