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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.

This question comes up frequently and needs to be addressed again. As shown in Figure 1, the brazing filler metal (BFM) has filled the inside of the tubular joint but has a slight recess at the top edge of the joint. There is no large external fillet (or “meniscus”) of BFM on the outside of the brazed joint. Notice in the photo how the recessed material has a concave shape to it. The “meniscus” of any liquid is the curved shape of the surface of that liquid caused by surface tension. A meniscus can be either concave (desirable in brazed joints) or convex. In Fig. 1, there is a concave recessed meniscus to the BFM at the top edge of the joint. Is this okay?

Many people who see such a joint may incorrectly think that any fillet, whether it is for welding or for brazing, must extend beyond (outside) the joint in order to be acceptable and that any joint that has a recessed-meniscus, as shown in Fig. 1, has to be rejected as being “incomplete”. This is what I call “weld-think”, and has resulted in many such joints being re-brazed (unnecessarily) in order to add more BFM to the joint until the resulting joint shows a large external fillet. This is erroneous thinking that can actually hurt the brazed assembly.

A number of years ago I wrote an article about the question of refrigerating brazing paste, something that has caused real problems for a number of people in the brazing industry, and which is still doing so today. A recent question about this topic was sent to me and indicated to me that it’s time to once again discuss this topic.

Many people in brazing shops today are still receiving brazing filler metal (BFM) pastes in containers (both large and small) indicating on the label that the BFM-paste must be refrigerated prior to use. Without proper explanation, a simple statement such as “Must be refrigerated prior to use” can lead to significant misunderstandings about what is meant by such a phrase, and has caused a lot of difficulties for brazing personnel who have erroneously believed that brazing paste, according to that warning, has to be “cold” when it is being used in the shop. Thus, before they use the brazing paste, they place it in a small refrigerator, such as that shown in Fig. 1, and then remove it the next day for use in their shop. That is completely wrong.