Journal
JOURNAL OF ALLOYS AND COMPOUNDS
Volume 781, Issue -, Pages 842-856Publisher
ELSEVIER SCIENCE SA
DOI: 10.1016/j.jallcom.2018.12.070
Keywords
Additive manufacturing; Nickel-based superalloys; Focus offset; Columnar grain microstructure; gamma '/gamma ''/gamma ' Sandwich structures
Categories
Funding
- Korea Institute of Industrial Technology [KITECH EO-18-0021]
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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 mu m and a columnar grain microstructure with a strong [001] texture in the built direction (BD). Unmelted powder and macro-voids of approximately 200 mu m were observed at higher focus offsets, and the columnar grain width decreased from 600 mu m to 150 mu m with increasing focus offset. gamma' precipitates with a length of 10 nm developed parallel to the BD, while gamma '' 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.gamma'/gamma ''/gamma' and gamma'/gamma ''/gamma'/gamma ''/gamma' sandwich structures on the columnar gamma 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 delta -> gamma '' and delta -> gamma' phase transformation. The volume fractions and size of the gamma' and gamma '' 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. (C) 2018 Elsevier B.V. All rights reserved.
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