Tomáš Babinský

In this work, we explore the feasibility of simplifying the heat treatment of IN939 superalloy prepared by laser-based powder bed fusion to a single step in the process called direct ageing, conducted at 800 °C for 8 hours. Tensile tests and strain-controlled low-cycle fatigue tests were conducted at room temperature using machined specimens with horizontal and vertical build orientations. Test results were compared with other material states of IN939, namely conventional cast, and additively manufactured with and without post heat treatment. Investigated material states were characterized before and after testing by means of scanning and transmission electron microscopy. We found that the inherent cellular microstructure is further strengthened by γ’ precipitation, while additional precipitation of carbides and γ/γ’ eutectics at internal interfaces was observed. These microstructural features significantly enhance tensile strength, with yield strength and UTS exceeding 1300 and 1550 MPa, respectively, but reduce ductility down to a few percent. Considering the low-cycle fatigue performance, direct aged samples perform relatively well only at high strain amplitudes. The cracking is rather abrupt with little to no indication of impending failure due to limited ductility. Therefore, omitting the solution heat treatment and performing direct ageing of as-built additively manufactured IN939 components is not recommended, in agreement with previous studies on similarly treated materials.

The Inconel IN939 superalloy is a suitable material for additive manufacturing. However, such a manufacturing process creates an anisotropic microstructure, which influences the material’s mechanical performance. The effect of microstructure on fatigue behavior at high temperatures is studied using the finite element method within the crystal plasticity framework. The spatial evolution of stresses and strains with cyclic loading is analyzed. The results show that material loaded perpendicularly to the building direction (BD) has higher strengthening and higher activity of slip systems in whole volume while loading parallel to the BD induces plasticity, preferably in grains with (101) and (111) orientation. Such differences can lead to different damage initiation and evolution.