Investigation of the microstructure and mechanical properties of dissimilar joints of PBF-LB/IN718 to AISI 316L by laser welding

Abstract

In this study, nickel-based superalloy, Alloy 718 (IN718), processed by powder bed fusion, was welded to wrought AISI 316L using laser welding. The effect of building direction, pre-weld heat treatment, and laser linear energy density on the weld strength of the dissimilar joints was examined. It was shown that employing a lower energy density improved the mechanical performance of the joints, resulting in the highest ultimate tensile strength. The joints in the as-built condition resulted in superior mechanical properties than the joints in heat treated condition. The hardness values of the joints were dependent on whether the PBF-LB/IN718 specimens were in heat-treated (HT) or as-built (AB) condition. The average hardness value at the fusion zone (FZ) of the HT joints was lower than that of the AB joints. Furthermore, the ductility of the HT joints was substantially reduced at higher energy densities while the AB joints showed relatively similar ductility at various energy densities. The loss of ductility was attributed to more pores formation at the FZ of the HT joints as a result of higher degree of oxide contamination following the heat treatment process. In this regard, these porosities can be deemed more detrimental to the joint than the solidification cracks (micro-cracks) following the secondary phases precipitations of Laves and carbides. Finally, the mechanical properties of the joints were unaffected by the build orientation of their additively manufactured base metal.