Don’t Vacuum-Braze Metals Containing Zinc

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.

Last Updated on Thursday, 12 November 2020 22:27

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Can Brazing Stop-Off Usage be Abused?

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.

Last Updated on Thursday, 12 November 2020 22:21

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Are Recessed Braze-Fillets Okay?

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.

Last Updated on Thursday, 21 March 2019 01:08

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Revisiting Refrigeration of Brazing Paste. Is Refrigeration Really Necessary?

Revisiting Refrigeration of Brazing Paste. Is Refrigeration Really Necessary?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.

Last Updated on Thursday, 12 November 2020 22:38

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Vacuum brazing of EGR Coolers

Vacuum brazing of EGR CoolersExhaust Gas Recirculation (EGR) coolers have been used extensively for many years on diesel truck engines, and are now also being used on gasoline engines. EGR coolers are specialized heat-exchanger assemblies that make extensive use of furnace-brazing (usually vacuum brazing) to create strong, leak-tight brazements that are capable of handling the very high temperatures involved in engine operations. EGR coolers used in diesel-engine applications help to reduce the formation of various nitrogen-oxides, such as N-O (nitrogen monoxide) or N2O (nitrogen dioxide), since such emissions are considered atmosphere pollutants, and are formed within a narrow temperature band in the combustion cycle. By recirculating some of the engine’s exhaust gases back to the engine through the EGR cooler, this cooled recirculated-gas gets mixed in with the incoming air entering the engine-cylinders and helps to reduce the combustion temperatures just enough so that less of these pollutants are formed.

Last Updated on Friday, 13 November 2020 00:12

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Don’t ever use a Brazing Paste-flux when Vacuum Brazing!

A number of years ago I wrote an article for this website about the use of a brazing flux in a vacuum furnace, in answer to an inquiry I had received about the advantages or disadvantages of using a paste flux to enhance the brazeability of components in a vacuum brazing furnace. It is time to once again bring up that subject for today’s brazing community, in order to be sure that everyone understands that you should NEVER put a brazing flux in a vacuum furnace!

The brazing fluxes that people usually refer to when asking about the use of “a flux” in a vacuum furnace are the same fluxes that they use when hand-brazing with a torch or induction coil out in the open air. Such fluxes, as shown in Fig. 1, are thick paste products, with the consistency of thick mayonnaise, and the paste is either white or black in color.

Last Updated on Thursday, 12 November 2020 22:27

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Torricelli’s Importance in Vacuum Brazing History

Vacuum brazing involves the removal of all gaseous atmosphere from the sealed chamber of a “furnace” used for high-temperature brazing, a “furnace” being defined as any enclosure that can be heated to a high enough temperature to accomplish a specific task, such as heating a home, or heating an atmosphere to a temperature that can melt particular substances, such as a solder or a brazing filler metal (BFM), etc. By removing the atmosphere from a furnace during brazing, oxidation risks are eliminated (or greatly reduced), and brazing success is enhanced greatly. Thus, brazing in a vacuum furnace continues to gain popularity in the brazing world every day, because of its ability to “create a vacuum”, i.e., significantly reduce the amount of atmosphere (thus, oxygen), inside the brazing furnace. But — where did our understanding of vacuum come from? One of the first practical experiments with vacuum was conducted by Evangelista Torricelli, an Italian scientist, back in 1643. Torricelli was a great thinker and put into action many of his theories related to both physics and mathematics

Last Updated on Thursday, 12 November 2020 22:40

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