Very-High-Cycle-Fatigue Property of IN 718 Manufactured by Selective Laser Melting at Elevated Temperature: Microstructure-Related Failure Behavior and Life Prediction

The high- and very-high-cycle-fatigue properties of IN718 Ni-based superalloy manufactured by selective laser melting (SLM) were investigated by axially loaded fatigue tests at room temperature (25 °C) and elevated temperature (650 °C) with a stress ratio of R = −1. Due to the high energy density and rapid cooling rate of the SLM process, the grains grew as dendrites surrounded by chain and dotted Laves phases. Subsequently, the results of monotonic tensile tests show that the SLMed IN718 has good resistance to elevated temperature. The stress–life characteristic curves revealed a continuous decline with no conventional fatigue limit. The fracture observation illustrated that surface flaw-induced failure is predominant at 25 °C, while internal failure, caused by crystallographic facets cracking, is prevalent at 650 °C under lower stress. Furthermore, the internal failure mechanism related with microstructure was summarized. Based on the evaluation of stress intensity factor ranges, the threshold values of long and small crack growth can be obtained, as well as the transition lengths from small to long cracks can also be calculated. Finally, a model based on fatigue indicator parameter was proposed to predict the fatigue life, and the predicted results were acceptable.