Researchers from South China University of Technology and Beijing Tsinghua Changgung Hospital analyzed the effects of heat treatment on laser powder bed fusion (LPBF)-fabricated CoCrMo alloys, focusing on microstructural changes, mechanical anisotropy, and residual stress reduction. The study examined how solution treatment at 1150°C for one hour followed by annealing at 450°C for 0.5 hours altered grain morphology, tensile strength, and elongation properties.
CoCrMo alloys are widely used in orthopedic implants due to their corrosion resistance, wear resistance, and mechanical durability. When fabricated using LPBF, the layer-by-layer solidification process produces columnar grain structures aligned along the build direction, resulting in significant differences in mechanical properties depending on orientation. The research quantified these variations and assessed how heat treatment modifies the material’s structure to address mechanical disparities.
Mechanical testing showed that as-printed CoCrMo samples exhibited 19.1% elongation along the build direction and only 9.3% perpendicular to it, a difference exceeding 100%. Ultimate tensile strength (UTS) was also higher along the build axis, measuring 1173.7 MPa compared to 1048.6 MPa in the perpendicular direction.
Columnar grains, typically aligned in the <001> direction, restricted dislocation movement across grain boundaries, resulting in lower ductility perpendicular to the build axis. High dislocation densities at melt pool boundaries further influenced stress distribution, contributing to premature failure in the weaker orientation.
Recrystallization Reconfigures Grain Morphology
Solution heat treatment at 1150°C for one hour induced full recrystallization, transforming columnar grains into equiaxed grains. Electron backscatter diffraction (EBSD) analysis measured a 94.6% recrystallization ratio, confirming that the high-aspect-ratio grain structure observed in as-printed samples was eliminated.
Tensile testing of heat-treated samples showed that UTS values equalized across orientations, measuring 906.1 MPa and 879.2 MPa, while elongation reached 20.2% and 17.9%, respectively. The grain morphology shift reduced anisotropic effects, improving uniformity in mechanical performance.
Following solution treatment, annealing at 450°C for 0.5 hours introduced additional microstructural changes. TEM imaging identified nanoscale martensitic laths and annealing twins in the heat-treated samples. Kernel average misorientation (KAM) mapping confirmed that residual stress decreased significantly after treatment.
Grain size analysis found that heat-treated samples retained slight anisotropy in grain diameter but eliminated elongation differences caused by grain morphology. X-ray diffraction (XRD) measurements showed that γ-FCC and ε-HCP phases remained stable following heat treatment, with no phase transformation detected.
Residual Strength Reduction and Further Research Considerations
Although heat treatment eliminated anisotropic mechanical behavior, post-treatment samples exhibited reduced strength compared to as-printed CoCrMo alloys. Solution treatment removed columnar grain structures, while annealing introduced new strengthening mechanisms through precipitate formation and twin boundaries.
Further research is required to evaluate how precipitate formation at grain boundaries influences long-term fracture behavior. The study did not examine wear resistance or fatigue properties, leaving open questions about how treated CoCrMo alloys perform under cyclic loads.
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