A MULTI-COUPLED FINITE ELEMENT ANALYSIS OF RESISTANCE SPOT WELDING PROCESS

A two-dimensional axisymmetric finite element model is developed to analyze the transient thermal and mechanical behaviors of the Resistance Spot Welding （RSW） process using commercial software ANSYS. Firstly a direct-coupled electrical-thermal Finite Element Analysis （FEA） is performed to analyze the transient thermal characteristics of the RSW process. Then based on the thermal results a sequential coupled thermo-elastic-plastic analysis is conducted to determine the mechanical features of the RSW process. The thermal history of the whole process and the temperature distribution of the weldment are obtained through the analysis. The mechanical features, including the distributions of the contact pressure at both the faying surface and the electrode-workpiece interface, the stress and strain distributions in the weldment and their changes during the RSW process, the deformation of the weldment and the electrode displacement are also calculated.

Study of joining mechanism of ABS polymer and steel/aluminum by resistance spot welding

The joining of metal and polymer is an increasingly important method to get lightweight components in the development of manufacturing industry- nowadays. In this artiele, metal and polymer lap joint was achieved by means of resistance spot welding （RSW） and ultrasonic assistance welding （UAW）. The joining mechanism of lap joint was analyzed by OM, TEM on microstructure at the interface of lap joints and XPS and IR spectra was discussed based on the following different ones： mechanical-interlocking, diffusion bond and coordination bond. The results showed that it was the combined action that played an important role in the effective joining work. Besides, ultrasonic assistance was used in the study to aid welding process based on its high-frequency ultrasonic vibration, which made joints shaping better and improved tensile strength visibly contrast to joints with the same lower heat input parameters.

Numerical simulation on temperature field for resistance spot welding of non-equal thickness stainless steel

An axisymmetric finite element model is developed to simulate the temperature field of resistant spot welding according to the process characters of nugget formation of non equal stainless steel sheets. A simulation method of the interaction of electrical and thermal factors is presented. The spot welding process of nugget formation is simulated using hard and soft welding technique norms. The heating characters of soft and hard norms determine the differences in the process of nugget formation and determine the finally shape and offset of nugget. Experimental verification shows that the model prediction agrees well with the practical.

Comparative study of resistance spot welding performance between cold-rolled DP980 and Q&P980 steels

Advanced high strength steel (AHSS) has been widely used in the automobile industry.The resistance spot welding performance of DP980 and Q&P980 steels was studied through comparing the two steels’welding current range,tensile shear strength (TSS),cross tension strength (CTS),weld spots’microhardness,etc.The following conclusions were achieved:It is easy for both DP980 and Q&P980 steels to get a nugget size bigger than 4 mm,they all have welding current ranges exceeding 2 kA and high weld strength.

Numerical and Experimental Study on Nugget Formation Process in Resistance Spot Welding of Aluminum Alloy

The weld nugget formation in the resistance spot welding(RSW) of aluminum alloy was investigated in the present study. The nugget formation process was directly observed by using a digital high-speed camera. Numerical simulation was also employed to investigate the nugget formation process. The results showed that for the RSW of two aluminum alloy sheets, a nugget was first formed in the workpiece/workpiece(W/W) interface and grew along the radial direction and axial direction of the sheets, and then it became a large elliptical nugget. For the RSW of three aluminum alloy sheets, two small nuggets were firstly formed in two W/W interfaces and grew along the axial direction and radial direction; finally they fused into one nugget. Besides, there existed a critical welding time, after which the nugget size remained nearly unchanged. This indicates that a long welding time is unnecessary for the RSW of aluminum alloy. In addition, the calculated nugget radius was compared with the experimental results, which showed that the simulation results agreed well with the experimental results.

Interfacial microstructure and property of resistance spot welding joints of titanium to stainless steel

Commercially pure titanium and stainless steel sheets were welded using the technique of resistance spot welding with an aluminum alloy insert. The interfacial microstructure of the joint was observed and analyzed using electron microscopy; the tensile shear strength was investigated. An approximate 160 nm thick layer of Al solid solution supersaturated with Ti was observed at the interface between titanium and aluminum alloy. The solid solution layer contained the precipitates TiAla. And an approximate 1. 5 μm thick serrate reaction layer was observed at the interface between stainless steel and aluminum alloy. The maximum tensile shear load of 5.38 kN was obtained from the joint welded at the welding current of 10 kA. The results reveal that the property of the joint between titanium and stainless steel can be improved by using an aluminum alloy insert.

Filling technique for keyhole of friction stir spot welding based on the principles of resistance spot welding

Keyhole at the end of a weld prepared by friction stir welding(FSW)is one of the major issues that impede the application of FSW.To address this issue,a keyhole filling technique was proposed in this paper,which is based on the principles of resistance spot welding(RSW).A three-phase secondary rectifier resistance spot welder was applied as the experimental instrument for filling the keyhole in the center of friction stir spot weld(FSSW).The test sheet is a 2024-T4 aluminium alloy with a thickness of 6.0mm.The experiments results show that the filled joint strength is improved by 26.12%since the area is increased for the plug in the keyhole.And there are two kinds of dimples in the tensile fracture-equiaxial dimples and long dimples.The filled joint involves the fusion welding zone(FWZ),pressure welding zone(PWZ),melted plug zone(MPZ),and plastic deformation zone(PDZ).The FWZ and the PWZ is the melting bond and diffusion bond between the plug and keyhole,respectively.The MPZ is the center part and the PDZ is upper or lower part of the plug.

Dynamic simulation of resistance spot welding of zinc-coated steels

A model was developed to simulate the temperature distribution and nugget formation during resistance spot welding （ RS1V） of zinc-coated steels. It employs a coupled thermal-electrical-mechanical analysis simulating the dynamic RSW process. Temperature-dependent thermal-electrical-mechanical material properties were considered including contact-resistance. The contact area was determined from a coupled thermal-mechanical analysis. A layer of transition elements was used to represent the change of contact area by killing or activating elements. The heat generation and temperature field were computed in a coupled thermal-electrical model. All these analyses were solved using the commercial finite element method （FEM） based on ANSYS code, and some advanced functions were used by writing a paragraph of codes by the authors. Compared with the results from only coupled thermal-electrical model in which contact area was uniform during the whole process, the result matches better to the experimental results.

A cooperative control strategy of resistance spot welding process by combining the constant current control with the DRC method

The modeling control method based on the dynamic resistance characteristics of good nuggets, that is the DRC method, is an improvement on the dynamic resistance threshold method for the quality control of resistance spot welding. But there is still a control blind area in the initial four cycles. For this reason, the quality of every weld nugget could not be fully ensured. Thus a new fuzzy cooperative control method is put forward. It uses a multi-information time-control mechanism by combining the constant current control technology with the DRC method in a relay way. This whole-process control strategy has led to a good control effect and produced the dual-identical results in the weld nugget quality and the welding time.

Quality estimation of resistance spot welding of stainless steel based on BP neural network

The end value of the dynamic resistance curve of stainless steel was proved to have strong correlation with nugget size by experiments, so it was an important factor for estimation of weld quality. BP neural network was employed to estimate the weld quality, The end value of the dynamic resistance curve, welding current and welding time were selected as the input variables while the nugget diameter, which is closely related to weld quality, was selected as the output variable. Testing results shows that such network has fine fault tolerance and real-time quality estimation is possible.

Reconstruction of Current Density Distribution in Weld Area During Resistance Spot Welding of Aluminum Alloy Based on Magnetic Field

Magnetic flux density around the weld area was used to reconstruct the current density distribution during resistance spot welding(RSW) of aluminum alloy according to inverse problem theory. A current-magnetic field model was established and the conjugate gradient method was used to solve this model. The results showed that the current density was low at the center of nugget while high on the edge of nugget. Moreover, the welding time of 30ms—60 ms is a key period for nucleation. The current density distribution can reflect whether the weld nugget is formed or splashed, therefore it has the potential to monitor the weld quality of RSW.

Effect of retained austenite and nonmetallic inclusions on the thermal/electrical properties and resistance spot welding nuggets of Si-containing TRIP steels

Five advanced high-strength transformation-induced plasticity(TRIP) steels with different chemical compositions were studied to correlate the retained austenite and nonmetallic inclusion content with their physical properties and the characteristics of the resistance spot welding nuggets. Electrical and thermal properties and equilibrium phases of TRIP steels were predicted using the JMatPro? software. Retained austenite and nonmetallic inclusions were quantified by X-ray diffraction and saturation magnetization techniques. The nonmetallic inclusions were characterized by scanning electron microscopy. The results show that the contents of Si, C, Al, and Mn in TRIP steels increase both the retained austenite and the nonmetallic inclusion contents. We found that nonmetallic inclusions affect the thermal and electrical properties of the TRIP steels and that the differences between these properties tend to result in different cooling rates during the welding process. The results are discussed in terms of the electrical and thermal properties determined from the chemical composition and their impact on the resistance spot welding nuggets.

Effect of welding current on strength and microstructure in resistance spot welding of AZ31 Mg alloy

In this paper, resistance spot welding were performed on lmm-thickness magnesium AZ31B plates. The effect of welding current on the microstructure and tensile shear force was investigated. It was found that the welding current governed the nugget growth, and the nugget could not form if current levels were insufficient. The nugget revealed a homogeneous, equiaxed, fine-grained structure, which consisted of non-equilibrium microstructure of α-phase dendrites surrounded by eutectic mixtures of α and β（ Mg17All2 ） in the grain boundaries. With increasing welding current, the size of grains in nugget would be more smaller and uniform, and the width of plastic rings would be larger. Tensile shear tests showed that tensile shear force of the joints increased with increasing welding current when the welding current was smaller than 17 000 A. The maximum tensile shear force was up to 1980 N.

Measurement of dynamic resistance in resistance spot welding

The conventional methods of determining the dynamic resistance were mostly done by measuring the voltage and current at secondary side of transformer in resistance welding machines, in which the measuring set-up normally interferes with the movement of electrode, and the measuring precision is influenced by inductive noise caused by the high welding current. In this study, the dynamic resistance is determined by measuring the voltage at primary side and current at secondary side. This increases the accuracy of measurement because of higher signal-noise ratio, and allows to apply to in-process system without any wires connected to electrodes.

On-line evaluating on quality of mild steel joints in resistance spot welding

A method was developed to realize quality evaluation on every weld-spot in resistance spot welding based on information processing of artificial intelligent. Firstly, the signals of welding current and welding voltage, as information source, were synchronously collected. Input power and dynamic resistance were selected as monitoring waveforms. Eight characteristic parameters relating to weld quality were extracted from the monitoring waveforms. Secondly, tensile-shear strength of the spot-welded joint was employed as evaluating target of weld quality. Through correlation analysis between every two parameters of characteristic vector, five characteristic parameters were reasonably selected to found a mapping model of weld quality estimation. At last, the model was realized by means of the algorithms of Radial Basic Function neural network and sample matrixes. The results showed validations by a satisfaction in evaluating weld quality of mild steel joint on-line in spot welding process.

Electrical-thermal interaction simulation for resistance spot welding nugget process of mild steel and stainless steel

A three dimensional finite difference electrical thermal model for resistance spot welding nugget process of mild steel and stainless steel is introduced. A simulation method of the interaction of electrical and thermal factors is presented. Meanwhile, calculation method of contact resistance and treatment method of heater structure is provided. The influence of the temperature dependent material properties and various cooling boundary conditions on welding process was also taken into account in the model. A method for improving the mild steel and stainless steel joint was analyzed in numerical simulation process. Experimental verification shows that the model prediction agrees well with the practice. The model provides a useful theoretic tool for the analysis of the process of resistance spot welding of mild steel and stainless steel.

Optimization design of resistance spot welding parameters of magnesium alloy

By means of the quadratic regression combination design process, the regression equations of nugget diameter and tensile shear load of spot welded joint were established. Effects of welding parameters on the nugget diameter and the tensile shear load were investigated. The results show that effect of welding current on nugget diameter is the most evident. And higher welding current will result in bigger nugget diameter. Besides, interaction effect of electrode force and welding current on tensile shear load is the most evident compared with others. The optimum welding parameters corresponding to the maximum of tensile shear load have been obtained by programming using Matlab software, which is 4, 7 kN electrode force, 28 kA welding current and 4 cycle welding time. Under the condition of the optimum welding parameters, the joint having no visible defects can be obtained, nugget diameter and tensile shear load being 6. 8 mm and 3 256 N, respectively.

Quality estimation of the resistance spot welding based on pattern feature of the electrode displacement signal

The electrode displacement signal of the resistance spot welding process is monitored and mapped into a binary matrix. Some welded spots, from different welding current specifications, are classified into five classes according to the prototypes of the pattern matrices. A reliable quality classifier is developed based on Hopfield network when the tensile shear strength of the welded joint is measured as the quality indicator. The cross validation test results show that the method utilizing pattern matrix of the displacement signal to characterize nugget formation process is feasible and it can provide adequate quality information of the welded spot. At the same time, under small sample circumstance, the classifier presents good classification ability and it also can correctly estimate the weld quality in some abnormal welding process according to the pattern feature of the displacement signal.

Effect of upsetting force on microstructure of welds in resistance spot welding of 400 MPa ultra-fine grain steel

The ultra-fine grain （UFG） steel is welded by using resistance spot welding technique with or without requirement of upsetting force. Metallographic inspection shows that the grain size of weld nugget is larger than that of the base metal and the microstructure is altered significantly. In addition, contracting defects such as air holes can be found in the nugget center. The experiments show that the defects can be effectively avoided by the technique of adding upsetting force during the nugget cooling and crystallizing processes. In tensile shear tests, the welding joint starts to crack from the inner edge of the corona bond. The results of micro-hardness tests show that the newly born martensite structure dramatically improves the hardness of the joint. Under the interactions between residual stresses and regenerated fine grains, the micro-hardness of the heat-affected zone （ HAZ ） is lower than that of the nugget, but evidently higher than that of the base metal.

An Electrothermal Model Based Adaptive Control of Resistance Spot Welding Process

Resistance Spot Welding (RSW) is a process commonly used for joining a stack of two or three metal sheets at desired spots. The weld is accomplished by holding the metallic workpieces together by applying pressure through the tips of a pair of electrodes and then passing a strong electric current for a short duration. Inconsistent weld and insufficient nugget size are some of the common problems associated with RSW. To overcome these problems, a new adaptive control scheme is proposed in this paper. It is based on an electrothermal dynamical model of the RSW process, and utilizes the principle of adaptive one-step-ahead control. It is basically a tracking controller that adjusts the weld current continuously to make sure that the temperature of the workpieces or the weld nugget tracks a desired reference temperature profile. The proposed control scheme is expected to reduce energy consumption by 5% or more per weld, which can result in significant energy savings for any application requiring a high volume of spot welds. The design steps are discussed in details. Also, results of some simulation studies are presented.