Patent Application
20060165547
The present invention relates to methods for applying dense and highly uniform refractory metal alloy coatings onto articles such as aerospace components and, more particularly, to methods for coating at temperatures below the melting points of such alloys.
The aerospace industry is continuously seeking to increase the operating temperatures for launch vehicle components and equipment, and to thereby enhance the performance and increase the operational life for such products. Materials are needed that can withstand the high temperatures and pressures existing in a rocket exhaust environment. It is further desirable for high temperature and high pressure component materials that also are sufficiently strong that they can be manufactured as thin members and thereby reduce the overall component weight. Excessive erosion and/or wear rates may cause a component to have a short operational life even though the component has other mechanical properties that-exceed functional requirements. Many refractory metals and refractory metal alloys have sufficiently high melting temperatures and high temperature strength and stability to maintain their desirable physical and chemical properties in extreme environments such as that produced by rocket exhaust.
Rhenium and rhenium alloys are particularly useful refractory metals for aerospace components, and also for a wide variety of components that require a long operational life in high temperature and/or high pressure environments. Some characteristics that make these materials useful include ultrahigh temperature strength, i.e. 6 to 9 ksi at 4000° F. Rhenium also high plastic deformation capability at low temperatures in comparison with many other high melting point metals, making it a relatively reliable structural material. Although laboratory-prepared very pure rhenium typically has high ductility, commercial grade rhenium tends to have low ductility at high temperatures.
If a rhenium component experiences a slight structural failure when exposed to elevated temperature, the structural weakness tends to lie in the inter-granular mode, indicating the grain boundary strength is lower than the bulk grain strength. Low boundary strength creates a brittle material due to the inability of granular material to distort when subjected to a load. Low boundary strength may be caused by accumulation of oxygen or other impurities at grain boundaries and interfaces.
Hence, there is a need for a rhenium-based alloy that is not adversely affected by exposure to oxygen, particularly at very high temperatures and pressures. More particularly, there is a need for a rhenium-based alloy that does not experience grain boundary weakness and consequent low strain fracture in such extreme environments. There is also a need for such an alloy that has a high melting temperature and also has relatively high ductility and strength at high temperatures. Further, there is a need for components made from such alloys for use in a variety of applications and technologies.
The present invention provides a refractory alloy that includes rhenium as the most abundant alloy component by atomic percent. The alloy also includes tungsten, and at least one element having a higher oxygen affinity than rhenium and having a melting temperature of at least 1650° C. One exemplary alloy includes at least 90% rhenium by atomic percent, about 0.25% to about 4% tungsten by atomic percent, and about 0.25% to about 2% tantalum by atomic percent.
The present invention also provides a method of manufacturing a refractory metal alloy component. A powder blend comprising rhenium, tungsten, and at least one element having a higher oxygen affinity than rhenium and having a melting temperature of at least 1650° C. is combined to form a mixture having rhenium as the most abundant mixture component by atomic percent. The mixture is compressed to form a green component, and the green component is sintered, cold compressed, and annealed to form the refractory metal alloy.
The present invention also provides a solid free form fabrication method of manufacturing a refractory metal alloy component. The method comprises determining cross-sectional shapes for successive parallel component layers, and building the component by iteratively forming the cross-sectional shapes upon one another in a continuous layer-by-layer formation from a refractory metal alloy comprising rhenium, tungsten, and at least one element having a higher oxygen affinity than rhenium and having a melting temperature of at least 1650° C., with rhenium being the most abundant refractory metal alloy component by atomic percent.
Other independent features and advantages of the preferred methods will become apparent from the following detailed description, taken in conjunction with the accompanying drawing which illustrates, by way of example, the principles of the invention.
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
The refractory alloys of the present invention can be made using a variety of processes including but not limited to powder metallurgy fabrication methods, solid free-form fabrication methods, and casting methods. These and other methods can be used to simply cause the individual alloy components to diffuse and react to form an alloy, or alternatively they can be used to produce a component having a predetermined shape and predetermined dimensions from the alloy.
The term solid free-form fabrication (SFFF) is a designation for a group of processes that produce components in their three dimensional shapes from additive formation steps. SFFF does not implement part-specific tooling. Rather, components are produced by iteratively building thin component layers upon one another such that the additive layers form a three-dimensional object. The design of the final product is based on a three dimensional representation devised in computer aided modeling. The computerized representation is a layer-by-layer slicing of a component shape into consecutive two dimensional layers, which can then be fed to control equipment to fabricate complex, net-shape objects. According to the present invention, an exemplary SFFF method includes determining cross-sectional shapes for successive parallel component layers, and building the component by iteratively forming the cross-sectional shapes upon one another in a continuous layer-by-layer formation process.
One SFFF process is a layer-additive manufacturing (LAM) process, disclosed in U.S. Pat. No. 6,680,456, is a selective laser sintering process that involves selectively depositing a material such as a laser-melted powdered material onto a substrate to form complex, net-shape objects. In operation, a powdered material feeder provides a uniform and continuous flow of a measured amount of powdered material to a delivery system. The delivery system directs the powdered material toward a deposition stage in a converging conical pattern, the apex of which intersects the focal plane produced by a laser in close proximity to the deposition stage. Consequently, a substantial portion of the powdered material melts and is deposited on the deposition stage surface. By causing the deposition stage to move relative to the melt zone, layers of molten powdered material are deposited. Initially, a layer is deposited directly on the deposition stage. Thereafter, subsequent layers are deposited on previous layers until the desired three-dimensional object is formed as a net-shape or near net-shape object.
In addition to process described above, there are numerous other SFFF techniques that can be used to manufacture the starting materials. One suitable SFFF technique is a selective laser sintering processes in which a laser is used to selectively melt layers of powder materials into a shape. Another SFFF technique is ion fusion formation (IFF) in which a plasma welding torch is incorporated in conjunction with a stock feeding mechanism to direct molten feedstock to a targeted surface such as a base substrate or an in-process structure of previously-deposited feedstock. A component is built using IFF by applying small amounts of molten material only where needed in a plurality of deposition steps, resulting in net-shape or near-net-shape parts without the use of machining, molds, or mandrels. The deposition steps are typically performed in a layer-by-layer fashion wherein horizontal slices are taken through a three dimensional electronic model by a computer program. A positioner then directs the molten feedstock across each layer at a prescribed thickness.
There are also several general powder metallurgy fabrication methods that may be used alone or in combination to form the alloy or a component from the alloy.
Alloying high temperature materials is conventionally performed-by developing phase diagrams for a number of rhenium binary alloys, and then adding relatively small concentrations of other elements or compounds to a rhenium matrix to enhance matrix properties. For example, small concentrations of rhenium may be added to tungsten or chromium matrices to improve ductility. An exemplary refractory alloy according to the present invention includes rhenium as the most abundant alloy component by atomic percent, and preferably includes at least 50% rhenium. In a preferred embodiment, the alloy includes at least 90% rhenium by atomic percent, and more preferably at least 95% rhenium by atomic percent.
The alloy further includes tungsten, which provides the benefit of increased alloy strength. Tungsten is added to solid solution-strengthen rhenium which increases the yield strength of the alloy compared to that of pure rhenium. This effect is consistent over a temperature range from room temperature to 3500 F. An effective tungsten concentration in the alloy ranges between about 0.25% and less than 50% by atomic percent, and is preferably between about 0.25% and about 4% by atomic percent.
The alloy also includes at least one element as an oxygen getter, meaning that the at least one element has a higher oxygen affinity than rhenium. One exemplary oxygen getter is tantalum, which has a higher thermodynamic driving force for oxide formation than does rhenium. Thus, a preferred embodiment of the invention includes both tungsten and tantalum in the rhenium-based alloy. An effective tantalum concentration in the alloy ranges between about 0.25% and less than 50% by atomic percent, and is preferably between about 0.25% and about 5% by atomic percent.
The at least one oxygen getter, such as tantalum, preferably has a melting temperature of at least 1650° C., and more preferably at least 2200° C. in order for the alloy to maintain its structural integrity and strength at high operational temperatures. In addition, the alloy may include at least one additional element having a higher oxygen affinity than rhenium. Exemplary additional elements include hafnium, chromium, nickel, aluminum, and rare earth metals. In a preferred embodiment having at least one additional element with a higher oxygen affinity than rhenium, hafnium is included at a concentration ranging between about 0.25% and about 5% by atomic percent.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/647,537 filed Jan. 26, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60647537 | Jan 2005 | US |
