Note: Descriptions are shown in the official language in which they were submitted.
2~9~2~
-- 1 --
TRANSLATION
D E S C R I P T I O N
"ALUMIN~M ALLOY SHEETS FOR A~TO BODY PANELS
AND METHOD OF MANUFACTURING THE SAME"
Field of the Invention
The present invention relates to an aluminum alloy
sheets for auto, particularly auto body panels, and to
a method of manufacturing the same.
Description of the Prior Art
Reduction in the weight of an auto body is
vigorously studied nowadays in an attempt to save the
fuel cost and to improve the performance of the auto.
To achieve the object, an aluminum material having a
specific gravity about one-third the specific gravity of
iron has come to be used widely in place of the conven-
tional material of steel. The aluminum material, which
is light in weight, is excellent in its corrosion
resistance, formability and capability of surface treat-
ment. In addition, the aluminum material can be repro-
duced without difficulty. Such being the situation, the
aluminum material attracts attentions as one of the most
desirable materials for auto. The aluminum material
is widely used nowadays for forming an auto body,
a wheel, a bumper, a heat exchanger, an engine, etc.
of an auto, and the range of application of the
aluminum material is being widened in the field of
auto.
209~25`
-- 2 ~
Where an aluminum alloy sheets are used for
manufacturing, for example, auto body panels, the
alloy sheets are required to be satisfactory in, for
example, formability, weldability, adhesive properties,
s and corrosion resistance and surface appearance after
coatings. In the case of using aluminum alloy sheets,
the auto body paneis are manufactured as follows, which
is substantially equal to the conventional method using
steel sheets:
~9 Forming
A coiled aluminum alloy sheet or an aluminum alloy
sheets cut into a desired size from a coiled body is
formed in a desired shape.
~ Bonding
Members of the auto are mounted to the auto
body by means of welding and/or adhering. In this step,
the aluminum-based members are transferred in combina-
tion with members formed of the conventional steel
material to the succeeding process.
~ Surface Treatment
i) Degreasing with an alkaline detergent
ii) Water rinse
iii) Surface conditioning by treatment with, for
example, colloidal titanate
iv) Chemical conversion treatment with zinc
phosphating
v) water rinse (in this step, an after-treatment
_ 3 _ 2~ 2~
with, for example, an aqueous solution of chromic acid
is applied, as desired.)
vi) Drying
~3 Coating
i) Under coating by means of electrolytic depo-
sition
ii) Intermediate coating
iii) Top coating
~ Fitting-out (each part is mounted to the skele-
tal structure)
Auto bodies are manufactured through steps 1 to 5
described above.
The aluminum alloy sheets used as a raw material
are prepared by the ordinary steps of casting, soaking,
hot rolling, cold rolling and finish annealing. The
annealing is performed during the cold rolling step in
some cases. The alloy sheets thus prepared are coiled
or cut into a predetermined size and, then, subjected to
forming.
However, the aluminum alloy sheets for auto body
panels prepared by the conventional method described
above gives rise to serious problems. First of all, the
aluminum alloy sheets prepared by the conventional
method leaves room for further improvement in
formability, compared with a steel material mainly used
nowadays as a material of an auto body. Specifically, if
severe working is applied to the aluminum alloy sheets,
_ 4 _ 2~53~
problems such as cracking and surface roughening take
place, with the result that the members of the auto
body, which can be formed by using the aluminum alloy
sheets prepared by the conventional method, are much
restricted.
A second problem is that the aluminum alloy sheets
prepared by the conventional method are not satisfactory
in its bonding strength with another member. If alumi-
num alloy sheets having a bonded portion is subjected
to a durability test under severe conditions, peeling
takes place in the bonded portion, or the bonded portion
fails to exhibit a desired bonding strength. It follows
that the aluminum alloy sheets prepared by the conven-
tional method leave room for further improvement in
safety and reliability.
A third problem is that the conventional aluminum
alloy sheets are not satisfactory in weldability.
Because of the poor weldability, the welding apparatus,
particularly the electrodes included in the apparatus,
is readily damaged when alloy sheets are welded to each
other. It follows that the welding apparatus, particu-
larly the electrodes included in the apparatus, must be
repaired frequently, leading to a low productivity.
A fourth problem is that the conventional aluminum
alloy sheets are not satisfactory in its bonding strength
with a coated film. If a durability test is applied
under severe conditions to the alloy sheets after the
209552~
-- 5
coating step, the coated film t:ends to peel off or to be
swollen so as to impair the surface appearance of the
alloy sheets.
Further, a fifth problem is that the conventional
aluminum alloy sheets are poor in its corroslon
resistance. If a durability test is applied under
severe conditions to the alloy sheets after the coating,
a filiform corrosion is brought about to impair the sur-
face appearance of the alloy sheets. If the filiform
corrosion further proceeds, the performance of the
alloy sheets are lowered.
An aluminum material is used singly for forming an
auto body in some cases or is used in combination
with a steel material in other cases. As described pre-
viously, the auto body material is treated with zincphosphate in many cases before the under coating step.
The poor bonding strength with the coated film and the
poor corrosion resistance inherent in the conventional
aluminum alloy sheets are related particularly to the
0 chemical conversion treatment with zinc phosphate.
Summary of the Invention
The present inventors have found that the various
problems inherent in the prior art are caused by an
oxide film formed on the surface of an aluminum alloy
sheets. It has also been found that the structure of an
oxide film of aluminum is greatly affected by the com-
position of the aluminum alloy. For example,
- 6 - 209~2~
an aluminum alloy sheets used for forming an auto body
contains 0.3 to 10% by weight of magnesium. The oxide
film formed on the surface of such an alloy sheets con-
tains not only aluminum oxide and/or hydroxide but also
magnesium oxide and/or hydroxicle. It has been found
that the weldability, bonding strength and corrosion
resistance of the alloy sheets are adversely affected by
the presence of magnesium oxide and/or hydroxide noted
above.
The aluminum oxide can be roughly classified into
two types, i.e., amorphous oxide (A~2O3) and crystalline
oxide. The crystalline aluminum oxide film is in the
form of various phases depending on the atmospheres
under which the oxide film is formed. The typical
phases of the crystalline aluminum oxide include, for
example, Gibbsite (y-A~(OH)3), Bayerlite (cl-A~(OH)3),
and Boehmite (y-A~OOH). The crystalline aluminum oxide
film exhibits a frictional resistance lower than that of
the amorphous aluminum oxide film and, thus, is superior
to the amorphous oxide film in lubricity. It should be
noted that these two kinds of oxides are present
together in the oxide film formed on the surface of the
aluminum alloy sheets after the rolling step. Also, the
crystalline oxide is formed when the alloy sheets is
exposed to a wet atmosphere of high temperatures.
It has been found that a crystalline oxide film is
formed in the hot rolling step in the manufacture of
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an aluminum alloy sheets. In general, an aqueous rolling
oil of 200~C or more is used in the hot rolling step,
which is considered to give rise to formation of the
crystalline oxide film. The crystalline oxide film is
broken in the subsequent cold rolling step. However,
crystalline oxide is embedded in the aluminum matrix so
as to remain within the product alloy sheet.
The present inventors have also found that the
weldability, adhesive properties, and corrosion resis-
tance of an aluminum alloy sheets are affected by thepresence of two kinds of oxides, i.e., alumi.num oxide
and magnesium oxide, contained in the oxide film
formed on the surface of the aluminum alloy sheets.
Particularly, it has been found that these properties of
the alloy sheets are markedly deteriorated in the case
where the oxide film contains a large amount of magne-
sium oxide.
An object of the present invention is to remove
magnesium oxide contained in an oxide film formed on the
surface of an aluminum alloy sheets as much as possible
to improve the properties of the alloy sheets and to pre-
vent formation of magnesium oxide even if the alloy
sheets having the magnesium oxide removing treatment
applied thereto is left to stand for a long period of
time, so as to provide aluminum alloy sheets which
permits suppressing the change with time in the proper-
ties and is suitable for forming auto body panels.
20~25
According to a first aspect of the present inven-
tion, there is provided aluminum alloy sheets for
auto bodies, comprising a metal aluminum substrate
formed of an aluminum alloy containing 2 to 10% by
weight of magnesium; an aluminum phosphate film formed
on the substrate; an aluminum oxide film formed on the
phosphate film; and, as desired, an oil film formed on
the aluminum oxide film.
Another object of the present invention is to pro-
vide a method of efficiently manufacturing aluminum
alloy sheets which permits suppressing the change with
time in the properties and is suitable for forming
auto bodies.
According to a second a,spect of the present inven-
tion, there is provided a method of manufacturing
aluminum alloy sheets for auto bodies, comprising the
steps of:
treating the surface of aluminum alloy sheets
containing 2 to 10% by weight of magnesium with an acid
having a pH value of at most 4 to remove magnesium oxide
contained in an aluminum oxide-based film formed on the
surface of the alloy sheets;
treating the surface of the alloy sheets with a
phosphate solution to form an aluminum phosphate film
between the metal aluminum substrate and the aluminum
oxide film; and, as desired,
coating the aluminum oxide film with an o.il to form
2~9~
an oil film.
Description of the Preferred Embodiments
An aluminum alloy containing 2 to 10% by weight of
magnesium is used in the present invention including,
for example, JIS A5052 alloy, J:[S A5182 alloy, JIS A5082
alloy, JIS A5083 alloy, JIS A5086 alloy, and
A~(8 wt%)-Mg alloy. It is important for the magnesium
content of the aluminum alloy to fall within a range of
between 2 and 10% by weight. If the magnesium content
is lower than 2% by weight, the alloy fails to exhibit a
sufficiently high mechanical strength. If the magnesium
content is higher than 10%, however, it is difficult to
prepare an aluminum alloy sheets.
As described previously, aluminum oxide and magne-
sium oxide are contained in an oxide film formed on the
surface of the aluminum alloy sheets manufactured by the
ordinary method. In the present invention, magnesium
oxide alone is selectively removed in the first step by
a treatment with an acid from among the mixture of alu-
minum oxide and magnesium oxide which are originally
present on the metal aluminum substrate consisting of
aluminum alloy sheets. The pH value of the acid used for
this treatment is set at at most 4. If the pH value is
4 or less, the acid permits selectively dissolving
magnesium oxide alone on the substrate surface without
dissolving the substrate metal of aluminum and aluminum
oxide on the substrate surface. It follows that it is
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possible to allow the crystalline aluminum oxide having
a high lubricity, which is formed in the hot rolling
step, to be left unremoved. The acids which can be used
in the present invention include, for example, 0.5 to
30 wt% nitric acid and 0.5 to 30 wt% sulfuric acid.
In the present invention, it is desirable to remove
magnesium oxide such that the amount of magnesium oxide
is made at most 20% by weight based on the total amount
of the oxides formed on the metal substrate surface. If
the amount of magnesium oxide is larger than 20% by
weight based on the total amount of the oxides, the
magnesium oxide film itself acts as a brittle layer
(peeling portion) in the bonding step of the sheets to
another member, leading to a low bonding strength.
Also, an electrical resistance is increased in the
welding step so as to impair the electrodes of the
welding apparatus. As a result, the fused portion
called nugget is diminished during the continuous
welding operation, resulting in failure to obtain a
desired mechanical strength. What should also be noted
is that, if the content of magnesium oxide exceeds 20%
by weight, the amount of zinc phosphate film which is
formed in the step of the pre-treatment for coating,
i.e., a treatment with a phosphate solution, is dimi-
nished. As a result, the zinc phosphate crystals are
rendered rough and large, leading to a low bonding
strength and a low corrosion resistance of the aluminum
2~95~
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alloy sheet after the coating.
After -the acid treatment for removing magnesium
oxide from the surface of the substrate, the aluminum
oxide originally present on the substrate surface mainly
remains on the metal substrate consisting of the alumi-
num alloy sheets. It is desirable for the aluminum oxide
film to have a thickness falling within a range of be-
tween 10 and 20C~. If the aluminum oxide film has a
thickness smaller than loA, the electrical resistance of
the substrate is unduly low in the spot welding step,
resulting in failure to obtain a sufficient heat genera-
tion. Thus, nuggets are not formed. If the thickness
exceeds 200A, however, the aluminum oxide film itself
acts as a brittle layer in the bonding step, leading to
a low bonding strength. Further, the electrical
resistance is too much increased in the spot welding
step, with the result that the electrodes of the welding
apparatus are impaired so as to decrease the number of
continuous welding points achieved by using the same
welding apparatus.
It is desirable for the aluminum oxide film to have
an average surface roughness Ra falling within a range
of between 0.1 and 2.5 microns and a maximum surface
roughness Rmax falling within a range of between 0.5 and
40 microns. If the average surface roughness Ra is less
than 0.1 micron and the maximum surface roughness Rmax
is less than 0.5 micron, it is difficult to hold
209~52.S
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sufficiently a lubricating oil supplied to the aluminum
alloy sheets in the forming step, leading to a low
formability of the sheet. Further, the aluminum alloy
sheet is rendered low in its adhesive properties
because the low surface roughness denotes a small bond-
ing area. On the other hand, where the average surface
roughness Ra exceeds 2.5 microns and the maximum surface
roughness Rmax exceeds 40 microns, the surface of the
coated film after the coating step is rendered poor in
its smoothness and appearance so as to decrease the com-
mercial value of the alloy sheets.
As described previously, the substrate having
magnesium oxide removed from the surface thereof by the
acid treatment is then treated with a phosphate solution
so as to form a phosphate film between the metal
substrate of the aluminum alloy sheets and the aluminum
oxide film. The thickness of the aluminum phosphate
film should be about 1 to 5~. Incidentally, a suf-
ficient effect can be obtained by the presence of the
aluminum phosphate film even if the phosphate film is in
the form of a monomolecular film. The thickness of the
aluminum phosphate film can be controlled by adjusting
appropriately the conditions such that the concentration
of the phosphate solution falls within a range of be-
tween 0.01 and 5% by weight, the temperature of thephosphate solution is 20C or more, and the treating
time is at least 2 seconds.
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A solution containing phosphate ions (or phosphate
compounds) can be used for forming the aluminum
phosphate film including, for example, a solution con-
taining at least 0.01% by wei~ht of at least one of
sodium phosphate, sodium phosphite and sodium
pyrophosphate.
The treatment with a phosphate solution makes it
possible to prevent formation of magnesium oxide even if
- the aluminum alloy sheet is allowed to stand over a long
period of time. As a result, the change with time in
the properties of the aluminum alloy sheet can be
suppressed. It should be noted that the aluminum oxide
film originally formed on the surface of the substrate
is porous. Thus, the phosphate solution passes through
the aluminum oxide film during treatment with the
phosphate solution to perform a chemical reaction with
the substrate aluminum to form a strong aluminum
phosphate film between the metal aluminum substrate and
the aluminum oxide film.
In the present invention, it is desirable to coat
further the aluminum oxide film with an oil. Although
the aluminum phosphate film serves to sufficiently pre-
vent the formation of magnesium oxide even if the alumi-
num alloy sheet is left to stand over a long period of
time ranging between the treatment with the phosphate
solution and the molding operation, the oil coating
further promotes the effect of suppressing the magnesium
2~9~52~
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oxide formation. The oil used in the present invention
includes, for example, an antirust oil in the form of
emulsion or wax. Concerning the coating amount of oil,
a sufficient effect can be expected as far as the entire
surface region is uniformly coated with the oil. In
practice, the oil is coated in an amount of at least
o.l g/m2, preferably about 1 g/m2.
In the manufacturing method of the present inven-
tion, a coil of the raw material alloy sheets is cut into
sheets of a predetermined size, followed by applying
each of the treatments described above to the cut sheets.
Alternatively, each of the treatments can be con-
tinuously applied to a coiled raw material alloy sheets.
In particular, the continuous treatment permits manufac-
turing aluminum alloy sheets with an improved efficiency
and with a high productivity.
The prominent effects produced by the present
invention are apparent from the Examples which follow.
Needless to say, the technical scope of the present
invention is not restricted at all by the following
Examples. Also, various modifications and improvements
can be achieved within the technical scope of the pre-
sent invention.
Example 1
An ingot was prepared by melting and casting JIS
A5182 alloy (A~ 0.3 wt%-Mn 4.5 wt%-Mg alloy)~ followed
by applying successively a homogenizing treatment, a hot
- 15 - 2~9~5~5
rolling treatment, a cold rolling treatment, and finish
annealing treatment to the ingot so as to obtain a sheets
having a thickness of l.o mm.
The resultant sheets was treated with a 5 wt~ nitric
acid for 10 seconds by a spraying method, followed by
washing the sheets with water so as to remove selectively
magnesium oxide contained in an oxide film formed on
the surface of the sheets. Then, the washed sheets was
dried.
In the next step, the sheets was treated with a
0.1 wt% sodium pyrophosphate solution at 40C for
300 seconds to form an aluminum phosphate film having a
thickness of 5~ between the metal aluminum substrate and
the aluminum oxide film formed on the substrate, thereby
lS to obtain an aluminum alloy sheet 1 of the present
invention.
About one week after preparation, various proper-
ties of the aluminum sheet 1 were measured, including
the total thickness of the oxide film (sum of the magne-
sium oxide film and the aluminum oxide film), the per-
centage by weight of the magnesium oxide film based on
the sum of the magnesium oxide film and the aluminum
oxide film, the formability, adhesive properties, welda-
bility, bonding strength with a coated film, and corro-
sion resistance. Table 1 shows the results. Theseproperties were also measured after the aluminum alloy
sheet 1 was left to stand for 90 days within a constant
20~25
- 16 -
temperature-humidity bath maintained at a temperature of
40C and a relative humidity of 95% so as to evaluate
the changes with time in these properties. Table 1 also
shows the results. The properties noted above were eva-
luated as follows:
(1) Total thickness of the oxide film and the per-
centage by weight of magnesium oxide contained in the
oxide film:
Determined by ESCA (Electron spectroscopy for
chemical analysis).
(2) Formability
Erichsen test A defined in JIS z2247 was applied
applied to the aluminum alloy sheets 1 to determine the
Erichsen value (mm) as the formability.
(3) Adhesive properties
The aluminum alloy sheet 1 was cut into small
pieces sized at 25 mm x 100 mm. Two of these small
pieces were bonded to each other with an epoxy series
adhesive available on the market with a lapping width
2~ set at 13 mm, followed by baking the bonded small pieces
at 70C for 30 minutes. Then, a saline water spraying
test specified in JIS Z2371 was applied to the resultant
sample for 90 days. The tensile shearing strength of
the sample was measured both before and after the saline
water spraying test, and a strength residual rate as the
adhesive properties was ca]culated by the formula given
below:
- 17 - 20~5~
R(%) = (A/B) x 100
where, R is the strength residual rate, A is the
shearing strength after the test, and B is the shearing
strength before the test.
(4) Weldability
Spot welding was applied to the sample continuously
until the electrodes of the welding apparatus were
damaged and, thus, the nugget formation was made
impossible. The welding was applied to determine the
number of nuggets which can be formed by the continuous
welding operation by the same welding apparatus.
~5) Bonding strength with a coated film
A small sheets sized at 70 mm x 150 mm was cut out
of the aluminum alloy sheets 1 of the present invention
and, then, subjected to a degreasing treatment at 45C
for 30 seconds using a weakly alkaline degreasing agent.
After rinsing of the degreased sample with water, the
surface of the sample was adjusted at room temperature
for 30 seconds with a colloidal titanium-based liquid
material, followed by applying a chemical treatment to
the sample under the surface-adjusted state with a zinc
phosphate solution available on the market. The chemi-
cal conversion treatment was performed for 2 minutes at
45C. Then, the sample was successively subjected to
rinsing with water, drying, under coating with a cation
electrolyte deposition, intermediate coating by blowing,
and top coating.
2~9~rj2~
- 18 -
The resultant sample was kept immersed in warm
water of 50C for 20 days, followed by applying a
cross cut adhesion test. Specifically, a peeling test
using a tape was applied to the sample in the form of a
checkerboard having 100 meshes each sized 2 mm x 2 mm.
The number of residual meshes which were not peeled off
was indicated in Table 1 together with the number of
test pieces (100).
(5) Corrosion resistance
A sample was prepared as in the bonding strength
test with a coated film. Then, a cross-cut (i.e., a
mark X) was applied to the surface of the sample such
that the cross-cut reached the aluminum alloy sheet.
Under this condition, a salt water spraying test spe-
cified in JIS Z2371 was applied to the sample for
24 hours, followed by allowing the sample to stand in a
wet atmosphere for 2,000 hours at a temperature of 50C
and a relative humidity of 95% so as to measure the
maximum length of a filiform corrosion extending from
the cross-cut portion.
Ex ample 2
An aluminum alloy sheets 2 of the present invention
was obtained substantially as in Example 1, except that
an aluminum phosphate film having a thickness of 2A was
formed in Example 2 between the metal aluminum substrate
and the aluminum oxide film by the treatment with a
0.05 wt% sodium phosphate solution for 10 seconds at
2~95~2~
- 19 -
90C, though an aluminum phosphate film having a
thickness of 5A was formed in Example 1 by the treatment
with a 0.1 wt% sodium pyrophosphate solution for
300 seconds at 40~C.
various properties of the aluminum alloy sheets 2
were measured as in Example 1, including the total
thickness of the oxide film, the percentage by weight of
the magnesium oxide film based on the sum of the magne-
sium oxide film and the aluminum oxide film, the
formability, adhesive property, weldability, bonding
strength with a coated film, and corrosion resistance.
Table 1 also shows the results together with the changes
with time in these properties.
Example 3
An aluminum alloy sheets 3 of the present invention
was obtained substantially as in Example 1, except that
an aluminum phosphate film having a thickness of 5~ was
formed in Example 3 between the metal aluminum substrate
and the titanium oxide film by the treatment with a
3 wt% sodium pyrophosphate solution for 120 seconds at
50C, though an aluminum phosphate film having a
thickness of 5~ was formed in Example 1 by the treatment
with a 0.1 wt% sodium pyrophosphate solution for
300 seconds at 40~C. Further, the aluminum oxide film
(the uppermost film) was coated with an emulsion type
antirust oil having a viscosity of 3 cSt in a coating
amount of 1 g/m2 in Example 3.
2~95525
- 20 -
various properties of the aluminum alloy sheets 3
were measured as in Example 1, including the total
thickness of the oxide film, the percentage by weight of
the magnesium oxide film based on the sum of the magne-
sium oxide film and the aluminum oxide film, theformability, adhesive property, weldability, bonding
strength with a coated film, and corrosion resistance.
Table 1 also shows the results together with the changes
with time in these properties.
Prior Art 1
An ingot was prepared by melting and casting JIS
A5182 alloy, followed by applying successively a homoge-
nizing treatment, a hot rolling treatment, a cold
rolling treatment, and finish annealing treatment to the
ingot so as to obtain a sheets having a thickness of
1.0 mm.
The resultant sheets was treated with a 5 wt% nitric
acid for 10 seconds by a spraying method, followed by
rinsing the sheets withn water so as to remove selectively
magnesium oxide contained in an oxide film formed on
the surface of the sheets. Then, the rinsed sheets was
dried.
In the next step, the residual aluminum oxide film
was coated with an emulsion type antirust oil having a
viscosity of 5 cSt in an amount of 1 g/m2 so as to
obtain an aluminum alloy sheets (prior art 1).
various properties of the aluminum alloy sheets
~9552~
- 21 -
(prior art 1) were measured as in Example l, including
the total thickness of the oxide film, the percentage by
weight of the magnesium oxide film based on the sum of
the magnesium oxide film and the aluminum oxide film,
the formability, adhesive property, weldability,
bonding strength with a coated film, and corrosion
resistance. Table l also shows the results together
with the changes with time in these properties.
Prior Art 2
An aluminum alloy sheets (prior art 2) was prepared
as in prior art l, except that a wax-type antirust oil
was in place of the emulsion type antirust oil having a
viscosity of 5 cSt, which was used in Prior Art l.
Various properties of the aluminum alloy sheets
(prior art 2) were measured as in Example l, including
the total thickness of the oxide film, the percentage by
weight of the magnesium oxide film based on the sum of
the magnesium oxide film and the aluminum oxide film,
the formability, adhesive property, weldability,
bonding strength with a coated film, and corrosion
resistance. Table l also shows the results together
with the changes with time in these properties.
Prior Art 3
An ingot was prepared by melting and casting JIS
A5182 alloy, followed by applying successively a homoge-
nizing treatment, a hot rolling treatment, a cold
rolling treatment, and finish annealing treatment to the
- 2~ - 2~ 2.~
ingot so as to obtain a sheets having a thickness of
1.0 mm.
The resultant sheets was treated with a 5 wt% nitric
acid for 10 seconds by a spraying method, followed by
washing the sheets with water so as to remove selectively
magnesium oxide contained in an oxide film formed on
the surface of the sheets. Then, the washed sheets was
dried so as to obtain an aluminum alloy sheets (prior art
3).
Various properties of the aluminum alloy sheets
(prior art 3) were measured as in Example 1, including
the total thickness of the oxide film, the percentage by
weight of the magnesium oxide film based on the sum of
the magnesium oxide film and the aluminum oxide film,
the formability, adhesive property, weldability,
bonding strength with a coated film, and corrosion
resistance. Table 1 also shows the results together
with the changes with time in these properties.
2 ~ 2 5
-- 23 -
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~ o a~ ~ ~ ~ ~ ~ ~ c ~ oO
Z~3 OC X O
3~1
A ~ a~ ~
._1 1:~ ~ tD Il') O ,_1 Lrl X O C O
a~ : , a~ cc~ OD ~ O O C1~ S~
E~ C ~ h ~
^ _ --1 ~ O ~ h
~o\ ~D 3 a),¢~
~c a~ ~ ~D ~D n ~D I_ U~
h ~ ~ ~ a~ a~ ~ Cl~ (1~ t~ X ~
:~:~ o~m
u~ ~
: :
~, ~ ~ ~. .
C I ~ ~c
a) C h
U~ ~ C O
~ ~ O -~
h C-~ h h
H ~ ~ ~
- 25 - 2 ~ 2 ~
As apparent from Table l, the aluminum alloy sheets
of the present invention is small in the changes with
time in properties. On the other hand, the thickness of
the oxide film was increased in the prior art aluminum
alloy sheets after the sheets was allowed to stand in a
humid atmosphere of a high temperature for a long time.
In other words, the properties of the aluminum alloy
sheets were greatly changed with time in the prior art.
As described above in detail, the present invention
provides aluminum alloy sheets for auto body panels.
In the present invention, magnesium oxide is removed
from the natural oxide film formed on the surface of the
alloy sheets, followed by forming an aluminum phosphate
film between the metal aluminum substrate and the alumi-
num oxide film. Further, an oil film is formed asrequired on the uppermost layer of the aluminum oxide
film. The particular construction of the present inven-
tion makes it possible to improve the characteristics
including the formability, adhesive property, and welda-
bility of the aluminum alloy sheets. In addition, for-
mation of magnesium oxide can be markedly suppressed
over a long period after manufacture of the alloy sheets.
In other words, the aluminum alloy sheets of the present
invention permits suppressing the changes with time in
the characteristics thereof. ~hat should also be noted
is that the aluminum alloy sheets for auto bodies pro-
vided by the present invention permits promoting the
- 26 - 209~2~
forming rate of a chemical conversion film during the
chemical treatment in the coating step so as to suppress
the elusion of aluminum ions into the coating solution.
As a result, a chemical conversion film can be formed
uniformly on the aluminum alloy sheet, leading to a high
bonding strength of the alloy sheets with a coating film
and to a high corrosion resistance of the alloy sheets.
The present invention also provides a method of
manufacturing an aluminum alloy sheets for auto bodies,
which makes it possible to manufacture efficiently an
aluminum alloy sheets which permits suppressing the
changes w:ith time in the characteristics of the alloy
sheets.
