Liquid Film Migration in Aluminium Brazing Sheet?
From literature and own observations it is known that the clad and core alloys that make up aluminium brazing sheet can show severe interaction during the brazing cycle. This interaction leads to a complete re-distribution of elements, changing essential properties like strength and corrosion resistance. This interaction has been reported many times but up to present time no clear explanation is given why this interaction is actually occurring. There are a number of publications addressing the circumstances under which the interaction is more severe. Chemistry and low levels of strain applied before brazing have a significant influence on the severity of the interaction. As a yet possible mechanism behind the interaction Liquid Film Migration is mentioned. The observations done so far are in line with this described mechanism but no ultimate proof has been given so far. The question why the interaction takes place cannot be answered yet, clearly a change of free energy of the system is involved but the mechanism or mechanisms behind the change is unclear.
W.J. Poole, M.A. Wells and D.J. Lloyd
A. Wittebrood et al., "Liquid Film Migration in Aluminium Brazing Sheet?", Materials Science Forum, Vols. 519-521, pp. 1151-1156, 2006