Study On Resistance Spot Welding With Rectangular Electrode

Resistance spot welding is highly efficient joining process and is widely used in automobile industry and urban rail vehicle industry. The developments of relevant industries claimed it to new challenges and demand. The shape of electrode is an important parameter, and the conventional electrodes can be divided to the taper and spheriform ones, both of which produce a round welding spot, which has limited effective connection length. The weld spacing used in production must exceed a certain value to avoid shunt current, which lowers nugget size or even results in incomplete penetration because of decreased current density and underheating in weld field. The stopping property of resistance spot welding construction with conventional electrodes is low; therefore the conventional electrodes are suitable for the production with low requirement of stopping property, but not suitable for production which has high requirement of strength and gas tightness. The conventional seamwelding can not satisfy the need of joint in small scale because of restrictions of the configuration. Considering that the above-mentioned problem can't be solved by seamwelding or related joint, one of the electrode couple is plane electrode and the other electrode shape was changed to rectangle, that is resistance spot welding with rectangular electrode. The effective connection length was prolong , the stopping property and productivity effect was increased by this means.Resistance spot welding with rectangular electrode widen the application range of resistance spot welding. The resistance heat which can melt metal form nugget and realize connection, it is also the working principle of resistance spot welding with rectangular electrode. The change of electrode shape from taper or spheriform to rectangle may lead to various variation, which include contact state of interface,contact resistance,thermogenesis or heat dissipation and the regularity of electrode wear and so on. But these factors above were important elements of formation process of nugget,size of nugget and joint strength. The resistance spot welding parameters with conventional electrodes are not suitable to resistance spot welding with rectangular electrode on the same materials and size. So, it is necessary to study the basic problem of resistance spot welding with rectangular electrode.The suitable welding criterion is helpful to obtain high quality spot welding nugget. Under the condition of same current density between resistance spot welding with rectangular electrode and conventional electrodes, an experiment about SUS304 stainless steel resistance spot welding with different size rectangular electrode( B×L=2 mm×10mm or B×L=5 mm×18mm) was conducted。The chosen welding parameters should be adjusted because of the changed radiating condition resulted from the change of shape and size of electrode tip. The result showed that the lengths in the long axis direction and in the short axis direction of nuggets almost reach 15.4 mm and 4.0mm, the joint strength can reach 12.41kN, the welding parameters adopted were: electrode force F, 7.4 kN; welding current I, 16.5 kA; welding time, 6 cyclesles and the size of workpiece is B×L=5 mm×18mm; the lengths in long axis direction and short axis direction of nuggets size almost reach 7.4mm and 4.0mm, the joint strength can reach 5.6kN, the welding parameters adopted were: electrode force F, 3.4 kN; welding current I, 8.5 kA; welding time, 6 cyclesles and the size of workpiece is B×L=2 mm×10mm. The nugget size and joint strength would decreased with too high electrode force; there are splash under high welding current; the nugget size increase to be saturated with more than 6 cyclesles; The too large or small electrode cone would lead to wear or deformation of electrode. The good spot-welded joint would be obtained under two different size of electrode ifθ=45o.The stainless steel spot-welded joint composed of nugget and heat affected zone those microstructure are all austenite owing to some unique properties such as high resistivity, thermal conductivity, good weldability and so on. The result showed that there are not crack and shrinkage cavity with correct preventive measures and reasonable spot welding parameters, but there are splash, deep indentation and low size of nugget size with too large welding current, too high or too low electrode force. The main purpose of spot welding using rectangular electrode is to improve the airtightness of spot-welded joint. The closed vessel made from resistance spot welding with rectangular electrode achieved good airtightness when the welding parameters adopted were: electrode force F, 7.4 kN; welding current I, 16.5 kA; welding time, 6 cyclesles and the size of workpiece is B×L=5 mm×18mm.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 tensile shear load were investigated. The results show that effect of welding current and electrode force on nugget diameter and tensile shear load is the most evident. besides, interaction effect of electrode force and welding current on tensile shear load is the most evident. It is favorable to saving costing of production and improving welding quality for optimizing welding parameters.Study on resistance spot welding means be limited for its unvisible and complex, but the FEM can fetch up above-mentioned a certain extent. The paper studied the pre-pressing contact and the temperature field of stainless steel resistance spot welding under rectangular electrode condition using finite element method. 1/4 three-dimensional axismetric finite element model was used to simulate the resistance spot welding with rectangular electrode ( B×L=2 mm×10mm or B×L=5 mm×18mm). The results showed that: in the earlier period of welding, the contact stress at the edges and corners of electrode was very high because of the special shape of electrode, and the highest temperature appeared in the electrode boundary vicinity the contact surface between workpiece and workpiece. As the welding time increased, the contact resistance decreased, the highest temperature shifted to the internal and the nugget began to form, which was the similar to the conventional spot welding. The lengths in long axis direction almost reach 9.4mm and 17mm when the size of electrode is B×L=2 mm×10mm and B×L=5 mm×18mm.