Effect of Electropulsing-Assisted Ultrasonic Strengthening on Fatigue Properties of HIP Ti-6Al-4V Alloy

Electropulsing-assisted ultrasonic surface strengthening (EUSS) is conducted in investigating the fatigue properties of HIP Ti-6Al-4V alloy. The results show that the minimum grain size reaches to nanometer scale and is accompanied by many dislocations. This should be attributed to the effective combination of the athermal effect of pulsed current and ultrasonic shock. Meanwhile, the microhardness value is increased to 530 HV and the hardening depth reaches to 300 & mu;m. The compressive residual stress value drops to - 950 MPa in the thick plastic deformation layer and has an influence depth of 520 & mu;m. Meanwhile, the smooth surface quality of the sample reduces the stress concentrations. The high compressive residual stress value decreases the effectiveness of the formation and propagation of crack initiation sites. Finally, the tensile strength of the sample treated by EUSS is increased to 1104 MPa, and the fatigue life is increased to 2.5 x 105 at 700 MPa and the critical fatigue strength at 107 cycles is increased to 600 MPa. Moreover, the thermal effect of the pulsed current effectively reduces the microscopic defects (crack source) in material surface-modified layer. Therefore, the fatigue crack initiation site originates from the interior of the sample, and rather than on the surface.