BRAZING & SOLDERING TODAY BNi-5a amorphous foil, it becomes apparent that the oxidation resistance of the nodes suffers from the preplaced braze foil. The impression is that the BFM poisoned the composition by in-diffusion of undesirable elements or by diluting the aluminum concentration locally. With the need for more precise metering of the BFM, methods other than filling the honeycomb with braze powder need to be applied. One such method is the use of braze tape. Braze tape containing the common BFMs is available in a variety of thicknesses and therefore braze filler amounts can be controlled per unit surface area and is preplaced in the honeycomb structure, as shown in Fig. 8. Another method of predetermining the amount of braze filler more precisely is the use of all-metallic braze foil, which can also be preplaced in the honeycomb structure (Brazcor™ honeycomb) as discussed above. However, using metallic braze foil significantly adds to cost and the complexity of manufacture so that the use of braze tape becomes the preferred method and increasingly popular. Fatigue Cracking Another degradation mechanism observed for brazed honeycomb is fatigue cracking in situations where mechanical loading and oxidation are superimposed such as in cyclic oxidation or burner rig testing. Figure 9 shows Haynes® 214 and MI 2100 honeycomb after cyclic oxidation at 900°C (1650°F) over 200 h. Both honeycombs were brazed with AMS 4783 BFM, which was selected for its high melting temperature to produce a hightemperature seal. While the combination MI 2100/AMS 4783 seemed to survive the cyclic oxidation well, the combination Haynes® 214/AMS 4783 showed significant deterioration in the structure and formation of cracks in the honeycomb wall metal. At higher magnification of the Haynes® 214 honeycomb, several features became apparent — Fig. 10. First of all, a thick surface oxide layer could be observed in parallel with a fair amount of internal oxidation along cracks that had formed and obviously propagated along grain boundaries of the coarsened structure of the honeycomb material. Secondly, a high number of spherical 48 FEBRUARY 2014 and needle-shaped precipitates were seen. It is worth noting that similar features were not observed for the unbrazed material in air or burner gas oxidation, which leads to the assumption that their occurrence is linked to the brazing of the structure. Clearly, the above example shows combining a material that reflects good high-temperature oxidation resistance (Haynes® 214) with a braze material of sufficiently high melting temperature (AMS 4783) does not automatically produce a well-performing high-temperature seal. The pairing MI 2100 and AMS 4783 resulted in a much more stable seal configuration that shows good performance at up to 1100°C and only starts to deteriorate at 1200°C (2190 °F) by a mechanism initiated by diffusion of silicon from the braze filler into the honeycomb material (Ref. 3). Summary The performance of honeycomb structures made from alumina-forming, high-temperature-resistant NiCrAl, and FeCrAlY materials is significantly impacted by the presence of less oxidationresistant braze filler materials necessary to join the abradable honeycomb to a supporting structure. Several failure mechanisms are observed that can be summarized as BFM-induced oxidation and/or braze filler oxidation-induced fatigue of seal honeycomb. Therefore, the need exists to develop braze filler materials with much improved oxidation resistance to provide honeycomb seal lands for use in turbine labyrinth seals at operating peak temperatures above 1100°C. Conclusions To improve the thermal efficiency of jet engines, there is a trend toward higher turbine entry temperatures while reducing the cooling effort. This leads to everincreasing temperatures that honeycomb seals in the low-pressure turbine module of modern engines will have to withstand. While metallic foil alloy materials that can withstand a turbine hot gas environment at temperatures in excess of 1100°C are available, suitable braze filler materials to join seal-type honeycomb fabricated from these high-temperature-resisting Fig. 10 — Haynes® 214 honeycomb at higher magnification showing cracks and significant deterioration in the structure. materials are not. Various degradation mechanisms of brazed seals at high temperature, all directly or indirectly linked to the presence of braze fillers with fairly limited high-temperature oxidation and hot gas corrosion resistance, can be detected. These can be summarized as braze filler metal-induced oxidation failure or braze filler oxidationinduced fatigue failure. Clearly, there is a need for more oxidation-resistant brazes for joining ultrahigh-temperature seal honeycomb. When designing honeycomb seals for use at extreme temperatures, the influences of honeycomb wall thickness on the seal lifetime as well as compatibility of braze filler and honeycomb material need to be taken into consideration. Minimizing the amount of excess braze filler covering the seal structure must also be achieved to provide sufficient durability of the seals in high-temperature service.♦ References 1. Smarsly, W., et al. 2005. Advanced high temperature turbine seals materials and designs. Materials Science Forum, Vols. 492, 493, pp. 21–26. 2. Sporer, D. R., and Shiembob, L. T. 2004. Alloy selection for honeycomb gas path seal systems. ASME Turbo Expo, Vienna, Austria, June. Paper number GT2004-53115. 3. Potter, D. J., Chai, Y. W., and Tatlock, G. J. 2009. Improvements in honeycomb abradable seals. Materials at High Temperatures 26: 27–35.
Welding Journal | February 2014
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