Numerical Simulation Of Nugget Forming Process In Resistance Spot Welding For Aluminum Alloys

Aluminum alloy is regarded as one of environmental protection materials and preferred in the fields of automobile manufacture and aviation with its advantages. The technology of resistance spot welding (RSW) for aluminum alloy has become a main reason which impedes the application of aluminum alloy in the fields. The high thermal and electrical conductivity of aluminum alloy and compact oxide film (Al2O3) on its surface make its RSW weldability poor. The process mechanism of aluminum alloy RSW is the basis of improving weld quality. The RSW process with high nonlinear and coupling effect of multivariate is influenced by a number of random and chancy factors. The invisibility and instantaneity of RSW process make it difficult to monitor and control weld quality. The scientific numerical simulation with a little experimental verification is implemented to investigate the process mechanism of aluminum alloy RSW in this thesis. The mechanical,thermal,electrical and flow behaviors during the process of aluminum alloy RSW are analyzed,which provides the theoretical basis for solving welding problems. The initialized conducting region and contact resistance distribution are determined in the pre-squeeze stage of RSW,which has great effect on the position of initialized nucleation,nugget forming process and weld quality. According to elastoplastic and contact theory,an axial symmetry finite element model of pre-squeeze contact analysis for aluminum alloy RSW with dome-shaped electrode is developed. The contact behaviors in pre-squeeze stage of aluminum alloy RSW are simulated and the initialized conducting region is decided by this model,which lays solid foundation for numerical simulation of nugget forming process. The treatment method of contact resistance is key technology in RSW numerical simulation. Due to the oxide film on the surface of aluminum alloy,the contact resistance of aluminum alloy RSW is different from that of carbon steel RSW,which makes the treatment method of contact resistance in aluminum alloy RSW inapplicable here. A new contact resistance model is put forward by data fitting,which relates to the effect of contact force,contact area between two sheets and temperature variation. According to the basic equation of reflecting RSW process,on the basis of pre-squeeze contact analysis , an axial symmetric finite element model of mechanical,thermal and electrical coupled analysis for the process of aluminum alloy RSW is developed in this thesis based on the environment of software ANSYS. The mechanical,thermal and electrical coupled analysis is accomplished by using the APDL tool in ANSYS. In this model,the thermal physical properties,the latent heat of phase change and thermoelectric effect are considered and the reasonable boundary conditions are determined. The said model can reflect real process of aluminum alloy RSW with accuracy. The results of numerical simulation show that the distribution of contact pressure and current density on three contact areas during RSW is uniform. The change event of contact force is correspondent to that of current density,which can reflect the nugget forming process. The amplification of contact area between two aluminum alloy sheets reach 40 percent. By research on numerical simulation,the variation rules of dynamic resistance and contact resistance are discovered quantitatively. The contact resistance between two sheets disappears during primary stage of RSW,while that between electrode and work piece exists during the whole welding process. The body resistance of work pieces is larger than that of electrodes. The main heat source during process of RSW is the heat production of body resistance. In aluminum alloy RSW with dome-shaped electrodes,fusion nugget is formed firstly in the center of contact area between two sheets,which is different from the fact that early fusion nugget is a fusion annulus in RSW with cone-shaped electrodes. This nugget growth mode depends on the characteristic of distribution of contact force and current density during RSW process. The scale of nugget has formed within 20ms and nugget grows slowly in subsequent stages. The thermoelectric effect is considered which illustrates the asymmetry wear of electrodes in aluminum alloy direct current RSW. The reasonable use of thermoelectric effect can ameliorate nugget offset in dissimilar thickness of...