Effects Of Ultrasonic Impact Treatment(UIT) On The Distribution Of Residual Stress In TA15 Titanium Alloy Sheet

As an important method of material connection, welding has been widely applied in the field of aerospace, automobile and shipping. TA15 is a near α titanium alloy which is mainly applied in high temperature conditions. It has excellent welding performance and good thermal stability. However, residual stress is generated in the welding process due to the heterogeneous distribution of temperature and has a negative effect on the stability, stiffness and fatigue property of the welded structures.The common means used to reduce residual stress are heat treatment and mechanical methods. Heat treatment is namely annealing, it can reduce the residual stress and rearrange the distribution of stress, and however, some large welded components cannot use the annealing method due to the size of furnace. Ultrasonic Impact Treatment(UIT) is the most widely used among the mechanical methods. Comparing with the traditional methods, such as annealing, UIT can not only introduce compressive stress in the welded structure, but also has greater advantage in reducing energy consumption and improving efficiency. Meanwhile, the device of UIT is simple and portable. In this study, the TA15 titanium alloy plates were welded by electronic beam welding method(EBW) and tungsten inert gas welding(TIG) method, UIT was employed to reduce the stress in the welded joints, effects of different technological parameters(for example impact method, impact times, shape of the impact head) on the distribution of residual stress were investigated. The main results were drawn as follows:The curve of residual stress in the welded plate distributed symmetrically along the center line of the welded joint. The largest value which existed in the center line was nearly 80% of the yield strength of TA15 titanium alloy. Stresses in the welded joint were tensile stress while compressive stress in the base metal. The impact method and shape of the impact head had significant influences on the distribution of residual stress. The shape of impact head should be chosen according to the welding method, the width of welded joint and the flatness of the surface. In addition, impact times and single/double-side impact method could also affect the stress in the welded joint. More times and double side impact could lead to better results.Comparing with the original alloys, the microstructure of TA15 titanium alloy after welding changed obviously. The fine equiaxial α grain transformed into coarse widmanstatten structure. The side of grain in the fusion zone was much larger than that in the heat-affected zone. After welding, the elongation of TA15 titanium plate decreased from 16% to 6.5%, while the yield strength and ultimate tensile strength changed little.After UIT, no plastic deformation layer existed in the surface of the welded joint and the microstructure didn’t change obviously. The mechanical properties, such as yield strength and ultimate tensile strength, were basically in consistent with that of the welded plate. But compared with annealing, the elongation of plates after UIT was much lower while the microhardness was much higher. These phenomena indicated that UIT could redistribution the residual stress in the joint, but it couldn’t eliminate the stress fundamentally.