Title: Strategies for the Mitigation of Aluminum Sensitization on U.S. Navy Ships
Author(s): David Schario, Enterprise Ventures Corporation (EVC) / Concurrent Technologies Corporation (CTC)
The U.S. Navy has been experiencing issues with cracking in aluminum ship structures for many years. The problem is inherent in the 5xxx series aluminum used for these structures, as this material becomes sensitized after several years of exposure to elevated in-service temperatures. When this occurs, the exposed, sensitized structures become particularly susceptible to pitting corrosion and stress corrosion cracking (SCC). Since 5xxx series aluminum is a mainstay in modern ship construction, the issue is ubiquitous across the fleet. While this material is used extensively in CG-class cruisers, 5xxx series aluminum is also being utilized on many current new construction programs, including Littoral Combat Ships (LCS, both Freedom and Independence variants), Amphibious Assault Ships, Joint High Speed Vessels, Landing Craft Air Cushions, Ship-to-Shore Connectors, and CVN-class aircraft carriers. On CG-class cruisers alone, sensitization of the aluminum superstructures is a class-wide problem and is extensive on each ship; in addition, both LCS designs have already exhibited sensitization in areas.
The Navy has been working on this issue with research and development efforts focused on repair of cracks, the use of ultrasonic impact treatments to reduce residual welding stresses, and the development of both a portable probe and an in-situ metallographic technology to detect sensitized microstructures. To date, however, no commercially available products have been developed to remediate a plate that has become sensitized; additionally, weld repairs of cracks or inserts in sensitized material requires special treatment and complex welding procedures. The only current correction method for sensitized plate has therefore been the removal and replacement of the entire affected plate, including the removal and replacement of all outfitting obstructing the repair. The material that must be removed is often significant. For example, during recent modernization activities on the USS Vella Gulf (CG-72), the entire pilot house overhead had to be cut away and replaced (an estimated 200 ft2 area). The removal and replacement of sensitized aluminum on ships has therefore become an extremely costly and time-consuming endeavor for the U.S. Navy.
Concurrent Technologies Corporation (CTC), in collaboration with Enterprise Ventures Corporation (EVC), the U.S. Navy, relevant shipyards, academia, and commercial industry, are engaged in several projects that will provide products and solutions that directly address the aluminum sensitization issue. Using this work, CTC, EVC, and its teammates are implementing a strategy to develop solutions for aluminum sensitization that address the issue from a number of standpoints:
1. New materials – CTC continues to study and evaluate new aluminum materials to determine their susceptibility to sensitization and their suitability for Navy applications.
2. Mitigation for Existing Uncracked, Sensitized Aluminum – Under a project sponsored by the Navy Metalworking Center and the Navy Surface Combatants Program Office (PMS 407), CTC has designed and fabricated a device that can reverse sensitize aluminum in place, eliminating the need to cut out and replace it. The device is currently being evaluated and optimized for both functionality and man-portability.
3. Mitigation for Existing Cracked, Sensitized Aluminum – Under funding for the Office of Naval Research, CTC is developing a portable friction stir welding process to repair cracks in sensitized aluminum structures.
4. Prevention – Under a project funded by the National Shipbuilding Research Program, CTC is investigating the viability of applying thermal spray coatings to sensitized aluminum as a preventative for SCC. It is anticipated that the application of this coating will significantly reduce the SCC susceptibility of sensitized aluminum on both new construction and in-service ship structures.
For each of the above initiatives, CTC is developing roadmaps and technology transition plans to integrate these new technologies into existing work plans and successfully transition them to fleet maintenance operations. By establishing participation and buy-in from the end-users (Navy Regional Maintenance Centers, shipyards, and their subcontractors) and inviting them to join the project team, these transition plans are better defined and implemented.
This presentation will further discuss the sensitization issue, the status of each of the four mitigation strategies that CTC / EVC is pursuing, and the plans that will be employed to ensure that the resulting products are successfully transitioned to the fleet.