Can Cadmium and Zinc Be Used in Brazing

Earlier this year, I wrote an article on this website about the use of Zinc (Zn) in brazing, and strongly warned readers to never vacuum-braze any metals/alloys that contained any Zn in their chemistry or if they had Zn-plating on their surface. The same warnings apply to Cadmium (Cd).

Zn and Cd are added to some brazing filler metals (BFMs) to help lower the melting point of those particular BFMs. Both Zn and Cd are very effective “temperature-depressants”, i.e., they significantly lower the melting temp of any silver-based BFMs into which they are added, and they also help those BFMs to “wet” (i.e., to diffuse into and spread out over) the clean base-metal surfaces that are being brazed. Thus, both of these elements began being added to silver-based BFMs early in the last century in order to enable low-temp torch-brazing (flame-brazing) that was not only easy to perform but produced high-quality joints. This is still the case today. However, both Zn and Cd are highly volatile, and can easily outgas from BFMs when heated. This must be clearly understood when considering which brazing methods to use for such BFMs so that the resulting brazed joints will be properly made.

Last Updated on Thursday, 12 November 2020 22:22

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Brazing-Drawing Challenge – Part 2 (Answers)

We have a winner in the brazing-drawing challenge that I presented to you last month in this column. Peter Ditzel, Principal Engineer (M&P) at Parker Hannifin in Mentor, Ohio was the first respondent to find at least seven (7) brazing errors in the drawing as it relates to vacuum-brazing of aluminum alloys, and thus qualifies to receive a 10% discount on a registration for himself (or for a fellow employee there at Parker in Mentor, Ohio) to either one of my two upcoming 3-day brazing-training seminars in Spartanburg, SC (October 1-3) or in Simsbury, CT (Nov. 12-14, 2019). Congratulations, Peter! Please contact me at This email address is being protected from spambots. You need JavaScript enabled to view it. to take advantage of this discount for yourself or for a co-worker!

Fig. 1 shows the challenge-drawing once again. A friend of mine in the brazing world created an unusual brazing assembly-drawing as a “spoof” or “gag”, and intentionally included at least seven (7) brazing “no-no’s” that he himself had learned during a seminar that he attended earlier that year. He sent this “gag” assembly-drawing to me, humorously suggesting it might be a tool that I could use in my teaching to help other people learn what they should NOT do when vacuum-brazing aluminum alloys!

Last Updated on Thursday, 12 November 2020 22:16

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Brazing-Drawing Challenge

Shown in Fig. 1 is a drawing of an aluminum part that is to be vacuum-brazed. Are all the drawing callouts correct from a brazing perspective? Please examine the drawing carefully, and see if you can correctly find seven (7) things that are wrong with that assembly drawing. You’ll find things in the procedure-notes, and perhaps in the design itself. Send your list of “findings” to dan.kay (at) A description of each of these seven items will be discussed in detail in next month’s article (“Brazing-Drawing Challenge, Part 2) so that you can see what I strongly recommend that you should NOT do when vacuum brazing aluminum parts.

If you have attended one of my intensive 3-day brazing-training seminars, then you should be able to locate all of the mistakes. But if you haven’t attended one of my programs, then I hope you’ll find this to be an interesting challenge for you.

The person who is the first to correctly identify the “problems” in this assembly-drawing will be recognized personally in this column next month, and will also qualify for a 10% discount on a seminar-registration (That’s worth more than US$ 195) for themselves (or for one of their fellow workers at the same work-location) to attend either one of my two (2) brazing training seminars this fall in either Spartanburg, South Carolina (Oct.1-3) or in Simsbury, Connecticut (Nov. 9-11).

Last Updated on Thursday, 12 November 2020 22:16

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Temperature Uniformity Survey (TUS) Fixturing

It is very important that any company operating a brazing furnace (be it an atmosphere furnace or a vacuum furnace) should know how uniform the temperature is throughout their brazing furnace during the entire brazing cycle. This temperature uniformity is typically measured by placing a special test-rack into the furnace chamber, as shown in Fig. 1, to which thermocouples (TC’s) are attached.

Accurately controlling temperature (and temp-uniformity) throughout a brazing cycle is essential, and you would think that anyone involved in operating brazing furnaces would not only understand this but would also take needed steps to ensure the accuracy and control of temperature throughout each of their brazing cycles. Surprisingly that is not always the case, as I have seen over the years.

Last Updated on Thursday, 12 November 2020 22:40

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“Dummy-blocks” used in Brazing Furnaces

When parts are to be furnace-brazed in commercial job-shop vacuum furnaces or in standard atmosphere furnaces, they are typically placed on a rack inside the furnace, and then the furnace door is closed, thus completely hiding those parts from view. Once the actual furnace brazing-cycle begins inside that furnace, you really don’t know what’s happening to those parts, since you can’t see them. The only practical way to determine if they are actually being successfully brazed is to watch the furnace’s instrument panel in order to find out what’s happening inside the furnace (temperature, vacuum level, leak-up rates, etc.). Obviously, when you open the furnace door after the brazing cycle is over you will quickly see the results of the brazing cycle, which you hope will be fine. But when (not IF, but WHEN) something goes wrong with one of your brazing runs, and you see that the parts did not braze well (or did not braze at all), then the importance of properly instrumenting your brazing load will become very clear to you!

By the phrase “properly instrumenting your brazing load” I am primarily referring to the proper use of thermocouples (TC’s) to accurately monitor and record the temperatures being experienced by the parts themselves as they are being brazed! This is very important. As shown in Fig. 1, TC’s that are attached to the parts being brazed are known as “load-TC’s” since each such assembly is part of the “load” going into the furnace to be brazed.

Last Updated on Friday, 13 November 2020 00:14

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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, 12 November 2020 22:22

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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.

Last Updated on Thursday, 12 November 2020 22:38

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