Study On The Dynamic Process Of Resistance Spot Welding Of Aluminum Alloy And Mild Steel

With increasing usage of Al alloys in manufacturing conventional steel-dominating vehicle bodies for reduced vehicle weight and ultimately improved fuel efficiency, joining Al alloys to steels becomes necessary. Because of the great discrepancies between aluminum alloy and mild steel in terms of physical properties, such as melting point, thermal and electrical conductivity, and the extremely low solid solubility of Fe in Al, the easily resulting cracks, shrinkage cavities as well as intermetallic compound layer(IMC) are significant defects in the Al-steel welded joint. Therefore, the welding problem of aluminum alloy and mild steel become a challenge for the usage of this welded joint of dissimilar materials in the domain of automobile industry.In this paper, the resistance spot welding of aluminum alloy and mild steel was studied. Based on the analysis of the characteristics of Al-Al and steel-steel resistance spot welding, the welding process of 6022-T4 aluminum alloy and mild steel was carried out using direct current. Considering the effects of the latent heat of phase change, temperature-dependent thermal, electrical and mechanical properties and contact electrical resistance and thermal resistance on resistance spot welding, a numerical model of Al-steel resistance spot welding was established. This model could be used to describe the structural, thermal and electrical coupling behaviors. With the help of the model, the dynamic physical process in the resistance spot welding of aluminum alloy and mild steel was further studied by revealing the changing rules of stress and strain, thermal and electrical field during welding process. According to the computational results, an equation to predict the thickness of IMC along Al/steel interface was presented. Finally, a comparison between modeling results and experimental data was made to verify the reliability of the model and predicting equation. The conclusions are as following:(1) Contact resistance is an important part of dynamic resistance at the beginning of welding stage, while the bulk resistance is the main part during the mid-to-late period. And the bulk resistance of mild steel is bigger than that of Al alloy. The bulk resistance firstly ramps up at the very beginning, and then ramps down.(2) During the whole welding process, heat generation of steel accounts for approximately 85 %, while Al resistance heat is the next.(3) Double nuggets forms during the welding process. Al starts to melts near the Al/steel interface at 55 ms, while steel in the centerpart reaches melting point at 166 ms. The growing rates of Al nugget width and thickness drop gradually, but the growing rate of width is always bigger than that of thickness.(4) The thickness of IMC along Al/steel interface could be given by where Q and k0 are 120 k J/mol and63.79 10-′ m2/s, respectively. The calculated data is in reasonable agreement with the experimental result.