Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR RECYCLING LIGHTWEIGHT METAL PARTS
The invention relates to a process for recycling light metal parts with gas
inclusions
or non- metallic particles.
The invention further has as part of its objects the use of die casting scrap
material
as well as a use of chips of passivating metals.
An efficient utilisation of resources is propagated nowadays not only from the
point
of view of environment protection but is also increasingly offered from the
operational-economic point of view. High raw stock prices and disposal costs
compel product manufacturers in various fields to reuse or recycle waste
created
within the framework of the production. For example, manufacturers of light
metal
components find themselves confronted with the requirement of purposefully
utilising large quantities of waste from casting processes and chip-forming
shaping
processes.
In an optimum manner, a raw stock-saving and energy-saving utilisation of
wastes
yields high-value products in a few process steps. This obviously holds good
also
for the preparation and re-utilisation or recycling of light metal wastes.
Depending
on the origin of the waste, light metal wastes can have gas inclusions and/or
non-
metallic particles, which makes it difficult or even impossible to integrate
such
wastes into a production process without complexity.
Light metal parts or wastes having gas inclusions that occur in large
quantities, for
example, during casting process, particularly during die casting, are not
suitable for
direct production of high-class light metal components on account of the
mentioned
gas inclusions. Therefore, in order to enable re-use even for light metal
components
of high class, such light metal parts are melted and the melt is degassed
under
overheating, so that after solidification of the melt one obtains a dense
light metal
that can be subsequently used e. g. in a casting process.
Just as in the case of light metal parts having gas inclusions, also in case
of light
metal parts having non-metallic particles like magnesium chips, a cumbersome
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cleaning/purification of the material is necessary before the light metal
material can
be used for producing high-class components. Here one proceeds in such a way
that the impure material is melted, the disturbing particles are removed from
the
melt and the melt is subsequently allowed to solidify. At a high degree of
purity, a
preliminary material thus obtained can be subsequently used for production of
light
metal parts of high quality.
Although it is possible to produce high-class preliminary materials for light
metal
parts from light metal parts with gas inclusions or non-metallic impurities,
complicated cleaning/purification operations are necessary for this.
Based on these factors, it is the problem of this invention to present a
process for
preparing light metal parts with gas inclusions or non-metallic particles,
whereby
such waste products can be transformed in a simple process-technical manner
into
high class products, without any requirement of cleaning/purifying the
material
used.
This problem is solved by a process of the type mentioned above, characterized
therein that from at least one light metal part with gas inclusions and at
least one
rather dense light metal part with non-metallic particles, a gas-containing
metal melt
is produced and the metal melt is allowed to solidify at vacuum for at least
sometime
under formation of a light metal foam body.
The advantages targeted by the invention can mainly be seen therein, that
impure or
contaminated light metal parts can be directly transformed into light-weight
metal
foam bodies in a simple process, without any necessity of cleaning/purifying
the light
metal parts used. Advantageously, in the process according to the invention,
light
metal parts with gas inclusions as well as rather dense, non-metallic
particles are
used, so that different contaminated light metal parts or types of light metal
wastes
can be re-used or recycled at the same time. The obtained metal foam bodies
are
suitable as energy-absorbing and sound-absorbing components for thermal
insulation or as reinforcing elements in the automobile industry. Thus low
class
waste products can be transformed directly into high class metal foam bodies
with
multiple uses according to the invention.
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With regard to the mechanism of metal foam formation, it is assumed that gas
inclusions or non-metallic particles introduced into the metal melt by the
respective
light metal parts act together: through the gas inclusions of a first light
metal part,
the gas required for metal foam formation is brought in, whereby during
melting of
the light metal part the gas inclusions are retained in their form; similarly,
the non-
metallic particles present in the metal melt deposit themselves on the surface
of the
gas inclusions or gas bubbles due to energetic reasons, whereby these get
stabilised and a coalescence of the same is prevented. Moreover, the non-
metallic
particles increase the viscosity of the metal melt and thus reduce a mobility
of the
gas inclusions or gas bubbles in the metal melt. This also reduces the
tendency of
the gas inclusions to rise up to the surface of the melt and exit there.
Applying
vacuum ultimately causes foaming of the gas-containing metal melt under
formation
of the light metal foam body.
As far as a comparison to traditional processes for producing metal foam
bodies
can be drawn, the special feature of the process according to the invention is
that,
neither are special measures required to introduce a gas nor is it necessary
to carry
out a complicated separate production of expanding agent components and light
metal powder components, as is the case in melt-metallurgical or powder-
metallurgical processes.
Basically, in a process according to the invention, light metal parts from
different
light metals can be used. However, it is favourable if light metal parts from
the same
metal or the same alloy are used. These light metals then melt within a small
temperature interval, which makes conducting and control of the process
simpler.
Furthermore, it is advantageous if a die casting scrap is used as light metal
part with
gas inclusions. Die casting scrap, for example in the form of so-called
overflows that
occur during die casting process, can have a share of gas inclusions of more
than
approx. 10vol. %, which has proved to be useful with respect to high
introduction of
gas in the metal melt.
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It is particularly advantageous if the die casting scrap part consists of
magnesium or
magnesium alloy. Parts of these materials are rendered passive on the surface,
which leads to formation of magnesium particles or magnesium films. If such
parts
are subsequently used in a process according to the invention, then
stabilising
magnesium oxide particles can be additionally fed. Besides, even gas
inclusions in
the interior portions are rendered passive at the same time; this contributes
positively to the stability of gas inclusions or gas bubbles in the melt.
Similarly, a part made of magnesium or a magnesium alloy can be used as dense
light metal part, so that stabilisation of gas bubbles in the metal melt is
mainly
caused by magnesium oxide particles.
It is advantageous if the non-metallic particles are essentially oxide
particles, as
these particles behave uniformly inert with respect to reactions with light
metal
melts. Contrary to carbides, for example, in case of presence of such
particles in a
material used, it can be assumed that these particles are present almost
unchanged
in the formed metal foam body. Thereby even properties of the formed light
metal
foam body can be reliably influenced. Thus, on this basis, the content of non-
metallic particles in the produced metal foam body can be controlled. If a
particle
content in the used material is known, then one merely has to select a weight
ratio
of light metal parts with gas inclusions to light metal parts with oxide
particles
according to the desired particle content in the metal foam body.
In the context of non-metallic particles it has been seen that it is
advantageous if
these have an average size of lesser than 200 pm.
Ideally, a metal melt is produced with a volume share of non-metallic
particles of 2 to
10%. Above a particle content of 2% a good stabilising of gas inclusions/gas
bubbles in the melt is attained; up to a particle content of 10% the gas-
containing
metal melts can easily be foamed up by applying vacuum: besides, increasing
viscosity would make foaming of the metal melts difficult.
The metal melt is foamed and transformed into a light metal foam body by
allowing it
to solidify at an vacuum of 10 to 400 mbar, particularly 50 to
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200 mbar.
It is also possible to additionally feed gas into the metal melt over the melt
surface
by application of gas pressure, in order to support subsequent foaming.
However,
process-technically and apparatus-wise it is particularly simple if the metal
melt is
produced at atmospheric pressure. In this case, during the entire process the
atmospheric pressure is not exceeded.
After creation, the metal melt should not be overheated by more than 20 C. The
viscosity of metal melt decreases with increasing temperature, which basically
favours a mobility of gas bubbles and hence degassing. It would therefore be
ideal
to keep overheating of the metal melt controlled and low.
It is preferable if the vacuum a applied on reaching a temperature in the
range of
5 C above to 5 C below the solidification temperature or the solidification
interval of
the metal melt. In this temperature range the fluid phase of the metal has a
high
viscosity, which proves to be favourable with respect to the structural
stability of the
formed metal foam.
With respect to the structural stability of the formed metal foam, it is
similarly
preferred if the vacuum that causes foaming of the metal melt is maintained
till
complete solidification of the metal melt. At the same time, the metal melt
can be
cooled during solidification, in order to discharge solidification heat that
is released
and at the same time to freeze an inner structure of the formed light metal
foam
body.
In order to design a light metal foam body with a particular shape, the metal
melt
can be transferred to a container giving shape to the light metal foam body
before
applying vacuum.
In a further embodiment of the invention, during formation of the light metal
foam
body, parts of the same or metal foam can be brought in contact with a metal
body.
In that case, the formed metal foam is similarly bonded in a process step with
a
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metal body by a metal bond. This allows an extremely simple production of
compound parts.
In another embodiment of the invention the metal melt is allowed to solidify
in an
essentially closed container that limits spatial expansion of the light metal
foam body
that gets formed. In this case, on the one hand, the volume of the ready light
metal
foam body is pre-given; on the other hand, even the mass introduced into the
container can be selected or is fixed and consequently the density is also
fixed. In
other words, the invention allows specific adjustment of the density of the
produced
light metal foam body.
The process according to the invention can be used for producing several
identic
light metal foam bodies, if the metal melt is introduced into the container
portion-
wise and allowed to solidify there.
Using die casting scrap for producing metal foam bodies has proved to be
extremely
useful. Chips of passivating metals can be considered as second light metal
component. Such chips that occur in large quantities as waste product in lathe
machining of aluminium or magnesium work pieces are particularly advantageous
to
the extent that, on account of a surface having oxide particles rendered
passive,
they are best suited for introducing stabilising particles. Besides, for low
volumes
these chips have a large surface, so that already a small quantity of such
chips is
sufficient to enable foaming of a metal melt from die casting scrap and
formation of
a stable metal foam.
The invention is further described below on the basis of examples. The
following
are shown:
Fig. 1: A cut up magnesium foam body;
Fig.2: A micrograph of the magnesium foam body from fig. 1 in approx.
25-times enlargement;
Fig.3: A micrograph of the magnesium foam body from fig. 1 in approx.
90-times enlargement;
Fig. 4: Stress and compression diagrams of metal foam bodies with a density of
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a) 0.56 gm/cm3, b) 0.41 gm/cm3, c) 0.36 gm/cm3 and d) 0.23 gm/cm3;
Fig.5a: Metal foam body of alloy AZ 91, where vacuum was applied
after reaching 580 C;
Fig.5b: Metal foam body of alloy AZ 91, where vacuum was applied after
reaching
600 C;
Fig.5c: Metal foam body of alloy AZ 91, where vacuum was applied after
reaching
620 C;
Fig.6a: Magnesium foam body that was produced in an open container;
Fig.6b: Magnesium foam body that was produced in a closed container;
Fig.7: A device for foaming hollow bodies;
Fig.8: A composite part consisting of an aluminium tube with a magnesium foam
core.
In a recycling process according to invention, die casting scrap parts made of
magnesium alloys AZ 91 and AM 50 were used for producing light metal foam
bodies. The die casting scrap parts revealed pore inclusions with a volume
share of
respectively approx. 20%. The chemical composition of the used scrap parts is
given in table 1.
Table 1
Chemical composition of alloys AZ 91 and AM 50
Alloy Al [%] Be [%] Cu [%] Fe [%] Mn [%] Ni [%] Si [%] Zn[%]
AZ91 9,2 <0,001 <0,005 0,007 0,19 <0,001 0,03 0,75
AM50 4,7 <0,001 <0,005 0,002 0,29 <0,001 0,02 <0,01
Die casting scrap parts made of AZ 91 or AM 50 were melted together with
magnesium chips of the same alloy in the weight ratio of die casting scrap
parts:
chips = 7:1 in an open crucible under atmospheric pressure. The thus produced
metal melts were transferred into steel moulds. Subsequently the filled steel
crucibles were placed in a vacuum chamber and subjected to a vacuum of 80 mbar
at a temperature of 600 C (AZ 91) or 630 C (AM 50), whereby the metal melts
foamed and light metal foam bodies got formed.
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A thus produced light metal foam body is shown in section in fig. 1. As one
can see,
the light metal foam body has several pores and a dense outer surface. Apart
from
the pores seen in fig. 1 that have a diameter of a few millimetres, there are
further
pores with smaller diameters as one can see from the micrographs in fig. 2 and
fig.
3. The non-metallic particles have an average size of under than 200 pm.
The pores structure shown more clearly in figures 1 to 3 result in an energy
absorption behaviour that makes light metal foam bodies produced from
recycling
material usable for many applications in vehicle manufacturing.
In fig. 4 stress/ compression diagrams in compression tests on parallelepipeds
(5 x
x 3 cm3) are depicted for light metal foam bodies made of alloy AZ 91 and
different
densities. These compression curves for parallelepipeds having a density of a)
0.56
gm/cm3, b) 0.41 gm/cm3, c) 0.36 gm/cm3 and d) 0.23 gm/cm3 prove that recycled
light metal foam bodies in the compression test show a pronounced plateau
after a
short linear ascension. In this plateau region, the so-called deformation
stress, the
light metal foam bodies are effective as energy absorbers. The deformation
stresses
attained in the reforming light metal foam bodies lie in the region of those
light metal
foam bodies that are produced in the traditional way, e.g. by powder-
metallurgical or
melt-metallurgical method.
In figures 5a to 5c the effect of temperature at the beginning of vacuum onto
the
shape of a light metal foam body made of AZ 91 alloy is shown. Figures 5a and
5b
show in comparison, that at 580 C the foam formation is partially suppressed,
which
is due to very high viscosity of the material to be foamed at this
temperature. At
600 C, which somewhat corresponds to the melting point of AZ 91, the foam
formation process is optimized with respect to a complete expansion of the
foam
body. Higher temperatures like 620 C (fig. 5c) result in reduction of
viscosity of the
metal melt. Consequently the formed metal foam body can partially collapse
because a wall structure of the metal foam is not sufficiently stable.
Apart from the selection of a foaming temperature or a temperature at which
vacuum is first applied, even specific cooling conditions promote homogeneous
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formation of light metal foam bodies: a uniform cooling on all sides is
advantageous
and can be achieved by insulating the container in which foaming is carried
out.
Fig. 6a and 6b juxtapose formations of light foam bodies under same condition,
especially same temperatures, vacuums and same used mass of metal melts, in an
open container (fig. 6a) and in a closed container (fig. 6b). If a container
is open,
then the formed metal foam can expand, unhindered and one obtains a light
metal
foam body as shown in fig. 6a. However, if a container is closed in such a way
that
even though a vacuum can be generated within it, an expansion of the formed
metal
foam body is restricted; thus the shape of the container defines the shape of
metal
foam body. In such a case, one can obtain a porous light metal foam body of a
desired shape and with an almost dense surface.
On basis of fig. 7 and 8 it is further explained how a process according to
the
invention can be applied for producing compound parts. fig. 7 shows an vacuum
chamber 1 with a gas inlet 6 and a gas outlet 7 that are mounted on a cover 8
of the
vacuum chamber 1. The cover 8 can be removed from a side wall 5 of the vacuum
chamber 1, so that a steel mould 2 can be introduced into the vacuum chamber
1.
After introducing the steel mould 2 this serves the purpose of taking up a gas-
containing metal melt 4. It is obvious that it is similarly possible to first
put the metal
melt 4 into the steel mould 2 and subsequently the steel mould 2 into the
vacuum
chamber 1.
Apart from the steel mould 2, a hollow body 3 is introduced into the vacuum
chamber 1. The hollow body 3 is thereby placed in such a way, that the metal
foam
formed from the metal melt 4 can come in contact with it. As shown in the
example
in fig. 7, this can take place in such a way that a hollow body 3 in the form
of a tube
is placed in a conically shaped steel mould 2 just above a melt surface.
The hollow body 3 can be of any metal or alloy. For example, combinations with
light
metal foam bodies of magnesium or magnesium alloys, steel profiles or
aluminium
profiles are suitable. It is particularly advantageous if the hollow body 3 to
be
foamed out is made of a metal or an alloy having a melting point/melting
interval that
lies approx. in the range of the metal melt. Then contact with the metal foam
at or
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just below the melting temperature effects melting of the hollow body 3 at the
contact face, which promotes solid metal bonding of light metal foams and
hollow
body 3. It is also advantageous to pre-treat the surface of the used hollow
body 3 in
the desired contact region, e.g. by removing passivating films/layers in order
to
achieve a full-faced metal bonding as far as possible.
Fig. 8 shows a section through a compound part produced according to the
arrangement shown in fig. 7. An outer region of the compound part is formed by
the
used hollow profile 3 and the inner region from light metal foam.
It is obvious to the expert that not only is it possible to foam out hollow
profiles, but
that similarly with compound parts produced with the same quality also re-
foaming
of metal bodies and combinations of foaming out/re-foaming are possible.
