Abstract: The very high cycle fatigue properties and fracture mechanism of the electron beam weldment for TC21 titanium alloy are investigated by the ultrasonic fatigue testing method. The results show fatigue strength of electron beam weldment is much lower than that of base metal. In the shorter life regime, fatigue crack initiates on the surface for both base metal and electron beam weldment. However, in the longer life regime, the site of fatigue crack initiation transforms from surface to interior. Fatigue crack mainly initiates at internal microstructure for base metal, while fatigue crack initiates at weld pore for electron beam weldment. When fatigue life is longer, the initiation zone would appear "fish eye" morphology and the area near initiation zone appears white granular characteristics named FGA. The stress intensity factor ranges from 2.90 MPa•m1/2 to 3.33 MPa•m1/2 at the front of FGA is regarded as the threshold value of internal crack propagation. The relationship among fatigue life, fatigue stress and pore size is discussed based on the short crack propagation theory proposed by AKINIWA.

Key words: electron beam weldment, pore size, TC21 titanium alloy, very high cycle fatigue