Direct laser fabrication of Inconel-718: effects on distortion and microstructure

Limited work is available in the literature on the influence of Direct Laser Fabrication (DLF) on the microstructural homogeneity and the structural integrity (porosity and distortion) of the deposited structures or "builds". These issues are addressed in the current study for Inconel-718 (1N718), focusing on the influence of the tool (deposition) path on the distortion of the substrate, as well as the microstructural development (grain, precipitates and texture). Differing tool paths are shown to have a significant influence on the distortion exhibited; a strategy for optimising the tool paths is suggested. Due to the good weldability of IN718, the builds were crack-free, but there was a minor fraction of volumetric porosity (-0.02), which spatially varied across the build. The microstructural investigations showed that the build has a heterogeneous microstructure, with coarse columnar and equiaxed grains at the bead centre surrounded by fine equiaxed grains at the bead boundary. Electron-Backscattered Diffraction (EBSD) was performed to rationalise the solidification behaviour and texture developed, and any influence of substrate microstructure. It is found that the grain orientation of the substrate has a significant influence on the build as the first few layers are deposited. However, there is no strong texture in the final microstructure. The build displays significant interdendritic segregation which promotes Laves, γ", Ti and Nb rich carbides and consequently somewhat lower hardness when compared with wrought 1N718.