The use of a selective laser melting (SLM) powder-bed method to manufacture Ni-based superalloys components provides an economic approach for low production run components that operate under a high-temperature and stress environment. A major concern with the SLM of precipitation hardenable Ni-based superalloys is their high susceptibility to cracking, which has been heavily documented in the field of welding. Weld cracking may occur either during processing (hot cracking, liquation cracking and ductility-dip cracking) or during the post weld heat-treatment stage (strain-age cracking). Due to the complex thermal history of SLM fabricated material there is the potential for all of these mechanisms to be active.In this investigation, cuboidal coupons of the Ni-based superalloy CM247LC were fabricated by the SLM of argon gas atomised powder. Parametric studies were performed to investigate the influence of the process parameters (laser scan speed, power and scan spacing) on the cracking density and morphology through conducting a stereological study of scanning electron microscope (SEM) micrographs. Further microstructural evidence is presented, illustrating the different crack morphologies observed as well as suggesting the responsible mechanisms. Finally a postfabrication Hot Isostatic Pressing (HIP) treatment was performed to investigate its utility in 'healing' the internal cracks, and providing a route to retro-fix the cracking problem in the heat treatment stage of production. The findings highlight the need for process models of the SLM method in order to understand the thermal history and the laser fabricated structures observed.

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 (IN718), 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 IN718.