Microstructural Refinement of Additive Manufactured Ti-6Al-4V By Thermohydrogen Processing

Additive manufacturing (AM) has the potential to significantly reduce buy-to-fly ratios and cost in manufacturing of airframe components. Metallic alloy (including Ti) parts built by melting AM methods, however, tend to form large columnar microstructure along the build direction, which causes anisotropic mechanical properties and lower fatigue strength. Refining the microstructure and thus improving the mechanical properties of net and near-net shape AM Ti alloys faces significant challenges. Thermal-mechanical processing (TMP), which works well for parts made by the conventional processes, is not applicable because of dimensional constrains. That leaves very few options. Thermohydrogen processing (THP) uses hydrogen as a temporary alloying element to refine microstructure and improve mechanical properties without applying metalworking. THP uses the reversible hydrogenation process in Ti to achieve microstructural refinement. The process has the potential to create a new capability of processing net and near-net shape AM components. In this work, THP of powder bed electron beam melted AM Ti-6Al-4V was studied. Ti-6Al-4V samples were first hydrogenated to various hydrogen contents at elevated temperatures, and subsequently annealed and dehydrogenated in vacuum. The hydrogen contents of the dehydrogenated samples were less than 40 wppm. Microstructure at different THP stages was examined by using optical and scanning electron microscope. Results showed that THP gave rise to significant microstructural refinement in the AM Ti-6Al-4V. Phases in the materials were studied by X-ray diffraction (XRD) analysis. Mechanical test results will be presented.