It has been well documented that various migration phenomena across the interface of a composite brazing sheet have a profound influence on the brazing process [1], [2]. These interactions are particularly important for the control of cladding flow at the peak brazing temperature [3]. Unfortunately, our current understanding of these phenomena is to a great extent restricted to the micro-scale materials science aspects of the brazing process. Modeling of the cladding flow and joint formation mechanisms involving fluid, thermal, and mass transfer phenomena at a large-scale is virtually unexplored. The complexity of highly nonlinear reactive flow of the micro-layer of a molten metal governed by surface tension is a major obstacle in any effort to formulate a robust model for use in industrial practice. One of the most distinct features of this flow is the interaction of the molten metal and the substrate, accompanied by a migration of Si from the silicon reach cladding layer to the core. This process leads to an enrichment of Si content in the core zone close to the interface and, at the same time, a reduction of its content in the Si-depleted zone (on the cladding side of the interface). These simultaneous processes have a profound influence on the local shifts of liquidus temperatures, followed by a residuum formation and/or eventual dissolution of the core metal. Molten metal penetration along the grain boundaries is often present also. These complex processes hamper the cladding flow and joint formation, and make the prediction of what the available cladding for the joint formation would be very unreliable. In a more practical sense, an engineering approach to the design of a brazed joint zone must often be based on pure empirical evidence and a trial-and-error procedure. The main objective of this paper is to explore the residuum formation and Si diffusion at the peak brazing temperature of aluminum alloys, and to relate it to joint formation.
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