Study On Microstructure And Properties Of Friction Stir Spot Welding-soldering Hybrid Weled Joint Between Titanium And Aluminum Dissimilar Metals

A new technology of Ti/Al dissimilar metal connection with "Friction Stir Spot Welding and Soldering,FSSW-S" composite welded joint was developed.In the Ti/Al dissimilar metal friction stir spot welding,through add the appropriate solder to the Ti,Al lap interface,use of heat and force around the mixing zone generated by the friction stir spot welding to achieve a soldering connection.A good connection of 3mm thick TC4 titanium alloy and 2A14 aluminum alloy dissimilar metal was realized by "FSSW-S" technology.The effects of solder in different addition modes on the formation of joints were studied.The influence of process parameters on the mechanical properties of FSSW-S joints with titanium alloy surface aluminizing and conventional FSSW joints was analyzed.The microstructures,element distribution and interfacial structure of the microstructures were analyzed by SEM and EDS.The results show that:For three ways to add solder:the direct addition of Zn foil solder,titanium alloy surface sprayed pre-Zn85 Al solder and aluminized on the titanium alloy surface and then add Zn foil solder.In the first two types of soldering filler,the solder layer is well bonded to the aluminum alloy and there is a gap between the titanium alloy and the soldering filler does not react with the titanium alloy in the shoulder area.The third type of soldering filler,the interface of the shoulder area close together,the fracture of the joint is a mixed fracture in this region.The fracture of the the keyhole area occurred in aluminized layer and titanium alloy bonding surface and the edge area of the shoulder breaks at the surface of the aluminized layer under the interface.The formation process of FSSW-S composite joints aluminized on the titanium alloy surface can be divided into three stages.The first stage is the formation of the Hook when the tool is pressed down.The second stage is the solder under the shoulder squeezing out with the action of heat.The third stage is the dissolution and diffusion reaction of the remaining solder in the shoulder area.Zn solder is heated to melt and extruded under the effect of a large number of shoulder force.The remaining liquid Zn is filled with the gap between the aluminum alloy and the aluminized layer.Under the action of temperature and pressure,both sides of the Al continue to dissolve in the Zn,but also accompanied by the diffusion of Zn to Al until the final formation of the interface without defects and gaps.Tensile shear force of conventional FSSW joint change slow with the rotation ratio speed and welding time increases,The tensile shear force(TFS)of FSSW-S joint aluminized on the titanium alloy surface increases with the increase of the tool rotation ratio,With the increase of welding time the tensile shear force of the joint increases first and then decreases,and then remain unchanged,When the rotational ratio at 1200 rpm and the welding time at 18 s,the ultimate tensile shear force is 13.87kN/p,Which is 2.1 times over the same welding conditions under the conventional FSSW joints,Which is 1.9 times over the maximum tensile shear force of domestic literature at home and abroad.The fatigue load and fatigue life of Ti/Al "Friction stir spot welding and soldering" composite welding joints are significantly higher than those of conventional FSSW,The presence of the soldering interface changes the way in which the crack is extended in the joint.The fatigue load of the traditional FSSW joint and FSSW-S composite joint is 1.45 kN and 3.65 kN respectively when the number of corresponding fatigue cycles is 2×106,The fatigue life of the composite joint is much higher than that of the conventional FSSW joint.The crack source of the conventional FSSW joint fatigue fracture is the root position of the keyhole,the crack extends to the root of the Hook along the direction of the fatigue load,and the instantaneous fracture zone is the ductile fracture.FSSW-S composite joint crack along the soldering interface expansion has gone through three stages,Began to expand from the soldering interface outside the shoulder to the center of the keyhole and then expand to the aluminum in the direction of 45° to the interface,and then expand along the soldering interface until it extends to the root of the Hook,Fractures show Hook peeling fracture,near the keyhole area is brittle fracture.