Towards the mechanism of in situ welding during electron beam powder bed fusion process

Electron beam powder bed fusion (EB-PBF) is an additive manufacturing process that can fabricate complex structural metallic parts in near net shape, with possibilities for a wide range of applications. However, there exists a problem in fabricating medium-to large-volume parts in which the scan-line length is too long. The fabricated component often suffers from degraded mechanical properties due to lack-of-fusion pores. In this work, we apply in situ welding to rectify this critical problem. Here we select the Inconel 718 (IN718), one of the most widely used nickel-based superalloys in aerospace as an exemplar alloy to demonstrate this technique. The results are compared with previous studies on Ti-6Al-4V, the workhorse titanium alloy. Results reveal that the overlap distance to eliminate defects for IN718 needs to be greater or equal to 0.70 mm, which is slightly larger than the minimum distance required for Ti-6Al-4V. Columnar-to-equiaxed transition is observed in the overlap regions of samples with 0.70 mm and 3.50 mm overlap distances. An increase in overlap distance leads to a reduction in equiaxed grains, promoting better consistency of the microstructure in EB-PBF IN718 between overlap and non-overlap regions. On the other hand, varying the overlap distance shows little influence on microhardness variation. The overlap regions of the two samples studied have similar microhardness, which are higher than those of the non-overlap regions. This renders the tensile deformation dominant in the non-overlap region and results in a similar tensile property for the studied samples. These findings provide a solid basis for applying in situ EB-PBF welding technology in the industry and suggest a new avenue to tailor the grain morphology during EB-PBF.