Stress controlled high-cycle fatigue (HCF) test and strain controlled low-cycle fatigue (LCF) test for electron beam welding (EBW) joints of forged TC4 alloy were investigated at room temperature.Fracture surfaces were observed by scanning electron microscope (SEM)

and the crack initiation mechanism of fatigue failure was analyzed.The results show that all HCF specimens are fractured in the base material (BM) observed by SEM

while LCF specimens are mainly ruptured at fusion zone (FZ) and BM.Due to low stress level in HCF test

the crack of joints is initiated near the surface

while the crack of the joint in LCF test is initiated not only near the surface but also in the interior defects.The crack initiation mode depends on the load.