Effects of scanning parameters on material deposition during Electron Beam Selective Melting of Ti-6Al-4V powder

Electron Beam Selective Melting (EBSM) is an additive manufacturing technique that directly fabricates three dimensional parts in a layerwise fashion by using an electron beam to scan and melt the metal powder. In this study, the scanning parameters including beam current, scanning velocity and scanning line length were varied in a wide range of 2-18 mA, 250-2000 mm/s and 2-50 mm, respectively. The built samples of Ti-6Al-4V were characterized regarding the upper surface appearance, macro and microstructures and composition change. It was found that the built samples can be classified into three types: (I) porous surface with internal cavities; (II) dense and flat surface with pores at edge; and (III) significantly wavy surface. An increase in beam current, a decrease in scanning velocity or a decrease in the scanning line length, led to an evolution from type I to type II and finally to type III. The effects of beam current and scanning velocity can be evaluated by a combined parameter: energy density. The scanning frequency also has a significant impact on the extent of heat concentration, and thus affects the material deposition. In the samples of type II and III, the α′-martensite within the top region proves that the primary β phase firstly transforms into α′-martensite and then decomposes into α/β phase in continued building cycle. The causes of defects during the EBSM were also discussed.