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 ensure that brazed joints will work.
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 ensure proper fit-up of the parts before brazing, costly mistakes often result!
Butt-Joints: 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.
Lap-joints: 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, the 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.
Joint clearances must be close together and parallel.
The amount of clearance between the faying surfaces (the mating surfaces inside a joint being brazed) should ideally be kept small, on the order of about 0.000″– 0.002″ (0.000-0.050 mm) total, so that capillary action can most effectively pull the molten brazing filler metal (BFM) completely into and throughout a braze-joint. The actual amount of clearance recommended between the faying surfaces will vary depending on the base-metal/BFM combination, but it is certainly safe to say that, in all cases, although capillary energy can be very strong, it will not operate effectively when the gap between the faying surfaces becomes too large. You may recall from last month’s column that one of the drawings showed a lap-joint that was not parallel, resulting in what is called a “capillary break” when the joint clearance got too large.
Of course, these recommendations are for clearances “measured” at brazing temperature, since that is the when the BFM is molten and able to flow through the joint. The “zero-clearance” (0.000″) basically means that the metal surfaces are in direct contact with each other, which is fine — unless you have purposely polished the faying surfaces prior to brazing (rarely, if ever, recommended). Note in the illustration on the left that the normal as-received, as-machined, as-rolled surfaces of metals provide enough surface “roughness” (hills and valleys) to allow space for capillary action to occur when the two faying surfaces are in actual direct contact.
Since the suggested ideal clearances for just about all the BFMs, when operated in an atmosphere (vacuum is considered as an atmosphere) is on the order of about 0.000″– 0.002″ (0.025-0.050 mm) total, as mentioned earlier in this article, assembly of parts need not be difficult, nor is it really necessary to provide “spacers” to keep the faying surfaces apart. The normal surface roughness already does that. And, experience over many years by many companies in the industry has shown this to be correct. But too many companies still violate these clearance guidelines every day, try to braze parts with very large gaps between the faying surfaces, and then wonder why they have brazing problems. Please understand that gap clearance is a vitally important aspect of brazing, and cannot be abused at the whim of the manufacturer unless they are willing to accept a significant amount of re-work and/or scrap as a part of their daily operations!
Effect of gap clearance on joint strength.
A major benefit of following these joint-clearance guidelines is that joint strength is significantly improved! The illustration on the right shows the effect of joint clearance on joint strength.
When the proper joint-clearance is used, the figure on the right shows that the joint strength can be greatly improved! Please do not misinterpret this famous H&H diagram from the late 1930s. This chart is based on tensile-tests with stainless-steel butt-joints that were torch-brazed using a silver-based BFM and flux. As the gaps between the faying surfaces were brought closer and closer in each subsequent test, it reached a point where the presence of the entrapped flux voids, etc., in the butt-joint began to “rear their head” so to speak, and joint strength was affected. Such a drop-off of joint strength does not occur with samples that are brazed in atmospheres with no flux!
Be aware, too, that a properly brazed joint should never fail in the joint. It should always fail in the base metal outside of, and far away from, the brazed joint. Since brazing usually anneals the base metals being joined, and failure can be considered to have begun when the base metal begins to “yield” under heavy stress, the simple answer for the “strength of a brazed joint” is that the brazed assembly should fail in the base metal and the level of stress needed to cause such failure is equivalent to the yield strength of the base metal in question in it’s annealed condition. That’s information readily available in any decent materials handbook.
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