Effect of process parameters on microstructure and mechanical properties of friction stir welded CoCrFeNi high entropy alloy

In this study, CoCrFeNi high entropy alloy was successfully joined by friction stir welding (FSW) and the influences of welding process parameters on the microstructure and mechanical properties were studied. Three distinct zones, namely, the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and base metal (BM), have been identified. The SZ showed an equiaxed microstructure because of the recrystallization and the TMAZ exhibited a mixed microstructure with elongated and equiaxed grains. Under different process parameters, the morphology of the SZ was changed from basin-shaped to elliptical accompanied with the occurrence of banded structures and small tungsten-rich particles. The grain size in the SZ increased with the increasing rotation speeds of the welding tool with other parameters unaltered. The hardness of the SZ in all welded samples was higher than that of the base metal due to the dynamic recrystallization induced by the FSW process and the hardness followed the Hall–Petch relationship. At the optimum welding conditions, the UTS and the ductility of the joints reached 627 MPa and 42%, which were comparable to those of the base metal (628 MPa and 63%). The results were analyzed in terms of the heat input, which was shown to correlate well with the average grain size and hence the strength of the joints.