Different post-processing methods to improve fatigue properties of additively built Ti-6Al-4V alloy

Abstract

The fatigue performance of the Ti-6Al-4V alloy built using the laser-powder bed fusion additive method was investigated in this study. Post-process methods such as heat-treatment (HT), machining (M), hot isostatic pressing (HIP), and different combinations thereof were explored to enhance the mechanical as well as the fatigue properties. Load-controlled uniaxial high-cycle fatigue tests were performed using a servo-hydraulic fatigue testing machine at room temperature. The machining (M) of the as-built samples increased its fatigue properties, irrespective of the HT and HIP conditions. The tensile strength decreased considerably after the HT, HIP, and HT + HIP treatment but with increasing ductility. On the other hand, the fatigue strength improved with the HT condition and was further enhanced by five times after the HT + HIP + M condition, in comparison to that of the as-built alloy. The X-ray computed tomography analysis revealed a drastic reduction in the percentage of internal defects after the HIP treatment, from 0.31% to 0.01%. The fractographic analysis revealed that the cracks were mostly initiated from the defects in the as-built and HT conditions while in the HIP condition, it was from the facets in the α-phase. The reasons for the improvement in fatigue properties after the different post-process treatments were attributed to the induced-microstructural changes, surface finish quality, relaxed or compressive residual stresses, and most importantly, the reduced fraction of the crack-initiating process-inherent defects, which are explained and argued in this article.