Correlation between microstructures and mechanical properties of a SLM Ni-based superalloy after different post processes

Abstract

The microstructure and its relationship with the mechanical properties of an Inconel 718 superalloy fabricated by selective laser melting (SLM) technology were investigated under different post processes, including HT1, HT2, HT2+HIP and as-built conditions. Specifically, HT1 involved solution treatment at 960 °C for 1 h followed by air cooling (AC) and aging treatment at 720 °C for 8 h with furnace cooling (FC) to 620 °C for 8 h. HT2 involved solution treatment at 980 °C for 1 h followed by AC and aging treatment at 760 °C for 1 h with FC to 650 °C for 1 h. HT2+HIP included the same solution and aging treatments as HT2, with an additional Hot Isostatic Pressing (HIP) at 1170 °C for 4 h at 100 MPa. The characteristics of the microstructures after different post process and the failure behaviours such as the fractography and the path of microcracks were examined by SEM, TEM and EBSD. The evolution of the manufacturing defects such as loss of diffusion and keyholes are tracked by XCT. After HT1 and HT2 treatments, plenty of rod-like and needle-like δ phase were captured within the grain and on the grain boundaries. There has been a notable enhancement in the yield and ultimate strength of the superalloy. The uneven microstructures formed by the interlacing of fine and coarse grains under HT1 and HT2 conditions, cause strain concentration at the microscopic level, leading to the initiation of microcracks. In addition, the phenomena of growth and merging of defects were captured during the deformation. After adding a HIP process to HT2, the grains become uniform and coarse accompanied by twins in grains and large δ phase on the grain boundaries. Meanwhile, the size and quantity of defects significantly decrease. The ductility at this condition was improved to 12%, but the yield strength decreased to 886 MPa. The coarse grains and the precipitation of large δ phase on the grain boundaries reduce the strength of the superalloy, leading to microcracks easily initiating and propagating along the grain boundaries, resulting in the final failure. This research identifies post-processing effects on the microstructure and properties of SLM-fabricated Inconel 718, offering valuable guidance for optimizing treatment processes.

Keywords

• Selective laser melting
• Inconel 718 superalloy
• Post processes
• Tensile property
• Failure behaviour