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The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius Rn and the contact radius Rc between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet. [ABSTRACT FROM AUTHOR]

Resistance spot welding (RSW) is typically used in automobile body assembly, where continuous efficiency depends on electrode life, which is closely related with the water cooling effect. In this paper, the finite element simulation and response surface analysis methods are adopted to optimize the RSW electrode cooling process. Aiming to optimal cooling parameters and improve cool effect, we present a special sphere-shape weld cap with inlet pipe and cooling cavity and model its cooling effect as non-linear polynomial equations, which can avoid numerous influential factors. Electrode parameters, including pipe diameter, pipe height and flow velocity, are adjusted according to these non-linear equations. Optimization is carried out subsequently and experimental measurements on electrode diameters are performed for validation. It is found that the response surface model not only indicates the direction of design modification, but also leads to an optimal method for electrode design. [ABSTRACT FROM AUTHOR]

Spot weld fatigue performance of dual phase steels is of great interest to worldwide automotive manufacturers due to their expanding use in automotive industry. Given that the majority of the fatigue life is spent propagating the crack through the microstructure, changes in microstructure of welding steels could result in significant changes in spot weld fatigue life compared to that of low carbon steel spot weld. In the present study, tensile shear spot welds of DP600GI, DP780GI with different thickness were used for fatigue test under various amplitude tensile loads. For comparison tensile shear fatigue test was also conducted for spot welds of low carbon steel. Microhardness was measured for spot weld of dual phase steels to study the hardness change across the weld nugget. Failure mechanism and crack propagation path of dual phase steel spot weld was studied. For spot welds of low thickness, the fatigue strength of dual phase steel spot weld is higher than that of low carbon steel specimen in high cycle fatigue test. When thickness increases, the fatigue strength of dual phase steel spot weld and low carbon steel spot weld are almost the same, and fatigue strength of dual phase spot welds also increases with the thickness. And it is also shown in the present study that static tensile interfacial fracture of spot weld does not necessarily lead to interfacial fracture of high cycle fatigue tests. Crack propagation path of tensile shear spot weld of dual phase sheet steels under fatigue test was examined. Crack initiates at the faying surface around the weld nugget, and then passes through the thickness of sheet, which leads to the failure of specimens. [ABSTRACT FROM AUTHOR]

Galvanised sheet steels are now widely used to be the substrate for body in white (BIW) construction in the automotive industry. Weldability of galvanised sheet steels much worsened compared to spot welding of low carbon steels. The present paper develops a 2D axisymmetric model and employs an incremental coupled thermal–electrical–mechanical analysis to predict the nugget development during resistance spot welding (RSW) of galvanised sheet steels. Temperature dependent contact resistance for faying surfaces was determined to take into account of the influence of zinc coat for spot welding galvanised sheet steels. The effect of dynamic contact radii on temperature distribution was studied and compared with results under constant contact area assumption. The predicted nugget shape and size agreed well with the experimental data. Higher current and longer welding time should be applied for galvanised sheet steels compared to low carbon steel spot welding. The proposed model can be applied to predict weld quality and choose optimal welding conditions for spot welding galvanised sheet steels. [ABSTRACT FROM AUTHOR]

More galvanized steels are being used in auto body assembly to improve safety and fuel efficiency of automobiles. However, spot welding these materials will result in a short electrode life due to serious and inconsistent electrode wear. Electrode radial wear during the electrode life test is the main factor leading to unacceptable nugget size. This paper firstly studied the feedback characteristics of servo guns to detect electrode axial wear online based on the developed system of an integrated servo gun with robot. Then a mathematical model of a spherical electrode was established to evaluate radial wear by detecting axial wear online at different electrode wear stages. The model was verified by experiments for welding different galvanized steels. It was shown that evaluation results of the model agreed well with the experiment results. The model revealed that a smaller radius of electrode face at beginning welds would result in a higher electrode wear rate. The method of evaluating electrode wear online based on the model also offered a guidance to determine the suitable time for tip dressing in real production. [ABSTRACT FROM AUTHOR]

Resistance spot weldability is defined as the acceptable welding current ranges as determined by the weld lobe in resistance spot welding. Nowadays many studies have focused on the effect of welding current and welding time under constant electrode force on the weld quality and weldability. There is little research on the influence of variable electrode force on the weld quality and weldability because of the difficulty in controlling variable electrode force using pneumatic gun. In the present study, first, the influence of three stages of electrode force, including squeeze force, welding force and forging force, on the quality of welds is analysed. Then a design of experiment approach is applied to analyse the influence of the three stages of electrode force on welding quality and thus to obtain optimum parameter of variable electrode force by controlling the electrode force with servo gun. The comparisons of tensile shear strength, nugget size, weld lobe width and wear rate of electrode tip between variable force and constant force are carried out. The results show that the weld quality and weldability can be increased evidently using optimum parameter of variable electrode force without accelerating the electrode wear rate. [ABSTRACT FROM AUTHOR]

To reduce weight and improve performance, hydroformed tubes are being widely used in automotive structure fabrication and the single sided sheet to tube resistance spot welding (SSRSW) is considered as a feasible method for joining a tube to other parts. However, in the sheet to tube SSRSW process, it is difficult to assure welding quality because of large welding deformation due to a lack of support inside the tube. The present paper investigates the influences of welding parameters, such as electrode force and welding current, on the welding deformation and quality of the sheet to tube SSRSW using electrode displacement and tensile shear tests. The effects of different electrode force patterns on the welding quality are investigated utilising the force characteristics of a servo gun. It is found that the welding deformation is influenced by both the electrode force and the welding current, and the tensile shear strength declines with larger electrode force and higher welding current. However, the tensile strength could be enhanced significantly and the welding deformation decreased greatly by reducing the electrode force in the welding stage or holding stage. In order to decrease manufacturing cost and improve weld quality, the reduced electrode force is recommended for the sheet to tube SSRSW process. [ABSTRACT FROM AUTHOR]

Resistance spot welding is one of the major joining techniques widely used for car body assembly. Weld quality may significantly influence the durability and reliability of the automobile body. Automotive manufacturers often rely on destructive testing and monitoring variables which indirectly reflected weld quality to assess the weld quality and control the welding process. However, these approaches have inherent limitations and are difficult to be implemented in plant environments. Therefore, it is imperative to develop an online inspection method to evaluate weld quality. In the present study, a method of producing a series of substantially uniform spot welds between two metal parts using a servomotor driven movable electrode and an axially opposing fixed electrode is proposed. The indentation in the workpiece surface is suitably measured by the displacement of the movable electrode as it applies an electrode force and welding current is passed through the weld site of the workpiece. The optimal indentation range is determined by peel test and metallographic examinations with respect to various sheet gages and grades. Consequently, online weld quality inspection results are achieved based on developed optimal indentation range. [ABSTRACT FROM AUTHOR]

Spot welds are used extensively by automobile manufacturers as an efficient method for joining sheet steel. The quality of them can be tested non-destructively by using ultrasound. But when spot weld defect happens, the current ultrasonic inspection methods of weld quality are difficult to achieve an ideal result especially for the stick weld defect which is one of the most important types of spot weld defects in the automotive body. At first, this paper detailedly analyses the echo characteristics of ultrasonic curves which can reflect different spot welding joint defects. After echo characteristics qualitative analysis of different spot weld defects, a peak value marking algorithm is developed to identify the joint defects especially the stick weld defect rapidly and efficiently through selecting and confirming many optimal characteristic parameters. Finally, a lot of experiments are performed to verify the proposed methods. The results indicate that this fast identification method is credible and the identification rate can reach 95% in total test samples. [ABSTRACT FROM AUTHOR]