Structure and Properties of EP741NP Heat-Resistant Nickel Alloy Produced by Selective Laser Melting

The structure of EP741NP alloy samples produced by selective laser melting (SLM) in various technological modes, with various types of defects (the volume fraction of which varies from 0.31 to 0.65%), is investigated using optical and scanning electron microscopy; the mechanical characteristics of the samples are determined by tensile tests. All investigated SLM samples are typified by low strength characteristics, which is associated with the formation of a metastable single-phase structure, as well as with the presence of structural defects in the form of cracks. To improve the mechanical properties, the postprocessing of various types is carried out, including hot isostatic pressing (HIP); the heat treatment (HT) of the “quenching + ageing” type; and complex processing, which combines HIP and HT. According to the research results, the influence of various types of postprocessing on the microstructure and properties of SLM samples is determined. It is established that the use of HIP contributes to a decrease in porosity down to 0.04 vol %, the recrystallization of the structure, and the precipitation of the strengthening intermetallic phase based on Ni3Al (γ' phase) in the form of large particles of different sizes that create agglomerates. HT leads to the recrystallization of the structure and the precipitation of the finely dispersed γ' phase, which is uniformly distributed in the alloy matrix. In this case, the strength characteristics of the samples after HIP and HT are approximately on the same level (σu ~ 1250–1290 MPa); however, the ductility of the samples after HT is significantly lower, which is associated with the retention of defects in the structure in the form of cracks and large pores. The maximum increase in mechanical characteristics (σu of up to 1460 MPa and δ of up to 21.3%) is recorded during the complex postprocessing (HIP + HT), which ensures the elimination of defects and the formation of an optimal alloy structure.