PROCEDE ET DISPOSITIF POUR LA FABRICATION DE PIECES METALLIQUES FORMEES

PROCESS AND DEVICE FOR MANUFACTURING SHAPED PARTS OUT OF METAL

Abrégé anglais

A process for manufacturing shaped parts out of metal by pressing a thixotropic metal billet
(20) by means of a piston (16) out of a holding chamber (12) into a mould cavity (30) is such
that, at least up to the point in time (tE) at which the metal enters the mould cavity (30), the
said mould cavity (30) is forcibly evacuated. A corresponding thixoforming unit is such that
the mould cavity (30) is connected to a vacuum chamber (48).
Shaped parts having reduced gas porosity can be manufactured using the process.

Revendications

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The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:-
1. Process for manufacturing shaped parts out of metal by pressing a thixotropic metal
billet (20) by means of a piston (16) out of a holding chamber (12) into a mouldcavity (30),
characterised in that,
at least up to the point in time (tE) at which the metal enters the mould cavity (30),
the said mould cavity (30) is forcibly evacuated.
2. Process according to claim 1, characterised in that the forcible evacuation is main-
tained until the mould has been completely filled.
3. Process according to claim 1 or 2, characterised in that the holding chamber (12) is
thermally insulated and/or heated.
4. Process according to one of the claims 1 to 3, characterised in that the speed of the
piston is increased after the point in time (tE) at which the metal enters the mould
cavity (30).
5. Thixoforming unit for carrying out the process of the invention according to one of
the claims 1 to 4 having a holding chamber (12) to accommodate a thixotropic metal
billet (20), a mould cavity (30) following on from the holding chamber (12) and a
piston (16) for pressing the metal billet (20) out of the holding chamber (12) and
into the mould cavity (30), characterised in that the mould cavity (30) is connected
to a vacuum chamber (48).
6. Thixoforming unit according to claim 5, characterised in that a regulating facility for
opening and closing the connection between the mould cavity (30) and the vacuum
chamber (48) is provided between the mould cavity (30) and the vacuum chamber
(48).
7. Thixoforming unit according to claim 6, characterised in that the regulating facility
features a regulating shaft (38) with a closure head (40), said closure head (40)
serving the purpose of opening and closing an evacuating channel (34) immediately
following the mould cavity (30).

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8 Thixoforming unit according to one of the claims 5 to 7, characterised in that the
holding chamber (30) is of a ceramic material, in particular Si3N4.
9. Thixoforming unit according to one of the claims 7 to 8, characterised in that the
holding chamber (30) features heating facilities.

Description

2 1 70~1~
,
Process and Device for Manufacturin~ Shaoed Parts out of Metal
The invention relates to a process for m~nl~f~cturing shaped parts out of metal by pressing a
thixotropic metal billet by means of a piston out of a holding chamber into a mould cavity.
5 Also within the scope of the invention is a thixoforming unit suitable for carrying out the
process according to the invention.
The process for mAmlf~cturing shaped parts out of thixotropic i.e. partially solid/partially
liquid metal billets is known as thixofo,mlng. Metal billets that come into consideration for
10 this purpose are all billets of metal that can be ~ nsru,lncd to the thixotropic state. In
particular, the metal billets may be of ~ mini~lm m~gnecillm or zinc and their alloys.
Thixoforming of thoxotropic materials is generally known. In this process the thixotropic
properties of partially solid and partially liquid metal alloys are exploited. By the tlfi~ollopic
15 behaviour of a metal alloy is to be understood that a suitably prepared metal behaves as a
solid, if not subjected to force, if shear forces are applied to it, however, its viscosity is
reduced to such a degree that it behaves in a manner similar to that of molten metal. For that
purpose it is necessary for the alloy to be heated to the solidification interval between the
solidus and the liquidus telllpelalllres. The temperature has to be adjusted such that e.g. 20 to
20 80% of the structure is molten while the rest is in the solid state. In thixofo",ling the partially
solid/partially liquid metal is introduced in the form of a thixotropic metal billet into a -
normally horizontal - holding chal"bel and, by means of a piston, introduced or injected into
a mould cavity in which the thixotropic metal alloy solidifies.
25 In the ~ oro"""~g process the filling of the mould cavity with the partial solid/partially
liquid metal takes place essentially under laminar flow conditions The metal forms a closed
front which pushes the air ahead of it in the direction of the evacuation channels and the air
can escape via these ch~nn~lc. Although the filling of the mould cavity takes place relatively
slowly, air and/or other gaseous conctit~lents may become trapped in the shaped part - which
30 can lead to porosity and blisters, especially after heat lre~ Also, with the passage of
time, the evacuation chA--n~lc may become at least partially blocked by deposits of mould
sepalalillg material. This leads to increased gas pressure ahead of the metal front and
~helero,e to inc,eased levels of gas in the shaped part. Consequently the gas porosity in the
shaped parts can vary, depending on the degree of co~ l ion
When m~nllf~cturing shaped parts of special shape e.g. with lugs, it is inevitable that two or
more metal fronts form, which on collision can lead to localised e"l,ap"~ent of gases between
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the fronts. This occurs especially if no evacuation is provided or is impossible at the region
where the two fronts meet; consequently this leads systematically to defects in the part.
In view of the above, the object of the present invention is to provide a process of the kind
5 mentioned at the start by means of which the gas porosity in the shaped parts may be reduced
further. To this end a thixorolllling unit suitable for carrying out the process has to be
provided.
The objective of the invention is achieved by way of the invention in that at least up to the
10 point in time at which the metal enters the mould cavity, the said mould cavity is forcibly
evacuated. Special and further developed forms of the invention are presented in dependent
patent claims.
Using the process according to the invention thixoformed parts can be m~nllf~ctured that
15 exhibit a reduced degree of porosity compaled with shaped parts that are m~n~lf~ctured by
state of the art methods.
Investigations show that the gas content of thixorolllled parts is already at a very low level so
that no problems regarding porosity should arise. In spite of this, in certain parts and when
20 the quality requil~lllenls are very high, the gas content may be too high in some regions of
the part in question. One reason for locally elevated gas porosity can be e.g. that the filling
speed for thin-walled parts has to be made so high that the metal front is no longer compact.
Also, in some specific shaped parts, it is practically impossible to remove the gases from
certain regions that cannot be evacuated. Provided no full solution heat lre~l."e.~l is required,
25 the gases remain in solution or colllpressed under very high pressure - which presents no
special problem. If, however, high lllech~Lcal propellies are required, and thererore heat
lle~ .l is n ecessa,y, then even small amounts of gases that are concentrated at particular
sites may cause problelns. Forcible evacuation of the mould cavity in accordance with the
invention is of ~csi~t~nce here.
Particularly good results may be oblail1ed if forced evacuation is l"A;~IA;~-ed practically right
up to the complete filling of the mould.
An advantageous further development of the process is such that the holding challlber is
35 thermally in~ ted and/or heated. This reduces the cooling of the metal billet, and so allows
more time for evacuation.
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Forced evacuation of the mould cavity makes it possible to accelerate the speed of the piston
after the point in time of metal entering the mould cavity; the filling of the mould can
therefore also be accelerated, making it possible to produce thin-walled parts.
A thixoforming unit suitable for carrying out the process according to the invention exhibits a
5 holding chanlber to accommodate a thixotropic metal billet, a mould cavity following on
from the holding ch~llber and a piston for pressing the metal billet out of the holding
chamber into the mould cavity. In accoldance with the invention, the mould cavity is
connected to a vacuum chamber.
10 Provided between the mould cavity and the vacuum is usefully a re~ ting facility for
opening and closing the connection b~lween the mould cavity and the vacuum ch~lber.
In a p-ere led version of the thixofolllllllg unit the re~ ting facility features a control shaft
with a closure head; the latter serves to open the and close an evacuation channel following
15 on immediately from the mould cavity.
In order to achieve good thermal insulation, the holding cl~,lber may be made of a ceramic
material, especially Si3N4. It is also possible to provide the heating challlber with heating
facilities. These heating facilities may e.g. be in the form of heating rods or challl-els in the
20 wall of the holding cha,lll~er through which a heated medium flows, for example oil.
Further advantages, features and details of the invention are revealed in the following
description of a pr~rell~;d exemplified embodiment and with the aid of the drawing showing
sçhem~tically in
Fig. 1 a longitll-lin~l section through a thixoforming unit with forced evacuation;
Fig. 2 a typical sequence of events during the filling of the mould during thixoforming.
30 A thixoforming unit 10 shown in figure 1 exhibits a ho.i~ol-lal, cylindrical holding chamber
12 with interior 14. An openillg 18 in the holding chamber 12 enables a thixotropic metal
billet 20 to be introduced into the interior 14. The displ~cement of the metal billet 20 in the
interior 14 of the holding chamber 12 is effected by a piston 16 which can move along the
axis x of the holding cha,llber 12.
The holding ch~llbel- 12 terminates at a stationary mould plate 22 which faces a moveable
mould plate 24. Each of the mould plates 22, 24 forms a mould half 26, 28 which, when
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closed, together form a mould interior 30 in which the shaped part is created when the metal
solidifies.
The mould interior 30 features one or more evacuating chatmels 34 which, if desired lead to a
5 main ch~nnel A reg~ ting unit 36 with re~ ting shaft 38 is provided in the moveable form
plate 24. The reg~ ting shaft 38 features a closure head 40 for ope~ g and closing the
ev~A.~u~ting channel 34. Displacement of the re~ ting shaft 38 takes place via an actuating
cylinder 42 flanged onto the outside of the moveable mould plate 24. This arrangement
allows forcible evacuation of the mould cavity 30 to be ,~ ed right up until the cavity
10 30 has been completely filled. Only when the filling has been completed is the eviqc~ tinE
channel 34 closed at the end of the mould cavity 30 by means of the closure head 40 on the
re~ ting shaft 38.
Connected to the re~ ting facility 36 is a vacuum pipeline 44 which is in turn connected to
15 a vacuum cha-nber 48 via a valve 46. The vacuum chamber 48 is evacu~ted by means of a
vacuum pump 50 and held at reduced pressure. Manometers 52 are provided for checking
the pressure.
The closure head 40 on the re~ tin~ shaft 38 acts as a valve and serves a number of
20 purposes:
- Before filling the mould, the valve is closed and the vacuum creates a reduced pressure in
the vacuum ch~llber 48.
25 - During the first filling phase the valve is opened in a controlled manner and effects the start
of the forced evacuation.
- When the filling of the mould has been completed, the valve is closed in order that no metal
may enter the unit. It is also neces~ry to close the valve in order that the mould halves 26,
30 28 may be separated and the mould opened, and to allow the reduced pressure to be formed
again in the vacuum ch~l.ber 48.
Forced evacuation starts at the earliest when the piston 16 has closed opening 18 in the
holding chamber 12 and at the latest when the tool is opened again by sep~ g the two
35 halves of the mould 26, 28. Usefully and advantageously, forced evacuation ends as soon as
the filling ofthe mould has been completed i.e. the piston 16 is no longer moving. The forced
evacuation may of course also be te".~ Ale~ earlier. The points at which forced evacuation
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starts and stops may be determined by displacement sensors on the piston rod. The starting
and stopping points may, however, also be controlled as a function of time, speed or
pressure. A further possibility is to employ sensors detecting the metal front i.e. sensors that
release a switch when the metal front reaches a particular site.
s
Shown in figure 2 is a typical sequence of events accompanying the filling of the mould in a
~hixofolllling unit with forced evacuation. During a first filling phase the piston delivers the
metal up to the gate of the mould; in a second filling phase that begins at the point in time
(t~) when the metal starts to enter the mould cavity, the mould is filled with metal. Both
10 filling phases in thixorolllling are typically of about the same duration e.g. 0.5 sec. The time
of evacuation is thelefore less than 1 sec. The removal of the gases may begin only after the
piston has closed off the entry port. On the other hand the speed of the piston can not be
reduced at will, as this could cause too much metal to freeze in the holding chamber.
15 It is not necess~y to ~ a very high vacuum i.e. Iow pressure. It is sufflcient to suck off
the gases present in the mould cavity before and during the filling of the mould so that no
counter-pressure is created by these gases. By means of trials it was possible to confirm that
it is not essential to provide a very high vacuum i. e. very low pressure, in order to achieve
extremely good results with respect to porosity.
case 2060

Dessin représentatif