| The resistance spot welding process involves the coupling of electrical, thermal, metallurgical and force phenomena, The welding heating process is instantaneous, and the formation of the nugget is invisible, Therefore to the formation, growth and solidification of nugget, the temperature field and stress field of weld zone and electrode tip along with the change of Spot welding cycle research bring great difficulties. Meanwhile, The electrode tip is easy to occur plastic deformation, pitting, adhesion, and alloying in the process of spot welding, lead to the electrode failure quickly, Especially, The failure of the electrode in spot welding of galvanized steel and aluminum alloy plate is more serious. Nowadays, the short life of spot welding electrodes has become an urgent problem to be solved in the field of welding technology application.In this paper,We use the professional welding finite element softwareSYSWELD to establish the Resistance spot welding finite element model of DP600 dual phase galvanized steel, the model is coupled of Electrical and thermal-metallurgy – force. The finite element simulation of spot welding process is carried out, The change process of the temperature, stress and residual stress distribution in the weld zone and electrode tip and the formation of the nugget are analyzed; Using multi-factor multi-index orthogonal experimental method to study the effect of welding process parameters on the nugget size, heat affected zone size and the heat and stress of electrode, To determine the influence degree and trend of each factor on every index, And using multiple linear regression analysis method,quantitative analysis of the influence of various factors on the various indicators; The effects of electrode type and shape of electrode tip, the distance from the electrode tip to the end of the cooling water hole, the cooling hole diameter,and the Convective heat transfer coefficient of cooling water on the size of the nugget and the heat and stress of the electrode were simulated; The electrode failure caused by plastic deformation is simulated and analyzed and the relationship between the electrode radius and the size of the nugget is established, forecasting the electrode life preliminary. The main conclusions of this paper are as follows:(1) Power on start,The temperature on the contact surface of electrode and workpiece is rising rapidly, the plastic loop is formed, and continuously extend outward with spot welding, The contact surface of the workpiece and the workpiece started nucleation and growing, eventually forming the ellipsoid. The electrode and the work piece begin to contact for local contact, As the electrode is pressed into the work piece, the stress on the center area of the electrode tip is gradually decreased, and the distribution tends to be uniform, The highest temperature of electrode tip close to the electrode softening temperature. In the welding process, the compressive stress of the plastic ring region is the largest, Nugget zone stress change from axial compressive stress to axial tensile stress, After welding, the residual stress exists in the joint, which is tensile stress in the axial direction, the tensile stress in the heat affected zone and the compressive stress in the fusion zone After welding, the joint presence of residual stress, tensile stress exists in the axial direction, at the radial direction, the heat affected zone exists tensile stress,the Fusion zone exists compressive stress and decreasing to nugget center.(2) Orthogonal test showed that: The welding current has a very significant effect on the radius of the nugget, the radius of the heat affected zone and the average of highest temperature on the electrode tip, Increase the welding current and welding time, the three indicators are increased, Increase the thickness of the workpiece, the radius of the nugget, the radius of the heat affected zone are increases, and the average temperature decreases, The welding pressure and electrode radius have a significant effect on the average of maximum stress on the electrode tip, the value increases with the increase of the welding pressure, and decreases with the increase of the radius of the electrode. The other factors have little influence on each index. Regression analysis of the relationship between the indicators and the various factors, in a statistically significant, can be initially used for spot welding quality prediction and analysis of service condition of electrode, but still need to be verified.(3)With the same parameters, the frustum type electrode can obtain a larger size of the fusion core than the dome type electrode. But the appearance of solder jointis worse, the indentation depth is deeper, and the warping is larger; in the presence of the arc of the electrode tip, the temperature of the frustum type electrode in the spot welding process is higher, and the area on the softening temperature is wider, the time is longer, and the electrode is more easy to failure; The nearer the distance between the electrode tip and the tip of the cooling water hole, the better the cooling effect are, which can reduce the times of electrode wear, but the total number of welding points decrease; Within a certain range, the greater the diameter of cooling channel is, the cooling effect is better; The greater the convective heat transfer coefficient of the cooling water is, the better the cooling effect is, so the electrode should be fully cooled.(4)In continuous spot welding and 6 mm to9 mm interval, the radius of nugget slightly decrease, and the volume is decreased. When the electrode diameter is larger than 9mm, the radius of the nugget decreases rapidly, and the electrode is invalid; the increase of electrode diameter has little influence on the radius of the heat affected zone, and its value is basically constant; the relationship between the electrode diameter and the diameter of nugget is established, which can be used to predict the service life of the electrode. |
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