Most people realize that metals expand when they are heated and contract when they are cooled. This fact has been thoroughly explored over the years, and data tables have been published showing how fast (and how much) each metal expands as temperature increases.

Unfortunately, some people forget to use this data properly in a brazing situation. They set up a brazing operation with a desired "brazing clearance" at room temperature for the parts being brazed together and don't think about the fact that as the metals are heated, the clearance can, and most likely will, change dramatically. In some cases, brazing filler metal (BFM) flow is even shut off into the joint completely if the gap closes up tightly upon heating.

"Coefficient of expansion" data is readily available for most materials and should always be used in developing braze procedures when different kinds of base metals are to be brazed to each other. The success or failure of a braze procedure may very well depend on it!

Dissimilar-Metal Brazing
As two different metals are heated to brazing temperature, the joint clearance will change because one metal will expand at a faster rate than the other. This might mean that the gap between the two metals will either get smaller (perhaps even close completely as it is heated) or get larger (perhaps becoming so wide that capillary action can no longer operate, and the BFM is unable to flow through the joint).

As an example, consider what would happen if you were asked to braze a 304-stainless steel rod into a large hole in a center of a disk of tungsten carbide (WC). The expansion rate for WC is much lower than the expansion rate for the 304-stainless as they are heated. Thus, the 304 rod could grow so much relative to the carbide that the gap will completely close, and the BFM cannot flow into the joint by capillary action.

Always remember that the optimal braze joint clearance that is needed for a successful braze occursat brazing temperature, since only at brazing temperature can capillary energy draw the molten BFM through the joint.

Thus, it is critical that you understand available metal-expansion data, available in charts such as those found in the American Welding Society (AWS) "Brazing Handbook," in order to be able to figure out what clearance is needed in the braze joint at room temperature. Then when the assembly is heated, the differential expansion that occurs between the two materials (such as between the stainless and the carbide in our example) will result in a correct joint clearance at brazing temperature. Correct joint clearance ranges from about 0.001 to 0.003 inches at brazing temperature (depending on the metals involved).

Similar-Metal Brazing
If the two metals being joined are the same, it is very important to remember that different mass of the same metal will grow at different rates also. A heavy piece will take longer to absorb heat than a very thin piece made from the same base metal. Thus, the thin piece may quickly rise to brazing temperature, whereas the very heavy mass of the same metal will take much longer to reach temperature. Therefore, it is quite possible that, if the two pieces were lightly tack-welded together to hold them in proper alignment prior to brazing, the different rates of growth of the thin and heavy parts could cause distortion of the assembly and might even cause the welds to break during heating of the parts to the brazing temp. I've seen this happen.

To avoid this problem, it may be necessary to slow down the heating rates (and cooling rates) to allow the different masses of the assembly to heat/cool at about the same rate. This can help to eliminate the distortion or cracking altogether. For assemblies involving two different metals, this becomes even more critical.

Conclusion
Always know the expansion rates of the metals involved in brazing. Pre-calculate total expansion of each component, and build that data into your brazing worksheets. It can save you lots of lost time and money.