Effect of heat treatment on residual stress in Selective Laser Melted CoCrMo

Abstract

Selective Laser Melting (SLM) of Cobalt Chrome Molybdenum (CoCrMo) alloyspresents major advantages for producing high-performance biomedical and aerospace components. However, the process induces significant residual stresses due to large thermal gradient development during the layer-by-layer manufacturing method. Residual stresses are important to mitigate as they can compromise dimensional accuracy, accelerate crack formation, and decrease component reliability. This study aims to evaluate the effect of stress-relief heat treatments on the reduction of residual stresses in SLM-fabricated CoCrMo alloys. Cantilever and cubic specimens were manufactured to evaluate the effect heat treatment has on residual stresses through the cantilever deflections (inherent strain method), XRD and microstructural analysis. Numerical simulations were implemented to predict the effects of 700°C,770°C, and 1065°C heat treatments. Experimental results showed substantial reductions in cantilever deflection, with stress relief of 54% at 700°C and 82% at 770°C. Heat treatment at 1065°C led to stress reversal, indicating significant redistribution of residual stress as inferred from cantilever deflection. Simulations highlighted the trend of decreasing residual stress with an increase in temperature, although they underestimated the magnitude of experimental stress relief, particularly at higher temperature, suggesting unmodelled phase-transformation effects. Microstructural analysis revealed microstructure development from elongated columnar grains in the as-built condition to partial recrystallised structures at 770°C and mixed elongated and equiaxed microstructures at 1065°C. XRD confirmed the presence of ɣ-phase and ε-phase across all conditions, with the ɣ-phase being the most dominant. The XRD also showed that heat treatment caused shifts and broadening in peaks, intensity changes, and the appearance of an additional peak when heat treatments were applied.The combined simulations and experiments show that heat treatment is an effective approach for residual stress reduction in SLM CoCrMo alloys. The study also found that calibrated finite element simulations can serve as a valuable predictive tool for optimising heat treatment strategies and minimising trial-and-error experiments in SLM. It should be noted that residual stresses were inferred indirectly from cantilever deflection measurements using the inherent strain method, and that the investigation was limited to selected stress-relief temperatures for a single SLM CoCrMo material system. Within these constraints, a heat temperature of 770°C provides a desirable balance between stress relief and dimensional stability.