A novel heat treatment of the additively manufactured Co28Cr6Mo biomedical alloy and its effects on hardness, microstructure and sliding wear behavior

Co28Cr6Mo alloy (ASTM F75 and F1537) is one of the standard biomaterials for permanent orthopedic implants, utilized especially in case of joint replacement, such as knee and ankle prostheses. At the present, innovative Additive Manufacturing (AM) technologies, such as laser-based powder bed fusion (LPBF), also known as selective laser melting (SLM), enable the production of customized medical devices with improved mechanical properties. When dealing with implants for joint replacement, wear resistance is critical and, unlike compressive and tensile properties, the knowledge on wear behavior of the LPBF Co28Cr6Mo alloy is currently limited. Furthermore, the effect of post-process heat treatment on tribological properties, that have to be customized on the peculiar microstructure induced by LPBF, needs to be assessed. In this view, the present work first focuses on a novel direct aging treatment of the LPBF Co28Cr6Mo alloy, performed in the range 600-900 degrees C up to 180 min, and investigates the effects on hardness and microstructural features, with the optimized heat-treated condition found in case of 850 degrees C for 180 min aging treatment. Then, the attention is driven to the dry sliding wear behavior of as-built and heat-treated LPBF Co28Cr6Mo alloy, considering the conventional wrought alloy as benchmark. For testing conditions closer to the in-service ones, the as-built LPBF alloy showed a wear resistance higher than the conventional wrought alloy. The optimized aging treatment significantly modified the as-built LPBF microstructure, it improved the alloy hardness and, in general, it positively affected its friction and wear behavior.