Microstructural characteristics and dislocation mechanisms governing the mechanical properties of GTAW-WAAM deposited Inconel 625 alloy

Abstract

The thermal cycling during the Wire Arc Additive Manufacturing (WAAM) process significantly influences the microstructure, texture, and mechanical properties of the components. In this study, an Inconel 625 block sample comprising three parallel deposits of eight layers each was fabricated, and four regions, top, middle, bottom, and side, were analyzed. Variations in grain morphology and orientation occur primarily due to differences in the solidification rate and temperature gradient across the build. Electron Electron Backscatter Diffraction (EBSD) analysis is employed to quantitatively characterize the grain geometry and to investigate the distribution of geometrically necessary dislocations across different regions of the build. Transmission Electron Microscopy characterization further reveals the presence of strengthening phases (γ/γ′) throughout the build, with the γ′-phase being most prevalent in the top region. X-ray Diffraction (XRD) analysis confirms the dominance of γ-phase (FCC) and γ/γ′ phases across different regions. The dislocation density calculated from XRD data indicates that the top region exhibits the highest, whereas the middle region has the lowest. The results indicate that regions with lower Kernel Average Misorientation (KAM) values correspond to lower dislocation density, typically representing recrystallized grains. Conversely, higher KAM values signify regions with greater dislocation density, indicative of deformed grains or areas with accumulated thermal strain. The ultimate tensile strength (UTS) was highest in the side region (780 ± 15 MPa) compared to other regions, while the lowest UTS was observed in the middle region (690 ± 10 MPa). The UTS in the side region was approximately 13 % higher than that in the middle region. A similar trend was also observed for hardness, stiffness, and elastic modulus.