铝/钢熔钎焊界面金属间化合物生长与分解动态调控机制

Effective control of intermetallic compounds (IMCs) formation is the key issue in Al/Steel dissimilar metals joining process. Given that, a research idea is proposed in this project that the formation of IMCs can be dynamically regulated by adjusting the heat input, and thereby minimizing the thickness of IMCs and improving the performance of Al/Steel joints. Based on the discovery that IMCs attenuates to some extent with the increase of welding current, a welding method with the combination of welding arc and assistive heat source is employed in this research work. In this process, the assistive heat source plays the role of regulating the temperature field of solid-liquid interface at brazing side. In this way, the welding arc, melting pool and temperature field of interface will exert joint effects on IMCs, rather than the sole effect from welding current. On the other hand, with numerous experimental results of short-time hot-dipping of steel in aluminum, the influence of reaction temperature and time on IMCs’ growth and decomposition in temperature field can be obtained. Therefore, the transformation laws of Al/Steel joints’ microstructure could be fully analyzed, and a dynamics model of IMCs formation and decomposition at multi-interfaces is so established. With above-mentioned relationships and models, the mechanism of IMCs’ formation and decomposition at Al/Steel interface under disequilibrium condition will be revealed, and thereby achieving the effective control of interfacial IMCs.

有效控制界面金属间化合物是铝/钢异种材料连接最关键的问题。本课题基于适当增大焊接电流可减小界面化合物厚度的新发现，提出化合物生长与分解动态调控的化合物控制思想，即通过调控热输入来同时影响界面化合物的生长和分解过程，实现化合物厚度的减薄和最小化。提出电弧+辅助热源的研究方法，利用辅助热源来调控熔钎焊过程中固液界面的温度场，将焊接电流对金属间化合物的影响分解为焊接电弧、熔池体积及界面温度场对金属间化合物的影响；结合钢的短时热浸镀铝实验，细化温度场中的反应温度及反应时间与化合物生长及分解之间的规律；深入分析接头微观组织结构特征及其变化规律，建立多界面化合物生长与分解的动力学模型，揭示铝/钢固液界面短时非平衡态金属间化合物生长与分解动态调控机理，实现界面化合物的有效控制。

本课题基于铝合金/不锈钢熔-钎焊连接界面适当增大焊接电流可减小界面化合物厚度的新发现，对比分析了熔-钎焊与热浸镀铝界面化合物，发现二者界面主要化合物均为Fe4Al13且变化规律相同，均为先降低后增加，说明反应温度在界面反应中起到最为关键的作用，基于不锈钢热浸镀铝界面的研究对揭示熔-钎焊界面化合物演变机制具有重要理论价值。根据熔-钎焊界面焊接热循环曲线设计了不锈钢的热浸镀铝实验，研究固液界面反应温度及反应时间对界面化合物的影响规律，反应时间为2-30s，反应温度为700-1000℃。透射电镜分析结果表明，不锈钢热浸镀铝界面化合物由钢侧1.5-5.5μm厚的FeAl2及铝侧5-35μm厚的Fe4Al13组成。对化合物厚度的拟合结果表明，FeAl2的生长符合抛物线规律，为扩散控制机制，激活能Q=66.5kJ/mol。铝侧化合物Fe4Al13的生长在700℃为扩散、界面反应及化合物溶解共同作用机制；在800-1000℃由于FeAl2化合物的生成，界面由单层化合物转变为双层化合物结构，Fe4Al13的生长机制转变为扩散与化合物溶解共同控制，生长速率显著降低。内层化合物FeAl2对外层化合物Fe4Al13生长的抑制作用及Fe4Al13在铝液中的溶解机制是界面化合物随反应温度升高而减小的主要原因。