Study On Mechanism And Process Of Electric-aided Self-piercing Riveting Of Dual-phase Steels

For realizing the development goals of energy saying and emission reduction of automobile,besides to promote the development of energy efficient and new energy vehicle,the lightweighting of auto-body has also been taken as great importance by automobile industry.Considering the weight loss effect,mechanical properties,safety,corrosion resistance,retrievability,and the application cost of advanced high strength steels(AHSS)and the other light alloys,it is inevitable that the extensive mixed application of high strength ratio materials,e.g.AHSS and aluminum alloys together in body-in-white manufacturing will and has already become the main method to achieve the vehicle lightweighting all over the world.However,the hybrid use of multi-materials in automotive industry poses great challenges on the conventional joining processes for manufacturing vehicle body.Nowadays,resistance spot welding is the most widely used joining process in traditional automobile body manufacturing.Nevertheless,the physical properties of lightweight alloys and advanced high strength steel are quite different such as electric conductivity,thermal conductivity and melting point.Furthermore,the hard and brittle Fe-Al intermetallic compounds(IMC)can form easily along the faying interface during the fusion welding process.Therefore,it is difficult to join the two materials effectively with conventional resistance spot welding process.Self-piercing riveting is a cold forming process with high level of automaticity,which could avoid the problems caused by the large heat input during fusion welding process and join the dissimilar materials like lightweight alloys and advanced high strength steel successfully.However,this process based upon forming technology faces the new challenges since the strength of advanced high strength steel increased constantly.Electric-aided technology is an energy-saving and efficient process which utilizes the electro-plastic effect of materials.This process applies external current intensively during the material forming process to achieve better deformation through reducing the deformation resistance of materials.Up to now,electric-aided technology is widely used in the fields of wire drawing,strip rolling and plate bending,etc.Based on the above background,a new joining technique named electroplastic self-piercing riveting process was proposed in this thesis through integrating the electric-aided technology and traditional self-piercing riveting process.This process is aimed to improve the riveting quality,which takes advantage of the electro-plastic effect of materials to reduce the deformation resistance by applying current with high density on the high strength steel during riveting.On account of this research object,a material electric-aided,uniaxial tensile testing system was built firstly to study the change regularity of the electric conductivity and mechanical properties as the current density varied.Therefore,the prediction model of temperature field and material electric-aided constitutive model which decoupled the effect of heat and electricity were obtained.Based upon the aforementioned models,the feasibility and principle of electroplastic self-piercing riveting process were investigated through the finite element simulation method.At last,an electric-aided self-piercing riveting process prototype system was built to validate the electro-plastic effect by riveting typical dissimilar materials used on the vehicle like dual phase steel and aluminum alloy.The specific research contents are as follows:1)Research on the electrical-thermal properties of electric-aided processingof dual phase steelsBased on the Matthiessen's rule of resistivity,the effects of temperature and strain on the material resistivity were investigated under the condition of low current density.Besides,the electric-aided uniaxial tensile test system was established to study the effect of current density on the material resistivity under the conditions of high current density and large material deformation.With the understanding of the particular characteristics of electric-aided process,a comprehensive resistivity model which considers temperature,strain,current density and electrical effects was established.Finally,the model which is used to theoretically predict the temperature evolution during the electric-aided processing of dual phase steel was established.2)Establishment of the electro-plastic material constitutive model of dualphase steelThe effects of applied current to the elastic and plastic characteristics of material were investigated through conducting the electric-aided uniaxial tensile tests of dual phase steel.Based on this and considering the results of isothermal tensile tests,the Johnson-Cook(JC)constitutive parameters of dual phase steel were determined.Then,the modified JC constitutive model which considers the electroplastic effects,was deduced through comparing the theoretical predicted value of JC model and the experimental value of the electroplastic uniaxial tensile tests.3)Simulation analysis of the joint formation mechanism of electric-aidedself-piercing riveting processBased on the temperature prediction model of electric-aided processing of dual phase steel and the modified JC constitutive model which considers the electroplastic effects,the coupled thermal-mechanical numerical model of electroplastic self-piercing riveting process was established.The effects of applied current to the joint geometry,stress and strain distribution of self-piercing riveted joints were analyzed.4)Typical application of the electric-aided self-piercing riveting processAccording to the simulation results,an electric-aided self-piercing riveting prototype system was developed from the traditional self-piercing riveting equipment.The effects of applying current during the self-piercing riveting process on the joint formation,geometry,microhardness and mechanical performance were investigated with the dual phase steels of different grades,which laid the foundation for the industrial applications of this method.