DIRECT LASER FABRICATION OF INCONEL-718: EFFECTS ON DISTORTION AND MICROSTRUCTURE Lakshmi Lavanya Parimi, Moataz M. Attallah, J.C. Gebelin and Roger C. Reed School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. Keywords: Inconel 718, distortion, laser deposition, texture, precipitation Abstract 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. Introduction There is a growing interest in the use of direct laser fabrication (DLF) as a repair method or for the production of near net-shape components, particularly for Ni-based superalloys [1]. DLF can also be utilised to add features to an existing cast, machined, or forged structure, in what is commonly referred to as hybrid manufacturing. It can also be used as an economic repair method for repairing worn or damaged parts, with minimum post- machining or stress-relief processes, when compared to replacing the whole component [1]. Accordingly, DLF has a number of advantages, compared to the conventional manufacturing processes, including the reduction in material waste, the design flexibility, as well as the cost saving benefits when using it as a repair method. Furthermore, DLF structures normally have fine microstructures and reduced heat-affected zone, compared to fusion welding processes, in addition to superior mechanical properties compared to the as-cast material properties due to the inherently rapid solidification rates [2, 3]. Although it is possible to optimise the process parameters to produce fully dense structures, the DLF process ultimately produces local changes in the microstructure and structural integrity, due to the thermal fields associated with the process during the solidification of the molten powder. These issues need to be assessed in order to minimise their influences on the component properties. In practice, the build quality and morphology of laser deposition can be affected by a number of DLF parameters, including the laser power, traverse speed, standoff distance (or laser focal distance), hatch spacing (in plane spacing or bead overlap), layer thickness, tool path, and powder flow rate. Considerable work has been performed to optimise these parameters, in order to obtain a defect-free build with a good surface finish [4-7]. However, only limited work is available on the build/substrate interaction, particularly the microstructure/texture development, and on the mitigation of distortion due to DLF. Due to the local variations in the solidification conditions within the melt pool, the microstructure becomes generally heterogeneous, with varying grain sizes and shapes (columnar and equiaxed), and a complex precipitates structure [8]. Similarly, several thermal management approaches were previously studied to assess their influence on the distortion generated by DLF [9, 10]. However, only limited work was performed on the use of the tool (deposition) path to control the thermal fields, and accordingly minimise the distortion[11]. The aim of the present investigation is to address some of the aforementioned shortcomings in the DLF literature on the distortion and microstructural homogeneity. IN718 is known to have a good weldability, due to the relatively sluggish γ′′ precipitation kinetics, which makes it suitable for DLF [6]. Nevertheless, the exact procedures by which DLF is used to produce minimal distortion in the build and substrate remain unclear. Moreover, the microstructural evolution, in the grain and precipitates structures, as well as the texture, needs further assessment, particularly since the mechanical properties are greatly influenced by it. It equally highlights the need for any post DLF heat treatments to homogenise the microstructure. Experimental Material Gas atomized IN718 was used in this investigation, with the chemical composition given in Table 1. The average particle size of the powder is ~ 60 µm, with 90% of the particles falling within the size range of 40-100 μm, which is the typical range for DLF[6]. The powder is almost spherical in shape with inherent porosity in few particles and very fine satellite particles infrequently attached to the particles, Fig. 1. The microstructure of the particles shows a fine dendritic network, which is caused by the rapid solidification during gas atomization, Fig. 2. 511 Superalloys 2012: 12 th International Symposium on Superalloys Edited by: Eric S. Huron, Roger C. Reed, Mark C. Hardy, Michael J. Mills, Rick E. Montero, Pedro D. Portella, Jack Telesman TMS (The Minerals, Metals & Materials Society), 2012