Brazing, as we've seen, uses the principle of capillary action to distribute the molten filler metal between the surfaces of the base metals. Therefore, during the brazing operation, you should take care to maintain a clearance between the base metals to allow capillary action to work most effectively. This means, in almost all cases, a close clearance.
The following chart is based on brazing butt joints of stainless steel, using Handy & Harman's Easy-Flo filler metal. It shows how the tensile strength of the brazed joint varies with the amount of clearance between the parts being joined.
Note that the strongest joint (135,000 psi/930.8 MPa) is achieved when the joint clearance is .0015" (.038mm.) When the clearance is narrower than this, it's harder for the filler metal to distribute itself adequately throughout the entire joint and joint strength is ultimately reduced. Conversely, if the gap is wider than necessary, the strength of the joint will be reduced almost to that of the filler metal itself.
Capillary Action In Clearances
Capillary action is also reduced so the filler metal may fail to fill the joint completely, again lowering joint strength. So, the ideal clearance for a brazed joint, in the example above, is in the neighborhood of .0015" (.038mm.) However, in ordinary day-to-day brazing, you don't have to be this precise to get a sufficiently strong joint.
Capillary action operates over a range of clearances, so you get a certain amount of leeway. Look at the chart again, and see that clearances ranging from .001" to .005" (.025 mm to .127 mm) still produce joints of 100,000 psi (689.5 MPa) tensile strength. Translated into everyday shop practice, an easy slip fit will give you a perfectly adequate brazed joint between two tubular parts. And if you're joining two flat parts, you can simply rest one on top of the other.
The metal-to-metal contact is all the clearance you'll usually need, since the average "mill finish" of metals provides enough surface roughness to create capillary paths for the flow of molten filler metal. Highly polished surfaces, on the other hand, tend to restrict filler metal flow.
The Coefficient of Thermal Expansion
However, there's a special factor you should consider carefully in planning your joint clearances. Brazed joints are made at brazing temperatures, not at room temperature. So you must take into account the “coefficient of thermal expansion” of the metals being joined. This is particularly true of tubular assemblies in which dissimilar metals are joined.
As an example, let's say you're brazing a brass bushing into a steel sleeve. Brass expands when heated, more than steel. If you machine the parts to have a room temperature clearance of .002"-.003" (.051 mm- .076 mm), by the time you've heated the parts to brazing temperatures the gap may have closed completely! The answer? Allow a greater initial clearance, so that the gap at brazing temperature will be about .002"-.003" (.051 mm-.076 mm.)
Of course, the same principle holds in reverse. If the outer part is brass and the inner part steel, you can start with virtually a light force fit at room temperature. By the time you reach brazing temperature, the rapid expansion of the brass creates a suitable clearance.
Allowance For Expansion and Contraction
It depends on the nature and sizes of the metals being joined and the configuration of the joint itself how much allowance for expansion and contraction there should be. Although there are many variables involved in pinpointing exact clearance tolerances for each situation, keep in mind the principle involved and that different metals expand at different rates when heated. To help you in planning proper clearances in brazing dissimilar metals, the COE's Comparison of Materials chart furnishes the coefficient of thermal expansion for a variety of metals and alloys.