Fig. 1 – The as-rolled, as-machined, as-drawn surface finish will add the appropriate amount of surface “roughness” to the part to aid in capillary action of BFM through the brazed joint.
Over the years it has shown that the best surface for brazing, generally speaking, is the “as-received” (as-rolled, as-drawn, as-machined, etc.) surface roughness of the material coming into the brazing shop. An illustration of what this surface roughness might look like, under high magnification, is shown in Fig. 1.
Surface roughness obviously increases the total surface area of each faying surface inside the joint, when compared to a flat, polished surface. And, due to this “roughness”, it can be seen that there are many capillary paths for brazing filler metal (BFM) to follow between all the valleys and “peaks” on that roughened surface.
Surface roughness refers to the “texture” of a surface, the measurement of which is often expressed in several different ways, including Ra (Roughness average), RMS (Root Mean Square), AA (Arithmetic Average), and CLA (Center Line Average). I’ve found RMS to be a frequently used measure in my brazing experience. Others may perhaps have found otherwise.
Is there a conversion-chart that people can use to compare these different surface-roughness values? Yes! The company, L.J. Star Inc., has published the following excellent chart, reproduced in Table 1, which provides comparisons between several different surface-roughness measures. What I like about this chart is that it also adds in a column for “grit-polishing” for approximate comparison to the other columns. “Ra” is in microns (metric), whereas the remaining three columns are all measured in micro-inches. Please note that this chart represents just one possible approach to this topic of roughness-comparisons. But I have found the chart to be helpful, and pass it along to readers here.
For the balance of this article I will be using RMS for my discussion of surface-roughness. In my experience, RMS surface-roughness may range from about 16 RMS, to 32 RMS, 64 RMS, or even 125 RMS or greater when it comes to the as-received, as-machined, or as-drawn surface roughness of the part to be brazed. Please remember once again that RMS means “root mean square”, and is a mathematical average between the peaks and valleys of the surface roughness.