Progress in adapting the Investment Casting of NiAl
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its thermal conductivity is four to eight times that of nickel-base superalloys; its density of 5.95 g/cm 3 is approximately two-third the density of state of the art superalloys; its good oxidation resistance.
However, the high strength is usually associated with poor ductility at room temperature. With respect to ductility, intermetallics lie between metals and ceramics. High temperature properties, thermal expansion and chemical behavior of the ceramic mold system has to be compatible with the properties of the cast alloy [2], [3]. A special ceramic mold was used to prevent cracking and excessive mold-metal reactions. The molds were manufactured in the standard shell mold process, using a steam autoclave for dewaxing. Various casting conditions (variation of the melt superheat, mold temperatures and alloys) were employed to investigate their effect on crack formation. After removal of the molds, the castings were visually inspected to evaluate their surface quality. CASTING EXPERIMENTS The mold for casting the heat shield was produced in the lost-wax process, Fig. 1. To process NiA1 alloys, the components of the mold are very close to their application limits. Thus, SiO 2 levels in the mold are kept as low as possible. The mold, must be very strong at the casting temperature, but also weak enough at low temperatures to avoid cracking of the low-plasticity alloy due to differential contraction during cooling. For NiAl alloys only SiO 2 free ceramic molds were used. A typical mold used to produce heat shields is shown in Fig. 2. This heat shield geometriy was selected to research the effect of the weakening of the shell mold especially at the hole to produce crack-free cast parts. KK5.21.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
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Fig. 1: Flow chart of the investment casting process to produce NiAI heat shields with a SiO 2 free ceramic mold. 4.~-9
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Fig. 2: SiO 2-free shell molds , produced by the lost wax method, used to cast NiAI-alloys.
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Fig. 3: The FG 75 a precipitation hardening, laves-phase reinforced alloy with 45at.% Ni, 45 at.% Al, 7,5 at.% Cr and 2,5 at.% Ta.
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Fig. 4: The FG 27 has a high Crcontent. The structure shows the NiAI-matrix and the a-chrome crystal. The alloy contains 36,5 at.% Ni, 36,5 at.% Al and 27 at.% Cr.
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For the cast process, the mold is preheated. The transition from ductile to brittle behavior which is typical for intermetallics, requires a rather small solidification rate. For the experiments two different NiAl-alloys were used. The first alloy is characterized as a precipitation hardening, laves-phase reinforced alloy (FG 75) [4], [5]. It contains 45at.% Ni, 45 at.% Al, 7,5 at.% Cr and 2,5 at.% Ta, Fig. 3 shows its typical structure. The second used alloy is characterized by the high Cr-content (FG 27) [6], [5]. It contains 36,5 at.% Ni, 36,5 at.% Al and 27 at.% Cr. The structure is shown in Fig. 4. RESULTS With t
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