Examination of the porosity in reactive air brazed joints by ultrasonic testing

Reactive Air Brazing (RAB) has been developed as a method for joining ceramics and steel using CuO as a reactive agent that interacts with the surface of the ceramic, enabling a wetting by the molten filler material. A major benefit of this method is the fact that the joining process can be carried out in an ambient atmosphere, in contrast to active brazing, which needs to be performed in a vacuum furnace. In the past, several investigations were conducted to improve the mechanical bonding. But a sealed gas tightness is also important for applications like solid oxide fuel cells. In this regard, the reduction of porosity which is necessary in order to achieve reproducible joints with a long lifetime, presents a challenge. Therefore, it is necessary to conduct fundamental analyses of the reasons and mechanisms of the formation of voids in the interfacial area to guarantee a reliable joint quality. In this study, the authors used an ultrasonic testing method to evaluate the porosity in brazements produced with varying process parameters, using a light furnace. A major goal was to assess the influence of the cooling stage on the pore formation. The advantage of this non-destructive method is the possibility to scan the entire joint area using just one scan. These investigations were flanked by SEM analyses on different cross sections.