Note: Descriptions are shown in the official language in which they were submitted.
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Process for Transforming a Metal Alloy into a Partially-SolidIPartially-
Liquid Shaped Body
The invention relates to a process for transforming a metal alloy having a
liquidus temperature and a solidus temperature into a partially/solid/par-
tially/liquid shaped body, in which process the metal alloy is poured in the
liquid
state at a pouring temperature into a mould having an essentially cylindrical
mould wall, whereby the mould at the start of the filling stage exhibits an
initial
temperature that lies below the liquidus temperature and the metal alloy is
kept
in the mould until it has cooled to a discharging temperature lying between
the
liquidus temperature and the solidus temperature corresponding to the desired
solid / liquid ratio in the shaped body, and the shaped body is removed from
the
mould at the discharging temperature.
Part-solid/part-liquid shaped bodies with thixotropic properties can be made
from metal alloys. Because of the thixotropic properties, the shaped bodies
can
be processed further e.g. on a die-casting machine.
In a first, known process, so-called thixocasting, a metal alloy is cast to
billet
form by means of continuous casting. In order to achieve the fine-grained
structure necessary for the thixotropic properties, the molten metal is
stirred
vigorously in the solidification range i.e. between the liquidus and solidus
temperature of the metal alloy, whereby in particular electromagnetic stirring
devices have proved to be effective for that purpose. The stirring action
causes
the dendrites that are forming to be sheared or retarded in such a manner that
these primary solidifying solid particles take on an essentially globulitic
form.
The solidified billet is divided into shaped bodies which, after heating to a
temperature between the solidus and liquidus temperature of the alloy, exhibit
thixotropic properties. In the thixotropic state, reached after heating the
shaped
body in this manner, the metal alloy contains the retarded dendritic, primary
solidified and essentially globulitic particles in a surrounding matrix of
liquid
metal.
- 2341 -
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In another known process - so called rheocasting - a metal alloy melt is
produced in a continuous manner with a solid fraction corresponding to the
solid/liquid ratio desired in the shaped body. As in the first mentioned
process,
the metal melt is stirred vigorously in the temperature range between the
alloy
solidus and liquidus in order to create the fine grained structure required
for the
thixotropic properties. Compared with thixocasting, rheocasting offers a
signif-
icant advantage in terms of energy and therefore costs; however, rheocasting
units require complicated and difficult processes in order to ensure co-
ordination with a subsequent casting machine to manufacture the final product.
In the case of a process known from EF-A-0 745 694 a metal alloy containing
nucleating crystals is cast in a thermally insulated mould. As soon as the
desired soiid/liquid ratio has been established in the metal alloy, after
approp-
riate cooling, the resultant part-solid/part-liquid shaped body is advanced
for
further processing.
In a process published in WO-A-01/07672 the metal alloy is cast in a mould,
cooled to the discharging temperature between the liquidus and solidus temp-
eratures corresponding to the desired solid/liquid ratio, and held for a
specific
time at the discharging temperature in order to form the structure in the
resultant shaped body. Thereby, the mould parameters are selected specifically
for the metal alloy such that the change in enthalpy of the mould on heating
from the starting temperature to a final temperature is the same as the change
in enthalpy of the metal alloy on cooling from the temperature of the melt
when
poured into the mould to the discharging temperature at which the desired
solid/fiquid ratio is established in the metal alloy. The final temperature
for the
mould corresponds to the discharging temperature i.e. the metal alloy and the
mould both reach their thermal equilibrium at the discharging temperature. In
order to accelerate the reaching of thermal equilibrium, the mould may e.g. be
subjected to eccentric rotation during the cooling of the metal alloy.
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The disadvantage of both of the above mentioned processes lies in the
relatively long process time i.e. the duration of time from pouring the metal
alloy
into the form or moving the molten metal until removal of the shaped body from
the mould. For efficient production of shaped bodies it is therefore necessary
to
provide several stations for manufacturing shaped bodies.
The object of the present invention is to provide a process of the kind
described
at the start which, using simple means, permits the cooling conditions to be
reached in an optimal manner with the result that a shaped body can be
produced in the shortest possible time and without forming a peripheral zone.
That objective is achieved by way of the invention in that, in order to set a
desired cooling rate of the metal alloy in the mould, the thickness of the
mould
wall, the material and the initial temperature of the mould are selected such
that
the change in enthalpy of the metal alloy during cooling from the pouring
temperature to the discharging temperature is smaller than the change in
enthalpy for an increase in temperature of the mould from the initial
temperature
to its final temperature.
As a result of the thermal non-equilibrium between the mould and the metal
alloy that prevails at the discharging temperature, setting the parameters
that
influence the cooling rate of the mould in a manner according to the invention
leads to an optimal and, in comparison to the state-of-the-art, short duration
of
the process. By duration of the process is to be understood, here and in the
following, the time of cooling the metal alloy from the pouring temperature
or, if
the molten melt is moved i.e. agitated from the temperature at the start of
agitation, to the discharging temperature. The discharging temperature is the
temperature of the mould at the point in time when the shaped body is removed
from the mould.
In a first preferred version of the process according to the invention the
temperature over a period of time is employed to determine the time for
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discharging. The discharging takes place on reaching a given target value of
temperature profile in the metal alloy and a given target value of discharging
temperature. The time required to reach the discharging temperature in order
to
obtain a good, homogeneous shaped body depends on the composition of the
alloy.
In a second preferred version of the process according to the invention the
temperature change as a function of time at a fixed point in the mould wall is
chosen to determine the time for discharging. The discharging takes place on
reaching a given target value of temperature gradient and a given target value
of discharging temperature.
The initial temperature of the mould lies preferably between room temperature
and approximately 320°C.
The shaped body is normally removed from the mould immediately on reaching
the discharging temperature and advanced for further processing. For the case
in which the discharging can not be carried out immediately, e.g. if the
production unit is defective, it is possible using the process according to
the
invention to maintain the shaped body at the discharging temperature by
heating the mould until the problem has been solved.
The duration of the process can be optimised further in that the metal alloy
is
agitated and the agitation is maintained until the metal alloy has cooled to
the
discharging temperature. The agitation of the metal alloy may be performed in
principle using all known means e.g. by electromagnetic stirring or by move-
ment of the mould. The purpose of moving the mould is to produce flow
behaviour in the molten melt or later in the partially solid / partially
liquid melt.
The primary aim is to achieve good mixing without causing vortices or
currents.
In the interest of the process, the agitation should be such that it starts as
soon
as possible after pouring metal into the mould as the viscosity of the
cooling,
partly solidified metal alloy increases continuously, and effective agitation
is
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increasingly difficult to achieve. The movement of the mould is set such that
at
the start the metal alloy does not splash out of the mould; this can be
achieved
e.g. by agitating the melt with low intensity in the initial phase and
increasing the
intensity with increasing viscosity.
5
An optimum process duration is achieved when immediately after completed
filling of the mould the mould is made to rotate in an eccentric manner and
the
rotational movement is maintained until the metal alloy has cooled to the
discharging temperature. An eccentric movement of the mould means that the
mould axis is in a distance from the axis of rotation and rotates about that,
whereby the mould itself does not rotate about its own axis.
The rotational movement is preferably started when the starting temperature of
the metal alloy lies in the area of the liquidus temperatures or slightly
above the
liquidus temperatures, whereby the starting temperature of the metal alloy
preferably lies 5 to 15°C above the liquidus temperature.
The speed of rotation normally lies approx. between 50 to 500 rpm and is
preferably increased with progressive cooling of the metal alloy because of
the
rising viscosity.
The rotational movement which is applied over the whole duration of the
process is preferably divided into two, preferably three cycles, whereby the
maximum speed of rotation in each cycle is greater than the speed of rotation
in
the previous cycle. Usefully, the start up procedure for each rotation cycle
is
such that the maximum speed of rotation is reached after 10 to 20 seconds.
Particularly good mixing of the metal alloy with optimum removal of heat
through the mould wall is achieved by the combination of each rotation cycle
with a shaking cycle, whereby the shaking cycle follows on from the rotation
cycle or overlaps the rotational movement.
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In a preferred version of the process according to the invention the first two
rotation cycles are carried out within an overall time of 30 to 50 sec.
The shaking cycle comprises a shaking movement with a frequency of 2 to 3 Hz
over a maximum interval of 10 sec, preferably 1 to 6 sec.
The rotational movement is preferably started when the starting temperature of
the metal alloy lies 5 to 15°C above the liquidus temperature.
In particular aluminium alloys can be processed using the process according to
the invention.
Hereby the alloy may exhibit a eutectic solidus temperature with a significant
volume fraction of eutectic melt. Such alloys belong e.g. to the AI-Si system
with
2.5 - 10 wt.% Si or to the quasi-binary AI-Mg2Si system with 4 wt.% Si and 2 -
6 wt.% Mg.
Other alloys which can be processed are, however, alloys which do not exhibit
a
eutectic melting point, e.g. an alloy of the AIMg3Mn type.
In order to obtain uniform withdrawal of heat in an essentially radial
direction,
the thickness of the mould wall in the region of the head and base of the
mould
can be reduced compared with the thickness of the wall between the head and
the base. Another possibility is to insulate the base and the head of the
mould
with respect to its surroundings.
In order to facilitate the removal of the shaped body from the mould, the base
of
the mould may be designed such that it is hinged. The shaped body can then
be removed
In order to make it easier to remove the shaped body from the mould, the mould
wall may be sonically broadened from the base of the mould to the top. In the
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case of a mould with a hinged i.e. openable base, the conical broadening of
the
mould wall may also run from top to bottom.
In a preferred version of the mould, this is divided in the longitudinal
direction
and the shaped body removed from the mould after separating the two parts of
the mould. This makes it possible to open the mould and to remove the shaped
body e.g. using a robot arm in such a manner that the shaped body can be
introduced horizontal into the chamber of a diecasting machine. Such a manner
of handling is important in order to reduce the amount of deformation of the
shaped body due to its own weight ( so called "elephant foot").
The time-dependent non-stationary temperature field in the mould wall may be
employed to supervise and regulate heat extraction for determining the optimal
discharging temperature and with that the optimal duration of the process from
the starting temperature to the discharging temperature.
By employing the functional relationship for a minimum process time tP~
tPr = f(~HM, T,ann~ TEM, d~~ Tw(t,d1), TAF, Fo) ~ min.
OHM Change in enthalpy of the metal rnelt between TAM and TEM
TAM Temperature of the melt at the start of the rotational movement (starting
temperature)
TEM Temperature for discharging the shaped body from the mould
d1 Thickness of the mould wall
TW Temperature of a mould wall element during the process
TAF Initial temperature in the mould wall (pre-heat temperature)
Fo Fourier coefficient
it is possible to simulate and regulate the whole process on the basis of
Fourier
coefficients for the conduction of heat in the mould wall.
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Further advantages, features and details of the invention are revealed in the
following description of preferred exemplified embodiments of the invention
and
with the aid of the drawing which shows schematically in:
Fig. 1 a longitudinal section through a part of a mould;
Fig. 2 a side elevation of a divided mould;
Fig. 3 plan view of an arrangement with a cylindrical mould subjected to
eccentric rotation.
A mould 10 shown in fig. 1 made e.g. of steel comprises a cylindrical mould
wall 12 with an axis of symmetry z~. The mould 10 is closed on one side by a
base 14. The head 16 which is open at the top is covered over by a lid 18 made
of thermally insulating material. The base sits in a lower part 20 made of
thermally insulating material. The liquid/solid metal mixture 28 is situated
in the
interior of the mould 10.
The cylindrical mould wall 12 is thicker between the head 16 and the base 14.
For example the thickness d, of the wall 12 in the thicker part of the mould
is
5 mm, while the thickness d2 of the base 14 and the head 16 is 3 mm.
With the arrangement shown in figure 1 the extraction of heat from the metal
melt into the mould wall is more uniform and takes place essentially in a
radial
direction.
The mould 10 shown in figure 2 is divided along its axis i.e. in the
longitudinal
direction. Both parts l0a,b of the mould can be separated for removal of the
shaped body from the mould.
The influence of eccentric rotation on the movement of the molten melt in the
mould 10 is clear from figure 3. The mould 10 is e.g. mounted on a plate. The
axis of symmetry z~ of the mould 10 is a distance a from an axis of rotation
z2.
The axis z1 of the mould 10 rotates around the axis z2, whereby, however, the
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mould itself does not rotate around its own axis. Shown in figure 3 are the
circular paths of a point on the mould wall 12 and a point in the centre of
the
mould 10. For a radius r in the cylindrical inner wall of the mould and a
value a
of eccentricity, which corresponds to the radius of the circular path, the
inner
wall of the mould rolls on a circular path with a radius R = r + a ab. The
described eccentric rotation of the mould 10, together with a resultant
rotational
movement of the melt, leads to thorough homogeneous mixing of the melt. This
thorough mixing concerns both the alloying elements and the temperature.
Examples
The advantage of the process according to the invention is illustrated in the
following by way of processing four different alloys 1 to 4 into shaped
bodies.
The chemical compositions of the AI 99.85 based alloys used for the trials are
summarised in table 1.
Table 1
AlloySi [wt %] Mg (wt %] Mn [wt %] Fe [wt Ti [wt
%] %]
1 7.0 0.3 0.1 0.06
2 2.2 5.2 0.6 0.1 0.08
3 0.1 3.0 0.1 0.08
4 4.5 0.3 0.1 0.06
The alloys were poured in the molten state into a steel cylindrical mould, the
inner wall of which had been coated with a slurry to prevent sticking. The
inner
diameter of the mould was 100 mm and the wall thickness d1 lay between 2 and
7 mm; the mould was filled to a depth of 260 mm. In some of the trials the
mould was pre-heated before receiving the molten metal. As soon as the metal
melt, which was cooling from the pouring temperature, reached the starting
temperature, the mould was set into an eccentric rotational movement and
maintained so until the discharging temperature was reached.
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The eccentric rotational movement was investigated under the following condit-
ions for a degree of eccentricity of 6.5 mm:
A 15 sec at 140 rpm + 15 sec at 200 rpm + 250 rpm until discharging
B constant at 140 rpm until discharging
On removal of the shaped body from the mould quality assessment is carried
out via simple mechanical testing of a prematurely solidified edge shell, the
appearance of which can lead to a reduction in quality of the final product
resulting from further processing of the shaped body. The process parameters
and the results achieved with these are summarised in table 2.
Table 2
T~ TS d, TMo TAM TAF Ro- tP~ TeM Quality
of the
Alloy
[C][C] [mm] [C] [C] [C] tat.[s] [C] shaped body
~
1 610566 2 640 615 RT A 420 585 very good
1 610566 5 640 615 RT A 102 - 578 - very good
~ 115 583
1 610566 7 640 615 RT A 52 586 edge shell
1 610566 7 640 615 50 A 57 595 edge shell
1 610566 7 640 615 150 A 70 590 very good
~
1 610566 7 640 615 200 A 85 590 very good
1
i
1 610566 7 640 615 300 A 140 590 very good
2 620594 5 660 635 RT B 50 - 600 edge shell
55
..2 620594 5 660 635 200 B 85 600 edge shell
1
2 620594 5 660 635 300 B 180 600 very good
3 640600 5 680 655 RT B 35 - 633 very good
I 40
3 640600 2 645 645 RT A 300 634 very good
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T~ TS d, TMo TaMTaF Ro- tP~ TEM Quality
of the
Alloy
[C] [C] [mm] [C] [C][C] tat.[s] [C] shaped body
4 630 566 5 635 ~3~~ B 58 - 607 - very good
_~ 70 ~ 610
~
T~ Liquidus temperature of the alloy
TS Solidus temperature of the alloy
dy Wall thickness of the mould
TMo Temperature of the molten metal poured into the mould
TAM Starting temperature at the start of the eccentric movement of the mould
TaF Initial temperature of the mould (pre-heat temperature)
tp~ Duration of the process (time from the start of the eccentric rotational
movement until discharging from the mould)
TEM Discharging temperature
