CA1045528A - Producing combined high strength and high corrosion resistance in al-zn-mg-cu alloys - Google Patents

Abstract

Abstract of the Disclosure The method of thermally treating an article composed of an alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5 magnesium, 1 to 2.5% copper, and at least one element selected from 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 includes the steps of solution heat treating the article, then precipitation hardening the article at 175 to 325°F, then subjecting the article to a time and temperature within the perimeter ABCD of Figure 4, and then again precipitation hardening at 175 to 325°F. This method yields an article resistant to stress corrosion cracking without appreciable, if any, sacrifice in strength as compared with the T6 condition.

Description

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The present invention relates to a method of thermally treating articles containing an a~by based on aluminum.
The precipitation hardened condition o~ alumin~m alloy 7075, referred to as the T6 condition of alloy 7075, has not given sufficient resistance to corrosion under certain service conditions. The T73 temper improves the resistance of precipi-tation hardened 7075 alloy to stress corrosion cracking, although it decreases strength significantly vis-a-vls the T6 condition.
An object of the present invention is to provide a new heat treating method to produce an aluminum alloy in a unique heat treated condition for providing favorable resistance to corrosion combined with high strength Another object is to provide a new method for providing resistance to stress corrosion cracking in 7075 aluminum alloy.
These as well as other ob~ects which will become apparent in the discussion which follows are achieved, according to the present invention, by the method of thermally treating an article composed of an alloy consisting essentially of aluminum, 4 to 8~ zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from 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 includes the steps of solutlon heat treating ~he article, then precipitation hardening the article at 175 to 325F, then sub~ecting the article to a time and tempera-ture within the perimeter ABCD of Figure 4, and then again precipitation hardening at 175 to 325~F.
Figures 1-3 are ~ransmission electron micrographs of section~s in a plate of aluminum alloy 7075. The distance equivalent to 0.1 micron is indica~ed on the micrograph~s. The -~
metal su~faces reproduced in ~he micrographs all were perpen-dicular to the direction of rolling of the plateO
Figure 1 shows a prior ar~ sslution heat trea~ted and . .
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1~455;~8 -stress relieved condition referred to as the W51 condition.
Figure 2 shows the prior ar~ precipi~ation hardened condition referred to as the T6 condition.
Figure 3 shows the prior art stress corrosion cracking resistant condition referred to as the T73 condition.
Figure 4 is a graph showing characteristics 4f the invention.
The alloys in the present invention have a composition containing 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2~5~/D copper, and at least one element selected from the group made up by chromium at 0~05 to 0.3%, manganese at 0.1 to 0.5%9 and zirconium at 0.05 ~o 0~3%O The balance of the composition is essentially aluminum.
Alloys designated 7075 by the aluminum industry are preferred or the present invention and have a compo~ition ` ;
containlng 5.1 to 6.1% zinc, 2~1 to 2.9% magnesium, 1.2 to 2.0%
copper, 0.18 to 0~35~/O chromium, 0.30% maximum manganese, 0.40%
maximum silicon, 0OS0% maximum iron9 0.20% maximum titanium, ~ -others each 0.05h maximum and others total 0.15% maximum, balance aluminum.
The alloys used in the present invention may also contain one or more of the group of grain re~ining elements -including titanium at 0.01 to 0.2% and boron at O.OO~S to 0.002%.
These elements serve to produce a fine grain size in the cas~ -form of the alloy. This is generally advantageous to mechanical propertLes.
In addition, Lt may be helpful to add 0.001 to 0O005%
beryllium for the purpose o minimizing oxidation at times when the alloy is molten.
Iron and silicon are generally present as impurities.
Up to 0.5% iron can be tolerated, and the s~licon conten~ should not exceed 0.4%, in order to avoid the formation of any

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substantial amount of the intermetallic compound Mg2Si.
A preferred heat treatment according to ~he present invention for obtaining improved stress-corrosion resistance is to immerse alloy, as above defined, in the precipltation-hardened, T6 condition into molten me~al for a time and tempera-ture within the perime~er of the quadrilateral EFGH in Figure 4, then precipitation harden again.
In its broader aspects, a T6 condition may be obtained by precipitation hardening solution heat treated alloy at 175 to 325F. Typical conditions may be:
a. For alloys containing less than 7.5%
zinc, heating a solutlon heat treated article to 200 to 275F and holding for a period of 5 to 30 hours;
b. For alloys containing more than 7.5%
zinc, heating a solution heat treated article to 175 to 275F and holding ~or a period of 3 to 30 hours.
A usual practice for obtaining the T6 condition is obtained by heating a speclmen ~or 24 hours at 250F in a ;
circulatory-air furnaceO
According to another preferred embodiment of the ~ -~
invention, the alloy is solution heat treated, then precipitation -~
hardened at a temperature of 175 to 325F, then subjected to a time and temperature withln the perimeter ABCD, more preferably EFGH, and then again precipi~ation hardened ~or a time o~ 2 to 30 hours at a temperature of 270 to 320F.
The article of J. T. Staley et alO entitled "Heat Treating Characteristics of High Strength Al-Zn-Mg Cu A~loys -With and Without Silver Ad~itions" appearing at pages 191 to 199 in the January, 1972 issue of Metallurgical Transactions, pubLished by ASM/AIME, shows that solution heat trea~ quench rate, the lapse of time between the solutlon heat treat quench and the beginning of heating foi precipitation hardenillg, and the heating rate for precipitation hardening may afeclt the

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'~L0915S;~8 maximum yield strength obtainable in 7075 aluminum alloys. It is intended that, wi~hin the concepts of the present invention, the teachings of Sta.~e~: et al~ be used in the present invention for optimizing results. Thus, it may be advantageous for increasing strength to immerse specimens, which have had their solution 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 3, transmission electron . :~
micrographs of various microstructures important for considera-tion of the present invention are presented. All of Figures 1 to 3 were taken from a single 1/4-inch thick 7075 aluminum alloy plate of composition A in Table Io Figures 1 to 3 are micro-structures of prior art conditions of 707~ aluminum. In Figure 1, an example of the WSl solution heat treated conditlon is given, A W51 solution heat treated microstructure is obtained : :
in 7075 aluminum alloy pLate by heating to 900F and then ~
quenching in water at room temperature. The plate material is .. :
then stretched ~o from 1-1/2 to 3V/o permanent set for stress .
relief. This gives the microstructure shown in Figure 1, inclu~ing E-phase particles of Al-Mg-Cr precipitate, matrix -regions R of single phase aluminum solid-solution material, grain boundaries B and dislocations D. The mottling effect appearing in the matrLx region of Figure 1 is an artifact of the action of the thinning solution used in prepari.ng thinned material for transition electron microscopy.

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Table I.
Composition of Alloys, in Weight-%.

_ . . Alloy Element _ B
. . . _ __ - - -I , Cu lo~ 1.~3 Fe 0.19 0.30 Si 0.09 0.12 Mn 0.02 0.07 Mg 2.40 2.48 Zn 5.92 5.68 Ni 0.00 O.OQ
Cr 0.18 0.19 Ti 0.02 Q.05 Be 0 OOL 0.001 ~,`' Figure 2 shows the 707S alloy material of Figure 1 after it has been brought to the T6, in particular the T651, temper by heating W51 material in a circulatory-air furnace ~or 24 hours a~ 250F. E-phase remains substantLally unchanged.
Dislocations D and a grain bounclary B are shown. Now in ~he matrix there has appeared many csmall black do~s, these are ~ ., referred to as G.P. zones and are clusterings o~ magnesium and zinc atoms generally in the ratio two zinc atoms for each magnesium atom.
Figure 3 shows a ~pecimen taken from the same plate of Figur~ 1 and 2 in the T73 condition, which is produced from W51 material by heating in clrculatory-air furnaces ~or, first 24 hours at 250F and, second, 8 hours at 350F. Grain boundary precipi~ate 10 has appeared~ and the G.P. zones have grown to greater size. The G~P~ zones have begun to exhibit cry~stallinity -- 5 -- .
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~1~45S28 by giving rise to X-ray diffraction patterns and are referred to by ~hose in the art as M' and M phases 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 parti~y coherent with the matrLx crystal structurel The M' phase then changes to M phase, which has a crystal stnucture different from the matrixO It is believed also that the progression through the M' phase to the M phase makes the origlnal G.P. zones increasingly anodic with respect to the matrix and that the resulting anodic particulate matter in the matrix protect:s against stress-corrosion cracking.
Further illustrative of the present invention are the following examples.
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For each example, two tensile blanks of dimensions 3/8 inch by 3/8 inch by 2-1/2 inches were cut from a single lot o 2-1/2 inch thick 7075-T651 (metallurgical history as described for Figure 2) alloy plate sueh that ~heir lengths were in the short-transverse direction, i.eO, in the direction perpendicular to ~he surface of the plate.
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Table II. ~ :
Times and Temperatures in Wood's Metal for Examples 1 to 8 and the Coordinates of Points A ~o H. ~
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Example Mo., Time3 Temperature, :
or Point min. F ~ ~ :
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4 4.0 445 7.0 ~00 :
6 . 15~ 400 7 7.0 375 :; :~
8 15.0 375 .
A 2000 360 . .

C 1.0 500 .......
: D 150.0 360 E 20.0 380 F 0.8 480 . G :L.2 480 H 4000 380 ;~
. _ _ _ , I , The chemical composition of the alloy is as presented ~.
for alloy:B in Table Io The tensile blanks for each example were:i~mersed in molten Wood's metal of composition 50% bismuth, ;.
25% lead, 12.5% tln and 12.5% cadmium. The immersion temperatures and times are presented in ~abular ~orm in Table II anc1 are plotted~in Figure 4O Following immersion in the molten Wood's :~
metal, the cooled specimens were ~hen precipl~tion hardened by : ;
heating them in a circulatory-air furnace ~or a time oi. 24 hours ' .:
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at 250F. In each of Examples l to 8, a tensile blank was machined to a 0Ol25 inch diameter tensile bar for exposure to 3-l/2% sodium chloride solution by alternate immersion a~ a str~s level of 42 ksi according to Military Specification MIL-A-22771B.
The specimens were held until failure with successive immersions ~or l0 minutes in the salt solution followed by 50 minutes in air.
The number of days until failure under such treatment is provided in Figure 4 above the ~ime-temperature point for each Example.
The remaining blank of each example was tes~ed ~or yield strength~
The yield strength data for Examples 1 to 8 are presented in Figure 4, below the time-temperature points, in terms of percentage of a yield strength of 62.3 ksi for the T651 condition.

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Further lllustrative of the pre~erred embodiment o the invention wherein the second precipitation hardening step is carried out for 2 to 30 hours at a ~emperature o 270 and 320F
are the following examples:
Examples 9 to 14 Procedure was as described for Examples l to 8, except that all examples utilized an immersion in molten Wood's metal ~or 90 seconds at 445F, before the second precipita~lon hardening. Other parameters and results were as presented in Table III. Examples 9 to ll form one group of comparative examp1es characterized by 3 hours at temperature in the second precipi~ation hardening step, wi~h Examples 12 to l4 forming a second group characterized by 24 hours at temperature in the second precipitation hardening step. The superior strength and corrosion resistance obtained when ~he second precipitation -harden~ng was done for 2 to 30 hours at 270 to 320F will be apparent from comparison of the examples wlthin the groups.

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The following definitions hold herein: ~ -a. The term "ksi" is equivalent to kilipounds per square inch.
b. Wherever percentages are given, re~erence is to % by weigh~, unless indica~ed ot~erwise c. The initials "G.P " stand for Guinier-Preston. . .
~ rious modifications may be made in the invention without departing from the spirit thereof, or the scope of the claims, and, therefore, the exact orm shown is to be taken as illustrative only and not in a limiting sense, and it is desired that only such limitations shall be placed thereon as are imposed by the prior art, or are ~pecifically set forth in the appended claims. ;

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of thermally treating an article com-posed of an alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese, and 0.05 to 0.3% zirconium, said method com-prising the steps of solution heat treating said article, then precipitation hardening the article at 175 to 325°F, then subject-ing said article to a time and temperature within the perimeter ABCD of Figure 4, and then again precipitation hardening at 175 to 325°F.

2. The method as claimed in Claim 1, wherein the step of again precipitation hardening is performed at 270 to 320°F for 2 to 30 hours.

3. The method as claimed in Claim 1, the step of subject-ing being for a time and temperature within the perimeter EFGH of Figure 4.

4. The method as claimed in Claim 1, wherein the step of subjecting comprises immersing said article in liquid having said temperature.

5. The method as claimed in Claim 4, wherein said liquid is molten metal.

6. The method as claimed in Claim 1, wherein the tem-perature within the perimeter ABCD of Figure 4 is less than 200°C.

7. The method as claimed in Claim 1, wherein the tem-perature within the perimeter ABCD of Figure 4 is 445°F.

8. The method as claimed in Claim 7, wherein the time within the perimeter ABCD of Figure 4 is from 1.5 to 4 minutes.