Research On Friction Stir Lap Welding Of Dissimilar Al/Ti Materials Assisted By Ultrasonic

Due to the high-performance,low-weight and low-cost design requirements in the aviation industry,the application of Al/Ti composite structures have been more and more used.Therefore,the research of joining technologies on Al/Ti dissimilar materials has been attracting more and more attentions.Friction stir lap welding?FSLW?,as a relatively new solid state welding technology,can be applied to the welding of Al/Ti dissimilar joints.In this paper,Al/Ti dissimilar FSLW with ultrasonic was studied.The effect of ultrasonic on joint formation and mechanical properties was mainly investigated.Firstly,Al/Ti dissimilar FSLW experiment was studied under the conventional condition.In this thesis,small plunging depth into lower sheet was adopted in the experiment in order to decrease heat input.The results showed that the inside defects of the joint decreased with the decrease of the rotating speed.When the process parameters were rotational speed of 300rpm and welding speed of 30mm/min,the defect-free joint was obtained.In the joints attained by lower rotational speed,the bonding mechanism of the lap interface was the diffusion bonding.The intermetallic compound of the interfacial diffusion layer at Al side was Ti Al₃.Secondly,Al/Ti dissimilar FSLW under the ultrasonic condition was performed,and the ultrasonic was applied to the lower sheet of titanium alloy.It was found that the ultrasonic can avoid the flash defects of Al/Ti dissimilar FSLW joint and reduce the inside defects of the joint.It is noted that the ultrasonic can also increase the thickness of interface diffusion layer,improving the bonding strength of interface.Finally,the mechanical properties of joints under different two conditions were tested.When the process parameters were rotational speed of 500rpm and welding speed of30mm/min,the addition of ultrasonic could avoid the void defect and then changed from shear fracture mode to the tensile fracture mode,greatly improving the tensile shear strength and elongation of joint.In joints without volume defects,the tensile fracture mode was attained,and the ultrasonic could increase the tensile shear strength of the joint due to the improved microhardness of thermo-mechanically affected zone.