Mechanical characteristics and microstructural investigation of CMT deposited bimetallic SS316LSi-IN625 thin wall for WAAM

Wire arc additive manufacturing (WAAM) has become increasingly popular over the past few years because of its higher deposition rates that allow fabrication of large, near-net-shaped components and associated reduction of machining time and cost. This work investigates the microstructure and mechanical behavior of a bimetallic thin wall structure composed of 316LSi stainless steel and Ni-base alloy 625 fabricated by the cold metal transfer (CMT) process. Microstructure analysis of the bimetallic wall cross section at different locations revealed a columnar solidification structure with epitaxial grain growth between layers. Microsegregation of Nb and Mo was observed in the interdendritic regions of the alloy 625 deposits leading to the formation of NbC and Laves phase. Microhardness traverses across the bimetallic interface revealed a hardness of 160 to 190 HV in the 316LSi while that of the alloy 625 deposit ranged from 220 to 245 HV with no abnormal hardness at the bimetallic interface. Tensile testing was conducted within the 316LSi and alloy 625 build layers and also transverse to the bimetallic interface. Tensile testing conducted across the bimetallic interface resulted in failure in the 316LSi deposit with an average ultimate tensile strength of 660 MPa, yield strength of 412 MPa, and 49.3% elongation. Fractographic analysis revealed ductile rupture in all cases. This study demonstrates that high-quality bimetallic thin wall deposits between an austenitic stainless steel and a Ni-base alloy can be successfully produced by CMT-WAAM.