Microstructure And Forming Mechanism Of Ultrasonic-assisted Transient Liquid Phase Bonded Joint Of Al/Mg

Aluminum,magnesium and their alloys are widely used in modern manufacturing due to their low density,high strength and good machinability.Therefore,there must be a mixture use for both of them in actual life.But,the bottleneck of joining Al to Mg is the formation of Al-Mg intermetallic compounds?IMCs?.However,the ultrasonic-assisted transient liquid phase bonding can not only realize joining relying on low temperature eutectic reaction between substrate and interlayer,but also combine ultrasonic cavitation effect to break oxide film and accelerate dissolution,diffusion and reaction between base metal and interlayer.Finally,it can achieve weld with lower temperature in the atmosphere.In order to avoid the generation of Al₃Mg₂ and Al₁₂Mg₁₇,the experiments utilize Zn foil as interlayer through this method.First,the microstructure and mechanism of Mg/Zn/Mg joints were studied.It found that the weld of joint consisted of Mg+MgZn eutectoid structure,MgZn₂ solid compound and a thin layer of MgZn.And compound hazards from strong to weak were MgZn₂,MgZn,Mg+MgZn in turn.Thus,brittle fracture occured in MgZn₂layer for typical joint.In addition,welding parameters had regular effects on microstructure of joints,namely,weld width and MgZn₂ thickness.With the increase of welding temperature and ultrasonic time,the weld width and thickness of MgZn₂layer decreased.With the increase of interlayer thickness,the weld width and thickness of MgZn₂ layer increased.With the increase of ultrasonic power,the weld width increased first and then decreased slightly and the thickness of MgZn₂ layer gradually decreased.With the increase of welding pressure,the weld width first increased and then decreased.Further,it was concluded that the Mg+MgZn eutectoid structure in the weld was formed by Mg-Zn eutectic and eutectoid transformation,while MgZn₂ was formed by solid state diffusion.Furthermore,the joints were mainly broken in the MgZn₂ layer,MgZn₂ and Mg+MgZn layer?mixed fracture?,Mg+MgZn layer,and the corresponding strength were 30.0MPa,37.9MPa,39.4MPa,respectively.Secondly,the microstructure and mechanism of Al/Zn/Al joints were studied.It found that the weld of joint included Al-Zn eutectic structure,?-Al and?-Zn.Besides,welding parameters also had regular effects on microstructure,that was,weld width.With the increase of ultrasonic power,welding temperature,welding pressure and ultrasonic time,the width of the weld seam decreased.With the increase of the thickness of the interlayer,the weld width increased.Besides,it was concluded that the Al-Zn eutectic reaction formed a eutectic liquid phase and ultrasound accelerated the dissolution,reaction,and diffusion between the base metal and interlayer.Finally,more?-Al was formed on both sides of the weld and grew toward the center of the weld.Finally,the microstructure and mechanism of Al/Zn/Mg joints were studied.It found that the generation of Al₃Mg₂ and Al₁₂Mg₁₇was successfully avoided.And joint welds mainly consisted of MgZn₂ with distribution of small Al-Zn eutectoid microstructures and Mg+MgZn eutectoids.In addition,a small amount of MgZn and Al-Mg-Zn ternary compounds were formed in the welds.Further,MgZn₂ was more harmful than Mg+MgZn.Hence,the joint fracture was brittle fracture.Besides,the influence of welding process parameters on the microstructure of joint s presented a certain regularity,mainly in the width of the weld seam and the thickness of the MgZn₂.With the increase of ultrasonic power,welding pressure and ultrasonic time,the width of weld and the width of MgZn₂ decreased.With the increase of interlayer thickness,the width of weld and the width of MgZn₂ increased.As the welding temperature increased,the MgZn₂ in the weld decreased gradually until it completely disappeared.Furthermore,the welds near the Mg side produced a transient liquid phase bonding by the eutectic reaction,while the welds near the aluminum side formed connection by solid diffusion reactions among Al,Mg,and Zn components at a certain temperature.In addition,the joints were mainly broken in MgZn₂ and Mg+MgZn layers?mixed fractures?,MgZn₂ layer,and Al-Mg-Zn ternary compound thin layer.The corresponding joint strengths were 24.6MPa,15.8MPa,10.9 MPa,respectively.Therefore,Al-Mg-Zn ternary compounds were more harmful than MgZn₂.