Problem: Does magnesium (Mg) vaporization in a vacuum furnace only occur as a so-called “mag burst” near the brazing temperature of aluminum, or can it occur at lower temperatures? How is this affected by the vacuum levels in the furnace?
Answer: Magnesium will outgas (vaporize/sublimate) at different temperatures, based on the heating rate used and on the level of vacuum being used in that furnace. Yes, a so-called magnesium burst (or “mag burst”) does seem to occur near the brazing temperature used for joining aluminum parts in the vacuum furnace, as shown in the furnace chart in Fig. 1 from California Brazing and published in an article by Jim Grann of Ipsen. However, it is important to know that some Mg volatilization does actually occur over a wide range of temperatures and vacuum levels.
Discussion. In order to better understand why magnesium volatilization is important in aluminum brazing, a person must first understand what Mg does during any brazing process.
Mg has a very strong affinity for oxygen, and it will aggressively seek to bond with oxygen to form a stable Mg-oxide (MgO) molecule. The oxygen can be present in the atmosphere as pure oxygen (O2) or in the form of moisture (H2O), and it can be present as an oxide layer on top of the aluminum (Al2O3). If magnesium is in the vicinity of any of these oxygen molecules, it will seek to grab that oxygen for itself. These chemical reactions can be expressed in the following three equations:
1. 2Mg + O2 → 2MgO
2. Mg + H2O → MgO + H2
3. 3Mg+ Al2O3 → 3MgO + 2Al
This shows why Mg is used in aluminum vacuum brazing, since these chemical reactions show that Mg is a very strong “getter” of oxygen in the furnace (i.e., it is strong enough to steal the oxygen from water molecules – eq. #2). It can even steal oxygen from the tenacious aluminum-oxide layer on the aluminum metal itself (eq. #3). So, because it is such a powerful oxygen getter, it is a standard component for any vacuum brazing of aluminum being done today.
Check back next week for the conclusion of this discussion.