Ten Reasons to Choose Bra...

Ten Reasons to Choose Brazing

BY W. DANIEL KAY
W. DANIEL Kay is President, Kay & Associates, Simsbury, Conn. Telephone (860) 651-5595; e-mail dankay@ct2.nai.net.


Knowing the strengths and limitations of brazing can help you decide whether it is the right joining process for your application.

Brazing dates back to at least the time of the Pharoahs of ancient Egypt. Over the millenia that have passed since then, brazing has found application in so many areas of human life that its continued, important role in our industrialized society is assured. Brazing is used to join materials in such diverse applications as jewelry, high-temperature ceramics, kitchen cutlery, bathroom faucets, automotive engines, jet aircraft engines and air-conditioning systems.

Why Choose Brazing?
Brazing is a joining process taking place above 840°F (450°C) in which the materials to be joined are commonly heated about 100°F (55.6°C) above the temperature at which the brazing filler metal (BFM) being used has turned liquid, so that the liquid BFM will cover all of the mating surfaces, and then alloy with and form a permanent bond between those surfaces. The BFM may be applied outside the mating surfaces, in which case, the liquid BFM will need to be drawn between the mating surfaces by capillary action. In other cases, the BFM may be formed in place by the metallurgical reaction (diffusion) between the base metals being joined and a plated metal on either or both of their mating surfaces. (For an official definition of the process, see the Brazing Handbook# [Ref. 1]).


FIG 1 - The casting shown at top needs to be faced, drilled and tapped in three placesw. Its is much easier to braze three threaded couplings/tubes into a machined block (bottom)

Brazing is, however, only one of several ways by which materials can be joined. Other important joining methods exist, such as fusion welding, soldering, adhesive bonding and mechanical joining, each of which has end-use applications for which it alone is optimally suited. Therefore, no single joining method, including brazing, should ever be viewed as being inherently superior to the others for all joining applications.

So then, why would someone choose brazing? This article will examine ten reasons for doing so. It is important to note, however, that when trying to decide on which one of possibly several joining methods to use, the final choice should be made only after thoroughly evaluating all the end-use conditions the part will see, such as service temperature, anticipated stress levels, fatigue, corrosion resistance requirements and any cyclic conditions it will encounter. Once these are known, certain joining methods may fall to the side and one or two others may become obvious preferred choices. With this in mind, letıs look at a number of factors that show the strengths ‹ and the limitations ‹ of brazing, so you can see where brazing fits into the overall joining picture.

1) Brazing permanently joins base materials. Brazing is, first of all, designed to permanently join materials together. Unlike some mechanically fastened parts joined with nuts, bolts or screws, brazed components are not usually taken apart after brazing. The intent is that when one or more parts are brazed together to form an assembly, that assembly will stay together permanently. And, when brazing is done properly, permanence can definitely be achieved for a wide range of end-use conditions.

2) Brazing is a high-temperatureprocess. As mentioned earlier, when brazing, component parts are heated to an elevated temperature (usually between 1000 and 2300°F [about 540 and 1250°C]), so the BFM will flow throughout the thin capillary space between the parts being joined. Since such high temperatures are used, one of brazingıs limitations becomes obvious, namely, that base materials must have melting temperatures above 1000°F. Therefore, plastics cannot be joined by brazing; instead, adhesives would be a more appropriate joining method.

3) Brazed joints are as strong as the base materials being joined. There is a common misconception in the marketplace that brazed joints are not strong, and only fusion welding can impart base-metal strength to an assembly. Not true. A properly designed and brazed assembly will be as strong as the base materials being joined. Period. That means, if the brazed component breaks (fails) in service, the failure will be in the base material, away from the brazed joint. Basically, in a lap joint (the most common type of brazed joint), as long as the faying surfaces overlap a distance equal to at least three times the thickness of the thinner of the two members being joined, and the clearance between the two parts (the joint thickness) is kept around 0.003 in. (0.075 mm) or less at brazing temperature, the brazed joint will be as strong or stronger than the base materials. Studies confirm this (Ref. 2).


FIG 2 - Several internal prazed joints are accomplished at the same time in one furnace braze by use of internal braze performs.

Further, for this to hold true for assemblies that will be subjected to shock and fatigue in service, no sharp corners (stress raisers) should be allowed at the edge of a brazed joint. Instead, brazed joints should be smoothly contoured so any service stresses are spread out over a large area and not concentrated right at the edge. Proper joint design is very important.

Even with butt joints (the other common type of brazed joint), failure will occur in the base metal, away from the joint when properly designed and brazed. In fact, to show why this occurs, tests were run many years ago in which two 304-stainless bars were brazed together in a butt joint configuration using a design that ensured a high stress concentration at the joint, and, therefore, part failure at that location in every test. However, it was surprising to find that, although breakage occurred in the joint in each test, it required higher and higher levels of loading to induce the failure as the joint clearances got narrower and narrower. The BFM tensile strength reached a peak of about 135 ksi when the clearance was approaching 0.001 in. The surprising thing about this is the same silver-based BFM in rod form has a tensile strength of only about 40 ksi when tensile tested on its own. Thus, joint clearance does, in fact, have a major effect on joint strength. Similar results will occur with many combinations of base metals and BFMs.

4) Brazing is economical for complex assemblies. Brazing allows complex assemblies to be made by brazing together basic component parts in one simple operation. Instead of making a complex casting, such as shown in Fig. 1 (top), the same assembly could be made by brazing readily available plate, bar and tubular structures together ‹ Fig. 1 (bottom). Time and material can be saved that might otherwise be consumed in designing and manufacturing complex molds and castings, or in machining away base metal stock to achieve a certain required shape. And, as compared to other joining methods, many parts can be made at the same time through bulk brazing procedures in furnaces, or via indexed work stations using torches or induction coils.

Figure 2 shows an assembly with several internal brazes, all of which can economically be brazed at one time in a single furnace-brazing operation, using pre-placed BFM. Many of these assemblies might be brazed at one time in one furnace load. Where these parts are used for high-temperature service, for corrosive service, in high-pressure applications or severe shock or cyclic fatigue, etc., brazing is the only way to effectively and safely join them.


FIG 3 - Brazing is efective in joining large surfaces. Channels were grooved into these two places prior to brazing to serve as internal cooling channels.
5) Large surface areas can be joined. For example, two large, flat copper plates each had cooling-water grooves machined into one of their surfaces. It was desired to put the two plates together, as shown in Fig. 3, so the machined grooves would form internal cooling channels when the plates were joined together into one assembly. Although adhesive bonding, soldering or brazing could have been considered, the strength and corrosion resistance requirements led to the selection of brazing as the best joining method to use. Large strips of brazing foil were preplaced between the plates, an inert furnace atmosphere was employed and the finished assembly came out flat and clean, with almost 100% braze of all internal faying surfaces (as determined by ultrasonic C-Scan testing).

Similarly, when it is desired to permanently join two different flat sheets or plates together via a lap joint, such as copper on stainless, or aluminum to stainless (as in the bottoms of frying pans), only brazing can do the job.

6) Stress and heat distribution. Whenever heat is used in a joining process, it can generate stresses in the assembly being joined. Since these stresses can affect dimensional stability of the part, and even its service life, they should be taken into account when the parts are designed so that service-related stresses can be minimized or spread out (no stress concentration points) as much as possible. Brazing has two advantages over fusion welding in this category. First, brazing temperatures are much lower than welding temperatures, so the induced stresses will be less. Second, brazing heat is more broadly distributed over the joint area, rather than being extremely localized, as in welding. In furnace brazing, the entire part is brought up to brazing temperature so there is virtually no temperature differential (also known as "delta T," or DT) throughout the part. Since DT within a part is what gives rise to stresses that can lead to distortion in the part, furnace brazing can be a very effective means to reduce or eliminate thermal stresses in a part.

Induction brazing and torch brazing (see lead photo) localize the brazing heat much more than furnace brazing, and it would, therefore, be expected induced thermal stresses will be higher with these two techniques than with furnace brazing, but theyıll still be a lot less than the stresses induced by fusion welding.

7) Brazing is very effective at joining dissimilar metals. As mentioned earlier, copper or aluminum can be readily brazed to stainless in the cookware industry. Tungsten carbide cutters are readily brazed onto tool steel shanks or drill bits, and pure graphite has been brazed to nickel-based superalloys (using layers of intermediate expansion materials to take up the differential thermal expansion stresses between the graphite and the superalloy). These are readily done with excellent results using standard, commercially available brazing equipment. Some of these base-metal combinations might be weldable (especially with either laser beam or electron beam welding), but not when large surface areas need to be intimately joined. Only brazing can accomplish this latter result.

8) Brazing can join metals to ceramics. It would be virtually impossible to fusion weld metals to ceramics, but such combinations can be readily brazed without much difficulty. This is usually accomplished in a furnace atmosphere in which the entire part can be slowly and uniformly heated and cooled so that thermal stresses are kept to a minimum. If needed, intermediate layers of materials can be brazed between the metal and the ceramic (or between two pieces of ceramics if that is what is being brazed) to help absorb any stresses resulting from differential material expansion.

9) Brazing can effectively join very thin metal to thick metal. Suppose someone wanted to join a 0.003-in.-thick strip of metal to one that was 3 in. (76.2 mm) thick. With brazing, that is not a problem. Although such a feat could be done with either laser beam or electron beam welding, it would be difficult to achieve with any other welding technique. Once again, depending on the service temperature involved and the corrosion resistance required, soldering or adhesive bonding might also be considered. But, if the part must survive corrosive conditions at 1200°F (650°C), then only brazing provides an inexpensive method to accomplish it.

10) Brazing can maintain precision dimensional tolerances. It is not uncommon for assemblies with extremely close dimensional tolerances to be furnace brazed and come out of the furnace still within the required tight dimensional tolerances. People make the incorrect assumption that any thermal processes associated with brazing will inherently cause parts to distort or to go out of dimensional tolerance. That just isnıt true. Since it has been demonstrated# (Ref. 3) that distortion relates almost entirely to the rate of heating and cooling, and very little to internal stresses in the part induced via machining, etc., one of the major benefits of furnace brazing is its ability to maintain tight tolerances throughout the entire process.

As you can see, brazing offers some unique qualities that set it apart from other joining methods.

References
1. Brazing Handbook, 4th edition. 1991. American Welding Society, Miami, Fla., p. xvii.
2. Brazing Handbook, 4th edition. 1991. American Welding Society, Miami, Fla., pp. 35­38.
3. Tennenhouse, C. 1971. Control of distortion during furnace brazing. Welding Journal 50(10): 701­711.