Whereas welding often depends on weld-fillets for strength, brazing does not!Ī properly designed and brazed structure should never fail in the brazed joint. Also, remember that joint strength is a direct function of the ability to fill the entire capillary space between the mating parts, and is not at all dependent on fillets outside the joint. For good joint strength, the faying surfaces of the lap joint should be close and parallel to each other, and not mismatched as shown at the bottom, the right side of that illustration. The joint strength of a brazed lap-joint is a function of overlap distance and the thickness of the brazed joint itself (more about this next month). Any greater overlap does not contribute to joint strength, and less than 3T might cause failure in the braze joint rather than in the base metal. Consequently, you will usually find that lap-joints have a higher load-carrying ability than butt-joints.įor lap-joints, the “reasonable’ amount of overlap is three-to-six times (3T-to-6T) the thickness (“T”) of the thinner of the two members being joined. As you can see in the illustration, the area covered by the BFM after it has melted and flowed through the joint is much larger for a lap-joint than for a butt-joint. As the name implies, the two parts intended for brazing are simply laid on top of each, and the capillary spacing between the two pieces will comprise the braze joint when the BFM melts and flows. Lap-joints: The easiest type of brazement to make is the lap-joint. Rounded edges can seriously reduce the effective braze area necessary for joint strength. Therefore, when brazing butt-joints, it is very important that joint edges be squared and parallel, and not rounded or chamfered (see illustration). The main weakness of butt-joints is the small braze area, which is limited to the cross-sectional area of the thinner of the two members being joined. However, when butt-joints are diffusion-brazed with high-strength BFMs, these joints can exhibit very high strength, adequate for most purposes. Special fixtures are usually employed in the furnace for this purpose.īutt-joints are usually used where strength requirements aren’t too critical, or where the use of a lap-joint would be objectionable (such as thickness constraints). If the BFM is pre-placed between the two parts being joined, then pressure may be required to force the parts together when the BFM becomes liquid, in order to close-up, or minimize, the gap between the parts. When the assembly is then furnace brazed, the BFM will melt and flow into the braze-joint by capillary action (but only if the joint spacing between the parts is correct, and the faying surfaces of the joint are clean). Then, the brazing filler metal (BFM) is either pre-placed between the two parts prior to assembly, or perhaps applied along the top edge of the joint after the two parts are already butted together. ![]() When shops do not take the time to ensure proper fit-up of the parts before brazing, costly mistakes often result!īutt-Joints: In this joint design, the ends of the two pieces of metal are butted-up against each other. These shortcuts can result in poorly brazed assemblies, or in premature field-failures when the parts are placed in service. The illustration below shows both good and bad ways to assemble such joints.Ī number of brazing shops today, unfortunately, take shortcuts or overlook important fit-up considerations in an effort to quickly make parts and braze them so that they can get them back to their customer as quickly as possible. All other joint designs are modifications of these two. ![]() There are basically two types of joint designs used in brazing: butt-joints and lap-joints. This brief article looks at the first of some important design considerations to ensure that brazed joints will work. For successful brazing to occur, the joints to be brazed have to be designed properly, and then properly manufactured to attain and maintain those shapes and dimensions.
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