TECHNOLOGY AREA(S): Materials/Processes

ACQUISITION PROGRAM: Frigate Program Office, PEO LCS PMS 515

OBJECTIVE: Development of a fieldable system and the associated algorithms to allow for in-situ characterization and quantification of the inherent variability of commercially available high-magnesium 5xxx series aluminum alloys.

DESCRIPTION: Aluminum alloys have become more prevalent in marine structural applications with the Navy�s need to build lighter, faster ships. The high magnesium 5xxx alloys, such as 5083 and 5456, are the best candidates, maximizing specific strength, corrosion resistance, and as-welded strength. Of particular concern, however, is the alloy�s susceptibility to sensitization. Aluminum sensitization has occurred when a nearly continuous network of beta-phase forms along the grain boundaries. The beta-phase is anodic to the aluminum matrix, and when exposed to the corrosive effects of seawater and sufficient loading, provides a clear pathway for stress corrosion cracking (SCC). The degree of beta-phase precipitation is driven by a combination of time and elevated temperature. The location and rate of beta-phase formation depends on material processing. The operating environment for Navy ships is particularly harsh, with wide variations in temperature and constant exposure to seawater. Aluminum alloys currently in service are subject to sensitization, which increases the potential to experience SCC, exfoliation, or inter-granular corrosion, often necessitating the repair or replacement of compromised material and has the potential to increase the total ownership cost across the life of a ship.� Both Littoral Combat Ship (LCS) variants utilize 5xxx series aluminum alloys. Due to cracking issues experienced on the CG-47 Cruiser class, the ability to predict sensitization and stress corrosion cracking onboard both LCS and the future Frigate (should it use aluminum structure) has become of particular interest. Current commercially available technologies are not predictive. Technologies such as the destructive ASTM G67 test have been developed and allow for the detection of sensitization at a single point in time, but do not allow for extrapolation to the likelihood of SCC. The Degree of Sensitization (DoS) Probe is a non-destructive test for determining level of sensitization, but is time consuming and is not capable of analyzing confined spaces (e.g., areas with corners/beams where the probe is unable to fit). In situ metallography images the microstructure of the material, but is qualitative in nature and unable to predict long-term susceptibility to sensitization and SCC.

This topic seeks robust SCC prediction tools for commercially available high-magnesium 5xxx series aluminum alloys procured under the ASTM B982 specification. The goal is to develop a system and the associated algorithms to allow designers and maintainers to perform in-situ characterization and quantification of the inherent variability in material microstructure to directly link variability to the actuality of stress corrosion cracking through confirmation testing. All algorithms should be developed using open architecture design principles, as practicable, with the larger objective of being incorporable into planned and future SCC prediction tools. Companies shall address their proposed methods and processes of gathering and displaying data as well as the ability for the developed algorithms to be integrated with future SCC predictive algorithms and tools.

Extreme variability in sensitization resistance adds significant complexity to the issue. As a result of aluminum plate processing (manufacturing), the level of sensitization can significantly differ between lots of material that are subjected to the same heat treatment, with the rate of beta-phase precipitation varying by up to 40 times for aluminum plates procured to the ASTM B928 specification. A more in-depth understanding of processing history and resultant material microstructure is necessary to quantify the rate of sensitization and the inherent variability in the procured materials. Quantifying specific microstructural features, such as, grain size, grain orientation, precipitates, dislocation density, etc., will lead to a better understanding of when SCC is expected to initiate during the lifetime of the ship. These algorithms will then allow for a more in-depth understanding of microstructural response, leading to a more robust ability for predicting the likelihood of material failure because of SCC. Outcomes could also help inform ship design processes and allow for more accurate predictions of when repair and maintenance decisions may be necessary.

Proposed solutions should be man-portable for demonstration and for use in an open-air ship, shipyard, or repair-yard environment. Concepts should be able to fit through a standard Navy watertight door with dimensions of 26 �� x 66� and must be weight compliant in accordance with MIL-STD-1472H. Additionally, the solution must be able to investigate deck and bulkhead areas as small as 12 inches in diameter. The test cycle should not be more than 1 hour per sample area from set-up to delivery of results. Of particular interest are final solutions that are non-destructive allowing for some method of surface preparation that does not require hot work to repair. Solutions must not use hazardous chemicals and should be usable by ship, shipyard, repair center, or industrial maintenance activities.

PHASE I: Develop a concept for a fieldable, man-portable system to allow designers and maintainers: 1) to perform in situ characterization and quantification of the inherent variability in material microstructure in high-magnesium 5xxx series aluminum alloys procured to the ASTM B982 specification and 2) to directly link such variability to the actuality of SCC. Propose a concept, conduct the supporting analysis and feasibility of the concept, and develop the initial concept design and model key elements of the proposed technology. The Phase I Option, if awarded, will include the initial design specifications and capabilities description to build and test prototype solutions in Phase II. Develop a Phase II plan.

PHASE II: Develop and deliver a fieldable, man-portable prototype system and calibrate prototype performance through confirmation testing and evaluation based upon the results of the Phase I and the Phase II Statement of Work (SOW).� As necessary, perform coupon testing in a laboratory environment to validate developed algorithms for accuracy and reliability; and establish a corresponding working materials database. A proposed test plan for U.S. Navy acceptance, business case analysis including a plan for manufacturing, and corresponding materials database must be included as part of the final report of Phase II accomplishments.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology to Navy use, and Navy test and evaluation done in accordance with the test plan developed in Phase II, ensuring that the delivered algorithms and associated prototype(s) perform as expected and are robust enough for use in the target environments referenced within the topic. The technology can expect to transition to LCS and potentially all platforms using 5xxx series aluminum for marine structural applications (e.g., Future Frigate, CG, Ship-to-Shore Connector [SSC], Expeditionary Fast Transport [EPF]).

5xxx series aluminum alloys are widely used in commercial marine structures.� Products resulting from this topic would have wide application for all commercial ships using marine grade aluminum.

REFERENCES:

1. Golumbfskie, W.J. Tran, K.T. et al. �Survey of Detection, Mitigation, and Repair Technologies to Address Problems Caused by Sensitization of Al-Mg Alloys on Navy Ships� Corrosion: The Journal of Science and Engineering. February, 2016. http://www.corrosionjournal.org/doi/abs/10.5006/1916?code=nace-prem-site

2. Zhang, S.P. Knight, R.L. Holtz, R. Goswami, C.H.J. Davies, N. Birbilis, �A Survey of Sensitization in 5xxx Series Aluminum Alloys� Corrosion: The Journal of Science and Engineering, September, 2015.� http://www.corrosionjournal.org/doi/full/10.5006/1787

3. ASTM International, ASTM G67, �Standard Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series Aluminum Alloys by Mass Loss After Exposure to Nitric Acid (NAMLT Test)�. May, 2013. https://www.astm.org/Standards/G67.htm

4. ASTM B928/B928M-15, "Standard Specification for High Magnesium Aluminum-Alloy Products for Marine Service and Similar Environments�. June, 2015. https://www.astm.org/Standards/B928.htm

5. MIL-STD-1472G, �Department of Defense Design Criteria Standard: Human Engineering (11-Jan-2012).� http://everyspec.com/MIL-STD/MIL-STD-1400-1499/MIL-STD-1472G_39997/

KEYWORDS: Corrosion in Aluminum Alloys; Sensitization of Aluminum Alloys; Stress Corrosion Cracking (SCC) of Aluminum; Aluminum Alloy; High Magnesium 5xxx Alloys; Processing Variability in Aluminum Alloys