Fig. 1 When the rapidly expanding aluminum base metal causes the lower-expanding oxide layer to break apart, any free oxygen in the atmosphere wants to quickly try to re-form new Al-oxide to “heal” the breach.
In last month’s article, we looked at the use of titanium-“getters” when vacuum-brazing high-temperature base-metals that are very sensitive to oxidation. In this month’s article, let’s look at how magnesium (Mg) is used as a “getter” when vacuum-brazing at temperatures of only about 1000-1100°F (540-600°C), as needed for joining aluminum base metals.
Magnesium (Mg), often referred to simply as “mag”, can be highly effective at gettering both oxygen and moisture that may be present in a vacuum-furnace atmosphere being used in aluminum-brazing operations.
Aluminum (Al) reacts readily with oxygen to instantly form a tenacious Al-oxide layer on its surface. This Al-oxide layer is very stable, and, if mechanically removed, will quickly re-form. Thus, in real life, a layer of aluminum-oxide will constantly be present on the aluminum surface before, during, and after aluminum brazing. Dealing with that oxide layer has proven to be a challenge to many brazing shops over the years.
It is interesting to note, first of all, that this stable layer of Al-oxide is actually a “ceramic” type of material, and as such, expands/contracts at much lower rates than most metals, and certainly at a far lower rate than aluminum itself. This big difference in expansion rates between aluminum and the oxide layer on its surface can actually become a real “plus” when vacuum-brazing aluminum components because as the rapidly expanding aluminum base metal heats up to brazing temperature, the lower-expanding oxide layer will crack and break apart (similar to ice breaking apart on rivers in early spring), exposing clean oxide-free aluminum base-metal to the BFM when the BFM melts and flows at brazing-temperature.
However, no vacuum brazing “environment” is actually a “perfect vacuum”. Instead, there will still be (relatively speaking) a lot of oxygen atoms and water-molecules (which also represents oxygen) present in that furnace atmosphere at brazing temp, and that oxygen, as shown in Fig. 1, will want to quickly move into those cracks/openings in the Al-oxide layer to form new aluminum-oxides that can “heal” the cracks in order to make the Al-oxide layer continuous once again.