Influence of the application of ultrasound on the microstructure of cemented carbide/steel joints brazed with Ag49ZnCuMnNi

The poor wettability of cemented carbides by molten metals as well as the different material properties compared to steel makes thermal joining process challenging. In this regard, innovative ultrasonic-assisted induction brazing is capable of joining the materials quickly, cost-effectively and without using environmentally hazardous fluxes. Instead, the oxide scales are broken up within the joining process due to ultrasonic-induced cavitation in the molten filler alloy and hence promote the wetting. However, since the use of ultrasound in brazing steel/cemented carbide joints is still insufficiently investigated, there is still a need to explore the fundamental influences of ultrasound on the brazing process. In this work, the effect of ultrasound on the microstructure of the brazed joints and the diffusion depth of different elements were analysed. The two ultrasonic parameters activation time and amplitude were varied during the experiments. Acicular or dendritic structures that have formed in the braze metal without ultrasonic stimulation were transformed into globular structures, when ultrasound was applied. In addition, a non-eutectic silver-rich phase formed, which was not observed in joints manufactured without the use of ultrasound. Furthermore, the use of ultrasound led to increased diffusion between the base material and the filler material as well as the dissolution and distribution of oxides in the edge regions of the brazing joints.