Ultrasonic-assisted TLP brazing of aluminum and aluminum to steel in air using inductive heating and PVD deposited filler metals

摘要

The application of ultrasound during brazing enables flux-free joining of aluminum and steel in ambient atmosphere. The ultrasonic waves induce cavitations in the molten filler metal, leading to a mechanical breakup of the oxide film at the faying surfaces. Prior to the Transient Liquid Phase (TLP) brazing process, the aluminum alloy AA 6060 and the deep drawing steel AISI 1008 were coated with pure copper by means of Arc-Physical Vapor Deposition (Arc-PVD). The application of the filler metal by Arc-PVD secures an oxide-free material contact between the filler metal and the substrate. This is necessary for diffusion between aluminum and copper before the formation of the liquid in the TLP brazing process. Induction heating was chosen in order to guarantee short heating cycles with high heating rates. It can be especially beneficial for the suppression of the formation of brittle iron aluminides in aluminum/steel joints. Within this study, the microstructure and the mechanical properties of the brazements in dependence to the brazing time, the surface conditions, and the deposited copper layer thickness were analyzed. The joints were brazed at a temperature of 550°C for varying dwell times and ultrasound was applied for 6 seconds directly after the brazing temperature was reached. The microstructural analyses of the aluminum joints proved the occurrence of a high level of diffusion during the brazing process and the joints reached an average tensile strength of 46 MPa. The aluminum/steel joints featured significantly thinner intermetallic phase bands compared to vacuum brazed specimens. The iron aluminides in the brazements conducted at air had only an uncritical thickness of 2μm.