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

铝和不锈钢的接头对于汽车工业、航空航天和高功率半导体器件技术具有特殊意义，这是由于不锈钢的良好机械性能与铝合金的低比重相结合。在中等强度铝合金的情况下，接头通常使用熔化温度为575°C的Al-Si钎焊填料制造。然而，先进的高性能产品的开发需要用高强度铝合金替代现有的合金，其熔点明显更低。使用现有的填充金属，几乎不可能钎焊高强度合金，因为可能发生铝合金的机械性能的劣化或甚至局部熔化。基于三元体系Al-Ge-Si的低熔点钎料的开发允许不锈钢和高强度（迄今为止不可钎焊的）铝合金的连接。这些钎焊合金是在MEPHI开发并进一步优化的。初步研究表明，有可能实现熔点低于铝硅合金，因为三元共晶组合物的存在。快速淬火技术基于熔体的超快速淬火，熔体凝固为具有均匀分布的合金元素的非晶或纳米晶体箔。这些箔相当薄（50-70 µm），这意味着对基材的侵蚀很低，因为钎焊过程中发生的溶解现象较少。钎焊采用两种不同的工艺进行：感应钎焊和真空炉钎焊。第一个确保了对结晶稳定的金属间相的抑制，第二个导致接头的良好均匀化。目前，对于高强铝合金/不锈钢钎焊接头组织的形成及其与钎焊工艺的关系还没有基础性的研究。感应钎焊（TUC）和真空炉钎焊（MEPHI）用于生产铝合金/不锈钢接头。由于感应钎焊时间短、局部热输入大，钎焊后可望获得良好的机械性能。与此相反，长保温时间的真空钎焊会导致反应区中完全不同的扩散过程。通过比较所得的微观结构，可以了解反应区形成的动力学及其对接头机械性能的影响。除了单调的机械性能的知识，有必要调查的疲劳行为，以确定潜在的接头寿命。研究了接头的断裂面，以了解接头的结构-性能关系。为高强铝合金/不锈钢钎焊接头的开发和应用提供了基础。

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.