Study On The Parameters And Microstructure Properties Of Ti/Al Friction Stir Spot Welded Joints

Ti/Al dissimilar metal structure has broad application prospects in the fields of aerospace,high speed railway and so on for the owning of two metals advantages.Because of the huge difference in physically and chemically of the two metals.Welding of this two metals are difficult.In this paper,the 3mm thick TC4 titanium alloy and 2A14 aluminum alloy dissimilar metals are joined by the friction stir spot welding technology.The influence of welding parameters on the forming and mechanical properties of the joint are studied.The microstructure of spot welded joints,the fracture types,the forming mechanism of the joints and the formation mechanism of the brittle phase in the joints were analyzed by using SEM,XRD and other analytical test methods.Research results show that: The connection of TC4 titanium alloy sheet and 2A14 aluminum alloy sheet was successfully achieved by using the stir head of a cylinder with a left screw thread.The optimum process parameters of spot welding joints are 950r/min,the residence time is 9s and the plunge depth is 0.3mm.Joint surface molding is good and the cross section of solder joints is in good condition,there are no macroscopic defects.With the increase of the rotation speed of the stir the plunge depth and the residence time,the surface forming and cross section forming of spot welding joints change from rough shaping to smooth forming.When the process parameter is too large,the spot weld joint is formed to smooth but form a conical keyhole or surface cracking appears and joint molding become bad.In the scope of this experimental study,such process parameters as rotation speed of stir-welding head,retention time and quantity for shoulder being press contribute the effect tendency on the tensile strength of joint shear: With the increase of variable parameter shear strength follow the tendency of first increases and then decreases;through the hook area EDS energy spectrum analysis and XRD analysis of micro area,Found that the hook area,titanium aluminum interface area composed into gray stripe?black streaks of TiAl3 and Ti/Al brittle phase in the banded structure,with the increase of stir-welding head rotation speed,The tendency of brittle phase formation in Ti/Al joint increasing;with the number and variety of brittle phase in the Ti/Al joints influence on the performance of the joint group.Friction stir spot welding of Ti/Al shear tensile fracture morphology show that,the fracture forms of spot welded joint are of type II fracture.Ti/Al is found to have a small scale combination only in the stir needle area.In the area of the shaft shoulder pressing,there is no material flow behavior,and only the impression of the formation of the heat transfer on the surface of the aluminum alloy.The connection mechanism is mainly based on mechanical connection,and at the same time there is a small amount of metallurgical adhesion.The material flow in Ti/Al Friction Stir Spot Welding between upper and lower is poor.