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. This brief article looks at the first of some important design considerations to insure that brazed joints will work. Next month we’ll look at joint clearance considerations in more detail.
Types of Brazed Joints
There are basically two types of joint designs used in brazing: butt-joints and lap-joints. All other joint designs are modifications of these two. The illustration below shows both good and bad ways to assemble such joints.
A 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. These shortcuts can result in poorly brazed assemblies, or in premature field-failures when the parts are placed in service. When shops do not take the time to insure proper fit-up of the parts before brazing, costly mistakes often result!
In this joint design, the ends of the two pieces of metal are butted-up against each other. 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 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).
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. Special fixtures are usually employed in the furnace for this purpose.
Butt-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). However, when butt-joints are diffusion-brazed with high-strength BFMs, these joints can exhibit very high strength, adequate for most purposes. 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. Therefore, when brazing butt-joints, it is very important that joint edges be squared and parallel, and not rounded or chamfered (see illustration). Rounded edges can seriously reduce the effective braze area necessary for joint strength.
The easiest type of brazement to make is the lap-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. 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. Consequently, you will usually find that lap-joints have a higher load-carrying ability than butt-joints.
For 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. 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.
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). 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, right side of that illustration. 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. Whereas welding often depends on weld-fillets for strength, brazing does not!
A properly designed and brazed structure should never fail in the brazed joint. If such a joint is stressed to failure, the failure should always occur in the base metal, not in the brazed joint! If asked “Just how strong will that joint be?” you should always reply that a brazed assembly, when stressed to the point of failure, should always fail at a strength level equal to, or greater than, the yield-strength (in the annealed condition) of the weaker of the two base metals being joined. That’s because the temperatures involved in brazing are usually high enough to fully anneal the metals being joined. And “failure” should usually be designated as the point where the weaker of the two metals begins to “stretch” (i.e., yield). Of course, if the brazed joints are heat-treated after brazing, then the assembly can become even stronger, in order to handle severe service conditions.
Next month I’ll discuss the issue of braze gap clearance for some different base-metal and brazing filler metal (BFM) combinations, and how that affects joint strength and hermeticity. In succeeding months we’ll address issues such as dissimilar metal brazing and how that affects joint design.