Transfer tape (Fig. 3) is made by mixing BFM powder with a suitable binder, cast/rolled out into flat sheets and rolled up and slit to various widths. The brazing practice consists of placing the tape onto the honeycomb material and pressing or rolling into each cell. The honeycomb surface is scraped to reveal the bottom edges (see right side of Fig. 3) for subsequent resistance welding to its substrate prior to brazing.
Amorphous foil is a 0.0015 to 0.003 in. (0.04 to 0.08 mm) thick rapidly solidified, flexible, Ni-base BFM used to join honeycomb in one of two ways: (1) Foil is placed on a substrate and the honeycomb placed on top of it for brazing. Top pressure is applied to the honeycomb to ensure that it makes intimate contact with the substrate by settling through the BFM when the foil melts. This process is not widely used because special tooling is required to keep the honeycomb from “skating” over the molten BFM and becoming misaligned.
(2) Foil is added during the honeycomb manufacturing process so it becomes a part of the honeycomb itself (Fig. 4). The foil is added as either straight foil strips through the centers of each cell (bottom honeycomb in the photo) or as a corrugated foil conforming to the sides of each honeycomb cell (the panel above the pencil in the photo). The primary advantages of this method are no binders to outgas during the brazing cycle, use of only the precalculated exact amount of BFM needed for the joint and no separate BFM application step needed during turbine honeycomb seal assembly (as is required with powder, paste and tape).
The higher cost of amorphous-foil honeycomb often is offset by the savings from fewer manufacturing steps, lower brazing-reject rates compared with other BFM forms, simpler honeycomb replacement during repair cycles, and even a reduction in engine weight from less BFM being used. However, the substrate surfaces must be very flat and smooth for correct use of this type of honeycomb. Thus, it is ideal for new construction, or for use in repair operations in which the substrate surfaces can be machined smooth. During rework and repair, if extensive hand grinding/shaping is done, it might not produce the required surface smoothness to allow the use of this form of honeycomb.
The art of resistance welding honeycomb into place for brazing is not well developed enough in some brazing shops. In some cases, it is assumed that just enough spot welds should be used to lightly hold the honeycomb in position on its backing member until the BFM melts and bonds the honeycomb in place. This can result in honeycomb that partially lifts up during the brazing cycle, because the thin, lightweight honeycomb heats up and expands faster than the heavier backing material, lifting up off its backing support and not brazing properly.