Provision of a thin layer of zinc on aluminium to provide the fillets during brazing has been described previously [1]. The process for zinc coating of the extruded aluminium multiport tubes has been further enhanced by utilisation of the zinc layer to carry flux for subsequent brazing operations, thereby making it possible to produce brazed heat exchanger assemblies utilising the thin zinc layer, without the need for a separate fluxing operation prior to brazing.
The flux is applied directly onto the molten zinc coating where it becomes mechanically locked in position when the zinc freezes. Because of this action, the flux cannot be easily dislodged, unlike other techniques where powdered flux has been sprayed directly upon the aluminium tube surface or a fused flux has been applied to the tube surface. The prefluxed product can readily be handled and subjected to simple forming operations without dislodging the flux.
Flux can be adhered to zinc coatings as thin as two micrometers and below, which is somewhat lower than required for the zinc-based joining process but, in this form, the flux layer is suitable for enhancement of conventional Al: Si brazing technology. In this case, the zinc is diffused into the aluminium tube wall during brazing to provide enhanced corrosion resistance. When the flux is applied to a heavier zinc coating, the need for Al:Si brazing is avoided, with the zinc providing the braze fillets through diffusion and partial melting of the diffused layer at normal braze temperatures.
The technology also offers potential for brazing in normal air rather than nitrogen atmospheres and more simplified braze cycles can be used, where the need to drive off water from the flux is reduced. It also has the added advantage that the flux is carried directly into the joint area where it will melt and act in situ obviating the requirement to melt and flow into the joints to become effective during brazing. Furthermore, the need to apply extra flux to certain joint areas can be eliminated.