We pick up where we left off last month with our final two posts on the subject of brazing heat- and corrosion-resistant alloys.
HCR Alloys Used in Honeycomb Brazing
Brazed metallic honeycomb structures are used in a wide number of high-temperature aerospace applications, with a number of HCR alloys having a long history of success in such usage, including Hastelloy, 321 stainless, Inconels and several titanium alloys.
One unique application developed in the 21st century is a special honeycomb manufacturing process in which a very thin strip of BNi-2 nickel-based BFM foil is embedded into the honeycomb structure as the honeycomb is being formed. This is clearly seen in Fig. 6, where the close-up of the honeycomb shows the BNi-2 foil within the honeycomb structure. This specialized product was designed to provide the exact amount of BFM needed to effectively bond the honeycomb to its substrate but with no excess BFM material to cause any erosion of the HCR honeycomb structure.
Titanium has been an HCR alloy of choice for aerospace applications for many years. Its light weight and high strength relative to steels and Inconels is a major advantage. Consequently, it has found major use in aerospace honeycomb structures as well.
However, titanium’s strong reaction with oxygen at elevated temperatures requires that brazing be done only in vacuum furnaces. Further, only vacuum furnaces in which the brazing hot zone is extremely clean and leak-tight should be used. As mentioned earlier, electrolytic-nickel plating is often required on faying surfaces of brazed joints so that the molten BFM can adequately flow and create a strong metallurgical bond.
Figure 8 depicts an example of a titanium-brazed honeycomb structure for use in commercial jet engines on many of the Airbus A340 and A380 aircraft today. The entire exhaust system is constructed from titanium (grade 21). Perforated sheets on the right side of the honeycomb structure in the photograph allow for excellent acoustic-attenuation (noise-abatement) characteristics for this material. The entire assembly handles exhaust gases up to about 1200°F (650°C).
We will conclude our discussion of this topic next time.
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