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 industry has shown this to be correct. But too many companies still violate these clearance guidelines everyday, 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!
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 1930’s. 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!