Laser powder bed fusion produced Ti-6Al-4V: Influence of high-energy process parameters on in-situ martensite decomposition and prior beta grain texture

Abstract

Since Ti-6Al-4V components fabricated by laser powder bed fusion achieve an undesired martensitic microstructure (α'), high residual stresses and columnar prior-β grains, post-process heat treating has become an essential operation to improve these properties. However, post-process heat treating could be considered an undesirable additional step. Thus, this study investigated the viability of in-situ heating by experimentally establishing the effect of in-situ heating on part properties. This was accomplished by incrementally increasing the laser-induced energy density (Ev) above the machine-default Ev while maintaining high part density> 99%. Microstructural changes were monitored using optical and scanning electron microscopy and quantified using X-ray and electron-backscatter diffraction analysis. The study found a decrease in α grain size with an increase in Ev, a full α' decomposition at an Ev of 167 J/mm3 and indications of prior-β grain columnar-to-equiaxed transition at higher Ev. Graded properties along the sample height, and an increase in oxygen and nitrogen pick-up was measured at high Ev, causing a decrease in material ductility. A discussion of the thermodynamics of the process parameters demonstrated the mechanisms by which a reduced laser hatch distance controls powder bed fusion texture. Process variables of interaction time and exposure rate are proposed to better quantify the interactions between process parameters and thermodynamics.