An integrated investigation of the effect of sub-transus treatment on the microstructure and corrosion behaviour of the LPBF Ti–6Al–4V alloy

Abstract

Biomedical grade Ti–6Al–4V alloy, is the best candidate material for bioimplants due to its outstanding corrosion resistance. Additive manufacturing processes such as laser-based powder bed fusion (LPBF) are trending technologies that challenge conventional manufacturing methods in producing customized implants. However, the Ti–6Al–4V alloy fabricated by LPBF consists of a non-equilibrium phase (α′-martensite), that tends to degrade the corrosion resistance, and therefore a post-treatment is required to stabilize the phases. In this work, the annealing is done in the α+β range of the Ti–6Al–4V alloy. The corrosion performances of the as-built (AB) and annealed samples in 0.9 wt% NaCl solution are investigated through electrochemical measurements. The relationship between the microstructure and its corresponding corrosion behaviour is elucidated. The rate of corrosion increased by around 12 % following low temperature annealing, but decreased by more than 50 % following high temperature annealing above 850 ᵒC. Overall, the AB structure shows an inferior corrosion behaviour compared to the annealed ones. The electrochemical results show that post-annealing has a positive effect on corrosion performance that can be ascribed to various factors such as phase fraction, chemical composition, morphological features of microstructural and presence of defects. The findings show that a microstructure with increased β-phase fraction, a fine lamellar structure with low defect density, and a comparatively coarser grain size is beneficial in improving corrosion resistance. This can be achieved by annealing the Ti–6Al–4V alloy at 850 ᵒC or higher.