Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to a cast bar of an alumi-
num alloy having improved mechanical properties and worka-
bility and a process for producing the same.
In the AA ~Aluminum Association) standard 2000 series
S aluminum alloys, which have been commercially used for
wrought products, the major alloying component of AA 2000
series is copper.
As is well-known, because of high specific tenacity,
i.e. high strength per unit weight, high impact resistance
and high fatigue strength, aluminum base alloys for wrought
products according to the above-mentioned standard have been
widely used in the production of parts for automobiles and
aircraft and various other machinery and apparatus. The
above-mentioned series alloys for wrought products can be
generally converted into forgings by using either an open
die forging process or a die forging process. In the case
of the open die forging process to be intended for producing
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large-sized forgings, it is usual that billets having a
diameter of from 150 to 300 mm which are produced by a
continuous casting method or a semicontinuous casting method
are used. Large diameter billets, which have been con-
ventionally used as a starting material for producing large-
-sized forgings, exhibit a significantly non-uniform cast
structure throughout the cross-sectional area thereof.
Particularly, the cast structure of the peripheral portion
is substantially different from that of the center portion
of the billets. For this reason, the mechanical properties
of the billet are variable in different portions of the
cross-sectional area of the billets. In addition, there is
a possibility that such billets possess defects, such as
pinholes, segregation, microshrinkage or microcracking.
Accordingly, in order to produce forgings having satis-
factory mechanical properties, it is not only necessary to
select non-defective billets after being inspected
throughly, but it is also necessary to subject the billets
to a repeated forging operation to completely eliminate the
above-mentioned defects and nonuniform cast structure during
the forging process. Accordingly, the production of
large-sized forgings from the conventional billets requires
a long period of time and much labor.
On the other hand, in the case of the die forging
process intended for producing small-sized forgings, it is
usual that starting material having a small diameter of from
5 to 70 mm is used. The small diameter starting material is
generally obtained by producing bil1ets having a diameter of
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from 150 to 300 mm by a continuous casting method, sub-
jecting the billet to a homogenizing heat treatment at a
temperature of from 450 to 600C over a period of 2 to
20 hours and hot-extruding the heat-treated billet at a
temperature of from 350 to 500C. In this manner, the
conventional process for producing the small diameter
starting material requires the extrusion step, which results
in an increase in the production cost.
In addition, the conventional processes for producing
the aluminum alloys for wrought products have the following
disadvantages.
(1) The AA 2000 series alloy
generally exhibits a high deformation resistance
and a low deformability at the extrusion. This feature
makes it difficult to set adequate extrusion conditions such
as an extrusion speed. If the extrusion conditions are
inadequate, the extruded articles have coarse recrystalliza-
tion grains at the region near the peripheral surface
thereof. Microcracks may be generated at and propagated
from the boundaries of the recrystallization grains. In
addition the rupture of the extruded articles may eventually
be caused due to the recrystallization grains and the
intergranular crack.
(2) AA 2000 series alloy exhibits a high
deformation resistance at the extrusion, as described above.
; When these alloys are subjected to extrusion, a temperature
increase by plastic working occurs because of friction
between the die and the workpiece. Accordingly, the working
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temperature i5 higher at a later stage of one extrusion
pass, which results in a change in the properties of the
extruded articles along the longitudinal direction of the
extruded articles. As a result, forgings produced from such
extruded articles exhibit nonuniformity in properties.
(3) When an alloy material is subjected to extrusion
working, the deformation degree in the portion near the
peripheral surface is different from that in the central
portion of the extruded material. As a result, the extruded
article shows a difference in the work structure between
these two portions. Particularly, in the case of AA
standard 2000 alloy, the extruded articles
exhibit a prominently fine work-structure at the portion
near the peripheral surface thereof because of its high
deformation degree, while the central portion thereof
exhibits a coarse work-structure because of its low de-
formation degree. If the extruded articles having such a
nonuniform structure are forged, the fiber structure of
resultant forgings is broken up into fragments and exhibit
poor fatigue and impact strengths.
(4) When billets made of the AA 2000 series
alloys are subjected to extrusion, the crystal grains
and precipitates such as intermetallic compounds in the
billets are forced to elongate in the extrusion direction.
As a result, the extruded articles have a crystal texture
having a particular directional property and are neither
isotropic nor homogeneous. For this reason, the extruded
articles must be forged in consideration of the extrusion
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direction o~ the billet. However, if a product having a
particular shape is to be produced, it is often difficult to
apply the forging to the entire parts of the extruded
article so as to provide these parts with the optimal
directional property. In such a case, the resultant
forgings may suffer from cracks and a local deterioration in
the mechanical properties including fatigue strength.
As described above, the small diameter material for
forgings produced by extrusion possesses inevitable dis-
advantages due to the extrusion, particularly, the dis-
advantages that properties of this material are anisotropic
and nonhomogeneous. For this reason, forgings produced
from such a material do not always exhibit satisfactory
mechanical properties, particularly, high fatigue strength
and high impact strength. The small diameter forging
material may also be produced by using a continuous cast- or
semicontinuous cast-bar. However, in this case, the same
problems as those described for the large diameter billet,
such as the nonuniform structure or structural defects,
arise. Apart from these problems, it is difficult under the
present circumstances to produce a small diameter bar having
a diameter of 100 mm or less by a continuous casting method
on an industrial scale.
The AA 2014 and AA 2017 Al-Cu alloys and AA 7075
Al-Zn-Mg alloys which have been commercially used as a high
strength aluminum base alloy for forgings are superior to
the AA 4000 series alloy in respect to mechanical strength,
while the former alloys are inferior to the latter alloy in
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respect to the heat- and wear-resistance. The AA 4000
series alloy exhibits excellent heat- and wear-resistances
due to the presence of silicon contained therein.
Under these circumstances, the inventors have made
extensive studies to develop a continuously cast bar of
aluminum base alloys for wrought products, particularly, AA
2000 series alloys, so that the bar is: homogeneous in
structure; free of defects; exhibits excellent mechanical
properties, and; can be subjected, as cast or after a heat
treatment, to a working operation such as forging. This
operation ~or producing forgings must be a single operation,
that is the bar is not subjected to any preliminary plastic
working having an adverse effect on the properties of the
cast bar, such as the above-mentioned hot extrusion. As a
lS result, the inventors have found that an adequate
combination of the composition of the aluminum base alloy to
be cast and the cast structure of the aluminum base alloy
produces an aluminum alloy cast bar capable of satisfying
the above-mentioned requirements and that when a melt of an
aluminum base alloy having a particular composition is cast
under particular conditions, it is possible to produce an
aluminum alloy cast bar having the desired structure.
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Embodiments of the present invention seek to provide a
continuously cast bar of aluminum base alloys for forglngs
equivalent to the AA standard 2000 sexies, which bar is of a
homogeneous structure; is free of defects; exhibits excel-
lent mechanical properties, and; can be subjected, as castor after heat treatment, to one of the working operations
for producing wrought products such as forging, without
applying any preliminary plastic working, such as hot extru-
sion, having an adverse effect on the properties of the cast
bar.
Embodiments of the present invention also seek to pro-
vide a process for producing a continuously cast bar of
aluminum base alloys possessing a satisfactory workability,
such as excellent forgeability, by selecting an adequate com-
lS bination of the composition of AA standard 2000 alloys withcast structure thereof and by selecting the casting condi-
tions.
Embodiments of the present invention also seek to
increase the workability of the continuously cast bar pro-
duced by the above-mentioned process by subjecting the bar
to a heat treatment.
Embodiments of the present invention also seek to pro-
vide an aluminum base alloy in cast state, which exhibits
mechanical properties equivalent to, because of no direction-
al properties, or, superior to that possessed by the conven-
tional extrusion of aluminum base alloys.
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The basic technical concept of the present invention
resides in the discovery that, a critical value of the secon-
dary dendrite arm spacing (hereinafter referred to as DAS)
and a critical value of the size of second phase particles
and a critical value of the grain diameter, which critical
values are substantially lower than the DAS, the size of the
second phase particles and the grain diameter in the conven-
tional aluminum alloy ingot, are responsible for fine cast
structure capable of remarkably enhancing the workability
and the mechanical properties of the aluminum base alloys
for forgings having the compositions described hereinbelow.
The process for producing an aluminum base alloy bar
according to the present invention is based on the technical
concept that, when an aluminum base alloy melt i5 cast into
a smaller diameter bar at a higher casting speed than in the
case of the conventional continuous casting method indus-
trially practiced, the resultant bar has a cast structure
which is microscopically fine, macroscopically fine, isotro-
pic and free from defects.
In accordance with one aspect of the present invention,
there is provided a cast bar of an aluminum base alloy forforgings (AA standard 2000 series for forgings), exhibiting
high tensile strength, impact strength and fatigue strength,
which comprises 2.0 to 9.0 wt% of copper, 0.2 to 1.2 wt%
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of magnesium, 0.2 to 1.2 wt% of silicon, 0.2 to 0.8 wt% of
manganese and optionally, titanium or titanium and boron in
a total amount of from 0.005 to 0.15 wt%, and the balance
consisting o~ aluminum and unavoidable impurities, and
wherein the secondary dendrite arm spacing is not greater
than 15 ~m, the grain diameter is not greater than 80 ~m
and the second phase particles comprising intermetallic
compounds have a size not greater than 10 ~m.
Preferably, the ratio of the concentration (a) of the
solute components in the matrix within the crystal grain to
the concentration (b) of the solute components in the grain
boundaries, i.e. a/b, is not smaller than 0.70.
In accordance with another aspect of the present inven-
tion, there is provided a process for producing a cast bar
of an aluminum base alloy, which comprises 2.0 to 9.0 wt% of
copper, 0.2 to 1.2 wt% of magnesium, 0.2 to 1.2 wt% of sili-
con, 0.2 to 0.8 wt% of manganese, and, optionally, titanium
or titanium and boron in a total amount of from 0.005 to
0.15wt%, said process comprising preparing a melt of the
alloy composition and continuously casting the melt at a
solidification speed of not lower than 25C/sec. The pro-
cess may include subjecting the cast bar to a homogenizing
heat treatment at a temperature of from 450 to 530C over a
period of from 0.5 to 20 hours.
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The cast structure of the aluminum base alloy cast bars
of the present invention is fine, homogeneous and isotropic
from the central portion to the peripheral surface through-
out the cross sections theerof taken in any direction. In
addition, the aluminum alloy cast bars exhibit high tensile
strength, impact strength and fatigue strength, and, fur-
ther, even if they may contain a large amount of alloying
elements, exhibit an excellent workability. The aluminum
alloy cast bars of the present invention can be subjected
directly to plastic workings such as forging or mechanical
workings such as machining without applying a preliminary
working, such as extrusion. This feature makes it possible
to produce various aluminum alloy parts at low cost and to
eliminate adverse effects on the parts due to a preliminary
working such as extrusion working. Forgings, therefore,
exhibit excellent properties and are produced at a low cost.
Also, in accordance with the process of the present
invention, aluminum base alloy for forgings exhibiting the
above-mentioned excellent properties can be simply and
easily obtained.
Embodiments of the invention will now be described by
way of example with reference to the accompanying drawings
in which:
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FIGURE 1 is a cross-sectional microscope view of a cast
bar consisting of one of AA standard 2000 series aluminum
alloy and being produced by casting at a solidification
speed of 25C/sec;
FIGURE 2 is the same view as in Fig. 1, except that the
solidification speed was 0.5C/sec;
FIGURE 3 is a graph showing a tensile strength and an
elongation versus sampling positions of a specimen in Exam-
ple l;
FIGURE 4A and 4B are perspective views of a wedge speci-
men and an illustration showing a method for testing, using
the wedge specimen, respectively;
FIGURE 5 is a graph showing a limitative working degree
versus solidification speed of a wedge specimen from Example
2, the limitative working degree being a working degree at
which forging cracks occur;
FIGURE 6 is a graph showing a deformation resistance
and a deformability versus solidification speed of a speci-
men in Example 2;
;20 FIGURE 7 is a graph showing a DAS versus solidification
speed of each of alloy ingots, A, B and C in Example 3;
FIGURE 8 is a graph showing a tensile strength and an
elongation versus solidification speed of an alloy B in Exam-
ple 3;
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FIGURE 9 is a graph showing an impact value and a reduc-
tion of area versus solidification speed of an alloy B in
Example 3;
FIGURE 10 is a graph showing a fatigue strength in
terms of test load of a specimen in Eample 4; and
FIGURES llA, llB AND llC are each an illutration show-
ing an upsetting test.
The term "working" as used herein is not always limited
to forging, and it is intended to include other plastic work-
ings such as rolling, drawing, wire drawing and extrusion
and also machining such as cutting. Accordingly, the alumi-
num base alloys for forgings of the present invention can be
subjected to various plastic workings and machining. It is
to be understood that the term "continuous casting" as used
herein is intended to include not only a so-called completed
continuous casting, but also a semi-continuous casting for
producing a certain length of cast articles.
The term "secondary dendrite" as used herein is used in
an ordinary sense in the field of metallography. The secon-
dary dendrite is distinguished from the dendrite cell mean-
ing a primary dendrite.
The fine structure of AA standard 2000 series alloy forforgings according to the present invention is explained
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first. This alloy should have the above-mentioned composi-
tion and a fine cast structure, such that the grain diameter
is not greater than 80 ~m, the DAS is not greater than 15
~m and the second phase particles comprising intermetallic
compounds have a size not exceeding 10 ~m.
The intermetallic compounds constituting the second
phase particles include Al-Cu, Mg-Si, Al-Mn-Fe, Al-Fe-Si com-
pounds and the like.
When the grain diameter, the DAS and the second phase
particle size are outside each of the above defined values,
even if the composition of the alloy is within the composi-
tional ranges as described hereinafter, the resultant cast
bar cannot exhibit desired properties, such as high tensile
strength, high fatigue strength and high impact strength.
In addition, the isotropic macrostructure of the alloy is
lost and the macrostructure tends to be inhomogeneous, with
the result that the alloy exhibits a poor workability.
The AA standard 2000 series homogeneous alloys of the
present invention are more homogeneous when the ratio of the
concentration (a) of the solute components, such as Cu, Cr,
Mg, Si atld Mn, in the matrix of the crystal grains to the
concer.tration (b) of the solute components, such as Cu, Mg,
Si and Mn, contained in the grain boundaries, i.e., a/b, is
0.70 or more. The concentration (b) can be determined by
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locally applying the X-ray over a specimen across the grain
boundaries by emission spectroanalysis or X-ray analysis,
and then comparing the detected concentration of the compo-
nents of the grain boundaries with the detected concentra-
tion of the components in the matrix of the crystal grains.In accordance with the present invention, enrichment of the
solute components in the grain boundaries is suppressed so
as to avoid a reduction in the ductility of the aluminum
base alloys.
It is sometimes difficult to maintain by the control of
the solidification speed the macroscopically isotropic pro-
perty of the cast bars. In such a case, if the cast bars
produced by a solidification speed of at least a critical
cooling speed is subjected to a homogenizing heat treatment
suitable for adjusting the distribution of the solute compo-
nents so as to obtain a ratio of a/b of at least 0.70, the
homogenized bars exhibit almost the same desired properties
as those attained by the aluminum base alloys for wrought
products, having the fine structure. In order to obtain the
concentration a ratio of a/b of at least 0.70, the
homogenizing heat treatment is carried out at a temperature
of from 450 to 530C over the period of 0.5 to 20 hours for
AA standard 2000 series alloy.
Ideally, not only the limitation of the grain diameter,
the DAS and the second phase particle size to the above-
mentioned maximum values but also the limitation of the
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ratio, a/b, to the value of at least 0.70 should be satis-
fied in AA standard 2000 series alloys for forgings. In
this case, the workability and the mechanical properties of
the cast bar are further enhanced as compared in the alloys
S satisfying grain diameter, DAS and second phase particle
size but not the concentration ratio, a/b.
The aluminum alloy bar satisfying the grain diameter,
DAS and the second phase particle size can be produced by a
continuous casting process wherein the solidification speed
at any site throughout the cross section of the bars is set
to at least a critical speed.
The critical solidification speed is 25C/sec for the
AA standard 2000 series alloy.
When the solidification speeds are gradually increased
lS from a level much lower than the critical solidification
speeds in the production of the AA standard 2000 series, the
grain diameter, the DAS and the second phase particles of
the alloys become extremely small or very fine in the vicin-
ity of the critical solidification speeds, whereby the
above-mentioned structure conditions are satisfied. In
other words, solidification of at least the critical solidi-
fication speeds permits to provide the cast bars with the
desired mechanical properties and workability. For this rea-
son, the solidification speeds should be set to values of at
least the critical speeds.
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The term "solidification speed" as used herein refersto a temperature dropping speed at the interface between the
solid phase and the liquid phase of an alloy placed in a
continuous casting mold. The temperature dropping speed can
be experimentally detected, for example, by inserting a
thermocouple into the liquid phase from above the mold and
determining a change in temperature of the place where the
thermocouple comes into a contact with the solid phase.
In the continuous casting production of the aluminum
base alloys for forgings of the present invention, a conven-
tional float type continuous casting process is difficult toapply for the production of bars having a diameter of 100 mm
or less. As far as the inventors know, a gas pressure~
applying type hot top continuous casting process, which has -
lS been proposed in USP No. 4,157,728 is the best process for
producing a bar with a small diameter of particularly from 5
to 70 mm. It is however, to be understood that if any con-
tinuous casting process other than the process of USP
No.4,157,728 allows a solidification speed equal to or
greater than the critical solidification speed of 25C/sec,
this process can be applied to the processes of the present
invention.
The cast bars thus prepared may be subjected directly
to plastic working or mechanical working. Alternately, the
cast bars may be subjected to a homogenizing heat treatment
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before being worked on by various procedures. Furthermore,
the cast bar may be subjected to a heat treatment such as T6
treatment before being worked on by various procedures.
The fact that the cast structure of AA standard 2000
series alloy becomes suddenly ~ine at a solidification speed
of about 25C~sec will be illustrated with reference to
Figs. 1 and 2. Fig. 1 shows a microscope view of a
structure of an aluminum base alloy bar consistinq of
4.5 wt% of Cu, 0.6 wt% of Mg, 0.6 wt% of Si, 0.4 wt% of ~n,
0.01 wt~ of Ti and the balance being Al and trace of
impurities. The aluminum base alloy was continuously cast
at a solidification speed of 25C/sec into the form of the
bar which was cut so as to observe the micro structure of
the bar cross section. Fig. 2 shows a microscope view of
the same aluminum base alloy bar as in Fig. 1 except that
the bar was produced by casting with the solidification
speed of 0.5C/sec. As being apparent from Fig. 1, the
granular crystals are uniformly distributed through the
structure shown in Fig. 1, the DAS is not greater than 15 ~m
and all of the second phase particles comprising inter-
metallic compounds have a size not greater than 10 ~m, with
regard to the alloy cast at the high solidification speed.
In contrast, as being apparent from Fig. 2, the DAS is
greater than 15 ~m, the second phase particles comprising
intermetallic compounds are significantly coarse with regard
to the alloy cast at the low solidification speed.
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As a result of our investigations of the relationship
between the solidification speed and the propertie~ of the
resultant alloy ingot, the lower limit of the solidlfication
speed was found to be 25C/sec for AA standard 2000 series.
It goes without saying that in order to produce an alloy
ingot at a high solidification speed, a continuous casting
is the most suitable. The most convenient continuous cast-
ing process which can be industrially used, today, is a ver-
tical semi-continuous casting process. In order to realize
a high solidification speed, for example 25C/sec or more,
in this vertical semi-continuous casting process, casting a
melt into a bar having a small diameter is appropriate. The
solidifcation speed 25C/sec or more can be realized by
determining the diameter of the resultant bar to a small
size of from 40 to 100 mm without making any substantial
modification in the cooling-water injecting conditions, such
as the water temperature, the water flow rate, and the
water-injecting position, usually employed in the operation
of the process according to USP No. 4,157,728.
The alloy compositions of the present invention will be
discussed, below.
First, the reasons for the limitation of the co~position
of the AA standard 200 series alloy will be illustrated. If
the copper content is less than 2.0 wt%, the resultant bar
exhibits an unsatisfactory mechanical strength. If the
copper content is more than 9.0 wt~, the copper is not
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79
solutionized to a satisfactory extent even by a solution
treatment and is precipitated as intermetallic compounds.
As a result, the mechanical properties, such a~ tensile
strength, elongation and impact strength, of the cast bar
become inferior, which leads to the formation of casting
cracks of the resultant bar during a continuous casting oper-
ation. However, by establishing a solidification speed of
25C/sec or higher at any site throughout the solid-liquid
interface, it is possible to carry out the continuous cast-
ing operation smoothly without causing any casting cracksdue to high copper content of up to 9.0 wt%. Since a high
cooling speed is maintained uniformly over the bar cross sec-
tion being cast, the segregation tendency seen in the bar
cross section is reduced and the solidification, which is
quite similar to an ideal unidirectional solidification,
takes place. Such solidification seems to make it possible
to increase the maximum copper content, at which no serious
copper segregation occurs, as compared with the prior art.
Also, if the magnesium content is less than 0.2 wt%,
the resultant cast bar exhibits a poor tensile strength. If
the magnesium content is more than 1.2 wt%, the Mg-Si inter-
metallic compounds are formed, which causes deterioration of
elongation and tensile strength of the resultant cast bar.
As a result of the intermetallic compounds, a cast bar exhi-
biting the desired properties, i.e. high fatigue strengthand high impact strength, cannot be obtained. Also, if the
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silicon content is less than 0.2 wt%, the resultant alloy is
not heat-treatable. On the other hand, a silicon content
more than 1.2 wt~ causes deterioration of a tensile strength
and impact strength of resultant cast bar. A manganese con-
tent less than 0.2 wt% is ineffective for obtaining high ten-
sile strength and impact strength. If the manganese content
is more than 0.8 wt%, coarse grains are formed, with the
result that the structure conditions described above cannot
be satisfied, which causes the resultant cast bar to exhibit
a poor tensile strength. The aluminum alloy of the present
invention may contain, if necessary, titanium. Alternately,
the aluminum alloy may contain both titanium and boron, in a
total amount of from 0.005 to 0.15 wt%. The titanium or
both titanium and boron is effective for further refining
the crystal grains and thus, for attaining more excellent
mechanical properties.
The present invention will be further illustrated by
the examples set forth below, which are provided for the pur-
pose of illustration and should not be interpreted as in any
way limiting the scope of the present invention.
Example 1
Run No. 1 (invention)
An aluminum alloy melt comprising 4.7 wt% of copper,
0.7 wt% of silicon, 0.6 wt% of manganese, 0.5 wt% of
magnesium, 0.015wt% of titanium and boron, and the balance
consisting essentially of aluminum was prepared and the melt
was subjected to a hot top continuous casting process at a
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solidification speed o~ 25C/sec, thereby to produce a round
bar having a diameter of 53 mm. The gas pressure was
applied to the molten metal body being cast as described in
USP No. 4,157,728.
Run No. 2 (control)
The same procedures as those described in Run No. 1
were repeated, except that a solidification speed of
0.15C/sec was used.
Run No . 3 (control)
The same procedures as those described in Run No. 1
were repeated, except that a solidification speed of 3C/sec
was used.
From each of the round bars obtained by Run Nos. 1, 2
and 3, specimens were cut out along the longitudinal
15 direction of the bars at varying distances from the outer
periphery thereof. Each of the specimens was heated at a
temperature of 505C for 6 hours and, then, cooled in hot
water. Thereafter, the specimens were aged at a temperature
of 170C for 8 hours to produce a so-called T6 material.
20 The tensile strength and elongation at normal temperature of
the specimens were determined and the results are shown in
Fig. 3. In Fig. 3, the cross symbol X denotes a specimen
from Run No. 1 (25C/sec), the black circle symbol denotes a
specimen from Run No. 2 (0.15C/sec) and the circle
25 symbol O denotes a specimen from Run No. 3 (3C/sec). It is
apparent from Fig. 3 that in the case of solidification
speeds of 0.15C/sec and 3C/sec, a considerable scattering
of tensile strength and elongation is noted between the
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outer periphery portion and the central portion. In the
case of the solidification speed of 25C/sec, there is
little scattering in both tensile strength and elongation
between the outer periphery and the central portions, which
suggests almost homogeneous properties of the cast bar from
the outer periphery to the central portions. Al so, the DAS,
the second phase particle size and the ratio a/b of solute
concentration within the matrix of the grain to solute
concentration in the grain boundary of the ingot bar
obtained by Run Nos. 1, 2 and 3 were determined. The
results are shown in Table 1.
Table 1
Solidification Si f secondary solute con-
s~eed DAS phase pa~ticles ratio a/b
C/sec m ~m
Run No.2 0.15 40 21 40
Run No.3 3 25 16 55
Run No.l 25 12 8 75
AS iS apparent from Table 1, in the case of the
solidification speed of 25C/sec, both DAS and second phase
particles are fine-sized and the ratio of the solute
component concentration ~a) to the solute component con-
centration (b), hereinafter referred to as the soluteconcentration ratio a/b, is high, while in the case of
solidification speeds of 0.15C/sec and 3C/sec, both DAS
and second phase particles are significantly coarse and the
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solute concentration ratio a/b, is low. In view of these
results and the test results shown in Fig. 3, it is apparent
that the structure factors have a great influence on the
tensile strength and the elongation o~ the cast bar.
Example 2
An alloy melt comprising 4.5 wt~ of copper, 0.6 wt% of
silicon, 0.6 wt~ of magnesium, 0.8 wt% of manganese,
0.015 wt% of titanium and the balance consisting essentially
of aluminum was prepared, and the melt was cast into a round
bar having a diameter of 78 mm by means of a hot top
continuous casting process. The gas pressure was applied to
the molten metal body being cast as described in USP
No. 4,157,728. This casting is hereinafter referred to as
the gas-pressure applying type hot top continuous casting.
The solidification speed was varied between 5 and 80C/secO
The bar was homogenized at a temperature of 505C for
8 hours to produce test specimens.
Wedge specimens as shown in Fig. 4A were cut out from
each of the test specimens. The wedge specimens were
hot-forged at a temperature of 300, 400 and 450C, re-
spectively, thereby to determine a limitative working degree
at which forging cracks were generated on the specimens.
The wedge test was carried out according to a testing
procedure described in "Metal Plastic Working", (in
Japanese) Kenzo Kato, published by Maruzen Co. Ltd. In this
testing procedure, a specimen in the form of a wedge as
shown in Fig. 4A is placed on a platen 2 as shown in
Fig. 4B. Then, a 1/2 ton hammer 3 was struck on the wedge
~ 67g
- 24 -
specimen 4. The working limit i~ determined on the basis of
the crack of the specimen 4 generated after the forging
operation. This procedure is very suitable and reliable for
evaluating the forgeability of a material.
The test results are shown in Fig. 5. The obliquely
line region of Fig. 5 above the polygonal lines indicates
the levels of forging temperature and wedge-shaped reduction
ratio allowing the forging without cracks. It is apparent
from Fig. 5 that the limitative working degree is increased
as the forging temperature is increased. Also, when the
solidification speed is high the limitative working degree
is high and forging cracks are difficult to occur at any
forging temperature. Particularly, the limitative working
degree is remarkably increased when the solidification speed
is increased to about 25C/sec. This fact corresponds to an
abrupt change in structure at the solidification speed of
about 25C/sec as described hereinabove. In addition, in
order to evaluate workability by gen~ral hot working in-
cluding forging and rolling of the cast bars produced at
various solidification speeds, the testing material was
subjected to a hot torsion test (cf. for example, "Light
Metal n ~ Horiuchi et al, (in Japanese) Vol. 20, No. 5) to
determine the deformation resistance and the deformability
thereof. The results are shown in Fig. 6. As is apparent
from Fig. 6, the deformation resistance change depending on
the solidification speed increase displays an abrupt de-
crease at about 25C/sec and the deformability change
depending on the solidification speed displays an abrupt
, . .. . . .
- 25 -
increase at a 501idification speed of about 25C/sec. In
view of these results, it is clear that workability of the
cast bars according to the present invention is excellent.
Example 3
Analloy melt comprising 2.3 wt% of copper, 0.3 wt~ of
magnesium, 0.3 wt% of silicon, 0.2 wt~ of manganese,
0.02 wt% of titanium and the balance consisting essentially
of aluminum (which alloy is referred to as alloy A), alloy
melt comprising 4.5 wt% of copper, 0.6 wt% of magnesium,
0.7 wt% of silicon, 0.6 wt% of manganese, 0.01 wt~ of
titanium and the balance consisting essentially of aluminum
(which alloy is referred to as alloy B) and an alloy melt
comprising 8.7 wt% of copper 1.0 wt% of magnesium, 1.0 wt%
of silicon, 0.7 wt% of manganese, 0.015 wt% of total of
titanium and boron and the balance consisting essentially of
aluminum (which alloy is referred to as alloy C) were
separately prepared. Each of these melts was cast into a
round bar having a diameter of 62 mm by means of a gas-
pressure applying type hot top continuous casting process at
a solidification speed varying between about 6 to about
80C/sec. The master alloy comprising 5 wt% of titanium,
0.7 wt% of boron and the balance consisting essentially of
aluminum was added into the melt of the alloy C so as to
incorporate titanium and boron.
The DAS of each of the round bars prepared above is
shown in Fig. 7.
As is clearly shown in Fig 7, the DAS change of each
of the alloys A, B and C depending on the solidification
D
11 7 76~9
- ~6 -
speed increase displays a prominent decrease at the solidi-
fication speed up to about 25C/sec, and DAS assumed an
almost constant value of about 6 ~m at a solidification
speed above 25C/sec which is a critical solidification
speed. This clearly indicates that a solidification speed
of 25C/sec is significant for the fine structure of the AA
standard 2000 series alloy according to the present in-
vention, that is, 25C/sec is a critical solidification
speed at which the structure of such alloy becomes fine.
In addition, as a representative example of the above-
-mentioned three alloys, the alloy B was cast at various
solidification speeds into the bars and the tensile strength
and elongation of the bars were measured. The results are
shown in Fig. 8. It was confirmed from Fig. 8 taking into
consideration Fig. 7, that both tensile strength and
elongation are increased with the increase in the DAS
values. Particularly, an increase in elongation is outstand-
ingly shown. Furthermore, the cast bars produced from the
alloy B at various solidification speeds were subjected to
an impact test and a drawing test. The results are shown in
Fig. 9. It was also confirmed that the solidification
speed of 25C/sec causes a prominent increase of the
mechanical properties, i.e., impact value and the reduction
of area.
Example 4
Run No. 4 (invention)
An alloy melt comprising 4.0 wt% of copper, 0.2 wt~ of
silicon, 0.6 wt% of magnesium, 0.6 wt% of manganese, 0.01 wt%
: - ' ,
. .
li77679
-- 2 7
of titanium and th~ balance con9isting essentially of
aluminum was prepared, and cast into a round bar having a
diameter of 35 mm. The casting was carried out by the
gas-pressure applying type hot top continuous casting and
the solidification speed was 30C/sec. The bar was forged
into a connecting rod by a conventional hot forging. The
connecting rod was heat-treated at a temperature of 505C
for 2 hours, and, then, water-cooled, for the purpose of
solution treatment. Thereafter, the connecting rod was aged
at room temperature (T4 treatment) for two days. The
resultant material in the form of the connecting rod was
subjected to a fatigue test.
Run No. 5 (control)
An alloy melt having the same composition as that of
Run No. 4 was cast by a conventional direct chill-casting
method. The obtained cast bar was extruded at an extrusion
ratio of 40, thereby to produce an extruded bar having a
diameter of 35 mm. The extruded bar was hot forged, solu-
tionized and was subject to a T4 treatment successively,
under the same conditions as those described in Run No. 4.
A fatigue test was carried out on the resultant bar.
The results of the fatigue tests carried out in Run
Nos. 4 and 5 are shown in Fig. 10. Since the stress levels
in the connecting rod are locally varied within the planes
across the connecting rod, the ordinate axis of Fig. 10
indicates not the stress but test load. It is clear from
Fig. 10 that the forged material produced by Run No. 4
exhibits a remarkably excellent fatigue strength as compared
~7~679
- 28 -
with the forged material produced by Run No. 5. The super-
iority of the material of Run No. 4 to the material of Run
No. 5 is probably ascribable to the following. The material
of Run No. 5 subjected to the forging is an extruded
material having a fibrous texture developed during the
extrusion, and the fibrous texture is broken up during the
forging, with the result that the forged product cannot
prossess a normal fibrous texture, while the material of Run
No. 4 subjected to the forging is a cast bar having a homo-
geneous internal structure and a macroscopically isotropicproperty, with the result that a normal fibrous structure is
formed at any direction of forging force applied to the
production of the connecting rod.
Example 5
Analuminum alloy melt comprising 4.0 wt% of copper,
0.6 wt% of magnesium 0.3 wt% silicon, 0.6 wt% manganese,
0.02 wt% titanium, and the balance consisting essentially of
Al was prepared, and the melt was cast into a round bar
having a diameter of 53 mm. The casting was carried out by
the gas-pressure applying hot top continuous casting and the
solidification speed was 30C/sec. For comparison purpose,
round bar having a diameter of 53 mm was produced according
to the same procedures as those described above, except that
the gas pressure was not applied to the cast melt body.
The existence and non existence of the gas-pressure
application caused a change in the surface of the cast bars.
The influence of the surface on the forgeability was in-
vestigated by the method illustrated in Figs. llA, B and C.
.~
~1..?
1~7~6q9
- 29
That is, a billet 11 having diameter of 53 mm and a length
of 140 mm was subjected to upsetting by using a hammer 10,
then, the free surface was observed to determine whether or
not lla, cracks were formed. In the case of no application
of gas pressure, the resultant cast bar has a lapping
surface or a bleb surface and tends to generate cracks on
the free surface of the ingot at the first and second steps
shown in Figs. llA and llB. Microscope inspection of the
structure of this cast bar indicated that defects such as
cracks, pinholes, and blow holes, were sporadically present
in the vicinity of the surface of cast bar. These defects
are responsible for the notch effect, causing cracks during
forging. In contrast, in the cast of application of gas
pressure, the resultant cast bar has a smooth surface and
tends not to crack in the three steps shown in Figs. llA
through llC. In this case, forging without surface defects
was obtained.
