CONTROL OF GRAIN STRUCTURE IN SELECTIVE-ELECTRON BEAM MELTING OF NICKEL-BASED SUPERALLOYS

The powder bed additive manufacturing process of selective-electron beam melting can produce near net shape parts with nickel based superalloys. Control of grain structure is the next step in research as site specific columnar or equiaxed grain structure can give the process further advantage over traditional processing. Previous work has used alloys that were designed for casting processes and have not tried to control the columnar to equiaxed transition (CET) by changing the composition. To determine if alloying for the CET is possible, two custom high gamma prime nickel alloys were designed using CALPHAD software and a CET model. After processing the custom alloys alongside traditional alloys using the Selective Electron Beam Melting (S-EBM) process multi-scale characterization was performed to determine the resulting grain and precipitate structure. From our findings, the process parameters and class of alloy have more control over the CET than expected. Alloying for the CET, without the context of final geometry and processing, is not recommended due to composition having a greater effect on precipitant structure than grain structure.