Effects of different heat treatments on the cyclic material behavior of additively manufactured Inconel®718

Abstract

The flexibility in design offered by advanced additive manufacturing (AM) technologies makes these processes more and more attractive for automotive and aircraft applications and also for the manufacture of safety relevant metal components. The high-strength, thermally resistant Nickel-based alloy Inconel®718 is widely used by the aircraft industry. Its reduced machinability makes it an optimal candidate for AM technologies. The challenge, together with improving the process, is now to build the path that will bring AM technologies from rapid prototyping to series production. Therefore, it is essential to investigate additively manufactured materials and the effect of different subsequent heat treatments on their properties and microstructure. While the static properties of additively manufactured Inconel®718 have already been investigated, this work aims to describe its cyclic stress-strain behavior, to be used for a fatigue assessment. Small scale flat specimens are produced by Laser Powder Bed Fusion (LPBF) of Inconel®718 powder. To state the initial condition, the specimens are printed with different orientations with respect to the build platform using standard process parameters and individually adapted support structures, when needed, are subsequently removed. The material, in the as-built state, shows columnar grains and a highly heterogeneous microstructure. Therefore, it undergoes three different heat treatments corresponding to maximum temperatures of 650°C, 720°C or 965°C. The specimens are then subjected to incremental, variable amplitude strain-controlled testing (Incremental Step Tests) in order to evaluate the cyclic stress-strain behavior of the material. The combined effect of the different heat treatments and build orientations on the cyclic stress-strain behavior and fatigue life are evaluated and discussed through metallographic investigations and analysis of the stabilized hystereses.