Prediction of fatigue limit in additively manufactured Ti-6Al-4V alloy at elevated temperature

Rotating bending fatigue tests were conducted at room temperature (RT) and at elevated temperature (ET) of 250 degrees C using a Ti-6Al-4V alloy fabricated by an additive manufacturing (AM) process, and the effect of defects on fatigue strength was investigated. Particularly, the applicability of Murakami's model for predicting the fatigue limit at ET was examined. Microstructure observations revealed that the AM Ti-6Al-4V alloy had acicular alpha + beta microstructures while the conventionally manufactured (CM) Ti-6Al-4V alloy had equiaxed alpha phases in beta matrix. Round-shaped defects and crevice-liked sharp defects with the size of up to 50 mu m were observed in the AM Ti-6Al-4V, which were not observed in the CM Ti-6Al-4V. The fatigue strengths at 10(7) cycles of the CM Ti-6Al-4V were 625 MPa at RT and 475 MPa at ET, and those of the AM Ti-6Al-4V were 300 MPa at RT and 250 MPa at ET. Scanning electron microscopy (SEM) on the fracture surfaces revealed that fatigue cracks in the AM Ti-6Al-4V specimens initiated from defects at ET as well as they did at RT. The fatigue strengths of the AM Ti-6Al-4V at RT and at ET were evaluated by Murakami's model and the predicted value agreed well with the experimental value.