Investigation of the grain deformation to orthogonal cutting process of the textured Alloy 718 fabricated by laser powder bed fusion

Abstract

In the laser powder bed fusion (LPBF), the grains grow in preferential directions depending on the scanning strategies, which results in layer-by-layer builds of particular crystallographic textures. The unique microstructure formed by LPBF results in anisotropic properties of the built structure at both macro and micro levels. To understand the grain deformation of the textured alloy fabricated by LPBF in the high-strain-rate shear process, Alloy 718 was used as an example in this work. Bulk samples with different metallurgical textures were deliberately fabricated by LPBF via three laser rotation angles, namely 0°, 67° and 90°, and then four thin slices obtained from bulks were subjected to “quasi-in-situ” grain deformation investigation through orthogonal cutting (a simple shear loading condition). The evolution of crystal orientations and morphologies, including size and shape, were traced before and after shear deformation. A full-field crystal plasticity simulation was used to quantify the stress status for grains obtained from EBSD data. This for the first time reveals the crystallographic level deformation history for hundreds of microns during a high strain rate shear removal deformation. Due to the carefully retained deformation history (i.e., typical bulges and slip bands) on the surface, a repeated deformation pattern was observed, attributing to the non-homogeneous deformation of typical build-directional blocks. The most active slip trace of deformed grain was calculated and verified based on the dominated slip bands within individual grains. The slip trace direction and intensity were quantified for different textured Alloy 718. Since the slipping-based deformation for an orientated grain is represented by its most active slip trace, a deformation tendency map is obtained by combining the shear direction, slip system and grain morphology. It reveals that grains in high texture intensity workpieces generally follow the macro shear-based deformation, while with the decrease in texture intensity, the plastic anisotropy is significant at the grain scale. Grains with similar orientations may also result in localised deformation anisotropy due to the different morphologies.