Phase transformation in additively manufactured Co-Cr-Mo alloy after solution and aging heat treatment

The unique structure and solute distribution of CoCrMo alloys produced using Laser Powder Bed Fusion (L-PBF) technique require custom heat-treating processes to achieve the targeted phase distribution and mechanical properties. The rise of multi-material additively manufactured structures also prompts further study at temperatures beyond standard aging and solution treatment conditions. In the present study, the phase transformation behavior, precipitate distribution, and gamma-epsilon phase boundary orientation relationships of as-printed and solution heat-treated CoCrMo samples after aging heat treatment at 940 degrees C were investigated. The results show a higher epsilon-phase fraction of 71.2% in directly aged samples compared to 49.2% in solution heat-treated samples. This discrepancy is attributed to band-like isothermal transformation, coherent grain face massive transformation, and recrystallization. Carbide nucleation sites shifted from disintegrated cellular structures in directly aged samples to grain boundaries and twin interfaces in solution-treated samples. The presence of transgranular epsilon-phase bands and adjacent grains with analogous orientations observed after direct aging were linked to the weak (110) // BD and (111) // BD texture after printing. Tensile tests demonstrated an increase in tensile strength from 1162 MPa to 1261 MPa after direct aging, while significant enhancement in ductility from 16.4% to 26.1% was attained after solution heat treatment. While direct aging is more suitable for applications requiring high strength and wear resistance, solution heat treatment is advantageous when a consistent morphology and stress-free isotropic structures are desired.

Automated summary

The unique structure and solute distribution of CoCrMo alloys produced using Laser Powder Bed Fusion technique require custom heat-treating processes to achieve the targeted phase distribution and mechanical properties. This study investigates the phase transformation behavior and precipitate distribution of CoCrMo samples after aging heat treatment. The results show differences in phase fraction and nucleation sites between directly aged and solution heat-treated samples.