Phase Field Simulations of Microstructure Formation in An Ag-Cu-O Brazing Filler for Reactive Air Brazing Conditions

The microstructure within the brazing filler is one of the governing properties for the mechanical strength of the joint. The initial microstructure of the filler material evolves during solidification and depends on the thermal process conditions, the brazing alloy composition, and may also be affected by reactions between the brazing alloy and the materials to be joined.

In this presentation, we will present a phase field simulation study of microstructure evolution during solidification of an Ag-Cu-O alloy under reactive air brazing (RAB) conditions. In particular, the reaction of the liquid brazing alloy with air, leading to the formation of oxygen related phases like CuO is considered in the model. For the simulations, a general multiphase and multicomponent phase field model is used. The model is linked to a Gibbs free energy database for the alloy, enabling the calculation of thermodynamic driving forces and phase compositions. The model thus allows quantitative simulations with respect to varying alloy compositions, but also a quantitative calculation of the enthalpy change during solidification. This enthalpy change is used to directly compare the simulation results with differential scanning calorimetry (DSC) measurements for a metal/ceramic joint. The comparison helps to elucidate the role of nucleation temperature, nucleation rate and preferential nucleation sites for the microstructure formation in the brazing joint. The perspective to simulate the aging of the microstructure at elevated temperatures under various atmospheric conditions will be discussed.