Enhanced corrosion fatigue strength of additively manufactured graded porous scaffold-coated Ti-6Al-7Nb alloy

Abstract

Current modifications of Ti-based materials with porous scaffolds for achieving biological fixation often decrease corrosion fatigue strength (σcf) of the resultant implants, thereby shortening their service lifespan. To resolve this issue, in the present, a step-wise graded porous Ti-6Al-7Nb scaffold was additively manufactured on optimally surface mechanical attrition treated (SMATed) Ti-6Al-7Nb (specifically denoted as S-Ti6Al7Nb) using laser powder bed fusion (PBF) technology. The microstructure, bond strength, residual stress distribution, and corrosion fatigue behavior of porous scaffolds modified S-Ti6Al7Nb were investigated and compared with those of mechanically polished Ti-6Al-7Nb (P-Ti6Al7Nb), S-Ti6Al7Nb, and porous scaffolds modified P-Ti6Al7Nb. Results showed that corrosion fatigue of porous scaffolds modified Ti-6Al-7Nb was propagation controlled. Moreover, the crack propagation behavior in the PBF scaffold's fusion zone (FZ) and heat-affected zone (HAZ), exhibiting insensitivity to the microstructural configurations characterized by columnar prior-β grain (PBG) boundaries and acicular α′ martensites, coupled with the PBF-induced residual tensile stresses in these regions, resulted in a considerable decrease in σcf for porous scaffolds modified P-Ti6Al7Nb compared to P-Ti6Al7Nb. In contrast, step-wise graded porous scaffold-modified S-Ti6Al7Nb demonstrated an improved σcf which was even higher than that of P-Ti6Al7Nb. Such an advancement in corrosion fatigue strength is primarily attributed to the presence of residual compressive stresses within the underlying S-Ti6Al7Nb substrate, extending beyond FZ and HAZ. These stresses increased the crack propagation threshold, leading to crack deflection/branching and increased crack-path tortuosity, thereby synergistically markedly enhancing the crack propagation resistance of porous scaffolds modified S-Ti6Al7Nb.