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?