Powder bed fusion with electron beam of Ti4822: assessing the benefits of contour on surface topography and part precision

The titanium aluminide alloy γ-TiAl Ti48Al2Cr2Nb (Ti4822) is, to a great extent, produced by powder bed fusion with electron beam melting (PBF-EB). Owing to the high working temperature and the vacuum environment, PBF-EB for Ti4822 maintains phase stability and minimises residual stresses. The standard melting procedure of PBF-EB considers the melt of the section in two steps: the contour of the area first and then the inner area through a hatching strategy. Despite this standard, Ti4822 is today processed only by melting the inner area of the sliced section through hatching. The reasons for such strategy or benefits in terms of, e.g. the component quality, are not clear. This paper covers this literature gap, showing the differences between a two-step strategy and a one-step using only hatching. Reference artefacts were produced using a PBF-EB machine and γ-TiAl Ti48Al2Cr2Nb alloy. The surface topography, dimensional accuracy and geometrical tolerances of the two strategies were observed and compared. The results provide preliminary guidelines for correcting and adjusting the geometry according to the PBF-EB capabilities. The contour in the two-step strategy significantly improves the surface roughness by modifying the topography of the vertical surfaces. On the other hand, the one-step strategy guarantees the production of more dimensionally accurate features for some size classes. The two-step strategy can better control the part dimensions but requires a longer time.