Peter the Great Saint Petersburg Polytechnic University, Saint Petersburg, Russia:

V. Sh. Sufiiarov, Leading Researcher, e-mail: vadim.spbstu@yandex.ru
A. A. Popovich, Professor, Director of Institute of Metallurgy, Mechanical Engineering and Transport, e-mail: popovicha@mail.ru
E. V. Borisov, Post-Graduate Student, Researcher, e-mail: evgenii.borisov@icloud.com
I. A. Polozov, Post-Graduate Student, Engineer, e-mail: igor.polozov@gmail.com

The paper describes the study of density, microstructure, phase composition and mechanical properties of samples made of heat resistant nickel alloy Inconel 718 by selective laser melting technology (SLM). The effects of the hot isostatic pressing (HIP) and heat treatment on microstructure, porosity and phase composition of the sample are studied. Hot isostatic pressing in both regimes significantly compacted samples after the SLM. In this case, the density of the samples exceeded 99.9%. This confirms the effectiveness of this method to increase the density of the samples. Differences in microstructure of samples are according to subsequent processing after SLM. Microstructure after HIP consists of equiaxed grains of -Ni, ranging in size from 55 to 110 μm. After the heat treatment microstructure of material is characterized by inequigranular grains with uniformly distributed precipitates of  γ"-phase (Ni₃Nb). It is observed in the sample coarse grains with size up to 200 μm and grains sized 50–80 μm. This irregularity is, presumably, a result of incompleteness of recrystallization process. Hardness measurements show that samples have high values of hardness after selective laser melting. Values of hardness after hot isostatic pressing are below hardness of samples after SLM, which appears to be associated with releasing of internal quench stresses generated during SLM. Formation during thermal processing of reinforcing particles γ"-phase (Ni₃Nb) increases the value of material hardness. Investigation of mechanical properties show that after hot isostatic pressing and heat treatment, the strength properties become higher and ductility is slightly lower, while values of properties become more stable. Strength characteristics exceed requirements for hot rolled samples of the alloy according a technical specification. Ductility of samples, thus, is placed at lower boundary of requirements.

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