Phase field modeling applied to reactive air brazing: Investigating reaction kinetics with focus on oxygen exchange

Abstract

The role of oxygen exchange for the microstructure formation and reaction kinetics in reactive air brazing (RAB) using Ag-Cu brazing fillers is investigated by means of phase-field modeling. In addition to the microstructure evolution, the simulations allow computing differential scanning calorimetry (DSC) curves from the overall enthalpy change as a good representation for the reaction kinetics. The reaction kinetics are investigated with a focus on the interplay between the phase formation and oxygen exchange between the brazing filler and the ambient atmosphere. For the oxygen exchange, a simple permeability model is introduced, which is based on the interface mobility of the gasmelt phase boundary and which describes the oxygen flux j proportional to the equilibrium partial pressure difference Δp. It is observed that the L1 liquid phase can serve as a "buffer," providing or absorbing oxygen for phase transformation reactions. This "buffering" changes the oxygen activity, which in turn affects reaction temperatures and thus allows controlling reaction temperatures by adjusting the oxygen partial pressure in the ambient atmosphere: Higher oxygen levels increase the temperature of CuO formation, which requires oxygen, and lower the transformation temperatures for fcc-Ag solidification, which itself releases oxygen possibly leading to gas pore formation.