Controlled “Heat-down” For Vacuum Brazing

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.

Last Updated on Thursday, 25 April 2019 15:43

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Magnesium (Mg) Vapor in Vacuum-Furnaces Used for Brazing Aluminum Components

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.

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 Tuesday, 12 March 2019 21:23

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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 Monday, 11 February 2019 21:15

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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.

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 Wednesday, 07 November 2018 18:19

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