The effect of beam scan strategies on microstructural variations in Ti-6Al-4V fabricated by electron beam powder bed fusion

Abstract

In electron beam melting (EBM) powder bed fusion (PBF), variations in electron beam scan strategies can be used to control thermal transients in the additive manufacturing process, both in the melt pool and in previously deposited layers. In this study, three different EBM beam scan strategies, i.e. the standard raster scan, ordered spot scan, and random spot scan patterns, were used to fabricate three identical Ti-6Al-4V blocks. Using scanning electron microscopy and electron backscatter diffraction, variations in microstructure and crystallographic texture, such as α lath thickness, prior beta β grain size and orientation, α/α lath boundary (LB) distributions, are investigated with respect to the applied scan strategy. Both spot scan strategies result in coarser α laths and smaller prior β grains with width and height < 1/3 of the value of the typical large columnar grains, observed in raster scan samples. The combined fraction of type 2 and type 4 α/α LBs measured in the three samples was found to be between 0.50 and 0.85, which is greater than the expected combined fraction of ~0.36 for a random distribution of α variants. This suggests the presence of a mild to weak variant selection in EBM Ti64.