An Old High-Temp Differential-Expansion Problem

Today’s brazing technology is based on a strong foundation of the brazing experiences of many people around the world over a period of many decades (even centuries). I’ve now been very active in the brazing world for almost 50-years, and, like my predecessors in the world of brazing, I’ve learned a lot about this fascinating joining process (and I’m still learning). In this article, I’d like to share with you one of my brazing experiences from many years back, one that involved high-temperature differential-expansion between an 18” (45 cm) diameter tool steel die and a thin carbide plate (round disc) that needed to be brazed to the die’s front surface for wear-protection.

When trying to braze together materials that have widely different Coefficients Of Thermal Expansions (COE’s), the material with the higher expansion rate (COE) will grow faster than the other when heated and shrink (contract) faster when cooled down from the brazing temperature. Once the two different materials have been brazed together and cooling begins, the shrinkage-rate differences between those two materials can produce significant shear stresses at the brazed interface between them. These stresses can, in some cases, be so strong that the thin brazed joint may be torn apart at either interface, or it might cause fracturing of either base metal, or perhaps open up a large crack through the BFM layer itself. It was this extreme — and very real — differential expansion problem that we had to address in today’s “history lesson”.

Last Updated on Thursday, 12 November 2020 22:12

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Feed BFM From One Side, Inspect On Other Side

There is an important guideline about brazing filler metals (BFMs) that needs to be followed when performing any kind of brazing — namely, it is very important to feed brazing filler metal (BFM) into only one side of the joint to be brazed, allow that molten BFM to be pulled completely through the joint by capillary action until the BFM is visible at the other side of the joint, and can be seen to produce a complete braze-fillet (meniscus) around (or along) that opposite end of the joint.

As shown in Fig. 1, a BFM wire is being hand-fed into one end of a joint being brazed (gap is shown much thicker than desired only for illustration purposes), and the BFM flows all the way to the other side of that joint where it can be inspected for complete pull-through. This is an example of proper brazing in which “you feed from one side, and inspect on the other”.

It is never wise to feed BFM from both sides of the joint, since by so doing you can trap air, moisture bubbles, outgassing materials, etc., inside the joint between the two incoming walls of molten BFM, thus greatly increasing the void content in that joint. Fig. 2 shows an example of a cross-section photo of a round electrical contact that was brazed onto a flat surface, where the BFM was applied completely around the disc-shaped part that was to be brazed to the substrate below it. When the BFM melted and tried to flow into the joint, it could not do so since the air and other contaminants inside the joint could escape to the outside since the “wall of liquid BFM” was blocking it. Thus, only a fillet formed around the outside of the joint but did not flow into the joint, as shown in that photo in Fig. 2.

Last Updated on Thursday, 12 November 2020 22:31

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Shell & Tube Heat Exchangers — Methods to Seal Tube-to-Header Joints

Shell & tube heat exchangers (S&THE) have been used for many years in a wide range of industries, including aerospace, oil refineries, chemical processing, beverage industries, and pharmaceutical industries, to name a few.

Brazing has become more and more the desired joining-method to “seal” these heat exchangers from any internal or external leaks. In this article I will look at some of the internal characteristics of S&THE’s, options for sealing them to make them leak-tight, and then at the end of the article, I will discuss some important requirements for proper joint design so that brazing can be used to cost-effectively make many tube-to-header leak-tight joints in S&THE’s at the same time! A typical shell & tube heat exchanger is pictured in Fig. 1, and a cross-sectional view of one is shown in Fig. 2, where it can be seen that an outer shell surrounds a bundle of tubes within that shell.

Last Updated on Wednesday, 08 April 2020 00:36

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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 Friday, 13 November 2020 00:13

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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, 12 November 2020 22:40

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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 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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