Effects of specimen size and hot isostatic pressing on high-cycle and very-high-cycle fatigue of additively manufactured Ti6Al4V

This study investigates the effects of specimen size and hot isostatic pressing (HIP) on the high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) performance of additively manufactured (AMed) Ti6Al4V. Three groups of specimens were designed and fabricated. Two of them had control volumes of 50 mm 3 and 8294 mm 3 , respectively, and the third group consisted of 50 mm 3 subjected to HIP treatment. The results indicate that fatigue performance demonstrates a notable size effect as the control volume increases, with smaller specimens exhibiting higher fatigue strength. However, this size effect gradually diminishes with increasing failure cycles. For the HIP-treated specimens, crack initiation is no longer dominated by void defects but occurs due to α-phase grain cleavage. For the two groups without HIP treatment, a statistical analysis of crack initiation defects reveals that, in both HCF and VHCF states, the fatigue cracks in larger specimens always originate from internal defects. The average defect sizes are 109 μm for large sized specimen group and 54 μm for small sized specimen group. By using the probabilistic control volume model, the fatigue strength of large specimens was predicted based on the fatigue data of small specimens, and the prediction was consistent with the experimental data.