Microscale Insight into the Deformation and Fracture of Post-Processed Additively Manufactured Ti-6Al-4V

Abstract

Additive manufacturing of metallic materials has become exceedingly popular due to their potential to more efficiently fabricate near-net geometries than conventional manufacturing techniques. However, additive manufacturing of metallic materials produces unconventional microstructures that can lead to inferior mechanical performance. Specifically, powder-based additive manufacturing of Ti-6Al-4V leads to anisotropic microstructures along with defects such as lack of fusion and keyhole porosity. One way to partially mitigate these issues is through post-processing like hot isostatic pressing, which involves the combined effects of pressure and heat-treatment. During this post-processing, it is mainly the heat-treatment that affects the material microstructure. Following this, the goal of this dissertation is twofold: first, understand the implications of post-processing heat-treatments on the microstructure and resultant mechanical performance of Ti-6Al-4V specimens fabricated via electron beam melting (EBM) additive manufacturing technique, and second, implement novel post-processing heat-treatment techniques to obtain rather isotropic microstructures with enhanced mechanical performance. To this end, in-situ mechanical tests that capture a large area with microstructural-scale resolution and microstructure-based digital image correlations are carried out on Ti-6Al-4V specimens fabricated via EBM and subjected to both sub- and super-transus post-processing heat-treatments. The results show that in the as-fabricated material and sub-transus heat-treated materials, the deformation is rather homogeneous, while in the super-transus heat-treated materials, depending on the cooling rate, deformation either tends to localize along the grain boundaries or in the preferentially oriented colonies. Furthermore, in the presence of a structural discontinuity, in the as-fabricated and sub-transus heat-treated materials, cracks emanate from the notch tip; but in the super-transus heat-treated material, depending on the cooling rate, cracks either nucleate along the prior-β grain boundaries or within the prior-β grains at the colony-colony boundaries. Finally, a novel post-processing treatment involving the temporary alloying of hydrogen is carried out on as-fabricated EBM Ti-6Al-4V specimens to successfully eliminate the anisotropic microstructure while enhancing the mechanical performance.