Speed Function Effects in Electron Beam Additive Manufacturing

In this paper, the process parameter effects on the thermal characteristics in powder-bed electron beam additive manufacturing (EBAM) using Ti-6Al-4V powder were investigated. The machine-specific setting, called "speed function" (SF) index that controls the beam speed and the beam current during a build, was utilized to evaluate the beam scanning speed effects. EBAM parts were fabricated using different levels of SF index (20 to 65) and build surface morphology and part microstructures were examined. A near infrared (NIR) thermal imager was used for temperatures measurements during the EBAM process. In addition, a thermal model previously developed was employed for temperature predictions and comparison with the experimental results. The major results are summarized as follows. The SF index noticeably affects the thermal characteristics in EBAM, e.g., a melt pool length of 1.72 mm vs. 1.26 mm for SF20 and SF65, respectively, at the 24.43 mm build height. This is because the higher the speed function index, the higher the beam speed, which reduces the energy density input and results in a lower process temperature. For the surface conditions and part microstructures, in general, a higher SF index tends to produce parts of rougher surfaces with more residual porosity features and large beta grain columnar widths.