Note: Descriptions are shown in the official language in which they were submitted.
CA 022203~7 1997-11-06
--1--
METHOD OF S~APING SEMISOLID METALS
Background of the Invention
This invention relates to a method of shaping
semisolid metals. More particularly, the invention relates
to a method of shaping semisolid metals, in which li~uid
alloy having crystal nuclei and at a temperature not lower
than the liquidus temperature or a partially solid,
partically liquid alloy having crystal nuclei and at a
temperature less than the liquidus temperature but not
lower than the molding temperature is poured into a holding
vessel, cooled at an average cooling rate in a specified
range and held as such until just prior to the start of
shaping under pressure, whereby fine primary crystals are
generated in the alloy solution and the alloy within said
holding vessel is temperature adjusted by induction heating
such that the temperatures of various parts of the alloy
fall within the desired molding temperature range for the
establishment of a specified fraction liquid not later than
the start of shaping and the alloy is recovered from the
holding vessel, supplied into a forming mold and shaped
under pressure.
The invention also relates to a method of shaping
semisolid metals, in which a molten aluminum or magnesium
alloy containing a crystal grain refiner which is held
superheated to less than 50~ above the liquidus temperature
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~_ -2-
is poured directly into a holding vessel without using any
cooling jig and held for a period from 30 seconds to 30
minutes as the melt is cooled to the molding temperature
where a specified fraction liquid is established such that
the temperature of the poured alloy which is either liquid
and superheated to less than 10~ above the liquidus
temperature or which is partially solid, partially liquid
and less than 5~ below the liquidus temperature is allowed
to decrease from the initial level and pass through a
temperature zone 5~ below the liquidus temperature within
10 minutes, whereby fine primary crystals are generated in
the alloy solution, and the alloy is recovered from the
holding vessel, supplied into a forming mold and shaped
under pressure.
Various methods for shaping semisolid metals are known
in the art. A thixo-casting process is drawing researcher's
attention these days since it involves a fewer molding
defects and segregations, produces uniform metallographic
structures and features longer mold lives but shorter
molding cycles than the existing casting techniques. The
billets used in this molding method (A) are characterized
by spheroidized structures obtained by either performing
mechanical or electromagnetic agitation in temperature
ranges that produce semisolid metals or by taking advantage
of recrystallization of worked metals. On the other hand,
CA 022203~7 1997 -11- 06
raw materials cast by the existing methods may be molded in
a semisolid state. There are three examples of this
approach; the first two concern magnesium alloys that will
easily produce an equiaxed microstructure and Zr is added
to induce the formation of finer crystals [method (B)] or a
carbonaceous refiner is added for the same purpose [method
(C)]; the third approach concerns aluminum alloys and a
master alloy comprising an Al-5~ Ti-l~ B system is added as
a refiner in amounts ranging from 2 - 10 times the
conventional amount ~method (D?]. The raw materials
prepared by these methods are heated to temperature ranges
that produce semisolid metals and the resulting primary
crystals are spheroidized before molding. It is also known
that alloys within a solubility limit are heated fairly
rapidly up to a temperature near the solidus line and,
thereafter, in order to ensure a uniform temperature
profile through the raw material while avoiding local
melting, the alloy is slowly heated to an appropriate
temperature beyond the solidus line so that the material
becomes sufficiently soft to be molded [method (E)]. A
me~hod is also known, in which molten aluminum at about 700
is cast to flow down an inclined cooling plate to form
partialy molten aluminum, which is collectd in a vessel
[method (F)].
These methods in which billets are molded after they
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are heated to temperatures that produce semisolid metals
are in sharp contrast with a rheo-casting process (G), in
which molten metals containing spherical primary crystals
are produced continuously and molded as such without being
solidified to billets. It is also known to form a rheo-
casting slurry by a method in which a metal which is at
least partially solid, partially liquid and which is
obtained by bringing a molten metal into contact with a
chiller and inclined chiller is held in a temperature range
that produces a semisolid metal [method (H)].
Further, a casting apparatus (I) is known which
produces a partially solidified billet by cooling a metal
in a billet case either from the outside of a vessel or
with ultrasonic vibrations being applied directly to the
interior of the vessel and the billet is taken out of the
case and shaped either as such or after reheating with r-f
induction heater.
However, the above-described conventional methods have
their own problems. Method (A) is cumbersome and the
production cost is high irrespective of whether the
agitation or recrystallization technique is utilized. When
applied to magnesium alloys, method (B) is economically
disadvantageous since Zr is an expensive element and
speaking of method (C), in order to ensure that
carbonaceous refiners will exhibit their function to the
CA 022203~7 1997-11-06
fullest extent, the addition of Be as an oxidation control
element has to be reduced to a level as low as about 7 ppm
but then the alloy is prone to burn by oxidation during the
heat treatment ~ust prior to molding and this is
inconvenient in operations.
In the case of aluminum alloys, about S00 ~m is the
size that can be achieved by the mere addition of refiners
and it is not easy to obtain crystal grains finer than 200
m. To solve this problem, increased amounts of refiners are
added in method (D) but this is industrially difficult to
implement because the added refiners are prone to settle on
the bottom of the furnace; furthermore, the method is
costly. Method (E) is a thixo-casting process which is
characterized by heating the raw material slowly after the
temperature has exceeded the solidus line such that the raw
material is uniformly heated and spheroidized. In fact,
however, an ordinary dendritic microstructure will not
transform to a thixotropic structure (in which the primary
dendrites have been spheroidized) upon heating. According
to method (F). partially molten aluminum having spherical
particles in the microstructure can be obtained
conveniently but no conditions are available that provide
for direct shaping.
What is more, thixo-casting methods (A) - (F) have a
common problem in that they are more costly than the
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existing casting methods because in order to perform
molding in the semisolid state, the liquid phase must first
be solidified to prepare a billet, which is heated again to
a temperature range that produces a semisolid metal. In
addition, the billets as the starting material are
difficult to recycle and the fraction liquid cannot be
increased to a very high level because of handling
considerations. In contrast, method (G) which continuously
generates and supplies a molten metal containing spherical
primary crystals is more advantageous than the thixocasting
approach from the viewpoint of cost and energy but, on the
other hand, the machine to be installed for producing a
metal material consisting of a spherical structure and a
liquid phase requires cumbersome procedures to assure
effective operative association with the casting machine to
yield the final product. Specifically, if the casting
machine fails, difficulty arises in the processing of the
semisolid metal.
Method (H) which holds the chilled metal for a
specified time in a temperature range that produces a
semisolid metal has the following problem. Unlike the thixo-
casting approach which is characterized by solidification
into billets, reheating and subsequent shaping, the method
(H) involves direct shaping of the semisolid metal obtained
by holding in the specified temperature range for a
CA 022203~7 1997 -11- 06
specified time and in order to realize industrial
continuous operations, it is necessary that an alloy having
a good enough temperature profile to establish a specified
fraction liquid suitable for shaping should be formed
within a short time. However, the desired rheo-casting
semisolid metal which has a fraction liqùid and a
temperature profile that are suitable for shaping cannot be
obtained by merely holding the cooled metal in the
specified temperature range for a specified period.
In method (I), a case for cooling the metal in a
vessel is employed but the top and the bottom portions of
the metal in the vessel will cool faster than the center
and it is difficult to produce a partially solidified
billet having a uniform temperature profile and immediate
shaping will yield a product of nonuniform structure. What
is more, considering the need to satisfy the requirement
that the partially solidified billet as taken out of the
billet case have such a temperature that the initial state
of the billet is maintained, it is difficult for the
fraction liquid of the partially solidified billet to
exceed 50~ and the maximum that can be attained practically
is no more than about 40%, which makes it necessary to give
special considerations in determining injection and other
conditions for shaping by diecasting. If the fraction
liquid of the billet has dropped below 40%, it could be
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reheated with a r-f induction heater but is is still
difficult to attain a fraction liquid in excess of 50~ and
special considerations must be made in in~ection and other
shaping conditions. In addition, eliminating any
significant temperature uneveness that has occurred within
the partially solidified billet is a time-consuming
practice and it is required, although for only a short
time, that the r-f induction heater produce a high power
comparable to that required in thixo-casting. In addition
it is necessary to install multiple units of the r-f
induction heater in order to achieve continuous operation
in short cycles.
Another problem with the industrial practice of
shaping semisolid metals in a continuous manner is that if
a trouble occurs in the casting machine, the semisolid
metal may occasionally be held in a specified temperature
range for a period longer than the prescribed time. Unless
a certain problem occurs in the metallographic structure,
it is desired that the semisolid metal be maintained at a
specified temperature; in practice, however, particularly
in the thixo-casting process where the semisolid metal is
held with its temperature elevated from room temperature,
the metallographic structure becomes coarse and the billets
are considerably deformed (progressively increase in
diameter toward the bottom) and, in addition, such billets
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_ 9 _
are usually discarded, which is simply a waste in
resources, unless their temperatures are individually
controlled.
The present invention has been accomplished under
these circumstances of the prior art and its principal
object is to provide a method that does not use billets or
any cumbersome procedures but which ensures that semisolid
metal (including those which have higher values of fraction
liquid than what are obtained by the conventional thixo-
casting process) which are suitable for subsequent shapingon account of both a uniform structure containing
spheroidized primary crystals and uniform temperature
profile can be produced in a convenient and easy way with
such great rapidity that the power requirement of the r-f
induction heater is no more than 50% of what is commonly
spent in shaping by the thixo-casting process, said
semisolid metals being subsequently shaped under pressure.
Summary of the Invention
The stated object of the invention can be attained by
the method of shaping a semisolid metal recited in claim 1,
in which liquid alloy having crystal nuclei and at a
temperature not lower than the liquidus temperature or a
partially solid, partially liquid alloy having crystal
nuclei and at a temperature less than the liquidus
temperature but not lower than the molding temperature is
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--10--
poured into a holding vessel having a thermal conductivity
of at least 1 kcal/mh~, cooled at an average cooling rate
of 0.01 ~/s - 3.0 ~/s and held as such until just prior to
the start of shaping under pressure, whereby fine primary
crystals are generated in said alloy solution and the alloy
within said holding vessel is temperature adjusted by
induction heating such that the temperatures of various
parts of the alloy fall within the desired molding
temperature range for the establishment of a specified
fraction liquid no later than the start of shaping and the
alloy is recovered from said holding vessel, supplied into
a forming mold and shaped under pressure.
According to claim 2, the induction heating mentioned
in claim 1 is for effecting thermal adjustment such that a
specified amount of electric current is applied for a
specified time immediately after the pouring of the molten
alloy before the representative temperature of the alloy
slowly cooling in the holding vessel has dropped to at
least 10~ below the desired molding temperature, so that
the temperatures of various areas of the alloy within said
holding vessel fall within the limits of +5~ of the desired
molding temperature.
According to claim 3, once the temperatures of various
parts of the alloy within the holding vessel have been
adjusted by induction heating to fall within the desired
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molding temperature range within a specified time, the
temperature of said alloy is held until just before the
start of the shaping step by induction heating at a
frequency comparable to or higher than the frequency used
in the induction heating for the preceding temperature
ad~ustment.
According to claim 4, either the top portion or the
bottom portion or both of the holding vessel are heat-
retained or heated to a higher temperature than the middle
portion or the top and bottom portions of the holding
vessel are smaller in wall thickness than the middle
portion.
According to clam 5, the alloy within the holding
vessel is cooled by blowing either air or water or both
against said holding vessel from its outside.
According to claim 6, either air or water or both
which are at a specified temperature are blown from at
least two different, independently operable heights
exterior to the holding vessel such that the blowing
conditions and times can be varied freely.
According to claim 7, the alloy to be supplied into
the forming mold has a fraction li~uid of at least 1.0~ but
less than 75%.
According to claim 8, the crystal nuclei are generated
by vibrating the alloy which builds up in the holding
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_ -12-
vessel by pouring in a melt superheated to less than 50~
above the liquidus temperature, the vibration being applied
to said alloy either by means of vibrating rod which is
submerged in the melt during its pouring so that it has
direct contact with the alloy or by vibrating not only the
vibrating rod but also the holding vessel as the alloy is
poured into said holding vessel.
According to claim 9, the crystal nuclei are generated
by pouring a molten aluminum alloy into the holding vessel,
said alloy being held superheated to less than 50~ above
the liquidus temperature and containing 0.001% - 0.01~ B
and 0.005% - 0.3% Ti.
According to claim 10, the crystal nuclei are
generated by pouring a molten magnesium alloy into the
holding vessel, said alloy being held superheated to less
than 50~ above the liquidus temperature and containing
0.01% - 1.5% Si and 0.005% - 0.1~ Sr or 0.05% - 0.30% Ca
alone.
The stated of the invention can also be attained by
the method of shaping a semisolid metal recited in claim
11, in which a molten aluminum or magnesium alloy
containing a crystal grain refiner which is held
superheated to less than 50~ above the liquidus temperature
is poured directly into a holding vessel without using any
cooling jig and held for a period from 30 seconds to 30
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minutes as the melt is cooled to the molding temperature
where a specified fraction liquid is established such that
the temperature of the poured alloy which is liquid and
superheated to less than 10~ above the liquidus temperature
or which is partially solid, partially liquid and less than
5~ below the liquidus temperature is allowed to decrease
from the initial level and pass through a temperature zone
5~ below the liquidus temperature within 10 minutes,
whereby fine primary crystals are generated in said alloy
solution, and the alloy is recovered from the holding
vessel, supplied into a forming mold and shaped under
pressure.
According to claim 12, the aluminum alloy mentioned in
claim 11 has 0.03% - 0.30% Ti added and superheated to less
than 30~ above the liquidus temperature as it is poured
into the holding vessel.
According to claim 13, the aluminum alloy mentioned in
claim 11 has 0.005~ - 0.30% Ti and 0.001% - 0.01% B added
and superheated to less than 50~ above the liquidus
temperature as it is poured into the holding vessel.
According to claim 14, the temperature of the alloy
poured into the holding vessel is held by temperature
adjustment through induction heating such that the
temperatures of various parts of said alloy within said
holding vessel are allowed to fall within the desired
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molding temperature range for the establishment of a
specified fraction liquid not later than the start of
shaping.
Brief Description of the Invention
Fig. 1 is a diagram showing a process sequence for the
semisolid forming of a hypoeutectic aluminum alloy having a
composition at or above a m~x~mllm solubility limit
according to the invention;
Fig. 2 is a diagram showing a process sequence for the
semisolid forming of a magnesium or aluminum alloy having a
composition within a m~ximum solubility limit according to
the invention;
Fig. 3 shows the process flow in Examples 1 - lO which
starts with the generation of spherical primary crystals
and ending with the molding step;
Fig. 4 shows diagrammatically the metallographic
structures obtained in the respective steps shown in Fig.
3;
Fig. 5 is an equilibrium phase diagram for an Al-Si
alloy as a typical aluminum alloy system according to the
invention;
Fig. 6 is an equilibrium phase diagram for a Mg-Al
alloy as a typical magnesium alloy system according to the
invention;
Fig. 7 is a diagrammatic representation of a
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micrograph showing the metallographic structure of a shaped
part (of AC4CH alloy) according to an example of the
invention;
Fig. 8 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (of AC4CH alloy) according to a comparative examples;
Fig. 9 is a graph illustrating the correlationship
between the temperature distribution of AC4CH alloy in a
holding vessel and its cooling rate according to an example
of the invention:
Fig. 10 is a graph showing the effect of r-f induction
heating on the temperature distribution of AC4CH allcy in a
holding vessel according to an example of the invention:
Fig. 11 is a graph showing the effect of r-f induction
heating on the temperature distribution of AC4CH alloy in a
holding vessel according to another example of the
invention:
Fig. 12 illustrates how holding by r-f induction
heating affects the compositional homogenization of a
semisolid metal after the molding temperature was reached
in an example of the invention;
Fig. 13 shows a process flow in the invention which
starts with the generation of spherical primary crystals
and which ends with the molding step;
Fig. 14 is a graph showing how the B content and the
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degree of superheating of a melt during pouring affect the
size and morphology of the primary crystals of AC4CH alloy
(Al-7~ Si-0.3~ Mg-0.15~ Ti) according to the invention;
Fig. 15 is a graph showing how the B content and the
degree of superheating of a melt during pouring affect the
size and morphology of the primary crystals of 7075 alloy
(Al-5.5~ Zn-2.5~ Mg-1.6~ Cu-0.15~ Ti) according to the
invention;
Fig. 16 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (from AC4CH-0.15% Ti) according to an example of the
invention;
Fig. 17 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (from AZ91-0.01% Sr-0.4~ Si) according to another
example of the invention;
Fig. 18 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (from 7075-0.15% Ti-0.002% B) according to yet another
example of the invention;
Fig. 19 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (from AC4CH-0.15% Ti) according to a comparative
example;
Fig. 20 is a diagrammatic representation of a
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micrograph showing the metallographic structure of a shaped
part (from AZ91) according to another comparative example;
Fig. 21 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (from AZ91-0.01~ Sr) according to yet another
comparative example; and
Fig. 22 is a diagrammatic representation of a
micrograph showing the metallographic structure of a shaped
part (from 7075) according to still another comparative
example.
Detailed Description of the Invention
In the present invention, liquid alloy having crystal
nuclei and at a temperature not lower than the liquidus
temperature or a partially solid, partially liquid alloy
having crystal nuclei and at a temperature less than the
liquidus temperature but not lower than the molding
temperature is poured into a holding vessel having a
thermal conductivity of at least 1 kcal/mh~, is cooled at
an average cooling rate of 0.01 ~/s - 3.0 ~/s and held as
ZO such until just prior to the start of shaping under
pressure, whereby fine primary crystals are generated in
said alloy solution and the alloy within said holding
vessel is temperature adjusted by induction heating such
that the temperatures of various parts of the alloy fall
within the desired molding temperature range for the
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establishment of a specified fraction liquid not later than
the start of shaping and the alloy is recovered from said
holding vessel, supplied into a forming mold and shaped
under pressure. Since the temperature control of the alloy
prior to the shaping step is performed in the ideal manner,
satisfactory shaped parts can be obtained that have a
homogeneous structure containing spheroidized primary
crystals.
It is also within the scope of the invention that a
molten aluminum containing Ti either alone or in
combination with B or a molten magnesium alloy containing
Ca or both Si and Sr, is held superheated to less than 50
above the liquidus temperature, poured directly into a
holding vessel without using any cooling jig and held for a
period from 30 seconds to 30 minutes as the melt is cooled
to the molding temperature where a specified fraction
liquid is established such that the temperature of the
poured alloy which is liquid and superheated to less than
10~ above liquidus temperature or which is partially solid,
partially liquid and less than 5~ below the liquidus
temperature is allowed to decrease from the initial level
and pass through a temperature zone 5~ below the liquidus
temperature within 10 minutes, whereby fine primary
crystals are generated in said alloy solution and the
temperatures of various parts of the alloy within said
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-
holding vessel are adjusted such that by means of induction
heating and local heating or heat retention of the vessel,
said temperatures will fall within the desired molding
temperature range for the establishment of a specified
fraction liquid not later than the start of shaping, and
said alloy is recovered from said holding vessel, supplied
into a forming mold and shaped under pressure. As a result,
satisfactory shaped parts are obtained that have a fine and
uniform microstructure.
Examples
Examples of the invention will now be described in
detail with reference to accompanying drawings Figs. 1-12,
in which: Fig. 1 is a diagram showing a process sequence
for the semisolid forming of a hypoeutectic aluminum alloy
having a composition at or above a maximum solubility
limi~; Fig. 2 is a diagram showing a process sequence for
the semisolid forming of a magnesium or aluminum alloy
having a composition within a ~x;mum solubility limit;
Fig. 3 shows a process flow starting with the generation of
spherical primary crystals and ending with the molding
step; Fig. 4 shows diagrammatically the metallographic
structures obtained in the respective steps shown in Fig.
3; Fig. 5 is an equilibrium phase diagram for an Al-Si
alloy as a typical aluminum alloy system; Fig. 6 is an
equilibrium phase diagram for a Mg-Al alloy as a typical
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-20-
magnesium alloy system; Fig. 7 is a diagrammatic
representation of a micrograph showing the metallographic
structure of a shaped part according to the invention; Fig.
8 is a diagrammatic representation of a micrograph showing
the metallographic structure of a shaped part according to
the prior art; Fig. 9 is a graph illustrating the
correlationship between the temperature distribution of
AC4CH alloy in a holding vessel and its cooling rate: Fig.
10 is a graph showing the effect of r-f induction heating
on the temperature distribution of AC4CH alloy in a holding
vessel; Fig. 11 is another graph showing the effect of r-f
induction heating on the temperature distribution of AC4CH
alloy in a holding vessel; and Fig. 12 illustrates how
holding by r-f induction heating affects the compositional
homogenization of a semisolid metal after the molding
temperature was reached.
Fig. 13 - 18 relate to Examples 11 - 14 of the
invention. Fig. 13 shows a process flow starting with the
generation of spherical primary crystals and ending with
the molding step; Fig. 14 is a graph showing how the B
content and the degree of superheating of a melt during
pouring affect the size and morphology of the primary
crystals of AC4CH alloy (Al-7% Si-0.3% Mg-0.15% Ti); Fig.
15 is a graph showing how the B content and the degree of
superheating of a melt during pouring affect the size and
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morphology of the primary crystals of 7075 alloy (Al-5.5%
Zn-2.5% Mg-1.6% Cu-0.15% Ti); and Fig. 16 - 18 are
diagrammatic representation of a micrographs showing the
metallographic structures of shaped parts within the scope
of the invention.
Fig. 19 - 22 are diagrammatic representation of a
micrographs showing the metallographic structures of a
shaped parts.
As shown in Figs. 1, 2, 3, 5 and 6, the first step of
the process according to the invention comprises
superheating the melt of a hypoeutectic aluminum alloy of a
composition at or above a maximum solubility or a magnesium
or aluminum alloy of a composition within a m~xi mum
solubility limit, holding the melt superheated to less than
50~ above the liquidus temperature as it is poured into a
holding vessel, with a vibrating rod being submerged within
the melt in the holding vessel and vibrated in direct
contact with the melt so as to vibrate the latter and,
after the end of the pouring, immediately pulling up said
vibrating rod so that it disengages from the melt.
Thus, there is obtained the liquid alloy having
crystal nuclei and at a temperature not lower than the
liquidus temperature or the partially solid, partially
liquid alloy having crystal nuclei and at a temperature
less than the liquidus temperature but not lower than the
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holding temperature. Subsequently, either alloy in said
holding vessel is cooled to the molding temperature, where
a specified fraction liquid is established, at an average
cooling rate of 0.01 - 3.0 ~/s with a cooling medium such
as air at room temperature being blown against said holding
vessel from the outside and the alloy is held as such until
~ust prior to the start of shaping under pressure, whereby
fine primary crystals are generated in said alloy solution
and the alloy within said holding vessel is temperature
ad~usted by induction heating such that the temperatures of
various parts of the alloy fall within the desired molding
temperature range for establishment of a specified fraction
liquid not later than the start of shaping and said alloy
is recovered from said holding vessel, supplied into a
forming mold and shaped under pressure.
Another process according to the invention is also
shown in Fig. 13 and the first step comprises superheating
the melt of a hypoeutectic aluminum alloy of a composition
at or above a m~ximum solubility or a magnesium or aluminum
alloy of a composition within a maximum solubility limit,
both alloys containing a crystal grain refiner (which is
hereunder referred to as "refiner"), holding the melt
superheated to less than 50~ above the liquidus temperature
as it is poured into a holding vessel 30. Then, the alloy
is held for a period from 30 seconds to 30 minutes as the
CA 022203~7 1997 11 06
,_
melt is cooled to the molding temperature whereas specified
fraction liquid is established such that the temperature of
either the poured liquid alloy superheated to less than lO~
above the liquidus temperature or the poured partially
solid, partially liquid alloy which is less than 5~ below
the liquidus temperature is allowed to decrease from the
initial level and pass through a temperature range 5~ below
the liquidus temperature within 10 minutes, whereby fine
primary crystals are generated in said alloy solution, and
the alloy is recovered from the holding vessel 30, supplied
into a forming mold 60 and shaped under pressure.
In practice, a molten alloy which has been poured into
the holding vessel is cooled by blowing air or water from
the outside of the vessel until the melt reaches the
predetermined temperature which is set above the
temperature of shaping, while the temperature of the upper
and the lower portions of the vessel is being maintained
constant. Further, the temperature of various portions of
the melt in the holding vessel is adjusted by induction
heating so that the melt may have a temperature within the
desired molding temperature range to establish a specified
fraction liquid before the start of shaping at latest.
The term "a specified fraction liquid" means a
relative proportion of the liquid phase which is suitable
for pressure forming. In high-pressure casting operations
CA 022203~7 1997-11-06
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such as die casting and squeeze casting, the fraction
liquid is less than 75%, preferably in the range of 40% -
65%. If the fraction liquid is less than 40~, not only is
it difficult to recover the alloy from the holding vessel
30 but also the formability of the raw material is poor. If
the fraction liquid exceeds 75%, the raw material is so
soft that it is not only difficult to handle but also less
likely to produce a homogeneous microstructure because the
molten metal will entrap the surrounding air when it is
inserted into the sleeve for in~ection into a mold on a die-
casting machine or segregation develops in the
metallographic structure of the casting. For these reasons,
the fraction liquid for high-pressure casting operations
should not be more than 75%, preferably not more than 65%.
In extruding and forging operations, the fraction
liquid ranges from 1.0~ to 70%, preferably from 10% to 65%.
Beyond 70%, an uneven structure can potentially occur.
Therefore, the fraction liquid should not be higher than
70%, preferably 65% or less. Below 1.0~, the resistance to
deformation is unduly high; therefore, the fraction liquid
should be at least 1.0~. If extruding or forging operations
are to be performed with an alloy having a fraction liquid
of less than 40%, the alloy is first adjusted to a fraction
liquid of 40~ and more before it is taken out of the
holding vessel and thereafter the fraction liquid is
CA 022203~7 1997-11-06
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lowered to less than 40~.
The "holding vessel n as used in the invention is
metallic nonmetallic vessel (including a ceramic vessel),
or a metallic vessel having a surface coated with
nonmetallic materials, or a metallic vessel composited with
nonmetallic materials. Coating the surface of a metallic
vessel with nonmetallic materials is effective in
preventing the sticking of the metal. The holding vessel
may be heated either internally or externally by means of a
heater; alternatively, a r-f induction heater may be
employed.
The term "the representative temperature" as used
herein refers to the center temperature of the alloy
charged into holding vessel. More specifically, it means
the temperature at the center of the alloy in the holding
vessel in both the height and radial directions. In
practical operations, however, it is difficult to measure
the temperature of the alloy center in both directions and,
instead, the temperature in a position a specified depth
(say, 1 cm) below the surface of a semisolid metal is
measured. From this temperature, the representative
temperature is estimated on the basis of the preliminarily
established relationship between the representative
temperature and the temperatures of various parts of the
alloy.
CA 022203~7 1997-11-06
-26-
According to the invention, two methods are proposed
for generating crystal nuclei, first by using vibrating jig
during the pouring of a melt into the vessel, and second by
using a low-temperature melt containing a refiner. Rnown
methods may of course be employed to generate crystal
nuclei, and they include the ~seed pouring" method
utilizing crystal liberation (the melt is cast to flow on a
water-cooled inlined cooling plate) and mixing two liquid
phases having different melting points. According to he
invention, the crystal nuclei are generated "by vibrating
the alloy which builds up in the holding vessel by pouring
in a melt, the vibration being applied to said alloy by
means of a vibrating rod which is submerged in the melt
during its pouring so that it has direct contact with the
alloy". This does not mean that the melt is poured on to
the vibrating rod placed in the holding vessel; rather, the
liquid alloy which is building up in the holding vessel
after it was poured in is vibrated by means of the
vibrating rod submerged in said alloy (when the pouring
ends, the vibrating rod is immediately disengated from the
melt).
The term "vibration" as used herein is in no way
limited in terms of the type of the vibrator used and the
vibrating conditions (frequency and amplitude) and any
commercial pneumatic and electric vibrators may be
CA 022203~7 1997-11-06
-27-
employed. As for the applicable vibrating conditions, the
frequency typically ranges from 10 Hz to 50 kHz, preferably
from 50 Hz to 1 k~z, and the amplitude ranges from 1 mm to
0.1 ~m, prefer~bly from S00 am to 10 am, per side.
The method of pouring the refiner-containing low-
temperature melt into the holding vessel 30 should be such
that crystal nuclei (fine crystals) can be generated in the
poured melt. In order to ensure that the refiner which
works as a foreign nucleus or as an element to accelerate
the liberation of crystals will manifest its effect, the
melt must be poured in at a specified rate and, in
addition, it must be superheated to a temperature that is
above the liquidus temperature by a specified degree. The
degree of superheating varies with the kind of the refiner
to be added and the amount of its addition (the criticality
of the degree of superheating will be described later in
this specification).
If the melt is poured in too fast, it is prone to
entrap the surrounding air; on the other hand, if the melt
is poured in too slowly, the intended effect of adding the
refiner is not achieved and it is not efficient from an
engineering viewpoint. Therefore, it is important that the
metal be poured in at an appropriate rate within the range
that does not cause entrapping of the surrounding air. The
appropriate rate is faster than what is determined by
CA 022203~7 1997-11-06
-28-
equation (1) but slower than the rate determined by
equation ( 2):
Eq. (1): Y = 0.015X + 0.02 (preferably Y = 0.03X + 0.02)
Eq. (2) :Y - 0.017X + 0.06
S where Y is the pouring rate (kg/s) and X is the weight of
the melt (kg).
Titanium (Ti) may be added to the aluminum alloy as a
refiner either alone or in combination with boron (B) in
order to produce fine spherical crystal grains. If Ti is to
be added alone, its refining effect is small if the
addition is less than 0.03~. Beyond 0.30%, coarse Ti
compounds well develop to reduce the ductility. Hence, Ti
is added in an amount of 0.03% - 0.30%.
If both Ti and B are to be added, the effect of Ti is
small if its addition is less than 0.005%. Beyond 0.30%,
coarse Ti compounds will develop to reduce the ductility.
Hence, Ti is added in an amount of 0.005% - 0.30% in
combination with B. Boron (B), when added in combination
with Ti, promotes the refining process. However, if its
addition is less than 0.001%, only a small refining effect
occurs. The effect of B is saturated if it is added in
excess of 0.01~. Therefore, the addition of B should range
from 0.001% to 0.01%.
Calcium (Ca) or the combination of Sr and Si may be
added to the magnesium alloy as a refiner. If Ca is to be
CA 022203~7 1997-11-06
-29-
_
added, its refining effect is small if the addition is less
than 0.05~. Beyond 0.30~, the effect of Ca is saturated.
Therefore, the addition of Ca should range from 0.05~ to
0.30%. In the case of combined addition of Sr and Si, only
a small refining effect occurs if Sr is added in an amount
of less than 0.005~. The effect of Sr is saturated if it is
added in excess of 0.1%. Therefore, the addition of Sr
should range from 0.005% to 0.1~. Silicon (Si), when added
in combination with Sr, promotes the refining process.
However, if its addition is less than 0.01%, only a small
refining effect occurs. If Si is added in excess of 1.5~,
its effect is saturated and, what is more, there occurs a
drop in ductility. Therefore, the addition of Si should
range from 0.01~ to 1.5~.
According to the invention, semisolid metal forming
will proceed by the following specific procedure. In step
(1) of the process shown in Figs. 3 and 4, a complete
liquid form of metal M1 is contained in a ladle 10. In step
(2), the alloy M1 is poured into a holding vessel 30 (which
is either a ceramic or a ceramic-coated metallic vessel) as
a vibrating rod 20 submerged in the alloy to have direct
'contact with it is vibrated to impart vibrations to the
alloy, with the holding vessel 30 being vibrated with a
vibrator 40 as required during the pouring of the melt.
After the end of the pouring operation, the vibrating rod
CA 022203~7 1997-11-06
-30-
20 is immediately pulled up so that crystal nuclei are
generated in the alloy which is either liquid or partially
liquid at a temperature near the liquidus temperature.
In subsequent step (3), the alloy is cooled at an
average cooling rate of 0.01 ~/s - 3.0 ~/s and held as such
within the holding vessel 30 until just prior to the start
of shaping under pressure so that fine primary crystals are
generated in said alloy solution; at the same time,
induction heating (i.e., energization of a heating coil 80
around the holding vessel 30) is performed to effect
temperature adjustment right after the pouring of the melt
such that the temperatures of various parts of the alloy in
the vessel will fall within the desired molding temperature
range for establishment of a specified fraction liquid not
later than the start of the molding step. For cooling the
alloy, air (or water) 90 is blown against the holding
vessel from its outside. If necessary, both the tip and
bottom portions of the holding vessel 30 may be heat-
retained with a heat insulator or heated so that the alloy
is held partially molten to generate fine spherical (non-
dendritic) primary crystals from the introduced crystal
nuclei. Metal M2 thus obtained at a specified fraction
liquid is inserted from the inverted holding vessel 30 [see
step (3)-d] into a die casting injection sleeve 50 and
thereafter pressure formed within an mold cavity 60a on a
CA 022203~7 1997-11-06
_ -31-
die casting machine to produce a shaped part [step (4)].
In the other method of the invention, semisolid metal
forming will proceed by the following specific procedure.
In step (1) of the process shown in Figs. 3 and 4, a
complete liquid form of metal M1 containing a refiner is
charged into a pouring ladle 10 (which is hereunder
sometimes referred to simply as Uladle"). In step (2), the
melt is gently but rapidly poured into a holding vessel 30
(which is either a ceramic coated or a ceramic vessel),
thereby forming either a liquid or a partially solid,
partially liquid alloy that contain crystal nuclei (fine
crystal grains) and which are at a temperature near the
liquidus temperature.
Subsequently in step (3), the temperature of the
poured alloy which is either liquid and superheated to less
than 10~ above the liquidus temperature of which is
partially solid, partially liquid and less than 5~ below
the liquidus temperature is allowed to decrease from the
initial level and pass through a temperature zone 5~ below
the liquidus temperature within 10 minutes, whereby fine
primary crystals are generated in said alloy solution; at
the same time, induction heating (i.e., energization of a
heating coil 80 around the holding vessel 30) iS performed
to effect temperature adjustment such that the temperatures
of various parts of the alloy in the vessel 30 will fall
CA 022203~7 1997-11-06
-32-
within the desired molding temperature range for the
establishment of a specified fraction liquid not later than
the start of the molding step.
Fig. 9 is a graph illustrating the correlationship
between the temperature distribution of AC4CH alloy in the
holding vessel and its cooling rate. In other words, Fig. 9
shows the effect of cooling rate (for cooling from 615~ to
585~) on the temperature distribution of AC4C~ alloy in the
holding vessel 30; obviously, the temperature distribution
becomes wider as the cooling rate increases.
Fig. 9a shows the case where the cooling rate was 0 .3 ~C
/s; in this case, the alloy was cooled with air being blown
from the outside of the holding vessel, the tip portion of
which was heat-retained with a heat insulator which was
also provided on the underside of the vessel. Fig. 9b shows
the case where the cooling rate was 0.2 ~/s; in this case,
both the top and bottom portions of the vessel were heat-
retained with a heat insulator and the alloy was cooled in
the atmosphere.
Fig. 10 is a graph showing the effect of r-f induction
heating on the temperature distribution of AC4CH alloy in
the holding vessel. According to the invention when the
representative temperature of the alloy (its center
temperature as it is in the holding vessel) has reached +3~5
above the desired molding temperature the blowing of air is
CA 022203~7 1997-11-06
- -33-
stopped and r-f induction heating is started when the
desired temperature is reached.
Fig. 11 is another graph showing the effect of r-f
induction heating on the temperature distribution of AC4C~
alloy in the holding vessel. According to the invention,
when the representative temperature of the alloy (its
center temperature as it is within the holding vessel) has
reached a temperature 11~ below the desired molding
temperature, the blowing of air is stopped and r-f
induction heating is started.
If the r-f induction heater is started to operate
before the temperature becomes unduly lower than the
desired molding temperature, the temperatures of various
parts of the alloy in the holding vessel 30 can be
maintained at the desired molding temperature in a short
time with small electric power. On the other hand, if the r-
f induction heater becomes operational after the alloy's
temperature has become at least 10~ lower than the desired
molding temperature, it is not easy to maintain various
parts of the alloy in the vessel at uniform temperature
without performing induction heating with high electric
power for a prolonged time. Therefore, the induction
heating should comprise at least one application of
electric current in a specified amount for specified period
of time before the representative temperature of the alloy
CA 022203~7 1997-11-06
-34-
slowly cooling in the holding vessel 30 has dropped to at
least 10~ below the desired molding temperature.
Fig. 12 illustrates how holding by r-f induction
heating affects the compositional homogenization of a
semisolid metal after the molding temperature has been
reached. Each of the diagrams in Fig. 12 is a vertical
sectlon of the alloy in the holding vessel 30; Fig. 12a
shows the state of the alloy which has attained the molding
temperature; Fig. 12b shows the state of the alloy which
was held for 20 minutes by heating with the r-f indication
heater at a frequency of 8 kHz; and 12c shows the state of
the alloy which was held for 20 minutes by heating with the
r-f induction heater at a frequency of 40 kHz.
The operating frequency of the r-f induction heater is
8 kHz before the alloy's temperature is adjusted to the
molding temperature. A peculiar phenomenon which does not
occur at the time the molding temperature has been reached
(Fig. 12a) is observed if the alloy is held for a prolonged
time; that is the uneven occurrence of the liquid phase in
the top peripheral portion of the semisolid metal which is
inherently a uniform mixture of the liquid and solid phases
tthe concentrated liquid phase is shown shaded in Fig.
12b).
This problem may be explained as follows: the metal in
the holding vessel 30 forms "mushrooms" during the
CA 022203~7 1997-11-06
-35-
._
induction heating and the liquid phase of the semisolid
metal floats in the top portion of the vessel mainly due to
the agi~ating force. To suppress this agitating force,
induction heating is performed at a higher frequency after
the semisolid metal in the holding vessel has been adjusted
to the molding temperature; consequently, the degree of the
uneven occurrence of the liquid phase can be reduced. To
this end, after the temperatures of the various parts of
the alloy in the hoiding vesse' have been ad~usted by
L0 induction heating to fall within the desired molding
temperature range within a specified time, the same alloy
is held within the stated range until just prior to the
start of the molding step by continuing the induction
heating at a frequency either comparable to or higher than
the frequency used in the preceding induction heating.
The semisolid metal forming process of the invention
shown in Fiss. 1, 2, 3, 4 and 11 has the following
differences from the conventional thixocasting and
rheocasting methods. In the invention method, the dendritic
primary crystals that have been generated within a
temperature range of from the semisolid state are not
ground into spherical gra-ns ~y mechanical or
electromagnetic agitation as in the prior art but the large
num~er of primary crystals that have been generated and
grown from the introduced crystal nuclei with the
CA 022203~7 1997-11-06
~_ -36-
decreasing temperature in the range for the semisolid state
are spheroidized continuously by the heat of the alloy
itself (which may optionally by supplied with external heat
hand held at a desired temperature). In addition, the
semisolid metal forming method of the invention is
characterized by the production of a uniform microstructure
and temperature distribution by r-f induction heating with
lower output and it is a very convenient and economical
process since it does not involve the step of partially
melting billets by reheating in the thixo-casting process.
The nucleating, spheroidizing and molding conditions
that are respectively set for the steps shown in Fig. 3,
namely, the step of pouring the metal into the holding
vessel 30, the step of generating and spheroidizing primary
crystals and the forming step, are set forth below more
specifically. Also discussed below is the criticality of
the numerical limitations the invention should have.
If crystal nuclei are to be generated by (1) applying
vibrations to the melt in the holding vessel 30 or (2)
pouring a Ti- and B-containing aluminum alloy or a Si and
Sr-containing magnesium alloy or a Ca-containing magnesium
alloy directly into the holding vessel, the melt should be
superheated to less than 50~, preferably less than 30~,
above the liquidus temperature. If crystal nuclei are to be
generated by pouring a Ti-containing aluminum alloy into
CA 022203~7 1997-11-06
-37-
the holding vessel, the melt should be superheated to less
than 30'C above the liquidus temperature. If the temperature
of the melt being poured into he holding vessel is higher
than these limits, the following phenomena will occur;
(1) only a few crystal nuclei are generated;
(2) the temperature of the alloy as poured into the vessel
is hlgher than the liquidus temperature and, hence, the
number of residual crystal nuclei is small and the size of
primary crystals is large enough to produce amorphous
dendrites.
If the upper or lower portion of the holding vessel 30
is not heated or heat-retained while the alloy Ml poured
into the vessel is cooled to establish a fraction liquid
suitable for molding, dendritic primary crystals are
generated in the skin of the alloy M1 in the tip and/or
bottom portion of the vessel or a solidified layer will
grow to cause nonuniformity in the temperature distribution
of the metal in the holding vessel 30; as a result, even if
r-f induction heating is performed, the alloy having the
specified fraction liquid cannot be discharged from the
inverted vessel 30 or the remaining solidified layer within
the holding vessel 30 either introduces difficulty into the
practice of continued shaping operation or prevents the
temperature distribution of the alloy from being improved
in the desired way.
CA 022203~7 1997-11-06
-38-
In order to avoid these problems, if the poured metal
is held in the vessel for a comparatively short time until
the molding temperature is reached, the top and/or bottom
portlon of the holding vessel is heated or heat-retained at
a higher temperature than the middle portion in the cooling
process; if necessary, both the top and bottom portions of
the holding vessel 30 may be heated not only in the cooling
process but also before the pouring step.
If the wall thickness of the holding vessel 30 is
reduced, the formation of a solidified layer can be
suppressed; hence, the wall of the holding vessel is made
smaller in the top and bottom portions than in the middle
to thereby facilitate the discharge of the alloy from the
holding vessel 30.
If the holding vessel 30 is made of a material having
a thermal conductivity of less than l.0 kcal/mh~, the
cooling time is prolonged to a practically undesirable
level; hence, the holding vessel 30 should have a thermal
conductivity of at least l.0 kcal/mh~. If the holding
vessel 30 is made of a metal, its surface is preferably
coated with a nonmetallic material (e.g. BN or graphite).
the coating method may be either mechanical or chemical or
physical. Both the magnesium and aluminum alloys are highly
oxidizable metals, so if the holding vessel 30 is made of
an air-permeable material or if the alloy is to be held for
CA 022203~7 1997-11-06
_ -39-
a long time in the vessel, the exterior to the vessel is
preferably filled with a specified atmosphere (e.g. an
inert or vacuum atmosphere). Even in the case of using the
metallic vessel, the magnesium alloy which is highly
oxidizable is desirably isolated by an inert of CO2
atmosphere.
For preventing oxidation, an oxidation control element
may be preliminarily added to the molten metal, as
exemplified by Be and Ca in the case of the magnesium alloy
and Be for the aluminum alloy. The shape of the vessel 30
is by no means limited to a tubular form and any other
shapes that are suitable for the subsequent forming process
may be adopted.
If the average rate of cooling in the holding vessel
30 is faster than 3.0 ~/s, it is not easy to permit the
temperatures of various parts of the alloy to fall within
the desired molding temperature range for establishment of
the specified fraction liquid even if induction heating is
employed and, in addition, it is difficult to generate
spherical primary crystals. If, on the other hand, the
average cooling rate is less than 0.014 ~/s, the cooling
time is prolonged to cause inconvenience in commercial
production. Therefore, the average rate of cooling in the
holding vessel 30 should range preferably from 0.01 ~/s to
3.0 ~/s, more preferably from 0.05 ~/s to 1 ~/s.
CA 022203~7 1997-11-06
- -40-
Crystal nuclei can also be generated by pouring a
refiner containing molten alloy directly into the holding
vessel 30. In this case, if the poured alloy is superheated
to more than lG~ above than the liquidus temperature, fine
spherical crystals cannot be produced no matter what
cooling rate is adopted. Hence, the as-poured metal should
be superheated to less than 10~ above the liquidus
temperature. If the temperature of the alloy which is
either liquid and superheated to less than 10~ above the
liquidus temperature or partially solid, partially liquid
alloy and less than 5~ below the liquidus temperature is
allowed to decrease from the initial level and pass through
a temperature zone 5~ below the liquidus temperature taking
a time longer than 10 minutes, it is impossible to produce
a fine spherical microstructure.
To avoid this problem, the temperature of the alloy is
allowed to decrease from the initial level and pass through
the temperature zone 5~ below the liquidus temperature
within 10 minutes, preferably within 5 minutes, to thereby
generate fine primary crystals in the solution of the
alloy, which is taken out of the holding vessel 30,
supplied into the forming mold 60 and shaped under
pressure.
If enhanced cooking of the holding vessel 30 is
necessary, either air or water or both are blown against
-41-
the holding vessel 30 from its outside. Depending on the
need, the cooling medium may be blown from at least two
different, independently operable heights exterior to the
holding vessel such that the blowing conditions and times
can be varied freely. The cooling medium to be blown, the
amount of blow, its velocity, speed, position and timing
are variable with the alloy in the holding vessel 30, the
material of which the vessel is made, its wall thickness,
etc.
If the temperature of the yet to be shaped alloy in
the holding vessel exceeds the limits of +5~ of the desired
molding temperature, a shaped part of uniform
microstructure cannot be produced by casting. Hence, the
temperature of the alloy in the holding vessel should be
adjusted by induction heating to fall within the limits of L
5~ of the desired molding temperature.
If the vibrating rod 20 is to be used for the purpose
of creating crystal nuclei in the alloy being poured into
the holding vessel, it preferably satisfied the following
two requirements: it should be coolable either internally
or externally in order to provide for its continued use and
generate many crystal; the surface of the vibrating rod 20
should be coated with a nonmetallic material. It should be
noted that the use of rod that can be cooled internally but
which is nonvibrating has the following disadvantage even
CA 022203~7 1997-11-06
-42-
if it is coated with a nonmetallic material: when the rod
is pulled up from the poured alloy, a solidified layer will
stick extensively to the surface of the rod or many
dendrites will form in the alloy in the holding vessel. To
avoid this problem, the coolable rod must be vibrated when
it is placed in contact with the molten metal.
The use of the vibrating rod 20 is effective in
generating fine primary crystals in the alloy in the
holding vessel but, at the same time, dendrites may
occasionally form in those parts of the alloy which contact
the inner surface of the holding vessel 30. To avoid this
problem, the holding vessel 30 is preferably vibrated
during pouring of the metal.
Table 1 sets forth the conditions for the preparation
of semisolid metal samples to be shaped, and Table 2 sets
forth the temperature distribution of yet to be shaped
metal samples in the holding vessel, as well as the quality
of shaped parts. As Fig. 3 shows, the forming step
consisted of inserting the semisolid metal into the sleeve
50 and subsequent treatment with a squeeze casting machine.
The forming conditions were as followed: pressure, 950
kgf/cm2; injection rate, 0.5 m/s; casting weight (inclusive
of biscuits), 1.5 kg; mold temperature, 230~.
Table 1 (Continued on the next page)
Conditions for Preparation of Semisolid Yetals to be Yolded
Pouring Temper- Yaterial Yold- Average Induction
Run Alloy temper- Nucle- ature of of hold- ing cooling heating
No. ature, ation metal in ing ves- Temper- rate, ~atterr
~ sel,~ --l ~ re,~ . B C
., ~ . . " . ~
. ., , ~ _ . . . J
. . ' , . ~ . O
Inven- .,~ ~ _ Ai . O
tion .. ' ' - ~ - O
O
~ , A V Ti -~ x
.. .. ~ . . ~
. ~ . Sc, Sr . . - ~ . x D
L X
. . J ; - ~
,. .. , J. ~. i .. ., . ~
- O o
~ .3 8
Compar- .. ' '
ison ~ , ~ Ai , .
., , , ( ~5 x
~ Ai ~ C. 5
Notes:
(n.p.) AC4CH Al-7XSi-0.3XYg-0.15XTi 615~ (heat-retained) Vessel was covered with a
AZ91 Yg-9XAl-0.7XYn-0.2XYa 595~ ceramic material having a thermal
AC7A Al-5XNg-0.4XNa 635~ conductivity of 0.3 kcal/mh~.
(nucleation) Y: based on claim 8; frequency 100 Rz; (heated) Heated with ir heater.
amplitude 0.1 mm per side (blowing of cooling air or water)
Ti: based on claim 9; 0.175X Ti and Air: blown from the outside of coil to
0.005X B after addition of refiners cool vessel within the coil.
(induction heating) ~ater: blown against the vessel before
pattern A: heated (-5 to t5~) after the representative it was placed within the coil.
temperature reached the desired molding temperature. (holding time) Holding time from the end of
pattern B: heated each time the decreasing representative metal pouring into vessel until the start
temperature reached a specified level. of shaping.
pattern C: heating started at a temperature at least 10~ (degree of spheroidization of primary crystals)
below the desired molding temperature. O, mostly spherical particles
(material of holding vessel) Designated in terms of the thermal ~, coarse spherical particles
conductivity (kcal/mh~) at 500~; 14 for stainless steel Sl; 18 x, many dendrites and amorphous particles
for cast iron S2; 0.3 for ceramic C.
Table 1 (Continued)
Conditions for Preparation of Semisolid ~etals to be Shaped
Frecqu-ncy Temperature Cooling medium to
Run Alloy Before After control be blown against Hold- hetal
No. adjustment adjust- of holding holding vessel ing weight,
to molding ment vessel Air or Temper- time, kg
temperature Top Bottom ~ater ature, ~ min
1 AC4Cll 8 8 heat- - Air 25 3.0 1.5
retained
2 AC4CH 8 8 heat- heat- Air 200 5.5 15.0
retained retained
3 AC4CH 8 8 heat- - - - 40.0 15.0
retained
Inven- 4 AC4CH 8 8 heat- heat- - - 4.3 1.5
retained retained D
tion 5 AC4CH 8 8 heat- - Air 25 2.0 1.5 O
retained ~ater
6 AC4CH 8 8 heat- - Air 25 3.0 1.5
retained ,~~
7 AC4CH 8 45 heat- heat- Air 25 30.0 1.5
retained retained
8 AC4CH 8 8 heat- heated Air 100 4.5 15.0
retained
9 AZ91 8 8 heat- 'heat- - - 3.0 0.9
retained retained
AZ91,Si,Sr 8 8 heat- heat- Air 25 1.1 0.9 ~o
retained retained
11 AZ91, Ca 8 8 heat- heat- Air 25 2.0 0.9
retained retained
12 AC7A 8 8 heated heat- Air 25 3.0 1.5
retained
13 AC4CH 8 8 heat- - Air 25 3.0 1.5
retained
14 AC4CH 8 8 heat- heat- Air 25 3.1 1.5
retained retained
AC4CH 8 8 heat- - - - 65.0 20.5
retained
Compar- 16 AC4Cn 8 8 heat- heat- - - 40.0 1.5
retained retained
ison 17 AC4CH 8 8 ~ater 25 0.3 1.5
18 AC4CH 8 8 heat- heat- - - 10.0 1.5
retained retained
19 AC4CH 8 8 - Air 25 2.8 1.5
CA 022203~7 1997-11-06
-45-
-
Table 2
Temperature of Semisolid ~etals and ~icrostructure
of Shaped Parts
No. Temperature Degree of
distribution shperoidi- Pemarks
of yet to be zation
shaped metal
t'', -: O
+ ~ O
+~, -: O
+", -', O
+., -" O
+', -" O
+ ', -: O
t~ O Top and bottom portions of the vessel were about two
thirds in thickness of the middle portion.
9 +'., -1 0
O +, -~ O
: +,, -- . O
+ , -: O
1 +". -~ O E~trusion molded
1~ - 0. ~ O Induction heating started as at a temperature at
least 10~ below the desired molding temperature.
-~, 5 ~ Cooling rate too slow.
: -~, -2 0 ~eld by induction heating for an unduly long time.
-~, 7 x Cooling rate too fast.
1 -', -5 x Pouring temperature too high.
1 - , 3 x Vessel heat-retained insufficiently.
CA 022203~7 1997-11-06
-46-
-
It should be noted that the data for Run No. 13 in
Tables 1 and 2 refer to the conditions for forming with an
extruding machine and the quality of the shaped part. The
forming step consisted of inserting the semisolid metal
into the container and extruding the same. The extruding
conditions were as follow: extruding machine, 800 t;
extruding rate (output rate), 80 m/min; extrusion ratio,
20; billet diameter, 75 mm.
In Run No. 14 (comparison) in Tables 1 and 2, the
representative temperature of the alloy cooling in the
holding vessel 30 had dropped to at least 10~ below the
desired molding temperature before induction heating
started and, hence, the temperature of the alloy could not
be adjusted to fall within the limits of +5~ of the desired
molding temperature, thus making it impossible to produce a
shaped part having a homogeneous microstructure.
In Run 15 (comparison), the cooling rate was slow and
caused no big problems in temperature distribution but, on
the other hand, the size of primary crystals exceeded 200
m and the slow cooling was inconvenience to continuous
production.
In Run No. 16 (comparison), the alloy in the holding
vessel which had the temperatures of various parts adjusted
to fall within the desired molding temperature range was
continuously held as such by induction heating for an
CA 022203~7 1997-11-06
-47-
-
unduly long time and without changing the frequency; as a
result, a liquid phase occurred extensively in the top
peripheral portion of the semisolid metal.
In Run No~ 17 (comparison), the cooling rate was so
fast that even when induction heating was performed, the
temperature of the alloy could not be ad~usted to fall
within the limits of t 5~ of the desired molding temperature
range and no shaped part having a homogeneous
microstructure could be produced; what is more, a
solidified layer formed within the vessel, making it
difficult to recover the semisolid metal from the vessel.
In Run No. 18 (comparison), the high pouring
temperature led to an unduly hot melt in the vessel and,
hence, there were no residual crystal nuclei and many
amorphous dendrites formed.
In Run No. 19 (comparison), the holding vessel was
heat-retained only insufficietnly so that the metal in the
top of the vessel cooled prematurely, making it very
difficult to recover the metal from the vessel.
In Run Nos. 1 - 13 according to the invention, there
were obtained shaped parts having a homogeneous
microstructure which, as shown in Fig. 7, had no
recognizable amorphous dendrites but comprised fine
spherical primary crystals.
Fig. 14 is a graph showing how the B content and the
CA 022203~7 1997-11-06
-48-
degree of superheating of a melt during pouring affect the
size and morphology of the primary crystals of AC4CH alloy
(Al-7~ Si-0.3% Mg-0.15% Ti). Unlike in the case of combined
addition of Ti and B, no spherical crystals can be obtained
at temperatures more than 30~ above the liquidus
temperature when only Ti was added as a refiner.
Fig. 15 is a graph showing how the B content and the
degree of superheating of a melt during pouring affect the
size and morphology of the primary crystals of 7075 alloy
(Al-5.5~ Zn-2.5~ Mg-1.6% Cu-0.15~ Ti). The 7075 alloy was
in contrast with the AC4C~ alloy in that fine spherical
crystals are obtained with high degree of superheating even
when only Ti is used as a refiner.
Table 3 (Continued on the next page)
Degree of Refiner, X Temper- Pass- Overall Method Mater- Medium for Induct-
Run Alloy Super- ature of ing holding adding ial of cooling ion
No. heating, metal in time, time, refiner ladle holding heating
-sel,~ ni m n vessel
3~ a ,er. - "es
. 3, c 'er. - 'es
~ .. . . a ,er. - "es
Inven- ~ ,.'L' -~ , ~ . . .:;, ~ . ~. d ~ron - "es
tion ,~ 3~ b ,er. Air "es
.' . 5. 0 a 'er. Iater "es
Ca . . - . ~. a ~ron - ~es
_t .-Si, Sn : :.l 0.01 . ~. a ~ron - "es
. . a 'er. - es
~)~ :. . 0.002 ~ . ~, a 'er, - o D
.. ~. - , .. . a ron - o
' . ~. ~. - ~ . .. a ::ron - o
a 'er. - es
. C~er~ - 'es ~
L - A ~4 5 r5. 5 a 'er. - "es
Compar- ~ 0~ 0 a ,er. - ''es
ison ~ 5 a 'er Air 'es
. 5.0 a 'er Air o
" '- - -. - -' .~ ~.7 a 'er. - ~es
t -. - . .. 00 a ~ron - "es
-~ +Sr 0.015... ~ 1.0 a ron - ''es ~
~2 r ~ L ~ 7 a ~er. - ''es
Alloy AC4CH Al-7%Si-0.3XMg (Ti not added) m.p.: AC4CH 6 ,~
AZ91 Mg-9XAl-0.7XZn-0.4XMn AZ91 5
7075 Al-5.5XZn-2.5%~g-1.6XCu (Ti not added) 7075 6~ ~
Temperature of metal in vessel: Temperature of as-poured metal Material of lac e:
Passing time : Cer.: Ceramics;
Time required for the as-poured melt to decrease in temperature Iron: Stainless steel or cast iron
from the initial level and through temperature zone 5~ below Heating or heat retention of holding vessel:
the liquidus temperature. both top and bottom portions of vessel
Overall holding time: were heated or heate-retained
Holding time required for the temperature of the as-poured melt Fraction liquid:
to decrease from the initial level to the molding temperature. Estimated from equlibrium phase diagram
Method of adding refiner: a, melted in holding furnace; and cooling curve.
b, melted in ladle; Metal temperature distribution:
c, diluted; O, within t5~ of the desired temperature.x, outside t5~ of the desired temperature.
Table 3 (Continued)
lleating or Fraction Temperature Amount of Size of
Run Alloy heat-retention liquid distribution spherical primary Remarks
No. of holding before of metal in particles crystals
v~ssel sh~ing,X ho:-ing vessel
' es _ x
''es
~ ,. "es - x~
Inven~ , "es ~ '~ 'x
tion .. ~ ''es ~ 'x .:
- '~ ~--. ''es -
-Ca "es
Si,Sn "es , _ ,J
.es ~ ,x D
- es
:.,''es . _' ~ O
r es
. AC4CR+Ti ''es : ~ x 1 Degree of superheating O
too high
14 AC4CH+Ti,B Yes 60 O x 100 Degree of superheating
too hig-
.. C~C -~ ,3 "es .: O x. :. : ~SsinR ,ime ,oo l011R
Compar- .C~C -~:.,.3 es . O x loldinR ,ime .oo lona
ison "C~.C--~ , 3 No - x O ~ Ineven d stri~ution of
metal temperature O
18 AC4CR+Ti,B No 60 x 0 110 Uneven distribution of
me,al temperature
:3 ~R ''es : ~' x ' : 'es:.ner absent
) ~O ~ I_ x 'l: e ner absent
.. +Sr 'es ~ x : n.y Sr added
"es ~ x ' . efiner absent
CA 022203~7 1997-11-06
-51-
Table 3 sets forth the conditions for the preparation
of semisolid metal samples and the results of examination
of the microstructure of shaped parts. As Fig. 13 shows,
the forming step consisted of inserting the semisolid metal
into the injection sleeve 170 and subsequent treatment with
a squeeze casting machine. The forming conditions were as
follows: pressure, 950 kgf/cm2; injection rate, 0.5 m/s;
casting weight (inclusive of biscuits), 1.5 kg; mold
temperature, 230~.
In Run Nos. 13 and 14 (comparisons) in Table 3, the
degree of superheating above the liquidus temperature was
so high that no fine spherical crystals were obtained but
only coarse primary crystals formed (see Fig. 19).
In Run No. 15 (comparison), the temperature of the
melt poured into the holding vessel 30 was allowed to
decrease from the initial level and pass through a
temperature zone 5~ below the liquidus temperature taking a
time longer than 10 minutes. In Run NO. 16 (comparison),
the holding time was unduly long. Hence, only coarse
primary particles were obtained in these runs.
In Run Nos. 17 and 18, neither top nor bottom portion
of the holding vessel 30 was heat-retained or heated, so
even when induction heating was effected, the alloy in the
holding vessel 30 had an uneven temperature distribution.
In Run Nos. 19 and 20, the alloy samples produced only
CA 022203~7 1997-11-06
-52-
coarse primary crystals since they did not contain a
refiner. See Fig. 20.
In Run No. 21 (comparison), only Sr was added as a
refiner and the shaped part was not much refined compared
to that of the alloy containing no Sr. See Fig. 21 for the
microstructure of the shaped part obtained in Run No. 21.
In Run No.22, the alloy sample did not contain a
refiner and the degree of its superheatlng above liquidus
temperature was unduly high; hence, only coarse primary
crystals formed as shown in Fig. 22.
In contrast, the alloy samples prepared in Run Nos. 1 -
12 according to the fine spherical primary particles asshown in Figs. 16, 17 and 18.
As will be understood from the foregoing description,
according to the method of the invention for shaping
semisolid metals, shaped parts having fine and spherical
microstructures can be produced in a convenient, easy and
inexpensive manner without relying upon agitation by the
conventional mechanical and electromagnetic methods.
