Field of the invention
[0001] The present invention relates to an aluminum alloy forged material suitably used
for strength members for transportation machines or the like, particularly chassis
members of an automobile, and a method for manufacturing the same.
Background of the invention
[0002] Conventionally, aluminum alloys composed of 6000 series (Al-Mg-Si series) aluminum
alloys according to the JIS standard or the AA standard, and the like, are used for
structural materials or structural parts of transportation machines such as vehicles,
ships, aircrafts, motorcycles, and automobiles. The 6000 series aluminum alloys have
relatively excellent resistance to corrosion. The 6000 series aluminum alloys are
also excellent in terms of their easy reusability of scraps as 6000 series aluminum
alloy molten raw materials.
[0003] In view of decreasing production cost and workability to components of complicated
structure, aluminum alloy cast materials and aluminum alloy forged materials are used
for structural materials or structural parts of transportation machines. Among them,
the aluminum alloy forged materials are used for structural members of automobiles,
particularly for underbody parts such as upper arms and lower arms as they require
mechanical properties including higher strength and toughness. Each of such aluminum
alloy materials is produced by performing hot forging (die forging) such as mechanical
forging or hydraulic forging after a homogenizing heat treatment of an aluminum alloy
cast material, and then performing a tempering treatment including a solution-quenching
treatment and an artificial aging treatment (sometime referred simply to aging treatment
hereinafter). As a raw material for forging, besides the cast material mentioned above,
an extruded material obtained by extruding a cast material after a homogenizing heat
treatment may also be used.
[0004] In recent years, increasing requirements of further weight (thickness) reduction
have been raised for strength members for the transportation machines due to higher
demand of low fuel consumption and low CO
2 emission. It is difficult, however, for 6000 series aluminum alloy forged materials
such as 6061 and 6151 alloys that have been conventionally used for the applications
to realize the high enough strength (0.2 % proof stress) and toughness.
[0005] In order to solve the problem, the present inventors proposed an aluminum alloy forged
material described in
Japanese Patent No. 3766357. Disclosed in the patent literature is an aluminum alloy forged material including
Mg: 0.6 - 1.8 mass%, Si: 0.8 - 1.8 mass%, Cu: 0.2 - 1.0 mass%, mass ratio of Si/Mg
is 1 or more, further including one or more of Mn: 0.1 - 0.6 mass%, Cr: 0.1 - 0.2
mass% and Zr: 0.1 - 0.2 mass%, and the remainder being A1 and inevitable impurities.
The aluminum alloy forged material of the composition has a thickness of the thinnest
portion of 30 mm or less, electrical conductivity measured at the surface of 41.0
- 42.5 % IACS after artificial aging treatment, and 0.2 % proof stress of 350 MPa
or more.
Summary of the invention
Problems to be solved by the invention
[0006] However, when one attempts to mass produce an aluminum alloy forged material having
a portion whose thickness is 10 mm or less, the one faces a problem that a forged
material having high mechanical strength such as 0.2 % proof stress of 360 MPa or
more and high toughness cannot be obtained in a reproducible manner by using the aluminum
alloy forged material disclosed in Japanese Patent No.
3766357.
[0007] In the mass production of 6000 series of aluminum alloy forged material, dispersions
in various conditions of homogenizing heat treatment and hot forging can be usually
accommodated to a certain extent. However, the dispersions in manufacturing conditions,
which are usually accommodated, affect the 0.2 % proof stress of an aluminum forged
material if the mechanical strength of the forged material is enhanced so that the
0.2 % proof stress is 360 MPa or more by containing an excess amount of Si as well
as Cu and Mn which are added in an increased amount to enhance the mechanical strength
of the thinned forged material. As a result, forged material of high mechanical strength
and high toughness cannot be secured as the 0.2 % proof stress of the products excessively
varies even if the manufacturing conditions are within the specified range.
[0008] Such a poor reproducibility of a forged material of high mechanical strength and
toughness damages reliability of the material for the use of strength members, reduces
the manufacturing yield of the forged material product, and increases the manufacturing
cost. Excessively narrow tolerances of the manufacturing conditions aiming to stabilize
the 0.2 % proof stress and toughness of the forged material also lead to increasing
the manufacturing cost.
[0009] The present invention has been made in view of the above-mentioned problem. An object
of the present invention is to stably provide an aluminum alloy forged material which
exhibits increased mechanical strength and high toughness even if the forged aluminum
alloy material is reduced in thickness, by containing an excess amount of Si and a
high amount of a strength-increasing element such as Cu and Mn, and a method for manufacturing
the same.
Means to solve the problem
[0010] The present invention to solve the above-mentioned problem is characterized in that
the aluminum alloy forged material comprising: 0.60 - 1.80 mass% of Mg; 0.80 - 1.80
mass% of Si; 0.20 - 1.00 mass% of Cu; 0.05 - 0.40 mass% of Fe; 0.001 - 0.15 mass%
of Ti; 1 - 500 ppm of B; further comprising one or more selected from 0.10 - 0.60
mass% of Mn, 0.10 - 0.40 mass% of Cr, and 0.10 - 0.20 mass% of Zr; and the remainder
being A1 and inevitable impurities, and the electrical conductivity measured at the
surface of the aluminum alloy forged material at 20 °C is greater than 42.5 % IACS
but not more than 46.0 % IACS, the 0.2 % proof stress is 360 MPa or more, and the
Charpy impact value is 6 J/cm
2 or more.
[0011] With the prescribed compositions and properties, mechanical strength and toughness
of the aluminum alloy forged material are improved by containing the specific amounts
of Mg, Si, Cu, Fe, Ti, and B, and further containing the specific amount of a strength-increasing
element such as Mn, and by controlling the 0.2 % proof stress and the Charpy impact
value to the specific value or larger. The mechanical strength and toughness of the
forged material are also improved by controlling the electrical conductivity measured
at the surface
of the aluminium alloy forged material to the specific range because the ratio of sub-grain
structure is increased while maintaining the corrosion resistance.
[0012] Mass ratio of Si/Mg is preferably 1 or more in the aluminum alloy of the present
invention. If the Si/Mg mass ratio is 1 or more, the 0.2 % proof stress of the aluminum
alloy forged material further increases.
[0013] Content of hydrogen gas is preferably 0.25 ml/100 g-Al or less in the aluminum alloy
forged material. By controlling the content of hydrogen gas to the specified value
or less, along with the aforementioned composition, forging defects such as bubbles
from hydrogen are diminished. As a result of the decreased density of fracture starting
points, the Charpy impact value of the aluminum alloy forged material is increased.
[0014] The method for manufacturing the aluminum alloy forged material according to the
present invention is comprising: a melting step of melting the aluminum alloy into
a molten metal, a casting step of casting the molten metal at a cooling rate of 10
°C/sec or more to form an ingot, a homogenizing heat treatment step of subjecting
the ingot to heating at a rate of 5 °C/min or less, and to a homogenizing heat treatment
at 450 - 550 °C, a forging step of subjecting the ingot having been subjected to the
homogenizing heat treatment to forging at 460 - 540 °C of the forging start temperature,
and after the forging step, a solution heat treatment step of subjecting the forged
material to a solution heat treatment at 520 - 570 °C, and an artificial aging treatment
step of subjecting the forged material to an artificial aging treatment at 170 - 200
°C for 4 - 9 hours.
[0015] According to the process described above, by conducting each of the step under the
specific condition, particularly by carrying out the forging step at the starting
temperature of 460 - 540 °C, precipitation of Mg
2Si is enhanced because of increase of ratio of sub-grain structure and hence the grain
boundaries in the forged structure. As a result, the electrical conductivity measured
at the surface of the aluminum alloy forged material after the artificial aging treatment
is controlled to the specific range.
[0016] The present inventors have conceived and found that by containing an excess amount
of Si and an increased amount of Cu and Mn or the like, electrical conductivity measured
at the surface of the aluminum alloy (sometimes referred to as surface electrical
conductivity hereinafter) is more closely correlated with the 0.2 % proof stress of
the forged material in an aluminum alloy forged material which exhibits high mechanical
strength of 0.2 % proof stress of 360 MPa or more, even if the forged material is
reduced in thickness.
[0017] If has been generally known that the surface electrical conductivity, reflecting
the structure of an aluminum alloy, is closely correlated with the 0.2 % proof stress
of the aluminum alloy. It is not necessarily limited to 6000 series aluminum alloy
forged material. However, in ordinary 6000 series aluminum alloy forged materials,
the relation between electrical conductivity on the surface of aluminum alloy forged
material and 0.2 % proof stress is moderately linear. Unless the surface electrical
conductivity drastically changes, 0.2 % proof stress is not significantly influenced
by the electrical conductivity in the ordinary 6000 series aluminum alloy forged materials.
[0018] On the other hand, in a 6000 series aluminum alloy forged material which is highly
strengthened to the 0.2 % proof stress of 360 MPa or more by containing an excess
amount of Si and an increased amount of Cu and Mn or the like, even if the forged
material is reduced in thickness, the 0.2 % proof stress tends to be maximized if
the surface electrical conductivity is more than 42.5 % IACS but not more than 46.0
% IACS. Moreover, the 0.2 % proof stress shows peculiar characteristics of sharp decrease
if the surface electrical conductivity is out of the range.
[0019] Therefore, in the 6000 series aluminum alloy forged material in which the 0.2 % proof
stress is increased to 360 MPa or more, and the thickness is reduced, the dispersions
in electrical conductivity on the surface of the aluminum alloy forged material due
to dispersions in manufacturing conditions more sensitively affect the 0.2 % proof
stress of the forged material. As a result, a forged material with 0.2 % proof stress
of 360 MPa or more is not stably produced due to the dispersion of the manufacturing
conditions that are usually accommodated as described above.
[0020] According to the present invention, by utilizing the above-described phenomenon,
0.2 % proof stress of 360 MPa or more in an A1 alloy forged material can be secured
and such material can be stably produced by controlling the surface electrical conductivity
to more than 42.5 % IACS but not greater than 46.0 % IACS. In other words, by controlling
the manufacturing condition so that the electrical conductivity is more than 42.5
% IACS but not greater than 46.0 % IACS on the surface of the aluminum alloy forge
material, a forged material with 0.2 % proof stress of 360 MPa or more can be stably
produced.
Effect of the invention
[0021] Provided by the present invention is an aluminum alloy forged material comprising
an excess amount of Si and a large quantity of a strength-increasing element such
as Cu or Mn, and by which high strength and high toughness can be stably obtained
while maintaining the corrosion resistance even if the forged material is thinned,
and also provided is a method for producing the same. The present invention therefore
has considerable industrial significance as it expands the use of aluminum alloy forged
material for transportation machines.
Brief description of the drawings
[0022]
FIG. 1 is a front view of a test specimen used for measurements of tensile strength,
0.2 % proof stress, and elongation.
FIGs. 2 are (a) a front view, (b) a side view, and (c) a magnified view of the notched
part of (b) of a test specimen to be used for Charpy impact test.
FIGs. 3 show (a) a side view and (b) a front view of a test specimen used for stress
corrosion test.
Description of the preferred embodiments
[0023] Firstly, the aluminum alloy forged material (referred to A1 alloy forged material
hereinafter) according to the present invention is explained. In the A1 alloy forged
material, electrical conductivity measured at 20 °C at the surface of the material
after an artificial aging treatment which is described later is controlled to a range
of greater than 42.5 % IACS but not more than 46.0 % IACS in order to secure and stably
obtain 0.2 % proof stress of 360 MPa or more.
(Electrical conductivity at 20 °C of greater than 42.5 % IACS but not more than 46.0
% IACS)
[0024] In a highly-strengthened A1 alloy forged material in which the 0.2 % proof stress
is enhanced to 360 MPa or more even if the thickness of the material is reduced, by
containing excess amount of Si and an increased amount of Cu and Mn or the like as
for the present invention, 0.2 % proof stress of 360 MPa or more can not be attained
if the electrical conductivity at 20 °C measured at the surface is either 42.5 % IACS
or less, or more than 46.0 % IACS.
[0025] The electrical conductivity of the A1 alloy forged material shows a similar behavior
not only at the surface but also within the bulk including the center portion. Because
of convenience in measurement, the electrical conductivity at the surface of the A1
alloy forged material is selected in the present invention.
[0026] Each of test specimen of A1 alloy forged material for measurement of electrical conductivity
is prepared from an A1 alloy forged material after an artificial aging treatment by
mechanical grinding the surface about 0.05 to 0.1 mm or by etching the surface about
a few micrometers. The electrical conductivity at the surface is measured by using,
for example, an Eddy current type electrical conductivity meter (AutoSigma 3000DL,
manufactured by GE Inspection Technologies Japan). By leaving the meter with a probe,
a standard specimen, and the A1 alloy forged material specimen for measurement in
the same inspection area so that temperature of the all are equal, it is confirmed
prior to the measurement that temperature of the specimens is within a range of ±
1 °C of the ambient temperature by a contact thermometer. In the present invention,
a measured value at 20 °C or a conversion value is used to represent electrical conductivity
of A1 alloy forged material. Hereinafter, "electrical conductivity at 20 °C" is simply
referred to as "electrical conductivity".
[0027] The electrical conductivity on the surface of A1 alloy forged material comprehensively
reflects the contents of each of alloy elements as well as structure of the material
including their dispersion and degree of crystallization. Further, in addition to
these material factors, the electrical conductivity also reflects total metallurgical
state of the material including factors in manufacturing conditions.
[0028] In an A1 alloy forged material which is highly-strengthened to 0.2 % proof stress
of 360 MPa or more, even if the forged material is reduced in thickness, by containing
an excess amount of Si and an increased amount of Cu and Mn or the like, the surface
electrical conductivity is not necessarily the same even if the content of each alloying
element or the process conditions such as temperature of homogenizing heat treatment
and starting temperature of hot forging are roughly in accord with each other.
[0029] Influencing factors of manufacturing condition, which affect surface electrical conductivity
of the A1 alloy forged material after the artificial aging treatment include; in addition
to the above-described temperature conditions, details of the cooling rate in the
casting step, the heating rate, the holding temperature, and the cooling rate of ingot
in the homogenizing heat treatment step, type of hot forging apparatus such as mechanical
forging and hydraulic forging, number of times of the forging, working ratio in each
of the forging steps, the condition of forging finish temperature, the temperature
and time conditions of the solution heat treatment, the quenching treatment, and the
artificial aging treatment.
[0030] This is because the surface electrical conductivity is significantly affected by
slight difference in the process condition in an A1 alloy forged material even if
it is reduced in thickness as the forged material is highly-strengthened to 0.2 %
proof stress of 360 MPa or more by containing an excess amount of Si and an increased
amount of Cu and Mn or the like.
[0031] Thus, if the conditions of material and manufacturing are roughly in accord and the
surface electrical conductivities of the Al alloy forged materials are similar, then
the problem to be solved by the present invention, the dispersion in the 0.2 % proof
stress in the mass-produced materials, would not arise.
[0032] According to the present invention, 0.2 % proof stress is controlled to 360 MPa or
more and Charpy impact value is controlled to 6 J/cm
2 or more in the A1 alloy forged material.
(0.2 % proof stress: 360 MPa or more, and Charpy impact value: 6 J/cm2)
[0033] By controlling the 0.2 % proof stress to 360 MPa or more and the Charpy impact value
to 6 J/cm
2 or more in the A1 alloy forged material to possess high enough mechanical strength
and toughness, it becomes possible to use the A1 alloy forged material for structural
material or parts for transportation machines such as an automobile and a ship.
[0034] Chemical components in the A1 alloy forged material of the present invention are
explained hereinafter. The A1 alloy forged material of the present invention comprises
Al-Mg-Si series (6000 series) A1 alloy, and the chemical components are specified
to secure high mechanical strength, high toughness, and high durability such as stress
corrosion cracking resistance for the material to be used for structural material
or structural parts for transportation machines such as an automobile and a ship.
The chemical components in the Al alloy forged material of the present invention are
ones of main factors to govern the electrical conductivity on the surface of the forged
material.
[0035] The chemical composition of the aluminum alloy forged material of the present invention
includes Mg: 0.60 - 1.80 mass%, Si: 0.80 - 1.80 mass%, Cu: 0.20 - 1.00 mass%, Fe:
0.05 - 0.40 mass%, Ti: 0.001 - 0.15 mass%, B: 1 - 500 ppm; further comprising one
or more element selected from Mn: 0.10 - 0.60 mass%, Cr: 0.10 - 0.40 mass% and Zr:
0.10 - 0.20 mass%; and the remainder being Al and inevitable impurities, accordingly.
[0036] It is to be noted here that the chemical composition of the aluminum alloy forged
material of the present invention may not necessarily be in accord with a standard
component of 6000 series A1 alloy of the JIS standard or the like. For the purpose
of further improvement or additional properties, other kinds of element are allowed
to be contained appropriately within a range which does not inhibit the characteristics
of the present invention. In addition, inevitable impurities which are inevitably
mixed from molten raw material scraps are also allowed within a range which does not
inhibit the qualities of the forged material of the present invention.
[0037] Next, critical significance and preferred ranges of each element constituting the
Al alloy forged material of the present invention are explained.
(Mg: 0.60 - 1.80 mass%)
[0038] Mg is precipitated as Mg
2Si (6' phase) in crystal grains together with Si by artificial aging treatment, and
is an essential element for imparting the high 0.2 % proof stress to the aluminum
alloy forged material. When the content of Mg is less than 0.60 mass%, the amount
of age hardening during the artificial aging treatment is decreased, resulting in
deterioration of Charpy impact value (referred to as an index of toughness hereinbelow)
and corrosion resistance which are essential properties for an Al alloy forged material
as for the high 0.2 % proof stress. On the other hand, when the content of Mg exceeds
1.80 mass%, the 0.2 % proof stress is excessively increased to inhibit forging properties.
Further, a large amount of Mg
2Si is liable to precipitate in the middle of a quenching step after a solution heat
treatment as explained below. It hinders decrease of average grain size of the Mg
2Si and Al-Fe-Si-(Mn,Cr,Zr)-based crystallized and precipitated products residing on
grain boundaries, which are formed by selective chemical bonding of Al, Si, Mn, Cr,
Zr, and Fe, and suppresses average distance between the crystallized and precipitated
products. As a result, the corrosion resistance of the A1 alloy forged material is
deteriorated. Moreover, when the content of Mg exceeds the specified range, it becomes
difficult to control the electrical conductivity of the surface of the Al alloy forged
materials to the range of greater than 42.5 % IACS but not more than 46.0 % IACS by
adjusting the manufacturing conditions. Accordingly, the content of Mg is adjusted
to the range from 0.60 to 1.80 mass%.
(Si: 0.80 - 1.80 mass%)
[0039] Si is combined with Mg to form Mg
2Si (β' phase) which precipitates during the artificial aging treatment. The precipitation
of Mg
2Si crystals contributes to increasing 0.2 % proof stress of the aluminum alloy forged
material. When the Si content is less than 0.80 mass%, the amount of temper hardening
decreases, and 0.2 % proof stress and corrosion resistance of the A1 alloy forged
material are deteriorated. On the other hand, when the Si content exceeds 1.80 mass%,
coarse single body Si particles are crystallized and precipitated in casting and in
the middle of quenching after the solution heat treatment. Further, too much amount
of the excessive Si prevents average grain size of the Al-Fe-Si-(Mn,Cr,Zr)-based crystallized
and precipitated products residing on grain boundaries from decreasing, and suppresses
average distance between the crystallized and precipitated products, resulting in
deterioration of corrosion resistance and toughness of the A1 alloy forged material,
as for Mg. Furthermore, the excessive Si deteriorates the workability of the A1 alloy
forged material by lowering its elongation. Moreover, when the content of Si exceeds
the specified range, it becomes difficult to control the electrical conductivity of
the surface of the Al alloy forged materials to the range of greater than 42.5 % IACS
but not more than 46.0 % IACS by adjusting the manufacturing conditions. The content
of Si is to be 0.80 - 1.80 mass%, accordingly.
(Cu: 0.20 - 1.00 mass%)
[0040] Cu contributes to enhancement of 0.2 % proof stress for the material by solid solution
strengthening. Furthermore, Cu has an effect to significantly promote age hardening
of A1 alloy forged material in the step of the artificial aging treatment. When the
content of Cu is less than 0.20 mass%, these effects cannot be expected and 0.2 %
proof stress cannot be obtained. In order to secure these effects, the content of
Cu is preferably controlled to 0.30 mass% or more. On the other hand, when the content
of Cu exceeds 1.00 mass%, it extremely increases the sensitivity of stress corrosion
cracking and intergranular corrosion of the structure of the aluminum alloy forged
material, and deteriorates the corrosion resistance of the aluminum alloy forged material.
Further, when the content of Cu exceeds the specified range, it becomes difficult
to control the electrical conductivity of the surface of the Al alloy forged materials
to the range of more than 42.5 % IACS and 46.0 % IACS or less by adjusting the manufacturing
conditions. Therefore, the content of Cu is to be in the range of 0.20 - 1.00 mass%,
and preferably 0.30 - 1.00 mass%.
(Fe: 0.05 - 0.40 mass%)
[0041] Fe is added to improve the toughness of the A1 alloy forged material. Fe forms Al
7Cu
2Fe, Al
12(Fe,Mn)
3Cu
2, (Fe,Mn)Al
6, and coarse Al-Fe-Si-(Mn,Cr,Zr)-based crystallized and precipitated products which
is a problem to be solved by the present invention. These crystallized and precipitated
products provide the start point of fracture and deteriorate the fracture toughness,
fatigue properties and the like. In particular, if the content of Fe is more than
0.40 mass%, more strictly 0.35 mass%, then the Al-Fe-Si-(Mn,Cr,Zr)-based crystallized
and precipitated products residing on grain boundaries become coarse, and average
distance between the crystallized and precipitated products decreases, resulting in
decrease of the toughness. If the content of Fe is less than 0.05 mass%, on the other
hand, cracking during the casting step and abnormal material structure are induced.
The content of Fe is to be in a range of 0.05 - 0.40 mass%, accordingly. The content
of Fe is preferably to be in a range of 0.05 - 0.35 mass%.
(Ti: 0.001 - 0.15 mass%)
[0042] Ti is added to the aluminum alloy to make crystal grains in the ingot finer to improve
the workability of the material in the extrusion, rolling, and forging steps. If a
content of Ti is less than 0.001 mass%, the effect of improved workability is not
obtained. On the other hand, if the content of Ti is higher than 0.15 mass%, coarse
precipitated crystalline particles are formed and the workability is deteriorated.
The content of Ti is to be in a range of 0.001 - 0.15 mass%, accordingly.
(B: 1 - 500 ppm)
[0043] Like Ti, B is added to the aluminum alloy to make crystal grains in the ingot finer
to improve the workability of the material in the extrusion, rolling, and forging
steps. If a content of B is less than 1 ppm, the effect is not obtained. On the other
hand, if the content of B is higher than 500 ppm, coarse precipitated crystalline
particles are formed and the workability is deteriorated. The content of B is to be
in a range of 1 - 500 ppm, accordingly.
(At least one element of Mn: 0.10 - 0.60 mass%, Cr: 0.10 - 0.40 mass%, and Zr: 0.10-0.20
mass%)
[0044] These elements forms dispersed particles (dispersed phase) collectively known as
(Fe,Mn,Cr,Zr)
3SiAl
12 system of Al-Mn, Al-Cr, and Al-Zr intermetallic compounds which precipitate by selective
chemical bonding of Fe, Mn, Cr, Zr, Si, and A1 or others according to their contents
during the homogenizing heat treatment step and subsequent the hot forging step.
[0045] Since these dispersed particles have an effect of preventing grain boundaries from
moving after recrystallization, coarsening of average crystal size in ST direction
of parting line structure in the forging step is prevented. Fine crystal grains or
fine sub grains are obtained as well throughout the A1 alloy forged material according
to the present invention. Further, enhancement of 0.2 % proof stress is also expected
by solid solution of Mn, Cr, and Zr.
[0046] The aluminum alloy according to the present invention contains one or more elements
selected from Mn, Cr and Zr, within the range of the contents. If the content of Mn,
Cr and Zr is too low, the effect cannot be expected. On the other hand, if an excessive
amount of these elements is contained, coarse Al-Fe-Si-(Mn,Cr,Zr)-based intermetallic
compounds or crystallized and precipitated products are liable to be formed in the
middle of melting and casting steps. They behave as fracture starting points and are
factors to deteriorate at least one of electrical conductivity, 0.2 % proof stress,
toughness, and corrosion resistance of the Al alloy forged material. Accordingly,
the content of each of the elements is adjusted to the range of Mn: 0.10 - 0.60 mass%,
Cr: 0.10 - 0.40 mass% and Zr: 0.10 - 0.20 mass%, and one or more kinds of them are
to be contained.
(Inevitably contained impurities)
[0047] Elements such as Zn, Be, and V may be assumed inevitably contained in the aluminum
alloy. Each of these elements may be contained as long as the amount is low enough
not to affect the feature of the present invention. Specifically, an amount of each
of these elements is to be 0.05 mass% or less and a total amount of these elements
is to be 0.15 mass%.
(Si/Mg mass ratio: 1 or more)
[0048] Mass ratio of Si/Mg is preferably 1 or more in the aluminum alloy of the present
invention. By controlling the Si/Mg mass ratio to 1 or more within the content of
the elements the 0.2 % proof stress further increases. If the Si/Mg mass ratio is
less than 1, such enhancement of the 0.2 % proof stress cannot be realized.
[0049] Further, the content of hydrogen in the A1 alloy forged material of the present invention
is preferably to be regulated to a range shown below.
(Hydrogen: 0.25 ml/100 g-Al or less)
[0050] Especially when the working ratio of an A1 alloy forged material is small, hydrogen
(H
2) is liable to cause forging defects such as blow holes and the like caused by hydrogen,
provides starting points of fracture, and therefore is liable to significantly deteriorate
the toughness and fatigue properties of the forged material. The effect of hydrogen
particularly becomes significant for the highly strengthened structural material of
transportation devices. The content of hydrogen, therefore, is to be regulated to
0.25 ml/100 g-Al or less, and preferably as low as possible.
[0051] Next, the method for manufacturing the aluminum alloy forged material in relation
with the present invention is explained. The manufacturing method in relation with
the present invention includes a melting step, a casting step, a homogenizing heat
treatment step, a forging step, and a tempering step. The Al alloy forged material
of the present invention may be manufactured in a usual manner, other than controlling
the surface electrical conductivity to the range of more than 42.5 % IACS but not
more than 46.0 % IACS, the 0.2 % proof stress, and the toughness of the A1 alloy forged
material. Explained hereinafter are conditions of each of the steps to improve the
characteristics of the A1 alloy forged material such as controlling the electrical
conductivity within the specified range.
(Melting step)
[0052] The melting step is a step to dissolve an aluminum alloy having the above-described
composition into a molten metal.
(Casting step)
[0053] The casting step is a step to cast the molten metal having the above-described composition
into an ingot. Ordinal melting and casting method, such as continuous casting and
rolling method, semi-continuous casting method (direct chill casting process), hot-top
casting method, or the like is suitably selected and the ingot is casted. Shape of
ingot is not particularly limited. It may be a round bar or a slab shape.
[0054] The molten alloy is cast to an ingot at a cooling rate of 10 °C/sec or more for the
purpose of refinement of crystal grains in the ingot, decreasing the mean crystal
grain size of the Al-Fe-Si-(Mn,Cr,Zr)-based crystallized and precipitated products
residing on grain boundaries, and increasing the average distance between the crystallized
and precipitated products. If the cooling rate is low, the mean crystal grain size
of the Al-Fe-Si-(Mn,Cr,Zr)-based crystallized and precipitated products residing on
grain boundaries increases, and the average distance between the crystallized and
precipitated products decreases, resulting in decrease of the 0.2 % proof stress of
the A1 alloy forged material after an artificial aging treatment. It is noted that
that the cooling rate of the molten alloy is defined as a mean cooling rate from the
liquidus temperature to the solidus temperature.
(Homogenizing heat treatment step)
[0055] The homogenizing heat treatment step is a step of subjecting the ingot to homogenizing
heat treatment. The treatment is conducted at a heating rate of 5 °C/min or less and
at a holding temperature ranging from 450 to 550 °C.
[0056] When the homogenizing heat treatment temperature is too high exceeding 550 °C, the
(Fe,Mn,Cr,Zr)
3SiAl
12 dispersed particles become coarse and the density decreases, failing to disperse
relatively high density of fine dispersed crystals among the crystal grains and to
obtaining fine crystal grains. As a result, the 0.2 % proof stress of the A1 alloy
forged material is decreased after the homogenizing heat treatment.
[0057] On the other hand, of the holding temperature is too low, i.e., less than 450 °C,
the number of the (Fe,Mn,Cr,Zr)
3SiAl
12 dispersed precipitates decreases, causing a shortage of density of the dispersed
grains. Further, sufficient solid solution of the Al-Fe-Si-(Mn,Cr,Zr)-based crystallized
and precipitated products cannot be realized. It becomes difficult to decrease the
mean grain size of Mg
2Si and Al-Fe-Si-(Mn,Cr,Zr)-based crystallized and precipitated products residing on
grain boundaries in the A1 alloy forged material after a tempering step explained
later, and to increase the average distance between the crystallized and precipitated
products. As a result, it becomes difficult to control the electrical conductivity
of the surface of the A1 alloy forged materials to the range of more than 42.5 % IACS
but not more than 46.0 % IACS (hereinafter referred to the range of the present invention)
after the artificial aging treatment.
[0058] The heating rate to the holding temperature is suppressed low as to 5 °C/min. or
less in order to maintain the 0.2 % proof stress of the Al alloy forged material after
the artificial aging treatment. Holding duration at the holding temperature is preferably
2 hours or more. For the homogenizing heat treatment, the air furnace, the induction
heating furnace, a niter furnace, or the like is used appropriately. The heating rate
of an ingot is defined as a mean heating rate from room temperature to the holding
temperature.
(Forging step)
[0059] The forging step is a step of using the above-mentioned ingot subjected to the homogenizing
heat treatment as a forging raw material, and performing predetermined hot forging
by forging using a mechanical press or by forging using a hydraulic press. For the
hot forging, starting temperature of the forging raw material is adjusted to 460 -
540 °C. If the starting temperature is less than 460 °C, precipitation of Mg
2Si is suppressed because a ratio of sub- grain and the density of grain boundaries
in the forged structure are decreased. As a result, the 0.2 % proof stress is deteriorated
as the electrical conductivity on the surface of the A1 alloy forged material after
the artificial aging treatment cannot be controlled within the range of the present
invention. On the other hand, of the starting temperature is more than 540 °C, there
are cases in which a portion of the material melts because of heat generated during
the forging step. As a result, the 0.2 % proof stress and the corrosion resistance
are deteriorated as the electrical conductivity cannot be controlled within the range
of the present invention.
[0060] The finishing temperature of the forging raw material in the hot forging step is
preferably to be 350 - 540 °C from a point of view to control the electrical conductivity
to the range of the present invention. For the purpose of diminishing residual forged
structure in the A1 alloy forged material and further improvement of 0.2 % proof stress
and toughness, an ingot which has been extruded and/or rolled after a homogenizing
heat treatment may also be used as a forging raw material.
[0061] In order to control the hot-forging finishing temperature to 350 - 540 °C, it is
necessary to exercise ingenuity such as reheating prior to hot forging and using a
die which can be held at high temperatures.
[0062] For the purpose of facilitating to control the electrical conductivity on the surface
of the A1 alloy forged material after the artificial aging treatment to the range
of the present invention, it is preferred to perform the hot forging by a mechanical
forging method, and to perform the forging not more than three times. The shape of
the A1 alloy forged material is not particularly limited, and may be near net shape
which is close to that of a final product.
(Heat treatment step)
[0063] A heat treatment step is a step of performing a solution treatment and an artificial
aging treatment after the forging step, in order to obtain 0.2 % proof stress, corrosion
resistance, and toughness necessary for an A1 alloy forged material. Specifically,
the heat treatment step includes T6 (an artificial aging treatment for obtaining the
maximum strength after the solution treatment at 520 - 570 °C), T7 (excessive artificial
aging treatment (over-aging treatment) surpassing the conditions of the artificial
aging treatment for obtaining the maximum strength after the solution treatment),
T8 (an artificial aging treatment for obtaining the maximum strength, in addition
to a cold forging after the solution treatment) or the like.
[0064] The solution heat treatment is conducted at a temperature range of 520 - 570 °C.
If the treatment temperature is excessively low, insufficient solutionizing prevents
the solid solution of Mg
2Si, controlling the electrical conductivity to the specified range of the present
invention, which results in decrease of 0.2 % proof stress. If the treatment temperature
is excessively high, localized melting and coarsening of crystal grains are induced,
resulting in decrease of 0.2 % proof stress. The holding duration and heating rate
of the solution heat treatment are preferably 20 minutes to 20 hours and 100 °C/hr
or more, respectively, in order to secure the 0.2 % proof stress. The heating rate
of the A1 alloy forged material is defined as a mean heating rate from the temperature
of the material immersed in the solution to the holding temperature in the present
invention.
[0065] A quenching step may be conducted after the solution heat treatment. The quenching
treatment may be conducted by cooling in either cold or hot water. The cooling rate
is preferably 40 °C/sec or more in order to prevent degradation of toughness and fatigue
property of the forged material. An air furnace, an induction heating furnace, a niter
furnace, or the like is used appropriately.
[0066] Temperature and duration of the artificial aging step significantly affect the electrical
conductivity on the surface of the Al alloy forged material after the artificial aging
step. It is thus necessary to select appropriate conditions in the step by considering
the process hysteresis of the material in order to secure the 0.2 % proof stress by
controlling the electrical conductivity within the range of the present invention
as well as to secure the essential toughness and corrosion resistance. Depending on
the amount of alloy elements and process hysteresis (manufacturing conditions) before
the artificial aging treatment step, it is necessary to confirm in each of the processing
step and manufacturing apparatus. In order to control the electrical conductivity
on the surface of the Al alloy forged material after the artificial aging treatment
into the range of the present invention, processing condition of the artificial aging
treatment is selected from a range of temperature of 170 to 200 °C and a duration
of 4 to 9 hours, considering the conditions aiming for the material having the maximum
strength by the tempering treatments of T6, T7, and T8 steps. For the artificial aging
treatment, the air furnace, the induction heating furnace, an oil bath, or the like
is used appropriately.
[0067] Further, the manufacturing method according to the present invention preferably includes
a degassing step between the melting step and the casting step.
(Degassing step)
[0068] The degassing step is a step of removing a hydrogen gas (degassing treatment) from
the above-mentioned molten metal of the aluminum alloy melted in the melting step,
and controlling a hydrogen gas concentration in 100 g of the aluminum alloy to 0.25
ml or less. The removal of the hydrogen gas is performed in a holding furnace for
adjusting the components of the molten metal, and removing inclusions by fluxing,
chlorine refining, or in-line refining of the molten metal. Preferably, the hydrogen
gas is removed by blowing an inert gas of argon or the like into the molten metal
using SNIF (Spinning Nozzle Inert Floatation) or porous plugs (Japanese Unexamined
Patent Application Publication No.
2002-146447) in an apparatus for removing the hydrogen gas.
[0069] Here, the determination of the hydrogen gas concentration is performed by measuring
a hydrogen gas concentration in an ingot produced in the casting step described later
or in a forging produced in the forging step, which is described later. The hydrogen
gas concentration in the ingot can be obtained by, e.g., cutting a sample out of the
ingot prior to the homogenizing heat treatment, subjecting the sample to ultrasonic
cleaning using alcohol and acetone, and measuring the hydrogen gas concentration in
the sample by, e g., the inert gas flow fusion-thermal conductivity method (LIS A06-1993).
On the other hand, the hydrogen gas concentration in the forged material can be obtained
by, e.g., cutting a sample out of the forging, immersing the sample in a NaOH solution,
removing an oxide coating on the surface thereof with a nitric acid, subjecting the
sample to ultrasonic cleaning using alcohol and acetone, and measuring the hydrogen
gas concentration in the sample by the vacuum heating extraction volumetric method
(LIS A06-1993).
[0070] In the manufacturing method of the present invention, a pre-forming step by forging
roll or the like may be included prior to the forging step.
Examples
[0071] Next, the present invention is specifically described based on examples. A1 alloy
ingots of compositions shown in Table 1 were casted at a cooling rate of 20 °C/sec
by a hot-top casting method into a round bar of 68 mm in diameter and 580 mm in length.
The ingots were then subjected to a homogenizing heat treatment at a heating rate
of 5 °C/min and a holding temperature of 550 °C for 4 hours.
[0072] Further, a hot forging was performed three times so that the total forging working
ratio reaches 75 % by a mechanical forging method using an upper and a lower dies
at the forging start temperature and forging finish temperature shown in Table 2.
Manufactured was an A1 alloy forged material in a shape of chassis members of an automobile.
The thinnest part of the forged material was 6 mm in thickness.
[0073] Next, the A1 alloy forged material was subjected to a solution heat treatment at
550 °C for 1 hour in an air furnace, followed by a water cooling (water quenching)
step. The forged material was subsequently subjected to an artificial aging treatment
at 190 °C for 5 hours in an air furnace.
[0074] Three test pieces were collected from each of the specimens, and subjected to evaluations
on electrical conductivity at the surface, tensile properties such as tensile strength,
0.2 % proof stress, elongation, which are measures of mechanical strength, and Charpy
impact value (mechanical property) which is a measure of toughness. Each of the values
shown in Table 2 is an average value of those collected from the three test pieces.
The measurements of tensile strength, 0.2 % proof stress, elongation were conducted
using test specimens S1 collected from each of the A1 alloy forged material as shown
in FIG. 1 and by a test method according to the provisions of JIS-Z-2241. The measurement
of Charpy impact value was performed using test specimens S2 collected from each of
the Al alloy forged material as shown in FIG. 2 and by a test method according to
the provisions of JIS-Z-2242. The test pieces having 0.2 % proof stress of 360 MPa
or more and Charpy impact values of 6 J/cm
2 or more were each evaluated as excellent.
[0075] Test pieces S3 of a C-ring shape illustrated in FIG. 3 were also collected from each
of the A1 alloy forged material, and subjected to a stress corrosion cracking resistance
test. The stress corrosion cracking resistance test was performed using the test pieces
S3 in the C-ring shape and the alternate immersion test method according to the provisions
of ASTMG47. Under added stress of 75 % of the yield stress in LT direction of the
test pieces S3, the C-ring shape test pieces were repeatedly subjected to a cycle
of immersion and pulling into and out of a salt water during a test period of 90 days.
Then presence/absence of stress corrosion cracking in each of the test pieces was
examined. The test pieces which did not undergo cracking nor grain boundary corrosion
including whole-surface corrosion on the surface were each provided with "good" representing
excellent stress corrosion cracking resistance, and evaluated as having excellent
corrosion resistance. The test pieces without stress corrosion cracking but with grain
boundary corrosion which could potentially leads to stress corrosion cracking were
each provided with "not very good" representing not very good stress corrosion cracking
resistance, and evaluated as having not very good corrosion resistance. The test pieces
which underwent cracking were each provided with "poor" representing poor stress corrosion
cracking resistance, and evaluated as having poor corrosion resistance. The results
are shown in Table 2.
[Table 1]
Alloy No. |
Al alloy composition |
(In mass%. Contents of B and H2 are in ppm and ml/100g-Al, respectively. The remainder bering Al) |
Mg |
Si |
Cu |
Fe |
Ti |
B |
Mn |
Cr |
Zr |
Zn |
H2 |
Si/Mg |
1 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
2 |
0.60 |
0.80 |
0.20 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.33 |
3 |
0.60 |
0.80 |
0.20 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.33 |
4 |
1.80 |
1.80 |
1.00 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.00 |
5 |
1.80 |
1.80 |
1.00 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.00 |
6 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
- |
- |
0.01 |
0.15 |
1.10 |
7 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
- |
0.15 |
- |
0.01 |
0.15 |
1.10 |
8 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
- |
- |
0.10 |
0.01 |
0.15 |
1.10 |
9 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
- |
0.01 |
0.15 |
1.10 |
10 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
- |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
1A |
1.00 |
0.80 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
0.80 |
1B |
1.00 |
0.90 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
0.90 |
1C |
1.00 |
0.80 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.26 |
0.80 |
1D |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.26 |
1.10 |
13 |
0.50 |
1.80 |
1.00 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
3.60 |
14 |
2.00 |
1.80 |
1.00 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
0.90 |
15 |
1.80 |
0.70 |
1.00 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
0.39 |
16 |
1.80 |
2.00 |
1.00 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.11 |
17 |
1.80 |
1.80 |
0.10 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.00 |
18 |
1.80 |
1.80 |
1.20 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.00 |
19 |
1.90 |
1.90 |
1.10 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.00 |
20 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
- |
- |
- |
0.01 |
0.15 |
1.10 |
21 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.7 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
22 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.42 |
0.10 |
0.01 |
0.15 |
1.10 |
23 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.5 |
0.15 |
0.22 |
0.01 |
0.15 |
1.10 |
24 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
50 |
0.7 |
0.42 |
0.30 |
0.01 |
0.15 |
1.10 |
25 |
1.00 |
1.10 |
0.50 |
0.45 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
26 |
1.00 |
1.10 |
0.50 |
0.03 |
0.03 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
27 |
1.00 |
1.10 |
0.50 |
0.20 |
0.17 |
50 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
28 |
1.00 |
1.10 |
0.50 |
0.20 |
0.03 |
600 |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
29 |
1.00 |
1.10 |
0.50 |
0.20 |
- |
- |
0.5 |
0.15 |
0.10 |
0.01 |
0.15 |
1.10 |
Note that underlined values indicate that they are out of the claimed range specified
in Claim 1. " - " represents the element is not contained in the alloy specimen. |
[0076] As shown in Tables 1 and 2, A1 alloy forged material examples Nos. 1 - 10, 10A -
10H were excellent in terms of 0.2 % proof stress, Charpy impact value, and stress
corrosion cracking resistance, satisfying the scope of claims of the present invention.
On the other hand, A1 alloy forged material comparative examples Nos. 11-34 did not
satisfy the scope of claims of the present invention, and are inferior in terms of
either 0.2 % proof stress, Charpy impact value, or stress corrosion cracking resistance.
[0077] Specifically, comparative example No. 11 was inferior in terms of the Charpy impact
value and the stress corrosion crack resistance because the content of Mg was less
than the lower limit. Comparative example No. 12 was inferior in terms of the stress
corrosion crack resistance and the electrical conductivity was less than the lower
limit because the content of Mg was more than the upper limit. Comparative example
No. 13 was inferior in terms of the 0.2 % proof stress and the stress corrosion crack
resistance because the content of Si was less than the lower limit. Comparative example
No. 14 was inferior in terms of the Charpy impact value and the stress corrosion crack
resistance and the electrical conductivity was less than the lower limit because the
content of Si was more than the upper limit. Comparative example No. 15 was inferior
in terms of the 0.2 % proof stress because the content of Cu was less than the lower
limit. Comparative example No. 16 was inferior in terms of the stress corrosion crack
resistance and the electrical conductivity was less than the lower limit because the
content of Cu was more than the upper limit. Comparative example No. 17 was inferior
in terms of the Charpy impact value and the stress corrosion crack resistance and
the electrical conductivity was less than the lower limit because the contents of
Mg, Si, and Cu were more than each of the upper limit.
[0078] Comparative example No. 18 was inferior in terms of the 0.2 % proof stress because
it did not contain either Mn or Cr or Zr. Comparative example No. 19 was inferior
in terms of the 0.2 % proof stress and the electrical conductivity was less than the
lower limit because the content of Mn was more than the upper limit. Comparative example
No. 20 was inferior in terms of the 0.2 % proof stress and the stress corrosion crack
resistance because the content of Cr was more than the upper limit. Comparative example
No. 21 was inferior in terms of the 0.2 % proof stress and the stress corrosion crack
resistance because the content of Zr was more than the upper limit. Comparative example
No. 22 was inferior in terms of the 0.2 % proof stress and the electrical conductivity
was less than the lower limit because the contents of Mn, Cr, and Zr were more than
each of the upper limit.
[0079] Although the chemical components satisfy the claimed range, comparative example No.
23 was inferior in terms of the 0.2 % proof stress because the cooling rate was less
than the lower limit. Although the chemical components satisfy the claimed range,
comparative example No. 24 was inferior in terms of the 0.2 % proof stress because
the heating rate was more than the upper limit. Although the chemical components satisfy
the claimed range, comparative example No. 25 was inferior in terms of the 0.2 % proof
stress because the duration in the homogenizing heat treatment step was more than
the upper limit. Although the chemical components satisfy the claimed range, comparative
example No. 26, which is the A1 alloy forged material according to the invention of
Japanese Patent No.
3766357, was inferior in terms of the 0.2 % proof stress because the forging start temperature
was less than the lower limit and hence the electrical conductivity became less than
the lower limit. Although the chemical components satisfy the claimed range, comparative
example No. 27 was inferior in terms of the 0.2 % proof stress and the resistance
to stress corrosion cracking because the forging start temperature was more than the
upper limit and hence the electrical conductivity became more than the upper limit.
Although the chemical components satisfy the claimed range, comparative example No.
28 was inferior in terms of the 0.2 % proof stress because the temperature of the
solution heat treatment was less than the lower limit and hence the electrical conductivity
became more than the upper limit. Although the chemical components satisfy the claimed
range, comparative example No. 29 was inferior in terms of the 0.2 % proof stress
because the temperature of the artificial aging treatment was more than the upper
limit and hence the electrical conductivity became more than the upper limit.
[0080] Comparative example No. 30 was inferior in terms of the Charpy impact value because
the content of Fe was more than the upper limit. Cracking occurred and comparative
example No. 31 could not be forged because the content of Fe was less than the lower
limit. Comparative example No. 32 was inferior in terms of the Charpy impact value
because the content of Ti was more than the upper limit. Comparative example No. 33
was inferior in terms of the Charpy impact value because the content of B was more
than the upper limit. Comparative example No. 34 cracked during the forging step because
the cast structure became coarse as it did not contain Ti and B.