Assessment of intra-build variations in tensile strength in electron beam powder-bed fusion Ti–6AL–4V part 1: Effects of build height

Abstract

In this work, rectangular blocks of electron beam powder-bed fusion (PBF-EB) additively manufactured (AM) Ti–6Al–4V were built, such that a total of 68 mini-tensile test coupons could be extracted for mechanical testing over a range of build height and build plate locations. These as-built tensile test coupons were uniaxially tested to fracture and subsequently studied via scanning electron microscopy, micro X-ray computed tomography, and inert gas fusion chemical analysis to systematically study the intra-build variations in tensile properties, microstructure, porosity, and chemistry. A strong correlation was observed between YS and build height, where YS was observed to decrease with increasing build height, up to 35 mm. There were no significant contributions to this variation in yield strength (YS) from build location, porosity, grain size, or crystallographic texture. Inert gas fusion oxygen measurements reveal the same trend, suggesting that oxygen content variation as a function of build height is contributing to the observed YS variation. The source of the oxygen content variation is thought to be a transient source of oxygen, such as moisture in the powder and build chamber. The establishment of this oxygen variation and its correlation to tensile properties motivates and facilitates improvements to PBF-EB Ti–6Al–4V processing, including powder mixing and handling.