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Essential Criteria for Brazing: Item 1 - Proper Design for Brazing

Over the almost 45-years of my brazing career I have discovered that there are a number of fundamental principles that must be understood and followed if successful brazing is to occur. Over the next few months we will look at each of these principles in more detail, but I will merely introduce them to the reader here.

Brazing is a wonderful joining process, and also a forgiving process. By this I mean that even when you do not follow all the brazing principles exactly, brazing can work pretty well for you, but within limits. Gross disregard for some of these principles will, in fact, lead to failure of parts in the field (or in your brazing shop before parts are to be shipped to your customers) and are responsible for most of the problems people face with poor-quality brazed joints. By comparison, when people understand these principles, practice them well in their shops, they usually find that there are very few, if any, problems with their brazing operations and, as a result, their customers are quite satisfied with the brazed products shipped to them.

Fig 1
Fig. 1 -- Tube-In-Fitting joint showing reasonable penetration of joint by the brazing filler metal (BFM). Notice the small, concave shape of the braze fillet on outside of joint (at right side).

Figure 1 shows a typical cross-section of a tube-in-fitting joint, which can be made from a wide variety of metals. The brazing filler metal (BFM), applied as either a paste or as a preformed-ring on the right-hand surface of the joint where the tube enters the fitting, has melted and flowed into the joint by capillary action, and has actually come out of the back-side of the joint at the far left of the photo. This is actually a good brazed joint.

So then, what are the principles of brazing that must be followed? Table 1 shows a list of the essentials – in order of importance – that I would call to the attention of the reader.

table 1
Table 1. Fundamental criteria that brazing personnel need to fully understand and follow in order to insure good brazing in their shops.

Criteria#1. Proper design for brazing.

This is number one on the hit-parade. Designing a braze-joint is very different from designing a weld-joint. People do not often understand this, and as a result, many brazing shops are asked to braze parts that were actually optimized for welding, and not for brazing! Unfortunately, braze-design is not taught in academic environments (colleges, universities, and trade-schools) and most principles of design taught in such environments are based on the welding world, since that is what most instructors have learned. I would like to change that.

To begin with, look at the diagrams in Fig. 2.

Fig 2
Fig. 2 -- Brazing usually involves either butt-joints or lap-joints, and sometimes a combination of the two.

Brazing depends on what is happening inside the joint, not on the outside. The two left-hand drawings (butt and lap) show close fitting, parallel joints into which the molten brazing filler metal (BFM) has flowed by capillary action. This is where all the “goodness” of a brazed joint lies – not in external fillets, but rather in the flow of molten BFM between the faying surfaces (the inside surfaces of the joint). Welding, by comparison depends on external fillets.

IMPORTANT: Notice that for most metals, lap joints are used (including tubular joints), and the amount of overlap of the parallel surfaces should typically be about 3-to-4T (6T max), where “T” is the thickness of the thinner member being brazed. This has been verified over many years, and when such overlap is used, any failure of the brazed assembly will typically occur in the base metal away from the joint, not in the joint itself! An exception to this 3-to-6T rule is aluminum, because of the strong interaction between the molten aluminum BFM and the base metal. Since aluminum BFM typically melts within 50-100F (30-60C) of the melting temperature of the base metal, there can be intense alloying between the two, and the molten BFM does not typically penetrate as far into the joint. So, for aluminum brazing we suggest an overlap of about 1-to-3T instead.

As we will see in a future article dealing with Criteria#3 (good fitup), the braze joints should typically have a gap clearance of about 0.001 to 0.005” (0.025 to 0.15mm).

Carrying this design analysis one step further, look at Fig. 3, where I show a comparison of welding design vs. brazing design. The two designs in the top row are used in welding. Notice that the tube-through-plate design on the top right is such that when welded, either full-penetration or partial-penetration welds might be applied from one side only, or perhaps from two sides (i.e., both top and bottom, etc.). Hmmmmm…….. don’t I see those identical tube-through-plate designs often used in brazing?

Fig 3
Fig. 3 -- Comparison of welding designs (top) versus brazing designs (on bottom row).

Note that proper brazing design (bottom row) should involve the use of overlapping surfaces (faying surfaces), such as those shown in Fig. 2. However, it is very common in most brazing shops that I visit to see tubular joints made in the manner as shown in the upper right hand side of Fig. 3, which is actually a welding-design.

Because holes are often punched through plates/sheets/tubes, etc., the surface around the punched hole is usually no longer flat, but may have a contour such as that shown in the top left drawing of Fig. 4. If such a design were to be brazed, it would involve the casting of a large amount of BFM into that joint to braze it together. This is NOT good brazing practice, since castings are not reliable when it comes to leak-tightness and/or strength.

Fig 4
Fig. 4 -- Note in top left drawing that because a typical punched hole in a sheet, plate, or tube is not square, then the molten BFM must create a “casting” to be able to encompass the entire joint area. But castings are notoriously weak and prone to cause leakers through the joint.

A typical application of these design principles can be seen in Figure 5, in which an automotive tubular fitting is being brazed into another tubular component, using a crimp in the tube for “bottoming out” the one tube into the other so that it does not block any fluid flow in the tube into which it is being brazed. It led to numerous problems in service, including leakers.

Fig 5b
Fig. 5 -- Actual application of poor design in the top drawing (“A”) of an automotive tubular situation a few years ago that resulted in braze problems. It was resolved when they changed their design to that shown in “B”.

The brazing design shown in portion “B” of Fig. 5 included a “flat” on the tube so that the inserted fitting could rest on the flat surface and capillary action could create a good braze joint.

CONCLUSION: Brazing design is unique, and very different from welding designs. As shown in this article, care must be taken to insure that brazed joints have sufficient faying surfaces, properly close and parallel, so that capillary action can draw the molten BFM into and through the joint, resulting in a strong, sound, leak tight assembly.  And don’t forget that when lap-joints are involved for tubular or overlapping sheet metal joints, that the amount of overlap should be about 3-6T for most metals (where “T” is the thickness of the thinner of the two metal-components being joined, but is only about 1-3T for aluminum, as discussed earlier in this article.

NEXT MONTH: We will examine joint cleanliness requirements in more detail. Yes, “cleanliness is next to godliness” in brazing, and ignoring this fact can cause a lot of problems and leaky joints..


DAN KAY BRAZING SEMINARS: 3-Day seminars provide intensive training in "The Fundamentals of Brazing", covering the brazing of materials ranging from aluminum to titanium to ceramics! FIND OUT MORE  

Dan Kay - Tel: 860-651-5595: - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/

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