Note: Descriptions are shown in the official language in which they were submitted.
CA 02434193 2003-07-09
METHOD FOR PROVIDING A PARTIALLY SOLIDIFIED ALLOY
SUSPENSION AND DEVICES
The present invention relates to a process in
accordance with the preamble of claim 1 and to the devices
having the features of the preamble of claim 6.
A process of the above type is known from
EP-A-0 745 694. This process uses an open casting ladle to
pour out the melt above an open channel, the intention being
that first nuclei for the formation of globular crystals
should be formed on the channel. To enable these nuclei to
multiply and grow, a number of individual crucibles are
guided past the exit from the channel and filled with
individual batches, the time which these crucibles require to
move along a path or a carousel being used to form globules
before the last crucible is then heated to facilitate pouring
and is then emptied into a forming machine, such as a die-
casting machine.
This known process is relatively complex and
disadvantageous, firstly because a large number of individual
crucibles have to be provided and moved along a path. This
alone entails considerable structural outlay. However, should
work be interrupted at the forming machine, the temperature
in the large number of crucibles will be different than the
desired temperature, leading to a different solids content,
and consequently it may no longer be possible to empty out
the material which has solidified in the crucibles. This then
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leads to a corresponding loss of material.
US-A-3,902,544 has disclosed another process in which
a furnace vessel is heated by induction coils at its
periphery and the liquid metal is fed to three discharge
pipes which are connected to the base wall and in which it is
stirred into a thixotropic state, forming degenerated
dendrites. This is relatively complex and the final
effect - as has been demonstrated - is relatively
ineffective. One factor in this respect is the fact that the
stirring entails a high level of outlay on both design and
energy and may give rise to operating shutdowns. Arranging
the discharge pipes in the region of the base also leads to
increased dendrite formation, since the base wall of the
furnace vessel is already subject to a certain cooling and
consequently a type of "sump" comprising dendritic primary
crystals was formed and was fed directly to the discharge
pipe in question, where the dendrite growth was then promoted
by continued cooling.
Electromagnetic stirring in continuous-casting
installations is also known from a very wide range of
documents. This stirring has always taken place using high
shear forces, since it was important to shear off and
"degenerate", i.e. comminute and round off, dendrites which
form at the edge. However, anyone who has ever stirred a cup
of coffee will know that during stirring a dead zone is
formed in the center of the stirring circle, in which no
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mixing takes place. However, this leads to temperature and
concentration gradients.
Therefore, the invention is based on the object of
making a process of the type described in the introduction
more efficient. This is achieved by the characterizing
features of claim 1.
Unlike in the prior art which has been described
above, the invention is based on the discovery that these
previous processes have been primarily based on the object of
destroying dendrites which form, but this destruction could
be substantially avoided if dendrite formation were to be
suppressed to a considerable degree from the outset.
Consequently, moveable stirrers or parts or other stirring
devices can be dispensed with.
To do this, it is necessary to consider the
"mechanism" of the solidification of metal. According to the
book "Metallurgie des Stranggie~ens" [Metallurgy of
Continuous Casting] by Prof. K. Schwerdtfeger, Verlag
Stahleisen GmbH, Dusseldorf, 1992, p. 59, the following
sequence of events results during cooling of a melt:
1. firstly, the formation of cells,
2. which are converted into dendritic cells,
3. which then become definite dendrites before
4. any slurry-like solidification at all, with
additional formation of globules, occurs.
Therefore, if the aim is to achieve a globular
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microstructure, according to these details it would be
impossible to get round the formation of dendrites. The
explanations given below will demonstrate that this can in
fact be achieved.
There will now follow an analysis of why so many
dendrites in fact form from the metal which is per se in
liquid form. These dendrites grow out of the cooler zone
toward the warmer zone, resulting in significant shifts in
concentration. Briefly, the considerations undertaken by the
inventors were as follows: the profile of the concentration
in front of a solidification front of this type can be
determined by the diffusion equation or Fick's 2nd law.
However, a boundary layer, the thickness 8N of which is
dependent on various factors, including the mixing, and which
likewise has a concentration difference with respect to the
melt, builds up in front of the solidification front. In the
known processes this leads to considerable mixing, for
example as a result of electromagnetic stirring, being set in
motion in order firstly to break up this segregation area and
secondly to shear. off the dendrites which have already
formed.
On the other hand, an area of what is known as
"constitutional supercooling" only occurs in a melt when the
gradient of the actual temperature is greater than or equal
to the gradient, which is induced by concentration
differences at the solidification front, of the liquidus
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temperature (which is predetermined for a specific alloy).
However, the aim, if a semi-solid material is desired, is to
achieve a solidification front with a thickness of virtually
zero. This then presents the question of how to achieve this.
This question then in each case led to the solution
described in the characterizing clause of claim 1, which is
carried out in one or other form but preferably in
combination. In actual fact, the three characteristics
mentioned are merely three different aspects of one and the
same solution, as will emerge below with reference to the
description of the drawings. Feature a) involves setting the
residence time in such a way that it corresponds to the cycle
time of the downstream forming machine. The forming machine
may optionally be a forging machine, an extruder, a heat-
rolling mill, a thixo-forming machine (with extruder), an
extrusion machine, but preferably a die-casting machine or a
continuous-casting device which operates with cycles (of
greater or shorter length). In any event, this setting of the
residence time therefore makes it possible to avoid the
downstream connection of a large number of crucibles in which
the operation of crystal growth is to take place in
accordance with the prior art, with all the inconveniences
which have been outlined above, since the desired suspension
is already obtained at the end of the suspending section.
Also, the invention if appropriate also allows a thixo-
forming machine to be of simpler configuration, since the
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extruder which is then generally provided no longer has to
break up dendrites, but rather is used primarily to introduce
the alloy suspension into a mold.
Feature b) describes the extent of cooling used to
achieve the desired suspension. The cooling can be set by
selecting the coolant or - if a flowing coolant, e.g. oil, is
used - by setting the quantitative flow of this coolant per
unit time. Evidently, strong cooling of this type has not
hitherto been attempted and it has therefore been necessary
to make do with a large number of crucibles connected
downstream of a cooling channel. However, the invention has
shown that this prejudice in the specialist field was
unjustified.
According to feature c), the method steps which have
previously been carried out involving nucleation and
increasing the number of or growing the nuclei, has been
advanced one station, specifically the initial nucleation
into the reservoir, this being based on the discovery that
precisely such initial nuclei, i.e. atom arrangements
corresponding to the later crystal, are already present in a
reservoir of this type (which is preferably a furnace).
However, the distribution and feed generates a flow which
enables nuclei of this type which are already inherently
present to be guided into the desired direction and in this
way moved to the suspending section, along which, by means of
a turbulent flow, which if appropriate is generated by static
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mixing, such a large number of crystallization nuclei are
formed that there is simply no space for dendrite growth,
i.e. in any event the basic idea of the invention is based on
simply not allowing dendrites, which would then have to be
destroyed, to form right from the outset.
Compared to the closest prior art, the characterizing
features explained above provide the advantage of allowing a
virtually continuous process without crucible-moving means
and without the risk of such high material losses, instead of
a batch process with a huge number of small batches
(crucibles). However, there is also no need for the alloy
suspension formed in this way to have any dimensional
stability whatsoever, as has been aimed for in the prior art.
It will also be understood that it is preferable if at least
two of the characterizing feature groups explained above are
used in combination with one another. It is then preferable
to provide the features of claims 26 and/or 27, which make it
particularly easy to match the metering of the melt to the
cycle time. This means in any event that the alloy suspension
is produced as required virtually simultaneously with the
feed to the forming means (irrespective of the type of
forming used, for example forging machine, die-casting
machine, etc.).
The turbulent flow which is established at the
suspending section on account of viscosity effects is
inherently sufficient, but it is also possible to provide the
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features described in claim 3. The static mixing makes it
easy for the nuclei formed at the cooling surface to be
homogeneously suspended in the melt. This suspending step at
the same time suppresses the formation of a diffusion zone at
the boundary layer between nucleus and melt and thereby
avoids the prerequisite condition for dendrite growth.
Therefore, there is no constitutional supercooling. It should
be pointed out once again at this point that in the present
context the term "nucleus" is to be understood as meaning a
preformed atom arrangement which corresponds to the crystal
lattice.
A further significant drawback of the prior art was
the large areas of the alloy suspension which were left open
to oxidation. Therefore, according to a refinement of the
invention, feature E) of claim 3 and/or the features of
claim 5 are provided.
A device according to the invention preferably has
the features of claim 8 or one of the associated subclaims.
However, a problem with the active cooling by means of a
cooling system which is preferably provided (but which can
also occur without the production of a partially solidified
alloy suspension) is that the metal tends to "cake" onto the
cooled walls. To avoid this, it is preferable to provide the
features of claim 10.
Further details of the present invention will emerge
on the basis of the following description of exemplary
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embodiments which are diagrammatically depicted in the
drawing, in which:
Fig. 1A shows a device which has been configured in
accordance with the invention for the purpose of
providing a partially solidified alloy suspension,
in order to give a more detailed explanation of the
process according to the invention;
Fig. 1B shows a variant of the device illustrated in
Fig. 1A together with a continuous-casting device
as forming machine;
Fig. 2 shows an outlet pipe, which has been constructed in
accordance with a second exemplary embodiment of
the invention, of a melting furnace upstream of a
die-casting machine with a die-casting die into
which material is cast centrally;
Fig. 3 shows an outlet pipe, which has been constructed in
accordance with a third exemplary embodiment of the
invention, of a melting furnace upstream of part of
an extrusion installation;
Figs. 4 and 5 show further alternative embodiments;
Fig. 6 shows a variant of Fig. 4 in a section on line VI-
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VI from Fig. 4, while
Fig. 7 shows a section on line VII-VII from Fig. 6;
Fig. 8 shows a device which is composed of individual
sections whose temperature can be controlled
separately, and
Fig. 8A shows an enlarged exerpt of a detail A from Fig. 8;
and
Fig. 9 shows a section on line IX-IX from Fig. 8.
Fig. 1A diagrammatically depicts part of the shot
sleeve 6 of a die-casting machine 1 having an injection
plunger 7. The shot sleeve 6 also in the usual way includes a
filling opening 8, through which metal which is to be cast
can be introduced in front of the plunger 7. An alloy is
introduced via a transfer vessel 10d, which is in this case
connected to the outlet pipe 9 of a metering furnace 9'. The
advantage of a vessel lOd of'this type is that its volume can
easily be used to determine the volume of metal which is to
be introduced into the filling hole 8 for a shot. For
example, if appropriate a single melting furnace 9',
which - as will be shown below - is advantageously formed as
a metering furnace 9', can if appropriate be assigned to one
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die-casting cell (which may comprise one or more die-casting
machines in the immediate vicinity, e.g. in a star
arrangement).
The transfer vessel lOd preferably has a discharge
means, preferably in the form of a plunger 28 (although in
principle it would also be possible to use an extrusion
screw, but a plunger 28 is simpler), so that the metal which
has collected therein can be forcibly pressed under pressure
into the shot sleeve 6. Since the metal is in the partially
solidified state; the pressure exerted in this way can if
necessary reduce its viscosity, making it easier to introduce
the metal into the shot sleeve. Moreover, the volume to be
introduced can easily be determined by the volume of the
transfer vessel 10d. If it is desired to change the volume,
the transfer vessel 10d can advantageously be removed from
the outlet pipe 9c by means of a releasable connection means
(not shown in detail here) and replaced with a transfer
vessel of larger or smaller volume.
The transfer vessel lOd may either simply be
correspondingly insulated, in order to ensure an isothermal
state of the metal which it contains after it has acquired a
desired partially solidified state over the suspending
section 9, or expediently it may also have at least one
cooling means having an inlet 12, for example arranged at the
bottom, and an outlet 13 and cooling pipes O. The mouth of
the transfer vessel 10d may be provided with a closure means
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10" which can be slid out of the opening position shown into
a closed position perpendicular to the direction running in
the plane of the drawing or can be pivoted about a hinge H,
as is known in the case of a tundish. A further particular
purpose of the transfer vessel lOd is to match the residence
time to the cycle time of the downstream forming machine 1.
However, it may arise that operating faults occur,
preventing the transfer vessel lOd from being emptied
immediately. In this case, there would be a risk of the
transfer vessel lOd ultimately only containing completely
solidified metal. To prevent this, it is preferable if the
transfer vessel lOd is also provided with heater coils X.
These heater coils X may be assigned a heat sensor (or a
sensor, for example an inductive sensor, for detecting the
state of aggregation of the metal therein, as has already
been described in the literature), in order for the heater
coils, if appropriate only in part, to be switched on when
the metal which has cooled to the desired partially
solidified state is to be kept in this state. Indeed, heater
coils X of this type can also be used to enable the partially
solidified material to be discharged from the transfer vessel
lOd more easily by liquefying its edge zones.
The metering furnace 9' has a forced-delivery pump
17. In the process according to the invention, this pump
performs a number of functions simultaneously: firstly, melt
is introduced, at least periodically continuously, into the
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outlet pipe 9, which forms a suspending section for the melt
flowing out. In this context, it should be mentioned that it
would per se also be possible to provide an open channel
instead of the outlet pipe 9, but a closed pipe offers better
protection against oxidation and also enables a shielding gas
atmosphere to build up therein. Since the metal in the
melting furnace is held in the liquid state, i.e. above the
liquidus temperature, it is necessary to have a cooling
intermediate step if it is desired to feed the shot sleeve 6
with partially solidified metal. It will therefore be
understood that it is preferable if the metal in the melting
furnace, by the outlet pipe 9, is only brought to a
temperature which is no more than 30°C, preferably no more
than 20°C, e.g. is only approximately 10°C, above the
liquidus temperature, in order in this way firstly to save
energy and secondly to accelerate the operation of cooling to
the partially solidified state. The furnace 9' is preferably
provided with a metering chamber 24 which, apart from a
connecting opening 23, is split by a partition 22, is
connected via the connecting opening 23 to a chamber
positioned in front of it, for example a chamber which is at
a higher temperature, and on the exit side is directly
connected to the outlet pipe 9.
A further function of the pump 17 is to generate a
flow in the melt in the furnace 9', so that unmelted
crystallization nuclei, whether these be external nuclei or
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small dendrites which have formed on the furnace walls, are
passed into the pump pipe 17" . A propeller 17' (or a screw)
acts as a distributor (mixer) and divider, so that further
nuclei are formed therefrom. In this context, reference is
made to Friedrich Ostermann, "Anwendungstechnologie
Aluminium" [Technical Applications for Aluminum ,
Springer-Verlag, 1999, p. 306, which describes the advantage
of stirring operations with a grain-refining action. A
further function of the pump 17 is that it can be used to
carry out precision metering (alloy suspension "on request")
when it is assigned at least one drive, whether as
transmission or as motor M, which can be switched on and off,
by means of a switch S which can be actuated by hand or by a
program control, for conveying purposes over the cycle time
of the die-casting machine (1 in Fig. 1B). Preferably or as
an alternative, the screw or propeller pump 17 is assigned a
variable-speed drive M, for which purpose a motor control
stage C may be provided.
To obtain additional primary nuclei, it may be
expedient for at least parts of the pump 17 to be cooled. By
way of example, the pipe 17" may be provided with a cooling
jacket. However, it is preferable if moving parts of the pump
17 are provided with a cooling means of this type. For this
purpose, the shaft 17a of the pump 17 is formed as a hollow
shaft, as is known, for example, for stirrer mills for
cooling purposes, and consequently the details of a cooling
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arrangement of this type do not need to be described.
Accordingly, coolant runs in in the direction indicated by an
arrow P through the central hollow part of the shaft 17a (a
pipe which is inserted into the shaft 17a, rotates with it
and, by way of example, does not extend all the way to the
bottom of the cavity in the shaft 17a), then at the bottom
end of this hollow part flows radially outward into an
annular passage and leaves the hollow shaft 17a through a
stationary rotation outlet 17b, which is known per se for
stirrer mills, with an outlet collection piece 17c. However,
if desired it is also possible for the propeller 17' or the
pump screw to be cooled in a manner which is known per se.
The rotation of the shaft 17a means that any primary
dendrites which form at the shaft are thrown off radially
into the melt and degenerate in the melt as has been
described in connection with the abovementioned prior art.
The advantage of the process according to the invention over
EP-A-0 745 694 resides in the fact that the nucleation
process is advanced in terms of both time and location, and
consequently it is unnecessary for a large number of moving
crucibles to be connected downstream for the subsequent
cooling and nucleus growth, and there is therefore also no
need for the equipment used to move these crucibles. Then, in
a second step, the nucleated volume which has been preformed
in this way is increased in size in such a manner that even
irrespective of the effects of the static mixing, the
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geometric boundary conditions also no longer allow dendrites
to grow.
Therefore, the pump 17 conveys the melt into a pipe
connection piece 26 which branches off from the pump pipe
17 " and to which the outlet pipe 9 is connected. The outlet
pipe 9 is internally smooth but, in a similar way to the
transfer vessel 10d, has cooling coils O and heater coils X
for the same purpose as has been described above in
connection with the vessel 10d. The melt which has been
provided with primary nuclei is therefore conveyed by the
pump 17, preferably in a thin film, over the base of the
outlet pipe 9. The cooling means 0 ensures that the layer of
the melt which is closest to the base becomes more viscous
and starts to flow more slowly, while a hotter layer above it
runs more quickly. However, the hotter layer melts the thin
film below it again, so that the ultimate result is
turbulence in the flow, producing a mixing effect. This
mixing effect in turn homogenizes the alloy suspension which
forms in this way, i.e. temperature and concentration
differences over the volume of the alloy transversely with
respect to the direction of flow are avoided and therefore so
is the tendency for dendrites to form. Rather, further nuclei
are formed, no longer allowing space for dendrites to grow,
and therefore increase in size in globular form, which is of
course desirable. Therefore, the desired alloy suspension is
already present in substantially finished form at the exit
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from the suspending section formed by the outlet pipe 9 and
there is therefore no longer any need for any further
crucibles.
Now, if the same alloy always with substantially the
same solids content is to be used in each instance, the
configuration of the arrangement shown in Fig. 1A which has
been described above is sufficient. However, if it is to be
possible to make changes to the alloy and/or the solids
content, a problem is that given a constant inclination a of
the suspending section 8 with respect to a horizontal plane
shown by dot-dashed lines, the viscosity of the alloy in
question will vary, which would then affect the cooling time
in the outlet pipe. To obtain a melt residence time in the
suspending section 9 which can be selected independently and
in this way to control the proportion of the melt heat which
is extracted from the melt, the angle of inclination a of the
suspending section is preferably adjustable. This allows the
residence time to be controlled in such a way that firstly it
can be matched to the cycle time of the downstream forming
machine, for example the die-casting machine indicated using
parts 6-8 in Fig. 1A, and that secondly the extraction of the
melt heat can if appropriate be set in this way, while a
further method of setting this extraction resides in
selecting the coolant which flows through the pipes O and its
throughput per unit time. For example, a proportion of 20% to
60%, preferably 30% to 50%, of the melt heat can be withdrawn
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from the melt as it passes along the suspending section 9, in
such a manner that the desired alloy suspension is present in
finished form at the lower end of the suspending section. It
will be understood that the angle of inclination a will
deviate from 90°, i.e. from the vertical, and will be less
than 90°.
To enable the angle of inclination a to be set, there
may be an adjuster device in the form of an eccentric 54,
which can rotate about a stationary axis 53 or shaft, inside
a frame 55 connected to the outlet pipe 9, so that the pipe 9
can be inclined to a greater or lesser extent . At the other
end, the pipe can be pivoted about an axis 58 which is
located preferably close to a wall of the furnace 9' or close
to the pipe connection piece 26. It will be understood that
the embodiment shown merely represents an example and that it
may even be preferable to use a fluidic adjustment system, a
rack system or a lever transmission in order to achieve
greater adjustment movements, for example to raise the
suspending section 9 above the horizontal plane shown by the
dot-dashed line, in order in this way to enable the alloy to
flow back into the furnace 9' should faults occur at the
forming machine.
In order to as far as possible avoid a skew position
of the transfer vessel 10d, the furnace 9' preferably stands
on a lifting frame, which is only diagrammatically indicated
and may be formed in a conventional way, for example as a
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frame which can be raised and lowered hydraulically or
mechanically. It is preferable for the adjustment of the
adjuster means 53-55 or the rotation of the shaft 53 to be
synchronized with the movement of the lifting frame 57. Of
course, it would theoretically also be possible for the
furnace 9' to be provided at such a high level that the
vessel lOd is under all circumstances above the filling
opening 8 (which is then of correspondingly large dimensions)
even if the inclination of the pipe 9 varies.
The cooling means 0 was mentioned above. However,
cooling may additionally or alternatively also take place in
such a way that shielding gas, e.g. nitrogen, is fed into the
outlet pipe 8 via an inlet 58 and is discharged via an outlet
59 at the end. In such a case, there is no need to heat the
gas, which can be supplied at room temperature, i.e.
approximately 20°C, or even in liquid form. This is
particularly advantageous when processing magnesium, in which
case the interior of the furnace 9' will also be filled with
an inert atmosphere of this type. Of course, a measure of
this type is also advantageous for aluminum or any other
metal, since it greatly reduces or virtually eliminates
oxidation.
According to Fig. 1B, the metal which is to be cast
arrives from the metering furnace 9' (Fig. 1A), of which only
the outlet pipe 9 is shown in Fig. 1B. In the embodiment
shown in Fig. 1B, the step of filling a forming machine 1a is
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carried out using a transfer vessel 10 which is separate from
the outlet pipe. The advantage of a separate transfer vessel
resides, inter alia, in the fact that it is simply no
longer necessaxy to assign each forming machine a dedicated
melting furnace with outlet pipe 9, but rather it is possible
for a single melting furnace to be positioned at a central
location, from which the individual forming machines can be
supplied via vessels 10 of this type.
A continuous-casting means la of a form which is
known per se, as the forming machine, is connected downstream
of a collection vessel or tundish 10e, beneath the transfer
vessel 10. This is a similar continuous-casting means to the
one which has been disclosed by DE-A-1 783 060, the only
difference being that in this continuous-casting machine it
was necessary to provide an electromagnetic stirring means in
order to destroy dendrites. This means can now be eliminated
by the invention, so that the continuous-casting device la
can be of simplex and less expensive construction. It should
be noted that a continuous-casting machine la of this type
can be operated either cyclically - to produce billets of
greater or lesser length - or continuously. It should be
noted that the combination of a continuous-casting device la
with a metering furnace 9' which includes a pump 17, as
described in detail below with reference to Fig. 4, is
particularly advantageous because a continuous-casting
installation and the quality which it achieves are by no
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means least also dependent on as far as possible achieving a
uniform static liquid pressure. By way of example, it is
known from US-A-4,358,416 or EP-A-0 095 596 to provide a
means for controlling the level in the tundish. However, if
the metering pump 17 is combined with the continuous-casting
means, a constant static liquid pressure is automatically
obtained, and under certain circumstances it is even possible
to dispense with the tundish 10e, enabling the metering pump
17 to supply the continuous-casting device la directly.
To make the mixing effect described above more
intensive, the vessel 10 now has a static mixing means,
preferably in the form of wall protrusions 11 which mesh with
one another, in each case turn the metal from one side to the
other and thereby mix the metal. These wall protrusions
therefore mix the metal while it is being introduced and
while it is flowing out. Unlike in the known batch process,
in which individual batches are introduced into a large
number of crucibles, for example moving along a carousel, in
this case, therefore, a throughflow process is employed or
the partially solidified metal is obtained in a through-flow
process, in which the closure 10" is always open or may even
be omitted altogether. At the same time, the vessel 10, in a
similar manner to the vessel lOd which has been described
with reference to Fig. 1a, may either simply be
correspondingly insulated, in order to keep the alloy
suspension which has been introduced isothermal, or
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alternatively it may, like the vessel 10d, expediently have
at least one cooling means with an inlet 12, for example
arranged at the bottom, and an outlet 13, as well as cooling
pipes 0 in the interior of the protrusions I1. This ensures
the transition from the (still) liquid state, as it flows out
of the pipe 9, into a cooled state; the mouth 10' of the
vessel 10 may once again be provided with a slideable closure
10" , but in the present case will not be, as has been
mentioned above. It will be understood that when a portable
transfer vessel 10 of this type is used, the cooling capacity
of the outlet pipe can at least be reduced, for example the
cooling means 0 can be dispensed with and it is possible to
make do with cooling by shielding gas alone. For the same
reason as has been described above with reference to the
vessel 10d, it is also preferable to provide a heater means X
which can be switched on as desired and can also be used to
melt solidified material, so that there is no need for any
separate shock heating 16a.
It will be explained below with reference to Fig. 8
how zoned monitoring of the temperature with corresponding
control can be achieved. However, in the embodiment shown in
Fig. 1B, it may be expedient for the vessel 10 (or the vessel
10d) to be divided, for example along its longitudinal axis
14, and to be dismantlable, so that it can be cleaned if
necessary. In such a case, it will be expedient if each half
is assigned a dedicated feed and discharge for the
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corresponding temperature-control medium (cooling or heating
medium), as is shown in the case of the four connections 15
( in each case one pair for each half of the vessel ) for the
electrical connections of the heater coils X.
Fig. 2 diagrammatically depicts the die-casting
machine I having a plurality of die parts 2, a fixed platen 3
for a stationary die part 4, and a stationary shield 5. The
shot sleeve 6, in which the injection plunger 7 can be moved
along the length, is clamped in between the platen 3 and the
shield 5. The shot sleeve 6 may be provided with a heater
means 16. It will be understood that as an alternative to the
die-casting machine 1 it is also possible to use any other
forming machine, for example an extruder on its own or as a
thixo-forming machine, for example in accordance with
WO 97/21509. However, while the extruder in that document has
the function, inter alia, of destroying any dendrites by
means of its shear forces (and therefore also has a
correspondingly high demand for energy), this is not the case
of the present invention, since in this case no dendrites are
formed.
In the present exemplary embodiment, mixing
protrusions lla of a gravity mixer or static mixer are
integrated in the outlet pipe 9a of the melting furnace 9',
which latter is only partially illustrated. It should be
noted at the present paint that although it is preferable fox
the mixing to be carried out under the force of gravity, it
CA 02434193 2003-07-09
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would also be conceivable for the mixing protrusions, e.g.
11a, to be accommodated in a rising cooling pipe through
which the metal is conveyed, for example by means of gas
pressure. If appropriate, the melting furnace 9' may be
displaceable, in order to be able to reach each forming
machine which is to be supplied, i.e. it does not need to be
assigned in a stationary position to the forming machine. In
this case, as in the following exemplary embodiments and in
the same way as in Fig. 1B, however, the reference symbols
are if appropriate provided with a suffix.
Therefore, if liquid metal, preferably at just above
the liquidus temperature, is conveyed out of the furnace 9'
with the aid of a metering pump 17 into the outlet pipe 9a,
it runs downward over the staircase formed by the protrusions
11a, the outlet pipe 9a either having a sufficient length for
natural, unforced cooling to occur or once again - as
illustrated - being provided with cooling pipes 0 in the
protrusions 11a, which is preferable. The desired mixing
effect results from the constantly recurring, cascade-like
pouring onto in each case the next step 11a down.
Should the outlet pipe be so thin that the flowing
metal fills its entire internal diameter, it is also possible
for mixing protrusions lla of this type to be provided at the
upper or side walls of the outlet pipe, in which case the
shape of the protrusions may be identical or different, but
in any case different shapes, for example of the type still
CA 02434193 2003-07-09
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to be shown below, may be present in mixed form. For example,
it would be conceivable for a disk provided with openings to
be provided transversely across the diameter of the pipe at
the upper end of the outlet pipe 9a in order to achieve a
static mixing effect, so that the flow of liquid metal is
divided into a plurality of partial streams which are
combined again and thereby mixed downstream of the disk.
However, a disk of this type constitutes a certain flow
resistance, and consequently it should only be arranged at
the upper region of the outlet pipe 9a.
As with the vessel 10 shown in Fig. 1B, in this case
too it may be advantageous to incorporate heater coils X, in
order to prevent metal from "caking" on the walls of the
outlet pipe 9a should a prolonged residence time of the metal
in the outlet pipe result, for example on account of
operating faults. Therefore, it is particularly advantageous
to use non-wetting materials for the static mixer and/or the
outlet pipe 9, 9a of the furnace. Examples which may be
mentioned include ceramic-coated metal or completely ceramic
components. In this case, a metal plate 19 of this type is
indicated by dashed lines at 19. The mixing protrusions lla
may then be arranged on the plate 19, which in Fig. 2 can be
pulled out of the pipe 9a for repair or cleaning purposes.
Additional strong heater coils X' may be advantageous for
shock-type heating if the suspension has completely
solidified, for example on account of a fault, and needs to
CA 02434193 2003-07-09
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be made to flow again.
Furthermore, it may be advantageous to fit a high-
frequency means, preferably an ultrasonic-frequency means 16a
to the suspending section 9a, in order to prevent melt from
penetrating into the pores of the ceramic material which
forms the protrusions 11a. Particularly when ultrasonic
vibrations are imposed, it has been found that such
vibrations force the metal out of the pores and thereby
extend the service life of the ceramic parts.
If appropriate, the lower end of the outlet pipe 9a
may also be provided with a similar closure to that indicated
at 10" for the transfer vessel in Fig. 1A or B. For this
purpose, it is possible to provide a slide housing 18. This
makes it possible to prevent alloy suspension from flowing
out in the event of faults at the forming machine (and
therefore a change in the cycle time). However, if
appropriate the outlet pipe 9 can be pivoted about the axis
56 (Fig. 1A), in such a manner that in such a case it is
pivoted upward above the horizontal plane shown by dot-dashed
lines in Fig. 1A and as a result the alloy suspension flows
back into the furnace 9' (or into another storage vessel). Of
course, it would also be conceivable for the outlet pipe 9a,
as in the case of the vessel 10 shown in Fig. 1B, to be
composed of two dismantlable halves, for example along its
longitudinal axis L, so that maintenance work can be carried
out more easily.
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Where the text above referred to the provision of an
ultrasonic means, it should be noted at this point that the
use of ultrasound, for example in a vessel 10, has also been
found by the inventors to have a positive effect on the
microstructure of the metal. It becomes finer and the
crystals become rounder. An ultrasound effect of this type
can also be applied, for example, to the forming machine,
such as a die-casting machine, since it then exerts a type of
"vibrator effect" in a similar manner to the compacting
action of concrete vibrators. Since ultrasonic vibration of
this type generally propagates to all sides, it may be
sufficient to fit a single ultrasound means at one location
and to operate this means with an energy which is such that
it has a favorable effect both on the ceramic lining of the
suspending section and on the forming machine. By way of
example, the ultrasound means could be arranged at a shot
sleeve - which is expediently also lined with ceramic - of a
die-casting machine, in which case the sound has its effects
both as far as the upstream suspending section and also as
far as the vessel 10, in one direction, and as far as into
the cavity of the casting die. However, this means that a
relatively high level of energy is required, and consequently
it will be preferable to provide a plurality of ultrasound
means of this type. Although an ultrasound means is
preferable, it is also conceivable to use other high-
frequency vibration-exciting means, such as for example an
CA 02434193 2003-07-09
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alternating electromagnetic field, which may be effective at
lower frequencies but is preferably operated at high
frequency. It will be understood that this application of
vibrational energy also constitutes an independent invention
on its own, irrespective of the other features.
Fig. 3 shows a similar illustration to Fig. 2, but of
a modified exemplary embodiment. In this case, the forming
machine provided may be an extrusion press with an extrusion
plunger 7b, but it may also be a shot sleeve 6a similar to
the shot sleeve 6 shown in Fig. 1A for a die-casting machine.
The form of this shot sleeve 6a now corresponds to a shot
sleeve as described in German Laid-Open Specification
100 47 735, the content of which is hereby incorporated by
reference. Specifically, it is appropriate for the shot
sleeve 6a to be heated, for example, by means of heater coils
16 in order not to change the alloy suspension. In this
context, it may be advantageous for the front part of the
shot sleeve 6a to a certain extent to be formed as a
"transfer vessel" and provided with a, for example likewise
heated, sliding closure 10" .
As in Fig. 2, in Fig. 3 the outlet pipe 9b itself is
likewise provided with a static gravity mixer along a
meandering center line L' between protrusions llb which
overlap one another, in a similar manner to that shown in
connection with the mixer illustrated in Fig. 1B, allowing
particularly intimate mixing and homogenization. Although the
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static mixer may in principle be formed with internals
provided with openings, in the manner of the type used for
bulk materials, so that part of the stream flows outward or
inward through the openings while another partial stream
moves past them, in the context of the present invention
protrusions which overlap one another are particularly
preferred for a number of reasons. Firstly, internals
provided with openings, like the abovementioned disk inserted
transversely, tend to become blocked, specifically as soon as
the temperature of the metal has dropped to a corresponding
degree and the metal has become more viscous. Secondly, an
overlap means that some of the metal flows along the wall but
some of it drops onto the next overlapping protrusion llb and
from there is discharged downward, where it is combined with
the metal flowing along the wall and therefore over a
different path, with a mixing effect.
It has already been mentioned above that it may be
advantageous to provide both cooling and heating means. Since
the metal emerges from the furnace 9' at a relatively high
temperature, it may be that further heating in the event of a
shutdown or interruption in operation is less necessary in
the upper part of the gravity mixer. This case is shown in
Fig. 3, in which only cooling coils 0 are provided in the
upper part of the outlet pipe 9b, but, in a tonal manner, an
increasing number of electric heater units X a,re provided the
closer the mixer space 21 comes along the line L' to the
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mouth 10'b or the slide housing 18 of the outlet pipe. The
shock heating 16a for emergencies, which has been explained
with reference to Fig. 2, can likewise be provided.
Although the cooling means or heat-conducting pipes
may per se be formed in various ways, for example may also
operate with evaporable coolant, with such strong cooling
there is a risk of local supercooling, which may then lead to
the "constitutional supercooling" which has been described in
the literature and to dendrites being formed. Therefore,
cooling by means of a flowing cooling medium is preferred, in
which case, although it is possible to carry out cooling in
countercurrent to the flow of the metal, in the manner which
has been shown with reference to Fig. 1B, it is preferable to
reverse the arrangement shown in Fig. 1B, with the cooling
inlet 12 at the top and the outlet 13 at the bottom, i . a . a
co-current cooling arrangement, since in this way the upper
region, where the liquid metal enters, is cooled to a greater
extent than the lower region. Although this nevertheless
overall leads to an approximately linear drop in the metal
temperature over the length of the path, e.g. along the line
L', in practice there tends to be a more or less gentle
depression in the cooling capacity.
It has been mentioned above that the metal which
emerges from the mouth 10' (or 10'a or 10'b) may if
appropriate also be in semi-liquid form, i.e. may have a
solids content of below SOo. Of course, this makes it easier
CA 02434193 2003-07-09
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for the cooled metal to flow out, although in certain cases
metal with a solids content of >_ 50 a by weight is preferred
in particular for forming machines, such as 1 or la. In this
case, however, it may under certain circumstances be more
difficult for the metal to be introduced into the forming
machine. Fig. 4 shows a way round this problem.
Fig. 4 once again illustrates the furnace 9' with the
metering chamber 24, which is divided by the partition 22
apart from a connecting opening 23 and is directly connected
to the outlet pipe 9c. For this purpose, the pipe 17" of the
pump 17 is immersed below a liquid level, defined for example
by a sensor 25, of the melting furnace 9' and conveys the
melt via the pipe connection piece 26 which projects into the
outlet pipe 9c into said pipe 9c. At the location of a sensor
25 which controls the pump 17, it is also possible to provide
an overflow edge which determines the liquid level without
any control outlay. By way of example, the inner, lower edge
of the connection piece 26 can be used as an overflow edge.
Within the outlet pipe 9c, the static mixer is in
this case formed as a fixed worm coil 11c, which if
appropriate may be provided with individual pins 27 over its
circumference or length in order to improve the mixing
action. However, the bottom end of the outlet pipe 9c now
opens out into a transfer vessel lOc which collects the alloy
suspension and can be docked or is fixed docked onto the
filling hole 8 in the manner indicated. The transfer vessel
CA 02434193 2003-07-09
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lOc is advantageously once again provided with a discharge
means or the plunger 26. As can be seen and as is indicated,
it is once again advantageous for the vessel lOc to be
provided with heating means X, if appropriate also with a
cooling means 0.
The embodiment in Fig. 5 is similar to that shown in
Fig. 4 to the extent that in this case too a static screw
coil llc is fitted. This embodiment merely illustrates that
it would be conceivable for the outlet pipe 9d to be held
rotatably in bearings 29 on a base 29' and to be driven, for
example, by means of an externally fitted toothed ring 30 and
a motor pinion 30 of a motor M1. Depending on the dimensions
selected, in particular the length of the outlet pipe 9d, the
direction of rotation may be either in the conveying
direction of the metal flowing down under the force of
gravity or advantageously in the opposite direction. In the
case of the opposite direction to the conveying direction
defined by the coil 11c, the metal remains in the region of
the temperature-control means X and/or 0 of the outlet pipe
9d for longer, i.e. this pipe 9d can then if appropriate be
made shorter. In addition, the gravity conveying in the
downward direction of the outlet pipe 9d and the simultaneous
rotation of this pipe in the opposite direction result in
improved mixing. Nevertheless, this embodiment is not
preferred in all cases, on account of the additional drive
which has to be provided.
CA 02434193 2003-07-09
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Fig. 6 illustrates a further embodiment in a
sectional view from above, approximately on line VI-VI from
Fig. 4. In this embodiment, an outlet pipe 9e, which is
connected to the isothermal collection vessel 10c, includes,
as suspending section, a series of cooling fins 31 which, in
order to achieve a mixing action, can, although do not have
to, be provided with diverter means, for example at 32,
and/or with interruptions 33 and/or thickened diverter
sections 34. It would also be possible for a diverter pin,
for example similar to the pins 27 shown in Fig. 4 or in the
style of Fig. 9 described below, to be provided in the
channel between two such fins 31, in the region of an
interruption, so that at least the streams of metal which
have been split by the cooling fins flow back into one
another or are backed up and then mixed with incoming metal.
Fig. 7 shows a section approximately on line VII-VII
from Fig. 6, in which fins 31 and 31a are provided offset
with respect to one another. It is clearly apparent that the
fins 31a are also provided with cooling passages 0. At this
point, it should be mentioned that fins which are parallel to
one another (as can be seen in the sectional illustration
presented in Fig. 7), i.e. which do not per se result in any
static mixing effect (apart from the turbulence of the layers
which has been described with reference to Fig. 1A), are able
to make a contribution to improving the cooling capacity, and
consequently internals 31, 32a of this type are advantageous
CA 02434193 2003-07-09
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with a view to increasing the size of the cooling surfaces.
Therefore, it is conceivable to take account of differing
cooling requirements for different alloys and/or solids
contents by replacing different sizes of the cooling surface.
For this purpose, the design which has been described above
with reference to the exchangeable plate 19 is once again
advantageous. However, if desired it would also be possible
to provide an exchangeable inner pipe instead of an
exchangeable plate 19.
A particular embodiment is to be shown with reference
to Figs. 8, 8A and 9, from which it can first of all be seen
that the outlet pipe 9f is running at an ever steeper
gradient, i.e. at a steeper angle to the horizontal, from the
top downward. This takes account of the fact that the metal
becomes increasingly viscous as it cools and therefore may
start to flow more slowly. The profile of the center line L'
illustrated preferably approximately corresponds to a
brachistochrone (cycloid), but other profiles, for example
with straight sections which in each case adjoin one another
at an angle, which may be comparable, for example, to a
cycloidal profile, are also conceivable.
Furthermore, these figures provide a more pronounced
illustration of something which has already been indicated in
the embodiment shown in Fig. 3, namely a different
temperature control in different regions. In the case shown
in Fig. 8, this is so advanced that the outlet pipe 9f is
CA 02434193 2003-07-09
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divided into individual rings 9.1 to 9.5 which can be fitted
together and each have separate temperature-control circuits
(only the cooling circuits are illustrated).
Therefore, there is a feed manifold pipe 35 and a
discharge manifold pipe 36 provided along the outlet pipe 9f,
both of which can be connected, via an associated connection
piece 37 or 38, respectively, to corresponding feed lines and
discharge lines. In each case one feed branch 39 at the upper
end of each ring and one discharge branch 40 with a control
valve V at the lower end of each ring 9.1 to 9.5 extend from
these manifold pipes 35, 36 to each of the rings 9.1 to 9.5.
Instead of being provided in the discharge branch 40, the
control valve V could also be provided in the respective feed
branch 39. Each ring 9.1 to 9.5 is assigned a temperature
sensor 41, which in this case is shown in the drawing at the
top side but preferably tends to lie in the region of the
discharge branch.
The sensors 41 may for example - as is known from
sensor cables - be connected to a bus 42 and they are
interrogated on an ongoing basis by a processor 43 about the
temperature which they measure. By way of example, each
sensor 41 has an addressing part with a dedicated address
and, after this address has been called up by the processor
43, transfers its temperature data to the latter. Then, after
comparison with a SET value, the processor 43 can emit a
corresponding control signal to the respectively associated
CA 02434193 2003-07-09
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control valve V to which it is connected via a further bus 44
(or via individual lines). Of course, in the case of a bus
44, it must be possible also to address each control valve V.
In accordance with the statement which has already been made,
the SET values will substantially decrease, either linearly
or slightly depressively, from 9.5 to 9.1. This means that
with a depressive temperature gradient profile from the top
downward, the temperature at the mouth or underside of the
outlet pipe 9f will be higher than if the temperature were to
be reduced linearly from ring to ring. To ensure optimum
cooling, it may be expedient it in each case screw-like coils
48, which lead from the feed branch 39 to the discharge
branch 40, are fitted in a lateral space 45 (cf. Fig. 8A) in
each ring 9.1 to 9.5.
In this case, the static mixer is provided by pins
27a which are offset with respect to one another, if
appropriate may be mixed with one of the embodiments
described above and may - as illustrated - be arranged only
at the base of the outlet pipe 9f or may alternatively also
be distributed over the circumference.
Fig. 8A shows how the individual rings 9.1, 9.2 can
be fitted together. To prevent metal from penetrating into
the gap between two rings, in each case the upper ring 9.2
has an inner, downwardly directed skirt part 47 which covers
the separating gap 47. It is then possible for a seal 48,
e.g. made from impregnated ceramic fibers, to be provided in
CA 02434193 2003-07-09
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corresponding grooves in the two rings 9.1, 9.2 in the
parting gap 46 itself, which for this purpose faces in the
axial direction rather than transversely to it, and this seal
already contributes to securely holding the two rings
together. A similar arrangement may also be provided at the
outer side, with an upwardly directed skirt part 99 in each
case on the lower ring 9.1, and a seal 50. Of course, this
type of seal is only an example which can be modified as
desired within the scope of specialist knowledge in the field
of seals and insulations. An insulation which effects thermal
decoupling between the individual rings is advantageous. To
ensure that they are held together, each ring 9.1, 9.2 may
have securing lugs 51 (preferably - as shown in Fig. 9 - on
opposite sides of the rings) for a securing bolt 52 to be
fitted through. Since it will be advantageous to form the
outlet pipe or the rings from ceramic, it is expedient for
the securing lugs 51 to bear flat against one another in the
manner shown in Fig. 8A, in order to avoid bending moments,
in which case the plug connection with the seals 48, 50 is in
any case substantially free of tensile forces on account of
the screw connection 51, 52.
A few examples are intended to explain the core of
the invention further in the text which follows.
Example 1:
A first series of tests was intended to investigate
CA 02434193 2003-07-09
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how, for a given setting of a specific metering weight, the
inclination of the channel affects the temperature of a
suspension comprising the Mg alloy AZ 91 downstream of the
channel. A metering weight of 1260 g (constant pump capacity
of approx. 50 cm3/s and pumping duration of approx. 15 s) was
set and a suspending section of a similar design to Fig. 4
was used. Instead of the forming machine, a thick-walled
steel vessel (collection vessel) modeled on the shot sleeve
of a die-casting machine was used as transfer vessel 10, from
the center of which the sampling, which is yet to be
described, took place. Instead of the discharge plunger 26, a
cover was fitted onto the transfer vessel 10, through which
two thermocouples had been led in order to record the
suspension temperatures. The entire surface of the Mg was
covered with shielding gas. It was known from preliminary
tests that the transfer vessel 10 should be set to
approximately 585°C in order in practice to provide the
isothermal conditions desired. The suspension was removed
from the transfer vessel 10 35 s after the metering pump 17
had been switched on, which corresponds to a die-casting
machine cycle time which is quite sufficient for the metered
quantity. The values determined are given in Table 1.
CA 02434193 2003-07-09
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Table 1
Test Melt temp. Inclination Removal Max. temp.
No. [C] in the temperature diff. [K]
furnace (9) [C] after between
40 s center and
edge
1 630 15 585 4
2 632 15 586 4
3 630 10 584 3
4 631 10 584 3
630 10 583 3
The influence of the inclination of the suspending
section 9, which had been varied within certain limits, on
the temperature distribution in the suspension is therefore
slight. The fluctuations in the temperature gradient in the
transfer vessel 10 (right-hand column) are correspondingly
slight.
To investigate the expected microstructure following
a forming operation, a cylindrical specimen was punched out,
removed and quenched immediately after the collection vessel
had been filled in each test. The microsections which were
then produced did not reveal any dendrites at all. The mean
grain size of the predominantly globular microstructure was
0 dun .
CA 02434193 2003-07-09
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Example 2:
A second series of tests was intended to investigate
how the temperature conditions in the transfer vessel 10
alter when the dissipation of heat in the suspending section
takes place via different forms of flow paths. The intention
was to demonstrate that the dissipation of heat can be
increased in order not to excessively lengthen the cycle time
of a downstream die-casting machine despite the increasing
metering weight.
For this purpose, in each test the same metering
quantity of approximately 2200 g of the Mg alloy AZ 91 was
used. Firstly, in two tests the cooling took place on the
suspending section 9 as in Example 1 (15° inclination, same
coolant temperature) to the target temperature by increasing
the pumping time (from 15 s to 30 s; heat extraction 1);
secondly, in two further tests, the cooling took place on the
suspending section by increasing the dissipation of heat
while keeping the original pumping time of 15 s (heat
extraction 2). The increased dissipation of heat was
substantially achieved by widening the suspending section and
increasing the throughput of coolant. The results of the
total of two times two tests are compiled in Table 2:
CA 02434193 2003-07-09
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Table 2
Test Melt Type of heat Pumping Removal Max.
No. temp. extraction time [s] temperature temp.
[C] in (cf. text) [C] after diff. [K]
the 40 s between
furnace center
(9) and edge
6 635 1 30 589 4
7 635 1 30 590 4
8 634 2 15 588 4
9 634 2 15 589 4
The temperature deviations in the transfer vessel
given in the last column are identical. The reason far the
slight increase compared to Example 1 could be that the
transfer vessel, as before, had been held at 585°C. These
tests were also repeated with aluminum alloys, and a similar
pattern emerged.
The example illustrates that by selecting a suitable
suspending section 9 it is possible to substantially adapt to
the cycle time of a die-casting machine (or other forming
device) and to the required metering weights.
As in Example 1, specimens were produced, The
microsections which were then made once again did not reveal
any dendrites. The mean grain size of the predominantly
CA 02434193 2003-07-09
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globular microstructure was likewise 100 Vim.
Example 3:
The intention of this example was to investigate the
influence of the residence time in the transfer vessel and
the influence of the temperature in the transfer vessel on
the microstructure. The tests were carried out using an Mg
alloy which contained only 6~ of aluminum and correspondingly
had a reduced solidification interval compared to the alloy
used in Examples 1 and 2. The temperature of the transfer
vessel 10 was preset to 570°C and the alloy was cooled along
the suspending section as in tests 8 and 9. The results are
compiled in Table 3 below.
CA 02434193 2003-07-09
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Table 3
Test Melt Time [s] Removal Max. Temp.
No. temp, between temperature temp. diff. [K]
[C] in start of [C] diff. [K] between
the metering and between center
furnace removal from center and edge
(9) the transfer and edge at the
vessel time of
removal
635 80 580 8 3
11 635 100 576 8 2
12 634 140 575 7 1
13 634 180 573 7 1
After removal of the suspension, it was tested at two
points (center and edge). Although no dendrites were found,
it was clearly apparent that eutectic inclusions were
noticeable in particular in the edge zones. With increasing
residence time in the transfer vessel, a tendency to form
larger crystals was also found. Nevertheless, it can be
concluded from this example that it is desirable but not
particularly critical to maintain isothermal conditions in
the transfer vessel 10. Slight temperature losses in the
vessel do not cause any significant changes in the
microstructure. However, prolonged actions of a relatively
CA 02434193 2003-07-09
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major temperature gradient should be avoided, since this can
lead to significant constitutional supercooling.
In all tests (Mg and A1), the typical thixotropic
behavior was found, namely it was found that a certain
contiguity (skeleton formation between the globules) ensured
the dimensional stability of the alloy discharged from the
collection vessel, e.g. in the form of a metal billet, but it
was easy to deform the material under the action of shear
forces.
