Unveiling the macrosegregation formation mechanism and its impact on properties in dissimilar welding between CoCrFeMnNi high-entropy alloy and 316 stainless steel

Abstract High-entropy alloys (HEAs) are increasingly preferred as structural materials in nuclear engineering and aerospace applications. These fields often require the design of dissimilar joints. Here, gas tungsten arc welding (GTAW) was used for the first time to join CoCrFeMnNi HEAs with 316 stainless steel. Microstructural characterization, including electron microscopy, high-energy synchrotron X-ray diffraction, and thermodynamic calculations, along with micro- and macroscale mechanical assessments, was utilized. These methods were instrumental in evaluating and clarifying the effects of the non-equilibrium solidification and weld thermal cycle on the microstructure evolution of the joint. In the fusion zone (FZ), distinctive peninsula-shaped macroscopic segregation area is observed, with its formation being related to the liquidus temperature differences between the base materials (BMs) and the welded metal, compounded by the Marangoni effect. The weld thermal cycle was found to promote multiple solid-state phase transformations in the heat-affected zone (HAZ) adjacent to the CoCrFeMnNi BM, leading to varying degrees of softening. The HAZ near the 316 stainless steel BM maintained its original microstructural and mechanical properties. Fracture predominantly occurred in the FZ, mainly due to the interplay of large columnar grains, macrosegregation effects, and emergence of BCC and σ brittle phases due to the complex chemistry within this region. Thermodynamic modeling validated the formation of these phases. The ultimate tensile strength and elongation at room temperature were approximately ≈493 MPa and ≈10.70%, respectively.