Mechanical properties of dissimilar laser welding of high-entropy alloy to stainless steel at room and cryogenic temperatures

The FeCoCrNiMn high-entropy alloy exhibits promising mechanical properties at cryogenic temperatures. It is of great practical significance to study the welding between the FeCoCrNiMn alloy and low-temperature steel represented by stainless steel. However, limited studies have focused on the mechanical properties of this welded joint at cryogenic temperatures, especially the cryogenic-temperature toughness. In this study, the tensile properties, impact toughness, and fracture toughness of the dissimilar metal joint between the FeCoCrNiMn alloy and 304 stainless steel were evaluated and compared at two temperatures: 298 K and 77 K. A notable finding of the study was that as the temperature decreased from 298 K to 77 K, the tensile strength of the welded joint increased by ∼73 %, while the fracture elongation rose by ∼94 %. Numerous twin boundaries were observed in the fusion zone (FZ) of the fractured tensile sample at 77 K, contributing to the emergence of the rising stage (stage II) in the work-hardening curve at this temperature. In contrast, twinning deformation occurred infrequently in the FZ during the tensile tests conducted at 298 K. The impact toughness of the FZ diminished from 45.5 ± 15.3 J/cm2 at 298 K to 25.7 ± 4.7 J/cm2 at 77 K, with the crack tip opening displacement (CTOD) of the FZ reflecting a similar trend. A significant number of low-angle grain boundaries, rather than twin boundaries, were observed in the fractured samples from the impact and CTOD tests.