Comparative study on microstructure, mechanical and high temperature oxidation resistant behaviors of SLM IN718 superalloy before and after heat treatment

Abstract

The microstructure, phase transformation behavior, heat treatment effects, and their impacts on the mechanical properties and high-temperature oxidation resistance of IN718 nickel-based superalloy processed by selective laser melting (SLM) both before and after heat treatment was systematically investigated. The heat treatment process involved a two-step aging at 720 °C for 8 h plus 620 °C for 10 h with a controlled slow cooling rate of 10 °C/min. Through the comprehensive use of scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM), the evolution of the microstructure and its mechanism of influence on material properties were revealed. The study identified the formation of γ phase, γ′ phase, γ'' phase, δ phase, and Laves phase during the SLM forming process. Heat treatment played a key role in improving the microstructure and properties of the SLM-processed alloy, with experimental results showing that the mechanical performance and high-temperature oxidation resistance were significantly enhanced after heat treatment. Specifically, the material after heat treatment exhibited a yield strength of 1472 MPa, a tensile strength of 1608 MPa, and an elongation of 9.2%, which represented increases of 76.5%, 38.4%, and 16.5% respectively, compared to the as-printed alloy. Moreover, the oxidation resistance at 1000 °C of the heat-treated samples improved significantly, with weight loss reduced from −0.17 mg/cm2 before heat treatment to −0.15 mg/cm2. Microstructural analysis further indicated that heat treatment optimized the crystal orientation, promoted the formation of more precipitated phases and reduced defects, thereby enhancing the overall performance of the material. This work provides significant scientific insights into understanding and optimizing the IN718 nickel-based superalloy processed by SLM, demonstrating the crucial role of heat treatment in improving its microstructure and properties.

Keywords

• Additive manufacturing
• Selective laser melting
• Heat treatment
• Mechanical property
• High temperature oxidation