Analysis of Microstructure and Mechanical Properties in As-Built/As-Cast and Heat-Treated Conditions for IN718 Alloy Obtained by Selective Laser Melting and Investment Casting Processes

Abstract

In this work, new customized heat treatments for selective laser melted (SLM) parts in IN718 alloy were analyzed. This was done through the evaluation of the mechanical properties and advanced characterization of the phases and microstructure obtained in as-built condition and after the application of standard and tailored heat treatments. The microstructure and mechanical properties were compared and discussed with results reported in the literature. Finally, strengthening mechanisms of IN718 alloy processed by SLM and its differences with mechanisms that occur in investment casting were analyzed. Both processes generate quite different microstructures, investment casting is composed mainly by a dendritic structure, and SLM is characterized by columnar and cellular structures with very thin cells. Due to the fine and homogeneous microstructure obtained from SLM processing and its specific strengthening mechanisms, it is not necessary to apply homogenization and solution stages as in standard heat treatment used for this type of alloy in casting or wrought. The pre-heating and process parameters selected, in combination with a direct stepped aging (at 720 °C/620 °C), provide the material with its best mechanical properties, which are superior to those obtained by standard heat treatment (AMS 5383F) applied to investment casting of IN718 alloy.