As we start out this new year, I think it would be good to review the definition of the word “brazing”, since it is somewhat complex, and some aspects of the definition are still being misused by a number of people in the brazing industry, not only in their speech but also in their writing. So, it’s time to take a fresh look at the word, especially for those who are somewhat new to brazing, and perhaps for some older persons who never really understood what that definition meant in the first place!

The American Welding Society (AWS), in the Fifth Edition (2007) of their AWS Brazing Handbook, defines brazing as “a group of joining processes that produces coalescence of materials by heating them to the brazing temperature in the presence of a filler metal having a liquidus above 840F (450C) and below the solidus of the base metal. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action.”

The handbook goes on to state in its Preface that brazing needs to meet the following three criteria: “(1) The parts must be joined without melting the base metals. (2) The filler metal must have a liquidus temperature above 840F (450C). (3) The filler metal must wet the base metal surfaces and be drawn into or held in the joint by capillary action.”

Even though it is a “technical definition” designed to be read and understood by “technical” engineering-type people, it still has some inaccuracies in the way it is worded, and may not be easily understood by lay-persons at all.

1. The temperature conversions involved

First of all, the specific temperatures mentioned in the definition are shown incorrectly as 840F (450C), whereas it really should be shown as 450C (840F). The 840F is merely an approximate conversion to Fahrenheit of the 450C. Actually, 450C converts to 842F, which has merely been rounded down to 840F for convenience sake. The agreed-to temperature, worldwide, for brazing is 450C, not 840F. Therefore, all publications should more accurately refer to brazing as occurring above 450C (840F), thus showing the approximated temperature conversion in parentheses, rather than the other way around.

I find it very interesting that people too often try to give “exact” conversions when they refer to temperatures, rather than just approximations. I remember seeing an article in which it was suggested that a particular brazing run should be done at 2100F (1149C), rather than rounding the 1149C up to 1150C. Rounding the temps to the nearest 5-or-10 degrees in either “F” or “C” is often “close enough” for most brazing applications since you can’t accurately control any brazing process to +/- 1-degree. Thus, saying that brazing filler metals must have a liquidus above 450C (840F) is close enough, without trying to convert to the nearest degree. Thus, when I saw an article that referred to brazing as occurring above 840F (449C), I knew that the author didn’t really grasp that temperature “standard” correctly, believing that 840F was an exact temp.

2. What does 450C (840F) really refer to?

Here’s another misunderstanding about that definition, namely, what does that 450C (840F) actually represent? This temperature refers to the “liquidus temperature” of the brazing filler metal (BFM), and doesn’t refer to the actual brazing temperature being used.

I’ve heard someone say that the definition is telling them that “…brazing is a process that is supposed to take place above 840F, whereas soldering is any process taking place below 840F.” Does that mean that if I’m joining metals together at a temperature of 900F then that process must be, by definition, “brazing”? No, not at all! That would be an incorrect conclusion!

Brazing

Important: The 450C (840F) refers only to the published “liquidus” temperature of the filler metal. What’s that? By definition, the “liquidus temperature” of a metal is the temperature at which, when being cooled, that liquid metal will begin to solidify. Thus, at temperatures higher than the “liquidus” temp of any metal, that particular metal should still be liquid, i.e., in its molten state. It should be free-flowing.

Looking, for instance, at pure copper, it has a liquidus temp of 1083C/1981F (please note that those temps are exact, and should be stated as such). When it is heated to a temperature above its liquidus the copper metal should be fully molten, and, if it is being used as a brazing filler metal (BFM), it could then flow into a joint that needs to be brazed together. At temperatures lower than its liquidus, the copper would not be fully liquid, and thus would not be able to flow.

Note: Remember, the “liquidus” temperature is a temperature, and not a “state of being” (liquid, solid, etc.). I often hear people say: “When the filler metal went liquidus…..” No, that is wrong! That person SHOULD have said: “When that filler metal became liquid after it was heated above its liquidus temp….” Please use the term correctly.

Recommendation: Always heat the brazing filler metal (BFM) to a temperature higher than its liquidus, and try to add enough extra heat to help make that liquid-metal even more free-flowing, so that it can easily move into a joint being brazed. For all metals other than aluminum I like to suggest that the BFM be heated to about 100F (50C) above its published liquidus. This will allow enough extra heating to make the BFM quite fluid so that it can easily flow into a joint being brazed. You cannot use this same temp range for aluminum, otherwise, you might melt the base metals, since the melting points of the aluminum BFMs are fairly close to the melting points of the aluminum base metals being joined.

Please remember that all thermocouples, furnace instruments, BFM compositions, etc., have “tolerances” to them, which must be taken into account. I remember one person who complained that he couldn’t get the BFM to flow, even though, according to his furnace charts, he had heated the furnace to almost 5F above the published liquidus temp of the BFM. He didn’t understand that he needed to take all these tolerances into account, and thus, needed to braze at a temp much higher than the published liquidus temp to ensure that the BFM would indeed flow as desired.

Important Thing To Remember From This: The temperature used in the definition of brazing is merely defining the physical melting characteristic of a brazing filler metal (BFM), and is not saying or suggesting anything about recommended brazing temps that someone should use when they are brazing. It is merely telling you that the actual brazing filler metal (BFM) must have a “liquidus” temp above 450C (840F) in order to be classified as a brazing filler metal.

3. “Coalescence” of materials

The word “coalescence” used in the definition of “brazing” means that there should be a permanent bonding/joining of two or more materials by this brazing process.

To ensure that “coalescence” will occur, it is first of all necessary that the mating surfaces inside the joint (also known as the “faying surfaces”) must be clean, i.e., free from surface contamination by oils, grease, dirt, oxides, finger-prints, etc. Clean surfaces are required for good brazing so that the molten BFM can flow into the joint between the faying surfaces and alloy with the materials on either side of the joint.

It is very important that the molten BFM’s chemistry be compatible with that of the metals (materials) being joined, i.e., it must be able to diffuse into, and alloy with, those base metals (materials) being joined. Thus, cleanliness and compatibility are two very important criteria to ensure that “coalescence” can occur between the BFM and the materials being joined.

Brazing

4. Brazing must be below the solidus of the base materials being joined

The “solidus” temperature of any material is the temperature, below which, any particular material is completely solid. When any metal is heated to its solidus temp, that metal will begin to melt. And at temps much higher than its solidus temp, that metal will melt more and more. Because you don’t want to actually melt any of the base metals that you’re trying to braze, it is always advisable to be sure that your brazing temperature is lower than the “solidus temp” of the base metals you’re trying to braze, since that solidus temp represents the temperature where that base-metal will start to melt! When brazing, you only want the BFM to melt and flow by capillary action into the joint between two or more SOLID metals. Whereas in welding I might want some of the base metal to melt, I do NOT want to melt any of the base metals when I am brazing! Thus, the brazing temp should always be below the solidus-temp of any of the metals (materials) I am trying to braze together.

5. Capillary action

BFMs can be pre-placed inside a joint, such as when a preform sheet is used, or when the base metal might be pre-clad with a roll-bonded layer of BFM. But BFMs can also be placed on the outside of a joint, such as when a brazing paste or a preform-ring is put on the outside of joint, or a BFM-rod/wire is fed into a joint during heating. Especially when the BFM is applied to the outside of a joint, that BFM must melt and then be drawn into the joint by capillary action. Capillary action is a strong surface-tension force that can, when properly used, draw molten BFM deeply into a joint, even upwards against gravity. For strong capillary action to occur, the joint faying surfaces should be quite close together, with a gap that is typically about 0.001-0.005” (0.025-0.125mm) thick. Further information about recommended gap-clearances can be found in a number of previous articles I’ve written for this site.

Conclusions

Hopefully, this article may help some of you to more fully understand the words and phrases in the official definition of the word “brazing”. Obviously the key is to be sure that the surfaces being joined are clean, the assembly is heated to a temperature high enough to ensure that the BFM will melt and flow nicely and be drawn by capillary action into the joint being brazed, but at a temp that does not cause any of the base-metals (materials) being joined to melt.

Problem solve and improve with our technical experts.