Microstructural control of alloy 718 fabricated by electron beam melting with expanded processing window by adaptive offset method

This paper presents microstructures of Inconel 718 fabricated by electron beam melting (EBM), a powder bed fusion (PBF), additive manufacturing (AM) process, under various conditions with an extended range of electron beam power (P) and scanning speed (V). An adaptive offset method (AOM) was used to optimize the beam scanning line offset adaptively to fabricate blocks without macroscopic defects. In the AOM, the line offset is changed depending on the geometry of melt-pools of adjacent scanning lines. The AOM is valid as long as melt pool depth is larger than layer thickness, and it greatly broadens the process window for building dense and even parts. The broadened process window extends the range of the solidification condition and the variation of the resultant microstructures. Fully columnar grains and mixtures of columnar and equiaxed grains were formed. Mixtures of columnar grains and equiaxed grains were formed under two different types of extreme conditions. The first type is with low line energy/small line offset, which causes equiaxed grains associated with a small number of defects. The second type is with high line energy/large line offset, which causes equiaxed grains via the columnar-to-equiaxed transition (CET). The fully columnar grains are preferably oriented to < 001 > direction in the build direction. The blocks with the different microstructure exhibited similar strengths but different elongations. A processing map was established by plotting the points indicating the build condition and resultant microstructure in the P-V space as a guide for controlling microstructure.