Significance of hot isostatic pressing (HIP) on multiaxial deformation and fatigue behaviors of additive manufactured Ti-6Al-4V including build orientation and surface roughness effects

Additive manufacturing (AM) technology has gained significant attention in recent years due to several important advantages. However, design of critical load carrying parts using this technique is still at its infancy, partly due to the inferior performance and lack of sufficient understanding of cyclic deformation and fatigue behaviors of AM metals as compared to their wrought counterparts. Similar to most other components in different industries, AM parts typically undergo cyclic loadings through their service life; therefore, fatigue performance is a key performance criterion. In addition, due to common presence of multiaxial stress states at fatigue critical locations, multiaxial fatigue is of special interest. In this study, thin-walled tubular specimens made of Ti-6Al-4V alloy fabricated with powder bed fusion (PBF) process were used to investigate their cyclic deformation and fatigue behaviors. The main focus of this study was on the effect of hot isostatic pressing (HIP) on fatigue performance under uniaxial, torsion, combined in-phase, and 90° out-of-phase axial-torsion loads. To study build orientation effect, both vertically built and diagonally built (at 45°) specimens were included. However, no substantial difference on the fatigue behavior of vertical and 45° built specimens was observed. Although the majority of specimens had a machined surface, some torsional tests with as-built surface were also included to evaluate surface roughness effect. It was found that even after HIP treatment, the rough surface is the dominant factor in shortening the fatigue life. In addition, to evaluate the effectiveness of the HIP treatment, torsion fatigue performance of the HIPed specimens were compared to torsion fatigue performance of annealed AM specimens, as well as to the multiaxial fatigue behavior of the conventional wrought material. Fatigue failure mechanism of the HIPed material was shear, similar to the wrought material, and fatigue performance was found to be similar to the wrought alloy. Fatigue test results were correlated well using a shear-based critical plane parameter approach, also similar to the wrought material. Comparison of multiaxial fatigue data in this study and a similar previous study by the authors indicates that fabrication machine-to-machine and the associated build parameter differences can result in substantial differences in the observed defect size and population, resulting in different failure mechanisms and subsequently different fatigue performance.