Fig. 1 Magnesium (Mg) plays a very important role in the vacuum-brazing of aluminum components because of its strong oxygen-gettering capability. Illustration © Kelly Brogan MD, and is reproduced courtesy of Kelly Brogan MD from her newsletter at kellybroganmd.com.
As mentioned in my article last month, it is critically important to be aware of the vapor pressures of any materials that are processed at elevated temperatures in a vacuum-furnace, because a vacuum can effectively lower the temperature at which a particular material will volatilize (outgas). We learned that you should never try to vacuum-braze brass, a copper-alloy which contains zinc (Zn), because Zn is a metallic element which can easily volatize when heated. The same is true for cadmium (Cd), a metallic element that is added to a number of silver-based brazing filler metals (BFMs) to lower its melting temp and improve wetting (such as in AWS A5.8, Class BAg-1).
Magnesium (Mg) is another metal (see Fig. 1) that, when heated in a vacuum, can also volatilize quite easily, and should therefore (like Zn and Cd) never be used in any vacuum furnace used for high-temp aerospace brazing of stainless or super-alloy base metals, since Mg contamination in such furnaces could ruin the furnace, rendering it non-useable ever again for any critical high-temp aerospace applications.
Does this rule out Mg from ever being used in any vacuum furnace? No, it does not. Some vacuum furnaces are built with the express purpose of allowing Mg to be used in them when brazing one specific type of base metal – aluminum (or aluminium as many prefer to spell it)! Vacuum furnaces built for brazing aluminum are unique – they are built to operate at just about half the temperature needed for high-temp aero brazing, they use different kinds of metals for their heating elements and hot-zones, and have much tighter temp-control than their higher-temp aerospace-brazing cousins. We’ve discussed all this before in previous articles on the subject.
IMPORTANT REMINDER: Aluminum should ONLY be brazed in vacuum furnaces specifically built for aluminum brazing. Aluminum should NEVER be brazed in standard high-temp aerospace-brazing furnaces!
Chemically, magnesium (Mg) has a very strong affinity for oxygen. In fact, it is much stronger than aluminum’s affinity for oxygen. Because of this, Mg can be intentionally added to an aluminum vacuum-brazing process in order to “getter” (i.e., react with and remove) as much oxygen as possible from the brazing environment, in order to keep it away from the aluminum and prevent the formation of any more aluminum-oxides. In fact, Mg can react with the aluminum-oxide layer on top of an aluminum base metal, and because of its stronger “gettering” action, can actually “break-down” some of these Al-oxides and remove some of the oxygen from the aluminum surface, resulting in a surface that can be more readily brazed. The typical reactions that occur with magnesium (Mg) during the aluminum vacuum-brazing process are:
Mg + H2O → MgO +H2 Mg + O2 → 2 MgO Mg+ Al2O3 → MgO + Al
From these one-way chemical-reactions shown here, it can be seen that when Mg reacts with oxygen and with water-vapor it will take the oxygen and “bond” the oxygen to itself, creating “clean” aluminum that can now be brazed. As discussed below, at aluminum-brazing temps in a vacuum furnace, the Mg will literally vaporize, forming a highly reactive Mg-rich “gaseous cloud” that actively looks for any oxygen to getter. One important place it goes is into aluminum-oxide “cracks” that form on the surface of the aluminum as it is heated to brazing temp. Let’s take a deeper look, once again, at how those “cracks” form, and their beneficial effects on aluminum brazing.
It is interesting to recall that Al-oxide, being a very stable “ceramic” type of material, expands/contracts at a much lower rate than the aluminum base-metal itself on which these oxides are sitting. This big difference in expansion rates can actually become a real “plus” when vacuum-brazing aluminum components because as the aluminum base metal rapidly expands when it is being heated up to brazing temperature, the aluminum-oxide layer on its surface cannot expand in a similar manner. Al-oxide can only expand at a rate that is about ⅓ to ¼ of the expansion rate of aluminum-alloys themselves. Thus, the Al-oxide’s much lower expansion rate will cause that extremely thin oxide layer (which is only about 40-60 Angstroms thick) to crack and break apart when the aluminum base-metal is heated (similar to ice breaking apart on rivers in early spring), thereby opening up gaps in the oxide layer (as shown in Fig. 2), which quickly exposes clean, oxide-free aluminum base-metal to the vacuum “atmosphere”.
But — because no vacuum brazing “atmosphere” is actually a “perfect vacuum”, there will always be some oxygen atoms and water-molecules (which also represents oxygen) present in that furnace atmosphere at brazing temp. That oxygen will want to quickly move into those cracks/openings in the oxide layer (as also shown in Fig. 2) to “heal” those cracks by trying to quickly form new aluminum-oxides in those cracks, so as to make the Al-oxide layer continuous once again.