Toward post-process-free fatigue performance: In-situ heating and heat treatment of additively manufactured IN718

Abstract

This study investigated the microstructural characteristics and mechanical properties of Inconel 718 (IN718) fabricated using laser powder bed fusion and focused on developing post-heating methods to enhance the mechanical properties. Three different heat treatments with various times and temperature ranges were examined, followed by standard aging. Samples were fabricated in horizontal and vertical orientations. Digital light microscopy, scanning electron microscopy, X-ray diffraction, hardness, and fatigue tests were performed to characterize the microstructure and mechanical properties of IN718. The findings revealed that heat treatment at 980 °C enhanced the hardness, tensile strength, and fatigue life, whereas a further increase in the heat treatment temperature led to an increase in grain size and undesired precipitates. In addition, the work utilized a pre-heated substrate to decrease thermal gradients, provide better control over cooling, and regulate the mechanical properties of the material. A heated bed with a temperature of 250 °C to 500 °C resulted in better mechanical properties compared to as-built samples and higher ductility compared to post-process heat-treated samples. This comprehensive study showed that optimizing the in-situ heating temperature has the potential to eliminate the need for post-process heat treatment for high-cycle fatigue performance. It also allows for a greater understanding of the fabrication and modification of the post-process heat treatment and in-situ heating to optimize the mechanical properties of additively manufactured IN718. • In-situ heating reduced Laves phase and promoted δ-phase formation, improving the microstructure which can enhance high-temperature performance. • Post-process heat treatment also promoted γ′ and γ″ strengthening phases, significantly increasing hardness and strength. • Combined effect of in-situ heating and post-process treatments produced refined microstructure and improved mechanical and fatigue properties. • Grain structure evolved from columnar to equiaxed grains due to thermal gradients induced by in-situ heating. • Optimizing in-situ heating parameters is crucial to balance strength, ductility and surface roughness in AM of IN718.