Gas pore-based fatigue strength and fatigue life prediction models of laser additive manufactured Ti-6Al-4V alloy in very high cycle fatigue regime

Due to the high-density energy input characteristics of laser additive manufacturing (AM), gas pores are often as high-frequency defects in additive manufacturing materials, which makes the long-life fatigue service of structures have potential safety hazards. However, the fatigue researches on AM materials mostly focus on the lack of fusion (LoF) defect induced damage. Therefore, we propose an idea whether we can customize an AM alloy only with pore defects, and explore the very high cycle fatigue behavior. Ti-6Al-4V alloy are widely used in aerospace key components, and the research in additive manufacturing is relatively in-depth. Here, we selected laser additive manufactured Ti-6Al-4V alloy as the model material for ultrasonic fatigue test, and carried out defect tomography reconstruction, defect stress field simulation, and fracture quantitative analysis. Based on this, we introduce a low fatigue stress sensitivity coefficient to modify Murakami 's fatigue strength prediction model, and control the prediction ability within the error range of 10 %. Meanwhile, considering the location, size and shape of the pores, the T parameter was established, and the Schmid factor was introduced in combination with the microstructure cracking near the pores, so that the FIP model was optimized, making the predicted lives distribution within 2 times line of actual lives.