Study On The Ring Nugget Formation And Performance In Sheet-to-Tube Single Sided Spot Welding

Concerns about fuel economy, safety, reliability and performance are continuously driving auto designers to use new materials, structures and techniques in auto designs. Hydroforming tubes as new structures are being currently used to manufacture several automotive parts included in car body systems due to its lower weight and increased structural integrity. How to integrate the hydroformed tubes without flanges into vehicle body structure becomes a new challenge.At present, resistance spot welding (RSW) technology is the most dominating joining and assembly method in auto body manufacturing process. As a result, it is significative to join sheet to tube using RSW technology. An innovation welding method——single-sided spot welding (SSSW) is developed to perform the joining of sheet and tube. However, new challenging issues limit the use of SSSW in auto industry. The SSSW approach attaches a sheet to a closed tube without inside support. A bottom block as an outside support is used for force transmission and current flow. The weak structure is the clearest characteristic of the sheet-to-tube SSRSW. As a new type of resistance welding process, the nugget formation mechanisms and its intensities of SSSW are greatly different from that of conventional RSW. The quality of the ring nugget is difficult to be guranteed under constant electrode force using traditional pneumatic gun.Depending on the issues mentioned above, this dissertation, firstly, analysed the difference between the sheet-to-tube welding and conventional sheet-to-sheet welding. Based on the characteristics of SSSW, a welding system using servo gun is built. Secondly, a multi-physical coupled finite element (FE) model for SSSW is proposed to study the nugget formation process based on commercial FE code ANSYS. Thirdly, a set of tensile-shear tests were designed based on Latin hypercube sampling and were performed by FE method. The objective is to research the effects of the physical and geometrical factors on the strength of the ring nuggets. Forthly, the effects of the welding process parameters on the quality of nuggets were deduced based on above models and the correlation rules were revealed by experimental methods. Last, considering the forcing characteristic of servo gun and the progress parameter rules of SSSW, a new method that the quality of SSSW welds could be improved by verifying electrode force during welding stage was presented. All those are critical to the research and application of SSSW. The following five works were done mainly:(1) SSSW physical process and testing systemThe structural stiffness of SSSW is much smaller than that of the conventional RSW. Consequently,there exists large deformation during welding process, which influences directly the nugget formation process and nugget form. The smooth feature in applying electrode force of servo gun meets the need of SSSW. So, servo gun experimental system including servo gun, robot and welding controller was set up. In order to fix the weldments during SSSW, a special flexible clamping fixture was designed. In addition, an acquisition system of electrode displacement including laser sensor with high precision, acquisition processor, and computer was built to supply testing parameters for verifying the solution of simulation.(2) FE multi-physics analysis of SSSW ring nugget formation processA parametric electro-thermal-mechanical coupled FE model was built to research the squeeze and welding process. Results show that two ring contact regions come into being at the interface of both electrode/sheet and sheet/tube. Smaller contact area and greater pressure is obtained around the bottom of electrode than that between the sheet and tube. In addition, there exists an obvious minor contact region between sheet/tube along the faying surface far away from the main contact region and its pressure is much smaller than that of the main contact region. At the beginning of welding,there is a high temperature region between electrode and sheet, which may accelerate the wear of upper electrode. During the welding stage, the high temperature softens material and more fierce deformation occurs, which leads to the contact region and temperature zone at sheet/tube interface transferring outward slowly and makes it difficult to form nugget. Owing to the shunting by minor contact region and the dynamic change of deformation, expulsion exists. At the end, a ring nugget formed.(3) Investigation on the mechanical performance of ring nugget in sheet to tube joiningIn SSSW for sheet-to-tube, a ring nugget is obtained. The form and size of ring nugget are different from that of button nugget in two sheets joining. Conventional button nugget quality inspection is not preferred for ring nugget quality prediction. In this chapter, a detailed computer model of ring nugget formed in sheet-to-tube joining was presented and a computer design of experiments method was used for carrying out to evaluate the strength of spot weld. The regression model between weld quality and weld attributes were established, depending on which the ring nugget strength could be estimated.(4) Study on the influencing factors of weld quality for SSSWBased on the above models and results, the effects of process parameters including welding current, time, and electrode force on the nugget formation were analyzed theoretically and tentatively. Aimed to get good welds, the characteristic of process parameters are achieved for workpieces with different physical dimensions including the width of tube cross section and the thickness of tube. All those are critical for practical application of SSSW.(5)Weld performance optimization of sheet-to-tube single sided spot joining based on electrode pressure changeDepending on the characteristics of servo gun and the effect rules of weld conditions on the weld strength of sheet-to-tube joining, it is investigated that the weld performance can be increased by changing electrode pressure during welding progress. The comparations among the weld performances with different electrode pressure styles are carried out. The results indicate that the electrode pressure decreasing during current stage can increase the strength of sheet-to-tube weld clearly, decrease the deformation of the weldment during the weld stage, improve the surface quality of weldment, and upgrade the weldability of sheet to tube.The dissertation researched and revealed the nature rules during SSSW process, the mechanical properties of nugget in sheet to tube joining, and the influencing factors of weld quality on numerical analysis and some necessary experiments validation. The research results have significant guidance meaning in further understanding SSSW process and its widely application in auto production-line. Above all, the research fruits in this dissertation have very important value and practice significance.