The biggest problem faced by operators new to aluminum torch brazing is the close proximity of the brazing alloy’s melt temperature to the melt temperature of the assembly. This can be especially problematic when brazing air-conditioning coils, where there are many braze joints in one concentrated area.
When using the most common aluminum brazing alloy, BrazeIt #718 (Al 718), there is less than a 100°F difference between the liquidus temperature of the braze alloy and the solidus temperature of the aluminum. The table below lists the melt and flow temperatures of the aluminum grades most commonly used in brazing applications. The solidus temperature is the last temperature at which the alloy is totally solid. Above the solidus, damage occurs in the base metal, regardless of visual appearance.
|Melting Range of Al Base Metals (F°)|
The next table shows the melting range of the two most common aluminum brazing filler metals:
|Melting Range of Aluminum Brazing Alloys (F°)|
|ChannelFlux 4047 (88% Al 12% Si – BAlSi-4)||1070||1080|
|ChannelFlux ZA-1 (78% Zn 22% Al)||826||905|
When comparing these two tables, the challenge is outlined – the standard Aluminum-Silicon alloy doesn’t allow for any overheating errors! ChannelFlux Za-1 is specifically designed for use on thermal transfer device like AC coils. The low melting temperature of the alloy is perfectly matched to the thermal activity of the cesium flux contained in the open channel and provides an adequate thermal window for most brazing applications, especially applications made from 6xxx series aluminum.
Tensile strength and burst tests on joints brazed with ZA-1 consistently demonstrate greater durability than the base metals. Saltwater spray tests to ASTM B117 for 2000 hours do not demonstrate any visible corrosion or deterioration of mass. Instances in which the high zinc content in ZA-1 might be offered sacrificially, independent tests verify the process is inhibited by the almost instantaneous formation of aluminum oxide.
Call Bellman-Melcor for technical support on your coil brazing project and a quote for ZA-1.