Effect of Heat Treatment on the Microstructural Evolution and Properties of 3D-Printed and Conventionally Produced Medical-Grade Ti6Al4V ELI Alloy

Medical-grade Ti6Al4V extra-low interstitial (ELI) alloy has widespread applications in the biomedical industry due to its excellent corrosion and wear resistance. Even though 3D printing offers geometry flexibility and rapid means of fabricating customized parts, 3D-printed parts are often plagued with defects including porosity, high residual stresses, and non-equilibrium structures. Thus, post-processing heat treatments may be required to optimize its properties for engineering applications. In this study, the effect of post-processing heat treatment on the microstructure, hardness, and wear properties of 3D-printed and conventionally produced medical-grade Ti6Al4V ELI alloy samples was investigated. In general, distinct α (alpha) lath and basket-weave lath structures with a high degree of orientation were observed within the microstructure of the as-printed samples. Heat treatment led to the growth of distinct continuous and discontinuous α-lath structures along prior β (beta) grain boundaries as well as basket-weave lath and the coalescence of V-shaped structures within the prior β-grains. The hardness of both the 3D-printed and conventionally produced samples increased after heat treatment (≥ 400 HV), regardless of the cooling rate and aging temperature. After being water quenched/aged, the 3D-printed samples at 500 °C had the highest hardness values owing to the presence of coarse V-shaped structures. Furthermore, the V-shaped structures were always harder than all other structures regardless of the heat treatment and manufacturing process used, indicating that these structures dictate the overall mechanical integrity of the material. X-ray diffraction and electron probe microanalysis indicated that the V-shaped structures are rich in aluminum and titanium content, which can form hcp α′ (AlTi3) intermetallic phases. The 3D-printed samples had higher wear resistance overall than the conventionally produced samples regardless of the heat treatment used. Aging at 500 °C led to a higher coefficient of friction after 3D printing owing to an increase in α-phases. Therefore, during heat treatment, the microstructure and properties of medical-grade Ti6Al4V ELI alloy are significantly affected by the starting microstructure, the rate of cooling below the β-transus, and aging temperature and time, regardless of the manufacturing process used.