Mechanical Behavior of Advanced Reactive Air Brazed Joints

The development of high-temperature electrochemical devices such as high temperature solid oxide fuel cells (SOFC), gas separators and reformers poses a great challenge in joining metals and ceramics. To maintain the operability of such equipment, the seals have to be reliable and robust in isothermal high temperature as well as in thermo-cyclic operation. As a solution for joining metallic and ceramic materials reactive air brazing (RAB) has gained increasing interest in recent years. Nowadays, the most commonly used reactive air-braze filler material is AgCu(O). However, the resulting joints are susceptible to aging in high temperature dual atmosphere application. During operation, oxidation and reduction of Cu-mixed oxides formed at the metal/braze interface during brazing can lead to degraded interfacial adhesion [1]. To develop a material system with less aging susceptibility AgAl brazes are currently under investigation because of their possibly enhanced aging properties and improved contour accuracy [2] in joining complex geometries. In this paper different Ag0.5Al (wt.-%) braze filler manufacturing processes (PVD coating of Al on the surface of the metallic joining partner / arc melting of AgAl with subsequent rolling) are compared and their influence on the mechanical behavior and microstructure of brazed joints is studied. Fracture mechanics experiments are carried out to characterize delamination resistance at ambient temperature and shear-strength at operating temperature. Discussion of the results focuses on the influence of microstructural evolution on mechanical properties and failure behavior.