Finishing our discussion from Part 1 in December…
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 (Fig. 2), where “T” is the thickness of the thinner of the two sheet-metal pieces being brazed 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.
As mentioned earlier, about 1,200 lap-joint specimens were tested in 10 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 1/8T 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 up 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 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 inch/0.000-0.050 mm) at brazing temperature.
Important Note: Aluminum Brazing
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 rather than being clad to the base-metal, the proper amount of overlap should only be in the range of 1T to 3T (instead of 3T) 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 temperatures, there is very intense interdiffusion 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 since the 3T overlap already contains a significant overlap safety factor. Thus, a 3T overlap is all that anyone needs in a lap joint.