This concludes our step-by-step discussion that we began last time.
5. Next, wrap some low-expanding metal strapping material around the outside of the honeycomb (top and bottom and perhaps in middle), as shown in Fig. 2. Do not allow any of the strapping material to cover any tape that may have been used in step 4.
This low-expanding metal strapping can be strips of Inconel or titanium sheet since, as is shown in Fig. 3, these metals will grow (expand) at a slower rate than stainless steel. The titanium alloy (Ti6Al4V) shown in Fig. 3 would be excellent for this application or perhaps Inconel 718 (or 738), not only because they have lower rates of expansion than the 304L stainless but also because they have aluminum and titanium in their chemistries, which can readily oxidize on heating and will not stick to the stainless honeycomb. If you use Inconel or titanium sheet metal for this application, I recommend that it be pre-oxidized in a prior furnace heating cycle (perhaps by placing that strip in a previous brazing-furnace run) since heavily oxidized sheet material will not be able to stick to the honeycomb.
Fig. 2. This drawing illustrates how low-expanding strapping can be tacked into place around the outside of a sheet of honeycomb that needs to be brazed to the OD of a stainless cylinder.
6. Hold the Inconel or titanium-alloy strapping tightly in place via small tack welds, being sure that you do not accidently tack weld the strips directly to the honeycomb). If a moly-alloy strip (or other alloy) is used, then an easy way to ensure that such a metal strap will not bond to the honeycomb is by painting a thin layer of brazing stop-off onto the back side of that strapping material prior to assembly onto the honeycomb.
7. The assembled cylinder is then placed in the furnace in a vertical position and processed through its brazing cycle. As the honeycomb tries to expand during the brazing cycle, the lower-expanding holding strips will put significant pressure onto the honeycomb, forcing it to bond tightly to the stainless cylinder as the BFM foil melts.
Fig. 3. Expansion-rate curves for a number of different metals. Notice that Inconel and titanium metals expand at a lower rate than does stainless steel.
Conclusions: By wrapping a metal with a sheet/strapping of a lower expansion-rate material, great amounts of pressure can be easily applied to the metal part being wrapped with very little weight of so-called “fixturing” being used. This process of fixturing parts using metals with lower rates of expansion is known as “metallurgical fixturing.” When used correctly, it can be highly cost-effective. Give it a try in your shop!