Effect of heat treatment on the anisotropic microstructural and mechanical properties of Co-Cr-Mo alloys produced by selective laser melting

The purpose of this study was to evaluate the effect of heat treatment on the anisotropy of the microstructure and mechanical properties of cobalt-chromium-molybdenum (Co-Cr-Mo) alloys fabricated by selective laser melting (SLM). Dumbbell samples were fabricated with the axes deviating from the build direction by 0 degrees (0 degrees-sample), 45 degrees (45 degrees-sample), or 90 degrees (90 degrees-sample) and were subjected to heat treatment at various temperatures (750, 900, 1050, or 1150 degrees C) for 6 h. In samples heat-treated at 750, 900, and 1050 degrees C, the microstructures exhibited columnar grains with a <001> fiber texture along the build direction, the same as in the as-built state. The mechanical properties showed anisotropy; the 0.2% offset yield strengths (YS) of the 0 degrees-samples were lower than those of the 90 degrees-samples, and the elongations of the 0 degrees-samples were significantly higher than those of the 45 degrees- and 90 degrees-samples. By contrast, in samples heated to 1150 degrees C for 6 h, the anisotropic columnar grains completely disappeared, and equiaxed grains with random orientations were found in all samples, indicating that recrystallization had occurred. Moreover, the specific microstructures and texture generated during SLM disappeared. Regarding tensile properties, the initially strong anisotropy exhibited by the as-SLM samples was significantly reduced. Thus, heat treatment at the recrystallization temperature produced uniform equiaxed grains with random texture, which contributed to reducing the mechanical anisotropy of the SLMed Co-Cr-Mo alloys.