Recrystallization induced by heat treatment regulates the anisotropic behavior of CoCrMo alloys fabricated by laser powder bed fusion

Abstract

The CoCrMo alloys fabricated by the laser powder bed fusion (LPBF) exhibit significant anisotropy due to the characteristics of layer-by-layer manufacturing. This study investigated the microstructural evolution and mechanical properties of CoCrMo alloys in both as-printed and heat-treated states. The results demonstrated that the elongation was 19.1% in the tensile direction parallel to the building direction, compared to only 9.3% in the perpendicular direction, showing a difference of over 100%. After solution heat treatment at 1150 °C for 1 h followed by annealing heat treatment at 450 °C for 0.5 h, the ultimate tensile strength (UTS) and elongation nearly equalized, reaching 906.1 MPa, 20.2% and 879.2 MPa, 17.9%, respectively. Further characterization indicated that the anisotropy was mainly caused by grain morphology. Solution treatment-induced recrystallization refined coarse columnar grains into equiaxed grains that accommodated orientation-related stresses, which tentatively achieves microstructural and mechanical homogeneity. Subsequent low-temperature annealing broke the trade-off between the strength and ductility while further promoting the isotropy. At this stage, the mechanical properties were strengthened by the synergistic interaction of nanoscale annealing twins and martensitic laths. This study provided valuable insights for optimizing the isotropic behavior in LPBF-fabricated CoCrMo alloys.