U.S. patent application number 14/614686 was filed with the patent office on 2016-04-21 for desensitization of aluminum alloys using pulsed electron beams.
The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Frank Hegeler, Ronald L. Holtz, Derek Horton, Alexis C. Lewis, Matthew C. Myers, John D. Sethian, Kathryn J. Wahl, Mathew Wolford.
Application Number | 20160108504 14/614686 |
Document ID | / |
Family ID | 55699938 |
Filed Date | 2016-04-21 |
United States Patent
Application |
20160108504 |
Kind Code |
A1 |
Sethian; John D. ; et
al. |
April 21, 2016 |
Desensitization of Aluminum Alloys Using Pulsed Electron Beams
Abstract
A method for desensitizing an aluminum alloy is presented. 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 a 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.
Inventors: |
Sethian; John D.; (Burke,
VA) ; Myers; Matthew C.; (Beltsville, MD) ;
Wolford; Mathew; (Woodbridge, VA) ; Hegeler;
Frank; (Vienna, VA) ; Holtz; Ronald L.;
(Lorton, VA) ; Horton; Derek; (Alexandria, VA)
; Lewis; Alexis C.; (Alexandria, VA) ; Wahl;
Kathryn J.; (Alexandria, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Washington |
DC |
US |
|
|
Family ID: |
55699938 |
Appl. No.: |
14/614686 |
Filed: |
February 5, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62017856 |
Jun 27, 2014 |
|
|
|
Current U.S.
Class: |
148/565 |
Current CPC
Class: |
C21D 1/34 20130101; C22C
21/06 20130101; C22F 1/047 20130101; C22C 21/08 20130101 |
International
Class: |
C22F 1/047 20060101
C22F001/047; C21D 1/34 20060101 C21D001/34; C22C 21/08 20060101
C22C021/08 |
Claims
1. A method for controllably desensitizing a metal alloy sample,
comprising: exposing a specific desired location on a surface of
the sample to a controlled pulsed electron beam having a voltage
greater than 100 kV to about 650 kV: wherein the electron beam is
controllably directed to the specific desired location without
exposing other areas on the sample to the electron beam; and
wherein the electron beam heats a shallow surface layer of the
metal alloy having a desired depth at the specific desired location
on the surface of the sample to a controlled temperature between a
solvus temperature and an annealing temperature of the metal alloy
without heating a bulk of the sample to controllably reduce a
degree of sensitization of the metal alloy sample at the specific
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, and a pulse frequency
of the electron beam.
2. The method according to claim 1, wherein a depth from the
surface of the sample at which the sample's sensitization is
reduced is 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.
3. The method according to claim 1, wherein the electron beam is
configured to heat a layer having a depth of between 10 and 200
microns at the surface of the metal alloy.
4. The method according to claim 1, wherein the electron beam is
configured to reduce the degree of sensitization in a layer having
a depth of about 10-200 .mu.m at the surface of the metal alloy
sample.
5. The method according to claim 1, wherein the electron beam
produces a controllably graded reduction in the degree of
sensitization in the metal alloy sample, the reduction in the
degree of sensitization being greatest at the surface of the sample
and decreasing at depths in the sample away from the surface, a
profile of the graded reduction in desensitization being
controllable by controlling at least one of a voltage a current
density, a pulse duration, a pulse frequency of the electron beam,
and a number of pulses of the electron beam.
6. The method according to claim 1, wherein the electron beam is
configured to produce a heated layer having a depth of 10 to 200
.mu.m at the surface of the metal alloy sample.
7. The method according to claim 1, wherein the electron beam is
configured to produce a heated layer having a temperature of 230 to
345.degree. C. at the surface of the metal alloy sample.
8. (canceled)
9. The method according to claim 1, wherein the electron beam is
configured to have a current density of 10 A/cm.sup.2 to 400
A/cm.sup.2.
10. The method according to claim 1, wherein the electron beam has
a pulse duration of 70 nsec to 2000 nsec.
11. The method according to claim 1, wherein the electron beam has
a pulse frequency of 0.1 Hz to 5 Hz.
12. The method according to claim 1, wherein the number of electron
beam pulses varies between 1 and 100.
13. The method according to claim 1, wherein the metal alloy is an
aluminum-magnesium alloy, and wherein the electron beam is
configured to produce a heated layer having a temperature of
between about 230.degree. C. and about 345.degree. C. at the
surface of the sample.
14. The method according to claim 11, wherein the metal alloy is a
5000-series aluminum alloy.
15. The method according to claim 1, wherein the electron beam is
fired for a total of 100 pulses at a pulse repetition rate of 5
pulses per second.
16. A method for controllably desensitizing a metal alloy sample,
comprising: exposing a specific desired location on a surface of a
metal alloy sample to a controlled pulsed electron beam having a
voltage greater than 100 kV to about 650 kV; wherein the electron
beam travels through an ambient atmosphere to the sample and is
controllably directed to the specific desired location without
exposing other areas on the sample to the electron beam; wherein
the electron beam heats a shallow surface layer of the sample
having a desired depth at the specific desired location on the
surface of the sample to a controlled temperature between a solvus
temperature and an annealing temperature of the metal alloy without
heating a bulk of the sample to controllably reduce a degree of
sensitization of the sample at the specific 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, and a pulse frequency of the electron
beam.
17. A method for controllably desensitizing a deckplate on a marine
vessel, comprising: exposing a specific desired location on a
surface of a deckplate to a controlled pulsed electron beam having
a voltage greater than 100 kV to about 650 kV; wherein the electron
beam is applied to the deckplate in situ on the vessel and is
controllably directed to the specific desired location on the
surface without exposing other areas on the deckplate to the
electron beam; and wherein the electron beam heats a shallow
surface layer of the deckplate having a desired depth at the
specific desired location on the surface of the deckplate to a
controlled temperature between a solvus temperature and an
annealing temperature of the metal alloy without heating a bulk of
the deckplate to controllably reduce a degree of sensitization of
the deckplate at the specific 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, and a pulse frequency of the electron beam.
Description
CROSS-REFERENCE
[0001] This application is a Nonprovisional of, and claims the
benefit of priority under 35 U.S.C. .sctn.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.
TECHNICAL FIELD
[0002] 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.
BACKGROUND
[0003] 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.
[0004] 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.
[0005] 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 FIG. 1, it can
combine with the aluminum to form second phase "precipitates ("beta
particles") with having an approximate stoichiometry of
Al.sub.3Mg.sub.2 at the grain boundaries. This environmentally
induced process, known as "sensitization," significantly reduces
the material's intergranular corrosion resistance, and leads to
stress corrosion cracking of the alloy.
[0006] 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.
[0007] 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 FIG. 2, which shows a crack in the aluminum deckplate of a
CG-47 Ticonderoga Class cruiser. The CG-47 class, which uses alloy
5456-H116 in their deck and superstructure plating, has experienced
severe degradation from sensitization. As can be seen in FIG. 2,
the crack is several millimeters wide and extends all the way
through the 5-millimeter-thick deck plate. See R. Schwarting, G.
Ebel, and T. J. Dorsch, "Manufacturing techniques and process
challenged with CG47 class ship aluminum superstructures
modernization and repairs," Fleet Maintenance & Modernization
Symposium 2001: Assessing Current & Future Maintenance
Strategies, San Diego, 2011. If such cracking occurs, the only
permanent remedy is to replace the parts, which is an expensive
activity and can only be done with the ship out of service.
Consequently, it is highly desirable to prevent cracking before it
occurs.
[0008] 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.
[0009] As illustrated in the plots shown in FIG. 3, such
desensitization occurs only over a limited temperature range. At
temperatures below about 230.degree. C., the aluminum remains
sensitized, while at temperatures above about 345.degree. C.,
aluminum will begin to anneal and soften (i.e. lose strength).
Consequently, the temperature of the aluminum alloy during
desensitization must be kept between about 230.degree. C. and about
345.degree. C. for sensitization to occur without loss of strength
in the metal.
[0010] 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.degree.
C., while commercial alloy 5083, which has additional constituents,
has an experimentally measured solvus value of 290.degree. 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.degree. C.; although the actual solvus has not been
experimentally measured.
[0011] 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.degree. 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.degree. C. and
345.degree. C., with specific, narrower temperature ranges for
alloy compositions being determined empirically in each case.
[0012] Various methods to heat the aluminum to a temperature
sufficient for desensitization while keeping the temperature within
this critical range have been proposed.
[0013] 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.
[0014] 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.degree. C. as shown in FIG. 3, it compromises
the strength of the material. The friction-stir process is somewhat
faster than pad heating and has the potential advantage of
preferentially heating a shallower layer, but it is still
impractical because the deck plating on a ship cannot support the
considerable mechanical forces required for such a process.
[0015] 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.
SUMMARY
[0016] 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.
[0017] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is an optical metallographic image showing examples
of the concentration of precipitated Al.sub.3Mg.sub.2 beta
particles at the grain boundaries of a 5000 aluminum sample.
[0019] FIG. 2 is a photographic image depicting damage and cracking
in the deckplate of a Navy vessel resulting from sensitization of
the 5000 aluminum forming the deckplate.
[0020] FIG. 3 is a plot illustrating that aluminum can be
desensitized by exposure to temperatures between about 230 and
345.degree. C., depending on the alloy.
[0021] FIG. 4 is a block diagram illustrating aspects of an
exemplary embodiment of an apparatus for desensitizing aluminum
using pulsed high voltage, high current electron beams in
accordance with the present invention.
[0022] FIG. 5 contains plots illustrating that the depth of
electron beam desensitization in accordance with the present
invention can be controlled by varying parameters of the electron
beam.
[0023] FIG. 6 contains a plot illustrating the DOS levels on the
surface of a exemplary 5000 aluminum alloy in the original
condition, after severe sensitization, and after desensitization in
accordance with the present invention by pulsed electron beams
having a current density of 130 A/cm.sup.2, 160 A/cm.sup.2, and 260
A/cm.sup.2.
[0024] FIG. 7 contains a plot illustrating the degree of
sensitization (DOS) at various depths in an aluminum sample and
after electron beam desensitization at two current levels in
accordance with the present invention.
[0025] FIG. 8 contains plots comparing the typical amount of beta
phase present on the grain boundaries in an untreated alloy as it
is aged, and a pulsed electron beam treated alloy as it is re-aged
after treatment.
[0026] FIG. 9 contains a plot illustrating how electron beam
desensitization of aluminum in accordance with the present
invention affects the rate of its resensitization compared to the
initial sensitization of the untreated alloy
[0027] FIGS. 10A-10C are optical metallographic images depicting a
5000 aluminum sample as received, after aging and resulting
sensitization, and after desensitization by exposure to an electron
beam in accordance with the present invention.
[0028] FIGS. 11A-11C illustrate aspects of Rockwell hardness
testing on a sensitized 5000 aluminum sample and on 5000 aluminum
samples that have been desensitized by exposure to an electron beam
in accordance with the present invention.
[0029] FIG. 12 is a block diagram illustrating aspects of an
exemplary embodiment of a compact, portable apparatus that can be
used for electron beam desensitization of aluminum in accordance
with the present invention.
DETAILED DESCRIPTION
[0030] 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.
[0031] 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.
[0032] 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 FIG. 3 described above, the aluminum
thus must be heated to a temperature above about 230.degree. C.
(depending on the alloy) for desensitization to occur while being
kept below a temperature of about 345.degree. C. to prevent the
aluminum from annealing. As described above, previously used
methods for heating aluminum, particularly aluminum that has
already been fabricated into, for example, a ship deck, are
unsatisfactory because they either end up heating the bulk of the
aluminum in order to treat undesirable sensitization that occurs
only at the surface, or require equipment that, in order to be
effective, must apply potentially damaging mechanical forces to the
material.
[0033] 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.
[0034] 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. FIG. 4 is a block diagram illustrating
aspects of an exemplary embodiment of an electron beam apparatus
that can be used to desensitize aluminum alloys such as 5000
aluminum in accordance with the present invention.
[0035] Thus, as illustrated in FIG. 4, an aluminum sample such as
deck plate 405 can be desensitized by applying a pulsed electron
beam 403 generated by applying a current produced by pulsed power
source 401 through a cathode 402, with target deck plate 405
absorbing the electron beam. In some embodiments, the apparatus can
be configured so that electron beam travels from the cathode 402 to
the target 405 in a vacuum (in which case the deckplate 405 is the
anode), while in other embodiments, the apparatus can be configured
so that the electron beam 403 travels through a foil window 404,
(in which case the foil window 405 is the anode) which allows the
beam to travel through, and the apparatus to operate in, the
ambient air.
[0036] 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.
[0037] 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
.mu.sec. 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.
[0038] 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.
[0039] 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.degree. 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 .mu.m) 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
.mu.m), 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.
[0040] 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.
[0041] FIGS. 5 through 9 further illustrate aspects of the way in
which a pulsed electron beam can be used to reverse the
sensitization of a metal such as 5000 aluminum in accordance with
the present invention.
[0042] The plots shown in FIG. 5 illustrate how the depth of
desensitization can be varied by adjusting the parameters of the
electron beam. As noted above, desensitization requires the heating
of the aluminum to a temperature between 230 and 345.degree. C.,
depending on the alloy, while the electron beam heats, and
therefore desensitizes, only a very shallow layer at the surface of
the aluminum.
[0043] As shown in FIG. 5, plot 501, an electron beam having a
voltage (energy) of 300 keV and a current density of 280 A/cm.sup.2
will heat an aluminum plate to the temperature range required for
desensitization (230.degree. C.) only up to a depth of about 40
.mu.m; beyond that depth, the temperature drops below the threshold
temperature very quickly, reaching a low temperature of about
20.degree. C. at a depth of about 230 .mu.m. In contrast, as shown
in FIG. 5, plot 502, an electron beam having a voltage (energy) of
480 keV and a current density of 350 A/cm.sup.2 will heat the metal
to a temperature above 230.degree. C. to a much greater depth of
about 200 .mu.m, with the result that desensitization will also
occur up to a depth of about 200 .mu.m within the metal. Although
the temperature of the metal heated by the higher power electron
beam drops more slowly than does the metal heated by the lower
power beam, in both cases, as shown in plot 502, the metal
temperature produced by that higher power beam drops to 20.degree.
C. at a depth in the metal of about 700 .mu.m.
[0044] 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.degree. C. and
treatment depth of 10 to 200 .mu.m) by varying the voltage,
current, pulse length, repetition rate and/or number of pulses of
the applied electron beam.
[0045] The plot in FIG. 6 illustrates that desensitization using a
pulsed electron beam in accordance with the present invention not
only removes the existing sensitization but can even place the
metal in a better condition, with even less sensitization, than in
the "as-received" state. To test the efficacy of the electron beam
desensitization treatment method in accordance with the present
invention, aluminum alloy samples were subjected to heat treatment
to sensitize the samples and then were exposed to three different
electron beams having different current densities. The DOS of the
"as-received" sample, the sensitized sample, and treated samples
was measured directly at the exposed surface.
[0046] 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 FIG. 6, in the present experiments, the as-received
sample had a DOS of approximately 8. A typical laboratory heat
treatment for evaluating the susceptibility of an alloy to
sensitization is to heat the material at 100.degree. C. for some
period of time. In the present case the as-received sample was
heated for 12.5 days, resulting in a DOS of 40, which is a high
level that typically would result in severely degraded stress
corrosion cracking and corrosion fatigue behavior.
[0047] The sensitized samples were treated with electron beams
having a current density of 130 A/cm.sup.2, 160 A/cm.sup.2, and 260
A/cm.sup.2. As can be seen from the plot in FIG. 6, in all cases,
treatment of the sensitized samples with such electron beams in
accordance with the present invention resulted in a significant
reduction in the DOS in the sensitized sample. In the case of
treatment by the 130 A/cm.sup.2 beam, the DOS was reduced nearly to
the "as-received" state, while in the case of treatment by the
higher energy 160 A/cm.sup.2 and 260 A/cm.sup.2 beams, the samples
were brought to a "better than new" state having a DOS of nearly
zero.
[0048] Thus, the plots in FIG. 6 show that by using the method of
electron beam desensitization treatment in accordance with the
present invention, it is possible to reduce the DOS at the surface
from 40 down to level comparable to the as-received material with a
low current treatment, and to DOS of essentially zero with higher
levels of current.
[0049] The plots in FIG. 7 further illustrate the depth into the
sample, below the surface, to which the desensitization effect
occurs. If the electron beam current is too low, even though there
may be a surface effect (for example, for the 130 A/cm.sup.2
treatment illustrated in FIG. 6), the treatment does not extend
below the surface. In contrast, if the current is very high, for
example 260 A/cm.sup.2, as can be seen in the plot in FIG. 7, the
DOS is reduced to essentially zero at the surface and is very low
(less than 5) even at a very deep depth within the sample, in this
case up to 0.5 mm below the surface. For intermediate current
levels, as can be seen for the plot of desensitization by a 160
A/cm.sup.2 electron beam, the reduction in DOS is graded with depth
in the sample, being reduced to essentially zero at the surface and
increasing--but still remaining below the sensitized DOS
level--below the surface.
[0050] FIG. 8 shows that it takes a longer time for a sample that
has been desensitized with the e beam treatment at 260 A/cm.sup.2
to "resensitize" to a given level, for example DOS of 15, than an
original "as received" sample. FIG. 8 gives grain boundary beta
phase coverage versus heat treatment time for the initial
as-received condition, and a previously sensitized then electron
beam treated condition. This illustrates a very large reduction of
the amount of beta phase on the grain boundaries in the electron
beam treated specimen, even considerably less than the original
as-received material. The horizontal lines mark reference values
relevant for ship service, as explained in more detail below with
reference to FIG. 9. Note that it takes 2 days for the original as
received sample to reach a DOS of 15, whereas it takes over 10 days
to reach a DOS of 15 for a sample that has been treated at 260
A/cm.sup.2.
[0051] FIG. 9 shows the same data as in FIG. 8 but in terms of the
DOS, rather than the beta phase coverage. The horizontal lines mark
reference values relevant for ship service according to ASTM B928
standard, which allows alloy with DOS below 15 to be used for ship
construction, but recommends against use if DOS is more than 25.
The DOS of 40 is the condition of the sensitized material used in
this study, and is a high DOS that will exhibit degraded corrosion
resistance. The as-received original material, with an initial DOS
of about 8, reaches a DOS of 15 within 2-3 days at 100.degree. C.,
and a DOS of 25 at about 6 days. In contrast, the material that has
been desensitized by electron beam treatment in accordance with the
present invention does not reach a DOS of 15 until 9-10 days of
aging, and does not read a DOS of 25 until at least 11 days. Thus,
the electron beam desensitization in accordance with the present
invention has considerably extended the usable life of the alloy,
from a factor of 5 at the DOS level of 15, to nearly a factor of 2
at the DOS level of 25.
Example
[0052] 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.
[0053] 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.degree. 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.
[0054] 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/cm.sup.2, 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/cm.sup.2, 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.
[0055] 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.
[0056] 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.degree. 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.
[0057] FIGS. 10A-10C shows a series of metallographs of samples of
the same alloy. The samples were taken from the same plate (1) as
received (FIG. 10A), (2) after it was aged (sensitized) in the
laboratory (FIG. 10B), and (3) after the aged sample was treated
with the pulsed electron beam (FIG. 10C). As can be seen in FIGS.
10A and 10C, desensitized aluminum is characterized by thin
discontinuous grain boundaries (appearing as faint, irregular
lines), whereas the sensitized aluminum shown in FIG. 10B exhibits
wide continuous boundaries. Moreover, as can be seen in FIG. 10C,
the sample that has been desensitized by electron beam treatment in
accordance with the present invention appears to have even fewer
sensitized boundaries than the original.
[0058] Electron beam desensitization of aluminum in accordance with
the present invention does not significantly affect the strength of
the bulk material. FIGS. 11B and 11C illustrate the results of
Rockwell Hardness B scale measurements for aluminum alloy samples
at the top surface (FIG. 11B) and the top, middle, and bottom
cross-sections (FIG. 11C) where the positions of these cross
section as shown in FIG. 11A,
[0059] As can be seen from the plot in FIG. 11B, the Rockwell
Hardness show that the hardness on the top surface is about the
same for the as-received and sensitized samples, and remains the
same after electron beam treatment at low and moderate current
densities of 130 A/cm.sup.2 and 160 A/cm.sup.2, respectively.
Although the top surface exhibits some softening after treatment by
a higher current density (260 A/cm.sup.2) electron beam, its
hardness still remains within 10% of the unsoftened condition.
[0060] Similarly, the plots in FIG. 11C show the hardness measured
in the top, middle, and bottom cross-sections of a sensitized and a
treated sample. As can be seen in plots in FIG. 11C, while the top
cross-section of a sample treated by electron beams having current
densities of 160 A/cm.sup.2 and 260 A/cm.sup.2 shows softening of
up to 10%, the middle cross-section shows that any change in
Rockwell Hardness for the treated samples is within 1 or 2 HRB of
the sensitized sample, while the bottom cross-section shows no
change in Rockwell Hardness after treatment by either electron
beam.
[0061] Thus, the Rockwell Hardness measurements show that while
there may be a small softening effect at the surface and the top
1/3 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.
ADVANTAGES AND NEW FEATURES
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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 FIG. 11,
where the apparatus is comparable in size to a 55 gallon drum
common on board ships, and is on a wheeled platform to easily move
to desired locations on board the vessel. The electron beam source
can be rigidly attached to the pulsed power system, or, in some
embodiments, can be located at the end of a flexible electrical
transmission line to allow easier access to parts of the
superstructure. In other embodiments, the apparatus can be mounted
on a vertical structure that can be varied in height, e.g., using a
forklift or scissors jack-like mechanism, so that it can treat
non-horizontal shipboard features such as superstructure
plating.
[0068] 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.
[0069] 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.
* * * * *