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The effects are described in aluminum brazing of a phenomenon known as Liquid Film Migration (LFM). This is primarily encountered when -O temper parts are formed and subsequently brazed. If the combination of material homogenization, alloy and cold work is such that the material does not recrystallize totally before the brazing temperature is reached, then the system begins to equilibrate by rapid solid state diffusional processes. These create a moving liquid interface which sweeps from the clad/core interface into the core of the material. This causes significant compositional changes in the volume through which the liquid film passes. Reduced brazeability (filler metal flow) results from the rapid diffusion of silicon into the core. Additionally, a coarse band of intermetallics is formed upon solidification of the enriched liquid zone which can impair corrosion resistance.

The phenomenon of liquid film migration (LFM) in aluminum brazing is described in a companion paper [1]. To clarify the mechanism, the effect of core alloy homogenization, cold work prior to brazing, and brazing temperature on the LFM kinetics has been studied. Based on microstructures, LFM kinetics, and concentration profile data, factors influencing the migration rate of liquid films have been analyzed. Several possible driving forces for LFM are proposed: (1) coherency strain energy, (2) reduction of particle/matrix interfacial energy, and (3) reduction of energy in recovered dislocation structure or subgrain boundaries in the case of lightly cold worked brazing sheet.