Structure and properties of EP741NP heat-resistant nickel alloy produced by selective laser melting

Abstract

EP741NP alloy samples featuring various types of defects with the volume fraction varying from 0.31 to 0.65 % were produced by the method of selective laser melting (SLM) at various process conditions. The structure of SLM samples was investigated using optical and scanning electron microscopy, and mechanical properties were determined by tensile tests. All investigated SLM samples featured by low strength characteristics due to the metastable single-phase structure formation, as well as structural defects in the form of cracks. To improve mechanical properties, various types of post-processing were carried out including hot isostatic pressing (HIP), heat treatment according to the «solution + aging» (HT) type, and comprehensive processing combining HIP and HT. According to the research results, the influence of various post-processing types on the microstructure and properties of SLM samples were determined. It was established that the use of HIP contributes to a decrease in porosity to 0.04 %, structure recrystallization, and the precipitation of a strengthening intermetallic phase based on Ni3Al (γ ′-phase) in the form of large particles of different sizes forming agglomerates. HT leads to the structure recrystallization and precipitation of a finely dispersed γ ′-phase uniformly distributed in the alloy matrix. In this case, strength characteristics of samples after HIP and HT are approximately at the same level (σв ~ 1250÷1290 MPa), however, the ductility of samples after HT is significantly lower. This is associated with the retention of defects in the structure in the form of cracks and large pores. The maximum increase in mechanical characteristics (σ up to 1460 MPa, δ up to 21.3 %) was recorded during comprehensive post-processing (HIP + HT) that ensures defect elimination and optimal alloy structure formation.