Simulation of the microstructure evolution in the brazing gap during reactive air brazing II

对于金属陶瓷构件的制造，“反应空气钎焊”（RAB）已成为一种具有很高潜力的连接工艺。反应性空气钎焊接头的机械性能通常受到结构变化以及在基材和钎焊合金之间的界面处形成氧化物相的限制。对Ag-Cu钎料钎焊接头的实验研究表明，在BSCF陶瓷中可能形成一个反应区，这与含Cu和Co的熔体沿沿着的渗透有关。在含Cr钢的界面处发现了Cu和Cr的氧化物，这些反应区的形貌和厚度影响着机械载荷下的失效，本课题的目的是建立反应区形成的数学模型，并定量模拟Ag-Cu钎料在空气中反应钎焊的组织演变动力学。 模拟应有助于识别那些控制形成动力学的过程，例如。G.氧或铜的扩散传输，或相边界的移动性。对微观结构形成的半定量理解可能会产生新的想法，即不希望的反应区的厚度如何受到合金成分或工艺条件变化的影响，例如通过施加不同的氧分压。在模型开发中，可以使用项目第一阶段的结果。至此，可以定量地模拟氧对Ag-Cu钎料合金相形成和凝固动力学的影响。相场模型已被扩展到计算DSC轨迹。这允许模拟的微观结构演变和测量的DSC扫描之间的定量比较。此外，还将利用计算机辅助热力学和相场法，特别是对以下几点进行模拟：钎料合金的熔化过程和伴随的相形成、液态钎料合金与BSCF陶瓷之间的相互作用以及熔融Ag-Cu合金与含Cr钢界面的反应和相形成。没有一个全面的热力学数据库可用于所涉及的材料。应根据可用的文献数据和适当的近似值，开发简化的热力学描述，并使用项目合作伙伴的实验结果进行校准。因此，这个项目也提供了一个方法论的贡献，对热力学方法的不同材料的连接过程建模。

For the fabrication of metal-ceramic components, "Reactive Air Brazing" (RAB) has emerged as a joining process with high potential. The mechanical properties of a reactive air brazed joint are often limited by structural changes and the formation of oxide phases at the interface between the base material and the brazing alloy. Experimental investigations of brazing joints with Ag-Cu braze alloys show that in BSCF ceramic a reaction zone may form, which is associated with the penetration of Cu-and Co-containing melt along grain boundaries. At the interface to Cr-containing steels Cu and Cr-containing oxides are found. Morphology and thickness of such reaction zones affect the failure under mechanical load.The aim of this project is to model the formation of such reaction zones and to simulate the kinetics of the microstructure evolution for reactive air brazing with Ag-Cu braze alloys quantitatively. The simulation should help to identify those processes that control the formation kinetics, e. g. diffusive transport of oxygen or cupper, or the mobility of phase boundaries. A semi-quantitative understanding of the microstructure formation may lead to new ideas how the thickness of the undesired reaction zones can be influenced by a change in the alloy composition or the process conditions, e.g. by applying different oxygen partial pressures.In the model development results from the first phase of the project can be used. So far, the effect of the oxygen intake on phase formation and solidification kinetics of Ag-Cu braze alloys could be simulated quantitatively. The phase-field model has been extended to calculate DSC traces. This allows a quantitative comparison between the simulated microstructure evolution and measured DSC scans. Furthermore, computer-aided thermodynamics and the phase-field method will be used.In particular, the following points will be addressed: Simulation of the melting process and accompanied phase formation for the braze alloy, the interaction between the liquid braze alloy and BSCF ceramic, as well as reaction and phase formation at the interface between the molten Ag-Cu alloy and Cr-containing steel. A comprehensive thermodynamic database is not available for the materials involved. Based on available literature data and appropriate approximations a simplified thermodynamic description shall be developed and calibrated using the experimental results of the project partners. Thus, this project also provides a methodological contribution towards a thermodynamic approach for the modeling of joining processes for dissimilar materials.