Alloying- and heating mode dependent microstructure of brazed AISI 304/AlGeSi/AA6082 joints and its influence on the static and cyclic performance

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Joints of aluminum and stainless steel are of special interest for the automotive industry, aerospace and highpower semiconductor device technologies due to the good mechanical properties of the stainless steel in combination with the low specific weight of aluminum alloys. In case of medium strength aluminum alloys, the joints are commonly manufactured using Al-Si brazing fillers with a melting temperature of 575°C. However, the development of advanced high-performance products requires the replacement of existing alloys with high-strength aluminum alloys, whose melting point is significantly lower. With existing filler metals, it is almost impossible to braze high-strength alloys, because a degradation of the mechanical properties or even localized melting of the aluminum alloys can occur. The development of low-melting brazing fillers based on the ternary system Al-Ge-Si allows the joining of stainless steel and high-strength, up to now non-brazeable, Al-alloys. These brazing alloys were developed and are further optimized at MEPHI. Preliminary investigations show the possibility to achieve a melting point lower than that of Al-Si alloys, because a ternary eutectic composition exists. The rapid quenching technology is based on ultra-fast quenching of the melt which solidifies as amorphous or nano-crystalline foils with homogeneously distributed alloying elements. These foils are rather thin (50-70 µm), which means that a low erosion of the base materials occurs, because less dissolution phenomena go on during brazing. Brazing is carried out using two different processes: induction brazing and vacuum furnace brazing. The first one ensures the suppression of thermodynamically stable intermetallic phases, the second one leads to a good homogenization of the joints. By now, there are no fundamental studies on the microstructure formation and structure-property relationships of high-strength aluminum alloys/stainless steel brazed joints in dependence on the brazing process. Induction brazing (at TUC) and vacuum furnace brazing (at MEPHI) are used to produce aluminum alloy/stainless steel joints. Due to the short time and local heat input, after induction brazing, good mechanical properties are expected. In contrast to that, vacuum brazing with long holding times leads to completely different diffusion processes in the reaction zone. By comparing the resulting microstructures, it is possible to understand the kinetics of the formation of the reaction zone and their influence on the mechanical properties of the joints. In addition to the knowledge of the monotonic mechanical properties, it is necessary to investigate the fatigue behavior in order to determine potential lifetimes of the joints. The fracture surfaces are investigated to understand the structure-property relationships of the joints. As a result, a fundamental basis for the development and application of high-strength aluminum alloys/stainless steel brazed joints will be generated.