Microstructure transition and mechanical properties of friction stir processed CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion

Abstract

In this work, Friction stir processing (FSP) was conducted on the CoCrFeMnNi alloy fabricated via laser powder bed fusion (LPBF) to modify the surface microstructure, during which the evolution of grain structures, dislocation structures, and texture as well as the dynamic recrystallization (DRX) mechanism were investigated. The experimental results demonstrated that continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) occurred simultaneously to refine the average grain size from 15.4 μm to 4.8 μm, while the distribution of grain size and area became more uniform in the stir zone (SZ). Due to the inhomogeneous distribution of material flow and friction heat, the thermo-mechanically affected zone (TMAZ) exhibits microstructure gradients and the grain structures are nonuniform throughout the SZ. In addition, the relationship between the microstructure and mechanical properties of the FSP region was analyzed. The hardness and yield strength (YS) of the SZ (∼190 ± 3 HV and 501 ± 2 MPa, respectively) are lower than those of the LPBF material (∼229 ± 10 HV and 563 ± 3 MPa, respectively). On the other hand, the elongation increased from 23.8 ± 0.4% to 37.5 ± 0.3%, while the ultimate tensile strength (UTS) increased from 665 ± 5 MPa to 719 ± 3 MPa. A better combination of strength and ductility than the LPBF-built CoCrFeMnNi alloy was achieved in the SZ after FSP.