The effect of hot isostatic pressing and heat treatment on the microstructure and properties of EP741NP nickel alloy manufactured by laser powder bed fusion

This paper addresses the effect of hot isostatic pressing (HIP) and heat treatment (HT) on the residual porosity, microstructure, and mechanical properties of the heat-resistant EP741NP nickel alloy produced by laser powder bed fusion (LPBF). The structural evolution was studied by X-ray powder diffraction, optical microscopy, as well as scanning and transmission electron microscopy. Special focus was placed on fine structure investigation to identify strengthening phases and alloying elements distribution. It was found that different Laves phases were formed in the interdendritic space during LPBF and were transformed to MC and M(23)C6 carbide phases during the subsequent HIP and HT. HIP resulted in healing of microcracks and large pores, as well as segregation of the strengthening gamma-phase consisting of irregularly shaped inclusions of heterogeneous size. Heat treatment led to a higher content of the gamma-phase, its refinement and more homogeneous distribution within the alloy matrix. Exposure to HIP and HT gave rise to a material characterized by ultimate tensile strength sigma similar to 1350 MPa and ductility delta similar to 12% (its strength characteristics were more than 30% superior to those of the LPBF samples).