Effect of split-sleeve cold hole expansion on the fatigue performance of an additive manufactured TC18 alloy

AN additive manufactured TC18 alloy was subjected to cold hole expansion and low-cycle fatigue tests. It was found that specimens expanded with 4.5% interference fit level had the longest median fatigue life, which increased to 3.3 times. After cold expansion, the initiation of fatigue cracks was delayed by a smoother surface, denser microstructure and increased hardness of the hole surface layer. Meanwhile, fatigue cracks tended to initiate at the hole chamfer where the stress was concentrated. Finite element simulation results showed that the residual compressive stress layer was formed around the expanded hole and then balanced by tensile stress. For specimens with defects, such as improperly melted particles, internal microcracks and surface spalling, the cold hole expansion process still effectively promoted their fatigue performance. It was demonstrated that residual compressive stress around the hole is the key factor to improve the fatigue life by prolonging the crack initiation stage.