Thermo-hydrogen refinement of microstructure to improve mechanical properties of Ti–6Al–4V fabricated via laser powder bed fusion

This paper describes the main results from an investigation into the consequences of thermo-hydrogen refinement of microstructure (THRM) after laser powder bed fusion (LPBF) of Ti–6Al–4V on the evolution of microstructure and mechanical properties using a set of experimental techniques. Porosity fraction, grain structure, phases, and crystallographic texture per phase are characterized using micro X-ray computed tomography, microscopy, and neutron diffraction. A hierarchical structure of acicular α-phase morphology formed inside the prior β grains by fast cooling during LPBF transforms into fine-grained globular microstructure by THRM, which facilitates homogeneous nucleation and growth of the low temperature phase with some retained β phase. Moreover, hydrogenated material during THRM has low activation energy for diffusion, which in conjunction with the surface energy of pores causes densification of the material, thereby closing porosity formed during LPBF. Such significant microstructural changes induced by the THRM treatment cause brittle material created by LPBF to become ductile. Significantly, the strength and ductility produced by THRM exceed the minimums set forth by the ASTM B348 standard for Ti-6Al-4V. Moreover, the treatment improves fatigue strength of the material. In particular, it improves the endurance limit and reduces the scatter in the measured fatigue strength data. Performance characteristics of the material can be further optimized for specific application requirements by tailoring microstructures using the LPBF and THRM processes.