U.S. patent application number 10/168416 was filed with the patent office on 2003-03-27 for pre-treatment of a thixotropic metal bolt.
Invention is credited to Arnold, Gregoire, Bagnoud, Christophe, Bolliger, Martin, Plata, Miroslaw.
Application Number | 20030056930 10/168416 |
Document ID | / |
Family ID | 8243209 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030056930 |
Kind Code |
A1 |
Plata, Miroslaw ; et
al. |
March 27, 2003 |
Pre-treatment of a thixotropic metal bolt
Abstract
The invention relates to a pre-treatment device and method for
production of a thixotropic metal bolt (10), in a casting chamber
(30) of a thixo-moulding unit. The pre-treatment device comprises a
container (14), for accommodating a metal bolt (10), an oven (20),
for converting the metal bolt (10) in the container (14) into a
partly fluid thixotropic state and a transport unit for
transporting and feeding the thixotropic metal bolt (10), into the
casting chamber (30). The container (14) is a cylinder-shape
heating tube (14), with closable sides. Furthermore, the
pre-treatment device is so arranged that, during the entire
pre-treatment, namely the heating process in the oven (20), the
transport into the casting chamber (30) and the period in the
casting chamber (30), the metal bolt (10) remains in the heating
tube (14).
Inventors: |
Plata, Miroslaw; (Vetroz,
CH) ; Bagnoud, Christophe; (Veyras, CH) ;
Arnold, Gregoire; (Miege, CH) ; Bolliger, Martin;
(Venthone, CH) |
Correspondence
Address: |
Bachman & LaPointe
Suite 1201
900 Chapel Street
New Haven
CT
06510-2802
US
|
Family ID: |
8243209 |
Appl. No.: |
10/168416 |
Filed: |
September 3, 2002 |
PCT Filed: |
December 12, 2000 |
PCT NO: |
PCT/EP00/12554 |
Current U.S.
Class: |
164/80 ; 164/113;
164/900 |
Current CPC
Class: |
F27D 3/00 20130101; F27D
2003/0075 20130101; B22D 17/007 20130101; F27D 3/12 20130101; F27B
17/0016 20130101 |
Class at
Publication: |
164/80 ; 164/900;
164/113 |
International
Class: |
B22D 017/00; B22D
023/06; B22D 025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 1999 |
EP |
99811196.7 |
Claims
1. Pretreatment device for provision of a thixotropic metal bolt
(10) in a casting chamber (30) of a thixoforming device, containing
a container to hold a metal bolt (10), an oven (20) to transfer the
metal bolt (10) in the container into a part-liquid thixotropic
state, and a transport device for transporting and introducing the
thixotropic metal bolt (10) into the casting chamber (30),
characterised in that the container is a cylindrical heating tube
(14) which can be closed at the sides, and the pretreatment device
is formed such that the metal bolt (10) can remain in the heating
tube (14) throughout the entire pretreatment, namely the heating
process in the oven (20) and transport to the casting chamber (30)
and its stay in the casting chamber (30) until the start of the
thixoforming process.
2. Pretreatment device according to claim 1, characterised in that
the internal diameter d.sub.R of the heating tube (14) as a
function of the bolt diameter d.sub.B is selected such that the
metal bolt (10) at its solidus temperature T.sub.solid
substantially has the same diameter d.sub.B as the internal
diameter d.sub.R of the heating tube (14).
3. Pretreatment device according to claim 1 or 2, characterised in
that the heating tube (14) consists of metal, preferably steel, in
particular stainless steel or tool steel.
4. Pretreatment device according to claim 1 or 2, characterised in
that the heating tube (14) consists of ceramic material.
5. Pretreatment device according to any of claims 1 to 4,
characterised in that when the metal bolt (10) is in a
substantially horizontal position during the heating process, the
heating tube (14) is tightly closed on both sides by sealing
elements (16, 18), where the sealing elements (16, 18) are designed
with regard to material choice and form such that their friction
properties in the heating tube (14) firstly allow a shift due to
thermal expansion of the metal bolt (10) during the heating process
in the direction of the longitudinal axis l of the heating tube
(14), and secondly prevent a shift due to the pressure exerted by
the metal bolt (10) on the sealing elements (16, 18) after reaching
the temperature necessary for the desired thixotropic state.
6. Pretreatment device according to any of claims 1 to 4,
characterised in that for a substantially vertical metal bolt
position during the heating process, the heating tube (14) is
closed on one side at the lower tube end (15), where on use of a
sealing element (16) the sealing element (16) is designed with
regard to material choice and form such that firstly during the
heating process in the oven (20) no liquid metal (24) can emerge
from the heating tube (14) and secondly the friction of the sealing
element (16) in the heating tube (14) is less than 10 N.
7. Pretreatment device according to claim 5 or 6, characterised in
that the sealing elements (16, 18) consist of ceramic material.
8. Pretreatment device according to any of claims 1 to 7,
characterised in that the oven (20) is an induction oven.
9. Pretreatment device according to any of claims 1 to 8,
characterised in that the transport device is a robot where the
clamping device of the robot to hold the heating tube at least on
the surface directed towards the heating tube consists of ceramic
material.
10. Use of the pretreatment device according to any of claims 1 to
9, for provision of a thixotropic metal bolt (10) in a casting
chamber (30) of a horizontal thixoforming device.
11. Use of a pretreatment device according to any of claims 1 to 9,
for provision of a thixotropic metal bolt (10) from an aluminium
alloy.
12. Process for provision of a thixotropic metal bolt (10) in a
casting chamber (30) of a thixoforming device, where a metal bolt
(10) in a solid state of aggregation is placed in a container, and
the metal bolt (10) in the container is heated in an oven (20)
until the metal bolt (10) is in a thixotropic state, and the
thixotropic metal bolt (10) is transported by means of a transport
device into the casting chamber (30) of a thixoforming device,
characterised in that the container is a cylindrical heating tube
(14), the metal bolt (10) remains in the heating tube (14)
throughout the heating process and subsequent transport into the
casting chamber (30), and the heating tube (14) containing the
metal bolt (10) is positioned in the casting chamber (30) such that
during the subsequent thixoforming process the plunger (34) of the
thixoforming device can push the thixotropic metal bolt (10) out of
the heating tube (14).
13. Process according to claim 12, characterised in that the
internal diameter d.sub.R of the heating tube (14) as a function of
the bolt diameter d.sub.B is selected such that at room temperature
the internal diameter d.sub.R is greater than the bolt diameter
d.sub.B and at the solidus temperature T.sub.solidus of the metal
bolt (10) the bolt diameter d.sub.B substantially corresponds to
the internal diameter d.sub.R of the heating tube (14).
14. Process according to claim 12 or 13, characterised in that
after the heating process but before insertion of the heating tube
(14) containing the thixotropic metal bolt (10) into the casting
chamber (30), the liquid metal (24) which emerged during the
heating process from the metal bolt (10) is at least partly removed
from the heating tube (14).
15. Process according to any of claims 12 to 14, characterised in
that the heating tube (14) is sealed on at least one side during
the heating process of the metal bolt (10).
16. Process according to any of claims 12 to 15, characterised in
that a heating tube (14) which is tightly closed at one end, at the
lower tube end (15), with a sealing element (16) and containing the
metal bolt (10), is placed in a vertical position vertically onto
the table plate (26), preferably onto a table plate (26) of ceramic
material, and the table plate (26) is introduced vertically,
preferably from below, into the oven (20), and the heating tube
(14) containing the metal bolt (10) is heated until the metal bolt
(10) is a thixotropic state, and subsequently the heating tube (14)
containing the thixotropic metal bolt (10) is removed vertically,
preferably by lowering the table plate (26), from the oven (20) and
separated from the sealing element (16).
17. Process according to any of claims 12 to 15, characterised in
that the heating tube (14) containing the metal bolt (10) is
tightly sealed on both sides with a sealing element (16, 18) which
can be moved in the direction of the concentric longitudinal axis l
of the heating tube (14) and with preset friction properties
between the heating tube (14) and the sealing element (16, 18), and
is introduced into the oven (20) in a substantially horizontal
position, where during the heating process the two sealing elements
(16, 18) due to thermal expansion of the bolt material (10) are
pushed apart and after reaching the thixotropic state the sealing
elements (16, 18) are held in their position due to friction, and
after removal of the heating tube (14) containing the metal bolt
(10) from the oven (20) the two sealing elements (16, 18) are
removed from the heating tube (14).
18. Use of the process according to any of claims 12 to 17 to
provide a thixotropic metal bolt (10) in a horizontal thixoforming
device.
19. Use of the process according to any of claims 12 to 17 to
provide a thixotropic metal bolt (10) from an aluminium alloy.
Description
[0001] The invention concerns a pretreatment device for provision
of a thixotropic metal bolt in a casting chamber of a thixoforming
device, containing a container to hold a metal bolt, an oven to
transfer the metal bolt in the container into a part-liquid
thixotropic state, and a transport device for transporting and
introducing the thixotropic metal bolt into the casting chamber,
and the use of the pretreatment device. The invention also concerns
a corresponding process according to the features of the preamble
of claim 12 and the use of the process.
[0002] Thixoforming concerns the production of mouldings from
thixotropic metal bolts. The metal bolts can be all bolts of a
metal which can be transferred to a thixotropic state. In
particular the metal bolts can consist of aluminium, magnesium or
zinc or alloys of these metals.
[0003] Thixoforming utilises the thixotropic properties of
part-fluid and part-solid metal alloys. The thixotropic properties
of a metal alloy mean that a correspondingly prepared metal behaves
as a solid when not stressed, however under shear stress its
viscosity is reduced so much that it behaves similar to a metal
melt. For this, heating of the alloy to the solidification range
between liquidus and solidus temperature is required. The
temperature must be set so that for example a structural part of 20
to 80 w. % is melted, but the rest remains in solid form.
[0004] In thixoforming, part-solid/part-liquid metal is processed
into mouldings in a modified diecasting machine, known as a
thixoforming device. The diecasting machine used for thixoforming
differs from diecasting machines for diecasting metal melts by for
example a longer design of casting chamber to hold the thixotropic
metal bolt and the greater piston stroke required as a result, and
for example a mechanically reinforced design of the parts of the
diecasting machine carrying the thixotropic metal alloy due to the
higher pressure loading on these parts during thixoforming.
[0005] The metal bolts are usually heated in a separate oven. The
oven can be heated with fuel, for example gas or oil, or electrical
energy such as for example by resistance heating or by means of
inductive energy supply.
[0006] The heating of the metal bolt has great significance in
relation to the great influence exerted by the state of the bolt
introduced into the casting chamber on the quality of the product
as:
[0007] the bolt state i.e. its part-solidity is normally only
present in small temperature ranges,
[0008] long heating times, for example to form a thick oxide skin
or avoid possible grain coarsening, must be avoided,
[0009] and to achieve a homogeneous end product, the temperature
distribution in the thixotropic metal bolt, the thixoblank, must be
as homogeneous as possible.
[0010] Therefore the metal bolt is suitably transferred to the
thixotropic state i.e. heated until the required alloy proportion
has melted, by an oven temperature regulated with sensors.
[0011] To heat the metal bolt this is usually placed in a dish-like
container, for example a metal dish of stainless steel or a
crucible of clay-graphite or clay-SiC, and brought into the
thixotropic state in a horizontal position.
[0012] The thixotropic metal bolt can then be transferred, for
example in the same container, by means of a gripper for example to
the casting chamber of a horizontal thixoforming device and by
tipping the container transferred to the casting chamber. In this
case the metal bolt remains in the same container during the
heating process and transport to the casting chamber.
[0013] EP-A-0 645 206 describes a device for production of
mechanically highly stressed parts by thixoforming which contains a
pretreatment device initially described. For this the metal bolts
are transferred in cup-like containers into a tubular passage oven
in the thixotropic state, and the containers containing the
thixotropic bolts are transported by means of a robot to the
casting chamber, and by tipping the container the thixotropic bolt
is transferred to the casting chamber.
[0014] EP-B-0 713 736 describes a holder device for inductive
heating of bolts of metal alloys with thixotropic properties and
for holding and transporting the bolts until casting. The holding
device is a specially designed trough-like dish.
[0015] The technical solutions applied in practice with regard to
heating and transport of thixotropic metal bolts often guarantee
neither adequate thermal homogeneity of the bolt introduced into
the casting chamber nor the repeatability of the heating process
necessary to achieve stable quality. The lack of thermal
homogeneity is manifested for example in a distribution of the
liquid part which is irregular in relation to the bolt
cross-section, where this often leads to local melting of the metal
bolt, in particular at the metal bolt ends. Consequently, the
geometry of the bolt can change substantially (ovalisation,
craters) which in the initial phase of the thixoforming process
usually leads to the inclusion of air and/or aluminium oxides in
the mould cavity and consequently an increase in the rejection
rate.
[0016] Another problem during the heating process can be caused by
the emergence of liquid metal from the bolt as such liquid metal
can seep outwards during transport of the container, for example
while still within the oven, which frequently damages the inside of
the oven, in particular when an induction oven is used. It must be
noted that the volume of liquid metal emerging from the metal bolt
during the bolt heating process can typically amount to up to 10%
of the bolt volume.
[0017] A further problem from the emergence of liquid metal during
the heating process can arise when liquid metal is poured into the
casting chamber as this can lead to premature solidification in the
casting chamber which can then cause a multiplicity of defects such
as air inclusions or non-homogenous structure in the
thixomoulding.
[0018] The purpose of the present invention is to avoid these
disadvantages in the state of the art and specify a pretreatment
device and process for reproducible provision of thixotropic metal
bolts in a casting chamber of a thixoforming device, where the
thixotropic metal bolt has a homogeneous temperature distribution
and a liquid part distributed homogeneously over the entire bolt
cross-section and entire bolt length, and avoids the emergence of
liquid metal in the heating oven and the introduction of liquid
metal into the casting chamber. A further task of the present
invention is to maintain the bolt shape during the heating process
and transport of the thixotropic bolt to the casting chamber, and
during its introduction into the casting chamber. Another task of
the present invention is the possibility of increasing the
transport speed as this will reduce the cooling occurring during
transport of the bolt from the oven to the casting chamber.
[0019] According to the invention this is achieved in that the
container has a cylindrical heating tube which can be closed at the
sides, and the pretreatment device is formed such that the metal
bolt can remain in the heating tube throughout the entire
pretreatment, namely the heating process in the oven and transport
to the casting chamber and its stay in the casting chamber until
the start of the thixoforming process.
[0020] Further advantageous designs of the pretreatment device
according to the invention are described in claims 2 to 9.
[0021] The new concept according to the invention, according to
which the metal bolt remains in the hollow cylindrical container
known as the heating tube during the entire pretreatment
process:
[0022] guarantees a homogeneous and symmetrical heating in the
axial and radial direction of the bolt;
[0023] minimises the non-homogeneity of the magnetic field at the
edge of the bolt during the heating process in an induction
oven;
[0024] prevents excess melting of the bolt surface;
[0025] guarantees retention of the bolt shape during the heating
process, transport to the casting chamber and in the casting
chamber;
[0026] eliminates the effect of any structural non-homogeneities
present and the chemical composition of the metal bolt on the
heating process;
[0027] allows the use of cheaper and more efficient heating by
means of an induction field with higher frequency than the state of
the art;
[0028] apart from metal bolts and the heating tubes, avoids the
requirement for and presence of other metal elements within an
induction oven, which minimises the disruption to homogeneity of
the magnetic field within an induction oven;
[0029] allows more precise control of the heating process than the
state of the art by direct measurement of the temperature of the
heating tube on the thermal length expansion of the metal bolt
occurring during the heating process, where the length expansion of
the metal bolt during the heating process above the solidus
temperature of the bolt material is proportional to the liquid
part;
[0030] eliminates the need to use compensation plates at the bolt
ends within an induction oven to homogenise the magnetic field,
which lowers the costs of the pretreatment device and increases its
functional reliability; and
[0031] allows faster transport than the known state of the art of
the thixotropic metal bolt from the heating oven to the casting
chamber.
[0032] The pretreatment device according to the invention is
suitable for all metal bolts of commercial alloys which can be
transferred to a thixotropic state. Particularly suitable metal
bolt materials are alloys of aluminium, magnesium or zinc. In
particular, aluminium casting and aluminium wrought alloys are
preferred. The pretreatment device according to the invention is
advantageously also suitable for processing of particle-reinforced
aluminium alloys which contain for example evenly distributed SiC
or Al.sub.2O.sub.3 particles. In particular the pretreatment device
according to the invention is suitable for aluminium alloys--which
have a pronounced solidification interval such as for example
AlSi7Mg.
[0033] The metal bolts suitably contain evenly distributed, primary
solidified particles which consist of individual degenerated
dendrites. The proportion of primary solidified particles is
preferably between 40 and 80 w. %. To achieve a good thixotropic
behaviour, for example with aluminium alloys the alpha mixing
crystals must be present in globulistic form in order to achieve an
even flow of the liquids and solids.
[0034] The degenerated dendrites of the metal bolts generally
preferably have a globulistic structure whereby an even homogeneous
flow of melt and solid can be achieved without separation of
mixture. Metal bolts with a structure with globulistic dendrites
are produced inter alia by an extrusion process combined with an
intensive electromagnetic agitation also during the solidification
phase. This leads to a melting and break-up of dendrite arms which
form close to the solidus temperature and form the globulistic
structure.
[0035] Before the thixoforming process, the metal bolts required
for thixoforming are heated by means of the pretreatment device
according to the invention to a temperature above the solidus
temperature and below the liquidus temperature, i.e. until they
reach a part-solid thixotropic state.
[0036] In the part-solid state the thixotropic alloy, known as the
thixotropic alloy pulp, contains the back-developed dendritic
primary solid particles in a surrounding matrix of liquid metal.
Preferably, the thixotropic alloy pulp contains a liquid part of 40
w. % to 50 w. % and in particular between 43 w. % and 48 w. %.
[0037] During the heating process the metal bolts can be in the
vertical or horizontal position in relation to their linear
axis.
[0038] To transfer the metal bolt to a thixotropic state,
preferably an induction oven is used. For example, a primary coil
is arranged around the container where the alternating current
flowing in the primary coil induces in the container and/or the
metal bolt an alternating magnetic field. The heating tube or metal
bolt is heated by the alternating magnetic field which provokes
powerful eddy currents and consequently heating in the heating tube
or metal bolt. The penetration depth of the eddy currents and hence
the depth of the heated layer is frequency-dependent; when high
frequencies are used, rapid heating occurs mainly of the layers
close to the surface.
[0039] In an advantageous design of the heating tube, this consists
of a metal. Preferred are metals of the series of the
iron-carbon-containing metals such as steel, stainless steel,
thermax steel, hot work steel, or from the series of the metals
tantalum, niobium, vanadium, tungsten or titanium or alloys
thereof. Also, preferably heating tubes are used which are made of
copper or its alloys. Particularly preferably, heating tubes are
made of steel and in particular stainless steel or tool steel.
[0040] If a metal heating tube, in particular a steel heating tube
is used for the heating process in an induction oven, the magnetic
field in essence penetrates only the heating tube so that the
induction oven in essence only heats the heating tube directly and
the heating of the metal bolt takes place almost exclusively by
heat conductance from the heating tube to the metal bolt. The bolt
material is thus heated by the heating of the heating tube and by
heat conductance from the heating tube to the bolt material.
Because of the high heat conductivity of a metal heating tube a
radially symmetrical heat conductance occurs from the heating tube
to the metal bolt insofar as the metal bolt is in thermal contact
with the heating tube over its entire periphery. The radially
symmetrical heat conductance then also causes a radially
symmetrical temperature distribution in the metal bolt whereby, due
to the high heat conductance of the metal bolt, a low radial
temperature gradient is rapidly established.
[0041] Due to the heating device according to the invention with a
metal heating tube, a very homogeneous temperature and hence very
homogeneous liquid metal distribution is achieved in the entire
metal bolt. This applies in particular also because the indirect
energy transfer takes place to the heating tube and the bolt
material is only heated indirectly by heat conductance so that any
local differences in the structure or chemical composition of the
bolt material have no effect on the direct energy application.
[0042] According to a further preferred embodiment of the heating
tube this can be made of ceramic material. When such heating tubes
are used in an induction oven, the magnetic field penetrates the
heating tube i.e. the ceramic material of the heating tube is
transparent to the magnetic field. The metal bolt is then heated by
direct interaction with the magnetic field of the induction
oven.
[0043] Suitable ceramic materials are for example Al.sub.2O.sub.3,
Al.sub.3O.sub.4, BN, SiC, Si.sub.3N.sub.4, MgO, TiO, ZrO.sub.2,
stabilised such as yttrium-stabilised ZrO.sub.2 glasses or
refractory cements or mixtures which contain the said materials.
Equally preferably, the heating tube can consist of
fibre-reinforced ceramic material or contain such materials, and
the fibres of the fibre-reinforced ceramic material can for example
be made of SiC, Al.sub.2O.sub.3, glass or carbon.
[0044] To guarantee the insertion of the metal bolt into the
heating tube, the internal diameter d.sub.R of the heating tube in
the cold state, in particular at room temperature i.e. a
temperature from 15.degree. C. to 30.degree. C., is suitably
slightly larger than the bolt diameter d.sub.B of the cold metal
bolt.
[0045] The internal diameter d.sub.R of the heating tube as a
function of the bolt diameter d.sub.B is preferably selected such
that the metal bolt in the cold state, in particular at room
temperature i.e. a temperature from 15.degree. C. to 30.degree. C.,
is smaller by a dimension .DELTA.d of around 0.5 mm, preferably
0.5.+-.0.3 mm, in particular 0.5.+-.0.1 mm, than the internal
diameter d.sub.R of the heating tube.
[0046] During the heating process the metal bolt expands radially
and axially so that at a particular time the bolt diameter d.sub.B
corresponds to the internal diameter d.sub.R of the heating tube,
where it should be taken into account that the internal diameter
d.sub.R is also temperature-dependent.
[0047] When a metal heating tube, in particular when a steel
heating tube is used, during the heating process of the metal
bolt--as soon as, due to thermal expansion, the bolt diameter
d.sub.B corresponds to the internal diameter d.sub.R of the heating
tube--a very even radially symmetrical heat transfer is guaranteed
from the heating tube to the metal bolt. An even radially
symmetrical heat transfer is of great importance, in particular
after reaching the solidus temperature of the metal bolt, as a
local excess temperature in a temperature range above the solidus
temperature causes a corresponding local melting of the bolt
material and consequently must be avoided.
[0048] As part of the inventive activity it has been shown that the
use of a thin lubricant layer to avoid adhesion of the metal bolt
to the heating tube has practically no effect on the heat
transmission between the bolt and the heating tube. It must be
noted that the lubricant used as a separating agent--where
necessary at all--is usually only used for heating tubes of metal,
i.e. when heating tubes of ceramic material are used no separating
agent is normally required.
[0049] In a preferred embodiment of the pretreatment device
according to the invention the material of the heating tube and the
internal diameter d.sub.R of the heating tube are selected such
that the metal bolt at a solidus temperature T.sub.solidus has
substantially the same diameter d.sub.B as the internal diameter
d.sub.R of the heating tube. Furthermore, preferably the bolt
diameter d.sub.B at T.sub.solidus corresponds to the relationship
0.996 d.sub.R.ltoreq.d.sub.B.ltoreq.d.sub- .R, particularly
preferably 0.998 d.sub.R.ltoreq.d.sub.B.ltoreq.d.sub.R and in
particular 0.999 d.sub.R.ltoreq.d.sub.B.ltoreq.d.sub.R.
[0050] The metal bolts are cylindrical and usually have a round or
oval cross-section but can however also have a polygonal
cross-section. The diameter of the metal bolt in the cold state is
for example 50 to 180 mm, suitably 75 to 150 mm and preferably 100
to 150 mm. The length of the metal bolt in cold state is for
example 80 to 500 mm.
[0051] When the metal bolt is heated the metal bolt expands as does
the heating tube. Different materials have different thermal
expansion co-efficients. The thermal expansion co-efficient of
steel or ceramic is substantially less than that of aluminium or
aluminium alloys for example. Consequently, a metal bolt of an
aluminium alloy for example expands more than a heating tube of
steel or ceramic material for example, so that starting from a
metal bolt diameter which in the cold state is smaller than the
internal diameter of the heating tube, at a particular temperature
the metal bolt has the same diameter as the heating tube.
[0052] According to the invention it is now preferred that at the
solidus temperature T.sub.solidus of the metal bolt material, the
diameter of the metal bolt substantially corresponds to the
diameter of the heating tube, so that in the temperature range in
which the thixotropic properties of the metal bolt are
substantially set, an optimum thermal contact is formed between the
heating tube and the metal bolt. When the heating tube containing
the metal bolt is heated above the solidus temperature
T.sub.solidus, a melting of the eutectic occurs associated with a
volume expansion of the metal bolt. The eutectic of the aluminium
alloy suitable for thixoforming is typically formed at around 550
to 570.degree. C. The volume expansion in the range between the
solidus and the liquidus temperature for thixotropic aluminium
alloys is typically around 1.0 to 5.5% and in particular 1.5 to 3%.
At the heating or warming phase above the solidus temperature
T.sub.solidus, which for aluminium alloys suitable for thixoforming
is typically around 520 to 550.degree. C., the bolt material can
expand only in the longitudinal direction of the heating tube
provided that the condition is fulfilled whereby at T.sub.solidus
the bolt diameter d.sub.B corresponds to diameter d.sub.R of the
heating tube. For aluminium alloys the volume expansion between the
solidus and liquidus states--depending on bolt length--is
manifested in an increase in bolt length of typically 3 to 16 mm
and in particular 3 to 6 mm. The length increase of an aluminium
bolt below the solidus temperature--depending on bolt length--is
typically between 1 and 2 mm.
[0053] Because of the length change of the metal bolt during its
transition to the thixotropic state and at least partial insertion
of sealing elements into the cavity of the heating tube, the length
of the heating tube must be greater than the bolt length.
Preferably, the length of the heating tube is selected so that the
heating tube in total is around 5 to 30 mm, in particular 10 to 20
mm longer than the metal bolt to be heated therein. Thus, when the
metal bolt is heated in a horizontal position on each side of the
heating tube the surface of the heating tube preferably projects
beyond the surface of the metal bolt by around 2.5 to 15 mm and in
particular by 5 to 10 mm. When the metal bolt is heated in the
vertical position the surface of the heating tube at the top tube
end projects over the surface of the metal bolt preferably by
around 5 to 30 mm and in particular 10 to 20 mm.
[0054] The wall thickness of the heating tube for steel tubes is
preferably 1 to 5 mm, for copper tubes preferably 4 to 10 mm and
for ceramic tubes 8 to 15 mm.
[0055] The heating tube according to the invention can be closed on
both sides. For this, sealing elements of ceramic material are
preferably used. Suitable ceramic materials for the sealing
elements are the same materials as described above for a preferred
embodiment of the heating tube of ceramic material. Ceramic
material has a low heat conductance towards the metal bolt so that
the radial temperature distribution at the surface edge areas of
the metal bolt is only slightly influenced by such sealing
elements.
[0056] When the metal bolt is in a horizontal position during the
heating process, the heating tube is suitably tightly sealed on
both surfaces by means of sealing elements, preferably by stopper-
or peg-like sealing elements. The seal here must prevent the
emergence of liquid metal from the heating tube during the heating
process.
[0057] Also, preferably the stopper- or peg-like sealing elements
are selected with regard to material and shape such that their
friction properties in the heating tube firstly allow a shift in
the direction of the longitudinal axis of the heating tube due to
thermal expansion of the metal bolt during the heating process, and
secondly prevent a shift due to the pressure exerted by the metal
bolt on the sealing element after reaching the temperature
necessary for the required thixotropic state. The sealing elements
are preferably pushed into the heating tube before the heating
process so far that they stand in direct mechanical contact with
the surfaces of the metal bolts. This causes the sealing elements
to move during the heating process in the heating tube due to the
thermal length expansion of the metal bolt.
[0058] When the metal bolt is in a substantially vertical position
during the heating process, the heating tube is tightly closed on
one side at the lower tube end. Here too, the seal must merely
prevent the emergence of liquid metals from the heating tube. The
heating tube can for this be placed directly on a preferably
height-adjustable table plate, preferably a table plate of ceramic
material, or the heating tube can be tightly sealed by a sealing
element, preferably a stopper- or peg-like sealing element, and by
means of this sealing element placed in a vertical position on a
preferably height-adjustable table plate made of any heat-resistant
material. In particular, when using a stopper- or peg-like sealing
element, the sealing element is designed with regard to material
choice and shape preferably such that firstly during the heating
process in the oven no liquid metal can emerge from the heating
tube and secondly the friction of the sealing element in the
heating tube is less than 10 N.
[0059] The friction of the sealing element must be slight so that a
gripper arm of a transport device, in particular a robot gripper
arm, can raise the heating tube from the sealing element on the
table plate without great force, i.e. without the gripper arm
needing to be exposed to great mechanical stress. Secondly, a high
friction is not necessary to achieve a high seal as the sealing
element need merely prevent the emergence of liquid metal during
the heating process, and because of the cohesion of metal melts in
particular aluminium melts, this does not require a high degree of
seal. Suitably, the friction of the sealing element in the heating
tube is less than 30 N, preferably between 2 and 20 N and in
particular between 5 and 10 N.
[0060] The pretreatment device according to the invention is
suitable for the provision of a thixotropic metal bolt in a casting
chamber of a vertical or horizontal thixoforming device.
Particularly advantageously this pretreatment device is however
used to provide a thixotropic metal bolt in a horizontal casting
chamber as here the form retention can be guaranteed particularly
well with the device according to the invention. In a horizontal
thixoforming device the casting chamber which holds the thixotropic
metal bolt is horizontal.
[0061] The device according to the invention is particularly
advantageous for the provision of thixotropic metal bolts of
aluminium or aluminium alloys. Aluminium bolts are quite
particularly preferably heated in an induction oven with a vertical
heating chamber.
[0062] The task for the process is solved in that the container
constitutes a cylindrical heating tube, the metal bolt always
remains in the heating tube during the heating process and
subsequent transport to the casting chamber, and the heating tube
containing the metal bolt is positioned in the casting chamber such
that during the subsequent thixoforming process the plunger of the
thixoforming device can push the thixotropic metal bolt out of the
heating tube.
[0063] Preferred embodiments of the process according to the
invention are described in the dependent claims 13 to 17. The
statements concerning the pretreatment device according to the
invention also apply accordingly, with regard to special features
and details, to the process according to the invention.
[0064] The heating tube containing the thixotropic metal bolt is
transported after the heating process and any dripping of the metal
melt emerging from the metal bolt during the heating process, by
for example a robot to a casting chamber and placed in the front
half-open part of the casting chamber. After the pretreatment i.e.
at the start of the actual thixoforming process, the plunger pushes
the metal bolt from the heating tube into a closed part of the
casting chamber; the thixotropic metal alloy is then passed through
a passage opening into sprues and hence into the moulding cavity.
After the thixoforming process the plunger is retracted so that
then a gripper arm of the transport device collects the heating
tube from the casting chamber and returns it for use in the
subsequent pretreatment process. The return of the heating tube for
re-use in the subsequent treatment process suitably takes place
during the solidification phase of the thixotropic metal alloy in
the moulding cavity. The casting structure which has formed during
the solidification of the thixotropic metal alloy in the moulding
cavity in essence determines the properties of the moulding. This
structure is formed characterised by phases such as mixed crystal
and eutectic phases, the casting granulation such as globulites and
dendrites, segregations and structure faults such as porosity (gas
pores, micro-pinholes) and contaminants for example oxides.
[0065] Preferably, after the heating process but before insertion
of the heating tube containing the thixotropic metal bolt into the
casting chamber, the liquid metal which emerged from the metal bolt
during the heating process is at least partly removed from the
heating tube. The liquid metal part emerging from the metal bolt
during the heating process is typically less than 1 w. % of the
bolt material.
[0066] The transport of the thixotropic metal bolt from the heating
oven to the casting chamber by means of robots typically takes
between 5 and 30 s and preferably 8 to 15 s. The time during which
the thixotropic metal bolt remains in the casting chamber is
typically between 3 and 5 s. This time is required to remove a
robot with the arm away from the casting chamber and for electronic
readiness checking of a thixoforming device.
[0067] The pretreatment process according to the invention brings
substantial advantages, in particular:
[0068] it leads to a substantial reduction in heat loss of the
thixotropic metal during the transport from the heating oven to the
casting chamber and in the casting chamber thanks to the heating
tube heated to the same temperature as the metal bolt;
[0069] by the settling of the metal bolt within the heating tube in
the casting chamber, in particular in the casting chamber of a
horizontal thixoforming device, it eliminates the shock of
dropping, guaranteeing retention of the form and homogeneity of the
metal bolt;
[0070] it avoids the separation of the liquid part on insertion of
the thixotropic metal bolt into the casting chamber, so that the
associated premature solidification of liquid metal in the casting
chamber is avoided;
[0071] to a considerable extent it prevents oxidation of the metal
bolt surface, as during the heating process and transport through
the casting chamber and during its stay in the casting chamber
until the start of the actual thixoforming process, the metal bolt
is not exposed to free atmosphere;
[0072] it reduces the risk of air inclusions and oxides in the
casting chamber during the filling phase of the moulding cavity, as
firstly the oxide formation is reduced and secondly by retention of
form of the metal bolt during pretreatment, air inclusions through
bolt deformations are avoided.
[0073] The said advantages have a direct influence on the quality
of the moulded parts produced; this reduces the rejection rate.
[0074] The pretreatment device according to the invention and the
process according to the invention are suitable for provision of
thixotropic metal bolts in vertical or horizontal casting chambers.
Preferred applications of the process according to the invention
are described in the application claims 18 and 19.
EXAMPLE
[0075] To verify the heating principle in a heating tube a circular
cylindrical aluminium bolt of diameter 100 mm and length 200 mm is
heated in the vertical position in an oven with resistance heating
to the necessary temperature above the solidus temperature, where
the final temperature and the time-dependent temperature profile of
the heating oven are selected such that at the end of the heating
process a thixotropic metal bolt is present with a liquid part of
around 50 w. %. During the heating process the aluminium bolt is in
a heating tube made of special stainless steel with a wall
thickness of 5 mm. The heating tube and hence also the metal bolt
lie at the lower end on a heat insulation plate. At the upper end
of the heating tube its circular upper edge projects around 5 mm
above the top bolt edge. The upper end of the heating tube is not
closed so that the length change can be measured by means of a
laser interferometer throughout the heating process.
[0076] The bolt temperature is assessed continuously during the
heating process by means of thermo-elements lying parallel to the
bolt longitudinal axis, where--in relation to the concentric
longitudinal axis of the aluminium bolt--a first thermo-element to
measure the edge temperature T.sub.0 is introduced in the edge area
of the aluminium bolt, a second thermo-element to measure
temperature T.sub.1 is positioned in the centre between the bolt
centre and bolt edge, and a third thermo-element is arranged around
5 mm from the bolt centre to measure temperature T.sub.2. The
thermo-elements are inserted around 50 mm deep into the bolt. The
time-dependent temperature profiles T.sub.0(t), T.sub.1(t) and
T.sub.2(t) measured with the said three thermo-elements are shown
in FIG. 3 and within a measurement accuracy of around 1%
substantially all show the same temperature development.
[0077] The length change in the metal bolt measured during the
heating profile shown in FIG. 3 is shown in FIG. 4. This shows that
the aluminium bolt until reaching the solidus temperature expands
in the linear direction by around 1.5 mm, where above the solidus
temperature the thermal length expansion increases greatly.
[0078] To study the dimensional stability of the thixotropic bolt
an aluminium bolt according to the invention was heated in the
vertical position until the thixotropic bolt had a liquid part of
around 50 w. %, then removed from the oven, transferred to a
horizontal position and pushed out of the heating tube. The test of
the geometric shape of the thixotropic aluminium bolt shows that
dimensional stability exists, i.e. the thixotropic bolt has in
essence, apart from the thermal expansion, the same shape as the
original aluminium bolt.
[0079] It is also found that during the heating process only very
little liquid metal emerges from the metal bolt. Also, the
thixotropic bolt retains its smooth surface even during the heating
process. No oxidation traces can be detected on the surface. The
test of the liquid metal distribution by means of a cutting test
also shows that the homogeneity of the thixotropic state is also
very well achieved.
[0080] Further advantages, features and details of the invention
arise from the following description of FIGS. 1 to 4 and the
drawings.
[0081] FIG. 1 shows diagrammatically the temporal sequence of the
substantial process steps for provision of a thixotropic metal bolt
in the casting chamber of a horizontal thixoforming device, where
the metal bolt is transferred to a thixotropic state in the
horizontal position;
[0082] FIG. 2 shows diagrammatically the temporal sequence of the
substantial process steps for provision of a thixotropic metal bolt
in the casting chamber of a horizontal thixoforming device, where
the metal bolt is heated in a vertical position;
[0083] FIG. 3 shows as an example a typical heating curve;
[0084] FIG. 4 shows as an example a typical temperature-dependent
deformation curve of a metal bolt during the heating process
according to the invention.
[0085] Drawings a) to c) of FIG. 1 each show a vertical linear
section along the concentric longitudinal axis l of a metal bolt
10, through the device elements 14, 20, 30 respectively, in which
the metal bolt 10 lies during pretreatment, where the heating
process of metal bolt 10 takes place in a horizontal position.
[0086] FIG. 1a) shows the loading of a metal bolt 10 in a solid
state of aggregation into a horizontal heating tube 14. The heating
tube 14 is closed with stopper-like sealing elements 16, 18, where
the sealing elements 16, 18 lie on one side on the surfaces 15 of
the heating tube 14 and on the other side close flush with the
metal bolt 10 i.e. the sealing elements 16, 18 lie within the
heating tube 14 on the surfaces 12 of the metal bolt 10.
[0087] The heating tube 14 containing the metal bolt 10 and closed
with the heating elements 16, 18 is inserted horizontally in the
heating chamber 21 of an induction oven 20. The heating tube 14 is
here in the centre of the heating chamber 21 surrounded by
induction coils 22, i.e. the concentric longitudinal axis of the
heating chamber 21 on the concentric longitudinal axis l of the
metal bolt 10 coincide. During the heating process the metal bolt
expands initially in all directions. When the metal bolt reaches
its solidus temperature T.sub.solidus, the metal bolt 10 presses
against the heating tube 14 so that the metal bolt 10 in essence
cannot expand further radially i.e. further radial expansion of the
metal bolt 10 is restricted to the normally very low radial
expansion of the heating tube 14. Thereafter, the further thermal
expansion of the metal bolt 10 after reaching the solidus
temperature T.sub.solidus is possible in essence only in the
direction of its concentric longitudinal axis l, where the sealing
elements 16, 18 are pushed apart corresponding to the thermal
expansion of the metal bolt 10 so that the stopper-like sealing
elements 16, 18 no longer lie on the surfaces 15 of the heating
tube 14.
[0088] FIG. 1b) shows the discharging of the induction oven 20 i.e.
extraction of the heating tube 14 containing the thixotropic metal
bolt 10 from the heating chamber 21 of the induction oven 20. The
sealing elements 16, 18 are separated from the heating tube 14
after discharging from the induction oven 20. The liquid metal melt
24 which emerged from the metal bolt 10 during the heating process
is then removed from the heating tube 14 by allowing the liquid
metal 24 to drip from the heating tube 14, where the liquid metal
is caught for example in a catchment dish (not shown).
[0089] FIG. 1c) shows the heating tube 14 introduced in a casting
chamber 30 of a horizontal thixoforming device. The heating tube 14
is positioned in the casting chamber cavity 32 of the casting
chamber 30 such that during the subsequent thixoforming process,
the plunger 34 can push the thixotropic metal bolt 10 out of the
heating tube 14 so that the thixotropic metal alloy is then
introduced through the passage opening 36 into the sprues (not
shown) and thence into the moulding cavity (not shown). The casting
chamber 30 has a recess to hold the heating tube. This recess
serves firstly to centre the heating tube 14 and secondly as a stop
for confining the heating tube 14 during the emergence of the
thixotropic metal bolt 10 at the start of the thixoforming
process.
[0090] Drawings a) to e) in FIG. 2 each show a vertical linear
section along the concentric longitudinal axis/of a metal bolt 10,
through device elements 14, 20, 30 respectively, in which the metal
bolt 10 is in the pretreatment phase, where the heating process of
the metal bolt 10 takes place in a vertical bolt position.
[0091] FIG. 2a) shows the introduction of a metal bolt 10 in a
vertical heating tube 14 into a vertical cylindrical heating
chamber 21 of an induction oven 20. The heating tube 14 is closed
at its lower tube end i.e. at the bottom surface 15 of the heating
tube 14 with a stopper-like sealing element 16. The sealing element
lies on a table plate 26. The heating tube 14 containing the metal
bolt 10 is inserted in the induction oven 20 through vertical
placement of the heating tube 14 on the table plate 26, where the
sealing element 16 comes to lie on the table plate 26, and by
raising the table plate 26 until the heating tube 14 comes to lie
fully in the heating chamber 21 of the induction oven 20. The
heating tube 14 is centrally positioned in the heating chamber 21
bordered by the induction coils 22 i.e. the concentric longitudinal
axis of the heating chamber 21 coincides with the concentric
longitudinal axis l of the metal bolt 10.
[0092] During the heating process the metal bolt initially expands
both in the radial and vertical directions. When the metal bolt
reaches its solidus temperature T.sub.solidus, the metal bolt 10
presses radially against the heating tube 14 so that the metal bolt
10 can in essence not expand further radially i.e. the further
radial expansion of the metal bolt 10 is restricted to the normally
very low radial expansion of the heating tube 14. Then the further
thermal expansion of the metal bolt 10 after reaching the solidus
temperature T.sub.solidus is possible in essence only in the
vertical direction parallel to its concentric longitudinal axis l.
The upper tube end 15 of the heating tube 14 is open so the metal
bolt 10 can expand thermally upwards unhindered.
[0093] FIG. 2b) shows the induction oven 20 after the heating tube
14 with thixotropic metal bolt 10 has been removed from the heating
chamber 21. The thixotropic metal bolt is here withdrawn by
lowering the table plate 26.
[0094] FIG. 2c) shows the heating tube 14 withdrawn vertically from
the oven and containing the thixotropic metal bolt 10, standing
vertically on the table plate 26 and still tightly sealed with the
stopper-like sealing element 16.
[0095] FIG. 2d) shows the heating tube 14 containing the
thixotropic metal bolt 10 separated from the table plate 26 and the
sealing element 16, in a horizontal position. Suitably, the heating
tube 14 is separated from the sealing element 16 and transferred to
a horizontal position by means of a robot. For this, the force
required by the robot arm for raising the heating tube 14 from the
table plate 26 and the sealing element 16 is very low.
[0096] The sealing element 16 closes the heating tube 14 as a tight
form fit. The seal however need merely prevent the emergence of
liquid metal so that, because of the surface tension of a liquid
metal, the sealing element 14 in essence need only engage as a
form-fit in the heating tube 14 and therefore no high friction is
required between the heating tube 14 and the sealing element
16.
[0097] The thixotropic metal bolt is clamped in the heating tube
i.e. its adhesion is sufficiently large that the heating tube can
be raised vertically from the sealing element without the
thixotropic metal bolt 10 falling out of the heating tube 14. The
vertical lifting of the heating tube 14 from the sealing element 16
defined on the table plate 26 also allows the dripping of the
liquid metal 24 formed during the heating process outside the
heating oven 20.
[0098] FIG. 2e) shows the heating tube 14 introduced into a casting
chamber 30 of a horizontal thixoforming device. Here the heating
tube 14 is positioned in the casting chamber cavity 32 of the
casting chamber 30 such that during the subsequent thixoforming
process the plunger 34 can expel the thixotropic metal bolt 10 from
the heating tube so that the thixotropic metal alloy can then be
introduced through the passage opening 36 into the sprues (not
shown) and thence into the moulding cavity (not shown). The casting
chamber 30 has a recess to hold the heating tube. This recess
serves firstly to centre the heating tube 14 and secondly as a stop
to confine the heating tube 14 during expulsion of the thixotropic
metal bolt 10 at the start of the thixoforming process.
[0099] The thixotropic metal bolt 10 is laid in the casting chamber
cavity 32 of the casting chamber 30 suitably by means of a robot.
The thixotropic metal bolt must be inserted so gently that the form
of the metal bolt 10 is retained after insertion in the casting
chamber 30.
[0100] FIG. 3 shows a typical heating curve, until the solidus
temperature T.sub.solidus is achieved, of an aluminium bolt 10 in a
heating tube 14 according to the invention in a resistance oven.
The heating curve concerns a circular cylindrical aluminium bolt 10
in a heating tube 14 of stainless steel, with a diameter of 100 mm
and a length of 200 mm, in a vertical position, where the heating
tube 14 has a wall thickness of 5 mm and the lower surface 12 of
the aluminium bolt 10 lies directly on a heat insulation plate 26
i.e. the lower surface 12 of the aluminium bolt 10 and the lower
surface 15 of the heating tube 14 lie in the same plane.
[0101] The heating curve, i.e. the time-dependent bolt temperature,
is determined continuously by means of thermo-elements lying
parallel to the bolt longitudinal axis l during the heating
process, where--in relation to the concentric longitudinal axis l
of the aluminium bolt 10--a first thermo-element to measure the
edge temperature T.sub.0(t) is introduced into the edge area of the
aluminium bolt 10, a second thermo-element to measure temperature
T.sub.1(t) is positioned in the centre between the bolt centre and
the bolt edge, and a third thermo-element to measure the
temperature T.sub.2(t) is arranged at a distance of around 5 mm
from the bolt centre. The thermo-elements are inserted around 50 mm
deep in the bolt 10. The time-dependent temperature profiles
T.sub.0(t), T.sub.1(t) and T.sub.2(t) measured with the said three
thermo-elements are drawn in FIG. 3 and--within a measurement
accuracy of .+-.1%--substantially all show the same temperature
development.
[0102] FIG. 4 shows as an example a typical temperature-dependent
deformation curve during the heating process according to the
invention of the aluminium bolt 10 shown by the heating curve in
FIG. 3. FIG. 4 shows that the aluminium bolt 10 expands
substantially linearly, temperature-dependent, by approximately 1.5
mm in the longitudinal direction until the solidus temperature
T.sub.solidus is reached at around 560.degree. C., where above the
solidus temperature the thermal linear expansion .DELTA.L(T)
increases suddenly.
* * * * *