Effect of Ultrasonic Impact on the Fatigue Properties of TC4 Titanium-Alloy Laser-Welded Joints

Ultrasonic impact treatment (UIT) as a surface improvement technology that is widely used in aerospace field. In order to study the effect of UIT on the properties of laser welded joints of titanium alloys. The effect of UIT on the fatigue properties of TC4 titanium-alloy laser-welded joints was studied. The surface roughness, surface topography, microstructural, microhardness and residual stress of specimens treated with two different UIT amplitude (18 and 25 mu m) were analyzed. Furthermore, fatigue performance test trials were carried out. The results show that the surface roughness and microhardness of UIT treated specimens increased by a maximum of 3.9 times and 10.88% comparing to the untreated specimen, respectively. Microstructure observation showed that a plastic deformation layer was formed on the surface of the specimen. Additionally, high level of compressive residual stress (the value around - 590 MPa) was introduced on the surface layer of the specimen. The S-N curve results showed that UIT improved the fatigue life of specimens, and the higher the UIT amplitude, the more obvious the effect of improvement. The fracture morphology of the specimens was analyzed by scanning electron microscopy. It can be observed that for the UIT treated specimens, the number of fatigue crack sources was reduced and shifted to the interior and the fatigue stripe spacing was reduced.