Influence of the eutectic interface on the fatigue behaviour of friction stir spot welds of aluminum with copper

The development of hybrid bonds between copper and aluminum is being pursued for reasons of cost, functionality, and weight, particularly in the field of electromobility, to achieve near net-zero emissions. Joining aluminum to copper is a challenge, as interfacial intermetallic compounds have a negative impact on the strength, ductility, and electrical properties of the joint. The development of brittle intermetallic compounds can be limited by targeted temperature control, making solid-phase joining processes particularly suitable. In this article, the fatigue behaviour of friction stir spot welded joints of copper CW004A and aluminum alloy AA1050A with probeless tools is studied. A melt film forms between the metals and the axial force displaces this film laterally, creating an intermediate layer of eutectic and Al2Cu in the joint area. The effect of this layer on the fatigue behaviour of the joint was investigated in this study. At high loads, failure occurs by a combination of Modes I and II with crack propagation in as well as around the bonding area, while at low loads only Mode I is observed. Typically, cracks origin at the spot outer diameter in aluminum, propagate at first in the laterally expelled melt and then through the aluminum sheet, causing unbuttoning.