Note: Descriptions are shown in the official language in which they were submitted.
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HEAT TREATMENT OF FORMED ALLTIvIlNUM ALLOY PRODUCTS
TECHNICAL FIELD
This invention relates to a heat treatment process for shaped articles,
particularly
those suitable for use in the fabrication of automotive body panels. More
particularly,
the invention relates to such articles made from aluminum alloy sheet material
that
exhibits an improvement of hardness after painting and baking operations have
been
carried out.
BACKGROUND ART
Aluminum alloy sheet is being used more extensively nowadays as a structural
and closure sheet material for vehicle bodies as automobile manufacturers
strive for
improved fuel economy by reducing vehicle weight. Traditionally, aluminum
alloy is
either direct chill cast to form ingots or continuous cast in the form of a
thick strip
material, and then hot rolled to a preliminary thickness. In a separate
operation, the strip
is cold rolled to the final thickness and wound into coil. The coil must then
undergo
solution heat treatment to allow strengthening of the formed panel during
painting and
baking (steps usually carried out on shaped automotive parts by vehicle
manufacturers or
others - also referred to as the paint bake or paint cure).
Several aluminum alloys of the AA (Aluminum Association) 2000 and 6000
series are usually considered for automotive panel applications. The AA6000
series
2 0 alloys contain magnesium and silicon, both with and without copper but,
depending upon
the Cu content, may be classified as AA2000 series alloys. These alloys are
formable in
the T4 or T4P temper conditions and become stronger after painting and baking.
Good
increases in strength after painting and baking are highly desirable so that
thinner and
therefore lighter panels may be employed.
2 5 It is highly desirable that the alloy sheet, when delivered to the
manufacturer, be
relatively easily deformable so that it can be stamped or formed into products
of the
required shapes without difficulty and without excessive springback. However,
it is also
desirable that the products, once formed and subjected to the normal painting
and baking
procedure, be relatively hard so that thin sheet can be employed and still
provide good
3 0 dent resistance.
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To facilitate understanding, a brief explanation of the terminology used to
describe alloy tempers may be in order at this stage. The temper referred to
as T4 is well
known (see, for example, Aluminum Standards and Data (1984), page 11,
published by
The Aluminum Association) and refers to alloy produced in the conventional
manner. i.e.
without intermediate batch annealing and pre-aging. This is the temper in
which
automotive sheet panels are normally delivered to parts manufacturers for
forming into
skin panels and the like. Material that has undergone an intermediate batch
annealing,
but no pre-aging, is said to have a T4A temper. An alloy that has only been
solution
heat-treated and artificially aged to peak strength is said to be in the T6
temper.
Material that has undergone pre-aging but not intermediate batch annealing is
said to
have a T4P temper, and material that has undergone both intermediate annealing
and
pre-aging is said to have a T4PA temper. T8 temper designates an alloy that
has been
solution heat-treated, cold worked and then artificially aged. Artificial
aging involves
holding the alloy at elevated temperatures) over a period of time. T8X temper
refers to
a T8 temper material that has been deformed in tension by 2% followed by a 30
minute
treatment at 177°C to represent the forming plus paint baking treatment
typically
experienced by formed automotive panels.
An objective has been to provide a good "paint bake response", i.e. a
significant
difference in hardness between the T4/T4P temper and the final T8X temper.
2 0 In the past, attention has been directed to steps carried out on the alloy
sheets
before the step of shaping the alloy sheets into products. For example, in US
patent
5,728,241 issued on March 17, 1998 to Gupta et al., assigned to Alcan
International
Limited, a process of producing aluminum sheet of the 6000 series is described
having
T4 and T8X tempers that are desirable for the production of automotive parts.
The
aluminum alloy sheet material is subjected before shaping to solution heat
treatment and
quenching and then, before substantial age hardening has taken place, the
sheet material
is subjected to one or more heat treatments involving heating the material to
a peak
temperature in the range of 100 to 300°C, holding the peak temperature
for a period of
time of less than one minute and then cooling the sheet material.
~.~~"s~"~ ~ u~:
3
Similarly, in US patent 5,616,189 issued on April l, 1997 to Jin et al.,
assigned to Alcan International Limited, a process is disclosed that involves
subjecting a sheet product, after cold rolling, to a solutionizing treatment
{heating
to 500 to 570°C) followed by a quenching or cooling process involving
carefully
S controlled cooling steps to bring about a degree of "pre-aging". This
procedure
results in the formation of fine stable precipitate clusters that promote a
fine, well
dispersed precipitate structure during the paintlbake procedure to which
automotive panels are subjected, and consequently a relatively high T8X
temper.
While such approaches have met with success, they require modification
of the traditional process for forming aluminum alloy sheet in strip form.
This is
inconvenient and may require expensive modification of existing fabrication
equipment. Moreover, the disclosed processes involve rather careful
temperature
control that can be diffcult or expensive to achieve.
It would be more convenient to be able to treat products made of
aluminum alloy sheet in some way after they have been formed into desired
shapes. This is convenient because such products must anyway be handled and
prepared for painting and baking, so additional steps at this point are easily
arranged.
p~ C'-1_.OSL1RR OF THE INVENTION
An object of the invention is to provide a process of producing a shaped
article of enhanced hardness response without modification of a conventional
procedure for produced aluminum sheet material in T4 or T4P temper.
Another object of the present invention is to provide a solution heat treated
aluminum alloy product that exhibits a good hardness response during shaped
article formation and finishing.
Yet another object of the invention is to produce a formed product from an
aluminum alloy sheet material that has a low yield strength in T4 temper and a
high yield strength in T8X temper.
According to one aspect of the invention, there is provided a process of
producing a painted shaped article, involving obtaining a sheet article made
of an
aluminum alloy of the 2000 or 6000 series in a T4 or T4P temper; shaping the
article by bending or stamping the article to form a non-planar shaped
article;
applying paint to the shaped article to form a painted shaped article; and if
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necessary to further enhance hardness of the painted shaped article andlor to
cure
the applied paint, baking the article at a temperature of at least
177°C;
characterized in that the shaped article is subjected to a thermal spiking
treatment
before painting involving heating the shaped article temporarily to a peak
temperature in the range of 150°C to 300°C.
The term "thermal spike treatment" means a step in which the article is
quickly raised in temperature from ambient (or other temperature at which the
sheet material may be heated on the part treatment line) to a predetermined
maximum temperature and is then quickly cooled or allowed to cool with or
without providing a holding period at the peak temperature.
The term "shaped article" includes any article obtained from sheet material
for use in fabricating an article or component. The term refers to a non-
planar
article produced by a bending or stamping step, e.g. for the production of an
automobile fender or door. The term does not include unformed or uncut sheet
material of indefinite length, e.g. coiled sheet produced directly from ingots
or
cast strip.
The present invention may be carried out with any precipitation hardening
aluminum alloy of the AA2000 or AA6000 series, i.e. alloys containing Al-Mg-Si
or Al-Mg-Si-Cu that are capable of exhibiting an age hardening response.
The invention also relates to a painted and shaped sheet article produced
by the above process.
While it has been usual in the past to refer to the desired increase in
hardness as the "paint bake response", this term is becoming somewhat less
appropriate as fabrication procedures advance. What is important is that this
increase in hardness (the hardness response) occur between the shaping step
(cutting/forming/stamping) initially carried out on the sheet form of the
shaped
product, and the finishing of the shaped product for delivery to the
automobile
manufacturer or the like. In modem processes, there may not be a traditional
paint bake step as paints of lower setting temperature may be employed. In the
present application, the term "hardness response" will consequently be used
instead of the more conventional term "paint bake response". This term refers
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to the change in tensile properties of the material at the end of a finishing
process
involving painting and optionally baking, compared to the properties prior to
shaping. In
the present invention, this increase may occur partially or fully during
painting and
baking, or partially or fully before such painting and baking, i.e. during the
heat spike
5 treatment itself, as will be explained more fully below.
The advantages of the invention, at least in preferred forms, include the
following:
(1) The thermally spiked sheet material parts (e.;. automotive panels) acquire
higher strength than those panels which have not been thermally spiked.
(2) In some forms of the invention, the maximum hardness response in the
formed part can be obtained through a thermal spiking alone without relying on
the paint
cure process (or without providing a paint cure at all).
(3) The thermal spiking process, at least in some forms of the invention, can
be
performed on a continuous basis in ovens typically used for paint cure
processes. The
process therefore may be integrated seamlessly into the conventional shaping
and
finishing processes of parts formation, thus leading to convenience,
efficiency and
economy.
(4) The process provides an alternative possibility to acquire strengths
higher
than those obtained from the T4P material.
2 0 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is graph illustrating a typical thermal spike treatment in accordance
with
the invention;
Fig. 2 is a graph as explained in the Examples below, showing the variation in
yield strength (YS) of conventional AA6111-T4 with (a) prestrain; and (b)
prestrain plus
2 5 '/~ hour at 177°C; and
Fig. 3 is a graph as explained in the Examples below, showing the variation in
yield strength (YS) of conventional AA6111, heat treated according to one form
of the
present invention, with (a) prestrain; and (b) prestrain plus '/~ hour at
177°C.
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BEST MODES FOR CARRYING OUT THE INVENTION
According to the present invention, at least in its preferred forms, in order
to
improve the hardness response of AA2000 or AA6000 series automotive alloy
sheet in
the T4iT4P temper, an article created from the sheet is subjected to a thermal
spike
treatment at a temperature in the range of 150-300°C after shaping
(e.g.
cutting/forming/stamping). The treatment may either involve a thermal spike
confined to
the lower part of the temperature range (e.g. 150-225°C), which then
relies on hardening
from a subsequent paint bake step, or may involve a thermal spike into the
upper part of
the temperature range (e.g. 225-300°C), which does not require
additional hardening
from a paint bake step (baking to the conventional temperature range may then
be
avoided, if desired, although conventional painting and baking is not
harmful). This
latter form of the invention is of special interest because, in the future as
new paints are
developed, paint bake temperatures are expected to fall below 160°C, a
temperature at
which hardening effects occur too slowly to fully strengthen the shaped
product during
normal curing times.
Conventional 6XXX materials in T4 or T4P tempers contain large number of fine
metastable clusters and zones uniformly distributed throughout a metal matrix.
In the
conventional process, during the paint cure, some fine unstable clusters/zones
re-dissolve
in the metal matrix, while other improve the material strength due to age
hardening. The
2 0 process of the present invention allows the alloy material to exhibit an
enhanced aging
response (hardness response), although the exact mechanism is not clear.
Without
wishing to be bound to a particular theory, it is believed that thermal
spiking between
150 and 225°C dissolves some of the clusters and zones and increases
the solute super-
saturation of the matrix of the formed part. Consequently, the formed part
softens
2 5 slightly, but the hardness response during subsequent painting and baking
is improved in
comparison with the conventional material. It should be noted that the formed
part does
not soften when the thermal spiking treatment is carried out at higher spiking
temperatures. This is largely due to the fact that the enhanced aging process
masks the
softening caused by the cluster dissolution. Surprisingly, the dislocations
produced
3 0 during part forming do not interfere with the precipitation process as
normally expected.
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This observation allows the thermally spiked panels to acquire the desired
enhanced
strength during the paint cure.
To achieve the desired hardness response, thermal spiking to temperatures in
the
lower part of the range (e.g. 150 to 225°C) may be carried out at
relatively slow heating
rates (e.g. about 1 to 70°C/minute), especially if the article is not
held at the peak
temperature for any time and is merely allowed to cool (or is forcefully
cooled) as soon
as the peak temperature is achieved. The relatively slow heating rate is often
found to be
necessary to improve the subsequent paint bake response; i.e. the desired
improvement
in hardness will often not materialize if the heating rate is any higher. As a
consequence,
the heating to the peak temperature in this form of the invention may take too
long for
the step to be incorporated into a continuous stamping and painting line. A
batch
treatment is therefore required.
If the thermal spiking extends into the upper temperature region (e.g. above
225°C), the heating rate may be quite rapid (e.g. 10 to
280°C/minute), even if there is
essentially no holding time at the peak temperature. It is found that the
desired increase
in hardness will occur whether the heating rate is in the lower part or the
higher part of
the range indicated above, but for the process to be incorporated into a
continuous
stamping and painting/baking line, the peak metal temperature (PMT) must
generally be
reached within about one minute. If the lowest ambient temperature likely to
be
2 0 encountered is 1 S°C, the effective range for a continuous
operation would likely be 210
to 285°C/minute, which is the preferred heating rate for the high
temperature thermal
spiking treatment.
The period of time for which the temperature is maintained at the peak thermal
spike temperature may range from zero to any time that is practical in the
circumstances.
2 5 From the metallurgical point of view, the longer the time at which the
temperature is
maintained, the better it is for achieving a desirable hardness response. In
practice the
period is usually from zero up to about 5 minutes.
Fig. 1 is a graphic representation of a preferred thermal spiking step showing
the
preferred PMT range, the overall heating rate range and the preferred time
range at
3 0 PMT.
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The invention is illustrated by the following Examples, which are not intended
to
be limiting.
EXAMPLE I
The invention was tested using a commercially produced AA61 I I material.
DC ingot 600 x 1600 mm double length of the AA611 I alloy containing
0.72°~0
Cu, 0.7% Mg, 0.6% Si, 0.25% Fe, 0.20% Mn and 0.06% Cr was cast on a commercial
scale. The ingots were scalped 12.5 mm per rolling face, fully homogenized,
hot rolled
and cold rolled to the final 0.93mm gauge, fully solutionized, rapidly cooled,
naturally
aged for > 48 hours and sampled for laboratory evaluation.
The paint bake response of the material was evaluated after subjecting it to a
heat
treatment according to the invention. Tensile samples were pre-strained by
different
amounts to simulate a typical forming operation, thermally spiked in a sand
bed furnace
at 240°C and aged for 30 minutes at 177°C. The results are
summarized in Table I
below.
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Table I
Tensile Properties of the Samples, with and without
Uni-Axial Pre-Strains, Thermally Spiked at 240°C in a Laboratory
Furnace
Tensile
Properties
After
Simulated
Paint
Cure
YS
@
(1/2
h
@
177
C)
.-. (%)
Pre-Strain
0
Inventive,ConventionalaterialInven tive erial
M Mat
~ o After .~ ~ o ~
_ s
'~' ' ikin
at i N
S ~
o w
~ g YS UTS YS UTS i
p ,
240C (MPa) (MPa) % (MPa) (MPa) %E1 ~
El
U o U
I
0 145 103 i 176 299 24.2 200 312 21.3 ~ 13.6
i
2 189 151 219 306 22.2 250 324 19.2 i 14.2
228 189 i 253 318 19.9 281 334 16.8 ~ 11.0
265 222 287 334 17.5 302 342 15.4 5.2
The variation in yield strength (YS) of the pre-strained and artificially aged
(1/2
hour at 177°C) material for both conventional and the inventive process
are plotted in
Figures 2 and 3, respectively, of the accompanying drawings.
Figure 2 shows that the paint bake response of the AA611 I -T4 material
5 increased about 30 MPa due to aging for 30 minutes at 177°C
(simulated paint cure). A
similar response is observed in pre-strained material, although the net yield
strength (YS)
in the 5 and 10% pre-strained product is slightly lower due to recovery. The
yield
strength (YS) of the thermally spiked material decreases about 40 MPa for all
levels of
pre-strain, although the paint bake response is about 90 MPa, which is greater
than their
10 conventional counterparts (compare Figures 2 and 3). The 10% pre-strained
material
shows slightly less paint bake response, which is related to the loss of
strength due to
recovery. In general, it is clear from Figures 2 and 3 that the inventive
process improves
the paint bake response of the material, with and without prior pre-strain,
quite
considerably. This means that the process can be used to heat-treat the formed
part
according to the invention and enhanced paint cure strength could be achieved.
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EXAMPLE 2
The tensile properties of the samples sheared from three different locations
of a
hood, formed from a T4P temper material, were determined in the as-received
and
5 artificially aged conditions. Table 2 lists the results of the tests carried
out in variety of
conditions.
Table 2
Yield Strength (MPa) of a Hood Outer
at Different Locations Before and After Aging at Different Temperatures
Samples
Near
Center
Line
Cut
(Longitudinal)
i
As
0 Formed
Plus
Aging
None 30 30 30
V min min min
O ;a; ia; ~a177C
140C 1~0C
v _.
-a ActualActualExpectedActualExpectedActualExpected
Front 219 231 2~2 236 263 -- 297
Middle 218 230 248 236 262 -- 296
Rear 219 230 249 236 263 -- 296
Driver
Side
Middle
(Transverse)
Front 226 -- -- -- -- 277 304
Middle -- -- -- -- -- 270 292
Rear -- -- -- -- -- 263 28~
It can be seen that the aging response of the hood material is about 20 MPa
lower than expected from the laboratory simulation experiments in all aging
conditions.
10 Table 3 compares the properties of the hood material with those subjected
to thermal
spiking at 240°C according to the inventive process.
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Table 3
Mechanical Properties of a Hood Outer
and the Effect of Thermal Spiking
Driver
Side
(Transverse
Direction)
As
Formed
+
PMT
;a;
As As
Formed Formed
+
1/2h
~'a,
;
240C
+'/
h
:a,
'
o i
177
177C
~
i
U
O
Thick% YS UTS YS UTS I~
YS
%E1 %E1 %E1
I
mm Red MPa MPa MPa MPa MPa MPa
Middle 0.97 3.0 218 309 19 267 348 18 281 3~2 16
It is clear that the strength of the thermally spiked material after aging 30
for
minutes at 177°C is about 14 MPa higher than its conventional formed
and aged
counterpart.
