Fig. 2 shows the expansion-rate curves for a number of common metals as they are heated. Notice that all these metals expand at different rates (each curve has a different slope). For the sake of our discussions, let’s assume that the components of an assembly to be brazed were made from 302-stainless steel. If, in Fig. 1b, the fixturing that surrounds the assembly were then made from thin molybdenum-alloy bar or rod stock, something very interesting will happen as the entire assembly is heated in a furnace. The stainless and the moly-alloy will both expand as the assembly is heated, but, according to the metal expansion-rate curves shown in Fig. 2, it should be apparent that the stainless will grow much faster than the moly-alloy. If the moly-fixturing is “loosely” fitted around the assembly, then, as it is heated in the furnace, the stainless will grow more rapidly than the moly-alloy, and finally catch up to it and start to press against it. As heating continues, the moly-fixture will exert more and more pressure on the stainless assembly as the stainless pushes out against it, and this pressure may even be sufficient to crush the stainless assembly! Experiments are often required with various degrees of “looseness” of the moly-fixturing (or whichever lower-expansion metal is used) to find out how much looseness at room-temp will provide just the right amount of pressure at brazing temperature that is needed to keep the braze joints tight.
Another option is to add a spring-loading feature to the fixturing just discussed, so that as the stainless expands more rapidly than the moly-alloy, it will force the springs to compress against the moly-fixturing. This may be “gentler” on the parts than fixturing that does not have this feature.
The use of what is often called “metallurgical fixturing”, i.e., surrounding an assembly with a fixture made from a small amount of a lower-expanding metal, is not only highly effective at providing lots of pressure (enough to actually compress the assembly), but is also light enough in weight so as not to add much weight to the total load going into the furnace! A double win!
Your goal should be to keep the weight of all external fixturing to no more than about 50% of the weight of the parts being brazed (thus the fixturing would represent no more than about one third of the total weight of the load going into the furnace). In that way, at least two-thirds of the heat going into the furnace is actually for production brazing, and not just for “heat-treating fixtures”!