摘要
铝合金电阻点焊广泛应用于航空航天、电子、车辆及轻工部门等领域,在现代制造业中具有重要的地位。然而,由于铝合金电阻点焊在连续点焊过程中存在焊点质量不稳定和电极使用效率低等问题,限制了电阻点焊在这些领域内的应用和推广。造成这一问题的主要原因是点焊过程中电极端面的铜铝合金化现象,而电极端面较高的温度与压力分布又对合金化反应起决定性作用。电阻点焊过程是一个高度非线性、多物理场耦合(热、电、力)作用的复杂过程。焊接时间的短暂性,熔池形成过程的不可见性,都给试验观察带来了困难,因此,本文拟采用数值模拟的方法,以ANSYS软件为工具,对铝合金点焊过程中铜铝合金化的反应机制进行模拟研究。具体研究成果如下:
本文根据弹塑性理论和接触性理论对电阻点焊预压阶段的接触行为进行数值分析,获得了铝合金点焊接触面的初始接触区域。并在此基础上,根据点焊过程的基本方程,建立了铝合金电阻点焊过程的热、电、力耦合模型。
利用所建立的耦合模型对铝合金点焊的温度场、应力场进行数值模拟分析,结果表明:在点焊初始阶段,由于接触电阻的分布很不均匀,造成接触面上的局部区域产生很高的温度,甚至能达到铜铝之间产生合金化反应的温度。但这种加热不均匀的现象持续时间非常短,且大部分分布在电极端面的边缘处,该处也为应力集中处。
为了进一步研究铜铝合金化反应的机理,本文建立了一种基于铜和铝扩散反应进行的有限元模型。由于热传导方程和扩散方程在表达形式上的相似性,所以用模拟热传导的方式来模拟铝在铜电极中的单向扩散过程,以温度分布表示扩散浓度的分布情况。通过对扩散反应的分析,提出了在点焊过程中发生铜铝合金化的可靠性理论:铜和铝在电极压力的作用下形成机械混合物,并以原子扩散的形式不断进行原子级别上的混合,形成铝在铜中的固溶体α(Cu),当铝元素在铜基中的含量超过5.65%,并且达到铜铝的共晶反应温度时,便生成了铜铝金属间化合物。另外,在高温高压下电极端部会产生塑性变形,提高电极内部的位错密度,利于铝原子的扩散。
Resistance spot welding (RSW) of aluminum alloys which are widely used in aerospace, electronics, vehicles and light industry and so on, has an important status in the modern manufacturing industry. Because the aluminum alloys resistance spot welding existence of instability in weld quality and electrode with low efficiency in the continuous welding process, it has limited the applications and development of the resistance spot welding in these areas. The alloying between copper electrode tip and aluminum alloy sheet surface during the spot welding is the main reason of decreasing electrode life, which is effected by the higher electrode tip temperature and pressure distribution. The resistance spot process is a highly nonlinear, multi-physical coupling complex process. It is difficult to observe the resistance spot process, due to the weld period temporary and the unobvious molten bath forming process. Therefore, the electrode collapse is researched by means of numerical simulation with ANSYS software. Specific results from the research in this paper are as follows:
According to the elastoplastic theory and contact theory,pre-squeeze contact analysis for RSW is developed by means of numerical simulation, and the initialized conducting region is decided by this model. According to the basic equation of depicting RSW process, an axial symmetric finite element model is developed of mechanical,thermal and electrical coupled analysis for the process of aluminum alloy RSW.
Temperature field and stress field of RSW are analyzed by numerical simulation with couple model. The results show that contact resistance distribution is not uniform in the initial stage, which cause a high temperature in contact surface of local and even can achieve the temperature of the alloying between copper and aluminum. This kind of phenomenon is changed in a very short time, and most of the energy is engendered on the edge of the electrode tip, where is also the stress concentration.
In order to further study the mechanism of the alloying of copper and aluminum, a finite element model based on the diffusion reaction of copper and aluminum was given. According to the similarity of diffusion equation and hears transfer equation, thermal analysis was used to simulate the diffusion process. Temperature was used to represent the concentration. Through the analysis of the diffusion reaction, some about the reliable theory of alloying of copper and aluminum is put forward: under electrode pressure, copper and aluminum form mechanical mixture, and both of them are mixed on atomic scales with the form of atomic diffusion. Finally, Al solubilizes in copper and the solid solution (α) is formed. When the content of copper element in aluminum matrix exceeds 5.65% and reaching the reaction temperature, intermetallic compound of copper and aluminum is generated. In addition, electrode tip is generated plastic deformation under high pressure and temperature, which improves internal dislocation density, and it is beneficial to the spread of aluminum atom.
引文
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