Low temperature hardening and toughening of CoCrFeMnNi and HSLA laser-welded joint by nano twin

Low strength and high cost are important factors hindering the development of large structural parts in modern industry. This study investigated the dissimilar laser welding between CoCrFeMnNi high-entropy alloy (HEA) and Hc340LA high-strength low-alloy steel (HSLA), with a focus on analyzing the microstructure, mechanical properties, and room/low-temperature deformation mechanisms of the welded joints (WJ). The WJ were defect-free, and the fusion zone (FZ) was primarily composed of a single FCC structure with no intermetallic compounds (IMCs). At room temperature, the tensile strength of the WJ was comparable to that of HSLA and the fracture did not occur in the FZ, benefiting from activation of the {111} < 110 > slip system. As the temperature decreased, the thermal activation and stacking fault energy (SFE) of alloy decreased, resulting in the generation of dislocation tangles (200 K), dislocation walls and dislocation cells (123 K) and nano twins (77 K) during deformation. The low temperature also raised the critical shear stress (CRSS) for intiating dislocation or twin, leading to yield strength increased. Ultimately, a high yield strength of 748 MPa was achieved at 77 K, while maintaining excellent work hardening ability and uniform elongation. These outstanding comprehensive mechanical properties were attributed to the dynamic Hall-Petch effect induced by the formation of nanoscale twins. The appearance of cross twinning and adaptive deformation via kinking also played a notable role. This study provided insights into the dissimilar welding of HEA and HSLA, demonstrating the potential of this welded combination for applications requiring high strength and low-temperature resistance.