WO2001045880A1

WO2001045880A1 - Pre-treatment of a thixotropic metal bolt

Info

Publication number: WO2001045880A1
Application number: PCT/EP2000/012554
Authority: WO
Inventors: Miroslaw Plata; Christophe Bagnoud; Grégoire Arnold; Martin Bolliger
Original assignee: Alcan Technology & Management Ltd
Priority date: 1999-12-22
Filing date: 2000-12-12
Publication date: 2001-06-28
Also published as: NO20022991L; ES2195954T3; EP1242205B1; EP1242205A1; DE50002685D1; NO20022991D0; AU3157201A; ATE243590T1; EP1110643A1; JP2003517933A; CA2395161A1; US20030056930A1

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).

Description

Pretreatment of a thixotropic metal bolt

The invention relates to a pretreatment device for providing a thixotropic metal bolt in a casting chamber of a thixoforming device, comprising a container for receiving a metal bolt, an oven for converting the metal bolt in the container into a semi-liquid, thixotropic state, and a transport device for transporting and Inserting the thixotropic metal bolt into the casting chamber and using the pretreatment device. The invention further relates to a corresponding method according to the features of the preamble of claim 12, and the use of the method.

Thixoforming relates to the production of molded parts from thixotropic metal bolts. All bolts made of a metal which can be converted into a thixotropic state are suitable as metal bolts. In particular, the metal bolts can consist of aluminum, magnesium or zinc and the alloys of these metals.

The thixotropic properties of partially liquid or partially solid metal alloys are used in thixoforming. The thixotropic properties of a metal alloy mean that a correspondingly prepared metal behaves unloaded like a solid, but reduces its viscosity under shear loads to such an extent that it behaves similarly to a metal melt. This requires heating the alloy in the solidification interval between the liquidus and solidus temperature. The temperature is to be set so that, for example, a structural proportion of 20 to 80% by weight is melted, but the rest remains in solid form.

In the case of thixoforming, partially solid / partially liquid metal is processed into molded parts in a modified die casting machine, a so-called thixoforming device. The die casting machines used for thixoforming differ from the die casting machines for die casting metal melts by, for example, a longer casting chamber for receiving the thixotropic metal pin and a larger piston stroke required as a result, and, for example, a mechanically reinforced design of the parts of the die casting machine that guide the thixotropic metal alloy as a result of the higher pressure load on these parts during thixoforming. The metal bolts are usually heated in a separate oven. The furnaces can be heated with fuel, for example gas or oil, or electrical energy, for example by means of resistance heating or by means of inductive energy input.

The heating of the metal bolts is of great importance in relation to the great influence that the state of the bolt inserted into the casting chamber has on the quality of the product, since:

- the bolt condition, i.e. its partial strength, usually only in a small temperature range,

long heating-up times, for example due to the formation of a thick oxide skin or possible coarsening of the grain, should be avoided,

- And to achieve a homogeneous end product, the temperature distribution in the thixotropic metal bolt, the so-called thixo blank, should be as homogeneous as possible.

Therefore, the metal bolt is brought into the thixotropic state, i.e. the stud is heated until the desired proportion of alloy has melted, expediently by means of an oven temperature controlled by sensors.

To heat the metal bolts, they are usually placed in a bowl-shaped container, for example in a metal bowl made of stainless steel, or a crucible made of clay-graphite or clay-SiC and transferred to the thixotropic state in a horizontal position.

The thixotropic metal bolt can then, for example in the same container, be transferred to the casting chamber of a horizontal thixoforming device by means of, for example, a gripper and introduced into the casting chamber by tilting the container. In this case, the metal bolt remains in the same container during the heating process and the transport to the casting chamber.

EP-A-0 645 206 describes a device for the production of mechanically highly stressed parts by thixoforming, which contains a pretreatment device described at the beginning. The metal bolts are placed in cup-shaped containers in a tubular continuous furnace in the thixotropic Transferred state, and the container containing the thixotropic bolt is transported to the casting chamber by a robot, and by tilting the container, the thixotropic bolt is conveyed into the casting chamber.

EP-B-0 713 736 describes a holding device for inductively heating bolts made of metal alloys with thixotropic properties and for holding and transporting the bolts until they are cast. The holding device is a specially designed trough-shaped shell.

The technical solutions used in practice with regard to bolt heating and the bolt transport of thixotropic metal bolts often neither guarantee sufficient thermal homogeneity of the bolt inserted into the casting chamber, nor the repeatability of the heating process necessary to achieve stable quality. The thermal inhomogeneity manifests itself, for example, in an irregular distribution of the liquid portion with respect to the bolt cross section, this often leading to local melting of the metal bolt, in particular at the metal bolt ends. As a result, the geometry of the bolt can change significantly (ovalization, crater), which usually causes air and / or aluminum oxide to penetrate into the mold cavity in the initial phase of the thixoforming process and, as a consequence, leads to an increase in scrap.

A further problem during the heating process can arise from the leakage of liquid metal from the bolt, since such liquid metal can escape to the outside during the transport of the container, for example still inside the furnace, which often affects the inside of the furnace, especially when used of an induction furnace, damaged. It should be noted here that the volume of liquid metal emerging from the metal bolt during the bolt heating process can typically be up to 10% of the bolt volume.

A further problem caused by the elimination of liquid metal during the heating process can arise if liquid metal is poured into the casting chamber, since this can cause pre-solidification in the casting chamber, which then causes a multitude of defects, such as air pockets or structural inhomogeneities, in the thixo Can cause molding. The object of the present invention is to avoid the aforementioned disadvantages of the prior art and to specify a pretreatment device and a method for the reproducible provision of thixotropic metal bolts in a casting chamber of a thixoforming device, the thixotropic metal bolts having a homogeneous temperature distribution and one over the entire bolt cross section and entire bolt length has a homogeneously distributed liquid component, and the leakage of liquid metal into the heating furnace and the introduction of liquid metal into the casting chamber are avoided. Another object of the present invention is to maintain the stud shape during the heating process and the transport of the thixotropic stud to the casting chamber and during its insertion into the casting chamber. A still further object of the present invention is seen in the possibility of increasing the transport speed, since this reduces the cooling that occurs during the transport of the billets from the furnace to the casting chamber.

According to the invention, this is achieved in that the container is a cylindrical, laterally closable heating tube, and the pretreatment device is designed such that the metal bolt during the entire pretreatment, namely the heating process in the furnace and the transport into the casting chamber and the lingering in the Casting chamber until the start of the thixoforming process, can remain in the heating tube.

Further advantageous refinements of the pretreatment device according to the invention are described in claims 2 to 9.

The new concept according to the invention, according to which the metal bolt is located in a hollow cylindrical container, the so-called heating tube, during the entire pretreatment process:

- guarantees homogeneous and symmetrical heating in the axial and radial direction of the bolt;

minimizes the inhomogeneities of the magnetic field at the edges of the stud during the heating process in an induction furnace;

- prevents excessive melting of the stud surface;

- ensures the shape of the pin during the heating process, the transport to the casting chamber and in the casting chamber; - eliminates the influence of any existing structural inhomogeneities and the chemical composition of the metal bolt on the heating process;

- allows the use of cheaper and more efficient heating by means of an induction field with a higher frequency than in the prior art;

- avoids - apart from the metal pin and the heating tube - the need and the presence of other metal elements within an induction furnace, which minimizes the disturbance of the homogeneity of the magnetic field within an induction furnace; - Enables a more precise control of the heating process compared to the prior art by direct measurement of the temperature of the heating pipe and the thermal linear expansion of the metal bolt that occurs during the heating process, the linear expansion of the metal bolt during the heating process being above the solidus temperature of the stud material proportional to the liquid portion is;

- eliminates the need to use compensation plates on the bolt ends within an induction furnace to homogenize the magnetic field, which lowers the cost of the pretreatment device and increases its reliability; and - enables faster transport of the thixotropic metal bolt from the heating furnace into the casting chamber compared to the known prior art.

The pretreatment device according to the invention is suitable for all metal bolts made of commercially available alloys which can be converted into a thixotropic state. Particularly suitable metal stud materials are alloys made of aluminum, magnesium or zinc. In particular, cast aluminum and wrought aluminum alloys are preferred. The pretreatment device according to the invention is advantageously also suitable for processing particle-reinforced aluminum alloys which contain, for example, homogeneously distributed SiC or Al ₂ O 3 particles. The pretreatment device according to the invention is very particularly suitable for aluminum alloys which have a pronounced solidification interval, such as AISi7Mg.

The metal bolts expediently contain homogeneously distributed, primarily solidified solid particles which consist of individual degenerate dentrites. Before- the proportion of primarily solidified solid particles is between 40 and 80% by weight. In order to achieve good thixotropic behavior, for example in aluminum alloys the alpha mixed crystal must be in a globulistic form in order to achieve a uniform flow of melt and solid.

The degenerate dentrites of the metal bolts generally generally have a globulistic shape, as a result of which a uniform, homogeneous flow of melt and solid can be achieved without segregation. The production of metal bolts with a structure with globulistic dentrites takes place, among other things. through a continuous casting process, combined with intensive electromagnetic stirring even during the solidification phase. This leads to melting and breaking off of dentrite arms, which form near the solidus temperature and form the globulistic structure.

The metal bolts required for the thixoforming are preceded to the thixoforming process by means of the pretreatment device according to the invention to a temperature above the solidus temperature and below the liquidus temperature, i.e. heated to a partially solid, thixotropic state.

In the semi-solid state, the thixotropic alloy, the so-called thixotropic alloy pulp, contains the reverse developed dentritic, primary-solid particles in a matrix of liquid metal surrounding them. The thixotropic alloy slurry preferably contains a liquid fraction of 40% by weight to 50% by weight and in particular between 43% by weight and 48% by weight.

During the heating process, the metal bolts can be in a vertical or horizontal position with respect to their longitudinal axis.

An induction furnace is preferably used to convert the metal bolt into a thixotropic state. For example, a primary coil is arranged around the container, the alternating current flowing in the primary coil inducing an alternating magnetic field in the container and / or in the metal bolt. The heating tube or the metal bolt is heated by the alternating magnetic field, which causes vigorous eddy currents in the heating tube or the metal bolt and thus causes a corresponding heating. The penetration depth of the eddy currents and thus the depth of the heated layer is frequency-dependent; if high frequencies are used, rapid heating predominantly of the layers near the surface takes place.

In an advantageous embodiment of the heating tube, this consists of a metal. Metals from the series of iron-carbon-containing metals, such as steel, stainless steel, Thermax steel, hot-work steel or from the series of metals tantalum, niobium, vanadium, tungsten or titanium or alloys thereof are preferably used for this purpose. Heating tubes made of copper or its alloys are further preferred. Heating tubes made of steel and in particular made of stainless steel or tool steel are particularly preferred.

If a metal heating tube, in particular a steel heating tube, is used for the heating process in an induction furnace, the magnetic field essentially only penetrates the heating tube, so that the induction furnace essentially only heats up the heating tube directly and the heating of the metal bolt almost exclusively by heat conduction happens from the heating pipe in the metal bolts. The stud material is thus heated by heating the heating tube and by conduction from the heating tube to the stud material. Due to the high thermal conductivity of a metal heating tube, there is a radially symmetrical heat conduction from the heating tube to the metal bolt, provided that the metal bolt is in thermal contact with the heating tube over its entire circumference. The radially symmetrical heat conduction then also results in a radially symmetrical temperature distribution in the metal bolt, a low radial temperature gradient being established very quickly due to the high thermal conductivity of the metal bolt.

The heating device according to the invention with a metal heating tube achieves a very homogeneous temperature and thus also a very homogeneous liquid metal distribution in the entire metal bolt. This is particularly because the direct introduction of energy into the heating tube occurs and the stud material is only indirectly heated by heat conduction, so that any local differences in the structure or chemical composition of the stud material have no influence on the direct introduction of energy.

According to a further preferred embodiment of the heating tube, this can consist of ceramic material. When using such heating tubes in an induction furnace, the magnetic field penetrates the heating tube, ie 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 furnace.

Suitable ceramic materials are, for example, Al ₂ 0 ₃ , AL ₃ 0 ₄ , BN, SiC, Si ₃ N, MgO, TiO, Zr0 ₂₎ stabilized, such as yttrium-stabilized Zr0 ₂ , glasses or refractory cements or mixtures containing the materials mentioned contain. More preferably, the heating tube can consist of fiber-reinforced ceramic material or contain such materials, and the fibers of the fiber-reinforced ceramic material can be made of SiC, Al ₂ O ₃ , glass or carbon, for example.

To ensure that the metal bolt is inserted into the heating tube, the inner diameter d _{R of} the heating tube in the cold state, in particular at room temperature, ie at a temperature of 15 ° C. to 30 ° C., is expediently somewhat larger than the bolt diameter d _{B of} the cold metal bolt ,

The inner diameter cfa of the heating tube as a function of the pin diameter αfe is preferably chosen such that the metal pin in the cold state, in particular at room temperature, ie at a temperature of 15 ° C. to 30 ° C., by a dimension Δd of approximately 0.5 mm , preferably 0.5 ± 0.3 mm, in particular 0.5 ± 0.1 mm, is smaller than the inner diameter d _{R of} the heating tube.

During the heating process, the metal bolt expands in the radial and axial direction, so that at a certain point in time the bolt diameter de corresponds to the inside diameter cfa of the heating tube, it having to be taken into account that the inside diameter d _{R is} also temperature-dependent.

When using a metal heating tube, especially when using a steel heating tube, a very even, radially symmetrical heat transfer from the heating tube to the metal bolt is ensured during the heating process of the metal bolt - as soon as the bolt diameter de equals the inside diameter d _{R of} the heating tube due to the thermal expansion. A radially symmetrical, even heat transfer is particularly important after reaching the solidus temperature of the metal bolt, since a locally elevated temperature in a temperature range above half the solidus temperature causes a corresponding local melting of the stud material and consequently should be avoided.

Within the scope of the inventive step, it has been shown that the use of a thin lubricating layer to prevent the metal bolt from adhering to the heating tube has practically no effect on the heat transfer between the bolt and the heating tube. It should be noted that the lubricants used as release agents - if necessary at all - are mostly only used for metal heating pipes, i.e. When using heating pipes made of ceramic material, no release agents are usually required.

In a preferred embodiment of the pretreatment device according to the invention, the material of the heating tube and the inner diameter d _{R of} the heating tube are selected such that the metal bolt at its solidus temperature T _SO ii _d u _{s has} essentially the same diameter d _B as the inner diameter d _{R of} the heating tube , More preferably, the bolt diameter d _B in Tsoii _d u _s fulfills the relationship 0.996 d _R d d _B d d _R , particularly preferably 0.998 d _R d d _B d d _R and in particular 0.999 d _R d d _B d d _R.

The metal bolts are cylindrical and generally have a round or oval cross section, but can also be a polygonal cross section. The diameter of the metal bolts in the cold state is, for example, 50 to 180 mm, advantageously 75 to 150 mm and preferably 100 to 150 mm. The length of the metal bolts is, for example, 80 to 500 mm when cold.

When the metal bolt is heated, the metal bolt and the heating tube expand. Different materials have different coefficients of thermal expansion. The coefficient of thermal expansion of steel or ceramic material is considerably smaller than that of, for example, aluminum or aluminum alloys. Accordingly, a metal bolt made of an aluminum alloy, for example, expands more than a heating tube made of, for example, steel or ceramic material, so that - starting from a smaller diameter of the metal bolt in the cold state compared to the inner diameter of the heating tube - the metal bolts at a certain temperature are the same Diameter as the heating tube has. According to the invention it is preferred now is that at the solidus temperature T _SO ii _d _ _ιs of the metal pin material of the diameter corresponds to the metal bolt substantially to the diameter of the heating tube, so that in the temperature range in which the thixotropic properties of the metal bolt to be adjusted substantially, optimal thermal contact is formed between the heating tube and metal bolt. When the heating tube containing the metal bolt is heated above the solidus temperature T _SO iidus, the eutectic is melted together with an increase in volume of the metal bolt. The eutectic of aluminum alloys suitable for thixoforming typically forms at approx. 550 to 570 ° C. The volume increase in the range between solidus and liquidus temperature is typically approx. 1.0 to 5.5% and in particular between 1.5 to 3% for thixotropic aluminum alloys. In the heating or heating phase above the solidus temperature T _SO ii _dus , which is typically around 520 to 550 ° C in the case of aluminum alloys suitable for thixoforming, the stud material can only expand in the longitudinal direction of the heating tube, provided that the condition according to which at T _SO ιidus the bolt diameter d _B corresponds to the diameter d _{R of} the heating tube is met. In the case of aluminum alloys, the increase in volume between the solidus and liquidus state - depending on the bolt length - manifests itself in an increase in the bolt length of typically approx. 3 to 16 mm and in particular 3 to 6 mm. The increase in length of an aluminum bolt below the solidus temperature is - depending on the bolt length - typically between 1 and 2 mm.

Due to the change in length of the metal bolt during its transition to the thixotropic state and for the at least partial insertion of closure elements into the cavity of the heating tube, the length of the heating tube must be greater than the bolt length. The length of the heating tube is preferably selected such that the heating tube is a total of approximately 5 to 30 mm, in particular 10 to 20 mm, longer than the metal bolt to be heated therein. When the metal bolt is heated in a horizontal position, the end face of the heating tube protrudes on each side of the heating tube, preferably by approximately 2.5 to 15 mm and in particular by 5 to 10 mm, relative to the end face of the metal bolt. When the metal bolt is heated in a vertical position, the end face of the heating tube at the upper end of the tube protrudes from the end face of the metal bolt preferably by approximately 5 to 30 mm and in particular by 10 to 20 mm. The wall thickness of the heating tube is preferably 1 to 5 mm for steel tubes, preferably 4 to 10 mm for copper tubes and 8 to 15 mm for ceramic tubes.

The heating tube according to the invention can be closed on both sides. Closure elements made of ceramic material are preferably used for this purpose. The same materials described above are suitable as ceramic materials for the closure elements as for the one preferred embodiment variant of the heating tube made of ceramic material. Ceramic material has a low thermal conductivity compared to the metal bolt material, so that the radial temperature distribution at the front edge regions of the metal bolt is only slightly influenced by such closure elements.

If the metal bolt is essentially horizontal during the heating process, the heating tube is expediently sealed on its two end faces by means of closure elements, preferably plug-like or peg-shaped closure elements. The tightness relates to the avoidance of liquid metal escaping from the heating tube during the heating process.

More preferably, the plug-like or peg-shaped closure elements are designed with regard to the choice of material and shape such that their friction properties in the heating tube on the one hand allow a displacement in the direction of the longitudinal axis of the heating tube caused by the thermal expansion of the metal bolt during the heating process and on the other hand a displacement by that of the metal bolt the closure elements applied pressure is avoided after reaching the temperature required for the desired thixotropic state. Prior to the heating process, the closure elements are preferably pushed into the heating tube to such an extent that they are in direct mechanical contact with the end faces of the metal bolt. As a result, the closure elements move during the heating process in the heating tube due to the thermal expansion of the metal bolt.

In the case of a metal bolt position which is essentially vertical during the heating process, the heating tube is sealed on one side and tightly at the lower end of the tube.

Here too, the tightness only refers to the avoidance of liquid metal flowing out of the heating pipe. The heating pipe can be side directly on a preferably height-adjustable table top, preferably on a table top made of ceramic material, or the heating tube can be sealed by means of a closure element, preferably by a plug-shaped or peg-shaped closure element, and by means of this closure element on a preferably height-adjustable table top any heat-resistant material in a vertical position. In particular when using a stopper-shaped or peg-shaped closure element, the closure element is preferably designed with regard to the choice of material and shape such that, on the one hand, no liquid metal can escape from the heating tube during the heating process in the furnace and, on the other hand, the friction of the closure element in the heating tube is less than 10 N.

The friction of the closure element must be low so that a gripper arm of a transport device, in particular a robot gripper arm, can heat the heating tube without great effort, i.e. without the gripper arm having to be designed for large mechanical loads, can lift off the locking element fixed on the table top. On the other hand, a high level of friction is not necessary to achieve a high level of tightness, since the closure element only has to prevent the leakage of liquid metal during the heating process and, because of the cohesion of molten metals, in particular aluminum melts, this does not require a high level of tightness. The friction of the closure element in the heating tube is expediently less than 30 N, preferably between 2 and 20 N and in particular between 5 and 10 N.

The pretreatment device according to the invention is suitable for providing a thixotropic metal bolt in a casting chamber of a vertical or horizontal thixoforming device. However, this pretreatment device is particularly advantageous for the provision of a thixotropic metal bolt in a horizontal casting chamber, since the shape retention can be ensured particularly well with the device according to the invention. In the case of a horizontal thixoforming device, the casting chamber, which receives the thixotropic metal bolt, lies horizontally.

The device according to the invention is particularly advantageous for the provision of thixotropic metal bolts made of aluminum or aluminum alloys. Aluminum bolts are very particularly preferably heated in an induction furnace with a vertical heating space. The object of the method is achieved according to the invention in that the container is a cylindrical heating tube, the metal pin always remains in the heating tube during the heating process and the subsequent transport into the casting chamber, and the heating tube containing the metal pin is positioned in the casting chamber in this way becomes that during the subsequent thixoforming process the casting piston of the thixoforming device can push the thixotropic metal bolt out of the heating tube.

Preferred developments of the method according to the invention are described in the dependent claims 13 to 17. The statements made for the pretreatment device according to the invention with regard to special features and details also apply analogously to the method according to the invention.

The heating tube containing the thixotropic metal bolt is transported to the casting chamber, for example by a robot, after the heating process and after any dripping of molten metal that has escaped from the metal bolt during the heating process, and is inserted into the front, half-open part of the casting chamber. After pretreatment, i.e. at the beginning of the actual thixoforming process, the casting piston pushes the metal bolt out of the heating pipe into a closed part of the casting chamber; The thixotropic metal alloy is then introduced through a through opening into the pouring channels and then into the mold cavity. After the thixoforming process, the casting piston is pulled back, so that a gripper arm of the transport device can then pick up the heating tube from the casting chamber and use it for further pretreatment processes. The return of the heating tube for further use for further pretreatment processes expediently takes place during the solidification phase of the thixotropic metal alloy in the mold cavity. The casting structure that forms in the mold cavity during the solidification of the thixotropic metal alloy essentially determines the properties of the molded parts. The microstructure formation is characterized by the phases, such as mixed crystal and eutectic phases, the cast grain, such as globulites and dendrites, segregations as well as structural defects such as porosity (gas pores, micro-voids) and impurities, such as oxides.

It is preferred after the heating process, but before inserting the heating tube containing the thixotropic metal bolt into the casting chamber, that Liquid metal escaping from the metal bolt during the heating process is at least partially removed from the heating tube. The liquid metal portion emerging from the metal bolt during the heating process is typically less than 1% by weight of the bolt material.

The transport of the thixotropic metal bolt from the heating furnace into the casting chamber by means of a robot typically takes 5 to 30 s and preferably 8 to 15 s. The time period during which the thixotropic metal bolt remains in the casting chamber is typically between 3 and 5 s. This time is required for moving a robot gripper arm out of the casting chamber and for the electronic readiness control of a thixoforming device.

The pretreatment method according to the invention has significant advantages, in particular: it leads to a substantial reduction in the heat losses of the thixotropic metal during the transport from the heating furnace to the casting chamber and in the casting chamber thanks to the heating tube heated up to the same temperature as the metal bolt;

- By depositing the metal bolt inside the heating tube in the casting chamber, in particular in the casting chamber of a horizontal thixoforming device, it eliminates the shock of the fall, which ensures the shape and homogeneity of the metal bolt;

- It avoids the separation of the liquid portion when inserting the thixotropic metal bolt in the casting chamber, so that the associated solidification of liquid metal in the casting chamber is avoided;

- It largely prevents oxidation of the metal stud surface, since the metal stud is not exposed to a free atmosphere during the heating process and the transport into the casting chamber and its stay in the casting chamber until the actual thixoforming process begins;

- It reduces the risk of air pockets and oxides in the casting chamber during the filling phase of the mold cavity, since on the one hand the oxide formation is reduced and on the other hand air pockets due to pin deformation are avoided due to the shape of the metal stud during pretreatment. The advantages mentioned have a direct influence on the quality of the molded parts produced; this reduces the scrap.

The pretreatment device according to the invention and the method according to the invention are suitable for the provision of thixotropic metal bolts in vertical or horizontal casting chambers. Preferred uses of the method according to the invention are described in use claims 18 and 19.

Example:

For the verification of the heating principle in a heating pipe, a circular cylindrical aluminum bolt with a diameter of 100 mm and a length of 200 mm in a vertical position is heated in a furnace with a resistance heater to a required temperature above the solidus temperature, whereby the final temperature and the time-dependent Temperature profile of the heating furnace can be selected such that at the end of the heating process there is a thixotropic metal bolt with a liquid content of approx. 50% by weight. The aluminum bolt is located in a stainless steel heating tube with a wall thickness of 5 mm during the heating process. The heating pipe and thus also the metal bolt lie on a heat insulation plate at the lower end. At the upper end of the heating tube, its circular upper edge protrudes about 5 mm above the upper edge of the bolt. The upper end of the heating tube is not closed, so that the change in length can be measured using a laser interferometer during the entire heating process.

The bolt temperature is recorded continuously during the heating process by means of thermocouples lying parallel to the longitudinal axis of the bolt, whereby - with respect to the concentric longitudinal axis of the aluminum bolt - a first thermocouple for measuring the edge temperature T _{0 is introduced} in the edge region of the aluminum bolt, a second thermocouple for measuring the Temperature Ti is positioned in the middle between the center of the bolt and the edge of the bolt and a third thermocouple for measuring the temperature T _{2 is arranged} approximately 5 mm from the center of the bolt. The thermocouples are inserted approx. 50 mm deep into the bolt. The time-dependent temperature profiles T ₀ (t), T ^ t) and T ₂ (t) measured with the three thermocouples mentioned are shown in FIG. 3 and show - within a measurement accuracy of ± 1% - essentially all the same temperature profile.

The change in length of the metal bolt measured during the heating profile shown in FIG. 3 is shown in FIG. 4. From this it can be seen that the aluminum bolt expands in the longitudinal direction by approximately 1.5 mm until the solidus temperature is reached, the thermal linear expansion increasing sharply above the solidus temperature.

To investigate the dimensional stability of the thixotropic bolt, an aluminum bolt according to the invention is heated in a vertical position until the thixotropic bolt has a liquid content of approximately 50% by weight, then removed from the furnace, transferred to a horizontal position and ejected from the heating tube. The examination of the geometric shape of the thixotropic aluminum bolt shows that the shape is stable, i.e. the thixotropic aluminum bolt has essentially the same shape as the original aluminum bolt, apart from the thermal expansion.

Furthermore, it is found that only very little liquid metal emerges from the metal bolt during the heating process. In addition, the thixotropic bolt keeps its smooth surface even during the heating process. No traces of oxidation can be observed on the surface. The examination of the liquid metal distribution by means of a cutting test further shows that the homogeneity of the thixotropic state is also very well fulfilled.

Further advantages, features and details of the invention emerge from the following description of FIGS. 1 to 4 and from the drawings.

1 schematically shows the chronological sequence of the essential method steps for the provision of a thixotropic metal bolt in the casting chamber of a horizontal thixoforming device, the metal bolt being converted into the thixotropic state in a horizontal position;

FIG. 2 shows schematically the chronological sequence of the essential method steps for the provision of a thixotropic metal bolt in the casting chamber of a horizontal thixoforming device, the metal bolt being heated in a vertical position; Fig. 3 shows an example of a typical heating curve;

4 shows an example of a typical temperature-dependent deformation curve of a metal bolt during a heating process according to the invention.

The drawings a) to c) of Fig. 1 each show a vertical longitudinal section along the concentric longitudinal axis £ of a metal bolt 10, respectively. through the device elements 14, 20, 30 in which the metal bolt 10 is located during the pretreatment, the heating process of the metal bolt 10 taking place in a horizontal position.

1 a) shows the loading of a metal bolt 10 which is in a fixed physical state into a horizontally lying heating tube 14. The heating tube 14 is closed with stopper-shaped closure elements 16, 18, the closure elements 16, 18 abutting the end faces 15 of the heating tube 14 on the one hand and on the other hand close flush with the metal bolt 10, ie the closure elements 16, 18 lie within the heating tube 14 on the end faces 12 of the metal bolt 10.

The heating tube 14 containing the metal bolt 10 and closed with the closure elements 16, 18 is inserted horizontally into the heating space 21 of an induction furnace 20. The heating tube 14 is located in the middle of the heating space 21 enclosed by induction coils 22, ie the concentric longitudinal axis of the heating space 21 and the concentric longitudinal axis 1 of the metal bolt 10 coincide. During the heating process, the metal bolt initially expands in all directions. When the metal bolt reaches its solidus temperature Ts _oüd u _s , the metal bolt 10 _abuts the heating pipe 14, so that the metal bolt 10 essentially does not expand any further radially, ie the further radial expansion of the metal bolt 10 is usually very small radial expansion of the heating tube 14 limited. Accordingly, the further thermal expansion of the metal bolt 10 after reaching the solidus temperature T _SO iidus is essentially only possible in the direction of its concentric longitudinal axis £, the closure elements 16, 18 being pushed apart from one another in accordance with the thermal expansion of the metal bolt 10, so that the plug-shaped ones Closure elements 16, 18 no longer rest against the end faces 15 of the heating tube 14. 1 b) shows the unloading of the induction furnace 20, ie the removal of the heating tube 14 containing the thixotropic metal bolt 10 from the heating space 21 of the induction furnace 20. The closure elements 16, 18 are separated from the heating tube 14 after the induction furnace 20 has been unloaded. Furthermore, the liquid metal melt 24 that emerged from the metal bolt 10 during the heating process is removed from the heating pipe 14 by letting the liquid metal 24 drip from the heating pipe 14, the liquid metal being collected, for example, in a collecting pan (not shown).

1 c) shows the heating pipe 14 inserted into a casting chamber 30 of a horizontal thixoforming device. The heating pipe 14 is positioned in the casting chamber cavity 32 of the casting chamber 30 such that during the subsequent thixoforming process the casting piston 34 removes the thixotropic metal bolt 10 from the heating pipe 14 butt, so that the thixotropic metal alloy can then be introduced through the through opening 36 into the sprue (not shown) and then into the mold cavity (not shown). The casting chamber 30 has a recess for receiving the heating tube. This cutout serves on the one hand for centering the heating tube 14 and on the other hand as a stop for fixing the heating tube 14 during the ejection of the thixotropic metal bolt 10 at the beginning of the thixoforming process.

The drawings a) to e) of Fig. 2 each show a vertical longitudinal section along the concentric longitudinal axis £ of a metal bolt 10, respectively. through the device elements 14, 20, 30, in which the metal bolt 10 is located during the pretreatment, the heating process of the metal bolt 10 taking place in a vertical bolt position.

2 a) shows the insertion of a metal bolt 10 located in a vertical heating tube 14 into a vertically lying, cylindrical heating space 21 of an induction furnace 20. The heating tube 14 is also at the lower end of the tube, ie on the lower end face 15 of the heating tube 14 closed a plug-shaped closure element 16. The closure element lies on a table top 26. The heating tube 14 containing the metal bolt 10 is introduced into the induction furnace 20 by vertically placing the heating tube 14 on the table top 26, the closure element 16 coming to rest on the table top 26, and by lifting the table top 26 until the heating tube 14 is complete comes to rest in the heating chamber 21 of the induction furnace 20. The heating tube 14 is located in the center of the heating space 21 delimited by the induction coils 22, ie the concentric longitudinal axis of the heating space 21 coincides with the concentric longitudinal axis £ of the metal bolt 10.

During the heating process, the metal bolt initially expands in the radial as well as in the vertical direction. When the metal bolt reaches its solidus temperature T _so ii u _s , the metal bolt 10 abuts the heating pipe 14 in the radial direction, so that the metal bolt 10 essentially does not expand any further radially, ie the further radial expansion of the metal bolt 10 is the usual one very small radial expansion of the heating tube 14 is limited. Accordingly, the further thermal expansion of the metal bolt 10 after reaching the solidus temperature T _SO ιidus i is essentially only possible in the vertical direction, parallel to its concentric longitudinal axis £. The upper tube end 15 of the heating tube 14 is open, so that the metal bolt 10 can expand thermally without hindrance.

2 b) shows the induction furnace 20 after the heating tube 14 with the thixotropic metal bolt 10 is led out of the heating space 21. The thixotropic metal bolt is produced by lowering the table top 26.

FIG. 2 c) shows the heating tube 14, which is led out of the furnace in the vertical direction and contains the thixotropic metal bolt 10 and is fixed vertically on the table top 26 and which is also tightly closed with the plug-shaped closure element 16.

2 d) shows the heating tube 14, which is separated from the table top 26 and the closure element 16 and contains the thixotropic metal bolt 10, in a horizontal position. The heating tube 14 is expediently separated from the closure element 16 and the heating tube is transferred into a horizontal position by means of a robot. The force to be used by the robot arm for lifting the heating tube 14 off the table top 26 and the closure element 16 is low.

The closure element 16 closes the heating tube 14 in a form-fitting and tight manner. However, the tightness is only to avoid the outflow of liquid Metal is required, so that due to the surface tension of the liquid metal, the closure element 16 essentially only has to engage in the heating tube 14 in a form-fitting manner and therefore no high friction between the heating tube 14 and the closure element 16 is required.

The thixotropic metal bolt is clamped in the heating tube in this way, i.e. its adhesion is so great that the heating tube can be lifted from the closure element in the vertical direction without the thixotropic metal bolt 10 falling out of the heating tube 14. The vertical lifting of the heating tube 14 from the closure element 16 fixed on the table top 26 also allows the liquid metal 24 formed during the heating process to drip outside the heating furnace 20.

2 e) shows the heating tube 14 inserted into 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 casting piston 34 removes the thixotropic metal bolt 10 from the heating tube 14 butt, so that the thixotropic metal alloy can then be introduced through the through opening 36 into the sprue (not shown) and then into the mold cavity (not shown). The casting chamber 30 has a recess for receiving the heating tube. This recess serves on the one hand for centering the heating tube 14 and on the other hand as a stop for fixing the heating tube 14 during the pushing out of the thixotropic metal bolt 10 at the beginning of the thixoforming process.

The thixotropic metal bolt 10 is expediently inserted into the casting chamber cavity 32 of the casting chamber 30 by means of a robot. The thixotropic metal bolt must be inserted so gently that the shape of the bolt 10 is guaranteed after insertion into the casting chamber 30.

FIG. 3 shows a typical heating curve until the solidus temperature T _so ii _{dus of} an aluminum bolt 10 located in a heating tube 14 _{according to the} invention is reached in a resistance furnace. The heating curve relates to a circular cylindrical aluminum bolt 10 with a diameter of 100 mm and a length of 200 mm in a heating pipe 14 made of stainless steel Position, the heating tube 14 has a wall thickness of 5 mm and the lower end face 12 of the aluminum bolt 10 rests directly on a heat insulation plate 26, ie the lower ledge side 12 of the aluminum bolt 10 and the lower end face 15 of the heating tube 14 lie in the same plane.

The heating curve, i.e. the time-dependent bolt temperature, is continuously recorded during the heating process by means of thermocouples lying parallel to the longitudinal axis of the bolt £, whereby - with respect to the concentric longitudinal axis £ of the aluminum bolt 10 - a first thermocouple for measuring the edge temperature T ₀ (t) in the edge region of the Aluminum bolt 10 is inserted, a second thermocouple for measuring the temperature Tι (t) is positioned in the middle between the center of the bolt and the edge of the bolt and a third thermocouple for measuring the temperature T ₂ (t) is arranged approximately 5 mm from the center of the bolt. The thermocouples are inserted approximately 50 mm deep into the bolt 10. The time-dependent temperature profiles To (t), Tι (t) and T ₂ (t) measured with the three thermocouples mentioned are shown in FIG. 3 and show - within a measurement accuracy of ± 1% - essentially all the same temperature profile.

FIG. 4 shows an example of a typical temperature-dependent deformation curve during the heating process of an aluminum bolt 10 according to the invention shown in FIG. 3 by the heating curve. From FIG. 4 it can be seen that the aluminum bolt 10 reaches about 560 until the solidus temperature T _SO iidus is reached ° C in the longitudinal direction essentially linearly temperature-dependent by approx. 1.5 mm, whereby the thermal linear expansion ΔL (T) increases abruptly above the solidus temperature.

Claims

claims

1. pretreatment device for providing a thixotropic metal bolt (10) in a casting chamber (30) of a thixoforming device, comprising a container for receiving a metal bolt (10), a furnace (20)

Transfer of the metal bolt (10) in the container into a partially liquid, thixotropic state, and a transport device for transporting and inserting the thixotropic metal bolt (10) into the casting chamber (30), characterized in that the container has a cylindrical, laterally closable heating tube ( 14), and the pretreatment device is designed such that the metal bolt (10) during the entire pretreatment, namely the heating process in the furnace (20) and the transport into the casting chamber (30) and the dwelling in the casting chamber (30) until Start of the thixoforming process, which can remain in the heating tube (14).

2. Pretreatment device according to claim 1, characterized in that the inner diameter C / R of the heating tube (14) is chosen in dependence on the bolt diameter d _B such that the metal bolt (10) at its

Solidus temperature T _SO ιi _d u _{s has} essentially the same diameter d _B as the inner diameter CR of the heating tube (14).

3. Pretreatment device according to claim 1 or 2, characterized 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, characterized in that the heating tube (14) consists of ceramic material.

5. Pretreatment device according to one of claims 1 to 4, characterized in that when the metal bolt (10) is in a substantially horizontal position during the heating process, the heating tube (14) is sealed on both sides by closure elements (16, 18). Closing elements (16, 18) with regard to choice of material and shape are formed in this way are that their frictional properties in the heating tube (14) on the one hand allow a shift in the direction of the longitudinal axis £ of the heating tube (14) due to the thermal expansion of the metal bolt (10) during the heating process and on the other hand a shift due to the from the metal bolt (10) to the Closure elements (16, 18) exerted pressure after reaching the temperature required for the desired thixotropic state is avoided.

6. Pretreatment device according to one of claims 1 to 4, characterized in that in 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), wherein when using a closure element (16) the closure element (16) is designed with regard to the choice of material and shape such that on the one hand no liquid metal (24) can escape from the heating tube (14) during the heating process in the furnace (20) and on the other hand the friction of the closure element (16) in the heating tube (14 ) is less than 10 N.

7. Pretreatment device according to claim 5 or 6, characterized in that the closure elements (16, 18) consist of ceramic material.

8. Pretreatment device according to one of claims 1 to 7, characterized in that the furnace (20) is an induction furnace.

9. pretreatment device according to one of claims 1 to 8, characterized in that the transport device is a robot, wherein the clamping device of the robot for holding the heating tube consists of ceramic material at least on the surface directed against the heating tube.

10. Use of the pretreatment device according to one of claims 1 to 9 for the provision of a thixotropic metal bolt (10) in a casting chamber (30) of a horizontal thixoforming device.

11. Use of the pretreatment device according to one of claims 1 to 9 for the provision of a thixotropic metal bolt (10) made of an aluminum alloy.

12. A method for providing a thixotropic metal bolt (10) in a casting chamber (30) of a thixoforming device, wherein a metal bolt (10) is placed in a solid state in a container and the metal bolt (10) in the container in an oven (20) as long is heated until the metal bolt (10) is in a thixotropic state and the thixotropic metal bolt (10) is introduced into the casting chamber (30) of a thixoforming device with the aid of a transport device, characterized in that the container is a cylindrical heating tube (14) represents, the metal bolt (10) during the heating process and the subsequent transport into the casting chamber (30) always remains in the heating pipe (14), and that

Heating pipe (14) containing metal bolts (10) is positioned in the casting chamber (30) such that during the subsequent thixoforming process, the casting piston (34) of the thixoforming device can push the thixotropic metal bolt (10) out of the heating pipe (14).

13. The method according to claim 12, characterized in that the inner diameter d _{R of} the heating tube (14) is selected as a function of the bolt diameter d _B such that the inner diameter d _{R is} larger than the bolt diameter d _B at room temperature and at the solidus temperature T _SO ιi _d u _{s of} the metal bolt (10) of the bolt diameter d _B im

Essentially corresponds to the inside diameter C / R of the heating pipe (14).

14. The method according to claim 12 or 13, characterized in that after the heating process, but before inserting the heating tube (14) containing the thixotropic metal bolt (10) into the casting chamber (30), which during the heating process from the metal bolt ( 10) leaked liquid metal (24) is at least partially removed from the heating tube (14).

15. The method according to any one of claims 12 to 14, characterized in that the heating tube (14) during the heating process of the metal bolt

(10) is closed at least on one side.

16. The method according to any one of claims 12 to 15, characterized in that a heating tube 5 (14), which is sealed at one end at the lower tube end (15) with a closure element (16) and contains the metal bolt (10), in a vertical position perpendicular to a table top (26) is placed, preferably on a table top (26) made of ceramic material, and the table top (26) is inserted in the vertical direction, preferably from below, into the oven (20) and the heating tube (14) containing the metal bolt (10) heating until the metal bolt (10) is in a thixotropic state and then the heating tube (14) containing the thixotropic metal bolt (10) in the vertical direction, preferably by lowering the table top (26), out of the oven ( 20) is led out and separated from the closure element (16).

15 17. The method according to any one of claims 12 to 15, characterized in that the heating tube (14) containing the metal bolt (10) on both sides with one in the direction of the concentric longitudinal axis £ of the heating tube (14) displaceable closure element (16, 18) with between the heating pipe (14) and the closure element (16, 18) predetermined friction properties

20 create a tight seal and are introduced into the oven (20) in a substantially horizontal position, the two closure elements (16, 18) being pushed apart from one another as a result of the thermal expansion of the bolt material (10) during the heating process and after the thixotropic state having been reached Closure elements (16, 18) due to friction

25 are held in position, and after the heating tube (14) containing the metal bolt (10) has been removed from the furnace (20), the two closure elements (16, 18) are removed from the heating tube (14).

18. Use of the method according to one of claims 12 to 17 for the provision of a thixotropic metal bolt (10) in a horizontal thixoforming device.

19. Use of the method according to one of claims 12 to 17 for providing a thixotropic metal bolt (10) made of an aluminum alloy.