Effect of heat treatment on the microstructural evolution of a precipitation-hardened superalloy produced by selective laser melting

Abstract

This study evaluates the microstructural evolution and mechanical properties of a selective laser melting (SLM) fabricated Inconel 718 (IN718) alloy under solution-aging treatment. It was demonstrated that the fraction of low angle grain boundaries (LAGBs) and the aspect ratio of the grains decreased with the increase of the solution temperature, while the average grain size increased after solution treatment. Aging treatment promoted full precipitation of the strengthening phase. The fraction of the γ′ and γ′′ phases exceeded 45.5% compared to 24% of the as-fabricated alloy. Heat treatment also eliminated the anisotropic mechanical properties of the SLM-processed IN718, as the columnar grains transformed to equiaxed grains with the increase of the solution temperature. High strength (1500 MPa) with good elongation was obtained in the low temperature solution- and double aging-treated IN718 alloy, which was attributed to the combination of hierarchically heterogeneous microstructures (solidification enabled sub-micron cellular structure and large fraction of LAGBs) formed in the SLM process and the full precipitation of the γ′ and γ′′ phases in the aging treatment. Furthermore, this study demonstrated that mechanical properties similar to those of wrought alloy can be produced in SLM-processed precipitation-hardened alloys by direct aging treatment without the energy-intensive solution treatments.