铝合金电阻点焊半熔化区的数值模拟研究

Numerical simulation of partial melting zone in resistance spot welding of aluminum alloy

半熔化温度区间是铝合金的固有属性。当温度位于固相线与液相线之间时,铝合金处于固液相共存状态,相应物理属性与其处于全固相或全液相相比存在巨大差异。铝合金进行电阻点焊过程中,由于半熔化温度区间的存在,会在熔核的外侧形成较明显的半熔化区,其固液相比例及传热属性随温度剧烈变化。文中基于相图计算技术配合专用材料数据库,得到相应牌号铝合金在半熔化温度区间的固相百分数及材料属性演化数据。建立有限元模型,并实现了力、热、电、磁、流5个物理场间的数据传输,进行耦合场数值模拟。结果表明,铝合金电阻点焊前期靠接触电阻产热,后期半熔化区及熔化区的体电阻产热使温度上升。随着温度的上升,电流出现了“绕流效应”。熔化后的流场会在电磁力的驱动下形成4个旋转核心,促使熔核沿厚度方向生长,使熔化区温度更加均匀。半熔化区在电阻点焊中起到电阻产热热源及热屏障的作用。在大厚度比异种铝合金点焊过程中,熔核生长至最终形态偏离常规的椭球形。

The partial melting zone is the inherent property of aluminum alloy and other alloy materials.The base metal with the temperature between the solid zone and the liquid zone will come to the partial melting state.The physical properties will change dramatically in this region.Resistance spot welding technology,as a common joining method of aluminum alloy sheet,has been widely used in automobile,aerospace and other industries.The joints in resistance spot welding will be affected by the dynamic change of material properties.In this paper,the CALPHAD technology was used to calculate dynamic evolution of material properties in the partial melting zone.Based on the numerical simulation tool ANSYS,three groups of finite element models of aluminum alloy spot welding was established.They were the pieces with the same thickness and the same material,the same material and different thickness,and the different thickness and the different material.The data transmission among the five physical fields of force,heat,electricity,magnetism and flow was realized.The results showed that the heat production of resistance spot welding depended on contact resistance in the early stage.As the welding process going on,the heat production of bulk resistance in the partial melting zone and melting zone contributed more to make the temperature rise.With the increase of temperature,the current appeared'bypass effect'.After melting,the flow field would form four rotating cores driven by electromagnetic force,which would promote the lateral growth of the molten core and made the melting zone temperature more uniform.Partial melting zone played a role of heat source and heat barrier in resistance spot welding,which made the subsequent nugget formation expand on the basis of partial melting zone.