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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.
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.
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.
Fig. 2 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.
For good brazing to occur, this “healing” action must be prevented! And that’s what the magnesium (Mg) does! The Mg can quickly react with the oxygen and water vapor, thus preventing those elements from reacting with the clean aluminum metal at the bottom of the cracks in the Al-oxide layer! Molten aluminum BFM can then flow into that oxide-free area to initiate bonding with the aluminum base metal, but will also continue to flow, even under the oxide layer, literally floating away the oxide layer, as verified by laboratory studies conducted many years ago!
How is Mg added to the furnace?
Mg can be added as a constituent of the aluminum base-metal chemistry, or perhaps added to the brazing filler metal (BFM) chemistry, or it may be added as a separate entity (as chips or powder), such as that shown in Fig. 3, and placed in a small crucible placed near the parts that are being brazed. As the temps increase in the vacuum furnace, the Mg (in powder or chip form) may be expected to volatilize and become most active at temperatures above approximately 1000°F (525°C). If the Mg is alloyed into either the aluminum base metal or the BFM, then the temperature needed for volatilization will be a bit higher, in the range of about 1050-1060°F (570°C), since the Mg is alloyed with the aluminum and silicon in the base materials.
Fig. 3. Magnesium turnings (chips). Courtesy of Firefox Enterprises
Then, as mentioned earlier, when Mg volatilizes, it forms a gaseous Mg-cloud that will grab onto any available oxygen (to form MgO) thus preventing that oxygen from reacting with the clean aluminum base metal surfaces revealed at the bottom of the cracked Al-oxide layer.
The temperature at which pure magnesium volatilizes, i.e., vaporizes to form a “gaseous cloud”, depends on a number of factors, one of the most important of which will be the level of vacuum in the furnace. As the chart in Fig. 4 shows, the stronger (the harder) the vacuum, the lower the temp at which pure Mg can begin to volatilize. If the Mg is alloyed into the base metal or the BFM then the temperature required for Mg-outgassing (volatilization/sublimation) will be higher at any given level of vacuum in order to first break the Mg-free from the metal into which it has been alloyed.
Fig. 4 -- Vapor Pressure curve for Mg (in center of chart, circled). Adapted from AWS Brazing Handbook (Third Ed., 1975), p. 113
How much Mg should be used?
The amount of magnesium needed for efficient gettering will vary according to the size of the vacuum-furnace hot-zone and the size of the part being brazed, and will probably need to be determined experimentally.
According to Duke Singleton of Singleton Technologies, Richmond, VA (Duke is an expert in vacuum-brazing of aluminum) the use of about 3-to-10 grams of magnesium (Mg) per cubic meter of furnace volume is a good place to start when trying to determine how much total Mg to use for effective gettering.
Heating rate for good gettering
Magnesium volatilizes/vaporizes (forming an active gaseous cloud, so to speak) at temperatures above about 1000°F (525°C), the formation and effectiveness of which depends on the level of vacuum and “leak-tightness” of the furnace, as well as the heating rate used.
If the heating rate is very fast, the mag-sublimation/volatilization might occur suddenly as a so-called “mag-burst”, and could cause problems with your vacuum-furnace pumping system. Since most of the magnesium vaporization is probably occurring in the 10-4 to 10-5 Torr range, that means that the diffusion pump is the piece of equipment that will be called upon to handle the gas load during that mag-burst, and it must be adequately sized (including backing pumps) for that purpose.
It’s also important that the rate and timing of the mag-volatilization be appropriate to allow for effective gettering at just the time when the BFM is becoming active as well. Otherwise, if the mag-burst occurs too soon and is then swept away by the pumping system, there may not be enough magnesium vapor in the chamber to effectively keep the aluminum base metal free of oxides during the brazing operation, and poor brazing might result.
The effectiveness of Mg-gettering is also dependent on the leak-tightness of the vacuum chamber, and the temperature at which the cooling water is operated in the furnace walls. If there is a significant leak-rate of outside atmosphere into the furnace, then the oxygen and moisture in the air leaking into the furnace can quickly destroy the effectiveness of the Mg-gettering.
IMPORTANT ADDITIONAL NOTE: The temperature of the furnace walls should be kept warm, approximately 180°F (80°C) or so, so that moisture will not condense on the inner walls of the furnace, especially when those walls are exposed to outside air when the furnace is being loaded/unloaded. Remember that any condensed moisture on furnace walls can be very difficult to remove during the aluminum brazing cycle, and can adversely affect both aluminum brazing and Mg-gettering.
PROBLEMS WITH MAGNESIUM IN A VACUUM FURNACE
Because the vacuum-furnaces that are used for aluminum-brazing are what are known as “cold-wall” vacuum furnaces, the Mg gas will condense on those cooler furnace walls, building up a layer of pure magnesium over time, which must be removed. Please note that magnesium can burn very easily in the presence of sparks or flame! Thus, furnace operators and maintenance personnel much always be very aware (and specifically trained to know) that Mg-buildup will occur over time in the vacuum furnace, and must be periodically removed VERY CAREFULLY! Fig. 5 shows an example of condensed Mg on a vacuum furnace component.
Fig. 5. Mg-condensation as a white/grey coating on component.
Removing Mg-Buildup from furnace walls
There are a number of methods available for handling the Mg condensation on furnace walls:
1. Special removable inserts along the furnace wall, as shown in Fig. 6, allow the Mg to condense on their surfaces. These inserts, when removed from the furnace, can easily be cleaned outside the furnace, and then re-inserted into place inside the furnace.
Fig. 6 Mg-condensation board inserted into furnace at left can be removed to scrape off Mg and then re-inserted into furnace. Notice the Mg coating on the board does not stick and is already wanting to peel off the surface of the insert. Photo courtesy of PV/T
2. Coat the inside walls of the furnace -- There are a number of high-temperature coatings that can be applied to the inside walls of vacuum-furnaces to keep the Mg from sticking to the walls:
A. Coat with a high-temp paint (see Fig. 7). These high-temp paints can handle the temperatures used for aluminum brazing, and present a non-bonding surface for the Mg vapors when the Mg vapors cool down and condense on those coated surfaces.
When the furnace hot-zone is removed from the vacuum-furnace chamber, the coated walls can be scraped with a non-metallic scraper (or a bronze tool), and the condensed Mg should come off without too much effort. Very thin Mg-residues that remain may also be scrubbed with abrasive cloth materials to remove the Mg, but re-painting of damaged areas may be required.
Fig. 7 Hi-temp coating on ID walls of vacuum-furnace used for brazing aluminum. Photo courtesy of VAC AERO International Inc.
B. Coat with a high-temp Brazing Stop-Off. Some brazing shops have used some of the commercially available brazing stop-offs to coat their vacuum-furnace walls that are used for aluminum brazing in order to prevent the Mg from sticking to the furnace walls, and have reported much success with it. Because of the proprietary nature of all brazing stop-off materials, you will need to evaluate different commercial stop-off materials to find out which one works best for you.
CONCLUSIONS: Although zinc and cadmium should never, ever be allowed to be used in any kind of vacuum brazing, magnesium (Mg) is one volatile metallic element that can be used in vacuum brazing, but ONLY when brazing aluminum base metals in vacuum-furnaces specifically designed and used ONLY for aluminum. This “Mg-gettering” process is a well-known part of the vacuum-brazing of aluminum, and once understood, can be effectively used and controlled.
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