The U.S. Army has experienced significant corrosion problems with magnesium alloys that are used to fabricate aircraft components. The most severe of these are associated with large and expensive transmission and gearbox housings for rotorcraft, which have to be removed prematurely because of corrosion. Many of the parts cannot be reclaimed because there is not an existing technology that can restore them adequately for service. The U.S. Army Research Laboratory (ARL) has developed a cold spray process to reclaim magnesium components that shows significant improvement over existing methods. Part of this program has demonstrated and validated a Cold Spray (supersonic particle deposition) process using aluminum and/or Al alloys as a cost-effective, environmentally acceptable technology to provide surface protection and a repair/rebuild methodology to a variety of magnesium alloy components for use on Army and Navy helicopters and advanced fixed-wing aircraft.
The cold spray repair has been shown to have superior performance in the tests conducted to date, is inexpensive, can be incorporated into production, and has been modified for field repair, making it a feasible alternative over competing technologies. A Cold Spray demonstration facility has been established at the Fleet Readiness Center East (FRC-E) North Carolina (formerly Naval Air Depot Cherry Point). Additionally, the Original Equipment Manufacturer (OEM), Sikorsky Aircraft Company is in the final approval stage for the implementation of the cold spray process developed by ARL for the H-60 family (Black Hawk, Seahawk, etc.) of helicopters.
The objectives of this program were: (1) to demonstrate and validate Cold Spray (supersonic particle deposition) of aluminum and/or Al alloys as a cost-effective, environmentally acceptable technology to provide surface protection and a repair/rebuild methodology to a variety of magnesium alloy components on Army and Navy helicopters and advanced fixed-wing aircraft, (2) establish a Cold Spray demonstration facility FRC-E, (3) validate the Cold Spray coatings through materials and component testing as defined by stakeholders and (4) demonstrate that Cold Spray can be used as a repair/rebuild methodology that can reclaim currently unsalvageable parts.
The Cold Spray process involves the introduction of a heated high-pressure gas such as helium or nitrogen together with 1- to 50-μm particles of a metal or alloy into a gun onto which is attached a nozzle designed such that the gas exits at supersonic velocities. The powder particles entrained in the gas flow are accelerated to velocities in the range of 200 to 3000 meters-per-second, considerably higher than what are achieved in any thermal spray process, including High Velocity Oxyfuel (HVOF). Because the temperature of the gas generally ranges from 0 to 800 ºC, no melting of the particles takes place, and there is no oxidation, decomposition, or other degradation of the powder material.
Other advantages of the Cold Spray process include:
- Extremely dense coatings with virtually no inclusions or cracks
- Retains properties and microstructure of initial powder particles allowing the deposition of nanostructured materials
- High deposition rates, up to 30 lbs/hour, equivalent or superior to other thermal spray processes
- Uniform microstructure and, for alloy coatings, powder phase structure is maintained
- Extremely thick coatings can be deposited as well as bulk material and free-standing structures/parts can be fabricated (many mm thick)
- Functionally graded coatings/bulk materials can be produced
- Unique and exotic coatings/materials can be formed that cannot be produced by conventional techniques, such as thermal spray or ingot metallurgy because consolidation is accomplished in the solid state
- Impact of particles on surface acts like shot peening, imparting favorable residual compressive stress
- Residual stress in coatings is neutral or slightly compressive
- Low heat input allows for coating broad range of materials, such as composites, including components with thin walls
- Process involves no toxic gases, radiation or chemical reactions
- Localized deposition possible, thereby eliminating most requirements for masking
Quantitative and qualitative objectives performance objects were established for this effort by the primary stakeholders who included representatives from the Tri-services, academia and industry. The quantitative performance objectives were approved by Naval Air Systems Command (NAVAIR), ARL and the Original Equipment Manufacturer (OEM), which was Sikorsky Aircraft Company.
The quantitative performance objectives involved the execution of various coatings testing and evaluation techniques, as well as comprehensive materials characterization and included: (1) Deposition rate, (2) Coating thickness uniformity, (3) Microstructure, (4) Hardness, (5) Fatigue, (6) Stress/strain, (7) Residual stress, (8) Uni-axial adhesion, (9) Shear adhesion, (10) Fretting Fatigue, (11) Salt fog, (12) Cyclic Corrosion, (13) Powder particle size distribution, (14) Powder chemical composition.
Test results from this program indicate that Cold Spray as a repair technology for the reclamation of magnesium aerospace components offers improved performance and permits the reclamation of material properties, as well as dimensional restoration, improving readiness since replacements would be less frequent. In conjunction, there is an anticipated reduction in logistics costs since fewer new gearboxes would be required in the field, and fewer would have to be shipped back and forth between depots and operating bases for repair. A Sikorsky Aircraft study also shows there is significant potential for cost reduction by reducing the number of condemnations, but in addition to reducing cost, reducing the number of condemnations will improve operational readiness.
Sikorsky Aircraft has invested over $1M toward the development of Cold Spray in collaboration with ARL and is planning on introducing the process for repair of H-60 sumps because it produces a superior quality repair than the current HVOF Al-12Si, permitting the reclamation of otherwise unsalvageable components. The UH-60 Main Sump Gearbox is currently repaired by Sikorsky using an HVOF Al-12Si coating together with a bond coat. This method is not satisfactory as the HVOF coating tends to crack on insertion of Rosan fitting and does not reclaim the mechanical properties of the magnesium alloy. It is expected that the use of cold spray coating will allow these gearboxes to be reclaimed, largely eliminating condemnations.
One of the objectives for this ESTCP project was to establish a fully functional Cold Spray facility at FRC-E, capable of performing component repair of magnesium aerospace components. A crucial step towards accomplishing this objective was achieved in January, 2011 when the cold spray system at FRC-E was used to produce a batch of aluminum coatings. The test results from the demonstration confirmed that the system is functioning correctly and that the operators are following the ARL Cold Spray procedure correctly. Follow on work has been continuous with the goal of flight testing repaired parts by the end of the calendar year.
A future research area should be the investigation of structural repair using the cold spray process. The mechanical test results of the 6061 He coating that ARL developed suggest there is potential beyond non-structural or cosmetic repairs using cold spray technology. ARL has generated preliminary data which shows that as-cold sprayed 6061 Al has a higher ultimate tensile strength, yield strength and hardness than conventional wrought 6061 Al that has been heat treated to the T-6 condition. Advancing the technology to the point where structural repair of gearboxes and other components is possible should be a priority of the Department of Defense (DoD) science and technology community. Structural repair capability would have a significant impact on the DoD in regards to cost savings, as well as increasing war fighter capability.