Effect of grain boundary irregularity on impact toughness in Inconel 718 alloy fabricated by laser powder bed fusion

This study comparatively evaluates the impact toughness of laser powder bed fusion (LPBF)-built and forged Inconel 718 alloys through instrumented Charpy impact testing, with particular emphasis on microstructural determinants of crack initiation and propagation resistance. The LPBF specimen exhibited significantly reduced impact energy (7.5 J) compared to its forged specimen (21.6 J), representing only 34.7 % of the wrought material's energy absorption capacity. Fracture energy partitioning analysis demonstrated that 85 % of the total energy in LPBF material was consumed during crack initiation, whereas 72 % of energy in forged alloy dissipated during propagation due to enhanced crack-tip blunting mechanisms. Microstructural characterization linked these disparities to grain morphology characteristics: the LPBF alloy's broad grain size distribution and low sphericity parameters promoted geometrically necessary dislocation (GND) accumulation at both fine-grained regions and large, irregular grains. Fractographic analysis identified distinct failure modes – quasi-cleavage fracture dominated by microvoid-limited coalescence in LPBF material versus ductile rupture through strain-hardening-assisted void growth in forged specimens. These findings provide critical insights for optimizing microstructure design in additively manufactured superalloys to enhance damage tolerance under impact loading conditions.