BRAZING & SOLDERING TODAY example, 80% minimum or d/e > 0.8 (Fig. 5) to ensure proper seal integrity and avoid honeycomb degradation through aerodynamically induced fatigue. While in a liquid state, the BFM will not only wet the nodal walls and base but also penetrate the backing member and create a braze diffused zone that may contain undesirable phases such as chrome borides (CrxBy) or silicides. Similarly, the liquid BFM may also alter the chemistry of the thin honeycomb cell walls by in-diffusion of elements from the braze filler into the honeycomb material or diluting the concentration of certain honeycomb material alloying elements by out-diffusion into the braze composition. By comparing the chemistries of the braze fillers and the honeycomb materials, it becomes evident that there is a high potential for diffusion processes based on the significant differences in chemical composition and large concentration gradients resulting from these. Oxidation of Plain and Brazed Honeycomb Oxidation Resistance of Plain Foil To provide seals that can withstand higher temperatures, first and foremost, the honeycomb material itself needs to be resistant to high-temperature oxidation and corrosion. To determine a base metal’s capability, a simple air oxidation test at 1100°C was carried out. The oxidation resistance was measured using the sample weight change per unit surface area after 24 h of exposure as a yardstick. Two performance classes were clearly revealed through the testing. The difference between the classes lies in the selfprotection mechanism of the different materials. Nimonic 86™, Inconel® 617, and Hastelloy® X primarily form chromia (Cr2O3) surface oxide layers. However, these layers are not stable and therefore not protective at the selected test temperature. The MI 2100/2200 and Haynes® 214, all containing a high percentage of aluminum, tend to form Al2O3 surface oxide layers that are stable and protective even at 1100°C. Weight change in the processed samples is a measure of the oxidation process 46 FEBRUARY 2014 Fig. 5 — Designations of geometrical characteristics of brazed honeycomb. Section through nodal walls (schematic). occurring to the engine components. Overall, the weight change is very small for the alumina formers compared to the results for the chromia formers in air oxidation at 1100°C. The oxide layers on the FeCrAlY foil samples are thin and well adhered. The materials MI 2100 and MI 2200 tend to form fine-grained α-Al2O3 layers and only partly Cr-rich oxide phases, while Haynes® 214 shows a higher degree of less desirable NiCr2O4 spinel oxides and coarse chromia embedded in the alumina (Refs. 1, 2). Apart from that, no significant internal oxidation of the foil materials was observed for the tests that were performed. Thickness Effects As the oxidation resistance of the honeycomb foil materials with high aluminum content relies on the formation of Al2O3, with Cr2O3 being a lot less resistant at high temperature, the amount of available matrix aluminum plays an eminent role. The formation of protective alumina requires aluminum from the matrix and as the protective oxide grows in thickness or needs to be replaced after having spalled off, matrix aluminum will be consumed. Once the aluminum concentration in the matrix falls below a certain critical level, formation of Al2O3 at the surface is no longer guaranteed and Fig. 6 — Condition of nickel-based seal honeycomb as returned from engine service. Cell size is 1.59 mm (1⁄16 in.).
Welding Journal | February 2014
To see the actual publication please follow the link above