Effect of processing on microstructure, mechanical properties, corrosion and biocompatibility of additive manufacturing Ti-6Al-4V orthopaedic implants

This study evaluates the microstructure, porosity, mechanical properties and corrosion resistance of Ti-6Al-4V hip implant samples manufactured by Selective Laser Melting (SLM) and Electron Beam Melting (EBM), followed by different post-treatments (heat treatment and hot isostatic pressing -HIP-). Hydroxyapatite (HA) coatings were applied to enhance biocompatibility. Post-treated samples exhibited a lamellar α + β microstructure, with the EBM sample showing a coarser Widmanstätten structure, while acicular α' martensite was predominant in the as-built sample. The degree of porosity was lower than 0.2% in all samples. Microhardness was highest in the as-built sample (~ 400 HV) and lower in post-treated samples (~ 370 HV). Open Circuit Potential (OCP) tests indicated superior corrosion resistance for heat-treated samples, confirmed by Electrochemical Impedance Spectroscopy (EIS), where Sample E-HIP (HIP) showed a polarization resistance (Rpol) of 2490 kΩ/cm². In vitro tests confirmed that HA-coated samples exhibited excellent biocompatibility, slightly surpassing the Ti-6Al-4V samples. The HA-coated samples exhibited increased metabolic activity over 7 days, indicating superior biocompatibility. Finally, additive manufacturing combined with heat treatments and HA coatings effectively enhances Ti-6Al-4V for orthopaedic implants by improving the mechanical performance, corrosion resistance and biocompatibility.