Influence of the focus offset on the defects, microstructure, and mechanical properties of an Inconel 718 superalloy fabricated by electron beam additive manufacturing

The defects, microstructure, and mechanical properties of Inconel 718 superalloy fabricated by electron beam additive manufacturing with different focus offsets were investigated. The samples built with a low focus offset exhibited numerous micro-voids of 10 μm and a columnar grain microstructure with a strong [001] texture in the built direction (BD). Unmelted powder and macro-voids of approximately 200 μm were observed at higher focus offsets, and the columnar grain width decreased from 600 μm to 150 μm with increasing focus offset. γ' precipitates with a length of 10 nm developed parallel to the BD, while γ" precipitates with length of 4 nm formed perpendicular to the BD owing to the higher thermal conductivity. Samples built with a low focus offset exhibited intergranular fractures, while samples built with a high focus offset exhibited brittle fractures because of the unmelted powder and macro-voids. Samples built with focus offsets of 9 mA and 12 mA exhibited the highest tensile strengths of 1.3 GPa and 1.0 GPa at room temperature and high temperature, respectively. γ'/γ"/γ' and γ'/γ"/γ'/γ"/γ' sandwich structures on the columnar γ grains developed parallel to the BD in the as-built samples because the preheating process between consecutive layers increases the dwelling time in the temperature range facilitating the δ → γ" and δ → γ' phase transformation. The volume fractions and size of the γ' and γ" precipitates increase with the focus offset owing to the prolonged dwelling time; however, micro-voids formed and unmelted powder was present because of insufficient heat of fusion.