Research On Process And Mechanism Of Ultrasonic Spot Welding For Al-Ti Dissimilar Metals

With the rapid development of aerospace and automotive industries,the requirement of light-weight structure is becoming more and more stringent.Titanium alloy is widely used in light-weight structures for its advantages in specific strength,fracture toughness and corrosion resistance.In consideration of economic efficiency and service character,aluminum alloy is a common substitute for titanium alloy in some parts of the light-weight structures,owning to its satisfactory comprehensive properties and low cost.Joining the dissimilar metals together is indispensable in designing the Al-Ti composite structures.Ultrasonic welding as a solid state welding technology,which can realize the joint of metal with the combined effect of clamping pressure and ultrasonic vibration.It has the characteristics of low temperature,low pressure,low energy consumption,which can achieve a good weld of aluminum alloy and titanium alloy.AA6061 aluminum alloy and commercial pure titanium were adopted for their good comprehensive mechanical properties,which are widely used in automobile and shipping industries.This research designs the proper scope of process parameters and investigates the mechanical properties and microstructure to reveal the mechanism of Al-Ti dissimilar metals ultrasonic welding.Al-Ti dissimilar metals were successfully joined under welding time from 600 ms to 1400 ms,welding pressure from 10 MPa to 20 MPa,amplitude from 27-37 ?m.With the increase of welding parameters,the welding heat input increased,the precipitated phases were dissolved,the grains were recrystallized and gradually coarsened.The top surface and the bottom surface near the interface of the aluminum side were subjected to high stress and friction,which lead to material plastic flow and grain refinement.No significant intermetallic reaction layer was found under all parameters.There is mutual diffusion of Al-Ti elements,and the maximum diffusion distance is about 5 ?m.The change trend of hardness presented similar U shape under different welding parameters.In the weld region,the aluminum alloy was softened by the weld energy,while the hardness of pure titanium did not change distinctly.As the welding parameters increased,the peak load of the joints firstly increased with the expansion of bond area,then decreased afterwards due to the fatigue crack in Al side.At the welding time of 1000 ms,welding pressure of 15 MPa,amplitude of 23 ?m,the tensile shear failure load was the maximum of 5128 N.With the increase of welding energy,fracture mode is changed from brittle fracture to tough-brittle hybrid fracture.The optimized tensile shear failure load of 5200 N can be obtained at the welding time of 1160 ms,welding pressure of 18 MPa,amplitude of 34 ?m by the response surface methodology.The thermal cycles of different welding parameters experienced the same changes,which can be divided into three parts: temperature rise period,high temperature holding period and cooling period.The peak temperature increased with increasing welding parameters and the highest temperature can reach 378?.Through kinetic and thermodynamic analysis,the reasons for the formation of non-metallic compounds were explained.The Al-Ti ultrasonic spot welding joint is formed under the combination of mechanical and metallurgical bonding.