| During the last few years, with the developments of the recognition of environmentsand the requirements for the safety, the automotive body structure and materials arechanging greatly. For vehicle lightweight and energy saving, hydroforming technique andhigh strength steels (HSS) have been widely used in vehicle structure. Because of highefficiency and low costs, resistance spot welding is the mainly method for joining.However, in sheet to tube spot welding, welding deformation often occurs because of lowtube stiffness, and elliptical nugget is formed even with cracks. Moreover, because ofmaterial properties and limitation for purpose of minimizing the weld flange, welds mustbe increased to assure welding quality. Through controlling the aspect ratio, ellipticalnugget can stabilize welding quality without increasing welds, and research on theinfluence of elliptical nugget on welding quality is seldom addressed. Therefore, in thepresent paper, research works are conducted on nugget formation mechanism and weldingquality inspection method of elliptical nugget.Based on the above backgrounds, experimental system of elliptical nugget isestablished firstly, and formation mechanism is studied by3D numerical simulation.Relationship between weld length and critical specimen size is determined combinedexperiments and logistic statistic analysis. Then relationships between weld attributes andwelding quality are established with numerical model of tensile-shear test and Latinhypercube design, and prediction of welding quality is achieved. Furthermore, ellipticalnugget size in sheet to tube spot welding is analyzed and influences of process andstructural parameters on welding quality are investigated. Last, decomposition of nuggetformation and tube deformation is proposed based on the electrode displacement, andcharacteristic of elliptical nugget can be analyzed, which provide reference for sheet totube spot welding quality control. Mainly research works are listed as follows1. Numerical simulation and experimental study on critical size of elliptical nuggetin spot welding In order to obtain elliptical nugget, experimental system of elliptical nugget isestablished, and formation mechanism is studied by simulation. Relationship betweenelectrode size and nugget shape is also verified. Combined experimental study and logisticanalysis, critical specimen sizes of DP600are determined and verified by multi-weldsstructure. Compared with circular nugget, elliptical nugget can minimize interfacialfracture and increase weld space.2. Prediction of tensile-shear strength for elliptical nuggetThere are many factors influenced tensile-shear strength, including nugget size andmaterial attributes. In order to analyze the effects of weld attributes on tensile strength,numerical model of tensile-shear test is established. The critical ellipticity is determinedand relationship between weld attributes and tensile strength is obtained. Last, ellipticalnugget size and influence of structural parameters on ellipticity are investigated further.3. Influence of process and structural parameters on elliptical nugget in sheet totube spot weldingSheet to cylindrical tube spot welding is investigated for complexity of vehiclecomponents. Manufacture dies and welding clamps are designed and experimental systemis established. Without support inside, welding deformation occurs and elliptical nugget isformed. The elliptical nugget is studied firstly by microstructure and hardness test.Influences of process parameters on welding deformation and tensile strength are alsoinvestigated. Then effects of electrode force, tube thickness and tube radius on ellipticalnugget welding quality are researched combined analysis of dynamic resistance.4. Characteristic analysis of elliptical nugget in sheet to tube single sided spotwelding based on electrode displacementBecause of special structure, welding deformation happens in sheet to tube spotwelding, and cold weld, expulsion and crack often occur. In order to inspect weldingquality of elliptical nugget in sheet to tube spot welding, the electrode displacement isdecomposed, and nugget formation and tube deformation are obtained.3D finite elementmodel of sheet to tube spot welding is established and the proportion of welding energy isdetermined to get the course of nugget formation. Through analysis of electrodedecomposition, characteristic of welding defects in sheet to tube spot welding can be inspected.Based on research above, combined numerical simulation, experimental design andstatistical analysis, relationship between nugget length and critical specimen size isdetermined, and weld space can be increased and interfacial fracture can be improved.Relationships between weld attributes and tensile strength are also established, and tensilestrength is evaluated. The influence of process and structural parameters on ellipticalnugget quality in sheet to tube spot welding is analyzed. And decomposition of nuggetformation and tube deformation is proposed based on the electrode displacement toanalyze elliptical nugget characteristic, which provide reference for process analysis,parameter selection and quality control. Research results have high theory foundation andapplication value. |
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