Multiscale modelling of additively manufactured Ti-6Al-4V alloy: Fatigue performance evaluation from material to structural level

Additively manufactured (AM) Ti-6Al-4V alloys face limitations in critical applications due to uncertainties in their fatigue performance. Despite extensive research on fatigue life prediction, the discrepancies in fatigue behaviour between AM structural components and material-level specimens remain poorly understood. In this study, the effect of hot isostatic pressing (HIP at 920 °C, 1000 bar Ar, 2 h) on the fatigue properties at both material and structural levels was investigated, aiming to bridge the gap in the cross-scale fatigue behaviour of AM Ti-6Al-4V alloys. Although HIP significantly improved microstructure and fatigue performance of material-level specimens, the presence of larger near-surface defects remained in the structural components, resulting in a poorer fatigue resistance. To quantitatively evaluate the impact of near-surface defects, a novel multiscale model for predicting fatigue behaviour of AM Ti-6Al-4V alloys was proposed. The model integrates finite element analysis with microstructural characteristics, where the grain boundaries (GBs) effect was quantified by the distance between GBs and the misorientations between adjacent grains. The input of this model requires only test conditions, tensile properties and microstructural information of the materials. The model was validated using S-N data from axial fatigue tests at both the material and structural levels, showing excellent correlation between predicted and experimental results. Overall, this study enhances the understanding of the relationship between processing techniques, microstructure and fatigue performance in AM alloys. Furthermore, it provides a multiscale analysis framework that integrates microstructural, material and structural information, offering a basis for the accurate prediction and design of fatigue-resistant AM components.