Effect of heat treatment on the corrosion behaviour of SLM fabricated CoCrMo alloys in simulated body fluid.

CoCrMo alloys are widely used as biomaterials to replace articular joints injured by trauma or degenerative disease due to their high corrosion resistance, good mechanical properties, and biocompatibility. In spite of this, several failures and discomfort in implanted patients are still reported. The main cause of these failures is related to the limited availability in size and shape of traditional prostheses which causes a mismatch between implanted prosthesis and bone. Selective Laser Melting (SLM) is an additive technique that can overcome these issues allowing the production of fully customizable prostheses with complex shapes, so improving osteointegration and bone adaptation. The high-temperature gradients developed during laser processing induce a non-equilibrium microstructure and residual stresses on the final product. Our previous researches showed that as-built alloys offer very good corrosion resistance in simulated body fluids, even when inflammatory conditions are established. Post-processing heat treatments are often used to change microstructure, with the aim of improving mechanical properties, applying stress relief or removing crystal defects. However, still, quite limited information is available about the effects of post-process heat treatments on the corrosion behaviour of SLM CoCrMo alloys. This research aimed at studying the effect of heat treatment on the corrosion behavior of SLM CoCrMo alloy obtained from ASTM F1527 powders, during exposure to simulated inflammatory conditions. Two different sets of SLM process parameters were used to prepare the samples and then a heat treatment originally designed for cast ASTM F75 CoCrMo alloy was applied to post-process the as-built materials. Microstructural, mechanical and electrochemical properties of heat-treated SLM CoCrMo samples were investigated and benchmarked against both as-built SLM and the cast (ISO 5832) CoCrMo alloys. The microstructure of the studied samples was characterized by using OM, SEM/EDS, and X-ray diffraction. Mechanical tests consisted of HRC hardness measurements. Moreover, the alloy corrosion resistance was monitored during 15 days of immersion in Phosphate-Buffered Saline (PBS) solutions at pH 4+30 mM H2O2 by Electrochemical Impedance Spectroscopy. Polarization curves were also recorded at both short immersion time and at the end of the 15 days of immersion. Evaluation of the microstructure and hardness revealed that the heat-treated specimens had significantly higher hardness than the as-built specimens, which was attributed to the occurrence of recrystallization and the disappearance of molten pool boundaries during heat treatment. The electrochemical tests showed that the heat-treated SLM CoCrMo alloy exhibited a higher corrosion resistance than the untreated alloy. Furthermore, depending on the adopted SLM process parameters, the corresponding heat-treated alloy reaches a corrosion resistance similar or higher than that offered by the traditional one.