Microstructure and mechanical properties of crack-free Inconel 738 fabricated by laser powder bed fusion

This study reports the successful crack-free fabrication of the non-weldable high gamma' Ni-superalloy IN 738 by laser powder bed fusion. The as-fabricated texture was composed of columnar grains with preferred orientation along < 100 > direction. Scanning electron microscopy and X-ray diffraction analysis revealed the presence of M(Ti, Ta, W, Mo, Nb)C and M(Cr, Mo, W)(23)C-6 carbides along the grain boundaries. A heat treatment used for IN 738 castings yielded a bimodal distribution of gamma' precipitates with a primary gamma' volume of 25% and size of similar to 226 nm and secondary gamma' with a volume of 43% and a size of similar to 88 nm. The carbides observed in the as-built condition were maintained. The heat treatment increased the hardness from 408 HV to 487 HV. The specimen exhibited an excellent room temperature yield strength, ultimate tensile strength, and elongation of 1010 MPa, 1444 MPa, and 13%, respectively. The coupons showed yield strength of 560 MPa and 388 MPa, the ultimate tensile strength of 765 MPa and 538 MPa, and an elongation of 17% and 14% at 850 degrees C and 927 degrees C, respectively. Finally, fracture analysis was used to better understand fracture behavior.