Effect of Initial Grain Size on Friction Stir Weldability for Rolled and Cast CoCrFeMnNi High-Entropy Alloys

This study investigated the influence of the initial grain size on the plastic deformation and tunnel defects that occurred from friction stir welding of CoCrFeMnNi high entropy alloys (HEAs). The rolled and cast HEAs had a grain size of 2.8 and 308 μm, respectively. After friction stir welding, the cast HEA weld had a grain size of 1.8 μm, which was coarser than that of the rolled HEA weld (1.4 μm). Therefore, the dynamic recrystallization ratios were 60.7 and 99.6% for the rolled and cast HEAs, respectively. The cast HEA weld with a large grain size contained a higher density of high-angle boundaries and twins than the rolled HEA weld with the small grain size. The cast HEA had a larger resistance to plastic deformation owing to the larger fraction of ∑3 twin boundaries than the rolled HEA during friction stirring. This was associated with the high strain hardening rate during tensile testing and to the significant amount of W dissolved from the stirring tool in the cast HEA weld. Thus, the cast HEA weld had a higher tunnel defects ratio than the rolled HEA weld. The total unbonded ratios of the rolled and cast HEA welds were 0.2 and 7.2%, respectively, indicating that the rolled HEA had better weldability than the cast HEA.