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Patent: Carbon fibers in ceramic cores for investment casting
Time:2020-09-28   Hits:845
The present disclosure relates to ceramic cores used to produce internal channels inside turbine blades in the investment casting process. A method of producing a ceramic core for investment casting is provided. The method includes injecting a slurry into a disposable die. The slurry includes ceramic particles, a binder, and carbon fibers. The method also includes a first heating to eliminate the disposable die, leaving a cured ceramic core comprising the ceramic particles, binder and carbon fibers, then the heating oxidizing and substantially removing the carbon fibers leaving voids. The present invention also provides a method of making a cast metal component including providing a non-reactive ceramic core including ceramic particles, a binder, and carbon fibers to contact with a reactive metal to produce the cast metal component. The present invention provides a method to produce core bodies with improved mechanical strength and increased porosity by introducing an optimum amount of carbon fibers to the non-reactive core fabrication process so that the core may be readily removed from the internal of a blade, and yet structurally strong enough to survive the core manufacturing process. The non-reactive cores according to an aspect of the invention include siloxane binders, fugitive species, and ceramic powders. The siloxane binders and fugitive species, upon thermal decomposition, create porosity in the final core body. Both pre- existing and newly formed ceramic phases give strength to a core body to maintain dimensional integrity. On the other hand, a porosity of more than 40 vol% with an interconnected pore structure helps to increase the leaching efficiency during the core removal process. In the invention, the slurry includes carbon fibers in a concentration not exceeding 20 wt% of the slurry. The carbon fibers have an average diameter of 200 microns or less, and have an aspect ratio of greater than 1:1 up to 100:1. These carbon fibers are critical in reducing cracks during the plastic die removing phase. The method further comprises at least one additional heating step that removes the disposable die and the carbon fibers. The fired ceramic core comprises ceramic particles and fiber-shaped voids, the fiber-shaped voids generally aligned with an axis of the core. Carbon fibers remain in the core body after thermal decomposition of plastic die around 500 °C (first fire), reinforcing the core body because carbon fibers do not oxide until at a higher temperature. During the subsequent second/setter fire up to 1650 °C in air, carbon fibers fully oxidize to CO and CO2. These gaseous species evaporate from the cores, creating interconnected pore structure within the cores. The resultant pore structure aids the leaching process in the core removal step.