Patent Application
20070274854
The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,
The foam material of the core, shown in
The cell walls 28 may be made of a ceramic material or a metallic material, or mixtures thereof. If ceramic is used for the cell walls 28, any operable ceramic may be used. An exemplary ceramic material is aluminum oxide (“alumina”). Aluminum oxide is of particular interest because of its low density. The ceramic material may be a mix of ceramics, with the ceramic that is present in the largest volume fraction being the “base ceramic”.
One or more modifying ceramics may be mixed with the base ceramic to alter its properties. For example, the modifying ceramic may be a ceramic material that is more abrasive than the base ceramic. Examples of abrasive-modifying ceramics that are more abrasive than aluminum oxide and may be mixed with the aluminum oxide base ceramic are cubic boron nitride and sol get alumina. The modifying ceramic may be a ceramic material that is less abrasive—that is, more abradable—than the base ceramic. Some examples of abradable modifying ceramics that are more abradable than aluminum oxide and may be mixed with the aluminum oxide base ceramic include silicon nitride and silicon carbide. The relative amounts of the modifying ceramics are selected according to the amount of modification desired.
The ceramic foam materials noted above and their construction are described in U.S. Pat. No. 6,435,824 issued to Schell et al. and assigned to the assignee of the present invention.
If a metal is used for the cell walls 28, any alloy which is suitable for the intended operating conditions may be used. Examples of alloys known to be suitable for turbine engine components include aluminum, titanium, iron, cobalt, and nickel alloys. Particular examples of alloys useful for turbine components include Ti-6Al-4V, nickel-based alloys such as INCO 718, UDIMET 720, and Rene 195, and iron-based alloys such as A286.
The metallic foam materials noted above are described in U.S. Pat. No. 6,443,700 issued to Grylls et al. and assigned to the assignee of the present invention.
Continuing at block 34 of
A metallic powder and a suitable binder are provided for injection into the mold. The metallic powder may be a single alloy or it may be a mechanical mixture of more than one alloy. As used herein the term “powder” refers to any generally free-flowing dry form of a metallic alloy, and is not limited to any particular shape or size of grains. Non-limiting Examples of known alloys suitable for constructing turbine blades include titanium alloys such as Ti-6Al-4V, nickel-based alloys such as INCO 718 or UDIMET 720, Rene 195, and iron-based alloys such as A286.
The binder may be any material which is chemically compatible with the metallic powder and which allows the required processing (e.g. mixing, injection, solidification, and leaching). Examples of known suitable binders include waxes and polymer resins. The binder may be provided in a powder form.
The binder and the metallic powder are thoroughly mechanically mixed together. The mixture is then heated to melt the binder and create a fluid with the metallic powder coated by the binder. Next, the mixture is formed into the shape of the turbine blade 10 at block 36, by using a known injection-molding apparatus to extrude the mixture into the cavity of the mold. The mold may optionally be heated to avoid excessively rapid solidification of the binder which would result in a brittle preform. During the injection process, the binder and metal powder flow around and through the core and fill the intracellular volume 30 of the core. The mixture is at a relatively low temperature, for example about 150° C. (300° F.). Accordingly, thermal stresses on the core are minimized. Once the mixture has solidified, the mold is opened and the resulting uncompacted or “green” preform with the core inside is removed (block 38).
The “green” preform comprises metal powder particles 42 suspended in the solidified binder 40 in the intracellular volume 30. The preform is not suitable for use as a finished component, but merely has sufficient mechanical strength to undergo further processing. At block 44 of
Next, at block 46, the “brown” preform is sintered. The preform is placed in a chamber which includes means for creating a suitable atmosphere to prevent undesired oxidation of the preform or other reactions during the sintering process. In the illustrated example a supply of inert gas such as argon is connected to the interior of the chamber. The sintering could also be performed under a vacuum.
The preform is heated to a temperature below the liquidus temperature of the metallic powder 42 and high enough to cause the metallic powder particles 42 to fuse together and consolidate, for example about 700° C. (1300° F.). The high temperature also melts and drives out any remaining binder. The preform is held at the desired temperature for a selected time period long enough to result in a consolidated turbine blade 10. The heating rate is selected depending on variables such as the mass of the preform and the desired cycle time of the sintering process. The metallic powder 42 will provide a support for the foam material during the initial sintering cycle, preventing the immediate thermal fatigue failure that normally occurs, and allowing the foam material to then become an integral part of the structure. The presence of the foam can then supply either stiffness or other mechanical property improvements to the turbine blade 10.
When the sintering cycle is complete, the turbine blade 10 is removed from the chamber and allowed to cool. When required, the turbine blade 10 may be subjected to further consolidation using a known hot isostatic pressing (“HIP”) process to result in a substantially 100% dense component. If desired, the turbine blade 10 may be subjected to additional processes such as final machining, coating, inspection, etc. in a known manner.
The foregoing has described a manufacturing process for metallic composite foam components. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.
