Numerical Simulation Study On The Role Of The Partial Melting Zone In The Resistance Spot Welding Of Aluminum Alloy

Aluminum alloy resistance spot welding(RSW)has been widely used in the field of aerospace,automotive and so on.Because of aluminum alloy RSW is a complex,transient and closed process,numerical simulation is a common method to reveal the nugget formation process during RSW.In the RSW of aluminum alloy,there is a zone outside the nugget where its temperature ranges from the solidus to the liquidus,called partial melted zone(PMZ).The solid-liquid phase ratio and material properties of PMZ change drastically with temperature,which affects the welding process.However,in the current numerical simulation studies,the PMZ is often simplified into a porous medium,and this zone is not separately studied.In order to study the influence of the PMZ on the nugget formation process of RSW,and further reveal the nugget formation process of aluminum alloy RSW,this paper conducts a numerical simulation of the aluminum alloy RSW process with consideration the PMZ.In this paper,based on the phase diagram calculation technology and the special material database,the solid percentage and material properties evolution data of the aluminum alloy in the partial melted temperature range are obtained.The data were imported into ANSYS numerical simulation software.Three groups of finite element models of same material with equal thickness,same material with unequal thickness and different material with unequal thickness were constructed,respectively.The data transmission among five physical fields of force,heat,electricity,magnetism and current is realized.Compared with the experimental data,the correctness of the calculation model is verified.The partial melted temperature ranges of the three materials were obtained by Calculation of Phase Diagram(CALPHAD).The results of CALPHAD showed that the properties of materials in the PMZ change sharply.The electrical resistivity and enthalpy increase,and the thermal conductivity decreases rapidly.The numerical results showed that the welding current of resistance spot welding of aluminum alloy will concentrated in the PMZ due to the "flow over effect".The induced magnetic field is affected by the current distribution.The intensity of magnetic field increases fastest in the PMZ.Driven by the induced magnetic force,the melted flow field will form four rotating cores,which will make the temperature of the liquid nugget more uniform and promote the expansion of the PMZ.The PMZ acts as a heat source and a heat barrier in the RSW process.During the welding,the outer boundary of the PMZ expands continuously.The nugget is formed on the basis of the shape of the PMZ.In the RSW of large thickness ratio dissimilar aluminum alloys due to the above impacts,the final shape of the nugget deviates from the conventional ellipsoid shape.For the welding combination of LD10 and 5A06,due to the sequence difference of PMZ,the nugget will shift to the side of 5A06 sheet.Taking PMZ into the consideration for the RSW simulation can simulate the phenomenon of nugget migration of different materials more accurately.