Effects of process parameters on microstructure and mechanical properties of Al0.5CoCrFeNi high entropy alloy thin sheets using pinless friction stir welding

The present study investigates the effects of welding procedure parameters on microstructure and mechanical properties through pinless friction stir welding (PFSW) for thin sheets of the Al0.5CoCrFeNi high entropy alloy (HEA). Results demonstrate that the joints comprise four different regions, that is, stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM). The heat index (HI) is shown as an important factor affecting the microstructure and mechanical properties of the joints. It influences recrystallization, the development of second phases, and even the weld tool's wear. The HI correlates significantly with welding process parameters, affecting the Al–Ni phase volume fraction (the B2 precipitate). The optimum amount of HI almost occurs at the welding speeds of 10 mm min−1, producing a high degree of B2 precipitates and leading to fine grains in the HAZ. EDS analysis shows that the high level of Cr in the SZ and TMAZ is ascribed to the emerging Cr-rich M23C6 carbides. At the optimum welding conditions and three specific temperatures of 25, 300, 500 °C, the UTS and the ductility of the best joint reaches 423 MPa and 20%, 495 MPa and 21%, and 646 MPa and 21%, respectively, comparable to those of the base metal (605 MPa and 28%, 642 MPa and 19%, and 702 MPa and 30%, respectively). Even at high temperatures, the higher tensile strength might be ascribed to the formation of extremely thermally stable B2 precipitates, resulting in small grains. Hence, the pinless FSW of the Al0.5CoCrFeNi is appropriate for high-temperature applications.