U.S. patent application number 15/124726 was filed with the patent office on 2017-01-12 for method for the manufacturing of products with anodized high gloss surfaces from extruded profiles of al-mg-si or al-mg-si cu extrusion alloys.
This patent application is currently assigned to NORSK HYDRO ASA. The applicant listed for this patent is NORSK HYDRO ASA. Invention is credited to Oddvin REISO, Ulf TUNDAL.
Application Number | 20170009322 15/124726 |
Document ID | / |
Family ID | 54196035 |
Filed Date | 2017-01-12 |
United States Patent
Application |
20170009322 |
Kind Code |
A1 |
TUNDAL; Ulf ; et
al. |
January 12, 2017 |
METHOD FOR THE MANUFACTURING OF PRODUCTS WITH ANODIZED HIGH GLOSS
SURFACES FROM EXTRUDED PROFILES OF AL-MG-SI OR AL-MG-SI CU
EXTRUSION ALLOYS
Abstract
Method for the manufacturing of products with anodized high
gloss surfaces from extruded profiles of Al--Mg--Si or
AS-Mg--Si--Cu, where the alloys initially are cast to extrusion
billets), containing in wt. % Si: 0.25-1.00 Mg. 0.25-1.00 Fe:
0.00-0.15 Cu: 0.00-0.30 Mn: 0.00-0.20 Cr: 0.00-0.10 Zr: 0.00-0.10
Se: 0.00 -0.10 Zn: 0.00-0.10 Ti: 0.00-0.05., and including
incidental impurities and balance A.L a) where the billet is
homogenised at a holding temperature between 480.degree. C. and
620.degree. C. and soaked at this temperature for 0-12 hours, where
after the billet is subjected to cooling from the homogenisation
temperature at a rate of 150.degree. C./h or faster, b) the billet
is preheated to a temperature between 400 and 540.degree. C. and
extruded preferably to a solid shape profile and cooled rapidly
down to room temperature, c) optionally artificially ageing the
profile, d) deforming the profile more than 10% by a cold roiling
operation, whereafter e) the profile is flash annealed with a
healing time of maximum two minutes to a temperature of between
450-530.degree. C. for not more than 5 minutes and subsequently
quenched, and f) optionally the profile after flash annealing is
further subjected to a cold deforming operation to remove residual
stresses from cooling and adjusting dimensional tolerances, and g)
the profile is finally aged.
Inventors: |
TUNDAL; Ulf; (Sunndalsora,
NO) ; REISO; Oddvin; (Sunndalsora, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NORSK HYDRO ASA |
Oslo |
|
NO |
|
|
Assignee: |
NORSK HYDRO ASA
Oslo
NO
|
Family ID: |
54196035 |
Appl. No.: |
15/124726 |
Filed: |
March 24, 2015 |
PCT Filed: |
March 24, 2015 |
PCT NO: |
PCT/NO2015/000005 |
371 Date: |
September 9, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 21/007 20130101;
C22C 21/08 20130101; B21B 3/00 20130101; B22D 17/00 20130101; B21B
2003/001 20130101; C22F 1/043 20130101; C22C 21/02 20130101; C22F
1/05 20130101; B22D 27/20 20130101 |
International
Class: |
C22F 1/043 20060101
C22F001/043; B21B 3/00 20060101 B21B003/00; B22D 27/20 20060101
B22D027/20; B22D 17/00 20060101 B22D017/00; C22C 21/02 20060101
C22C021/02; B22D 21/00 20060101 B22D021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2014 |
NO |
20140383 |
Claims
1-12. (canceled)
13. Method for the manufacturing of products with anodized high
gloss surfaces from extruded profiles of Al--Mg--Si or
Al--Mg--Si--Cu alloys, where the alloys initially are cast to
extrusion billet(s), containing in wt. % Si: 0.25-1.00 Mg:
0.25-1.00 Fe: 0.00-0.15 Cu: 0.00-0.30 Mn: 0.00-0.20 Cr: 0.00-0.10
Zr: 0.00-0.10 Sc: 0.00-0.10 Zn: 0.00-0.10 Ti: 0.00-0.05, and
including incidental impurities and balance Al, a) where the billet
is homogenised at a holding temperature between 480.degree. C. and
620.degree. C. and soaked at this temperature for 0-12 hours,
whereafter the billet subjected to cooling from the homogenization
temperature at a rate of 150.degree. C./h or faster, b) the billet
is preheated to a temperature between 400 and 540.degree. C. and
extruded preferably to a solid shape profile and cooled rapidly
down to room temperature, c) optionally artificially ageing the
profile, d) deforming the profile more than 10% by cold rolling
operation, whereafter e) the profile is flash annealed with a
heating time of maximum two minutes to a temperature of between
450-530.degree. C. for not more than 5 minutes and subsequently
quenched, and f) optionally the profile after flash annealing is
further subjected to a cold deforming operation to remove residual
stresses from cooling and adjusting dimensional tolerances, and g)
the profile is finally aged.
14. Method according to claim 13, wherein the composition of the
alloys measured in wt. % lies preferably within Si: 0.35-0.6 Mg:
0.35-0.6 and the following maximum levels (wt. %) of the following
elements Fe: 0.09 Cu: 0.15 Mn: 0.06 Cr: 0.04 Zn: 0.03 Ti: 0.02, and
including incidental impurities and balance Al.
15. Method according to claim 13, wherein the composition of the
alloys measured in wt. % lies preferably within Si: 0.35-0.6 Mg:
0.35-0.6 and with the following maximum levels (wt. %) of the
following elements Fe: 0.06 Cu: 0.12 Mn: 0.06 Cr: 0.04 Zn: 0.03 Ti:
0.02, and including incidental impurities and balance Al.
16. Method according to claim 13, wherein the profile in accordance
with step c) optionally is overaged to a T7 condition at
200-230.degree. C. for 1-5 hours.
17. Method according to claim 13, wherein the profile according to
step d) is deformed more than 20%.
18. Method according to claim 13, wherein the profile according to
step d) preferably is deformed between 30 and 50%.
19. Method according to claim 13, wherein the profile is flash
annealed according to step e) with a maximum heating time of 20
seconds to a temperature of between 450-530.degree. C. for more
than 1 minute.
20. Method according to claim 13, wherein the flash anneal heating
according to step e) is obtained by induction heating of the
profile.
21. Method according to claim 13, wherein the flash anneal heating
according to step e) is obtained by subjecting the profile to a
salt bath or other convection or radiation heating means providing
high heating rates.
22. Method according to claim 13, wherein the alloys are cast
without the use of grain refiner, except in the start-up of the
casting operation.
23. Method according to claim 13, wherein the ageing, step g) is a
one step or dual rate ageing operation to a final hold temperature
between 160.degree. C. and 220.degree. C. and where the total
ageing cycle is performed in a time span of between 3 and 24
hours.
24. Method according to claim 13, wherein the optical cold
deforming after flash annealing, step f) is a rolling operation, a
stretching operation or a forging operation.
Description
[0001] The present invention relates to a method for the
manufacturing of products with anodized high gloss surfaces from
extruded profiles of Al--Mg--Si or Al--Mg--Si--Cu alloys.
[0002] The oxide layer (Al.sub.2O.sub.3) formed during anodizing is
build up by dissolving the outer layer of the aluminium. For each 3
.mu.m of oxide layer formed 2 .mu.m of the aluminium is dissolved.
Since the oxide layer is bulkier than the aluminium the total
thickness will then increase by 1 .mu.m. In order to obtain high
gloss of an anodized aluminium product it is important to keep the
amount of constituent particles with a diameter larger than
approximately 0.3 .mu.m (S. Wernick, R. Pinner and P. G. Sheasby,
The Surface Treatment and Finishing of Aluminium and its Alleys,
ASM INTERNATIONAL, FINISHING PUBLICATIONS LTD, fifth Edition Vol 1,
1987, p. 143) at a low level, since these particles will be
embedded to the anodized layer and cause a reduction in the gloss.
The most important factor to achieve this is to keep the amount of
Fe at a low level, since primary AlFeSi particles are insoluble in
the aluminium matrix. Typically, alloys used for high gloss
products have a maximum limit of Fe around 0.12 wt %. Gloss is thus
also reduced with increasing thickness of the oxide layer formed
during anodizing since more particles then will be embedded.
Moreover the process parameters used during anodizing also affect
the gloss.
[0003] Hardening precipitates are formed during the artificial
ageing process (e.g. (.beta.''-MgSi) from the addition of Mg and
Si. If Cu is added in sufficient amount other phases than .beta.''
may form (e.g. Q' and L) (Calin D. Marioara, et. al., Improving
Thermal Stability in Cu-Containing Al--Mg--Si Alloys by Precipitate
Optimization, METALLURGICAL AND MATERIALS TRANSACTIONS A, March
2014). These hardening precipitates are much smaller than 0.3 .mu.m
and are therefore net reducing the gloss in the same way as the
primary AlFeSi particles. The strength requirement for the alloy
determines the necessary amount of Mg, Si and Cu in the alloy. In
order to maximize the gloss it is necessary to process the material
in a way where precipitation of larger non-hardening phases (e.g.
.beta.'-MgSi and .beta.-Mg.sub.2Si) of Mg, Si and Cu is avoided.
This is easiest to obtain for 6060 and 6063 type of alloys where
the Mg and Si contents are relatively low. Higher alloyed material
requires higher temperatures in the extrusion or solutionsing
processes and faster cooling afterwards to avoid precipitation of
such particles.
[0004] Alloying elements such as Mn, Cr, Zr or Sc can be added to
form dispersold particles during homogenisation. Frequently, these
elements are added in high amounts in order to prevent
recrystallization in the extruded profile. However, it can be
beneficial to add these elements in smaller amounts to only have
some dispersold particles in the alloy in order to avoid grain
growth during homogenisation and after the recrystallization
process occurring in the extrusion process or in a separate
recrystallization and solutionising process for the cold deformed
material. The size of these particles is typically between 0.01-0.2
.mu.m. Thus, such particles can be added, at least in a relative
low number, without significantly affecting the gloss. However, the
number of dispersoid particles should not be so high that the
exposed areas of the profile surface get a mixture of a
non-recrystallized and a recrystallized structure or a fully
recrystallized structure with a large and uneven grain size.
Addition of elements that form dispersold particles can also give
an unwanted colour of the anodising layer, or they can give an
unwanted surface appearance due to a strong texture of the
recrystallized grains.
[0005] If an anodized surface contains large grains the individual
grains can be detected by the naked eye. This surface defect is
frequently called mottling. The best surface appearance is obtained
when the average grain size Is smaller than approximately 70 .mu.m
and the grains mainly are randomly orientated.
[0006] If the processing of the material is satisfactory there will
be no large .beta.'-MgSi or .beta.-Mg.sub.2Si particles present in
the extruded and aged profile samples. In such a case the gloss
will be more or less proportional to the amount of Fe in the alloy
for a given anodizing process. To maximize the gloss one would like
to minimize the Fe content. Reducing the Fe content will the price
of the aluminium since it will be more costly to produce. It will
require alumina with low Fe and low contribution of Fe from the
anodes. The processing in the electrolysis and the casthouse also
has to be adapted in order to produce aluminium with very few Fe
content. The main problem by using very low Fe contents is,
however, the ability to control the grain size in the billet and in
the extruded profile.
[0007] From Japanese patent publication No. 10-306336 is known an
aluminium alloy extruded material having high surface gloss after
anodic oxidation treatment where the surface gloss allegedly is
made uniform fey specifying the number of the particles of
Mg.sub.2Si participated in the matrix. This is obtained with a
specific heat treatment procedure prior to and after extrusion.
[0008] With the present invention is provided a method tor the
manufacturing of products with anodized high gloss surfaces from
extruded profiles of Al--Mg--Si or Al--Mg--Si--Cu alloys with
excellent mechanical properties and at low costs.
[0009] The method according to the invention is characterized by
the features as defined in the accompanying independent claim
1.
[0010] Further embodiments are defined in the subordinate claims
2-12.
[0011] The invention will be further described in the following by
way of examples and with reference to the drawings and figures
where:
[0012] FIG. 1 is a photo of a quarter of a macro etched billet
slice (o228 mm in diameter) with abnormal grains.
[0013] FIG. 2 is a light optical micrograph showing a typical grain
structure through the thickness of a thick solid shape extruded
profile which is anodised and viewed in polarised light
[0014] FIG. 3 is a principal sketch of an industrial processing
line for performing the cold rolling and the annealing process
described in the present invention.
[0015] FIG. 4 shows light optical micrographs of samples from
example 1 showing the grain structure in the middle of the cross
section for the as extruded profile and for the samples that were
cold rolled to give 10, 20, 40 and 60% reduction in the thickness
prior to annealing. All samples are anodised and viewed in
polarized light.
[0016] FIG. 5 shows grain structure in an as cast billet (o95 mm
diameter) without grain refiner, which was used in example 2 of the
present application. Picture of a macro etched billet slice to the
left and anodised sample viewed in polarized light in a light
optical microscope to the right.
[0017] FIG. 6 are light optical micrographs showing the AlFeSi
particles in a homogenised billet cast without grain refiner (upper
picture) and in a homogenised billet cast with grain refiner (lower
picture). The position of the samples in the billet is
approximately half radius.
[0018] FIG. 7 is alight optical micrograph of an as extruded sample
in example 2 of the application, showing the grain structure close
to the surface. Anodised and viewed in polarized light.
[0019] FIG. 8 shows light optical micrographs of samples from
example 2, showing the gram structure in the middle of the cross
section for the as extruded profile and the samples that were cold
rolled to give 20, 30, 40 and 50% reduction in the thickness poor
to annealing. All samples are anodised and viewed in polarized
light.
[0020] FIG. 9 shows further light optical micrographs of samples
from example 2 of the present application, showing the grain
structure in the middle of the cross section for samples that were
cold rolled to 40% reduction in the thickness prior to annealing in
air (upper) and in a salt bath (lower). Both samples are anodised
and viewed in polarized light.
[0021] When the Fe content is below approximately 0.10 wt % the
chance of getting abnormal grains (grains that grow and consume
other grains that were formed during casting) in the billet during
homogenisation becomes very high. Therefore a grain size of several
centimeters is very common in billets of alloys with very low
amounts of Fe. An example of abnormal grains in a homogenised
billet with low Fe content is shown in FIG. 1.
[0022] The billet grain size will probably not affect the grain
size in the extruded profile much if the extent of deformation is
high, for example when extruding thin walled hollow profiles. For
solid shapes, and especially for thick walled profiles, the billet
grain size will most likely affect the grain size in the extruded
profile. An additional challenge is that the billet temperature
needs to be rather high in order to dissolve the Mg.sub.2Si
particles, and a high billet temperature makes it more difficult to
obtain a small gram size after extrusion.
[0023] In an extruded profile one usually sees a surface layer of
mainly randomly oriented grains and typically one or a few grains
in thickness. Underneath this layer one typically finds a region of
larger grains. The thickness of this layer varies, and is usually
thicker for a thick walled solid shape profile and thicker towards
the back end of the extruded length. An example of a typical grain
structure in a cross section of a thick walled industrially
extruded profile can be seen in FIG. 2. Below the layer of larger
grains the grain structure is typically more homogeneous. The
grains in the homogeneous center region of the cross section are
predominantly aligned in one direction, with a strong cube texture.
This is often seen in s micrograph of the grain structure in the
cross section by small differences in the colour of the grains.
[0024] More and more consumer electronics like mobile phones,
tablets and lap tops are made of aluminium from extruded profiles.
If the profile surface could have been used without any machining
the grain structure in the anodised surface would probably be okay
in most cases. However, very often them is a need to machine the
extruded profile to make the shape and the dimensional tolerances
of the final product. In that case the exposed surface can consist
of grains from the coarse grain layer beneath the surface layer of
the extruded profile. Due to this the entire coarse grain layer has
to be removed before starting to machine the shape of the final
product. The thickness of the layer that has to be removed due to
coarse grains will very with the size of the profile and the
extrusion conditions and is typicality in the range of 0.2 to 1
mm.
[0025] The present invention deals with the task to get a
homogeneous grain structure with an average grain size below
approximately 70 .mu.m irrespective of the Fe content, the grain
size in the billet prior to extrusion and the extrusion
conditions.
[0026] Solid shape profiles which are blanks for consumer
electronics will be more or less flat, but could possibly have some
features in the cross section in order to save material and
machining Such profiles are therefore very well suited for cold
rolling after extrusion. By cold rolling a profile by a minimum of
10% followed by flash annealing a new recrystallization process
will take place. With sufficient deformation and a proper annealing
process the resulting grain structure will be homogeneous over the
cross section with a much more random orientation of the grains
than in the as extruded profile. The grain size will in addition to
the alley content, depend on the degree of cold deformation, the
annealing temperature, the heat up conditions and the time at the
annealing temperature. In an alloy with very few Fe and no
dispersoid particles the recrystallization will take place at a low
temperature, most likely during heating to the annealing
temperature. One issue will then be to avoid grain growth at the
annealing temperature when there are almost no particles in the
material to pin the grains.
[0027] The annealing temperature should preferably be above the
solvus temperature for Mg.sub.2Si particles in order to avoid
particles that can reduce the strength and the gloss of the
anodised material. In addition, the time at this annealing
temperature should be as short as possible in order to avoid grain
growth. Therefore, the material should be processed through
extrusion in a way that Mg.sub.2Si particles are avoided. This
means sufficiently high billet temperature in combination with a
high enough exit temperature from extrusion and fast cooling of the
profile after extrusion. With no particles in the material prior to
cold rolling and annealing there is no need for a holding time for
the material at the annealing temperature.
[0028] The consequence of annealing at temperatures below the
solvus temperature will be that Mg--Si containing precipitates
larger than approximately 0.3 .mu.m may form. These particles will
contribute to a reduction in the gloss and in the strength of the
material. The amount of this reduction will depend on the actual
time-temperature history during the flash annealing and cooling
operation and the composition of the alloy.
[0029] An industrial process to perform the cold rolling and the
annealing process could be done as shown schematically in FIG. 3.
The cold rolling station should be followed by a station for
performing fast hasting to the annealing temperature. Using
induction heating is probably the best way to do this. With enough
power and induction coils that fit the shape of the profile and
good process control, it should be possible to heat the material to
a temperature around 500.degree. C. (depending on the composition
and thereby the solvus temperature of the alloy) within a very
short time and with sufficient accuracy in temperature.
[0030] In order to avoid precipitation of Mg--Si containing
particles larger than approximately 0.3 .mu.m the profile needs to
be cooled rather rapidly down to room temperature. The reason for
this is described in a previous section. Thus, preferably according
to the present invention, the profile is flash annealed wild a
heating time of maximum two minutes to a temperature of between
450-530.degree. C. for not more than 5 minutes and subsequently
quenched.
[0031] After the annealing operation one option could be a second
cold rolling operation to remove residual stresses from the
quenching operation. An alternative to cold rolling to remove
residual stresses would be to stretch the material in way similar
to what is done after extrusion, or performing a cold forging
operation en blanks from the flash annealed and cooled
material.
[0032] Further, to obtain a more homogeneous distribution of
deformation and more accumulated energy in the material the profile
could optionally be subjected to ageing after extrusion and prior
to cold deforming. Preferably the profile could be overaged to a T7
condition, for example at 200-230.degree. C. for 1-5 hours.
[0033] After the annealing process the final ageing of the material
can for example be done with the patented dual rate ageing cycle (U
Tundal and O. Reiso, EP 1 155 161 B1) to get maximum strength with
minimum amount of alloying elements.
[0034] The invention will be further described in the following by
way of examples.
Example 1
[0035] Billets with diameter 95 mm were cast in a lab casting
facility using the Hycast hot-top gas-slip technology (as described
in EP 0 778 097 B1) and a TiB.sub.2 based grain refiner. The
composition of the alloy is shown in Table 1.
TABLE-US-00001 TABLE 1 Chemical composition of the alloy used in
example 1 Mg Si Fe Mn Cr Cu Zn Zr Ti B Al 0.354 0.539 0.110 0.001
0.001 0.001 0.002 0.001 0.012 0.002 98.95
[0036] The billets were homogenised at 575.degree. C. for 2 hours
and 13 minutes followed by cooling at a rata of approximately
400.degree. C. per hour. Extrusion of the billets was performed at
an 8 MN laboratory extrusion press with a 100 mm diameter container
to a profile with 5.times.40 mm.sup.2 cross section. The billet
preheating temperature was approximately 500.degree. C. and the
extrusion speed 20 m/min. After extrusion the profile was quenched
in water.
[0037] A 50 cm long piece from the front part of the extruded
profile was cold rolled to give 10, 20, 40 and 60% reduction in the
thickness. The samples that were cold rolled to different
thicknesses were then annealed in a salt bath which had been
preheated to 500.degree. C. A hole was drilled into each of the
samples to fit a thermocouple. The heating time to temperature was
in the range 5-10 seconds, depending on the thickness of the
sample. When a sample was put Into the salt bath a holding time of
10 seconds started when the temperature reached 490.degree. C.
After annealing the samples were quenched in water.
[0038] Prior to extrusion the billets had an even and small grain
size. The as extruded sample in FIG. 4 shows a homogeneous grain
size throughout the cross section. In this case there is no coarse
grain layer below the surface. This is maybe because the sample is
smaller then the sample shown in FIG. 2 and maybe also because it
is taken from the front part of the extruded length. It is evident
that the grains under the randomly oriented layer of grains in the
profile surface area are predominantly aligned in one direction
since the colour contrast between the grains is low.
[0039] As can be seen from the large colour contrast, the cold
rolled and annealed samples show a much more random orientation of
the grains than the as extruded sample. This confirms that these
samples are fully recrystallized after annealing. The samples that
were cold rolled to 10 and 20% reduction in thicknesses clearly
have an uneven grain structure with the largest grains in the
middle of the cross section. The samples that were cold rolled to
40 and 60% reduction m thicknesses hove an even grain structure
throughout the cross section. The grain sizes of the samples shown
in FIG. 4 (measured 250 .mu.m below the surface of the cross
sections) are shown in Table 2.
TABLE-US-00002 TABLE 2 Average grain sizes as measured 250 .mu.m
below the surface of the cross section. The as extruded grain size
is very uncertain due to the very low contrast between the
individual grains. 10% cold 20% cold 40% cold 60% cold As rolled +
rolled + rolled + rolled + extruded annealed annealed annealed
annealed ~87 .mu.m 79 .mu.m 60 .mu.m 44 .mu.m 33 .mu.m
Example 2
[0040] Billets with diameter 95 mm were cast in a lad casting
facility using the Hycast hot-top gas-slip technology without using
a grain refiner. A picture of a macro etched billet since is shown
in FIG. 5 together with a micrograph showing an anodized sample
viewed in polarized light in the light optical microscope. Towards
the surface there am some relatively lame equiaxed grains, but a
large pad of the cross section of the billet slice consists of
feather crystals. The composition of the alloy is shown in Table
3.
TABLE-US-00003 TABLE 3 Chemical composition of the alloy used in
example 2 Mg Si Fe Mn Cr Cu Zn Zr Ti B Al 0.380 0.473 0.092 0.002
0.001 0.001 0.006 0.000 0.004 0.000 99.00
[0041] The cast billets were homogenised at 575.degree. C. for 2
hours and 15 minutes followed by cooling at a rate of approximately
400.degree. C. per hour. Micrographs of the particle structure in
the billets from the two different alloys in examples 1 and 2 are
shown in FIG. 6. The material cast without grain refiner (upper
picture) shows Fe containing particles (mainly .alpha.-AlFeSi) that
are smaller and mush more evenly distributed than the Fe containing
particles (mainly .beta.-AlFeSi) in material cast with grain refine
(lower picture). In the latter case the AlFeSi particles mainly are
located at the grain boundaries. In both cases the Fe/Si ratio is
very low, which makes .beta.-AlFeSi particles very stable in the
homogenising process. A particle structure as shown in the material
cast without a grain refiner would be beneficial in avoiding
alignment of particles and possible visible dark lines in the
extruded and anodized high gloss surface.
[0042] The billets where extruded at an 8 MN laboratory extrusion
press with a 100 mm diameter container to a profile with a cross
section of 5.times.40 mm.sup.2. The billet preheating temperature
was approximately 500.degree. C. and the extrusion speed 20 m/min.
After extrusion the profile was quenched in water.
[0043] A 100 cm long piece from the back pad of the extruded
profile was cold rolled to give 20, 30, 40 and 50% reduction in the
thickness. The samples that were cold rolled to different
thicknesses were then annealed in a salt bath which had been
preheated to 500.degree. C. A hole was drilled Into each of the
samples to fit a thermocouple. When a sample was put into the salt
bath the holding time of 10 seconds started when the temperature
reached 490.degree. C. After annealing the samples were quenched in
water, in addition one sample of the material cold roiled to 40%
reduction in thickness was held 5 minutes at 500.degree. C. Yet
another sample of the material cold rolled to 40% reduction in
thickness was heated in an air circulating oven at a considerably
lower heating rate to the annealing temperature than that obtained
in a salt bath.
[0044] A micrograph of the as extruded sample is shown in FIG. 7.
It seems like some of the grains below the surface are considerably
larger than 100 .mu.m, which could give some unwanted effects in
the surface appearance. Inside the surface region the grains are
strongly aligned in one direction, which gives very little contrast
between each individual grain in the micrograph.
[0045] FIG. 8 shows micrographs of the grain structure in the as
extruded sample as well as samples that have been cold roiled 20,
30, 40 and 50% and thereafter annealed. As also seen in example 1,
one can see from the large colour contrast that the cold rolled and
annealed samples show a much more random orientation of the grains
than the as extruded sample. The samples that were cold rolled to
20% reduction in thickness clearly have an uneven grain structure
with the largest grains in the middle of the cross section. The
sample cold rolled to 30% reduction in thickness has smaller grains
and a more even grain structure, but the grains in the middle still
are somewhat larger than those towards the surfaces. The samples
that were cold rolled to 40 and 50% reduction in thicknesses have a
smaller grain size and an even grain structure throughout the cross
section. As also shown in Table 4 the grain size seems to be
similar for the samples cold rolled to 40 and 50% reduction in
thicknesses.
TABLE-US-00004 TABLE 4 Average grain sizes as measured 250 .mu.m
below the surface of the cross section. The as extruded grain size
is very uncertain due to the very low contrast between the
individual grains. 20% cold 30% cold 40% cold 50% cold As rolled +
rolled + rolled + rolled + extruded annealed annealed annealed
annealed ~88 .mu.m 101 .mu.m 95 .mu.m 52 .mu.m 57 .mu.m
[0046] The sample that was cold rolled to 40% reduction in
thickness and held at 500.degree. C. for 5 minutes did not show any
grain growth. The reason for this is probably that the number of
AlFeSi-particles is high enough to prevent grain growth. With even
lower Fe contents than 0.09 wt % a holding lime of 5 minutes at
this temperature could cause grain growth in the sample
[0047] FIG. 9 shows that the sample heated in an air-circulating
furnace (6-7 minutes heating time) has a more uneven grain
structure and a slightly larger grain size than the sample that was
rapidly heated (5-10 seconds) in a salt bath up to the
solutionising temperature. The reason for this is probably linked
to precipitation of Mg--Si particles at the grain boundaries, which
are pinning the nuclei for new grams during the heat up process. In
a sample which is slowly heated in air there is enough time for
precipitation of Mg--Si particles to prevent the nuclei tor new
grains from growing until the particles start to dissolve again,
i.e., when the sample is approaching the solves temperature of the
alloy. In this process some grains will probably start to grew
earlier than others and therefore get larger, resulting in an
uneven grain structure when the recrystallization process is
complete.
[0048] Example 2 shows that it is beneficial to heat the cold
rolled sample fast to the solutionising temperature to obtain en
even grain size and that a holding time of only 10 seconds is
sufficient to obtain a fully recrystallized grain structure.
[0049] Example 2 also shows that the final grain structure in the
blanks could be perfect for providing attractive high gloss
anodized surfaces even though the billet grain structure is
regarded as being far from optimum when it is cast without grain
refiner.
[0050] The main benefit of the present invention is a grain
structure with an even grain size end a dose to random texture
throughout the cross section o the profile irrespective of the
grain size prior to cold rolling (and thus also of the grain
structure of the billet). An extruded thick welled flat profile
will in most cases have a coarse grain layer that has to be removed
in order to obtain a smooth anodized surface with a minimum of
defects in the final product. The amount of material that would
have to be removed in the as extruded cross section is typically in
the range 7-15%.
[0051] Moreover, the cold rolling will ensure a very accurate
thickness and flatness of the profile, and for that reason
considerably reduce the need for machining. An extruded profile
will have much more variation in the thickness, typically .+-.0.1
mm.
[0052] Since the grain size in the billet end the extruded profile
is of little importance for the resulting grain size in the cold
rolled and annealed blanks there is a possibility of casting the
billets with a minimum or even completely without the use of a
grain refiner. In order to avoid centre cracks in the billets in
the startup of the cast it could be beneficial to add some grain
refiner in the first metal in cast. The grain refiner itself could
be a source for inclusions that can cause failures in the anodized
surface. Another benefit of not using a grain refiner is that the
melt cleaning with the use of ceramic foam filters will be more
effective on other type of inclusions (Nicholas Towsey, Wolfgang
Schneider and Hans-Peter Krug, A comprehensive study of ceramic
foam filtration, 7th Australasian Asian Pacific Course &
Conference, Aluminium Cast House Technology Theory & Practice,
P Whiteley and J. Grandfield (TMS: 2001)
[0053] The possibility of reducing the Fe content and still obtain
an adequate grain structure will significantly improve with the use
of the present invention. The lower Fe content can either be used
to improve the gloss, or to keep the current gloss but add a
thicker and more wear resistant oxide layer to the anodized
product. The latter will make the product more durable.
[0054] Even though there is extra cost associated with the cold
rolling and annealing process to obtain the uniform and random
grain structure, this will probably ho more than compensated for by
the savings due to reduced machining and reduced material
consumption.
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