Study On Process And Mechanism Of Refill Friction Stir Spot Welding For Al/Ti Dissimilar Metals

In recent years,the urgent demanding requirements for lightweight of transportation industries are increasing.Al/Ti hybrid structures could combine the advantages of high strength and low weight.However,it is a challenging task to join them because of their physical and metallurgical differences.In the welding processes,the intermetallic compounds（IMCs）will significantly lowered the mechanical properties of the joints.The spot welding joints are widely used in industry.Refill friction stir spot welding was carried for 2 mm thick6061 aluminum alloy and 1.5 mm thick TC4 titanium alloy.The effects of different welding process parameters on the mechanical properties of the joints were analyzed.The welding joint with great comprehensive performance was obtained by optimizing the welding parameters.In addition,the influence of welding parameters on the microstructure and mechanical properties of the joint is explained.The welding temperature field and material flow behavior in the welding process were analyzed by numerical simulation.We explored the formation mechanism of Al/Ti dissimilar metal intermetallic compounds by thermodynamic and kinetic.The influence of welding process parameters on the weld surface was analyzed.Because of the different degrees of thermomechanical behavior,the microstruction of joint can be divided into stir zone（SZ）,heat affected zone（HAZ）and thermal-mechanically affected zone（TMAZ）.Dynamic recrystallization and grain refinement occurred in the stir zone.The grains in the thermal-mechanically affected zone were deformed,and the grains in the heat affected zone were growth.The diffusion of atoms at the interface resulted in the formation of a uniform diffusion structure with a thickness less than 50 nm.The intermetallic compound of Ti Al₃ was produced.A nanoscale amorphous phase was found at the joint interface.The joint hardness test results showed that the joint hardness presented a W-shaped distribution along the horizontal direction on the aluminum side.Hardness was symmetrically distributed along the center of the joint.The hardness of the joint area on the side of the aluminum alloy was lower than that of the base metal.The stir zone has the highest hardness except for base metal.The hardness between the heat affected zone and the thermal-mechanically affected zone was the lowest.At the rotation speed of 1300 rpm,the dwell time of 6 s,and the plunge speed of 0.9 mm/s,the failure load of the joint reached the maximum.Under the optimal process parameters,the fracture mode was tough-brittle hybrid fracture,while only brittle fracture was occurred under other process parameters.The connection strength at the interface of the pin stirred area was low.The optimized tensile shear failure load of 6601.19 N can be obtained at the rotation speed of 1378 rpm,the dwell time of 6.5 s,and the plunge speed of 1.1 mm/s by the response surface methodologyThe welding temperature was increased rapidly at the plunge process.And,the temperature was increased slightly in the dwell stage due to the accumulation of heat.After the tool withdrawn,the temperature was decreased slowly at room temperature.The numerical simulation results showed the temperature field which was axial symmetrical with respect to the weld axis.The pin and the sleeve were the main areas of heat generation and scattering,where the highest heat diffused to all sides.The heat transfer in titanium alloy was slower than that in aluminum alloy.The analyzation of thermodynamics and dynamics show that only one intermetallic compound（Ti Al₃）phase was formed during the refill friction stir spot welding process,which was consistent with the experimental results.During the strong plastic deformation,the diffusion of atoms was accelerated and the growth of intermetallic compounds was promoted.