Research On The Effects Of Ultrasonic Impact Treatment On The Distribution Of Residual Stress And Microstructure Of The Weldment TA15/BTi-6431S Titanium Alloy Sheet

With the high match requirement for space vehicles,the engineering structures have to withstand extremely high heat and pressure,the weldments of similar material are unable to satisfy the demand.The weldments of dissimilar metal alloys can make full advantage of properties of each component material,reduce production costs,which have been widely applied in the field of aerospace,automobile petrochemical and so on.I Iowever,the residual stress is generated due to the heterogenceous distribution of temperature formed during the welding process.The mainly methods used to reduce residual stress are classified into heat treatment and mechanical methods.Compared with the traditional methods,the Ultrasonic Impact Treatment(UIT)can not only introduce compressive stress,but has advantages of easy operation,high working efficiency,low energy consumption,and it's easy to carry.In this study,the TA15/BTi-6431S titanium alloy plates are welded by tungsten inert gas welding(TIG)and electronic beam welding(EBW),UIT was employed to reduce the stress in the welded joints,and effects of different technological parameters(impact times,moving velocity of the impact head,the material and the shape of the impact head,working electric current,etc)on the distribution of residual stress,microstructure characterization and mechanical properties were investigated.The main results were drawn as follows:The curve of residual stress distributed symmetrically along the center line of the welded TA15/BTi-6431S plates joint.The largest value existed in the center line was nearly 60%of the yield strength of the base metals.The longitudinal residual stress was obviously higher than that in the transverse direction.The different parameters of the UIT(impact times,moving velocity of the impact head,working current,the material and the shape of the impact head)could affect the distribution of residual stress of the welded joint.The residual stress was reduced dramatically with the increase of impact times and working electricity current and the decrease of the movement velocity of the impact head.The similar mechanical properties between the impact heads and the original materials should be considered,which may be helpful to reduce the residual stress and extend the service life of the impact head.In the case of the impact head shape,the point-to-point contact between the impact head and the welded joint should be guaranteed to reduce the residual stress uniformly.Compared with the original alloys,different microstructure characterizations in the fusion zone and the heat-affected zone were observed.Additionally,the welding methods also led to various microstructure features.The fusion zone was featured by coarse widmanstatten microstructure after TIG welding but martensite was observed after EBW welding.Under room temperature tensile tests,the elongation of TA15/BTi-6431S titanium plate welded by TIG decreased to 6%dramatically,while the yield strength and the ultimate tensile strength changed with a slight variation compared with the original plates.However,after EBW welding,the elongation was reduced to 12%but the ultimate tensile strength and yield strength was measured to be in the same level with the tensile properties of original plates.It is noted that ultimate tensile strength measured at 550?declined sharply but the elongation underwent an obviously improvement than the counterparts at room temperature.The region with a depth of ?m level below the surface of the TIG welded joint were observed but not existed near the EBW welded joint.The impact of welded joint had a minor effect on the mechanical properties,indicating that the depth of the region effected by UIT was limited and far less than the thickness of the plates.In the case of that.UIT is suitable to be employed to eliminate the residual stress retained near the surface of welded joint.