合金元素对铝合金点焊铜电极寿命的控制机理研究

Using aluminum in car body is an important means for lightweighting of automobile. At present, self-piercing riveting is the dominant technology used for joining aluminum, but it is so expensive that cannot be used for large-scale applications. Thus, the traditional resistance spot welding technology becomes a new choice for aluminum car body welding. However, the resistant spot welding of aluminum is confronted with great challenges. When using copper electrode cap to weld aluminum, because of the fast chemical reaction between aluminum and copper, which produces brittle, high resistivity and high thermal resistivity Al-Cu intermetallics. The intermetallic layer leads to overheating of the copper electrode cap and thus causes the rapid wear-out failure and the short-life of electrode. As a consequence, it also causes harmful effect on the weld quality. Aiming at the short-life problem of electrode and the rapid growth of brittle Al-Cu intermetallics, the present project will try to suppress the intermetallic layer growth by adding alloying elements to the copper base metal of the electrode material. This project would reveal the mechanism of how different alloying elements inhibit the growth of the brittle Al-Cu intermetallics in nano-micro scales by using transmission electronic microscopes and other advanced analysis techniques. Also this project would build an intermetallic layer growth kinetic equation which considers the influence of alloying elements. Through the comprehensive optimization of alloy composition, the present project will make a copper alloy design solution. According to this solution, the electrode life will be significantly improved, at the same time the electric conductivity and the strength will meet the requirements of industrial applications. This project will lay an important theoretic foundation for the industrial applications of resistance spot welding of aluminum car body.

铝合金是汽车车身轻量化的重要手段。自冲铆接是现阶段铝合金连接的主要工艺，但是成本很高，无法大规模应用，使得传统的电阻点焊工艺成为铝车身焊接的新选择。然而铝合金点焊时铝与铜电极之间的快速化学反应会在铜电极表面生成一层脆性大、电阻率高、导热性差的铝铜金属间化合物，这层化合物会导致焊接过程中铜电极表面温度过高，进而导致铜电极迅速磨损失效、寿命极短，同时对焊接质量产生不良影响。针对铝合金点焊时铜电极帽寿命极短、电极表面铝铜脆性金属间化合物快速生长的问题，本项目提出向铜电极材料中添加特定合金元素抑制金属间化合物层生长，通过使用透射电子显微镜等先进分析手段，在微纳尺度上揭示不同合金元素对铝铜金属间化合物生长的抑制机理，建立考虑合金元素影响的化合物层生长动力学方程。通过合金成分综合优化，形成电导率和强度满足使用要求同时寿命显著提高的铜电极合金设计方案。本项目将为铝合金电阻点焊工艺的工业应用奠定理论基础。

在汽车车身结构轻量化趋势的主导下，铝合金因其较高的比强度，以及优异的耐蚀性能正在逐步应用于汽车白车身的制造。电阻点焊因为其低成本、高效率、容易实现自动化及较好的适应性等特性被广泛应用于钢材的焊接中。然而铝合金的特有物理化学性质却给汽车工业应用最广泛的连接技术，电阻点焊，带来了极大的挑战。自冲铆接是现阶段铝合金连接的主要工艺，但是成本很高，无法大规模应用，使得传统的电阻点焊工艺成为铝车身焊接的新选择。然而铝合金点焊时铝与铜电极之间的快速化学反应会在铜电极表面生成一层脆性大、电阻率高、导热性差的铝铜金属间化合物，这层化合物会导致焊接过程中铜电极表面温度过高，进而导致铜电极迅速磨损失效、寿命极短，同时对焊接质量产生不良影响。本项目主要针对铝合金电阻点焊传统电极极易合金化和点蚀的问题，开发了一种由新型高导碲铬铜合金制备的电阻点焊铜电极。通过对比研究新型碲铬铜电极与现有铜电极经过焊接得到焊点的熔核直径、拉剪强度、接触电阻以及通过激光共聚焦CLSM扫描得到的点蚀体积等数据随着焊接次数的变化规律，分析证明了新开发的碲铬铜电极相较于传统铬锆铜电极具有更长的电极寿命和更稳定的焊点强度。最终通过铝铜界面金属间化合物的生长规律揭示了电极寿命延长的原因。本项目通过EDS、XRD、EBSD和TEM等手段鉴定了铝铜界面IMC层的物相组成为：Al4Cu9 (γ2)，Al2Cu3 (δ)和Al2Cu (θ)。通过不同温度和扩散时间下的IMC层厚度计算了铝铜IMC扩散激活能，为研究铝铜界面IMC层的生长奠定了几次。通过IMC厚度、材料的导电性和硬度等数据的对比，确定了碲Te和铬Cr元素作为铜合金的添加元素，并设计出了适宜的合金元素添加方案。本项目通过上述的合金成分综合优化，形成了电导率和强度满足使用要求同时寿命显著提高的铜电极合金设计方案。项目研究成果将为铝合金电阻点焊工艺的工业应用奠定理论基础。