Problem: An aerospace brazing company uses a high-temperature AMS-4782 nickel-based brazing filler metal (BFM) when vacuum brazing a high-temperature nickel-based honeycomb material onto the substrate to hold it in place for brazing. During inspection after brazing, it was noted that a number of outside cell walls appeared to be missing (Fig. 1), raising the question of what might have caused this.

Discussion: Compared to the close-up view of the missing cell walls in Fig. 1, the more general photograph of the honeycomb brazement shown in Fig. 2 seems to reveal a well-brazed structure overall. Therefore, the brazing process used does not appear to be in question at all since almost the entire brazement looks excellent. The question then seems to focus more specifically on the appearance of those questionable outer honeycomb cells both prior to and then after the brazing was finished.

The photo in Figure 1 shows that some of the honeycomb cells are open (or have been damaged), which can, in my experience, be caused by any of the following four possible reasons.

  1. Cutting off some of the edges of the cells when the honeycomb was first sized (i.e., when it was cut or machined to the desired final shape and size needed to fit onto the substrate to which it will be brazed).
  2. Localized overheating of the honeycomb sheet metal where it was being tack-welded onto the substrate since a spot-welding machine is used to lightly tack the honeycomb in many places to the substrate. If the current is too strong, localized overheating/melting of the honeycomb might occur.
  3. Erosion of the honeycomb by molten BFM. 
  4. Physical damage to the honeycomb during initial setup, in which a few of the cell walls were broken by tools used when first putting the honeycomb onto the substrate.

The first thing to do to locate the answer to this problem is to carefully look at all the edges of the honeycomb prior to and, then again, after the honeycomb has been cut to fit onto the substrate to which it will be brazed. Do the edges of the honeycomb show any cut-off cells following machining that were not there prior to that machining operation?

Next, prior to putting the tack-welded honeycomb into the vacuum furnace for brazing, carefully examine the honeycomb edges to see if any cells along those edges have been damaged by handling or if the tack-welding operation has burned the edges of any of the honeycomb cells.

Finally, carefully examine that same honeycomb assembly immediately after brazing to see if the brazing operation itself has caused any of the cells along the outside edge of the honeycomb to melt and open up.

A careful examination of the honeycomb edges at the various stages just described can help quickly identify the source of the missing cell walls.