Ebrahim Asadi

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.

Laser shock peening (LSP) uses plasma shock waves to induce compressive residual stress at the surface of a component which has the potential to improve its fatigue properties. For AM parts, the existence of internal defects, surface roughness, and tensile residual stresses leads to noticeably lower fatigue strength compared to materials produced through conventional processes. Furthermore, there is a tendency for greater scatter in the fatigue behavior of these parts when compared to traditionally manufactured components. In this study, the effect of LSP on the roughness and fatigue behavior of Ti-6Al-4V alloy constructed through Laser Powder Bed Fusion (L-PBF) technique was investigated. Two types of samples were designed and tested: as-built surface air foil samples for four-point bending tests and machined surface straight gage samples for uniaxial fatigue testing. Two sets of process parameters, optimized and non-optimized, were also used for the fabrication of each sample type. It was found that LSP had negative effects on the smooth (i.e., machined) surface samples, whereas for as-built surfaces the roughness was enhanced by decreasing the sharpness of the deep valleys and partially remelting the loosely bonded particles on the peaks. It was found that the scatter of the fatigue data decreased for optimized machined samples, while no clear improvement was observed in their lives. However, all non-optimized samples showed improvements in fatigue lives after the LSP process.