What Causes “Spatter” of BFM in Furnace Brazing?

Braze-spatter is occasionally seen in furnace brazing and usually occurs when someone is using brazing paste. I have not personally seen it happen when solid forms of brazing filler metal (BFM) such as wire, sheet or solid rings are used. When braze-spattering does happen, it might look something like the weld-spatter shown in Fig. 1, or perhaps like the soldering spatter shown in Fig. 2.

People are often surprised when they see braze-spatter on parts coming out of their brazing furnace and wonder if the spatter was due to careless application of the paste, or perhaps due to sloppy parts-handling by personnel in the brazing shop, etc., and fail to grasp that the vast majority of braze-spattering has to do with furnace heating rates in combination with the size of the brazing-paste bead that is applied to the joint area.

Last Updated on Wednesday, 27 November 2019 14:35

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The Famous Joint-Strength vs. Joint-Clearance Chart

One of the most widely used charts in the field of brazing is the strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930’s. This chart is shown below, in Fig. 1:

Notice that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the tensile strength (as shown on the vertical axis on the left-side of the chart) gets higher and higher. Although there is a lot of experience with this over the years, and general acceptance of this information is widespread, it must be pointed out that this chart is very specific only to the actual testing performed in making this particular chart, and may not be identical to tests performed by others using similar materials or conditions. But the general principal of increased joint strength with tighter gaps can be accepted.

Last Updated on Thursday, 07 November 2019 14:23

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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 Saturday, 07 December 2019 19:22

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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 Friday, 06 September 2019 17:27

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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 Wednesday, 14 August 2019 16:44

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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, 11 July 2019 16:39

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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 Thursday, 13 June 2019 16:32

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