Patent Application
20160108504
The present invention relates to treatment of aluminum, particularly the 5000 series aluminum alloys used in Navy ships and other maritime vessels, to reduce its susceptibility to corrosion and other damage.
Aluminum-magnesium alloys are important technological alloys for marine applications. With magnesium concentrations of 3 to 6%, along with other alloying additions and appropriate thermomechanical processing, the alloys are high strength, light weight, resistant to seawater corrosion, and weldable. These characteristics make these alloys attractive for lightweight, high speed, fuel efficient ships, amphibious craft, and land vehicle armor.
These qualities make aluminum a particularly useful metal for marine vessels. An important class of aluminum alloys that are widely used in Navy and commercial ships are the 5000-series aluminum alloys, often referred to as “5000 aluminum.” These alloys contain magnesium to enhance their strength, where the magnesium forms a solid solution having a magnesium concentration of between 3 and 6% in the aluminum bulk.
However, over time, and particularly under prolonged in-service exposure to high temperatures, the magnesium in these alloys migrates to the grain boundaries in the material, where, as can be seen in the optical metallography shown in
The degree of sensitization (“DOS”) is related to the density of beta particles present at the grain boundaries. A DOS near zero corresponds to a beta particle density of about 60% or less, while a DOS of 40 or more corresponds to a nearly 100% beta particle density at the grain boundaries. If the beta particle density on the grain boundaries exceeds about 60 to 65%, continuous networks of the particles may form, resulting in accelerated intergranular corrosion rates. It has been observed that if the DOS exceeds about 30, significant degradation of the corrosion fatigue and stress corrosion properties can occur, which rapidly gets worse with further increase of DOS.
Such sensitization affects a large class of Navy ships, including the DDG 963, CG, and FFG classes, which use 5000 series aluminum alloys in their deck plates and/or superstructures, as well potentially the Littoral Combat Ship (LCS), Joint High Speed Vessel (JHSV), and Joint Maritime Assault Connector (JMAC) that also will use this alloy of aluminum to achieve their performance. An example of sensitization-induced cracking on a Navy ship can be seen in
Studies show that the sensitization of aluminum can be reversed by heating the aluminum to a temperature which both causes the beta phase particles to dissociate and causes the magnesium to dissolve back into the aluminum bulk. This process is known as “desensitization.” See L. Kramer, M. Phillippi, W. T. Tack, and C. Wong, “Locally Reversing Sensitization in 5xxx Aluminum Plate,” Journal of Materials Engineering and Performance (2012) 21:1025-1029.
As illustrated in the plots shown in
Dissolving the beta phase requires that the temperature be raised above the solvus temperature of the alloy, which depends upon exact alloy composition and temper condition. Generally, the solvus temperature for the 5000 series alloys that experience sensitization will be higher than that for a pure binary aluminum-magnesium alloy, see Y. Zuo and Y. A. Chang, “Thermodynamic Calculation of the Al—Mg Phase Diagram,” CALPHAD, Vol. 17, No. 2, pp. 161-174 (1993), and will increase with additional concentrations of other alloying elements. For example, a pure binary alloy of aluminum and magnesium at 4.5 percent magnesium (i.e., an alloy having the same magnesium concentration as alloy 5083) has an estimated solvus temperature of 230° C., while commercial alloy 5083, which has additional constituents, has an experimentally measured solvus value of 290° C. See Y. K. Yang and T. R. Allen, “Determination of the beta Solvus Temperature of the Aluminum Alloys 5083,” Metallurgical and Materials Transactions A—Physical Metallurgy and Materials Science, Vol. 44A, Issue 11, pp. 5226-5233 (2013). Commercial alloy 5456, which has a nominal magnesium concentration of 5.5 percent, should have a solvus temperature above the binary alloy value of about 260° C.; although the actual solvus has not been experimentally measured.
In addition, as noted above, desensitization should not be performed at temperatures high enough to anneal the alloy. Although such high temperatures will desensitize the alloy, they also will considerably soften the alloy, reducing its strength. Standard reference sources list 345° C. as the typical annealing temperature for 5000 series alloys including 5083 and 5456. See, e.g., Heat Treating of Aluminum Alloys, American Society for Metals Handbook, Vol. 4, ASM International, Materials Park, Ohio, pp. 841-879 (1991). Thus, the temperature needed to achieve desensitization without softening in marine service alloys will generally be within the broad range between 230° C. and 345° C., with specific, narrower temperature ranges for alloy compositions being determined empirically in each case.
Various methods to heat the aluminum to a temperature sufficient for desensitization while keeping the temperature within this critical range have been proposed.
In one method, a flexible ceramic pad heater is used to apply heat to the surface of the sensitized aluminum. See L. Kramer, et al., supra. In another method, friction-stir processing is used to heat and thereby desensitize the metal. See, e.g., A. P. Reynolds and J. Chrisfield, “Friction Stir Processing for Mitigation of Sensitization in 5XXX Series Aluminum Alloys,” Corrosion, Vol. 68, No. 10 (2012), pp. 913-921.
However, there are significant problems with these approaches. Both approaches require intimate contact with the aluminum, so their efficiency can be compromised by the presence of surface irregularities such as weld seams. In addition, the pad heater is a slow process and locally heats the entire structure. Large-scale heating of the structure is undesirable because it potentially increases sensitization levels in areas around the zone being treated, it introduces residual stresses in weld connections to the underlying framing which can result in local fatigue cracking, and it exposes the interior of the ship, including sensitive electronics and equipment, to potentially damaging temperatures. Finally, if it heats the aluminum above the anneal temperature of 345° C. as shown in
Neither these nor any other approach has so far been deployed in the fleet, and the sensitization and the resulting susceptibility of 5000 aluminum to corrosion and other damage, remains a significant issue.
This summary is intended to introduce, in simplified form, a selection of concepts that are further described in the Detailed Description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Instead, it is merely presented as a brief overview of the subject matter described and claimed herein.
The present invention provides a method for desensitizing an aluminum alloy. In accordance with the present invention, a desired location on the surface of an aluminum alloy sample is exposed to a controlled pulsed electron beam. The pulsed electron beam heats a shallow layer of the metal alloy having a desired depth at the desired location on the surface of the sample to a temperature between the solvus temperature and an annealing temperature of the metal alloy to controllably reduce a degree of sensitization of the metal alloy sample at the desired location, an extent of a reduction in the degree of sensitization being controllable by varying at least one of a voltage, a current density, a pulse duration, a pulse frequency and a number of pulses of the electron beam.
The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.
For example, the electron beam desensitization treatment of the present invention is described herein in the context of desensitization of an aluminum alloy, often referred to herein simply as “aluminum” or “alloy,” and is of particular interest in connection with the 5000-series aluminum alloys commonly used for maritime applications such as deckplates for Navy ships.
As noted above, it has previously been discovered that sensitization of aluminum can be reversed by heating the aluminum to a point above its solvus temperature while being kept below the point at which it begins to anneal. See Kramer, supra. As illustrated by the plots in
The present invention overcomes the problems of the prior art method by using a pulsed high voltage, high current electron beam to provide the heat necessary to desensitize an aluminum alloy such as the 5000 series aluminum alloy used in Navy ships heat in a localized, depth-controlled manner.
Thus, as described in more detail below, in accordance with the present invention, environmentally induced corrosion susceptibility in an aluminum alloy can be reversed by applying a properly configured pulsed high voltage, high current electron beam to the alloy's surface.
Thus, as illustrated in
Any suitable pulsed power supply can be used, such as the repetitive pulsed power supply based on spark gap switches as used in the Electra repetitive pulsed electron beam facility at the Naval Research Laboratory (NRL). See J. D. Sethian, M. Myers, I. D. Smith, V. Carboni, J. Kishi, D. Morton, J. Pearce, B. Bowen, L. Schlitt, O. Barr, and W. Webster, “Pulsed power for a rep-rate, electron beam pumped, KrF laser,” IEEE Trans Plasma Sci., 28, 1333 (2000). In other embodiments, the power supply can be based on other systems such as the more advanced all solid-state system demonstrated by NRL. See F. Hegeler, M. W. McGeoch, J. D. Sethian, H. D. Sanders, S. C. Glidden, and M. C. Myers, “A durable, gigawatt class solid state pulsed power system,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 18, Issue 4, pp. 1205-1213, August 2011, both of which are hereby incorporated by reference into the present disclosure in their entirety.
A typical pulsed electron beam generated by an apparatus configured for use in the method of the present invention will have a voltage of about 100 to about 600 kV, a current of about 1 to about 100 kA, and a pulse duration of about 100 nsec to about 1 μsec. with the electron beam source having an ability to operate in bursts of 10 to 100 pulses at 0.1 to 5 pulses per second.
The electron beam can be controllably directed to specific areas on the surface of the aluminum, e.g., areas that have been identified as having an unacceptably high degree of sensitization. Thus, the present invention enables controlled, localized desensitization of specific areas on the aluminum surface without the need for unnecessarily treating large areas not suffering from the effects of sensitization.
In addition, a pulsed electron beam incident upon the surface of an aluminum sample deposits its energy only into a shallow layer, e.g., to a depth of 10 to 200 microns, depending on the energy of the electron beam. See J. A. Halbleib, R. P. Kensek, G. D. Valdez, S. M. Seltzer, and Martin J. Berger, “ITS: The Integrated TIGER Series of Electron/Photon Transport Codes—Version 3.0,” IEEE Trans. Nucl. Sci, Vol. 39, pp. 1025-1030, 1992. Thus, any heating of the metal that results from this added energy will also occur only within this shallow layer at the surface, and will quickly attenuate at greater depths. Because desensitization of commercial 5000 series alloys requires temperatures between 230-345° C., depending on the alloy, desensitization will not occur at depths in the metal where the electron beam does not raise the temperature to a sufficient degree. In addition, using a pulsed beam allows the surface to cool slightly between pulses, limiting the heating of the metal caused by this added energy and allowing it to be controllably heated to a desired depth without excessively heating its interior or backside. In the case of an electron beam being used to desensitize an aluminum deckplate on a ship, this means that a shallow (10-200 μm) surface layer of the deckplate can be treated and desensitized while the bulk of the deckplate, which has a thickness of 5 mm to 8 mm (5000 to 8000 μm), and thus the interior of the ship, remain relatively cool. It also means that the bulk material properties (strength, yield), which can be compromised by heat, will remain unchanged. In some embodiments, the back side of the material (i.e., the side opposite the electron beam exposure) can be actively cooled by flowing air or a water cooled plate.
As described in more detail below, this depth within the metal at which desensitization occurs can be controlled by varying the power and/or the current of the applied electron beam.
The plots shown in
As shown in
Thus, in accordance with the present invention, the treatment temperature, the duration for which the treatment temperature is maintained, and the depth of the treatment layer can be controlled across the entire range of conditions needed for desensitization (i.e., temperature of 230 to 345° C. and treatment depth of 10 to 200 μm) by varying the voltage, current, pulse length, repetition rate and/or number of pulses of the applied electron beam.
The plot in
The as-received condition of the material, which is the condition of the material as it is manufactured, typically is already partially sensitized with a DOS of 15 or lower. As shown in the plot in
The sensitized samples were treated with electron beams having a current density of 130 A/cm2, 160 A/cm2, and 260 A/cm2. As can be seen from the plot in
Thus, the plots in
The plots in
These and other aspects of the invention will now be described in the context of the following Example. It will readily be appreciated by one skilled in the art that the following description is merely exemplary, and that 5000 series aluminum and/or other aluminum alloys may be desensitized in accordance with the method of the present invention through the application of electron beams having other voltage, current, and/or pulse parameters thereto.
In an exemplary case, samples of the aluminum-magnesium alloy 5456-H116 Alcoa Aluminum (Lot #357543) meeting the Navy standards for shipboard use were procured. The samples as delivered exhibited some degree of sensitization which is a normal characteristic of such metal alloys resulting from the natural migration of the dissolved magnesium to the grain boundaries. The samples were subsequently aged by heating the samples to 100° C. for 12-and-a-half days, using standard heating techniques accepted in the industry to produce a high degree of sensitization in the samples, as confirmed by standard metallographic techniques.
The samples were then exposed to 100 electron beam pulses produced by the NRL Electra repetitive pulsed electron beam facility. See J. D. Sethian, M. C. Myers, J. I. Giuliani, Jr., R. H. Lehmberg, P. C. Kepple, S. P. Obenschain, F. Hegeler, M. Friedman, M. F. Wolford, R. V. Smilgys, S. B. Swanekamp, D. Weidenheimer, D. Giorgi, D. R. Welch, D. V. Rose, and S. Searles, “Electron beam pumped krypton fluoride lasers for fusion energy,” Proc. IEEE, 92, (2004) 1043-1056, the entirety of which is incorporated by reference into the present disclosure, for a description of this system. In this exemplary application of the method of the present invention, each electron beam pulse had a voltage of 500 kV, a current density ranging from 160 to 260 A/cm2, a beam diameter of 3.6 cm, a pulse length of 100 nsec (flat top), a repetition rate of 5 pulses per second, and a total number of 100 pulses. However, any one or more of these parameters can be varied significantly as needed to achieve the desired DOS, with typical ranges being electron beam energy of 100 to 650 kV, current density of 100 to 400 A/cm2, and pulse length of 70-140 nsec. The cathode (electron beam emitter) used in this Example was a disk of graphite, though it will be well appreciated that other emitters may also be used, such as an array of carbon fibers pyrolized to a carbon base, a velvet fiber cathode, or one made of a ceramic honeycomb over a fiber array emitter.
In this Example, the sample itself served as the electrical anode. In other cases, a thin metal (titanium, stainless steel, or aluminum) foil may be used as the anode, and in such cases, the electrons pass through the foil before impinging on the sample; such an approach may have advantages in the final application, as it prevents having to maintain a vacuum on the surface of the aluminum to be treated.
After the samples were aged, the level of their sensitization was assessed. Since the desensitization does not occur through the entire thickness of the specimens, standard techniques such as the ASTM G67 Nitric Acid Mass Loss Test are not applicable. Instead an alternative method was developed. In this alternative assessment method, the samples were subjected to a metallographic etching procedure and then examined with optical metallography to determine the amount of beta phase present on the grain boundaries. The etching is based on a general technique studied initially at the University of Virginia (see J. Buczynski, “Electrochemical analysis of etchants used to detect sensitization in marine-grade 5xxx aluminum-magnesium alloys,” M.S. Thesis, University of Virginia (2012)) but modified specifically for this project. The specimens were immersed for 60 minutes in ammonium persulfate at 0.2 M concentration with pH adjusted to 1.2 using sulfuric acid in a temperature controlled bath at 35° C. The etchant selectively dissolved the alloy phase responsible for sensitization, and the relative level of sensitization is apparent by the continuity and thickness of etched areas in the sample grain boundary microstructure.
Electron beam desensitization of aluminum in accordance with the present invention does not significantly affect the strength of the bulk material.
As can be seen from the plot in
Similarly, the plots in
Thus, the Rockwell Hardness measurements show that while there may be a small softening effect at the surface and the top ⅓ cross-section for the highest electron beam exposures, such softening is not of a magnitude that would compromise the suitability of the material for its intended structural purpose. These results support the claim that the e-beam desenstizes the surface only, without affecting the strength of the bulk material.
As noted above, the 5000 series aluminum alloy which can be desensitized using the pulsed electron beam treatment of the present invention is a key component of maritime vessels used in both civilian and military applications, and electron beam desensitization of such alloys in accordance with the present invention has significant advantages over conventional desensitization methods currently being used.
Because the pulsed electron beam treatment of the present invention heats only a shallow layer having a thickness of 10 to 200 microns at the surface of the metal, the bulk of the material remains relatively cool. For example, an electron beam having energy of 100 kV to 600 kV deposits its energy in, and hence heats only, a very shallow layer having a thickness of 50-100 microns at the surface. Heating the aluminum at this depth is sufficient to reduce the detrimental effects of sensitization, as corrosion caused by sensitization is a surface phenomenon. This can provide a particular advantage when desensitization of aluminum that has already been incorporated into a ship is desired. Typical 5000 series aluminum shipboard structures are on the order of 5-8 mm thick, but they can be as much as 10 mm, so even if the temperature of the backside of the structure does increase, it should be readily straightforward to deal with this additional heat using straightforward thermal management techniques, possibly as simple as circulating fans or water cooled contact plates.
In addition, electron beam desensitization in accordance with the present invention provides a non-contact method for applying heat and desensitizing a sensitized alloy, with the electron beam source being separated from the aluminum by a distance of 1-5 mm, depending on the conditions and the particular configuration of the beam apparatus. In addition, although the electrons carry energy, they carry virtually no momentum, so there is no mechanical loading of the structure.
Moreover, the electron beam desensitization method in accordance with the present invention is not a chemical process and does not apply a new material or coating to the alloy surface. Instead, the electron beam simply reverts the grain structure of the material to its original state. Thus there should be no need for retesting and certification of the alloy, as would be the case if the surface chemistry was altered or a coating was applied.
The electron beam desensitization method of the present invention can be used either to remediate in-service material that has become sensitized, or to treat new material to reduce the initial degree of sensitization.
It is also believed that an appropriate electron beam system could be made small enough to be transportable. An exemplary embodiment of such a portable apparatus is illustrated in
Thus it is anticipated that this invention could perform shipboard reversal of sensitization in situ, before the onset of cracks. As corrosion repair is a significant cost for the Navy, this could meaningfully lower total ownership costs for the fleet.
Although particular embodiments, aspects, and features have been described and illustrated, it should be noted that the invention described herein is not limited to only those embodiments, aspects, and features, and it should be readily appreciated that modifications may be made by persons skilled in the art. The present application contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein, and all such embodiments are within the scope and spirit of the present disclosure.
This application is a Nonprovisional of, and claims the benefit of priority under 35 U.S.C. §119 based on, U.S. Provisional Patent Application No. 62/017,856 filed on Jun. 27, 2014, the entirety of which is hereby incorporated by reference into the present application.
| Number | Date | Country | |
|---|---|---|---|
| 62017856 | Jun 2014 | US |
