Question: We are brazing honeycomb for a turbine seal in an aircraft jet engine using nickel filler metal. The filler metal wicks up the nodes in the honeycomb during brazing, however, forming a hard crown-like surface on the top of each of the honeycomb cells. This prevents the honeycomb from wearing away when the engine blades move across the top of it. Would increasing the brazing temperature or time help? What do you suggest?
Answer: Here are some important things to consider.
1. Time and temperature: Because capillary action is strong in tight joints, the molten brazing filler metal (BFM) will flow along any capillary path as far as it can, and it will do so more and more with higher temperatures and longer hold times. Thus, lower temperatures and shorter hold times might limit flow of BFM.
2. Quantity: The honeycomb-to-substrate joint is the most important, and it is a very small area. It only requires a small amount of molten BFM to join the honeycomb to the substrate. Additionally, some BFM is needed to join the honeycomb nodes (sidewalls of the honeycomb) to each other. That’s it. You certainly do not need, or want, to fill up the inside of any of the honeycomb cells! People put on far too much BFM when honeycomb brazing and often fill up the cells. This is not necessary at all! Calculate the amount of BFM needed to join the honeycomb to the substrate, and only about half to two-thirds up the nodes. That’s all the BFM you need.
3. Gap clearance: The molten BFM will readily flow throughout a tight capillary space in all directions as far as it can go. Thus, it will flow to the top of the honeycomb nodes every time if the node surfaces are closely fitted together. As a result, a hard cap of solid BFM can form at the top of the honeycomb cells, defeating the purpose of the cells to be soft sacrificial surfaces that are supposed to wear away rather than the blade tips that are sweeping by.
Recommendation: Some honeycomb manufacturers have actually experimented with, and produced, honeycomb that has gaps at the top that are a bit too wide for the BFM to flow there (Fig. 1). Additionally, some brazing shops have lightly rubbed some stop-off on the top of the honeycomb to prevent rise of BFM beyond a certain height. Be very careful with this option!
When manufacturing honeycomb, the nodes of each corrugated sheet of metal are spot-welded to the nodes of the next layer, thus forming a wider and wider sheet of honeycomb (Fig. 2). Corrugated sheet with slightly deformed top surfaces are possible to use in this kind of manufacture, even though it is nonstandard. Honeycomb suppliers/manufacturers may not readily offer this kind of honeycomb since the smooth, straight nodes are easiest and cheapest to make.
Summary: In my opinion and experience, it is best to significantly limit the amount of BFM applied, and keep the time at brazing temperatures minimal. Try an experiment using a brazing temperature of only 60°F (30°C) above the published liquidus of the BFM. Use “no hold time” when your load thermocouple arrives at temperature (VERY IMPORTANT) as recorded by a thermocouple on the honeycomb part, not just what is recorded by the furnace thermocouple.