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

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 ( E <sub>v</sub>) above the machine-default E <sub>v</sub> 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 E <sub>v</sub>, a full α ` decomposition at an E _v of 167 J/mm ³ and indications of prior-β grain columnar-to-equiaxed transition at higher E _v. Graded properties along the sample height, and an increase in oxygen and nitrogen pick-up was measured at high E _v , 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.