The effective use of “metallurgical fixturing”, instead of a lot of dead-weights, to effectively “load” parts with enough pressure to keep braze joints close together for effective brazing is described in detail.
The use of heavy weights on top of parts being brazed is a common practice. Its purpose is to load the top of the assembly with enough weight so as to insure that the components of the assembly will be pressed together sufficiently to keep the joints from opening up during furnace brazing. This should then insure that good capillary action of the brazing filler metal (BFM) into those joints will occur during the furnace cycle. Unfortunately this procedure does not always work (which can be very frustrating for the furnace operators).
Look at the two drawings in Fig. 1a and 1b, which shows two ways by which some heat-exchanger components might be fixtured for brazing. The assembly-layers on the left are being held together by placing a lot of dead-weights on top of a heavy flat plate located on top of the heat-exchanger corrugated assembly. It is very important to know how much weight per square inch (or per square centimeter) is applied to this load, and if that amount of loading would be sufficient to “yield” the base metals enough to keep the joint surfaces close together for good capillary action to occur.
One brazing shop had placed hundreds of pounds of dead weight on top of a flat, large-surface-area heat-exchanger assembly, only to find that the pressure that resulted from that loading only amounted to about 2-lbs per square inch, and wasn’t enough to insure flatness of the assembly during brazing.
A big disadvantage of using dead weights is that the ratio of the dead-weight fixturing to the weight of the assembly being brazed is sometimes extremely high, perhaps 50-to-1 or more. I’ve seen furnace loads in which the fixturing going into the furnace weighed more than 100 times the weight of all the components being brazed. Thus, the fixturing accounted for almost 99-percent of the furnace load, and that furnace cycle was indeed just a “fixturing heat treat cycle”, for which the parts being brazed were merely going along for the ride! Such furnace cycles are highly NON-productive, since almost all the heat (as measured by BTUs or calories) being expended in that entire furnace cycle was merely being used to heat and cool the load of fixturing materials, with little of that heat energy cost being for the parts that were being brazed.
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”!
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