Vacuum brazing is growing in many brazing shops today, to handle the complex needs of the aerospace, medical, and consumer-goods industries. The primary reason for this is that the amount of oxygen in any good vacuum atmosphere is so small that no oxidation of parts should ever occur, and thus, parts can be brazed without the need for any protective fluxes, and those surfaces can be kept bright and clean throughout the brazing process with no post-braze cleaning needed. That’s great! But, are there any “negatives” to brazing in a vacuum furnace?
WARNING: Yes, there are some significant potential problems that must be understood (and avoided) when vacuum brazing and the first one relates to the chemistry of each of the metals involved in the brazing process! Some metals contain zinc, such as the brass components in Fig. 1 that have been assembled for brazing. Zinc can easily volatilize (turn into a gaseous vapor) when heated, which could then badly contaminate (and possibly ruin) your vacuum furnace!
Cadmium (Cd), lead (Pb), and magnesium (Mg) are additional examples of metals that also easily volatilize when heated, and must be avoided when doing high-temp vacuum-brazing with silver-based, copper-based, and nickel-based BFMs. However, there is a unique situation in which Mg is actually used in vacuum furnace brazing, i.e. when brazing aluminum. In next month’s article, I’ll discuss this very specialized situation in more detail.
Let’s examine this potential volatilization problem further by studying the chart shown in Fig. 2. This chart shows a series of what are called “vapor pressure curves” for a number of common metallic elements.
Each curve on the chart represents a different metal. Notice that each curve slopes downwards from upper-right down to lower-left of the chart. Please also notice that there is a small circle on each curve, this circle representing the melting temperature of that particular element at one standard atmosphere of pressure (for informational purposes only). The left-hand axis of the chart represents pressure in mm of Hg, “Hg” being the symbol for the metal mercury, since mercury (Hg) is liquid at room temperature, and was, therefore, the liquid metal used to fill the glass tubes used in the early experiments by the Italian scientist Evangelista Torricelli to determine atmospheric pressure. To honor him, the scientific community agreed to use his name for certain pressure designations, and so they used an abbreviation of his name (Torr) to represent each millimeter (mm) of pressure. Thus, the amount of pressure needed to lift a column of mercury (Hg) by 1 mm was named after him, i.e., 1 mm of Hg = 1 Torr. Thus, the left-hand axis of the chart in Fig. 2 can also be designated as “Pressure in Torr”.
By comparison, the right-hand vertical axis represents pressure in millitorr (or microns), i.e., one-thousandth of the pressure shown on the left-hand axis. The horizontal dotted line that crosses the chart at 760-mm (just below 103 on that left-hand vertical axis) represents one standard atmosphere of pressure. It is important to note that the point at which any curve crosses that dotted line represents the approximate boiling-point temperature of that particular metal (i.e., the temperature at which that metal wants to turn to a gas) at one standard atmosphere.
For example, take a look at the curve for Chromium (Cr) — you will notice that it crosses the dotted line at a temperature of approximately 4352°F (2400C). Many people see this and believe that since it takes such a high temp to volatilize (vaporize) chromium, they should never have any problem with Cr outgassing in a vacuum furnace. Really? Many people braze stainless steels (which contains chromium) at vacuum levels approaching 10-5 Torr. Please follow the Cr-curve down to the left as the pressure measured on the left axis goes down from the dotted line (room pressure) down to 10-5 Torr. You can readily see that at 10-5 Torr the temperature at which Cr volatilizes has dropped down to only about 1800F (950°C). Since nickel-brazing of stainless typically takes place at about 2000-2100°F (1095-1150°C), please understand that you will indeed be volatilizing chromium during this brazing operation, which will condense on the furnace walls, giving them a greenish/bluish coloration.
As the vacuum pumps create a stronger and stronger vacuum in a vacuum furnace and the pressure continues to drop, a “harder” vacuum is created, to use one of the older words in vacuum terminology. However, as just noted, this can create problems for metals, since they can more easily vaporize (outgas) as the pressure in the vacuum furnace decreases.
Look at the curve for zinc (Zn) on the upper left side of this chart in Fig. 2. Zinc is a major ingredient in brass, and in some of the commonly used silver-based BFM. Note that the curve for Zn crosses the dotted line at the top of the chart around 1650°F (900°C), indicating that zinc starts to vaporize at standard room pressure at this temperature and that this vaporization temp drops quickly as the pressures decrease in a vacuum furnace. At a pressure of only about 10-2 Torr, the Zn would begin outgassing at only about 660°F (350°C) or so. Since it would be necessary to heat the vacuum furnace to about 1750°F (950°C) for a BAg-8 silver-based BFM to melt and flow properly, it can be seen that the zinc would outgas so much that perhaps all of it will have volatilized by the time brazing temp was reached, and not only would the brass part be a porous mess, but the furnace and pumping systems would be grossly contaminated and perhaps ruined!
Some people may suggest that instead of BAg-8, why not then use one of the much lower melting silver-based BFMs such as BAg-1, which melts at temperatures far lower than that needed for BAg-8. Unfortunately, the reason that BAg-1 melts at that low temp is because of significant additions to the BFM of temperature-lowering ingredients such as zinc and cadmium! Notice on the chart shown in Fig. 2 where the cadmium (Cd) curve is – it’s actually to the left of the Zn curve, which means it would be a worse actor in a vacuum furnace than Zn!
Zinc and cadmium are what is known as “high vapor pressure” metals, which simply means that they can easily volatilize when heated in air, or when heated in the slightest vacuum. Other “high vapor pressure” metals include lead (Pb), and even magnesium (Mg).
WARNING — Do not think that perhaps you can “suppress” this outgassing of high vapor pressure metals by operating your vacuum furnace with a partial pressure of argon, or nitrogen, etc. The vaporization characteristics of these high vapor pressure metals is such that no amount of partial-pressure in a vacuum furnace can ever suppress their outgassing and the problems caused by that outgassing.
CONCLUSIONS: Never vacuum-braze any zinc-containing metals — that includes any base metal parts (such as brass), or any BFMs that contain zinc or cadmium — since these high vapor pressure metals will readily outgas when being heated in a vacuum furnace, even if partial-pressure brazing is attempted, and can ruin your vacuum furnace and its pumping system. Any zinc or cadmium containing metals should ONLY be brazed out in open air, using a torch or induction setup (manual or automated), and using flux to prevent oxidation of the parts.
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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 47-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. Dan can be reached via e-mail at firstname.lastname@example.org, and his website can be visited at http://www.kaybrazing.com/
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