Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~353~L
Production of Aluminum Alloy Sheet
and Articles Fabricated Therefrom
Background of the Invention
This invention relates to processes for producing
Al-Mg-Si alloy sheet and articles fabricated therefrom,
and to the products of such processes.
Al-Mg-Si alloys as herein contemplated are alloys
having a major content of Al and a minor content of My
and Six and are exemplified by known alloys identified
by Aluminum Association designations in the 6000 series,
e.g. the alloys having Aluminum Association BAA) de-
signa~ions 6009, 6010, 6011, 6061, and 6063. The term
"sheet" is broadly used herein to mean rolled
products, without limitation to any particular gauge;
thus it includes products at plate and foil gauges as
well as products at conventional sheet gauges.
More particularly, the invention is directed to
processes for producing Al-Mg-Si sheet in so-called To
temper, which-is the temper achieved by performing sue-
cessively the steps of solution heat treatment,
quenching, cold working, and artificial aging, some-
times with a natural aging period interposed between the
quench after solution heat treatment and the following
cold working step. It has heretofore been known to
provide Al-Mg-Si products, including sheet, in To
temper, for various purposes.
In one important specific aspect, to which de-
tailed reference will be made herein for purposes of
illustration, the invention is dlrecte~ to the pro-
diction of aluminum alloy can body and lid stock, viz.
aluminum alloy sheet for forming one-piece drawn and
ironed can bodies and can lids for such bodies, as well
as to the formation of can bodies and lids from such
sheet and to the articles thus formed.
- -
-
353~
Presented metal cans as used for beverages such
as soft drinks, beer and the like are commonly keenest-
tuned of a seamless one-piece body (which includes the
bottom end and cylindrical side wall of the can) and a
top end bearing a ring or other opening device. The
body is produced from a blank of cold-rolled aluminum
alloy sheet (having a gauge, for example, of about
0.014 inch) by a now-conventional forming technique
known as drawing and ironing, which involves drawing
the blank into a cup and then passing it through a sue-
cession of dies to achieve the desired elongated cry-
lindrical body configuration, with a side wall of no-
duped thickness relative to the bottom end. The top
end is separately produced from another sheet aluminum
alloy blank, by different but also conventional forming
operations, and is secured around its circumference to
the top edge of the side wall of the body to provide a
complete can.
The severity of the forming procedure employed
in producing a drawn-and-ironed can body as described
above, and in particular the reduction in thickness of
the can side wall (which must nevertheless be able to
withstand the internal and external forces exerted on
it in use), as well as the fact that the formed can is
usually lacquered in an operation necessitating a
strength-reducing exposure to heat, require a special
combination of strength, formability, and tool wear
properties in the alloy sheet from which the can body
is made. Significant among these properties are multi-
mate tensile strength, yield strength, elongation, endearing. Attainment of the requisite combination of
properties is dependent on alloy composition and on
the processing conditions used to produce the sheet.
Heretofore, a conventional sheet for can body
blanks has been constituted of the alloy having the
SLY
-- 3
Aluminum Association designation AYE, and has been
produced from conventionally direct-chill-cast ingot up
to 24 inches thick by scalping and homogenizing the
ingot, and successively hot roiling and cold rolling to
the desired final gauge; often an anneal treatment is
used between the hot and cold rolling operations, with
the annealing gauge so selected that the amount of cold
reduction to final gauge after annealing is about 85%,
thereby to provide can body blanks in Hl9 (extra hard)
temper. This practice imparts the combination of pro-
parties currently required for commercial can body
stock. The aluminum alloy designated AA 5182 is ox-
pensively used for the manufacture of the can top ends
or lids, can lid stock (sheet) of such alloy being pro-
duped in a manner similar to that described above for production of AYE on body stock in that similar
steps of direct chill casting; homogenization, hot
rolling, annealing and cold rolling to the Hl9 temper
are employed; cold rolling may also be performed be
tweet the hot rolling and annealing steps. The final
can lid stock, ego at a gauge of about 0.013 inch, is fag-
queued and when formed into lids the lacquering operation
again involving a stowing (heating) step.
Although satisfactory cans are provided my the
foregoing conventional procedures utilizing different
alloys for the body and lid respectively, it would be
desirable to produce cans having both body and lid
formed of the same alloy, to facilitate recovery and
reuse of the metal when the cans are recycled. Such
an alloy requires a combination of high strength and
good formability. Aluminum alloy sheet having such a
combination of strength and formability properties
would be advantageous for use, at various gauges, in a
wide variety of other applications as well.
~L%3S3~l
Summary of the Invention
.
The present invention broadly contemplates the
provision ox a process for producing aluminum alloy
sheet of predetermined final gauge, comprising the
steps of ~rovidlng a sheet article of a heat-treatable
Al-Mg-Si alloy (having a composition as defined below)
at an intermediate gauge from which a reduction of be-
tweet about 25% and about 71~ is required to achieve the
predetermined final gauge; solution-heat-treating the
sheet article at the intermediate gauge by heating and
~enc~g, for effecting it least Shelley ~n~lete Lyon of ye My Ed
So therein; aster quenching, and without intervening
heat treatment, naturally aging the sheet article by
maintaining it at ambient temperature for at least about
one day; a~tPr natural aging, and without intervening
heat treatment, cold rolling the sheet article to the
final gauge (i.e. with a reduction ox between about
25% and about 71~); and artificially aging the sheet
article at the final gauge or increasing the yield
strength thereof by heating the article to a prude-
termined temperature for a time shorter than that no-
squired to achieve the maximum yield strength attainable
by artificial aging of the article at that predetermined
temperature, and such that the % elongation value ox the
article after artificial aging is within 20~ ox the
maximum value attainable by artificial aging OX the
sheet article at that temperature following the same
extent of cold reduction performed after solution heat
treatment. The alloys used in the process, in its broad-
eat aspects, are those having a major content of I
and a minor content of jig and available So such that
on a rectangular graph of % My slotted against % avail
able Six the point representing tune lug and available
at content lies within the area of a pentagon defined
by the coordinates 0.2% Six I lug 0.2~ Six 0.9~ lug
~2353~i~
-- 5
0~4% Six Lo lug 1.2~ Six 1.2~ go and 1.2% Six 0.4~~g, all composition percentages here and elsewhere set
forth in the present specification being expressed as
percentages by weight. As used herein, the term "avail-
able Six means So which has not been taken up by Fish is ordinarily present in the alloy. It is usual
to assume that a percentage of So equal to one third
or the Fe content is lost to the inter metallic compounds.
Thus, with this assumption made, the available So con-
tent of an alloy (in weight percent) is equal to teetotal So content of the alloy (in weight percent) less
one-third of the Fe content (in weight percent).
The process of the invention differs from pro-
seeders heretofore known (for producing Alms So en-
tides in To temper) in that, in the artificial aging step, heating is terminated before the article attains
its maximum yield strength. Specifically, it has now
been found that when a solution-heat-treated and worn-
hardened Al-Mg-Si sheet is heated to effect artificial
aging, the formability (represented by % elongation)
as well as the yield strength initially increases at-
though with continued heating, the % elongation begins
to decrease at a time when the yield strength is still
increasing. Thus, termination OX artificial aging be-
fore the peak yield strength is reached affords beneficial improvement in strength without substantial imp
paramount of formability, and indeed, in
many cases, with actual enhancement of formability.
More particularly, the steps of natural aging
after solution heat treatment, subsequent cold rolling
between about 25% and about 71%, and artificial aging
with observance of the special conditions just de-
scribed, cooperatively provide artificially aged sheet
having a superior combination of strength and format
ability properties. In one specific sense, the process
..
~23S36~
-- 6 --
of the invention further includes the step of forming the artificially aged sheet article into a component of
a can, viz. a one-piece drawn and ironed can body have
in an open end or a lid for closing the open end. In
some instances, the stowing (heating) operation per-
formed after lacquering of the lid stock may be con-
dueled under conditions selected to constitute the anti-
filial aging step of the present process, although it
is at present preferred to perform the artificial
lo aging on the sheet stock prior to lacquering. As will
be understood, in these embodiments of the process of
the invention, tile predetermined final gauge to which
the sheet is reduced before artificial aging is a de-
sired and e.g. conventional gauge for can body or lid
stock. Advantageously, the invention can be embodied
in a process for the production of cans wherein both
lid and body are fabricated of sheet of the same alloy
produced by the foregoing sequence of steps so that the
metal of the can (when recycled) may be remelted and
reused to produce new can bodies and lids without
major adjustment of alloy composition.
In a broader sense, the sheet products of the in-
mention may be produced at various final gauzes, since
the combination of strength and formability achieved
by the present process is beneficial for diverse uses.
A preferred upper limit of final sheet gauge for pro-
ducts of the present process is l/2 inch.
Preferably, the alloy composition employed in the
practice of the invention is selected to have at least
a slight excess of available So over that statue-
metrically required for combination (as Mg2Si) with
all the 21g present, and (especially for production of
can body or lid stock) the amount of My in the alloy it
selected to insure a total Mg2Si content between about
1.35 and about 1.50~. Preferably also, the amount of
-`
~23S3~
-- 7
cold reduction between solution heat treatment and
artificial aging is at least about 35~, and most pro-
fireball (again, for production of can body and lid
stock) the amount of such cold reduction is between
about 50 and about 71%, this condition being provided
by appropriate selection of the aforementioned inter-
mediate gauge with reference to the desired predetex-
mined final gauge.
The invention also embraces sheet articles, and
lo can components, produced by the foregoing process, and
possessing the advantageous combination of mechanical
properties thereby achieved.
Further features and advantages of the invention
will be apparent from the detailed description herein-
below set forth, together with the accompanying draw-
ins.
- 8 -
Brief Description of the Drawings
Fig. 1 is a rectangular graph on which % My is
plotted against available Six in illustration of the
; My and available So content of alloys suitable for the
practice of the present invention;
Fig. 2 is a rectangular graph of ultimate tensile
strength (US), yield strength (YE), elongation and
Erickson cup depths of artificially aged (To temper)
AYE alloy sheet, plotted against artificial aging
time, for sheet subjected to 35~ cold reduction after
solution heat treatment, to a final gauge of 0.030
inch, and then artificially aged at 160C.
Fig. 3 is a graph similar to jig. 2 for AWAIT
sheet subjected to 71% cold reduction after solution
heat treatment, to a final gauge of 0.0135 inch, and
then artificially aged at 160C: and
Fig. 4 is a graph similar to Figs 2 and 3 for
AYE (To) sheet subjected to 71% cold reduction after
solution heat treatment, to a final gauge of 0.0135
inch, and then artificially aged at 185%C.
Detailed Description
The invention will ye described, with reverence
to the drawings, as embodied in a process for product
in Al-Mg-Si alloy sheet from reroll stock by the sue-
cessive steps of providing a sheet article of inter-
mediate gauge, solution heat treating, natural aging,
- cold rolling, and artificial aging, and in the pro-
ducts. of that process. Par oculars of the alloys em-
plowed, the preparation of the reroll stock, the per-
pheromones of each of the aforementioned steps, and
their combination in the complete. process, are set
forth below.
35.3~S~
g
Alloy Composition
, . . . .
Alloys suitable for the practice of the present
invention broadly include Al-Mg-Si alloys having a
minor content of My and available So such that on a
S rectangular graph of % My plotted against available
So (i.e. the graph of Fig. 1) the point representing the
My and available So content of the alloy lies within
the area of pentagon 10 in Fig. 1, Vim. a pentagon de-
fined by the coordinates 0.2% Six 0.4~ My; 0.2~ Six
C.9~ My; 0~4~ Six 1.2~ My; 1.2% Six 1.2% My; and 1.2~
Six 0.4~ My. referred alloy compositio~s,within this
broad definition, are those or which the point repro-
setting My and available So content lies not only within
the aforesaid pentagon but also to the right of a line
12 which represents the theoretical Mg2Si weight ratio,
i.e. McCoy = 1.73/1. Preferably, also, the alloy con-
sits essentially of My and available So in amounts (~)
defined by pentagon 10, optionally also containing up to
0.9% Cut up to lo Fe, up to 0.8~ Len, up lo 0.35% Or,
up to 0.25% Zen, up to 0.20% Tip balance essentially Al
with usual impurity levels not materially affecting the
combination of strength and formability properties with
- which the present invention is concerned
Specific examples of known alloys within the
foregoing broad definition, and suitable for the pray-
lice of the invention are the alloys having the
Aluminum Association designations AA 6009, 6010, 6011, 6061,
and 6063, the registered compositions of which are as
follows:
~2353~
-- 10 --
Range or Maximum (% by weight) __
. . _ . _ . .
AYE AYE AYE AYE AYE
. . _
So 0.6-L.0 0.8-1.2 0.40-0.8 aye
Fe 0.50 0.50 0.7 0.35 1.0
Queue 0.15-0.6 0.15-0.40 0.100.40-0.9
My 0.20-0.8 0.20-0.8 Owls 0.10 0.8
My 0.40-0.8 0.6-1.0 0.8-1.2 0.45-0.90.6-1.2
Or 0.10 0.10 0.04-0.35 0.100.30
Zen 0.25 0.25 0.~5 0.10 1.5
1.0 To 0.10 0.10 0.15 0.100.20
No - - - - 0.20
other
(each/
total) 0.05/0.15 0.05/0.15 0.05/0.15 0.15/0.15 0.05/0.15
Al balance balance balance balance balance
Alloys with the composition limits of AYE as given
above are particularly-preferred, especially for em-
bodiments of the invention providing drawn-and-ironed- --
can body stock and can lid stock; currently most pro-
furred for these embodiments is an alloy having the
nominal composition 0.25% Fe, 0.30% Cut 0.65~ Six
0.05~ (max.) I, 0.90~ My, Mecca.) Zen, 0.17~ Or,
0.25~ (max.) Tip other 0.10% (max.), balance aluminum,
the designation "(Mecca' being used to indicate that the
value given is a maximum and that the element so design
noted is merely optional or tolerable as an impurity up
to the stated maximum. For good age-hardening response,
the alloy should contain a slight excess of available
So (at least about OOZE) over that needed to statue-
metrically form Mg2Si with a weight ratio McCauley of
1.73/1; as mentioned above, when making this calculi-
lion, it is usual to assume that a percentage of the
total So content equal to 1/3 of the Fe content is
lost to the inter metallic compounds It is also usual
with AYE to ensure a total Mg2Si content between
about lo 35 and about 1.50.
. ' .
353~1
A further example of alloys suitable for can stock
are those having a minor content of My and available
So such that on the graph of Pig. 1, the joint repro-
setting the go and available So content ox the alloy
lies within the area of a parallelogram defined by the
coordinates 0.3% Six 0.8~ My; 0.5S% Six 1.2~ My; 1.05
Six 1.2~ My; and 0.8% Six 0.8% My, this parallelogram
being represented in Fig. 1 by the chain lines 14 and
a portion of the top horizontal) line ox pentagon 10.
Preferred alloy compositions within this parallelogram
are those. for which the point representing My and avail-
able So content lies to the right of the aforementioned
line 12; of these, the most preferred compositions are
those (again within the parallelogram) for which the
point representing My and available So content lies above
and to the left of the dotted lines 16 and to the right of
line 12, i.e. within the quadrilateral defined by the
coordinates 0.7% Six 0.9~ My; 0.875% Six 1.2% My; 0.69% Six
1.2% My; 0.52~ Six owe% My.
Preparation of Reroll Stock
The starting material for the practice of the
present process, in illustrative embodiments thereof,
is a Dow of an alloy having a composition as defined
above, in the form or a strop of appropriate gauge or
the initial cold-rolling step ox the process, such
strip being herein termed "reroll stock." Typically,
the reroll stock is prepared my casting a convention-
ally dimensioned sheet ingot of the alloy, e.g. by so-
30 called direct chill casting, scalping and homogenizing
the ingot, and hot rolling to the reroll gauge, all
in accordance with well-known and wholly conventional
procedures. Alternatively, the reroll stock can be
produced by continuous strip casting techniques, viz.
: 35 by casting the alloy as a continuous, relatively thin
: .. .:
~23S3~
- 12 -
strip in a casting cavity defined between chilled end-
less moving steel bolts, between chilled rolls, or be-
tweet chilled walls of a stationary mold, again as is
well-known in the art. Such continuously cast strip
either can be cast sufficiently thin to enable direct
cold rolling, or can be hot-rolled to reroll gauge.
The reroll stock, however produced, is cooled and or-
dinarily coiled; thus, preferably in at least most
instances, the reroll gauge is sufficiently thin to
lo enable direct coiling.
Provision of Intermediate Gauge Sheet Article
In specific exemplary embodiments of the invention,
reroll stock prepared as described above is cold roiled
(employing procedure entirely conventional for cold roll-
in of Al-Mg-Si alloys to reduce it to strip of an inter-
mediate gauge at which the strip is to be solution heat
treated. This intermediate or solution-heat-treatment
gauge is selected, with reference to the predetermined
desired final gauge of the sheet to be produced, such
that a reduction of between about 25% and about 71%
from the intermediate gauge is required to achieve the
final gauge. That is to say, the intermediate gauge is
selected to provide for further cold reduction of about
25% to about 71% by cold rolling after solution heat
treatment, as described below; preferably, the amount
of cold reduction after solution heat treatment is be-
tweet about 35% and about 71% and indeed most prefer-`
ably (especially for production of can body or can lid
stock) between about 50% and about 71%, and for such
preferred practice the intermediate gauge is selected
accordingly. The reason for selecting the intermediate
gauge to provide for the specified amount of cold no-
diction after solution heat treatment is to enable de-
US velopment of desired properties in the strip by post-
~23S3~
solution-heat-treatment cold work. Selection of a
particular intermediate gauge Raytheon the stated ranges
is dependent on the specific properties sought to be
attained in the final product.
It will be appreciated that the reroll gauge is
not critical but is conveniently selected to be appear-
. privately larger than the aforementioned intermediate
gauge so that a substantial amount of reduction will
be performed in the initial cold rolling step. Merely
by way of illustration, in one example of production of
can lid stock of 0.013 inch final gauge by the process
of the invention, the intermediate (soluti.on-heat-treat-
mint) gauge is selected to be between 0.0~6 and 0.045
inch, such that the cold reduction to final gauge after
solution heat treatment is between 50% and about 71~,
depending upon the particular final properties desired;
the reroll gauge in this instance it
conveniently between about 0.120 inch and about C.160
inch.
It will be also be appreciated that, in its
broader aspects, the invention does not require that
- the sheet article be brought to intermediate gauge by
cold rolling, but embraces the provision of the sheet
article in intermediate gauge in other ways as well;
25 for example, in some instances the intermediate gauge
can be attained directly by hot rolling, without any
cold rolling before solution heat treatment.
~3~i3~
Solution float Treatment
The initially cold-rolled strip article, at the
aforementioned intermediate gauge, its solution heat treated
(by heating and quenching) under conditions selected to
effect at least substantially complete solution of the My and So
therein. I've steps and conditions employed may, again, be
entirely conventional, and as such are well known to persons
of ordinary skill in the art. ~atch-type solution heat
treatment may be used; although the time/temperature
conditions are dependent on the coarseness of the Mg2Si phase,
a batch process wherein the strip is heated for one hour at
530C is completely satisfactory. Alternatively and
preferably, continuous solution heat treatment of the
intermediate gauge strip (e.g. performed on a continuous
annealing line) may be employed, a high temperature being
required in view of the short soaking time involved. For
instance, in continuous solution heat treatment a peak
metal temperature of 570C, with a very short soak period of
less than one minute, has been found adequate.
high temperature being required in view of the short
soaking time involved. For instance, in continuous
solution heat treatment a peak metal temperature of
570C, with a very short soak period of less than one
minute, has been found adequate.
To retain the My and So in solution, the metal
must be rapidly cooled to room temperature (quenched)
from the solution heat treatment temperature, viz. in
a time of no more than 60 seconds, and preferably less
than 30 seconds. If the intermediate gauge is suffix
ciently small, air quenching can be em Poe us
water quenching is necessary for heavier gauges and
is suitable lo r all gauges.
Nature Aging
- Aster solution heat treatment and quenching, and
without any intervening subsequent heat treatment, the
as-cuenched strip article at the intermediate gauge is
subjected to natural axing by standing at ambient tempera-
lure (ens. about 0 to about 40C) for at least about
~;~353~
- 15 -
one day, and preferably for a-t least about three days.
Natural aging periods in excess of three days
(regardless OX how long) are also acceptable.
The reason for performing this natural aging step, in
the process of the invention, is to attain a state
wherein the strength of the strip becomes relatively
stable owing to the formation of lattice coherent
nuclei of the ~g2Si phase.
Cold Rolling to Final Gauge
After natural aging, and again without any inter-
vexing heat treatment, the strip is subjected to
cold rolling to effect work hardening while reducing it
to the predetermined final gauge. The extent of cold
reduction in this cold rolling step, in act
cordons with the invention, is between about 25~ and
about 71%, preferably at least about 35% and, as at-
ready stated, most preferably (especially for product
lion of can body or lid stock) between about 50% and
about 71~, the intermediate gauge being selected to
provide for thy extent of cold reduction after
solution heat treatment and natural aging. As before,
the equipment and procedures employed to perform
the cold reduction may be entirely conventional for
cold rolling of aluminum alloy strip. This cold roll-
in operation after solution heat treatment produces a
strip or sheet article which is at the final gauge and
has been enhanced in strength by work hardening tithe
as-rolled final gauge sheet being in To temper), and
which has not been subjected to any applied heat
treatment following the quench from the solution heat
-treatment. Typical or exemplary final gauges are 0.013
inch for can lid stock and 0.014 inch for can body
stock, or higher gauges (e.g. 0.040 inch) for other end
products.
;
~l~3S36~
16 -
Between this step of cold rolling to final gauge
and the subsequent artificial eying step described be-
low, there is almost inevitably some further natural
aging, since in the ordinary course of commercial opera-
Tony the cold-rolled strip article is not immediately
; artificially aged but sits for some period at ambient
temperature. Such further natural aging, of whatever
duration, is not material to. the process of the in-
mention.
~ti:ficial Aging
Further in accordance with the invention, and as
a particular feature thereof, the as-rolled strip at
final gauge (usually, as noted, after some further in-
cidental natural aging) is subjected to artificial aging, for increasing the yield strength thereof, by
heating the strip to a predetermined elevated tempera-
lure for a time shorter than that required to achieve
the maximum yield strength attainable by heating the
same strip to the same temperature, and such that the
elongation of the strip aster artificial aging is
within 20% of the maximum value atonal by heating
the same strip to. the same temperature. The expression
"heating to,`' as used herein, will be understood to em-
brace both raising the strip to, and maintaining the strip at, the predetermined elevated temperature.
In this connection it may be explained that the
yield strength and % elongation (as well as other prop-
reties) of Al-Mg-Si strip artificially aged from To temper
are both dependent on time of heating to elevated them-
orator, for any given elevated temperature, in the
artificial aging step. More particularly, it has now
been found that during such heating, the % elongation
(a measure of formability) as well as the yield strength
' '
~.%353~
- 17 -
initially increases to a maximum and when declines,
although the peak elongation is achieved earlier than
the peak yield strength. Thus, by the present step ox
artificially aging the strip by heating to an elevated
temperature for a time shorter than that required to
achieve peak yield strength (in contrast to the prior
conventional practice of heating at least long enough
to achieve peak yield strength), there is provided an
advantageous combination of high strength and good
formability. the relatively short heating time of-
feats beneficial enhancement of yield strength (as
compared to the yield strength in To temper) without
undue impairment of % elongation (as compared to the
% elongation in To temper). Fully adequate enhancement
of strength for such purposes as the fabrication of
drawn-and-ironed cans can be achieved by artificial
aging for a time such that the % elongation is within
20% of the maximum value attainable upon artificially
aging the same strip at the same temperature. Indeed,
preferably in many cases, the artificial aging time can
be selected to provide an actual increase in elongation
(as compared to the % elongation of the strip in To temper,
viz. just before artificial aging) as well as a satisfac
tory enhancement ox yield strength. Other pertinent mock-
apical properties are also wound to be at suitable levels
(e.g. for can stock and other uses) in To strip after sub- -
section to this duration of artificial aging.
The relationship between aging time and yield strength
and % elongation is illustrated, for exemplary treatments,
in Figs. 2-4. These figures show properties obtained upon
artificial aging of strip of an PA 6061 alloy having the
hollowing composition: 0.26% Cut 0.26% Fe, 0.89% My,
0.04% My, 0.64% Six 0.027% Tip 0.20% Or, balance essentially
- aluminum. The strip was produced from a direct chill cast
ingot which was homogenized, hot rolled and coiled at
0.13 in. (reroll gauge), cold rolled to an intermediate
gauge of 0.046 in., and solution heat treated on a continuous
~3S3~
- 13 -
annealing line t60 seconds, 570C). Thereafter, the strip
was naturally aged at ambient temperature for at least one
day, and cold rolled to final gauges of 0.030 in. strip
sample ox Fig. 2) or 0.0135 in. (strip samples of Figs
3 and 4). The 0.030 in. final gauge strip sample was ax-
tificially aged at 160~C, different portions of the sample
being thus heated for different times; the ~.0135 in.
final gauge samples were artificially aged a-t 160C (Fig.
3) or 185C (Fig. 4), with different portions of these
lo samples again being heated for different times. The
curves shown in Figs. 2-g represent the values of the
indicated properties, measured in the transverse direction,
for the strip portions in To temper after various dip-
fervent artificial aging times. The values of properties
lo indicated at 0 aging time ("as ruler) are the values
measured for each sample in To temper, before artificial
aging. For all aging times, strengths measured in a
longitudinal direction are generally higher than the
transverse values represented in the figures, but exhibit
essentially the same dependence on heating time. These
and other properties of the samples of Figs. 2-4 are
summarized in Table I.
It will be seen that for each of the samples rep-
resented by Figs. 2-4, both yield strength and elongation
exhibit an initial increase (compared to the To temper
values) during artificial aging. As the aging (heating)
treatment continues, elongation begins to decrease, while
yield strength continues to increase for some further
period before starting to decline. In each instance, it
is possible to select a time at which the elongation is
within 20% of its maximum value and yet the yield strength
is greater than that of To temper, although this time
varies depending on such factors as artificial aging them-
portray and % cold reduction (35% in Fig. 2; 71% in jigs.
3 and 4) after solution heat treatment.
By way of specific example, for 0.013 in. gauge
strip of AA 6061 alloy work hardened by cold reduction
of 71% after solution heat treatment, an aging time of
::.
.
:
~%353~L
-- 19 --
. o
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~23S3~
- 20 -
three hours at 160C (or a shorter aging time at
a higher temperature) produces a significant in-
crease in both elongation and yield strength
(as compared to the values in To temper) as well
as achieving satisfactory levels of such other prop-
reties as Erickson Cup height and ultimate tensile
strength. More generally, the times to peak % slog
anion and peak yield strength during artificial aging
are dependent on alloy composition; efficiency of soul-
lion heat treatment, as affected by time, -temperature,
quenching rate, and prior influence from homogenization
treatment of the ingot (e.g., whether the homogenization
treatment dissolved all coarse Mg2 So); % of cold no-
diction following solution heat treatment; extent of
natural aging, and whether natural aging precedes or
follows the cold reduction after solution heat treatment;
and aging temperature. The above factors, and also the
final gauge, affect the magnitude of the peak elonga-
lion during artificial aging. Accordingly, in the practice
of the invention, the proper heating time for the artificial
aging step is determined, after selection of the foregoing
factors, by aging for different periods a series of samples
of strip for which all these factors are nerd constant,
thereby to establish the artificial aging time dependency
of the yield strength and % elongation of such strip. An
aging time at which suitable values of yield strength and
% elongation are achieved can then be immediately determined,
and employed as the artificial aging time for commercial
production of the same strip. The procedure involved in
thus establishing the aforementioned time dependency is
simple and straightforward and can readily be practiced by -
persons of ordinary skill in the art.
Conveniently, the artificial aging step of the
present process can be performed US a batch artificial
aging treatment, by heaving a coil of the strip at final
:. :
-- , .
, .
~23S3fii~
-- 21 -
gauge (and initially in To temper) to a temperature
in the range of, say, 160C for a period of 1-3 hours.
Alternatively, aging can be performed by stowing the
To strip for a much shorter time at a substantially
higher temperature, e.g., by stowing for about 10 to
about 20 minutes at about 200C. In particular in-
stances, such a stowing step may also be used to perform
some other function; for example, in the production of
can lids, the stowing of the lid stock after lacquering
10 can be performed under the just-mentioned conditions so
as to constitute the artificial aging step OX the present
process. gain, in this rapid stowing treatment, the
artificial aging step effects. an increase in yield strength
and provides a % elongation (in the artificially aged strip)
within 20~ of the maximum value attainable during stowing
at the selected temperature, such value being commonly or
preferably higher than the % elongation of the To temper
strip before stowing.
The product of the present process, after completion
of the artificial aging step, is a sheet article of Alms
So alloy in To temper, exhibiting a combination of high
strength and good formability achieved by the above described
succession of steps, in particular including the artificial
aging step performed under the specified conditions of aging
for a time less than that required to achieve peak yield
strength Such sheet may be produced in various final
gauges, for a wide variety of different end uses for which
this combination of strength and formability properties
is necessary or advantageous.
Production of Cans
- In specific and presently preferred embodiments of
the process of the invention as employed for the production
of components of cans (viz. drawn-and-ironed can bodies or
lids therefore), the final gauge of the To strip resulting
from the practice of the above-described steps is selected
~23S3~
22
to be appropriate for direct formation of can bodies
(e.g. 0.014 inch final gauge strip) or lids (e.g.
0.013 inch final gauze strip), and the artificial ago
in treatment is hollowed by a step of forming the To
strip into a one-piece can body or a can lid, in act
cordons with forming procedures now wholly conventional
for forming such bodies and lids. Ordinarily the process
in each instance (bodies and lids) will include a lacquer-
in step, followed by stowing.
It is conventional, in the case of lids, to lacquer
and stove the sheet stock from which the lid is made prior
to the lid-forming operation. The lacquer in such case may
be applied while the sheet is in To temper and as already
stated, the subsequent stowing of the lacquered sheet may
be performed under conditions (e.g. heaving for about 10-
20 minutes at about 200C) selected to effect the special
artificial aging treatment of the invention. Thereafter,
the lacquered and stowed (To temper) sheet is conventionally
wormed into can lids. Alternatively, the To temper sheet
may first be artificially aged in accordance with the in-
mention and subsequently lacquered, stowed, and formed
into lids.
In the case of can body stock, the forming
(drawing and ironing operations precede lacquering and
25 stowing, and the stock in final gauge is subjected to
the artificial aging step of the invention before be-
in wormed into can bodies, i.e. the stowing after
lacquering is a separate heat treatment performed sub-
sequent to artificial aging. Where stowing after lacquering
30 is performed as a separate treatment, it ordinarily coca-
sons some reduction in strength, but causes relatively less
.
.;"
'
~L~3~;3~
- 23
strength reduction than is caused by stowing of
AA 3004 lacquered cans.
The products of these embodiments of the present
process are, respectively, a drawn-and-ironed can body
and a can lid of Alex alloy having the beneficial
properties developed by the combination of treatments
described above. Lucite advantageously, a lid and body
of the same alloy composition are produced and assembled
to provide a can wherein both components (lid and body)
lo are constituted of a single composition as desired to
facilitate recycling and reuse of the metal.
The invention affords further important advantages,
as well, for the production of can lids and bodies. Con-
lain of these advantages will be apparent from the compare
iron of AA 6061 can stock prepared in accordance with the
present process, and conventional AA 3004 and A 5182
can body and lid stock, set forth in Table II
wherein the AA 6061 stock at To temper (which is at a
gauge of 0.013 inch) represents the product ox the
invention:
.
. .
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us o own Lo
Jo aye I_
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- 25 -
The AA 6061 stock represented in the foregoing table
was produced by successively direct chill casting and homogen-
icing an ingot, hot rolling, cold rolling to an intermediate
gauge, solution-heat-treating and quenching, natural aging
for at least one day, and cold rolling (with 50-71% reduction)
to final gauge, followed by artificial aging, as indicated for
3 hours at 160C. The lacquering treatment referred to in the
table followed artificial aging (in the case of the AA 6061
stock and in each instance involved stowing the lacquered
metal at SKYE for 10 minutes.
As is apparent from the table, the AA 6061-~r8 strip
produced by the invention has earing and Erickson values
comparable to conventional AA 3004 body stock, better bend-
ability, and yield strength 14 ski (thousands of pounds per
square inch) higher than the AA 3004 body stock before lacquer-
in; after lacquering, though yield strength falls in both
instances, the yield strength differential is even greater
(17 ski in favor of the AA 6061-T8 stock. This lacquered
strength is particularly important for can bodies as it directly
affects the pressure at which the bottom of the filled can will
buckle outwardly. Because of pasteurization after filling, a
minimum bottom buckle pressure of 90 pi is commonly required
for drawn-and-ironed can bodies. 3004-Hl9 can bodies generally
develop buckle pressures between 95 and 110 pi in one test,
6061-T8 can bodies were shown to develop bottom buckle pressures
in excess of 130 pi Thus, 6061-T8 can body stock produce
by the process of the invention may be reduced in gauge, as
compared to 3004-H19 stock, with consequent reduction in metal
cost per can, and still exceed buckle pressure requirements.
Compared to 5182-Hl9 can lid stock, lacquered 6061-T8
stock produced in accordance with the invention has higher yield
strength (7 ski higher, in the example represented by the
table), higher Erickson cup values, and the same bendability,
although the 6061-T8 stock may be slightly less formable than
5182-Hl9 stock under severe draw conditions, and the higher
'foe
- 26 -
yield strength of 6061-T8 does not provide improved
buckle pressure performance, owing to the higher work
hardening rate of the 5182 alloy, which results in a
strength equivalent to 6061 in the formed areas of the
lid which actually control buckle performance. Never-
the less, as the comparison of properties in the table
illustrates the properties exhibited by the 6061-T8
sheet are fully adequate for use as both lid and body
stock, and are generally equivalent to or better than
the properties of the conventional alloys used for lids
and bodies.
It is to be understood that the invention is not
limited to the features and embodiments hereinabove
specifically set forth but may be carried out in other
ways without departure from its spirit.
