Effect of welding energy on interface zone of Al-Cu ultrasonic welded joint

Ultrasonic welding is attracting increasing attentions in joining of dissimilar materials. The effect of welding energy on joint strength, failure behaviour and microstructure of Al-Cu ultrasonic welded joint has been experimentally investigated. The results showed that joint strength increased with welding energy initially and reached its maximum at 1000 J, then dropped significantly instead. Meanwhile, the failure mode changed from interfacial debonding to nugget pullout, and then back to cleavage failure. Various microstructures with different morphologies and properties were also observed at the interfacial region. At lower energy, the joint was only partly bonded by numbers of dispersed microbonds. A swirl-like structure appeared at the weld interface and led to a mechanical interlocking between the materials at higher energies (similar to 1000 J). However, cavity defects and intermetallic compound (IMC) were more likely to form under excessively high energies. A 0.5 mu m thick IMC layer with dominant phase of Al4Cu9 was found in 2000 J joint.