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

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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.