Note: Descriptions are shown in the official language in which they were submitted.
Process for producing die-cast parts
The invention relates to a process for producing die-
cast parts made of an aluminum alloy.
Die-cast parts made of aluminum alloys are being used
ever more frequently, inter alia, in the automotive
industry for reasons of an increasing demand for weight
reduction. For casting technology reasons, it is
generally the case that a cast part wall thickness of
about 2 mm cannot be undershot, for example in the case
of nodes for space frame structures, with conventional
die-casting processes. The filling of the die-casting
mold with partially solid metal melts by using
thixocasting or rheocasting leads to better filling of
the mold and, as a result, to a possible further
reduction in the cast part wall thickness to about
1 mm. As the wall thickness decreases, however, the
reduced force-absorption capability increasingly
becomes a limiting factor. This disadvantage by itself
could be countered by the addition of nanoparticles to
an aluminum alloy matrix. However, there is a lack of
suitable processes for cost-effectively producing
aluminum alloys reinforced with nanoscale particles and
for the preparation thereof to form partially solid
metal melts for die casting.
The invention is based on the object of providing a
process of the type mentioned in the introduction, with
which process a partially solid aluminum alloy melt can
be provided continuously in a cost-effective manner and
further processed to form die-cast parts. It is a
further object of the invention to provide a process
for producing die-cast parts which are reinforced with
nanoparticles and are made of an aluminum alloy, with
which process a partially solid aluminum alloy melt can
be provided continuously in a cost-effective manner
under the action of shearing forces typical of the
2 -
process with a high fine dispersion of nanoparticles
and further processed to form die-cast parts.
The first object is achieved according to the invention
in that the aluminum alloy is exposed to high shearing
forces in a mixing and kneading machine, having a
housing with a working space, which is surrounded by an
inner housing sleeve, and a worm shaft, which rotates
about a longitudinal axis and moves to and fro
translationally in the longitudinal axis in the inner
housing sleeve and is provided with kneading blades,
and with kneading bolts, which are fastened to the
inner housing sleeve and protrude into the working
space, wherein liquid aluminum alloy is fed to the
working space at one end of the housing and, at the
other end of the housing, is removed from the working
space as partially solid aluminum alloy with a
predefined solids content, is transferred into a
filling chamber of a die-casting machine and is
introduced into a casting mold by means of a piston,
wherein the solids content of the aluminum alloy in the
working space is set to the predefined solids content
by cooling and heating the working space in a targeted
manner. Here, the high shearing forces present in the
kneading process in the partially solidified phase
state continuously comminute dendritic branches which
form, and this leads to an increased ductility of the
die-cast parts. The high compression forces
additionally lead to a greater transfer of heat, which
ultimately makes it possible to set the solids content
in the aluminum alloy more precisely.
The second object is achieved according to the
invention in that nanoparticles are mixed with the
aluminum alloy and finely dispersed in the aluminum
alloy by high shearing forces in a mixing and kneading
machine, having a housing with a working space, which
is surrounded by an inner housing sleeve, and a worm
3 -
shaft, which rotates about a longitudinal axis and
moves to and fro translationally in the longitudinal
axis in the inner housing sleeve and is provided with
kneading blades, and with kneading bolts, which are
fastened to the inner housing sleeve and protrude into
the working space, wherein liquid aluminum alloy and
nanoparticles are fed to the working space at one end
of the housing and, at the other end of the housing,
are removed from the working space as partially solid
aluminum alloy with a predefined solids content and
with nanoparticles finely dispersed in the aluminum
alloy, are transferred into a filling chamber of a die-
casting machine and are introduced into a casting mold
by means of a piston, wherein the solids content of the
aluminum alloy in the working space is set to the
predefined solids content by cooling and heating the
working space in a targeted manner. Here, in addition
to the comminution of dendritic branches which form and
the resultant higher ductility, the high shearing
forces present in the kneading process in the partially
solidified phase state finely disperse the
nanoparticles, which is required for the strength-
increasing effect thereof.
It is expedient that the inner housing sleeve is
surrounded by an outer housing sleeve such that an
intermediate space preferably in the form of a hollow
cylinder is formed, and cold and/or hot gases are
conducted through the intermediate space for cooling
and heating the working space. Air, preferably
compressed air, is preferably conducted through the
intermediate space for cooling, and hot gases,
preferably combustion gases, are preferably conducted
through the intermediate space for heating.
The gases are preferably conducted through the
intermediate space in countercurrent to the direction
in which the aluminum alloy is transported.
- 4 -
The solids content of the aluminum alloy is preferably
set to 40 to 80%, in particular to more than 50%.
In a preferred embodiment of the process according to
the invention, the partially solid aluminum alloy is
removed from the working space as a partially solid
metal strand. The continuously emerging, partially
solid metal strand is split into partially solid metal
portions and the partially solid metal portions are
transferred into the filling chamber of the die-casting
machine.
The content of the nanoparticles in the alloy is
preferably between about 0.1 and 10% by weight.
Suitable, cost-effective nanoparticles consist
preferably of fumed silica, such as e.g. Aerosil .
However, it is also possible to use other
nanoparticles, such as e.g. the known carbon nanotubes
(CNT) and also further, nanoscale particles which are
produced, for example, by the known Aerosil process
and are made of metal and semimetal oxides, such as
e.g. aluminum oxide (A1203), titanium dioxide (TiO2),
zirconium oxide (Zr02) , antimony(III) oxide,
chromium(III) oxide, iron(III) oxide, germanium(IV)
oxide, vanadium(V) oxide or tungsten(VI) oxide.
Further advantages, features and details of the
invention will become apparent from the following
description of preferred exemplary embodiments and with
reference to the drawing, which serves merely for
elucidation and is not to be interpreted as having a
limiting effect. Schematically, in the drawing,
figure 1 shows a longitudinal section through a die-
casting machine with an upstream mixing and
kneading machine;
-
figure 2 shows a longitudinal section through part of
a mixing and kneading machine;
figure 3 shows a cross section through the mixing and
5 kneading machine shown in figure 1;
figure 4 shows characteristic shearing and stretching
flow fields in a product mass, triggered by a
kneading blade moving past a kneading bolt;
figure 5 shows the continuous production of partially
solid starting material for die casting with
an arrangement according to figure 1.
A plant, shown in figure 1, for die casting die-cast
parts which are optionally reinforced with
nanoparticles and are made of an aluminum alloy has a
die-casting machine 10 and a mixing and kneading
machine 30 upstream of the die-casting machine 10.
The die-casting machine 10, which is shown only in part
in the drawing, is a commercially available machine for
conventionally die casting aluminum alloys and has,
inter alia, a filling chamber 12, which is connected to
a stationary side 18 of a casting mold, with an opening
16 for receiving the metal which is to be ejected from
the filling chamber 12 and introduced into a mold
cavity 14 of the casting mold by means of a piston 20.
The mixing and kneading machine 30 is shown in detail
in figures 2 and 3. The basic design of such a mixing
and kneading machine is known, for example, from CH-A-
278 575. The mixing and kneading machine 30 has a
housing 31 with a working space 34, which is surrounded
by an inner housing sleeve 32 and in which there is
arranged a worm shaft 36, which rotates about a
longitudinal axis x and moves to and fro
translationally in the longitudinal axis x in the inner
6 -
housing sleeve 32. The worm shaft 36 is interrupted in
the circumferential direction such that individual
kneading blades 38 are formed. Axial through openings
40 are thereby formed between the individual kneading
blades 38. Kneading bolts 42 protrude from the inner
side of the inner housing sleeve 32 into the working
space 34. The kneading bolts 42 on the housing side
engage into the axial through openings 40 of the
kneading blades 38 arranged on the main or worm shaft
36. A drive shaft 44 arranged concentrically to the
worm shaft 36 is guided out of the inner housing sleeve
32 at the end and is connected to a drive unit (not
shown in the drawing) for executing a rotational
movement of the worm shaft 36. A device interacting
with the worm shaft 36 for executing the translational
movement of the worm shaft 36 is likewise not shown in
the drawing.
The cylindrical inner housing sleeve 32 of the mixing
and kneading machine 30, which delimits the working
space 34, is surrounded by a cylindrical outer housing
sleeve 46. The inner housing sleeve 32 and the outer
housing sleeve 46 form a dual sleeve and thereby
enclose an intermediate space 48 in the form of a
hollow cylinder.
An introduction opening 50 for feeding liquid aluminum
alloy and optionally nanoparticles into the working
space 34 is provided at that end of the housing 31
which is close to the drive side of the worm shaft 36.
Although only one introduction opening 50 is shown in
the drawing, two separate introduction openings for the
aluminum alloy and for the nanoparticles can be
provided. In principle, it is also possible to admix
the nanoparticles with the liquid aluminum alloy even
before the metal is introduced into the kneading and
mixing machine 30. An outlet opening 52 for removing
partially solid aluminum alloy optionally with
- 7 -
nanoparticles dispersed therein is provided at that end
of the inner housing sleeve 32 which is remote from the
drive side of the worm shaft 36.
Inlet openings 54, 56 for introducing cold or hot gases
into the intermediate space 48 are provided in the
outer housing sleeve 46 at that end of the housing 31
which is remote from the drive side of the worm shaft
36. Correspondingly, outlet openings 58, 60 for the
discharge of the gases from the intermediate space 48
are provided at that end of the housing 31 which is
close to the drive side of the worm shaft 36. In order
to ensure a maximum throughflow of gas, which is
distributed uniformly over the circumference of the
inner housing sleeve 32, from the inlet openings 54, 56
to the outlet openings 58, 60, and thus a uniform
discharge of heat from the working space 34 or a
uniform introduction of heat into the working space 34,
the inlet and outlet openings 54, 56 and 58, 60,
respectively, are according to figure 3 arranged
distributed uniformly about the circumference of the
outer housing sleeve 46.
Figure 4 shows, in a schematic illustration,
characteristic shearing and stretching flow fields in a
product mass P, as triggered by a kneading blade 38
moving past a kneading bolt 42 in the case of a mixing
and kneading machine 30 formed according to the prior
art. The direction in which the kneading blade 38
rotates is indicated schematically by a curved arrow A,
whereas the translational movement of the kneading
blade 38 is indicated by a double-headed arrow B. The
rotational movement of the kneading blade 38 means that
its tip splits the product mass P, as indicated by
arrows C, D. There is a gap 41, the width of which
varies depending on the rotation and translational
movement of the worm shaft 36, between the kneading
bolt 42 and the main face 39 of the kneading blade 38,
8 -
which faces toward the kneading bolt 42, and the
kneading blade 38 moving past the latter. A shearing
process is brought about in the product mass P in this
gap 41, as indicated by arrow E. The product mass P
expands and reorientates itself both upstream and
downstream of the kneading bolt 42, as indicated by
rotation arrows F, G. As already mentioned in the
introduction, there is a maximum convergence of the
kneading blade 38 and the kneading bolt 42 and thus a
maximum shearing rate in the product mass P per
shearing cycle owing to the sinusoidal axial movement
of the respective kneading blade 38 on a line.
In the text which follows, the mode of operation of the
plant for die casting die-cast parts which are
optionally reinforced with nanoparticles and are made
of an aluminum alloy is explained in more detail, by
way of example, with reference to figures 1 and 2.
An aluminum alloy melt kept just above the liquidus
temperature of the alloy is fed to the working space 34
in metered form alone or together with nanoparticles
via the introduction opening 50. The pinching of the
partially solidified aluminum alloy with nanoparticles
between the kneading blades 38 and the kneading bolts
42 results in the application of high shearing forces,
which both lead to the comminution of dendritic
branches and finely disperse the nanoparticles present
in the form of agglomerates. Efficient, homogenizing
mixing results from the combination of a radial and
longitudinal mixing effect. By controlling the flow of
cold and hot gases through the intermediate space 48
between the inner housing sleeve 32 and the outer
housing sleeve 46, the solids content of the aluminum
alloy in the working space 34 is set such that it is in
the desired range when the metal is removed through the
outlet opening 52.
9 -
The desired solids content of the aluminum alloy is set
by measuring the change in viscosity of the metal melt
in the kneading and mixing machine 30. The viscosity,
which rises as the solids content of the partially
solid aluminum alloy increases, can be determined, for
example, by measuring the rotational resistance at the
drive shaft 44 of the worm shaft 36. By determining the
rotational resistance for defined solids contents, it
is possible to specify appropriate setpoint values, to
which measured actual values are regulated by
controlling the flow of cold and hot gases through the
intermediate space 48 between the inner housing sleeve
32 and the outer housing sleeve 46.
The aluminum alloy having the desired solids content
and optionally comprising finely dispersed
nanoparticles is introduced via the introduction
opening 16 into the filling chamber 12 of the die-
casting machine 10, and is injected intermittently from
the latter into the mold cavity 14 of the casting mold
from the filling chamber 12 in a known manner by means
of the piston 20.
With reference to figure 5, the text which follows
provides a more detailed explanation, by way of
example, of the continuous production of partially
solid, bar-shaped starting material for die casting
die-cast parts which are optionally reinforced with
nanoparticles and are made of an aluminum alloy. The
mode of operation explained above with reference to
figures 1 and 2 is retained.
The aluminum alloy having the desired solids content
and optionally comprising finely dispersed
nanoparticles is continuously ejected via the outlet
opening 52 in the form of a partially solid metal
strand 70. Partially solid metal portions 72 are cut to
-
length from the partially solid metal strand 70, for
example using a rotating blade. The partially solid
metal portions 72 usually each correspond to the
quantity of metal required for producing an individual
5 die-cast part and, for each shot, are transferred
individually into the filling chamber 12 of the die-
casting machine 10 and injected intermittently from the
latter into the mold cavity 14 of the casting mold from
the filling chamber 12 in a known manner by means of
10 the piston 20.
The partially solid metal strand 70 usually leaves the
mixing and kneading machine 30 in the direction of the
longitudinal axis x of the worm shaft 36 in a
horizontal direction, although another, e.g. vertical,
outlet direction is also conceivable. The cross section
of the metal strand 70 is determined by the cross
section of the outlet opening 52, and is usually
circular. The partially solid metal portions 72 can be
grasped by tongs, for example, and transferred into the
filling chamber 12 of the die-casting machine 10.
