Fracture Mode Variation Mechanism of Al/Steel Dissimilar Overlap Joint Made Using Variable Polarity Cold Metal Transfer-Based Arc Brazing

Rapid development of new energy vehicle desires more advanced lightweight design; such need makes the use of more Al continuously attractive to automotive industry. Since welding is the most commonly used method to build a car, the quality of Al/steel dissimilar joint becomes a key factor influencing the vehicle reliability. There is a common sense that Al/steel dissimilar joint would fracture either along Al/steel interface or Al heat-affected zone (HAZ), but the mechanism of such fracture mode variation is lack of experimental support. And as the joint interface is inevitably composed of Fe2Al5 and Fe4Al13 intermetallic layers, the detailed fracture procedure is usually too fast to be captured in situ and therefore rarely reported. In the present work, the fracture mode of Al/steel dissimilar overlap joints made using variable polarity cold metal transfer-based arc brazing is investigated using high-speed camera under special instrument setup. And by combining the high-speed camera results with interface material characterization results, the mechanism of fracture mode variation in Al/steel dissimilar overlap joint under axial tensile force is clarified. It is found that along with the increase in arc welding heat input, the fracture propagation path would experience a variation in “Al/steel interface → Al HAZ.” And such variation is practically induced by the synergistic effect of both overlap joint stress concentration and Al/steel interface strength, which is subsequently confirmed by numerical model analysis and a specially designed interface shear strength test.