Fig. 1. The average unit shear stress (circles) and base metal tensile strength (triangles) as a function of overlap distance. The open symbols represent failure in the brazed joint, whereas the dark symbols represent failure in the base metals. Overlap distance was shown in inches, mm, and in overlap “T” distances.

A half century ago (back in the early 1960’s) a lot of research work was done by The American Welding Society (AWS) Committee on Brazing and Soldering to determine appropriate criteria for brazing lap joints (the preferred type of joint design for assemblies requiring the ability to withstand high pressure in service, such as gas bottles, etc.). The results were published in their committee report: AWS C3.1 in 1963, one of the recommendations of which was that joints should have an overlap of 3T or more, where “T” is the thickness of the thinner of the two sheet metal pieces being brazing together.

Here’s how that recommendation came about. The AWS C3 committee arranged to conduct a series of round-robin testing in ten different laboratories around the country, using two different shear-type joint designs, four different base metals, and three different types of brazing filler metals (BFMs), for a total of about 1200 brazed shear test specimens. Their intent was not only to find out what constituted a satisfactory joint overlap design for brazing, but also to develop an easily reproducible test specimen that was “realistic” to the real-life world of brazed components in industry and which could become a “standard” that everyone could (and would) use to evaluate joint strength.

The data generated by the ten labs was plotted on graphs such as that shown in Fig. 1.

Much of their testing showed that with overlaps exceeding 2T overlap, failure in shear-testing always occurred in the base metal, and not in the brazed-joint, whereas most of the brazed joints with overlaps less than 2T failed in the brazed joint itself.

Based on the results of that extensive study, the brazing industry has adopted the guideline that lap-joints should be designed with an overlap of at least 3T (as shown in Figure 2) where “T” is the thickness of the thinner of the two sheet metal pieces being brazing together.

For lap-joints in brazing, I always recommend the “3T-to-6T Rule,” the 6T being the limit beyond which, in my experience and opinion, overlaps merely become wasteful of BFM and base metals, since no additional strength or leak-tightness benefit is obtained by such long overlaps.

A simple lap-joint design, in which the overlap is about 4.5 T, where “T” is the thickness of the thinner member of the joint. The 3T-to-6T rules applies to all metals except for aluminum

Fig. 2 A simple lap-joint design, in which the overlap is about 4.5 T, where “T” is the thickness of the thinner member of the joint. The 3T-to-6T rules applies to all metals except for aluminum (see discussion about aluminum later in this article).

As mentioned earlier, about 1200 lap-joint specimens were tested in ten different labs around the country using different base metals (stainless, carbon-steel, and copper), different types of brazing filler metals (silver, copper, and nickel) and different amounts of joint overlap (from about 1/8 T up to 6T). These tests showed that brazing sheet-metal with overlaps less than 2T commonly failed in the brazed joint itself when subjected to tensile tests, but when overlap averaged about 2.3T or more, the failures invariably occurred in the base metal away from the joint and not in the brazed joint itself. So, the brazing community at that time decided to recommend an overlap of 3T (rounded upwards from 2.3 to provide some safety factor) as the minimum overlap to use in a lap-joint design, so that failures should occur in the base metal and not in the brazed joint. Such an overlap also should also provide full hermaticity (leak-tightness) and fatigue-resistance to the joint. Of course this all presupposes that the joints are properly made with clean faying surfaces, and tight braze-joint clearances (0.000-0.002”/0.000-0.050mm) at brazing temperature.

Important Note Re Aluminum Brazing

Instead of 3T overlap, when brazing an aluminum-to-aluminum lap-joint in which the aluminum BFM will be applied as a preform ring, or hand-fed in wire form, etc., rather than being clad to the base-metal, the proper amount of overlap should only be in the range of 1T-to-3T in such situations, because of the extensive interaction of the aluminum BFM with the aluminum base metals at the brazing temperatures used.

Remember that when joining aluminum, the brazing temperatures are very close to the melting point of the aluminum base metals. At such high temps, there is very intense inter-diffusion of the BFM into the base-metal, and base-metal into the BFM — so much so that there is little capillary action running along the joint surfaces. Instead, the BFM wants to diffuse/alloy almost immediately into the base-metal rather than merely flow over the faying surfaces into the joint. Because of all this, we recommend a much shorter amount of joint overlap for aluminum brazing, knowing that 1T-to-3T will indeed be fully satisfactory to give us full-strength, hermaticity, and fatigue resistance in the aluminum brazement.

Please note once again that for all other metals (except aluminum) a 3T-overlap is sufficient for full-strength, etc., since the 3T overlap already contains a significant overlap safety factor — thus, a 3T-overlap is all that anyone needs in a lap joint!

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