Functional performance evaluation of laser powder bed fusion fabricated and sub-transus annealed Ti-6Al-7Nb alloy for acetabular implants

Abstract

Additive manufacturing specifically laser powder bed fusion (LPBF) also termed selective laser melting (SLM) has revolutionized the implant production landscape and is a pivotal research topic as it offers the perfect platform for producing complex, personalized implants with precise dimensions. Ti-6Al-4V extra low interstitial (ELI), designated as Grade 23 titanium alloy, is extensively utilized in acetabular implants. However, its corrosion resistance and biocompatibility are compromised by the cytotoxic nature of vanadium, one of its alloying elements. To address these limitations, Ti-6Al-7Nb, a vanadium-free titanium alloy, was developed as a viable alternative. The reduced service life of acetabular implants makes it critical that new biomaterials are explored and microstructural properties are designed to mitigate these failure mechanisms. Despite the potential of Ti-6Al-7Nb, limited studies have investigated how microstructural design through heat treatment of LPBF-fabricated Ti-6Al-7Nb influences its microhardness, wear performance, corrosion resistance, biocompatibility, and compressive behaviour in the context of acetabular implant applications. Thus, this study comprehensively investigated the impact of laser powder bed fusion process and annealing heat treatment (stress-relief annealing at 650°C with a holding temperature of 2 hours followed by air cooling, and full annealing at 850°C with 2 hours holding temperature followed by air cooling) of Ti-6Al-7Nb alloy on the microstructure, compressive behavior, microhardness, wear resistance, corrosion resistance, and invitro apatite-forming capability. The microstructure of as-built LPBF Ti-6Al-7Nb consisted of columnar prior β grains, characterized by fine acicular αʹ martensite and an intricate basket-weave morphology leading to high compressive yield strength (1230 MPa) and ultimate compressive strength (1634 MPa), enhanced microhardness (461±19.69 HV0.5), lower wear rate (1.33×10−3𝑚𝑚3𝑁.𝑚), and higher corrosion rate (2.285×10−4𝑚𝑚𝑦𝑟) in simulated body fluid. Stress-relief annealing initiated the precipitation of the β phase while full annealing completely transformed the αʹ martensite to a lamellar α+β microstructure. The lamellar microstructure exhibited a reduced compressive yield strength (973 MPa) and ultimate compressive strength (1183 MPa) critical for minimizing the stress shielding in implants, 19.7% reduction in microhardness (370±4.99 HV0.5), and the lowest corrosion rate in simulated body fluid (1.885×10−4𝑚𝑚𝑦𝑟). Furthermore, during the 14-day immersion period in simulated body fluid, the fully annealed specimens displayed faster apatite nucleation and proliferation, demonstrating superior biocompatibility and bone tissue integration than the as-built and stress-relief annealed specimens. This study provides new xxiv insight, for acetabular implant manufacturers, on how laser powder bed fusion process and annealing heat treatment can be adopted to design microstructural properties possessing superior biocompatibility, excellent tribological performance and corrosion resistance, as well as good compressive biomechanical properties for improved service life of the implant.