Brazing depends on capillary action to draw the liquid BFM into the brazing joint, and tight clearances are needed for best brazing to occur. If the BFM is pre-placed in the brazing joint prior to assembly of the parts, capillary action is not a major factor since the BFM will melt in-situ and join the two members without the need for flowing any distance through tight capillary spaces. But when the BFM is applied externally or if a preform ring of BFM is placed in a groove internally and must melt and flow through the joint by capillary action, then clearance become a critically important factor.
Too many brazing shops have to send parts through their brazing operations twice, three times or more because the brazing gap (clearance) does not properly fill during the brazing run and additional brazing runs must be made, with additional BFM each time, in order to finally be able to fill the large gap that exists between parts. This can lead to weak joints, grossly distorted parts, excessive grain growth, etc.
If a large gap exists and it must be brazed, it is quite possible to braze it successfully using a "wide-gap brazing" process. The steps to follow to do it successfully in only one brazing run are:
1. Be sure the gap is very clean (free from oils, grease, dirt and oxides). If necessary, submit the parts for state-of-the-art fluoride-ion cleaning prior to brazing the assembly. This is commonly done for parts coming back for repair from extensive aero-engine service.
2. As shown in Figure 1, pack base-metal powder into the wide-gap between the parts. It is important to match the powder with the base metal. Thus, if brazing 304-stainless parts, use 304 S/S powder to fill the gap. Bear in mind that this base-metal powder is a gap-filler, and it is not intended to be melted. The powder size (mesh) should be such that it can be easily placed into the gap, leaving spaces between the powder particles for BFM to flow. Packing density of the base-metal powder will be assumed to be approximately 50%.
3. Apply the desired BFM powder or paste on the outside of the joint, equal to about 60% of the volume of the gap, since about 50% of the joint will be taken up by the base-metal powder and the 10% extra BFM applied will be needed to compensate for any variations in gap clearance and also to provide a slight meniscus (filleting) on the outside of the joint.
4. Braze the assembly, causing the BFM to melt and flow by capillary action between the base-metal powder particles in the joint, joining them firmly in place and allowing the gap to be successfully brazed in one attempt only.
When done properly, large gaps can be brazed successfully this way with a high degree of strength and integrity. Many companies have learned to do this successfully over the years, but many more companies need to learn this technique.
If you are uncertain about how to perform this operation in your plant operations, please let me know, and I'll be happy to guide you.