| The demands for lightweight bodies and high safety requirements have promotedthe widespread use of Advanced High Strength Steels (AHSS) in car bodymanufacturing. Owing to its combination of high strength and ductility, Dual-Phase(DP) steel has become one of the most promising AHSS used in lightweight bodies.Spot welding is the major joining method in car body assembly due to its advantagessuch as low cost, high efficiency and easy automation. However, DP steel added withmore alloying elements causes that the spot welding process has a large coolingtemperature gradient and that the spot-welded DP steel is prone to exhibit the weldinterfacial fracture. It will reduce the cross tension strength by about10%and lowcycle fatigue strength by about25%. The development of toughened adhesive hascontributed to an increasingly important role of adhesive bonding in mechanicalfastening because of its high static and fatigue strength, and being convenient forjoining of dissimilar composite materials, etc. However, the adhesive-bondedspecimens are often easily damaged in certain conditions, such as low temperatureand high humidity. Therefore, weld-bonding, a combination of spot welding andadhesive bonding, has been the subject of particular attention in automotive industrysince it displays all the advantages of these two processes. However, some prominentissues occur in the weld-bonded DP steel such as being prone to exhibit the weldexpulsion and a narrow weld lobe. The weld expulsion can not only result in somedefects in the nugget but decrease the joint strength by about20%. The weld lobewidth of the weld-bonded DP steel is lower than the acceptable width,2kA. Adiscrepant weld emerges when welding current fluctuates or electrode caps wearduring welding, which causes a bad robustness in the weld-bonded DP steel. Theunderlying reason is the complicated coupling between the adhesive flow and heattransfer behavior of the weld nugget, and the difficult cooperative control between thetemperature and force field in the nugget region of weld-bonding. Therefore, it is imperative to explain the narrow weld lobe of the weld-bonded DP steel, explore theeffect of the electrode force on the mechanism of the weld-bonded nugget formationand widen the weld lobe through controlled electrode force. This dissertation isconducted under this situation.Based on the background mentioned above, DP steel is selected as the researchmaterial. The nugget quality of the weld-bonded DP steel is analyzed and thecontrolled electrode force is proposed to settle the narrow weld lobe. Firstly theexperiments about mechanical properties and microstructure of the weld-bondednugget are conducted. The influence of the adhesive layer on the nugget quality isanalyzed to show the underlying reason of the narrow weld lobe in the weld-bondedDP steel. Based on it, the balance equation of the electrode force during weldingstage, the adhesive bonding force and the effect of the temperature field in thenugget region on the force from the liquid nugget are set up. Moreover, a finiteelement model is established to simulate the temperature field in the weld-bondednugget region. The effect of the electrode force on the mechanism of theweld-bonded nugget formation is discovered. Then the effect of the electrode forceduring squeeze stage on the contact resistance between the sheet metals, and thecharacteristics of different regions in the dynamic resistance between electrodesduring welding stage are analyzed in order to discover the mechanism of theweld-bonded nugget formation. Finally using the regression method, the relationshipbetween the critical electrode force and welding parameters is established. Thestrategy of the controlled electrode force is determined and verified throughexperiments. This dissertation is aiming at settling the narrow weld lobe in theweld-bonded DP steel through controlled electrode force and developing a newmethod to control the nugget quality of the weld-bonded DP steel. The main researchis listed as follows.（1） Experimental study of the weld-bonded nugget qualityIn order to explore the narrow weld lobe in the weld-bonded DP steel, thispaper introduces the procedure and characteristic of the weld-bonded DP steel andthen compares mechanical properties (including tensile-shear force, fracture mode, energy absorption and weld lobe) and microstructure (including nuggetmicrostructure and microhardness) of the weld-bonded nugget with those of thespot-welded nugget. The underlying reason of the narrow weld lobe in theweld-bonded DP steel is obtained. It is proposed that the controlled electrode forcecan be used to widen the weld lobe. Results show that the weld-bonded specimenhas a larger nugget size, a higher tensile-shear force and energy absorption, andexhibit the button-pullout fracture more easily at lower welding currents while atrelative higher welding currents weld-bonding is prone to exhibit the weld expulsiontogether with the occurrence of the partial interfacial fracture. The weld expulsionhas a negative influence on the tensile-shear force and energy absorption. Thismakes the weld lobe of the weld-bonded DP steel too narrow. Based on it, thepreliminary experimental results show that the change in the electrode force canimprove the weld lobe in the weld-bonded DP steel, which will provide the datasupport for the research on the nugget quality of the weld-bonded DP steel based oncontrolled electrode force.（2） Analysis on the effect of the electrode force on the mechanism of theweld-bonded nugget formationIn order to discover the effect of the electrode force on the mechanism of theweld-bonded nugget formation, this paper analyzes the effect of the electrode forceon the process of the weld-bonded DP steel. Through the fundamental mechanicaland thermodynamic analysis, the balance equation of the electrode force duringwelding stage, the formula of the critical electrode force to balance the liquid nuggetformation, the adhesive bonding force and the relational model between the forcefrom the liquid nugget and temperature field in the nugget region are set up.Moreover, a finite element model is established to simulate the temperature field inthe weld-bonded nugget region. Results show that the balanced state is decided bythe interaction between the electrode force during welding stage, the adhesivebonding force and the force from the liquid nugget. The force from the liquid nuggetis determined by material properties of DP steel at solid and liquid state, the radiusand thickness of the liquid nugget, and the temperature field in the nugget region. This will provide the theory basis for determining the amplitude of the controlledelectrode force.（3） Research on the mechanism of the weld-bonded nugget formation basedon dynamic resistanceThe electrode force decides the contact resistance between the sheet metals. Thedynamic resistance between electrodes during welding stage can be used to discoverthe mechanism of the nugget formation. This paper uses the contact theory andexperimental results to investigate the influence of the electrode force on the contactresistance between the sheet metals. With the help of the solutions of the finiteelement model focusing on the temperature field in the weld-bonded nugget region,the characteristics of different regions in the dynamic resistance between electrodesduring welding stage are analyzed in order to discover the mechanism of theweld-bonded nugget formation. The difference of the nugget formation between theweld-bonded and spot-welded DP steel is analyzed. Results show that the higherelectrode force during squeeze stage can increase the amount and diameter of a-spotand thus decrease the contact resistance between the sheet metals. The presence ofthe adhesive layer at the indentation of electrode caps mainly influences the contactresistance and contact area between the sheet metals before the nugget formation.This leads to a large heat produced during the initial welding stage. Based on it, theelectrode force needs to be adjusted between the squeeze and welding stage. Inaddition, the electrode force during welding stage also needs to be adjusted to ensurethe weld-bonded nugget size and avoid the weld expulsion.（4） Determination of the strategy of the controlled electrode force focusingon controlling the weld-bonded nugget qualityBased on the above theory and experiment analysis, the influence of thecontrolled electrode force on the nugget quality of the weld-bonded DP steel isestablished. In order to determine the strategy of the controlled electrode force in theweld-bonded DP steel, this paper compares the solutions of the finite element modelwith experimental results to calculate the critical electrode force, and thus analyze the effect of the welding parameters (i.e. welding current, welding time and electrodeforce) on the temperature field in the nugget region, the force from the liquid nuggetand the critical electrode force. The relational model between the critical electrodeforce and welding parameters is set up to determine the moment and amplitude ofthe controlled electrode force.1.2mm DP780steel is used to conduct thecomparative experiments using the controlled and constant electrode force in termsof nugget size, dynamic resistance and weld lobe. Results show that the controlledelectrode force can inhibit the weld expulsion and then increase the nugget size byalmost10%at high currents. Moreover, the weld lobe of the weld-bonded DP steel iswidened by almost50%when the controlled electrode force is used.Based on the contents mentioned above, a general and systemic understandingof the nugget formation of the weld-bonded DP steel is conducted. The issues aboutthe nugget quality of the weld-bonded DP steel and corresponding solutions are fullyinvestigated. This dissertation focuses on controlled electrode force to improve thefluctuation in the nugget quality of the weld-bonded DP steel and develop a newmethod to control the nugget quality of the weld-bonded DP steel based on electrodeforce. It will provide the theory and experiment basis for the further implementationof the weld-bonded DP steel in automobile manufacturing. |
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