Thomas Schläfer

Thermochemistry of brazing ceramics and metals in air

Abstract Reactive air brazing offers economically and technologically advantageous joining of ceramics to metals. Solid oxide fuel cells and membranes for oxyfuel combustion are recent fields of application. However, it remains a problem that strong metallurgical reactions between brazes and base materials occur. These reactions were analysed by differential scanning calorimetry tests to get a better understanding. Therefore, three braze alloys (Ag8Cu, Ag8Cu0.5Ti and Ag4Cu4Ni) and five base materials (alumina, 3YSZ partially stabilised zirconia, BSCF perovskite ceramic, X1CrTiLa22 and X15CrNiSi25-20) were investigated. The reaction peaks correlate with the formation of reaction layers, which were observed in metallographic analysis of brazed specimens. The results help to explain the reaction mechanisms and allow optimised selection of filler metals and brazing temperature.

Development of an integrative simulation method to predict the microstructural influence on the mechanical behaviour of semi-crystalline thermoplastic parts

Abstract The mechanical properties of injection moulded plastic parts depend on the morphology, the degree of crystallinity and the molecular orientation of the formed microstructure. In order to take the variation of the microstructure into account in a structural analysis, a novel multi-scale, integrated simulation approach is presented here. At first, a coupled mould filling and heat transfer analysis is achieved at the macroscale and its temperature field is transferred to the micromodel. Based on the concept of cellular automata, a 3-D microstructure evolution model is developed. It specifies the nucleation of the spherulite germs and describes their expansion rate. To evaluate the effective mechanical properties of the simulated microstructures, the homogenisation method is applied directly to the spherulites, assembled in few classes according to their crystallinity degree. These local properties are then introduced into a new multilinear material model for structural analysis of thermoplastics. Finally, the influence of the microstructure on macroscopic behaviour is outlined for a polypropylene tensile bar, extracted from an injection moulded plate.