Phase transformations drive microstructure control in titanium alloys for additive manufacturing

Abstract

Metal-based additive layer manufacturing (ALM) is resulting in a paradigm change that could revolutionize manufacturing across multiple industries such as the aerospace, biomedical and automotive sectors. However, limited alloys are compatible with ALM. Today, most of the alloys being used for this technique were developed for other manufacturing processes such as casting and forging processes where the mechanical properties can be controlled using thermal and thermo-mechanical processing, but not by the rapid solidification conditions of ALM (steep heating and cooling rates between ~ 103-108 °C/s). In that context, the activation of phase transformations in ALM alloys plays a key role to obtain microstructures with direct properties in the as-built state. This work uses time-resolved characterization to reveal the influence of phase transformations on microstructure formation and deformation mechanisms, which take place in Ti-alloys particularly developed for ALM. The phase evolution in these alloys, fabricated by the powder-bed ALM technique selective laser melting, is investigated using in situ synchrotron tomography and in situ high energy synchrotron X-ray diffraction. They are developed to tackle the main drawbacks occurring in Ti-alloys fabricated by ALM, namely the formation of microstructures with anisotropy and poor ductility.