Autogenous laser welded joint of Inconel 625 and AISI 316L steel: Microstructure and mechanical properties

Abstract

The study focused on a dissimilar laser-welded joint between heat-resistant AISI 316L stainless steel and Inconel 625. It included optical and scanning electron microscopy (SEM) analysis of the weld metal, heat affected zone, and fusion interface, along with microhardness measurements, Charpy impact toughness tests, and tensile property evaluations at room and elevated temperatures. Microstructural examination revealed an asymmetric solidification behavior across the weld metal of the dissimilar welded joint. On AISI 316L side, weld metal near the fusion boundary predominantly exhibited cellular and columnar solidification structure. In contrast, Inconel 625 side showed the formation of columnar dendritic structures, indicating directional solidification driven by thermal gradients and compositional differences between the base metals. Within the inter-dendritic regions and along cellular grain boundaries of the weld metal, the precipitation of Laves phases and NbC was evident in various morphologies including spherical, chain-like, and rectangular, suggesting non-equilibrium segregation during rapid solidification. The tensile strength of the dissimilar weld metal was significantly lower than that of Inconel 625 base metals and close to AISI 316L steel base metal, with an average ultimate tensile strength of 600 MPa and elongation of 46%. The tensile strength of the welded joint was measured to be 336 MPa at 650 °C and 280 MPa at 700 °C. Similarly, Charpy impact testing at room temperature revealed lower energy absorption in weld metal compared to the base metals, with an average toughness value of 72 J.