The Top 3 Challenges of Aluminum Brazing

February 21, 2018

Aluminum base metals present a number of challenges during brazing.  First, the low melting temperature of aluminum base metals requires more precise heat control, and the lack of color change that occurs during heating doesn’t give braze operators any visual indication that the base metal has reached the proper brazing temperature.

Second, many of the challenges associated with aluminum brazing are related to heat transfer.  Manufacturers do not want premature melting of brazing alloy (most often in ring form) or localized areas of heat that create “hot spots”.  Heating must be even across the base metals and the ring, and the flux must be activated in advance of the ring melting.  Even heat transfer ultimately makes processing success easier to achieve in difficult applications like 3 and 4-row coils.

Last, because aluminum forms a tough oxide during brazing, the use of flux is essential.  Fluxing can add an entirely new step to the brazing process and can affect the appearance of the finished braze product.  Depending upon the type of flux used (corrosive v. non- corrosive), an additional step may be required if the flux residue must be removed after brazing.  Flux is available in a variety of forms including paste, slurry, powder, and “combined” forms.

Brazing filler metals appropriate for use with aluminum fall into two categories – Al/Si-based filler metals and Zn/Al-based filler metals.  The Al/Si formulation, as contained in ChannelFlux 4047 (also known as Al718 or BAlSi-4), contains 88% Al and 12% Si, is probably the most commonly used alloy when joining aluminum to aluminum in residential HVAC and automotive components.  It provides excellent joint quality, good mechanical properties, corrosion resistance, and minimal metal erosion.  However, since this alloy has a narrow process range (Melt = 1070˚F, Flow = 1080˚F), temperature control must be well established and controlled. Bellman-Melcor uses a “combined form” flux in its ChannelFlux family of alloys.  All ChannelFlux alloys contain a cesium flux that is non-corrosive, and therefore requires no post-braze cleaning.  The cesium flux also provides a clean, white joint that is easily visually inspected for voids and leaks.

The ChannelFlux family of alloys includes two formulations of Zn/Al-based filler metals:  ZA-1 and ZA-2.  ZA-1 contains 78% Zn and 22% Al and can be used on copper to aluminum joints in HVAC components.  It is frequently used in the aftermarket repair of damaged evaporator coils, on wire feed applications, and in other miscellaneous aluminum applications.  The lower melting point of ZA-1 (Melt = 826˚F, Flow = 905˚) provides a wider processing window than that of 4047.  ZA-2 contains 98% Zn and 2% Al, (Melt = 715˚F, Flow = 725˚), and is used primarily for hand braze repairs in production environments.  The proprietary nature of the general-purpose zinc alloy ZA-3 prevents us from providing the composition, (Melt =788˚F, Flow =824˚);  ZA-3 provides a processing window less narrow than 4047 or ZA-2, but not as wide as that of ZA-1.

ChannelFlux was designed with 3 and 4-row coils in mind, and as such provides maximum contact between the flux-integrated ring and the base metals, increasing conductive heat transfer.  The cesium flux activates well in advance of the alloy, providing proper wetting and aiding in the capillary action required for proper flow-through of the alloy into the braze joint.   ChannelFlux preform rings also allow manufacturers to apply a consistent weight and flux percentage for process stability, and the elimination of applying a separate flux minimizes process variables, mess, equipment maintenance, additional purchasing costs, and operator exposure to hazardous materials.

As always, Bellman-Melcor is happy to provide brazing solutions or technical support…click here for a quote on ChannelFlux for your aluminum to aluminum or aluminum to copper application.

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