Note: Descriptions are shown in the official language in which they were submitted.
~1037296
This application is directed to aluminum base alloys
- of the type containing both zinc and magnesium, together with
a controlled addition of copper, yet substantially free of
recrystallization-inhibiting elements such as chromium,
manganese and zirconium, and adapted for making wrought
articles having a recrystallized metallurgical structure
characterized by its resistance to stress corrosion cracking.
In general, the present invention relates to Al-Zn-Mg-Cu
alloys consisting essentially of aluminum, 3.5 to 5.5% zinc,
0.5 to 2% magnesium and 0.3 to 1.5% copper by weight, with
silicon, iron and other residual elements and impurities up
to about 0.5% total, including manganese, chromium, nickel,
titanium and zirconium not exceeding.05% each and 0.15% total. -~-~
In accordance with the present invention it has been
;` found that wrought articles made from such alloys, and having
a recrystallized structure, not only exhibit excellent resis-
tance to stress corrosion cracking, in artificially aged
condition, but, particularly in the case of preferred alloy
compositions hereinafter described, are readily anodized to
achieve a bright surface appearance. This is contrary to
what would be expected on the basis of published literature
regarding the role of copper, which generally is considered
as an undesirable addition to Al-Zn-Mg alloys for anodizing
purposes, except at very low levels of about 0.1% by weight.
- It is also unexpected and surprising that a recrytal-
- lized structure of such an Al-Zn-Mg-Cu alloy would be resistant
~ to stress corrosion cracking, even in underaged condition,
- since it has been considered by some experts in the art that
a fibrous structure would be necessary in this regard. ~
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1037296 .
The present invention is a result of efforts to
develop a heat-treatable alloy system suitable for making
automotive vehicle bumpers, or other wrought articles, and
adapted for extruding, solution treatment, hot working,
quenching, aging and anodizing to achieve a bright surface
appearance in artificially aged temper. In accordance with
a preferred aspect of the invention, an aluminum base alloy
(type X7016) is provided which meets all of these criteria,
and is believed to be the only known 7000 series alloy capable
of providing a recrystallized structure which exhibits the
property of being resistant to stress corrosion cracking, in
combination with the ability to be bright anodized. The
registered composition limits of alloy X7016 are silicon 0~10%
(max.), iron 0.10% (max.), copper 0.6 to 1.4%, manganese .Q3%
(max.), magnesium 0.8 to 1.4%, zinc 4 to 5%, titanium .03%
(max.), others (including chromium, nickel and zirconium) not
exceeding .03% each and 0.10% total.
The use of copper in 7000 series sheet and plate alloys
containing both zinc and magnesium has been known and prac-
ticed commercially for many years. However~ apparently due to
~; concern about its adverse effect on weldability, copper has
been used typically at very low levels where welding was
anticipated. It has been reported, for example, that small
controlled additions of copper (~ 0~20/o) to a range of Al-Zn-Mg
alloys in a variety of wrought forms improved the stress
corrosion resistance of parent metal and weldments without
reducing weldability. The literature further notes that copper
has been found to promote microfissuring in the heat-affected
zone of weldments, but that copper up to 0.4% is satisfactory
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~037;~6
in the presence of zirconium. Others have also suggested
` that minor additions of copper in medium strength weldable
alloys may be beneficial in reducing stress corrosion
susceptibility of recrystallized weldments of such alloys.
; -
; Apparently no studies have been reported on the effect of
copper levels above 0.4% in recrystallized structures.
However, as hereinafter discussed, our work shows that a
higher copper content is effective to increase resistance
to stress corrosion cracking.
On the other hand, with regard to copper-bearing
alloys of the 7000 series, it has been proposed that factors
such as controlling the heat treatment to achieve overaging,
and including alloying additions of various elements
(especially Cr, Mn~ Zr) to inhibit recrystallization, are
helpful in achieving stress corrosion resistance of Al-Zn-
Mg-Cu systems; also, that the presence of copper in these
alloys is detrimental to their general corrosion resistance,
as indicated by increased tendency toward pitting, although
a small amount of copper (such as 0.1%) is generally advan-
tageous for bright anodizing purposes. Although the general
. .
corrosion resistance of base Al-Zn-Mg-Cu alloys is impaired
by increasing the copper content from 0.1% to 1.0%, we find
that the corrosion resistance of anodized X7016 alloy is not.
.."
Consequently, in view of the recognized disadvantages
: .
; of copper with respect to impairment of weldability, and the
general reluctance to use recrystallized structures, it is
,; . .
surprising and unobvious to find, in accordance with the
present invention, that Al-Zn-Mg-Cu alloys typically containing
about 0.3 to 1.5% copper, and substantially free of
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1037Z~fi
;; recrysta~ization-inhibiting elements, can be used successfully
- to produce wrought articles having a recrystallized structure
characterized not only by its resistance to stress corrosion
cracking, but also adapted for anodizing in solution treated
and artificially aged condition to achieve a bright surface
finish.
Without being bound to any theoretical explanation of
the invention, it may be noted that the MgZn2 phase is highly
anodic to solid solutions of Al-Zn-Mg. When it is precipi-
tated predominantly in the grain boundaries, the alloy can be
highly susceptible to cracking. Consequently, additions of
copper may have at least two significant effects. First, some
copper apparently coprecipitates in solid solution in the grain
boundary MgZn2 phase, making its corrosion potential more
- cathodic. second, copper in the grain matrix promotes pitting,
and thus provides an alternate mechanism for the discharge of
corrosive agents such as dissolved oxygen or hydrogen ions.
When it occurs, pitting can actually retard stress corrosion
cracking either by providing active anodes throughout the
~` 20 matrix which, in turn, afford cathodic protection to less
active but susceptible grain boundaries, or by inhibiting
:,........................................................................ .
localized dissolution at the boundary and thus prevent the
. :: . .
- formation of cor~-osion paths of sufficiently small radius of -~
J ' ' ' curvature to propagate as a crack under the existing streæs
condition.
The foregoing analysis is compatible with and helps
`- to explain the observed phenomenon that using an accelerated
Boiling Salt Test to predict stress corrosion susceptibility,
although found to be an acceptable approach for copper-free
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~037Z96
alloys of the 7000 series, does not provide reliable data for
` X7016 alloy. In a standard Boiling Salt Test, the dissolved
oxygen content is low, which may tend to minimize pitting and
accentuate cracking tendencies, so that the results do not
adequately reflect the presence of copper.
For purposes of the invention, the upper limit of
copper, at least below the level of about 1.9% for which the
alloy's propensity to stress corrosion cracking becomes severe,
depends upon the acceptable degree of yellow coloration
developed during conventional H2SO4 anodizing. A maximum of
about 1.1% copper is preferred in this respect. It has also
been found that using an anodizing current density of about
10 amps per square foot (asf), plus or minus about 2 asf, is
helpful in minimizing the depth of coloration without requiring
prolonged treatment. A distinctive characteristic of X7016
alloy is its ability to be anodized to provide a clear bright
;~ coating exhibiting a specular reflectance factor of about 85
to 95% for anodic film thickness up to about one-half mil.
Alloys suitable for purposes of the invention include
those containing approximately 4 to 6% total of zinc and
magnesium, preferably about 5.30 to 5.85% total, and particu-
larly with a copper content in the range of about 0.7 to 1.1%.
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; The alloy may be solution treated at about 900-925F.,
for example, and quenched in water or by air blast, but a
relatively ast quench rate of about 100F./sec. is preferable
; for best specularity and optimum stress corrosion resistance
- properties as determined by Alternate Immersion Testing.
Suitable aging practices are about 3 to 8 hours at
200-225 F. plus 3 to 8 hours at 275-325F., to achieve an
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` 1037Z96
underaged temper and, for alloy X7016, a minimum yield strength
o~ 42 Ksi.
When the fabricating sequence involves extruding,
solution treating, hot forming, quenching, aging and anodizing
the alloy, it is preferable for best anodizing results (and
freedom from grain growth) to avoid cold working the extruded
alloy, such as by stretch straightening the extrusion. However,
routine cold finishing operations may be performed between the
quenching and aging operations.
The following exemplary practices of the invention and
its presently preferred alloy compositions are provided for the
purposes of illustration:
ExamPle I -
Tests were conducted by alternate immersion treatment
(using Test Method 823 of Fed. Std. No. 151) and by exposure
to the atmosphere at an outdoor location in Richmond, Virginia.
Samples taken from an extruded section were tested in the long
transverse direction, which was considered permissible because
.
; the structure was fully recrystallized and exhibited no signi-
; 20 ficant anisotropy.
(a) Test Material
A 30-inch length by 10-inch round billet (S-23891),
homogenized 16 hours at 1075F, was extruded to form a bumper
blank. The composition was .04 Si, .04 Fe, .26 Cu, C.01 Mn,
7' - '
1.06 Mg, C.ol Cr, C.01 ~i, 4.65 zn and <.01 Ti, balance
aluminum.
(b) Press Practice:
Extrusion Ratio - 98/1
- Cylinder temperature ~ 800F.
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1037296
Billet temperature - 920F.
; Ram Speed - 12 IPM ram speed
Cooling - air quench (fans)
(c) Testing Conditions
In all tests, specimens were strained by three-
point loading to a permanent set at a value of 12 mils/inch.
Specimen dimensions were 0.250" wide, about 3" long, and
0.126" thick. Test results for various conaitions noted are -
given in Table 1.
Table 1: Deflected Beam Stress-Corrosion Test Results
Condition Alternate ImmersiOn Richmond AtmosPhere
Failure ~o Failure NO
- Time Failure Time Failure
ys) 500 days (Days) 500 Davs
A 42
" 69
` B x x
. . .
:.
- " x x
". C 19
. . .
~ 7
:. ~
~ 20 D x x
...
,. " x x
E x x
~-
" x x
F x
" x
` Condition A - As press-fan quenched, stretched and naturally
aged for 11-1/2 months, followed by a solution heat treatment
(one hour at 900F ~ CWQ) and a 16-20 day natural age.
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1~37296
Condition B - condition A followed by a two-step age of 7 hours
at 225F + 8 hours at 300F (50F/hr. heat-up rates). TS =
54.2 KSI, YS = 48.0 KSI, Elong. = 16.5%.
condition C - as press-fan quenched, stretched and naturally
aged for 11-1/2 months.
Condition D - naturally aged for 12-1/2 months, followed by a
solution heat treatment (one hour at 900OF ~ CWQ) + 20 hours
at 250F. (5ooF/hr. heat-up rate).
- Condition E - naturally aged for 12-1/2 months, followed by
8 hours at 250F + 8 hours at 300 F. TS = 54.5 KSI,
YS = 49.1 KSI, Elong. = 13.0%.
Condition F - naturally aged for 12-1/2 months, followed by
- 20 hours at 250F. (5ooF~hr. heat-up rate).
EXAMPLE II
Stress corrosion resistance of Al-Zn-Mg-Cu alloys
having various copper levels (.51, .75, .99, 1.46, 1.93) was
determined by both alternate immersion testing and following
exposure in the Richmond atmosphere.
(al Test Material
Test material was extruded from 14-inch
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round by 30-inch length billets which had been homogenized as
noted below. Alloy compositions were as follows:
Alloy Compositions
Sample
I.D. Si Fe Cu Mn Mq Cr Ni Zn Ti
S-28902 .04 .06 .51 ~ 01 .98 <~01 ~01 4.65 <.01
S-28903 ~04 .06 .75 ~01 .98 ~.0~ <.01 4.61 <.01
S-28898 .04 .06 .99 ~01 1.03 ~01 ~.01 4.64 <.01
S-28904 .04 .07 1.46 ~01 ~96 ~01 ~.01 4.75 <.01
S-28888 .04 .06 1.93 ~ 01 .95 <.01 ~.01 4.55 ~01
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103~296
~omogenizinq Treatments
- Alloy I.D. Practice
S-28902 14 hours at 1030F
S-28903 14 hours at 1030F
S 28898 14 hours at 10650F
S-28904 14 hours at 1030F
S-28888 14 hours at 10650F
(b) Extrusion data were as follows:
.. ~,, ;
Break Extrude Indicated Table Billet
Charge Pressure Pressure Ram SPeed SPeed TemPerature
- 10 S-~8902 2800 2400 5 30 875
' S-28903 - 26 875
- S-28898 - - 5 - 875 -
, . . .
S-28904 2700 2400 5 32 875
, -
, ; S-28888 3600 3000 5 28 850
~' Container Temp. ~ 8000F.
;` COOling - air quench (fans)
` Structure - fully recrystallized
(c) Sample Preparation and Results
: -.
Residual stresses were introduced by indenting
specimens with the Olsen Ball Penetrator. Samples were
indented with various loads in both the T4 and aged conditions,
and after bright anodizing. The T4 condition was obtained
~- using a 10-minute solution heat treatment at 900F followed
by a fan quench and a caustic etch. Aging was for 8 hours
at 225F followed by 8 hours at 300OF using a 50F/hr. heat-up
,` rate. The order of treatments is shown in the appropriate
table. Aging was done within four days after either quenching
- ~r indenting. Indenting was done within ten days after quen-
ching and within ten days after aging.
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1037~6
All samples were indented under the following
:`
conditions after facing off to .240-.250 inches:
7/8" ball diameter
1-1/2" I.D. top die
1-3/8" I.D. bottom die ;
2000# clamp down load
Tests results are shown in Tables 2 and 3, for
the high and low.copper content alloys having the following
physical propertieæ:
:,
S-28902 - (.51 Cu) Aged properties - UTS = 51.3 KSI,-
YS = 47.2 KSI, Elong. = 16.8%. .
.;, .. ~ .
*: - .~ .
S-28888 - (1.93 Cu) Aged Properties - UTS = 54.6 KSI,
YS = 50.1 KSI, Elong. = 17.0%.
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1037Z96
(d) Others (.75, .99, 1.46 Cu)
~ o failures occurred in alternate immersion or
in the atmosphere over 220 days on test at these three inter-
mediate copper levels.
Additional comments on the Results of Example II.
., ,
Susceptibility at the highest Cu level (1.9%) was
found in alternate immersion for various sample conditions,
including the aged + indented specimens. With only one
exception, other samples which were deformed after aging have
not failed. This one exception applied to a comparison
specimen at lower copper content (.1 Cu) in the atmosphere
test using the highest load (12 KIP). Even then, the failure
had more the appearance of accelerated intergranular corrosion
than cracking. Contrary to the results found with the Boiling
- Salt Test, samples indented in the T4 condition and exposed to
alternate immersion showed some susceptibility, while all other
. ~ -.
conditions did not (with the exceptions at the 1.9 Cu level).
Similarly, in the atmosphere, three failures have been observed,
none of which are indented-after-aged conditions (again with
the exception of the 1.9 Cu alloy). Below the 1.9 Cu level,
.. :
five failures total have occurred in both alternate immersion
- and the atmosphere at the .1 Cu level, one at the .51 Cu level,
.
and none at .75, .99, and 1.46% CU. This indicates increasing
resistance with increasing copper content.
Perhaps the two most relevant sample conditions
tested were those that were aged after indenting (simulates
aging after cold forming) and those that were indented after
aging (simulates field damage to the fully hardened bumper).
Belsw the 1.9 Cu level, no failures have occurred for either
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1037Z~6
condition in alternate immersion, and with the one exception
previously mentioned of the indented after-aging sample, no
failures have occurred in the atmosphere.
Time on test (~ 200 days) has been sufficiently
long to allow these unfailed specimens to be described as very
resistant and possibly immune.
Example III
In forming several automotive wrap-around bumper
components of X7016 Alloy on prototype production tooling,
extruded sections (F-temper) were preheated at 900-9250F. for
twenty minutes, cooled by air blast to a suitable hot working
temperature of about 800F., hot formed to shape the wrap-
around end portions, and quenched in water. After final
finishing operations cold, i.e., at ambient temperature, the
`- thus fabricated components were sectioned at the most heavily
cold worked portions and aged for 8 hours at 225F. plus 8 hours
at 30CF.
The alloy contained approximately 4.69% zinc,
1.01% magnesium and 0.93% copper, with .04% silicon, 0.6% iron
i 20 and less than .01% each of manganese, chromium, nickel and
titanium, balance aluminum.
The heat treated specimens exhibited good resis-
tance to stress corrosion cracking under Alternate Immersion
` Testing, after being deformed to the point of incipient fractureand releasing the load just beyond the stress level producing
instability.
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