The influence of post heat treatments on the evolution of microstructure and mechanical properties of EP741NP nickel alloy produced by laser powder bed fusion

EP741NP superalloy was fabricated by laser powder bed fusion (LPBF) and subsequently subjected to different heat treatments including hot isostatic pressing (HIP), solution + aging, HIP + solution + aging. The structural features of the alloy in the as-built condition as well as after heat treatment were studied and compared to its mechanical characteristics such as hardness, tensile strength, impact strength, and compression strength at elevated temperatures. It was found that LPBF samples have a fine-grained columnar-cellular microstructure formed from colonies of primary dendrites and poor mechanical properties caused by structural defects (pores and isolated microcracks). The Laves phases of different compositions (Cr2Nb, Co2Nb and Cr2Hf) were detected in the interdendritic space. HIP contributes to elimination of microcracks, precipitation of the gamma ' phase (0.5-7 mu m in size), dissolution of the Laves phases, and formation of MeC carbide phases. A combination of these events increases strength and ductility of the alloy. Solution followed by aging without HIP results in formation of the fine-grained gamma ' phase with size ranging from 150 nm to 2 mu m, elimination of the Laves phases, and precipitation of MeC carbides (Cr23C6), which enhances strength. Ductility, however, remains low as the alloy structure still contains microcracks. The LPBF samples subjected to combined post-treatment (HIP + solution + aging) exhibit the maximum mechanical properties due to the elimination of structural defects during HIP and formation of the fine-grained gamma/gamma' microstructure during solution and aging.

Automated summary

EP741NP superalloy fabricated by laser powder bed fusion underwent different heat treatments, with hot isostatic pressing showing to eliminate microcracks, precipitate the gamma' phase, dissolve Laves phases, and form MeC carbide phases. The combination of these events increased strength and ductility of the alloy. Samples subjected to combined post-treatment (HIP + solution + aging) exhibited the best mechanical properties due to elimination of structural defects and the formation of a fine-grained gamma/gamma' microstructure.