Patent Application
20060134320
(1) Field of the Invention
The present invention relates to a method for repairing components, such as turbine engine parts, using a powdered cold sprayed aluminum containing material.
(2) Prior Art
Current repair methods allow for dimensional restoration of aluminum and magnesium structures through a variety of techniques such as plasma spray, HVOF, and epoxy bonding. However, these repairs offer no structural benefit when applied to the affected area. Therefore, although the part may be restored dimensionally, these existing repair methods are of no use when the underlying structure violates minimum dimension conditions because the repaired part will not have the required strength. Fusion welding processes, while capable of producing structural repairs, often result in unacceptable distortion. Additionally, fusion welding and similar processes generate high temperatures that may negatively impact the material properties of the substrate material.
Accordingly, it is an object of the present invention to provide a method for repairing components in a way which restores dimensions.
It is a further object of the present invention to provide a method as above which allows for some structural credit to be claimed for the repair without causing unacceptable distortion or negatively impacting the properties of the underlying material.
The foregoing objects are attained by the method of the present invention.
In accordance with the present invention, a method for repairing components broadly comprises the steps of providing a component having a defect to be repaired and depositing an aluminum containing repair material over an affected area on a surface of the component so that the repair material plastically deforms, without melting, and bonds to the affected area upon impact with the affected area and thereby covers the affected area in the solid state.
Other details of the structural repair method using cold sprayed aluminum materials in accordance with the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawing wherein like reference numerals depict like elements.
The FIGURE illustrates a system for repairing a component using the method of the present invention.
The present invention relates to the repair of components or parts, such as components and parts used in turbine engines, using powdered aluminum containing repair materials.
In the past few years, a technique known as cold gas dynamic spraying (“cold spray”) has been developed. This technique is advantageous in that it provides sufficient energy to accelerate particles to high enough velocities such that, upon impact during an initial pass, the particles plastically deform and bond to the surface of the component on which they are being deposited so as to build a relatively dense coating or structural deposit. On subsequent passes, the particles bond to the previously deposited layer. Cold spray does not metallurgically transform the particles from their solid state. The cold spray process has been found to be most useful in effecting repairs of components formed from ductile materials. For example, the cold spray process may be used during the repair of turbine engine components, such as a fan exit inner case or a gearbox, formed from aluminum or magnesium based materials.
When repairing a turbine engine component, corrosion pits and/or damaged areas are mechanically removed through grinding, machining, or other applicable techniques and the resulting surface may or may not be grit blasted prior to depositing a repair material, such as an aluminum containing material, using a cold spray method. Suitable aluminum containing materials which may be used to effect repairs in accordance with the present invention include, but are not limited to, pure aluminum, aluminum alloy 6061, aluminum alloy 2219, Al-12Si alloy, Al—Sc alloy, and aluminum alloy 6061/B4C. In a preferred embodiment of the present invention, the aluminum containing material comprises a material which has a composition that includes more than 50% by weight of aluminum.
Referring now to the FIGURE, there is shown a system for effecting repairs on an affected area 24 of a component or a part. The system includes a spray gun 22 having a converging/diverging nozzle 20 through which the repair material is sprayed onto an affected area of a surface 25 of the part or component 10 to be repaired. During deposition of the repair material, the part or component 10 may be held stationary or may be articulated or translated by any suitable means (not shown) known in the art. Alternatively, spray nozzle 20 may be held stationary or may be articulated or translated. In some situations, both the part and the nozzle may be manipulated.
In the method of the present invention, the repair material feedstock is a powdered aluminum containing material. The powdered aluminum containing material may be a powdered aluminum containing material of −325 mesh with particle sizes in the range of from 5 microns to 50 microns. Smaller particle sizes such as those mentioned before enable the achievement of higher particle velocities. Below 5 microns in diameter, the particles risk getting swept away from the surface 25 and/or the affected area 24 due to a bow shock layer above the surface 25 and/or the affected area 24. This is due to insufficient mass to propel the particles through the bow shock. The narrower the particle size distribution, the more uniform the particle velocity will be. This is because if one has large and small particles (bi-modal), the small ones will hit the slower, larger ones and effectively reduce the velocity of both.
The fine particles of the aluminum containing repair material may be accelerated to supersonic velocities using compressed gas, such as helium, nitrogen, or other inert gases, and mixtures thereof. Helium is a preferred gas because it produces the highest velocity due to its low molecular weight.
The bonding mechanism employed by the method of the present invention for transforming the powdered aluminum containing repair material into a deposit is strictly solid state, meaning that the particles plastically deform. Any oxide layer that is formed on the particles and/or on the component surface is broken up and fresh metal-to-metal contact is made at very high pressures.
The powdered aluminum containing repair material used to form the deposit may be fed to the spray gun 22 using any suitable means known in the art, such as modified thermal spray feeders. For example, a Praxair powder feeder at a wheel speed of 1-5 rpm may be used.
In the process of the present invention, the feeder may be pressurized with a gas selected from the group consisting of helium, nitrogen, or other inert gases, and mixtures thereof. For example, the feeder may be pressurized using helium at a pressure in the range of from 200 psi to 400 psi, preferably from 300 to 350 psi. The main gas is preferably heated so that gas temperatures are in the range of from 250 degrees Centigrade to 550 degrees Centigrade, preferably from 350 degrees Centigrade to 450 degrees Centigrade. The gas may be heated to keep it from rapidly cooling and freezing once it expands past the throat of nozzle 20. The net effect is a surface temperature on the part being repaired of about 115 degrees Fahrenheit during deposition. Any suitable means known in the art may be used to heat the gas.
To deposit the aluminum containing repair material, the nozzle 20 may pass over the affected area 24 of the part 10 being repaired more than once. The number of passes required is a function of the thickness of the repair material to be applied. The method of the present invention is capable of forming a deposit having any desired thickness. If one wants to form a thick layer, the spray gun 22 may be held stationary to form a thick deposit over the affected area 24. When building a deposit layer of the aluminum containing repair material, it is desirable to limit the thickness per pass in order to avoid a quick build up of residual stresses and unwanted debonding between deposit layers.
The main gas that is used to deposit the particles of the repair material over the affected area 24 may be passed through the nozzle 20 via an inlet 30 at a flow rate of from 0.001 SCFM to 50 SCFM, preferably in the range of from 15 SCFM to 35 SCFM. The foregoing flow rates are preferred when helium is used as the main gas.
The pressure of the spray gun 22 may be in the range of from 200 psi to 400 psi, preferably from 300 psi to 350 psi. The powdered aluminum containing repair material is preferably fed from a hopper, which is under a pressure in the range of from 10 to 50 psi higher than the specific main gas pressure, preferably 15 psi higher to the spray gun 22 via line 34 at a feed rate in the range of from 10 grams/min to 100 grams/min, preferably from 15 grams/min to 50 grams/min.
The powdered aluminum containing repair material is preferably fed to the spray gun 22 using a non-oxidizing carrier gas. The carrier gas may be introduced via inlet 30 at a flow rate of from 0.001 SCFM to 50 SCFM, preferably from 8 SCFM to 15 SCFM. The foregoing flow rate is useful if helium is used as the carrier gas.
The spray nozzle 20 is held at a distance from the affected area 24. This distance is known as the spray distance. Preferably, the spray distance is in the range of from 10 mm. to 50 mm.
The velocity of the powdered repair material particles leaving the spray nozzle 20 may be in the range of from 825 m/s to 1400 m/s. preferably from 850 m/s to 1200 m/s.
The deposit thickness per pass may be in the range of from 0.001 inches to 0.030 inches.
Using the method of the present invention, the aluminum containing repair material, such as aluminum alloy 6061, may be sprayed over the affected area on the part or component 10 to a thickness above the original wall thickness. After the aluminum containing material has been deposited, the part or component 10 may be heat treated to recover the ductility of the cold sprayed aluminum containing repair material. The heat treatment may be carried out at a temperature which achieves the desired ductility for the part or component 10. For example, the heat treatment may be one in which the part or component with the cold sprayed aluminum containing material deposit is heated in an air oven to a temperature of 500 degrees Fahrenheit for a time period of 1 hour to 2 hours. When some aluminum containing repair materials are used, no heat treatment may be needed. When other aluminum containing repair materials are used, the heat treatment may be at a temperature which varies from 100 degrees Fahrenheit to a temperature greater than 500 degrees Fahrenheit for a time period in the range of 1 hour to 24 hours. When a heat treatment is required, it may be applied globally to the entire component or locally in the area of the repair.
After any heat treatment has been completed, the part or component 10 with the deposited repair material may be mechanically smoothed in the region of the affected area 24 and structural credit may be claimed for the repaired area. Structural credit as used herein refers to the fact that the cold sprayed aluminum alloy repair material has a percentage of the base material strength and repaired thickness is considered as part of the measured wall thickness. It is not necessary that the deposits of the cold sprayed aluminum containing repair material have parent metal strength, only that the structural credit be sufficient such that the effective wall thickness is above the required minimum. If the effective wall thickness is above the required minimum, the part or component 10 can be salvaged. As used herein, the term effective wall thickness means that if one considers the sprayed deposit to have 75% of the strength of the base material forming the part or component 10, then the repair thickness is credited 75%. Thus, if the current wall thickness of the part or component 10 is 0.100 inches and a deposit of 0.050 inches is applied to make total thickness of 0.150 inches, the effective wall thickness of the repair is 0.1375 inches since the repair material has 75% of the strength of the base material of the part or component.
Cold spray offers many advantages over other metallization processes. Since the aluminum containing powders used for the repair material are not heated to high temperatures, no oxidation, decomposition, or other degradation of the feedstock material occurs. Powder oxidation during deposition is also controlled since the particles are contained within the oxygen-free accelerating gas stream. Cold spray also retains the microstructure of the feedstock. Still further, because the feedstock is not melted, cold spray offers the ability to deposit materials that cannot be sprayed conventionally due to the formation of brittle intermetallics or a propensity to crack upon cooling or during subsequent heat treatments.
Cold spray, because it is a solid state process, does not heat up the part appreciably. As a result, any resulting distortion is minimized. Controlling part temperature also allows certain base material properties, obtained through prior heat treatments, to be retained. Cold spray induces compressive surface residual stresses, which can increase resistance to fatigue and brittle failures, resulting in more robust repairs.
While the present invention has been described in the context of using a cold spray process, other processes can be used to effect the repairs. These processes provide sufficient energy to accelerate particles to a high enough velocity such that, upon impact, they plastically deform and bond to the surface and build a relatively dense coating or structural deposit. These processes also do not metallurgically transform the particles from their solid state. These processes include, but are not limited to, kinetic metallization, electromagnetic particle acceleration, modified high velocity air fuel spraying, and high velocity impact fusion.
As-deposited bond strengths in excess of 10 ksi have been achieved for spraying 6061 aluminum alloy onto 6061 aluminum alloy substrates. Specimens were prepared using helium carrier gas at 300 psi heated to 400 degrees Centigrade. The nozzle was traversed over the test specimen at 125 mm per second, and the powder feed rate was such that approximately a 0.010 inch layer of material was deposited each pass. A total of six passes were made over the specimen, resulting in a final deposited thickness of 0.060-0.070 inches.
It is apparent that there has been provided in accordance with the present invention a structural repair method using cold sprayed aluminum materials which fully satisfies the objects, means, and advantages set forth hereinbefore. While the present invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.
