Exploring heat treatment as an alternative to hot isostatic pressing in laser powder bed fused Ti-6Al-4V

Abstract This study aims to develop a heat-treatment protocol to replace hot isostatic pressing (HIP) for Ti-6Al-4V (Ti64) biomedical interbody structures fabricated via laser-powder bed fusion (PBF-LB/M) in their as-built surface condition. The PBF-LB/M components go through surface machining and HIP to minimize surface roughness and volumetric defects, resulting in enhanced mechanical properties. However, in the as-built surface condition, the benefits of HIP become insignificant and inconsistent. In this context, a tailored heat treatment protocol may offer similar benefits while being significantly more cost-effective. Therefore, the proposed heat treatment is designed to replicate the HIP microstructure and mechanical properties. The heat treatment schemes were developed by combining temperatures from either the sub-transus (925 ± 30 °C) or the super-transus (1050 ± 60 °C) regime, with predetermined holding durations designated as moderate (≤ 3 h) or extended (> 3 h), followed by furnace cooling. HIP was conducted at the temperature and pressure of 900 °C and 100 MPa, respectively, for 2 h, followed by furnace cooling. The heat-treated specimens were examined using optical microscopy, scanning electron microscopy, Vickers microhardness testing, and monotonic tensile testing, and the results were subsequently compared against the HIP specimen properties. The specimens exhibiting the closest resemblance to HIP were subjected to further characterizations, including electron backscatter diffraction and fatigue testing. Through gradual elimination based on the aforementioned characterizations, it was concluded that a heat treatment at 925 ± 30 °C for a moderate duration, followed by furnace cooling, provides an equivalent alternative to HIP as a post-processing method for PBF-LB/M, under as-built surface conditions.