| As an important strategy to save oil and reduce emissions,the effective way of automobile lightweight is the combination of aluminum alloy and high strength steel materials.However,the physical and chemical properties of steel and aluminum vary greatly,among which the brittle compounds with large grain size are easily formed with low mutual solubility,and the aluminum is easy to oxidize and form weld slag inclusion in the weld seam.The ultrasonic wave is put into the traditional resistance spot welding(RSW),which can effectively remove the oxide on the surface of workpieces,increase the fluidity and spreading property of liquid aluminum at the steel/aluminum interface,control the growth distribution of the intermetallic layer(IMC)and refine the grains in the nugget and heat affected zone.There are few researches on ultrasonic resistance spot welding(URW)technology at present.The welding mechanism,IMC growth and welding parameters on the quality of solder joint in the process of URW are systematically analyzed in this thesis,which provides theoretical basis for the application of URW.Taking ultrasonic resistance spot welding of TRIP780 steel and AA6061 aluminum alloy as the research object,the two-way indirect coupling finite element model of spherical electrode steel / aluminum resistance spot welding and ultrasonic resistance spot welding is established.By introducing ultrasonic softening rate,ultrasonic vibration pressure,plastic deformation heat and dynamic contact resistance,the comprehensive effects of electric field,temperature,ultrasonic wave and force field are fully analyzed.The deformation of welding joint and diameter of steel and aluminum nugget obtained from calculation are compared with those from experiment.Based on the finite element model of RSW and URW,this paper analyzes the dynamic changes of structure field and electric heating field,as well as the difference and relation between them.The main factors of heat analysis in welding stage are explored and the causes of "nugget migration" and "steel side protrusion" are revealed.Based on the temperature history and diffusion theory,the growth and distribution of the IMC at the interface are analyzed,and the causes of the RSW defects and the influence of ultrasonic wave on them are revealed.Based on the theory of response surface methodology optimization,the welding parameters such as welding current,electrode force,ultrasonic amplitude and welding time are used as the research object,and the welding quality is characterized by aluminum nugget diameter,indentation rate and comprehensive index based on both.By using the BoxBehnken method to design the experiment,the response equation of welding parameters and welding quality is obtained,and the influence law of welding parameters first term and interaction term on welding quality is studied.The response surface optimization methodology of welding parameters is carried out based on entropy method,and the predicted value of optimization results is in good agreement with the experimental value. |
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