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When parts are brazed in a vacuum furnace, distortion of those brazed assemblies can easily happen. To prevent parts from distorting, people have tried a variety of things, including extended stress-relieving of components prior to assembling those parts for brazing, the use of rigid fixturing to try to keep parts from moving during a brazing cycle, and even making components heavier and more massive in order to make them more distortion-resistant. Distortion still occurs.

Yes, some of these things, such as stress-relief heat-treatment prior to brazing might help to some extent, but it is not the answer to controlling distortion during any furnace brazing cycle. The real key to controlling distortion is to control the heating and cooling rates used in the brazing cycle. Shown in Fig. 1 is an illustration of what a typical furnace brazing cycle might look like, in which the furnace temp is controlled by the “Furnace thermocouple (TC)” shown on the left side of the chart, and three (3) load TC’s are used on one part to see the temperature-spread (temp-differential, or delta-T) within that one part.

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 aluminum 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.

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 vacuum-brazing. I’ll look at the magnesium problem further in next month’s article. 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.

This topic has surfaced again in one of my client’s brazing shops, as brazing personnel encountered a brazing problem actually caused by the misuse of a brazing “Stop-off” in their braze-prep area. As the name of this product-type indicates, a brazing “stop-off” is supposed to be a paintable product that when applied to a metal surface, such as shown in Fig. 1, will “STOP” a molten brazing filler metal (BFM) from flowing into areas where it is not supposed to be, thus keeping it “OFF” any critical surface that is supposed to remain free from the presence of any BFM.

First of all, it is very important that the reader should understand that molten brazing filler metals (BFMs) do not like to bond to (or flow over) oxides, dirt, or lubricants. The presence of any of these contaminants on the surface of parts to be brazed can literally prevent the BFM from alloying with (i.e., bonding to) any surfaces on which any of these contaminants are found, and can prevent any capillary action of BFM from occurring.

Because oxides are so good at stopping the flow of molten BFMs, brazing stop-off compounds are made as blends of a variety of metallic-oxides and are packaged in a variety of forms: liquids, pastes, powders, sprays, or tapes (to name just a few). Fig. 2 shows some typical containers of paintable stop-off, which can be supplied in spray-cans, or containers (plastic or glass) ranging in size from very small to very large.