Ultrasonic spot welded CP Ti/AA2024/CP Tialloy joint

Ultrasonic welding (USW) is a promising technique of solid state joining of similar and dissimilar materials, including aluminum and titanium alloys. Differences in the physical and mechanical properties of the materials to be joined make it difficult to adjust the welding conditions to form high-quality joints. The development of natural aging in aluminum alloys may cause a change in the fracture mode and strength of Al/Ti ultrasonically welded joints. In present study, cryorolled and naturally aged AA2024 alloy sheet and annealed commercially pure titanium sheets were subjected to USW. The pre-treatment provided similar values of the hardness of the initial sheets. To avoid the adhesion of aluminum to the welding tool during welding, the AA2024 alloy sheet was sandwiched between the titanium sheets. Microstructure and properties of the welded joints after natural aging were examined. No cracks, discontinuities, pores were observed near the contact surfaces at magnifications of the microscope up to 1000 times. No diffusional or intermetallic reaction layers were revealed in the welded joints. The microstructure and microhardness of titanium sheets did not change after ultrasonic welding. Recovery/recrystallization development led to a decrease in microhardness of AA2024 alloy sheet. All welded samples fractured along the interface between the AA2024 alloy sheet and the bottom titanium sheet adjacent to the anvil. The average lap shear failure load of the welds was 1612 +/- 278 N (shear strength of approximately 57 +/- 10 MPa).