The Simulation Of The Nugget Forming Process In Resistance Spot Welding

From 1960's, The numerical simulation method of resistance spot welding had been continually improving, but up to now we haven't got a satisfactory solution to some problems, such as how to build the electrical-thermal-structural coupled model, how to deal with the contact resistance and how to simulate the pole's changing contact area with dome-shaped electrode spot welding. This thesis's works have been going on with those points. Based on commercial finite element software ANSYS, dissection of contact resistance, a new method to determine the contact resistance is raised and the electrical- thermal- structural coupled model with dome-shaped electrode is built and tested practically by experiments. The nugget growth mechanism and the nature rules are studied and discussed most thoroughly.In this study a structural model with dome-shaped electrode and any electrical-thermal models with different contact area between electrode and work piece are built. The structural model is used to simulate the transformation of electrode and work piece during the whole welding process and the electrical-thermal models are used to simulate the changing temperature field.After simulate the pre-squeeze process with structural model, the initial contact condition of the welding process is given. The result show: the larger radial stress is appeared on the interface between electrode and work piece and the larger axial stress on the interface between two work pieces. Generally, contact area and the uniformity coefficient of contact press is proportional to the sphere radius of the electrode, the thickness of the work piece and welding force;stress concentration is easier appeared at the edge of the electrode-work piece interface with cone-shaped electrode than dome-shaped electrode.The electrical-thermal-structural coupled analysis in going on with small time step. The contact area between electrode and work piece are obtained from pre-squeeze analysis or thermal-structural analysis at every time step, and then the corresponding electrode-thermal model is selected and used in current time step. The contact between two work pieces is treated by element birth and death, after current step of electrode-thermal analyses, the structural model is selected, restarted and loaded with every node's temperature, after one time step's calculation, the contact condition is given and used to the next time step. The contact resistance is calculated based on the signals collected during welding process and used in the electrical-thermal coupledanalysis. The result show: when the electric is loaded with a very little time, the electrode tip's temperature is higher than work pieces' interface. But soon the temperature at work pieces' interfaces is ascent and higher than the electrode tip's temperature. When the nugget grows to a certain degree, the temperature at the middle of the nugget rises very slowly. At weld process the contact radius of the electrode and the work piece increase gradually but the contact radius of the work pieces descend very quickly in the earlier period and than increase slowly. The nugget is surrounded by plastic metals with high tempurature, the axial plastic deformation is gradually descent from negative value to positive, the contact of the work pieces are welded together at contact area and a plastic metal ring is formed to make the nugget enclosure. During the nugget formation process the influence of the electrode pressure is greatly, along with the increment of the electrode pressure, the contact area enlarges, the dynamic electric resistance decreases, the temperature's highest value and rising speed descend correspondingly.As a preliminary study of a on-line quality testing system, the signal of the electrode displacement and welding current are sampled practically during welding process and loaded to the electrical-thermal-structural model to simulate the whole welding process, the corresponding information of the nugget which have a close relationship to the welding quality are obtained.