Effect of Post‐heat Treatment on the Tensile and Cryogenic Impact Toughness Properties of Inconel 718 Manufactured by Selective Laser Melting

Abstract

This study investigates the effect of post‐heat treatments (solution treatment + aging [SA] and hot isostatic pressing + aging [HIPed]) on the tensile property as well as the room temperature and cryogenic impact toughness of Inconel 718 alloy manufactured by selective laser melting (SLM). The porosities of the three specimens measure SLM SA: 1.10% and SLM HIPed: 0.06%, and wrought SA: 0.01%. Initial microstructural observation shows γ″ + γ′ phase in all three alloys (SLM SA, SLM HIPed, and wrought SA). SLM SA and wrought SA show δ phase of greater fraction compared with SLM HIPed. SLM HIPed and wrought SA show a generally uniform microstructure, and equiaxed grains are found. In tensile tests, SLM HIPed shows greater strength compared with that of wrought SA, but SLM SA shows lowest yield strength among them. The elongations of SLM alloys are ≈1/3 of that of wrought SA. An impact test result shows that impact toughness value at cryogenic temperature of SLM HIPed alloy shows 80% of that of wrought SA. However, SLM SA shows relatively lower impact toughness. In addition, this study attempts to identify the deformation behavior and fracture mechanism of SLM Inconel 718 alloy based on the microstructural characteristics.