Abstract
The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2°) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS.
Highlights
IN718 is a Ni-based superalloy widely used in aviation and energy industries due to its exceptional combination of high-temperature mechanical stability, good fatigue life and high resistance to degradation in corrosive or oxidizing e nvironments[1]
In the scanning strategy denoted as Y-scan, the scan vector was parallel to the Y axis and the Scanning Direction (SD) was parallel to the X-direction
The stripe-hatching was performed along the Y-direction, with a Strip Width (SW) of half the width (W/2) of the sample and W/4 offset on consecutive layers
Summary
IN718 is a Ni-based superalloy widely used in aviation (e.g., aircraft engines) and energy industries (e.g., power generation turbines) due to its exceptional combination of high-temperature mechanical stability (up to 650 °C), good fatigue life and high resistance to degradation in corrosive or oxidizing e nvironments[1]. One important shortcoming of LPBF is the formation of complex Residual Stress (RS) states, with values in the range of the yield strength of a material when connected to the baseplate This RS state results from the steep spatial and temporal thermal gradients, locally induced by the laser during p rocessing[4,5]. In the case of IN718 alloys, characteristic epitaxial columnar grains with strong texture parallel to the BD have been observed by numerous authors[20,26,27,28,29,30,31] These features result from the combined effect of a heat flux that is strongly directed toward the baseplate and of the tendency of cubic crystals to grow with parallel to the heat flux[24,25]. The influence of the microstructure on the build-up of RS has been examined using different Scanning Electron Microscopic (SEM) techniques, in particular Electron Back Scattered Diffraction (EBSD)
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