Note: Descriptions are shown in the official language in which they were submitted.
~0 47 9 ~ 1 .
The present invention relates to a method of ~hermally
treating articles containing an alloy based on aluminum and to
an aluminum alloy in a particular heat ~rea~ed condition.
The precipitation hardened condition o~ aluminum alloy
7075, referred to as the T6 condition of alloy 7075, ha~ no~
given su~ficient resistance ~o corrosion under certain service
conditions. The T73 temper improves ths resistance of precipi-
tation hardened 7075 alloy to stress eorrosion cracking. The
process needed to obtain a T73 temper considerably increases the
time required for heat treatîng the 7075 alloyO
An object of the present invention is to provide a
new heat treating method to produce an alumlnum alloy in a
unique heat treated condltion for providing favorable re~istance
to corrosion.
A further object of the invention is to provide a new
heat treating method for creating in an aluminum alloy favorable
resistance to corrosion whlle allowing reduction of the heat
~reating time needed to reach the T73 conditLon.
Another object is to provide a new method for providing
resistance to stress corrosion cracking in 7075 aluminum alloy.
Yet another ob~ect is to obtain an aluminum alloy,
such as 7075, in a unique heat treated condition.
These as well as obher objects which will become
apparent in the discu~sion which follows are achieved, according
to the present invention, by
1) the metho~ of thermally treating an article
composed of an alloy consisting essentlally of aluminum, 4 to
8% zinc, 1.5 to 3.5% magnesium, 1 to 2~5~/o copper, and at least
one element selected ~rom the group consisting of 0.05 to 0.3% ~
chromium~ 0~1 to 0.5% manganese, and 0.05 to 0.3% zirconium, `
which method comprises solution heat treating the article and
subsequently subjecting the article to a time and temperature
: .. . . : . , . ." . . .
lO~g~
effective for increaslng the resistance to corrosion of ~ e alloy
over its resistance in the T6 condition, the time and temperature
being from 10 seconds to 10 minutes and from 350 to 520F
respectively:
2) the alloy consisting essentially of aluminum, 4 to
8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and a~ least
one element selected from the group consisting of 0.05 to 0,3~/0
chromium,sO.l to 0.5% manganese and 0.05 to 0.3% zirconium,
having a corrosion resistance increased over ~hat of its T6
condition, with a solution potential lying in the range 830 to
minus 935 millivolts, a yield strength lying in ~he range 46 to
72 ksi, a dislocation density above that exhibited by 7075 in the
T73 condition, denuded grain boundary regions 9 and grain boundary
precipitate. ;
Figures 1-7 are transmission electron micrographs of
sections in a plate of aIuminum alloy 7075. The distance
equivalent to 0.1 micron is indicated on the micrographs. The
metal surfaces reproduced in the micrographs all were perpen- -~
dicular to the direction of rolling of the plate.
Figure 1 shows a prior art solution heat treated and
stress relieved condition referred to as the W51 condition.
Figure 2 shows the prior art precipitation hardened
condition referred to as the T6 condition.
Figure 3 shows the prior art stress corrosion cracking
r~sistan~ condition re~erred to as the T73 condition.
Figures 4 and 5 illustrate one embodiment of the
present invention. -~
Figures 6 and 7 illustrate a second embodimen~ of the
present invention,
Figure 8 is a graph of data taken from Examples 31 to
42 illustrating the present invent~on.
Figure 9 is a graph ~ho~ng additional characteris~ics
790
of the invention.
The alloys in the present inventlon have a composition
con~aining 4 to 8% zinc, 1.5 to 3.S% magnesium, l to 2.S% copper,
and at least one element selected from the group made up by
chromium at 0O05 to 0.3%, manganese at O.l to 0.5%, and zirconium
at 0.05 to 0.3%. The balance of the compo~ition is essentially
aluminum.
Alloys designated 7075 by the aluminum indu&try are
preferred for the present invention and have a composition
containing 5.1 to 6.lV/o zinc, 211 to 2.9% magnesium, 1.2 to 2.0~/O
copper, 0~18 to 0.35% chromium, 0.30/O maximum manganese, 0.40% -~
maximum silicon, 0.50% maximum iron9 0.20~o maximum titanium,
others each 0.05% maximum and others to~al 0.15% maximum, ~ ;
balance aluminum.
The alloys used in the present inventlon may also
contain one or more of the group of grain refining elements
including titanium at O.Ol to 0.2% and boron at 0.0005 to 0.002%. ~ ~ -
These elements serve to produce a fine grain si2e in the cast
form o~ the alloy. This is generally advantageous to mechanical
properties.
In addition, it may be helpful to add O.OOl to 0.005% ;
beryllium for the purpose of minimizing oxidation at times when
the alloy i8 molten.
Iron and silicon are generally present as impurities.
Up to 0~5% iron can be tolerated, and the silicon content should
not exceed 0.4%, Ln order to avoid ths formation of any sub-
stantial amount of the intermetallic compound Mg2Sio
A preferred heat treatment accordlng to the present
invention for obtaining improved stress-corrosion resistance is
to immerse alloy, as above defined, in the precipitation~
hardened, T6 condition into m~lten metal at 400 to 500F for l
to 7 millutes.
~0~9~
In i~s broader aspects, a T6 condition may be obtained
by precipitation hardening soLu~ion heat treated alloy at 175 ~o
325F Typical conditions may be:
a. For alloys conta~ning les~ than 7.5~/O
zinc, heating a solution heat treated
article to 200 to 275F and holdlng
or a period o~ 5 to 30 hours;
b. For alloys containing more than 7.5%
~inc3 heating a solution heat treated
article to 175 to 275F and holding
or a period of 3 to 30 hours.
Preferably, the T6 condition i5 obtained by heating
a specimen for 24 hours at 250F in a circulatory-air furnace.
Th~ article of J. T. Staley e~ al. entitled "Heat
Treating Characteristics of High Strength Al-Zn-Mg-Gu Alloys
With and Without Silver Additions" appearing at pages 191 to
1~9 in the January, 1972 issue of Metallurgical Tran~a ons,
published by ASM/AIME, shows that solution heat treat quench
rate, the lapse of time between the solution heat ~rea~ quench
and the beginning o heating for precipitation hardening, and
the heating rate ~or precipitation hardening may affect the
~ ,.
maximum yield strength obtainable in 7075 aluminum~alloys. It
is intended that, within the concepts of the present invention,
the teachings of Staley et al. be used in the present lnvention
for optimizing results. Thus, it may be advantageous for
increasing strength to immerse specimens, which have had their
.~
Bolution heat treatment quench, for example, 1-1/2 years ago,
into molten Wood's metal according to the invention.
Referring now to Figures 1 to 7, ~ransmission electron ;~
micrographs of various microstruetures important for consideration
of the present invention are presented. All o~ Figures 1 to 7
were taken from a single l/4-inch thick 7075 aluminum alloy
plate. Figures 1 to 3 are micro~tructures of prior art `
conditions of 7075 aluminum. In Figure 1, an example of the W51 ;~
solution heat treated condition is given. A W51 solution heat ~-
- 4 ~ ~
. . ~ .
~ 4 ~ ~ ~
trea~ed mlcrostructure is ob~ained in 7075 aluminum alloy plate
by heating to 900F and then quenchlng in water at room
temperature. The plate ma~erial is then stretched to ~rom 1-1/2
to 3% permanent se~ for stress relief. This gives the micro-
structure shown in Figure 1, including E-phase particles of
Al-Mg-Cr precipitate, ma~rix regions R of single phase aluminum
solid-solution material, grain boundaries B and dislocatlons D.
The mottling effect appearing in the matrix region of ~igure l
is an ar iact of the action of the thinning solution used in
preparing thinned material for transition electron microscopy.
The specimen for Figure 1 was taken rom the same 7075 alloy
plate used in Examples 1 to 29 below.
Figure 2 shows the 7075 alloy material of Figure 1
after it has been brought to the T6, in particular ~he T651,
temper by heating W51 material in a circulatory-air furnace for
24 hours at 250F. E-phase remains ~ubstantially unchanged.
Dislocations D and a grain boundary B are shown. Now in the
matrix there has appeared many small black dots; these are
referred to as G.P. zones and are clusterings of magnesium and
zinc atoms generally in the ratio two zinc atoms for each
magnesium atom.
Figure 3 shows a specimen taken from the same plate o
Figures 1 and 2 in the T73 condition, which is produced from W51
material by heating in circula~ory-air ~urnaces for, first, 24
hours at 250F and, second, 8 hours at 350F. Grain boundary
precipitate 10 has appeared, and the G.P. zones have grown to
greater size. The G.P. zones have begun to exhibit crystallinity
by giving rise to X-ray diffraction patterns and are referred to
by those in the art as M' and M phase. Solution potential
studies indicate that the M' and M phases contain some copper ~ -
atoms. It is believed that the G.P. zones progress toward
crystallinity by becoming first M' phase, which is still
~047~
partlally coherent with the matrlx crystal structure. The M'
phase then changes to M phase, which has a crystal structure
different from the matrlx. It is believed also that the pro-
gression through the M' phase to the M phase makes the original
G.P. zones increasi~gly anodic wi~h respect to the matrix and
that the resulting anodic particuLate ma~ter in the matrix
protects against stress-corrosion cracking.
The microstructure of Figure 4 was obtained according
to the invention by aging a l/~" x 3/8" x 4" blank of the W51
material of Figure l first to the T6 condition using 24 hours
at 250F in a circulatory-air furnace and then immersing the
blank for lO seconds in Wood's me~al molten ~t 490F. Upon
removal from the molten Wood's metal the blank was allowed to
air cool. Appearing in Figure 4 are G.P. zones 3 E-phase, grain
boundary precipitate ll and denuded ~free of G.P. zones) grain
boundary material 12. Because of the particular orientation of
the grains in Figure 4, dislocations do not show. They are,
, : ~
however, present, as is clear ~rom the presence of dislocations
D shown in Figure 5 illustrating another gra1n in the same blank
used ~or Figure 4. The grain of Figure 5 is more favorably
oriented than that of Figure 4 for showing dislocations. ~
Figure 6 illustrates a blank of the same size as used ,! '
for Figures 4 and 5, heat treated in the same manner except
that, upon removal from the Wood's metal, the blank was quenched
in cold water. Present again are grain boundary precipitate 13,
denuded grain boundary material 14, E-phase, and G.P. zones.
Dislocations D appear in the lower, favorably oriented grain in ~ -
.. .
Figures6. Figure 7 shows another grain in the same blank as
used for Figure 6 for further illustrating the dislocation density. ;
Further illustrative of the present invention are the ~;` ;
following examples. Examples l to 29 use as starting material
the same plate used for obtaining Figures l to 7.
. . .
- 6 - ; ``
.. .
~(~479t)1
Examples 1 to 3
Data for Examples 1 to 3 appear ln Table I. Examples
1 to 3 represent different prior art processes and conditions
or a 7075 aluminum alloy composition, an alloy composltion whioh
may be used in the presen~ invention. The alloy composition was
as given in Table II for alloy A. Data W~9 collected from blanks
measuring l/4-inch thick by 4-inches. These blanks were taken
from a l/4-inch thick plate of alloy A in the W51 condition.
The longest, 4-inch dimension o ~he blanks was parallel to the
longitudinal direction of the plate, i.e., the direction of
rolling. The T6 temper was obtained by heating W51 blanks in a
circulating-air furnace for 24 hours at 250F. The T73 temper
was carried out also in circulating-air furnaces, first at 250F
for 24 hours and then for 8 hours at 350F. Measured were
solution potential, yield strength, and degree of ex~olia~ion9
as given in Table I.
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Table II.
Composition o:f Alloys, in Weight-%.
. . _ _
Alloy
_ . . .
Element A B
____ ~ . ,_
Cu 1~45 1~81
Fe 0.19 0.31
S~ 0.09 0.08
Mn 0.02 0.02
Mg 2.40 2.38 -~
Zn 5.92 6.02 ~ `
Ni 0.00 ____
Cr 0.18 0.19
Ti 0.02 0.03
Be 0.001 0 00
Examples 4 to 21 ;
Blanks as in Examples 1 to 3 were aged to a T6 temper
using 24 hours at 250F. They were then vapor degreased and
subjected~to additional treatmen~ in molten Wood's metal as ~;
indicated in Table I. Measured were solution potential, yield
stren8th9 and degree of exfoliation.
Examples 22 to 29
Specimens as in Example 1 of Alloy A were subjected to
various treatments in molten Wood's metal without firs~ being
brou~ht to the T6 temper. Measured were solution potential,
yield strength, cnd degree o ex~oliation. ; `
* * * ;~
A plot o~ yield strength versus solution potential or
the dcta of Examples 1 to 29 reveals thct the data for Examples -
4 to 29 according to the present invention lie in an area
occupied neither by the T6 data of Example 2 nor by the T73 data
. ;; ''
~ 47 g ~ ~
o~ Example 3. Examples 7 ~o 29 had exfoliation resistances
better than that for the T651 data of Example 2. Examples 8, 9,
10 to 15, 17 to 19, 21 to 23, 25, 26, and 27 to 29 had exfolia- :
tion resistances better than the data ~or the T73 condition of
Example 3. Each o Examples 7 to 22, 25~ 2B, 27, and 28 had
both higher yield strength and more anodic (more negative milli-
volt value) solution potential than the corresponding values for '.!
the T73 condition o~ Example 3. The solu~ion poten~ial obtained
by any particular heat treatment followed by a cold water quench
was considerably more anodic than that obtained by air cooling.
In general9 resistance to exfoliation and pitting increases as
solution potential becomes more anodic, i.e.~ progresses toward
greater negative value. ~.
* * . : .
Examplas 30 to 35 - Cold Water Quenched ;
For each example, four tensile blanks of dimensions
, :.~ . .. .
3/8 inch by 3/8 inch by 2-1/2 inches were cu~ rom a sin~le lot ~ ~ :
of 2-l/2 fnch thick 7075-T651 (metallurgical history as described
for Figure 2) alloy plate such that their lengths were in the
short-transverse direction7 i,e., in the direction perpendicular
to the surace of the plate. The mechanical properties of this
.
material were as presented in Table III. ~
Table III. . ~.
Mechanical Properties of Plate . .
Used for Examples 30 to 41.
... .
Tensile Yield
Strength Strength ~/O
ksi ksi Elongation
.... . . .
Long 80.2 71.7 8.0
Transverse
Short : 74.8 66.6 2.0
Transverse
The chemical composition of the alloy is as presented for alloy
B in Table II. The tensile blanks for each example were ~.
..
immersed in 445F molten Wood's metal of composition 50% bismuth,
- 12 - ~ -
~ , .
~ .. . . ... . .. ... .. . . . . .
14~9~
25% lead, 12.5% tin and 12.5% cadmium. The immersion times ~or
Examples 30 to 35 were 30, 60, 90, 120, 240, and 420 second6,
respec~lvely. Following immersion in the molten Wood's metal,
the specimens were quenched in cold water Erom the tap. The
dlfference between cold water temperature in ~he summer and in
the winter does not affect the results to any significan~ extent.
Two tensile blanks were machined to a 0.125 inch diame~er tensile
bar for exposure to 3-1/2% sodium chloride solution by alternate ~;
immersion at stress levels o~ 42 and 35 ksi, respectively,
according to Military Specification MIL-A-22771B. The specimens
were held until failure under a given stress level with
successive immersions for 10 minutes in the salt solu~ion ~ollowed
by 50 minutes in air~ Examples 31-35 lasted more than 30 days
under such treatment, thus meeting the standard of the Military
Specification. The remaining ~o blanks of each example were
tested for yield strength and solution potential, respectively. ~;
The yield strength and solution potential data for Examples 30
to 35 are presented in Figure 8 as curves originating respectively,
from the yield strength and solution pstential of the plate in
the T651 condition. It is to be noted that the yield strength
data passes through a minimum, termed herein a "first minlmum",
at Example 30. Measurements of the conductivities of Examples
~0 to 35 showed that a stress corrosi.on resistance (as measured
by the alternate immersion test~ obtainable only at a conductivity ~;
of 38% IACS with the T73 temper is obtained at 35 to 37% IACS in
the present invention. Conductivity data appears in Table IV.
~'~
- 13 -
. - :
~6347~ : 7 '
Table IV. ~ -
Conductivity Data for Examples 30 to 41.
. . ~ . _ ":,
Cold Water Quench
. _ _ , .: .
Immersion . -
Time In Electrical ..
Seconds ExampleConductivity ~:
No. ~/o IACS
~ :---~ -. ': '
33 3 ~:-
31 34 7 -~
32 35.2
120 33 35.8 -~.
~40 34 36.7
420 35 38.2 ``'i -~ :
_ ~ _. . .
Air CooIed
ElectricaI :~
ExampleConductivity
No. % IACS : ::~
: . - ~ .... ..
: 30 36 34.2 ~ ;
37 35.2 ~:
38 36.4 ~ ~ :
. 120 39 36~7 :: ` :
: 240 4Q 37.7
: 4~0 41 38.8 .
..... .. :~ ~ . .. ,-:
''~` ' `-
amples 36 to_41_- Air Cooled
Tests~were~run as for Examples 30 to 35, the only
differ~nce being that the specimens were allowed to air cool
~: following~immersioD, Diferences in room temperature from:day
....
: ~ to day~or season to aeason do ~ot produce significant variations
in results. The data for yield strength and solution potential
are presented in Figure 8. Here, all of Examples 37 to 41
.
passed the alternate immersion, aqueous sal~-solution test of ;:
- 14 - -
~. ',7 ~,
~O~ 79 ~ ~
the Military Speciication mentloned in Examples 30 to 35. Here
again, lt was noted that a stres~ corrosion resistance (as
measured by the al~ernate immersion test) obtainable only at a
conductivity of 38% IACS with the T73 temper is obtained at 35
to 37% IACS in the present invention. The data for conductivity
is presented in Table IV. All of Example~ 37 to 41 lying beyond
the fir~t minimum at Example 36 in Figure 8 in the yield strength
curve passed the 30-day standard of the Mil~ary Speclfication.
Additional Examples 4~ to 55
Tests were run as for Examples 36 to 41 using additional
variations in time and temperature of immersion in mol~en Wood's
metal. The points for these additional tests (as presented in
Table V) plus the ~ests of Examples 36 to 41 are plotted in
Flgure 9. Above every point in the Figure, the mean time to
failure in the alternate immersion, aqueous salt-solution test ~ -
of the Military Specification mentioned in ~xamples 30 to 35 is ;
glven in days. Below every point, the yield s~reng~h is given,
expressed in percent of the T651 yield strength. Times and
temperatures of the Wood's metal immersion according to one
aspect of the present invention showing combined high yield
strength and resistance to stress corrosion cracking fall within
the perimeter of irregular pentagon ABCDE in Figure 9. Preferably~
the times and temperatures lie within the perimeter of the
quadrilateral FGHI.
- 15 - ~; ,,:
,.-.
. . . . . .. .
i(J 4~9~
Table V.
Tlmes and Temperatures in :: ~
Wood's Metal for Example3 42 to 55 ;- -
and the Coordinates of Point~ A to I
. . _ ~ " ' .
Example No., Timeg Temperature,
or Point min. F
. .. , . . __
42 0.5 500
4~3 0.75 500
44 1.0 500
1. 5 500
46 2. (3 500
47 0. 5 475
b,8 1. 0 475
49 0.5 400
S0 ~. 0 400
51 1.5 400
52 200 400
5 3 4 . 0 ~ 4 0 0
54 6 . 0 400
5 5 ~ 7 . 0 ~ 375
A 3. 0: 390
: B 0. 2 500
C 1. 0 500
D : 10.0 438
E lO. 0 390
F 4.0 400
~ G 0.67 ~ 476
: H ~ 1.05 476
I 8. 0 400
~ . . i
::
.
- 16 ~
1047g~
The ~ollowing definitions hold herein:
a. The term "ksi" is equivalent to
kilopounds per square inch.
b. Wherever percentages are given,
re~erence is to % by weight, unless
indicated otherwise,
c. The initials "G.P,-I stand for
Guinier-Preston.
Various modi~lcations may be made in the inventlon
without departing from the sp-lrit thereof, or the scope of the
claims, and, therefore7 the exact orm shown is to be taken as
illustrative only and not în a limiting sense~ and it is
desired that only such limitationsshall be placed thereon as
are imposed by the prior art, or are specifically set forth in
the appended claims. `~
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- 17
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