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.
To prevent this from happening, magnesium (Mg) is often added to the brazing process, since magnesium will react with oxygen more actively (strongly, aggressively) than aluminum can. The magnesium (Mg) can be added to the aluminum base-metal, or to the brazing filler metal (BFM), or both. Or, the Mg may be added as a separate entity such as Mg-chips (as shown in Fig. 2), or as Mg-powder, either of which can be placed in a small ceramic crucible placed near the parts being brazed.
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. 3 shows, the stronger (the harder) the vacuum, the lower the temp at which pure Mg can begin to volatilize.
It should be noted, however, that if the source of Mg for gettering is to be the Mg that is already alloyed into the base metal or the BFM, then the temperature required for Mg-outgassing (volatilization/sublimation) will be much higher at any given level of vacuum in order to break the Mg free from the metal into which it has been alloyed.
For commercial brazing, pure magnesium (in the form of chips, powder, etc.) may be expected to volatilize and become most active at temperatures above 1000°F (540°C). When the Mg is alloyed into either the aluminum base metal or the BFM, then the temperature needed for volatilization may be in the range of about 1050-1060°F (565-570°C), since the Mg is alloyed with the aluminum and silicon in the base materials.
When Mg volatilizes, it forms a gaseous Mg-cloud that will grab onto any available oxygen (to form MgO) before the oxygen can react with the clean aluminum base metal surfaces revealed at the bottom of the cracked Al-oxide layer.
Note that 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 equations it can be seen that when Mg reacts with oxygen and water-vapor it does so by taking the oxygen and “bonding” the oxygen to itself. Mg is even able to reduce some of the aluminum-oxide directly! Yes, magnesium is indeed a strong and effective gettering material for use when vacuum brazing aluminum components.
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 a gaseous cloud, so to speak) at temperatures above 1000°F (540°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/un-loaded. 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.
Please send to me your comments about what you have found works well in your shop regarding Mg-gettering for your aluminum vacuum-brazing operations.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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