U.S. patent application number 11/489941 was filed with the patent office on 2007-01-25 for aluminum alloy extruded product exhibiting excellent surface properties, method of manufacturing the same, heat exchanger multi-port tube, and method of manufacturing heat exchanger including the multi-port tube.
Invention is credited to Yoshiharu Hasegawa, Tatsuya Hikida, Masaaki Kawakubo, Tomohiko Nakamura, Naoki Yamashita.
Application Number | 20070017605 11/489941 |
Document ID | / |
Family ID | 37188799 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070017605 |
Kind Code |
A1 |
Nakamura; Tomohiko ; et
al. |
January 25, 2007 |
Aluminum alloy extruded product exhibiting excellent surface
properties, method of manufacturing the same, heat exchanger
multi-port tube, and method of manufacturing heat exchanger
including the multi-port tube
Abstract
An aluminum alloy extruded product exhibiting excellent surface
properties, comprising 0.8 to 1.6% of Mn and 0.4 to 0.8% of Si at a
ratio of Mn content to Si content (Mn %/Si %) of 0.7 to 2.4, with
the balance being Al and inevitable impurities, the number of
intermetallic compounds with a diameter (circle equivalent
diameter) of 0.1 to 0.9 .mu.m dispersed in a matrix being
2.times.10.sup.5 or more per square millimeter. The aluminum alloy
extruded product allows extrusion of a thin multi-port tube at a
high limiting extrusion rate, prevents deposits from adhering to
the surface of the extruded tube, and may be suitably used as a
constituent member for an aluminum alloy automotive heat
exchanger.
Inventors: |
Nakamura; Tomohiko; (Obu
City, JP) ; Kawakubo; Masaaki; (Chita City, JP)
; Hasegawa; Yoshiharu; (Obu City, JP) ; Yamashita;
Naoki; (Nagoya City, JP) ; Hikida; Tatsuya;
(Nagoya City, JP) |
Correspondence
Address: |
FLYNN THIEL BOUTELL & TANIS, P.C.
2026 RAMBLING ROAD
KALAMAZOO
MI
49008-1631
US
|
Family ID: |
37188799 |
Appl. No.: |
11/489941 |
Filed: |
July 20, 2006 |
Current U.S.
Class: |
148/550 ;
148/437; 420/548 |
Current CPC
Class: |
F28F 2255/16 20130101;
C22F 1/08 20130101; F28F 1/022 20130101; F28F 21/084 20130101 |
Class at
Publication: |
148/550 ;
148/437; 420/548 |
International
Class: |
C22C 21/02 20060101
C22C021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2005 |
JP |
2005-212069 |
Claims
1. An aluminum alloy extruded product exhibiting excellent surface
properties, comprising 0.8 to 1.6% (mass %; hereinafter the same)
of Mn and 0.4 to 0.8% of Si at a ratio of Mn content to Si content
(Mn %/Si %) of 0.7 to 2.4, with the balance being Al and inevitable
impurities, the number of intermetallic compounds with a diameter
(circle equivalent diameter; hereinafter the same) of 0.1 to 0.9
.mu.m dispersed in a matrix being 2.times.10.sup.5 or more per
square millimeter.
2. The aluminum alloy extruded product according to claim 1,
further comprising 0.05% or less of Cu.
3. The aluminum alloy extruded product according to claim 1,
further comprising 0.2% or less of Mg.
4. The aluminum alloy extruded product according to claim 1,
further comprising 0.30% or less of Ti.
5. A heat exchanger multi-port tube comprising the aluminum alloy
extruded product according to claim 1.
6. A method of manufacturing an aluminum alloy extruded product
exhibiting excellent surface properties, the method comprising:
melting and casting an aluminum alloy having the composition
according to claim 1 to obtain an ingot; subjecting the ingot to
homogenization which includes a first-stage heat treatment in which
the ingot is maintained at 550 to 650.degree. C. for two hours or
more and a second-stage heat treatment in which the ingot is cooled
to 400 to 500.degree. C. at an average temperature decrease rate of
20 to 60.degree. C./h and maintained at that temperature for three
hours or more; heating the ingot at 480 to 560.degree. C.; and
extruding the ingot.
7. A method of manufacturing an aluminum alloy extruded product
exhibiting excellent surface properties, the method comprising:
melting and casting an aluminum alloy having the composition
according to claim 1 to obtain an ingot; subjecting the ingot to
homogenization which includes a first-stage heat treatment in which
the ingot is maintained at 550 to 650.degree. C. for two hours or
more and a second-stage heat treatment in which the ingot is cooled
to room temperature, heated to 400 to 500.degree. C. at an average
temperature increase rate of 20 to 60.degree. C./h, and maintained
at that temperature for three hours or more; heating the ingot at
480 to 560.degree. C.; and extruding the ingot.
8. A method of manufacturing a heat exchanger comprising extruding
a heat exchanger multi-port tube using the method according to
claim 6, and joining the multi-port tube to a heat exchanger by
brazing.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an aluminum alloy extruded
product exhibiting excellent surface properties, a method of
manufacturing the same, a heat exchanger multi-port tube, and a
method of manufacturing a heat exchanger including the multi-port
tube.
[0003] 2. Description of Related Art
[0004] As a constituent member for an automotive heat exchanger
such as an evaporator and a condenser, an aluminum alloy which has
a reduced weight and exhibits excellent thermal conductivity has
been generally used. In the manufacture of automotive heat
exchangers, an aluminum alloy tube (hereinafter called "tube"),
such as an aluminum alloy extruded flat multi-port tube
(hereinafter called "multi-port tube") having a plurality of hollow
portions divided by a plurality of partitions, is used as the
material for a working fluid passage. After applying a
fluoride-type flux to the surface of the multi-port tube, the
multi-port tube and other members such as a fin material are
assembled into a specific structure and joined by brazing in a
heating furnace containing inert gas.
[0005] In recent years, in order to reduce the fuel consumption of
automobiles from the viewpoint of reducing the environmental
impact, the weight of heat exchangers has been reduced. Along with
this trend, the thickness of the tube has been increasingly
reduced. An attempt has also been made to reduce the
cross-sectional area of the tube. In this case, since the extrusion
ratio (cross-sectional area of container/cross-sectional area of
extruded product) of the multi-port tube reaches several hundred to
several thousand, a pure Al material exhibiting excellent
extrudability has been used for the multi-port tube. It is expected
that the weight of exchangers and the thickness of the tube will be
more and more reduced. Therefore, it is necessary to increase the
strength of the tube.
[0006] It is effective to add Si, Cu, Mn, Mg, and the like in order
to increase the strength of the tube. On the other hand, when the
Mg content of the brazing target material exceeds 0.2%, a
fluoride-type flux containing potassium fluoroaluminate which is
melted during heating reacts with Mg in the material to produce
compounds such as MgF.sub.2 and KMgF.sub.3. This reduces the
activity of the flux, whereby brazeability deteriorates. The
operating temperature of a heat exchanger using a carbon dioxide
refrigerant is as high as about 150.degree. C. As a result,
intergranular corrosion susceptibility significantly increases when
Cu is contained in the material. Therefore, Si and Mn must be
necessarily added in order to increase the strength of the
tube.
[0007] In an alloy containing Mn and Si at high concentrations, Mn
and Si dissolved in the matrix increase the deformation resistance
of the alloy. For example, when the extrusion ratio reaches several
hundred to several thousand such as when manufacturing the
multi-port tube, the alloy exhibits significantly inferior
extrudability in comparison with a pure Al material. In this case,
extrudability is evaluated using the ram pressure necessary for
extrusion and the maximum extrusion rate at which the partition
wall of the hollow portion of the multi-port tube is completely
formed (i.e. limiting extrusion rate) as indices. A material which
requires a high ram pressure or exhibits a low limiting extrusion
rate is determined to have poor extrudability. An alloy containing
Mn and Si at high concentrations requires a ram press higher than
that of a pure Al material, whereby the die tends to break or wear.
Moreover, productivity decreases due to a decrease in the limiting
extrusion rate.
[0008] As a method for improving the extrudability of an aluminum
alloy containing Mn and Si, a method has been proposed in which the
amount of solute elements dissolved in the matrix is decreased by
performing homogenization in which high-temperature heat treatment
and low-temperature heat treatment are combined, thereby decreasing
deformation resistance (see JP-A-11-335764). However, extrudability
is not necessarily sufficiently improved when extruding a tube such
as a thin multi-port tube. Therefore, a further improvement is
required.
[0009] It was found that a phenomenon occurs in which an aluminum
alloy is deposited in the shape of a film on the bearing of the die
during extrusion and the deposit adheres to the surface of the
extruded tube. A fluoride-type flux is applied to the surface of
the extruded tube before brazing by roll coating or the like. In
this case, the portion to which the deposit adheres is not provided
with the flux. As a result, a brazing failure occurs in the portion
which is not provided with the flux. There may be a case where
potassium fluorozincate is applied as a flux and Zn produced by the
subsequent brazing is diffused in the thickness direction and
allowed to function as a sacrificial corrosion protection layer, In
this case, a Zn diffusion layer is not formed in the portion which
is not provided with the flux, whereby the corrosion protection
performance cannot be ensured.
[0010] The film-shaped deposit on the bearing of the die is
increased in thickness and amount during continuous extrusion. The
deposit is finally removed from the bearing and adheres to the
surface of the extruded tube. The deposition, removal, and adhesion
process then repeatedly occurs. As a result, the deposit adheres to
the surface of the extruded tube at specific intervals.
SUMMARY OF THE INVENTION
[0011] The present invention was achieved after further experiments
and investigations conducted on the relationship among the alloy
composition, heat treatment of an unextruded ingot, and
extrudability in an attempt to improve the extrudability of an
aluminum alloy to which Mn and Si are added to obtain high strength
and to solve the problem in which the deposit adheres to the
surface of the extruded tube. Accordingly, an object of the present
invention is to provide an aluminum alloy extruded product
exhibiting excellent surface properties which exhibits improved
strength and excellent extrudability, allows extrusion of a thin
multi-port tube at a high limiting extrusion rate, prevents the
deposit from adhering to the surface of the extruded tube, and may
be suitably used as a constituent member for an aluminum alloy
automotive heat exchanger, and a method of manufacturing the
same.
[0012] In order to achieve the above object, a first aspect of the
present invention provides an aluminum alloy extruded product
exhibiting excellent surface properties, comprising 0.8 to 1.6%
(mass %; hereinafter the same) of Mn and 0.4 to 0.8% of Si at a
ratio of Mn content to Si content (Mn %/Si %) of 0.7 to 2.4, with
the balance being Al and inevitable impurities, the number of
intermetallic compounds with a diameter (circle equivalent
diameter; hereinafter the same) of 0.1 to 0.9 .mu.m dispersed in a
matrix being 2.times.10.sup.5 or more per square millimeter.
[0013] This aluminum alloy extruded product may further comprise
0.05% or less of Cu.
[0014] This aluminum alloy extruded product may further comprise
0.2% or less of Mg.
[0015] This aluminum alloy extruded product may further comprise
0.3% or less of Ti.
[0016] A second aspect of the present invention provides a heat
exchanger multi-port tube comprising the above aluminum alloy
extruded product.
[0017] A third aspect of the present invention provides a method of
manufacturing an aluminum alloy extruded product exhibiting
excellent surface properties, the method comprising: melting and
casting an aluminum alloy having the above composition to obtain an
ingot; subjecting the ingot to homogenization which includes a
first-stage heat treatment in which the ingot is maintained at 550
to 650.degree. C. for two hours or more and a second-stage heat
treatment in which the ingot is cooled to 400 to 500.degree. C. at
an average temperature decrease rate of 20 to 60.degree. C./h and
maintained at that temperature for three hours or more; heating the
ingot at 480 to 560.degree. C.; and extruding the ingot.
[0018] A fourth aspect of the present invention provides a method
of manufacturing a aluminum alloy extruded product exhibiting
excellent surface properties, the method comprising: melting and
casting an aluminum alloy having the above composition to obtain an
ingot; subjecting the ingot to homogenization which includes a
first-stage heat treatment in which the ingot is maintained at 550
to 650.degree. C. for two hours or more and a second-stage heat
treatment in which the ingot is cooled to room temperature, heated
to 400 to 500.degree. C. at an average temperature increase rate of
20 to 60.degree. C./h, and maintained at that temperature for three
hours or more; heating the ingot at 480 to 560.degree. C.; and
extruding the ingot.
[0019] A fifth aspect of the present invention provides a method of
manufacturing a heat exchanger comprising extruding a heat
exchanger multi-port tube using the above method, and joining the
multi-port tube to a heat exchanger by brazing.
[0020] According to the present invention, an aluminum alloy
extruded product exhibiting excellent surface properties which
exhibits improved strength and excellent extrudability, allows
extrusion of a thin multi-port tube at a high limiting extrusion
rate, prevents the deposit from adhering to the surface of the
extruded tube, and may be suitably used as a constituent member for
an aluminum alloy automotive heat exchanger, a method of
manufacturing the same, a heat exchanger multi-port tube made of
the aluminum alloy extruded product, and a method of manufacturing
a heat exchanger including the multi-port tube can be provided.
BRIEF DESCRIPTION OF THE DRAWING
[0021] FIG. 1 is a cross-sectional view of an aluminum alloy flat
multi-port tube extruded in the examples of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT
[0022] The meanings and the reasons for limitations of the alloy
components of the aluminum alloy extruded product according to the
present invention are given below. Mn and Si are dissolved in the
matrix during heating for brazing to improve the strength of the
alloy. The Mn content is preferably 0.8 to 1.6%, and the Si content
is preferably 0.4 to 0.8%. If the content of Mn and Si is greater
than the upper limit, extrudability deteriorates to a large extent
to impair the strength improvement effect. If the content of Mn and
Si is less than the lower limit, a sufficient strength cannot be
obtained.
[0023] The ratio of the Mn content to the Si content (Mn mass %/Si
mass %) is preferably 0.7 to 2.4. If the ratio of the Mn content to
the Si content is within this range, Mn and Si dissolved in the
matrix during casting of the alloy can be mainly precipitated as an
Al--Mn--Si intermetallic compound during homogenization of the by
ingot, whereby the solid solubility in the matrix can be minimized.
The dispersion state in which a number of minute Al--Mn--Si
intermetallic compounds are precipitated reduces the deformation
resistance of the alloy during hot extrusion performed after
homogenization heat treatment, whereby the extrudability of the
alloy can be improved.
[0024] If the ratio "Mn %/Si %" is less than 0.7, since Si is
contained in the alloy in an amount exceeding the range of the
ratio "Mn %/Si %" which can minimize the solid solubility of Mn and
Si in the matrix, Si remains dissolved in the matrix after the
homogenization heat treatment, whereby the deformation resistance
of the alloy during the subsequent hot extrusion is not reduced. As
a result, the extrudability of the alloy cannot be improved. If the
ratio "Mn %/Si %" exceeds 2.4, since Mn is contained in the alloy
in an amount exceeding the range of the ratio "Mn %/Si %" which can
minimize the solid solubility of Mn and Si in the matrix, Mn
remains dissolved in the matrix after the homogenization heat
treatment, whereby the deformation resistance of the alloy during
the subsequent hot extrusion is not reduced. As a result, the
extrudability of the alloy cannot be improved.
[0025] The Cu content is preferably limited to 0.05% or less. This
reduces intergranular corrosion during use of an automotive heat
exchanger manufactured by brazing the aluminum alloy extruded
product according to the present invention. If the Cu content
exceeds 0.05%, since the operating temperature of a heat exchanger
using carbon dioxide as a refrigerant becomes as high as about
150.degree. C., Al--Mn compounds and the like are significantly
precipitated at the boundaries, whereby intergranular corrosion
susceptibility increases.
[0026] Mg improves the strength of the alloy when contained in an
amount of 0.2% or less. Moreover, when manufacturing an automotive
heat exchanger by brazing using a fluoride-type flux contain
potassium fluoroaluminate, excellent brazeability can be stably
obtained. If the Mg content exceeds 0.2%, when manufacturing an
automotive heat exchanger by brazing, a fluoride-type flux
containing potassium fluoroaluminate which is melted during heating
for brazing reacts with Mg in the material to produce compounds
such as MgF.sub.2 and KMgF.sub.3. This reduces the activity of the
flux, whereby brazeability deteriorates. Moreover, the
extrudability of the alloy decreases when the Mg content exceeds
0.2%.
[0027] Ti form a high-Ti-concentration area and a
low-Ti-concentration area in the alloy. These areas are alternately
distributed in layers in the direction of the thickness of the
material. Since the low-Ti-concentration area is preferentially
corroded in comparison with the high-Ti-concentration area,
corrosion occurs in layers. This prevents corrosion from proceeding
in the direction of the thickness of the material. As a result,
pitting corrosion resistance and intergranular corrosion resistance
are improved. Moreover, the strength of the material at room
temperature and a high temperature is improved by adding Ti. The Ti
content is preferably 0.06 to 0.3%. If the Ti content is less than
0.06%, the effect is insufficient. If the Ti content exceeds 0.3%,
coarse compounds are produced during casting, whereby workability
is impaired.
[0028] Fe is contained as an inevitable impurity. The Fe content is
preferably limited to about 0.7% or less, and still more preferably
0.3% or less. When adding B aiming at ingot grain refinement or the
like, the B content is preferably about 0.01% or less. Impurities
such as Cr, Zr, Ni, and Zn may be contained in the alloy in an
amount of 0.25% or less in total.
[0029] In the aluminum alloy extruded product according to the
present invention, it is important that intermetallic compounds
with a diameter (circle equivalent diameter) of 0.1 to 0.9 .mu.m be
dispersed in the matrix in a number of 2.times.10.sup.5 or more per
square millimeter (mm.sup.2). These intermetallic compounds are
mainly Al--Mn--Si intermetallic compounds. The above dispersion
structure is obtained by homogenizing an unextruded ingot (billet),
which reduces adhesion of the deposit to the surface of the
aluminum alloy extruded product and improves the strength of the
aluminum alloy extruded product after heating for brazing.
Specifically, the extruded aluminum alloy is deposited on the
bearing of the die in the shape of a film. When extruding a billet
in which the above intermetallic compounds are dispersed, since the
surface of the film-shaped deposit formed on the bearing of the die
is continuously scraped off by the dispersed minute intermetallic
compounds during extrusion, the deposit is formed in the shape of a
thin uniform film. Since the deposit is maintained in the shape of
a thin uniform film during continuous extrusion, removal of the
deposit is prevented. As a result, adhesion of the deposit to the
surface of the aluminum alloy extruded product is significantly
reduced. Since the deposit is maintained in the shape of a thin
uniform film, the extruded product is provided with excellent
surface properties to exhibit a gloss.
[0030] The extruded tube is attached to a heat exchanger (e.g.
automotive heat exchanger) and joined by brazing. In this case,
since the Al--Mn--Si intermetallic compounds dispersed in the
matrix are redissolved in the matrix, the strength of the tube
after joining by brazing is improved due to solid solution
hardening. Since the operating temperature is as high as about
150.degree. C. when using carbon dioxide as a refrigerant, the
aluminum alloy extruded product is required to exhibit creep
strength. Since Mn and Si (solute elements) are redissolved in the
matrix after joining by brazing, these elements hinder the motion
of dislocation in the matrix to improve the creep strength of the
aluminum alloy extruded product.
[0031] The aluminum alloy extruded product according to the present
invention is manufactured by melting an aluminum alloy having the
above composition, casting the aluminum alloy by semicontinuous
casting or the like to obtain an ingot (billet), and homogenizing
and hot-extruding the ingot. A structure in which the above
intermetallic compounds are dispersed is obtained by specifying the
homogenization conditions, whereby adhesion of a deposit to the
surface of the aluminum alloy extruded product is reduced, and the
strength of the aluminum alloy extruded product is improved after
heating for brazing. Moreover, an improved hot extrudability is
obtained by combining specific homogenization conditions and hot
extrusion conditions.
[0032] It is preferable to perform homogenization which includes a
first-stage heat treatment in which the billet is maintained at 550
to 650.degree. C. for two hours or more and a second-stage heat
treatment in which the billet is cooled to 400 to 500.degree. C. at
an average temperature decrease rate of 20 to 60.degree. C./h and
maintained at that temperature for three hours or more.
Homogenization may be performed which includes a first-stage heat
treatment in which the billet is maintained at 550 to 650.degree.
C. for two hours or more and a second-stage heat treatment in which
the billet is cooled to room temperature, heated to 400 to
500.degree. C. at an average temperature increase rate of 20 to
60.degree. C./h, and maintained at that temperature for three hours
or more.
[0033] Coarse crystals formed during casting/solidification are
decomposed, granulated, or redissolved during the first-stage heat
treatment in which the billet is maintained at 550 to 650.degree.
C. for two hours or more. If the temperature is less than
550.degree. C., the above reaction proceeds to only a small extent.
The rate of reaction increases as the homogenization temperature
becomes higher. On the other hand, local melting occurs when the
homogenization temperature is too high. Therefore, the upper limit
is preferably set at 650.degree. C. The temperature range of the
first-stage heat treatment is still more preferably 580 to
620.degree. C. The reaction proceeds to a larger extent as the
treatment time increases. Therefore, it is preferable to set the
treatment time at 10 hours or more. On the other hand, a further
effect cannot obtained even if the treatment is performed for more
than 24 hours. This is disadvantageous from the viewpoint of cost.
Therefore, the treatment time is preferably 10 to 24 hours.
[0034] The first-stage heat treatment performed at a high
temperature is effective for decomposing, granulating, or
redissolving coarse crystals formed during casting/solidification.
On the other hand, the first-stage heat treatment promotes
dissolution of Mn and Si (solute elements) in the matrix. If the
solid solubility of these solute elements in the matrix is high,
the moving speed of dislocation in the matrix decreases, whereby
the deformation resistance of the aluminum alloy increases.
Therefore, the extrudability of the aluminum alloy decreases when
the aluminum alloy is hot-extruded after homogenization including
only the first-stage heat treatment. In the present invention, the
second-stage heat treatment is performed after the first-stage heat
treatment at a temperature lower than that of the first-stage heat
treatment to precipitate Mn and Si dissolved in the matrix, whereby
the solid solubility of Mn and Si is decreased. This reduces the
deformation resistance of the aluminum alloy, whereby the
extrudability of the aluminum alloy is improved.
[0035] The second-stage heat treatment is preferably performed at
400 to 500.degree. C. for three hours or more. If the temperature
is less than 400.degree. C., only a small amount of Al--Mn--Si
intermetallic compounds precipitate, whereby the effect of
decreasing the deformation resistance becomes insufficient. If the
temperature exceeds 500.degree. C., the intermetallic compounds
precipitate to only a small extent, whereby the effect of
decreasing the deformation resistance becomes insufficient. If the
treatment time is less than three hours, since precipitation does
not sufficiently proceed, the effect of deceasing the deformation
resistance becomes insufficient. The reaction proceeds to a larger
extent as the treatment time increases. On the other hand, a
further effect cannot be obtained even if the treatment is
performed for more than 24 hours. This is disadvantageous from the
viewpoint of cost. The treatment time is still more preferably 5 to
15 hours.
[0036] In order to achieve the above effects during homogenization,
it is important to control the temperature decrease rate from the
first-stage heat treatment temperature to the second-stage heat
treatment temperature (the temperature increase rate from room
temperature to the second-stage heat treatment temperature when the
billet is cooled to room temperature after the stage heat
treatment) in order to precipitate Mn and Si dissolved in the
matrix to decrease the solid solubility of Mn and Si and to achieve
the above dispersion state of the intermetallic compounds. The
average temperature decrease rate from the first-stage heat
treatment temperature to the second-stage heat treatment
temperature is preferably 20 to 60.degree. C./h. If the average
temperature decrease rate is less than 20.degree. C./h,
intermetallic compounds are grown to a large extent due to the
progress of precipitation, whereby it is difficult to obtain a
structure in which intermetallic compounds with a diameter of 0.1
to 0.9 .mu.m are dispersed in a number of 2.times.10.sup.5 or more
per square millimeter. Moreover, it is not economical because the
treatment requires time. If the average temperature decrease rate
exceeds 60.degree. C./h, the temperature distribution of the billet
becomes nonuniform, whereby precipitation tends to become
nonuniform. It is also preferable that the average temperature
increase rate to the first-stage heat treatment temperature and the
average temperature decrease rate from the second-stage heat
treatment temperature to 300.degree. C. be 20 to 60.degree.
C./h.
[0037] When the billet is cooled to room temperature after the
first-stage heat treatment and then heated to the second-stage heat
treatment temperature, the average temperature increase rate is
preferably 20 to 60.degree. C./h. If the average temperature
increase rate is less than 20.degree. C./h, since precipitated
intermetallic compounds are grown to a large extent, the number of
intermetallic compounds is decreased, whereby the above
intermetallic compound dispersion structure may not be obtained.
Moreover, it is not economical because heating requires time. If
the average temperature increase rate exceeds 60.degree. C./h, it
is difficult to obtain the above intermetallic compound dispersion
structure since precipitation does not proceed. It is also
preferable that the average temperature decrease rate from the
second-stage heat treatment temperature to 300.degree. C. be 20 to
60.degree. C./h.
[0038] In the present invention, the solid solubility of the solute
elements in the matrix is decreased by homogenizing the billet by
combining the above specific high-temperature heat treatment and
low-temperature heat treatment. This reduces the deformation
resistance of the aluminum alloy during the subsequent hot
extrusion, whereby the extrudability of the aluminum alloy can be
improved. The heating temperature of the billet before hot
extrusion is preferably 480 to 560.degree. C. If the heating
temperature exceeds 560.degree. C., the precipitate mainly
containing Al--Mn--Si intermetallic compounds formed during
homogenization is redissolved to increase the solid solubility in
the matrix. This results in an increase in deformation resistance
during hot extrusion, whereby the extrudability of the aluminum
alloy is decreased. If the heating temperature is less than
480.degree. C., deformation resistance is increased due to too low
a temperate, whereby the extrudability of the aluminum alloy is
decreased. The heating temperature is still more preferably 480 to
530.degree. C. The holding time at the above heating temperature is
preferably 30 minutes or less. If the holding time exceeds 30
minutes, the intermetallic compounds precipitated during
homogenization are redissolved to increase the solid solubility in
the matrix. This results in an increase in deformation resistance
during hot extrusion, whereby the extrudability of the aluminum
alloy is decreased. The holding time is still more preferably 10
minutes or less.
[0039] The aluminum alloy extruded product according to the present
invention has been described above taking a tube as an example.
Note that the extrusion shape is not particularly limited. The
extrusion shape is appropriately selected depending on the
application such as the form of the heat exchanger. Multi-port
tubes of various shapes may be extruded using a porthole die. When
using the aluminum alloy extruded product as a working fluid
passage material for a heat exchanger, the aluminum alloy extruded
product and other constituent members (e.g. fin material and header
material) are assembled and integrally joined by brazing. An
automotive heat exchanger in which the working fluid passage is
formed using the above multi-port tube exhibits excellent corrosion
resistance and exhibits excellent durability even under a severe
corrosive environment.
EXAMPLES
[0040] The present invention is described below by way of examples
and comparative examples to demonstrate the effects of the present
invention. Not that these examples illustrate one aspect of the
present invention, and should not be construed as limiting the
present invention.
Example 1 and Comparative Example 1
[0041] An aluminum alloy having the composition shown in Table 1
was melted and cast by semicontinuous casting to obtain a billet.
The resulting billet was homogenized. The billet was homogenized by
increasing the temperature of the billet to a first-stage heat
treatment temperature of 600.degree. C. at an average temperature
increase rate of 50.degree. C./h, maintaining the billet at the
first-stage heat treatment temperature for 15 hours, decreasing the
temperature of the billet to a second-stage heat treatment
temperature of 450.degree. C. at an average temperature decrease
rate of 50.degree. C./h, maintaining the billet at the second-stage
heat treatment temperature for 10 hours, and decreasing the
temperature of the billet from the second-stage heat treatment
temperature to 300.degree. C. at an average temperature decrease
rate of 50.degree. C./h. After homogenization, the billet was
heated at 510.degree. C. for eight minutes and hot-extruded to
obtain a multi-port tube having a shape shown in FIG. 1. The
resulting multi-port tube was used as a test specimen.
[0042] The extrudability of the aluminum alloy during hot extrusion
was evaluated according to the following method. Likewise, the
number of deposit portions adhering to the surface of the extruded
multi-port tube was calculated, and the gloss of the multi-port
tube was observed. The distribution of intermetallic compounds
precipitated and dispersed in the matrix was also determined. The
multi-port tube was subjected to joining by brazing, and
brazeability, tensile strength after heating for brazing, and
intergranular corrosion susceptibility were evaluated. The results
are shown in Table 2. In Tables 1 and 2, values outside the
conditions according to the present invention are underlined.
[0043] Evaluation of extrudability: The limiting extrusion rate
(i.e. the maximum extrusion rate at which the partition wall of the
hollow portion of the extruded multi-port tube (see FIG. 1) is
completely formed) was taken as the extrudability index. The
limiting extrusion rate indicates the ratio of the limiting
extrusion rate of the aluminum alloy to the limiting extrusion rate
of a known alloy (see Table 1) (ratio when the limiting extrusion
rate of the known alloy is 1.0). An aluminum alloy with a limiting
extrusion rate ratio of 0.9 or more was indicated as "Excellent",
an aluminum alloy with a limiting extrusion rate ratio of 0.8 or
more and less than 0.9 was indicated as "Good", an aluminum alloy
with a limiting extrusion rate ratio of 0.7 or more and less than
0.8 was indicated as "Fair", and an aluminum alloy with a limiting
extrusion rate ratio of less than 0.7 was indicated as "Bad".
[0044] Measurement of number of deposit portions adhering to
surface and observation of gloss of surface of extruded product: A
portion to which foreign matter adhered was detected using an eddy
current test, and the number of portions of the surface of the
extruded product to which an aluminum alloy deposit adhered was
determined to calculate of the number of deposit portions per unit
length of the extruded product. The gloss of the surface of the
extruded product was evaluated by naked eye observation, and was
also taken as the index of adhesion of deposit to the surface of
the extruded product.
[0045] Evaluation of distribution (dispersion structure) of
intermetallic compounds: the cross-sectional microstructure of the
extruded product was observed, and the number of precipitated
intermetallic compounds with a diameter (circle equivalent
diameter) of 0.1 to 0.9 .mu.m was determined by image analysis.
[0046] Measurement of tensile strength after heating for brazing:
The multi-port tube obtained by extrusion was heat-treated at
600.degree. C. for three minutes in a nitrogen atmosphere as
simulated heating for brazing, cooled at an average temperature
decrease rate of 50 to 250.degree. C./min, and subjected to a
tensile test to determine the strength of the multi-port tube. A
multi-port tube with a tensile strength of 110 MPa or more was
determined to have a sufficient tensile strength.
[0047] Evaluation of brazeability: A fluoride-type flux containing
potassium fluoroaluminate was applied to the surface of the
extruded multi-port tube in an amount of 10 g/m.sup.2. The
multi-port tube and a fin were assembled and joined by brazing by
heat-treating the product at 600.degree. C. for three minutes in a
nitrogen atmosphere and cooling the product at an average
temperature decrease rate of 50 to 250.degree. C./min. The joining
state of the multi-port tube with the fin was then observed. A case
where the multi-port tube and the fin were sufficiently joined was
indicated as "Good", and a case where the multi-port tube and the
fin were not sufficiently joined was indicated as "Bad".
[0048] Evaluation of intergranular corrosion susceptibility: In
order to simulate the use at 150.degree. C., the multi-port tube
subjected to the above simulated heating for brazing was
heat-treated at 150.degree. C. for 120 hours and immersed for 24
hours in a solution prepared by adding 10 ml/l HCl to a 30 g/l NaCl
aqueous solution. The cross section of the multi-port tube was then
observed. A multi-port tube in which intergranular corrosion did
not occur was indicated as "Good", and a multi-port tube in which
intergranular corrosion occurred was indicated as "Bad".
TABLE-US-00001 TABLE 1 Composition (mass %) Alloy Si Fe Cu Mn Mg
Mn/Si Invention A 0.6 0.2 0 1.2 0 2 B 0.5 0.2 0 1 0.1 2 C 0.45 0.2
0 1 0.15 2.2 D 0.7 0.2 0 1.4 0.1 2 E 0.8 0.2 0 0.8 0 1 Comparison F
1.5 0.2 0 1.9 0 1.3 G 0.05 0.2 0 0.1 0 2 H 0.6 0.2 0.3 1.2 0 2 I
0.6 0.2 0 1.2 0.6 2 J 0.05 0.2 0.4 0.1 0 2
[0049] TABLE-US-00002 TABLE 2 Number of intermetallic compounds
with Number of diameter of Limiting Tensile Intergranular deposit
Test 0.1 to 0.9 .mu.m extrusion rate strength corrosion portions
Surface specimen Alloy (10.sup.5/mm.sup.2) ratio Brazeability (MPa)
Susceptibility (/10000 m) gloss 1 A 3.2 1.0 (Excellent) Good 114
Good 0 Good 2 B 3.5 0.95 (Excellent) Good 120 Good 0 Good 3 C 3.1
0.9 (Excellent) Good 110 Good 0 Good 4 D 3.8 0.8 (Good) Good 130
Good 0 Good 5 E 4.1 0.85 (Good) Good 110 Good 0 Good 6 F 5.0 0.4
(Bad) Good 145 Good 0 Good 7 G 0.5 1.0 (Excellent) Good 68 Good 2.4
Bad 8 H 2.8 0.7 (Fair) Good 122 Bad 0 Good 9 I 3.1 0.6 (Fair) Bad
168 Good 0 Good 10 J 0.5 1.0 (Excellent) Good 72 Bad 3.6 Bad
[0050] As shown in Table 2, the test specimens 1 to 5 according to
the present invention exhibited excellent extrudability, did not
show adhesion of deposit to the surface, and exhibited excellent
brazeability, intergranular corrosion resistance, and strength. On
the other hand, the test specimens 6 to 9 and the test specimen 10
(known alloy) were inferior in at least one of extrudability,
adhesion of deposit, strength, brazeability, and intergranular
corrosion resistance.
Comparative Example 2
[0051] An aluminum alloy having the composition A shown in Table 1
was melted and cast by semicontinuous casting to obtain a billet.
The resulting billet was homogenized under the conditions shown in
Table 3. The billet was homogenized by increasing the temperature
of the billet to a first-stage heat treatment temperature at an
average temperature increase rate of 50.degree. C./h, maintaining
the billet at the first-stage heat treatment temperature,
decreasing the temperature of the billet to a second-stage heat
treatment temperature, maintaining the billet at the second-stage
heat treatment temperature, and decreasing the temperature of the
billet to 300.degree. C. at an average temperature decrease rate of
50.degree. C./h. Table 3 shows the first-stage heat treatment
temperature, the average temperature decrease rate from the
first-stage heat treatment temperature to the second-stage heat
treatment temperature, and the second-stage heat treatment
temperature. After homogenization, the billet was hot-extruded
under the conditions shown in Table 3 to obtain a multi-port tube
shown in FIG. 1. The resulting multi-port tube was used as a test
specimen.
[0052] The extrudability of the aluminum alloy during hot extrusion
was evaluated in the same manner as in Example 1. Likewise, the
number of deposit portions adhering to the surface of the extruded
multi-port tube was calculated, and the gloss of the multi-port
tube was observed. The distribution of intermetallic compounds
precipitated and dispersed in the matrix was also determined. The
multi-port tube was subjected to joining by brazing, and
brazeability, tensile strength after heating for brazing, and
intergranular corrosion susceptibility were evaluated. The results
are shown in Table 4. In Tables 3 and 4, values outside the
conditions according to the present invention are underlined.
TABLE-US-00003 TABLE 3 Homogenization First-stage Average
Second-stage Extrosion heat treatment temperature heat treatment
Billet Test (temperature decrease (temperature Billet heating
heating time specimen Alloy (.degree. C.) .times. time (h)) rate
(.degree. C./h) (.degree. C.) .times. time (h)) temperature
(.degree. C.) (min) 11 A 530 .times. 15 50 450 .times. 10 510 8 12
A 600 .times. 15 50 530 .times. 10 510 8 13 A 600 .times. 15 50 450
.times. 1 510 8 14 A 600 .times. 15 15 450 .times. 10 510 8 15 A
600 .times. 15 50 450 .times. 10 580 35
[0053] TABLE-US-00004 TABLE 4 Number of intermetallic compounds
with Number of diameter of Limiting Tensile Intergranular deposit
Test 0.1 to 0.9 .mu.m extrusion rate strength corrosion portions
Surface specimen Alloy (10.sup.5/mm.sup.2) ratio Brazeability (MPa)
susceptibility (/10000 m) gloss 11 A 2.5 0.75 (Fair) Good 114 Good
0.3 Fair 12 A 2.1 0.7 (Fair) Good 114 Good 0.3 Fair 13 A 1.6 0.7
(Fair) Good 115 Good 0.3 Fair 14 A 3.1 0.75 (Fair) Good 113 Good
0.3 Fair 15 A 1.5 0.7 (Fair) Good 114 Good 0.3 Fair
[0054] As shown in Table 4, the test specimens 11 to 15 homogenized
under the conditions outside the conditions according to the
present invention were inferior in at least one of extrudability,
number of deposit portions, strength, brazeability, and
intergranular corrosion resistance.
Example 2 and Comparative Example 3
[0055] An aluminum alloy containing 0.6% of Si, 0.2% of Fe, and
1.0% of Mn (Mn %/Si %: 1.7) was melted and cast by semicontinuous
casting to obtain a billet. The resulting billet was homogenized
under the conditions shown in Table 5. The billet was homogenized
by increasing the temperature of the billet to a first-stage heat
treatment temperature at an average temperature increase rate of
50.degree. C./h, maintaining the billet at the first-stage heat
treatment temperature, decreasing the temperature of the billet to
room temperature, increasing the temperature of the billet to a
second-stage heat treatment temperature, maintaining the billet at
the second-stage heat treatment temperature, and decreasing the
temperature of the billet to 300.degree. C. at an average
temperature decrease rate of 50.degree. C./h. Table 5 shows the
first-stage heat treatment temperature, the second-stage heat
treatment temperature, and the average temperature increase rate
from room temperature to the second-stage heat treatment
temperature. After homogenization, the billet was hot-extruded
under the conditions shown in Table 5 to obtain a multi-port tube
shown in FIG. 1. The resulting multi-port tube was used as a test
specimen.
[0056] The extrudability of the aluminum alloy during hot extrusion
was evaluated in the same manner as in Example 1. Likewise, the
number of deposit portions adhering to the surface of the extruded
multi-port tube was calculated, and the gloss of the multi-port
tube was observed. The distribution of intermetallic compounds
precipitated and dispersed in the matrix was also determined. The
multi-port tube was subjected to joining by brazing, and
brazeability, tensile strength after heating for brazing, and
intergranular corrosion susceptibility were evaluated. The results
are shown in Table 6. In Tables 5 and 6, values outside the
conditions according to the present invention are underlined.
TABLE-US-00005 TABLE 5 Homogenization First-stage Average
Second-stage Extrusion heat treatment temperature heat treatment
Billet heating Test (temperature increase (temperature temperature
Billet heating specimen (.degree. C.) .times. time (h)) rate
(.degree. C./h) (.degree. C.) .times. time (h)) (.degree. C.) time
(min) 16 600 .times. 15 50 450 .times. 10 510 8 17 530 .times. 15
50 450 .times. 10 510 8 18 600 .times. 15 50 530 .times. 10 510 8
19 600 .times. 15 50 380 .times. 10 510 8 20 600 .times. 15 15 450
.times. 10 510 8 21 600 .times. 15 50 450 .times. 10 580 20
[0057] TABLE-US-00006 TABLE 6 Number of intermetallic Limiting
Tensile Intergranular Number of Test compounds with diameter of
extrusion rate strength corrosion deposit portions specimen 0.1 to
0.9 .mu.m (10.sup.5/mm.sup.2) ratio Brazeability (MPa)
susceptibility (/10000 m) Surface gloss 16 3.0 1.0 (Excellent) Good
113 Good 0 Good 17 2.5 0.75 (Fair) Good 113 Good 0.2 Fair 18 2.0
0.75 (Fair) Good 114 Good 0.3 Fair 19 1.5 0.7 (Fair) Good 116 Good
0.3 Fair 20 3.0 0.75 (Fair) Good 112 Good 0.4 Fair 21 1.4 0.7
(Fair) Good 113 Good 0.4 Fair
[0058] As shown in Table 6, the test specimen 16 according to the
present invention exhibited excellent extrudability, did not show
adhesion of deposit to the surface, and exhibited excellent
brazeability, intergranular corrosion resistance, and strength. On
the other hand, the test specimens 17 to 21 were inferior in at
least one of extrudability, adhesion of deposit, strength,
brazeability, and intergranular corrosion resistance.
[0059] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that, within the scope of the
appended claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *