EP1110643A1 - Pretreatment of thixotropic metal billets - Google Patents

Abstract

Pretreatment device comprises a container for a metal bolt (10), an oven (20) for transferring the metal bolt in the container into a partially liquid thixotropic state, and a transporting device for transporting and feeding the thixotropic metal bolt into a casting chamber (30). The container is a cylindrical laterally closable heating tube (14) and the pretreatment device is formed in such a way that the metal bolt can remain in the heating tube during the whole pretreatment. An Independent claim is also included for a process for the preparation of a thixotropic metal bolt in a casting chamber of a thixoforming device Preferred Features: The heating tube is made of steel or a ceramic material.

Description

The invention relates to a pretreatment device for providing a containing thixotropic metal bolt in a casting chamber of a thixoforming device a container for holding a metal bolt, an oven for Transfer of the metal bolt in the container into a partially liquid, thixotropic state, as well as a transport device for transporting and Inserting the thixotropic metal bolt into the casting chamber and using it the pretreatment device. The invention further relates to a corresponding one Method according to the features of the preamble of claim 12, and the use of the procedure.

Thixoforming relates to the production of molded parts from thixotropic metal bolts. As metal bolts, all bolts come from one into a thixotropic one Condition transferable metal in question. In particular, the metal bolts can be made of Aluminum, magnesium or zinc and the alloys of these metals exist.

With thixoforming, the thixotropic properties become more fluid or more solid Metal alloys used. The thixotropic properties of a metal alloy mean that a suitably prepared metal is unloaded like a solid behaves, but reduces its viscosity so far under shear stress, that it is similar to a molten metal. This is done by heating the Alloy in the solidification interval between liquidus and solidus temperature required. The temperature is to be set so that, for example, a structural component from 20 to 80% by weight is melted, but the rest in solid Shape remains.

With thixoforming, semi-solid / semi-liquid metal is used in a modified die casting machine, a so-called thixoforming device, processed into molded parts. The die casting machines used for thixoforming differ from each other die casting machines for die casting metal melts for example, a longer casting chamber to accommodate the thixotropic Metal pin and a larger piston stroke required thereby, and for example a mechanically reinforced design of the thixotropic metal alloy leading parts of the die casting machine due to the higher pressure load of these parts during thixoforming.

The metal bolts are usually heated in a separate oven. The furnaces can be heated with fuel, such as gas or oil, or electrical energy, such as by means of resistance heating or by means of inductive energy input.

• der Bolzenzustand, d.h. seine Teilfestigkeit, üblicherweise nur in einem kleinen Temperaturbereich vorhanden ist,
• lang dauernde Aufheizzeiten, beispielsweise der Bildung einer dicken Oxidhaut oder einer möglichen Kornvergröberung wegen, vermieden werden sollen,
• und zur Erzielung eines homogenen Endproduktes die Temperaturverteilung im thixotropen Metallbolzen, dem sogenannten Thixo-Rohling, möglichst homogen sein soll.

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 state, ie its partial strength, is usually only present 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.

This is why the metal bolt is brought into the thixotropic state, i.e. heating the bolt until the desired alloy content is melted is expediently by an oven temperature controlled by sensors.

To heat the metal bolts, they are usually shaped into a bowl Container, for example in a metallic bowl made of stainless steel, or given a crucible made of clay graphite or clay SiC and in a horizontal position in transferred the thixotropic state.

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

EP-A-0 645 206 describes an apparatus for producing mechanical highly stressed parts through thixoforming, which is described at the beginning Contains pretreatment device. The metal bolts are cup-shaped Containers in a tubular continuous furnace in the thixotropic Condition transferred, and the container containing the thixotropic bolt is by means of transported to the casting chamber by a robot, and by tilting the container the thixotropic bolt is transported into the casting chamber.

EP-B-0 713 736 describes a holding device for inductive heating of bolts made of metal alloys with thixotropic properties and for holding and transporting the bolts until casting. The holding device is there a specially designed trough-shaped bowl.

The technical solutions used in practice with regard to bolt heating and the bolt transport of thixotropic metal bolts often ensure neither sufficient thermal homogeneity of what is introduced into the casting chamber Bolzens, still the necessary to achieve a stable quality Repeatability of the heating process. The thermal inhomogeneity manifests for example in an irregular with respect to the bolt cross-section Distribution of the liquid portion, which often leads to a local melting of the Metal bolt, especially at the metal bolt ends, leads. As a consequence of this the geometry of the bolt can change significantly (ovalization, crater), which is usually an intrusion in the initial phase of the thixoform process caused by air and / or aluminum oxides in the mold cavity and as a consequence leads to an increase in the committee.

Another problem during the heating process can arise from the leakage of liquid metal resulting from the bolt, since such liquid metal during the transport of the container, for example still inside the oven, can penetrate to the outside, which is often the inside of the furnace, especially at Using an induction oven, damaged. It should be noted that the Volume emerging from the metal bolt during the bolt heating process of liquid metal is typically up to 10% of the bolt volume can.

Another problem with the elimination of liquid metal during the The heating process can occur when liquid metal is poured into the casting chamber , as this can cause pre-solidification in the casting chamber, which then a variety of defects, such as air pockets or structural inhomogeneities, in the thixo molded part.

The object of the present invention is to avoid the aforementioned disadvantages the state of the art and the specification of 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 bolt a homogeneous temperature distribution and one over the entire bolt cross-section and the 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 in the casting chamber can be avoided. Another task at hand Invention is the preservation of the bolt shape during the heating process and the transportation of the thixotropic bolt to and during the casting chamber Introduce into the casting chamber. Yet another object of the present invention is seen in the possibility of increasing the transport speed, because this means that during the bolt transport from the furnace to the casting chamber resulting cooling is reduced.

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

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

• garantiert eine homogene und symmetrische Erwärmung in achsialer und radialer Richtung des Bolzens;
• minimiert während dem Aufheizprozess in einem Induktionsofen die Inhomogenitäten des magnetischen Feldes an den Rändern des Bolzens;
• verhindert übermässiges Aufschmelzen der Bolzenoberfläche;
• gewährleistet den Formerhalt des Bolzens während dem Aufheizprozess, dem Transport zur Giesskammer und in der Giesskammer;
• beseitigt den Einfluss von allfällig vorhandenen, strukturellen Inhomogenitäten und der chemischen Zusammensetzung des Metallbolzens auf den Erwärmungsprozess;
• ermöglicht die Anwendung einer billigeren und effizienteren Erwärmung mittels Induktionsfeld mit einer gegenüber dem Stand der Technik höheren Frequenz;
• vermeidet - abgesehen vom Metallbolzen und dem Aufheizrohr - das Erfordernis und die Anwesenheit anderer Metallelemente innerhalb eines Induktionsofens, was die Störung der Homogenität des magnetischen Feldes innerhalb eines Induktionsofens minimiert;
• ermöglicht eine gegenüber dem Stand der Technik präzisere Kontrolle des Erwärmungsprozesses durch direkte Messung der Temperatur des Aufheizrohres und der während dem Aufheizprozess sich einstellenden, thermischen Längenausdehnung des Metallbolzens, wobei die Längenausdehnung des Metallbolzens während dem Aufheizprozess oberhalb der Solidustemperatur des Bolzenmaterials proportional zum Flüssiganteil ist;
• eliminiert die Notwendigkeit der Verwendung von Kompensationsplatten an den Bolzenenden innerhalb eines Induktionsofens zur Homogenisierung des magnetischen Feldes, was die Kosten für die Vorbehandlungsvorrichtung senkt und ihre Funktionssicherheit erhöht; und
• ermöglicht einen gegenüber dem bekannten Stand der Technik schnelleren Transport des thixotropen Metallbolzens vom Aufheizofen in die Giesskammer.

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;
• enables 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 bolt 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 proportional to the liquid content above the solidus temperature of the stud material;
• eliminates the need to use compensation plates at 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 a 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 also advantageously 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 AlSi7Mg.

The metal bolts expediently contain homogeneously distributed, primarily solidified Solid particles consisting of individual degenerate dentrites. Prefers the proportion of primarily solidified solid particles is between 40 and 80% by weight. To achieve good thixotropic behavior, for example, aluminum alloys the alpha solid solution is available in a globulistic form to achieve even flow of melt and solid.

The degenerate dentrites of the metal bolts generally generally have one globulistic shape, resulting in a uniformly homogeneous flow of Melt and solid can be achieved without segregation. The production metal bolts with a structure with globulistic dentrites are used, among other things. by a continuous casting process combined with an intense electromagnetic Stir also during the solidification phase. This leads to melting and breaking off dentrite arms that mold near the solidus temperature and form the globulistic structure.

The metal bolts required for thixoforming become 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.

The semi-solid state contains the thixotropic alloy, the so-called thixotropic alloy Alloy slurry, the reverse engineered dentritic, primary strength particles in one this surrounding matrix of liquid metal. The thixotrope preferably contains Alloy slurry has a liquid content of 40% by weight to 50% by weight and in particular between 43% and 48% by weight.

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

To convert the metal bolt into a thixotropic state, a is preferred Induction furnace used. For example, there is a primary coil around the container arranged, the alternating current flowing in the primary coil in the container and / or induces an alternating magnetic field in the metal bolt. The warming the heating tube or the metal bolt is done by the magnetic Alternating field, which has strong eddy currents in the heating tube or in the metal bolt and thereby causes a corresponding warming. The depth of penetration the eddy currents and thus the depth of the heated layer is frequency-dependent; when using high frequencies, rapid heating predominantly occurs near-surface layers.

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

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

Through the heating device according to the invention with a metal heating tube becomes a very homogeneous temperature and thus also a very homogeneous Liquid metal distribution achieved throughout the metal bolt. This especially because the direct introduction of energy into the heating pipe and the stud material is only indirectly heated by heat conduction, so that any local existing differences in the structure or in the chemical composition the bolt material has no influence on the direct energy input to have.

According to a further preferred embodiment of the heating tube this consist of ceramic material. When using such heating pipes in an induction furnace, the magnetic field penetrates the heating tube, i.e. the ceramic material of the heating tube is for the magnetic field transparent. The metal bolt is then heated by direct heating Interaction with the magnetic field of the induction furnace.

Suitable ceramic materials are, for example, Al ₂ O ₃ , AL ₃ O ₄ , BN, SiC, Si ₃ N ₄ , MgO, TiO, ZrO ₂ , stabilized, such as yttrium-stabilized ZrO ₂ , glasses or refractory cements or mixtures, which the mentioned Materials included. More preferably, the heating tube can be made 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 inside 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 inside diameter d _{R of} the heating tube as a function of the pin diameter d _B 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 ▵d of approx. 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 directions, so that at a certain point in time the bolt diameter d _B corresponds to the inside diameter d _{R 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 d _B corresponds to 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 the solidus temperature causes a corresponding local melting of the stud material and should therefore be avoided at all costs.

Within the scope of the inventive step it has been shown that the use a thin smear layer to prevent the metal bolt from sticking on the heating pipe practically no influence on the heat transfer between the Bolts and the heating pipe causes. It should be noted that the release agent Lubricants used - if necessary at all - mostly only with heating pipes made of metal, i.e. when using heating pipes No release agents are usually required made of ceramic material.

In a preferred embodiment of the pretreatment device according to the invention, the material of the heating tube and the inside diameter d _{R of} the heating tube are chosen such that the metal bolt at its solidus temperature T _{solidus has} essentially the same diameter d _B as the inside diameter d _{R of} the heating tube. The bolt diameter d _B at T _{solidus more} preferably fulfills the relationship 0.996 d _R ≤ 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 usually have a round or oval cross-section, but can also be polygonal cross-section. The The diameter of the metal bolts in the cold state is, for example, 50 to 180 mm, expediently 75 to 150 mm and preferably 100 to 150 mm. The The length of the metal bolts in the cold state is, for example, 80 to 500 mm.

When the metal bolt heats up, the metal bolt as well as the heating tube expands out. Different materials have different coefficients of thermal expansion on. The coefficient of thermal expansion of steel or ceramic material is much smaller than that of, for example, aluminum or aluminum alloys. As a result, for example, it expands Metal bolts made of an aluminum alloy are stronger than a heating tube made of, for example Steel or ceramic material, so that - starting from an im cold condition compared to the inner diameter of the heating tube smaller Diameter of the metal stud - the metal stud at a certain temperature has the same diameter as the heating tube.

It is preferred according to the invention that at the solidus temperature T _{solidus of} the metal stud material, the diameter of the metal stud essentially corresponds to the diameter of the heating pipe, so that in the temperature range in which the thixotropic properties of the metal stud are essentially set, optimal thermal contact between the heating pipe and the metal stud is formed. When the heating tube containing the metal bolt is heated above the solidus temperature T _solidus , the eutectic is melted, combined with an increase in volume of the metal bolt. The eutectic of aluminum alloys suitable for thixoforming typically forms at approximately 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 _solidus , 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 the condition according to which at T _solidus Bolt diameter d _B corresponds to the diameter d _{R of} the heating pipe is fulfilled. 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 length increase 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 transfer into the thixotropic state and for the at least partial insertion of closure elements the length of the heating tube must be in the cavity of the heating tube be longer than the bolt length. Preferably the length of the Heating tube selected such that the heating tube a total of about 5 to 30 mm, in particular 10 to 20 mm, is longer than the metal bolt to be heated therein. When the metal bolt is heated in a horizontal position, everyone stands on it Side of the heating tube, the front side of the heating tube opposite the front side of the metal bolt preferably by approximately 2.5 to 15 mm and in particular by 5 to 10 mm in front. When the metal bolt heats up in a vertical position, the front side is upright of the heating pipe at the upper end of the pipe opposite the face of the metal bolt preferably by about 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, for Copper pipes preferably 4 to 10 mm and for ceramic pipes preferably 8 to 15 mm.

The heating tube according to the invention can be closed on both sides. To do this preferably used closure elements made of ceramic material. As a ceramic Materials for the closure elements are the same as described above Materials as for the one preferred variant of the heating tube made of ceramic material. Ceramic material points towards the metal stud material a low thermal conductivity, so that the radial temperature distribution at the front edge areas of the metal bolt through such closure elements is little influenced.

With a substantially horizontal position of the The heating tube on both ends is expediently made of metal bolts by means of closure elements, preferably by plug or peg-shaped Closing elements, tightly closed. The tightness refers to the Avoiding the escape of liquid metal from the heating tube during the heating process.

The plug-shaped or peg-shaped closure elements are further preferred with regard to Material choice and shape designed such that their friction properties in the heating tube, on the one hand, due to the thermal expansion of the metal bolt shift in the direction of the longitudinal axis during the heating process allow the heating tube and on the other hand a shift by the Metal bolts exerted pressure on the closure elements after reaching the the desired thixotropic state temperature is avoided. Prior to the heating process, the closure elements are preferred so far pushed into the heating tube so that this is in direct mechanical contact the end faces of the metal bolt. This causes the locking elements to shift during the heating process in the heating tube due to the thermal Linear expansion of the metal bolt.

With a metal bolt position that is essentially vertical during the heating process the heating pipe is sealed on one side, tightly at the lower end of the pipe. Here, too, the tightness only refers to the prevention of the leakage of liquid metal from the heating tube. The heating tube can do this with one end directly on a preferably height-adjustable table top, preferably on a table top made of ceramic material, or the heating tube can by means of a closure element, preferably by a plug or cone-shaped closure element, tightly closed and by means of this closure element on a preferably height-adjustable table top made of any heat-resistant material can be set in a vertical position. Especially at The use of a stopper-shaped or peg-shaped closure element is the closure element with regard to choice of material and shape, preferably designed such that on the one hand no liquid metal is released during the heating process in the furnace can escape the heating tube and on the other hand the friction of the closure element is less than 10 N in the heating tube.

The friction of the closure element must be low so that a gripper arm Transport device, in particular a robot gripper arm, the heating tube without great effort, i.e. without the gripper arm for large mechanical Stresses must be interpreted by the specified on the table top Closure element can lift off. On the other hand, there is a high level of friction for achievement A high level of tightness is also not necessary because the closure element only prevent the leakage of liquid metal during the heating process must and due to the cohesion of molten metals, in particular aluminum melts, this does not require a high level of tightness. Conveniently the friction of the closure element in the heating tube is 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 provision of a thixotropic metal bolt in a casting chamber of a vertical or Horizontal thixoforming device. This pretreatment device is particularly advantageous however, for providing a thixotropic metal bolt in one horizontal casting chamber, since here the shape retention with the inventive Device can be guaranteed particularly well. With a horizontal thixoforming device lies the casting chamber that holds the thixotropic metal bolt picks up, horizontally.

The device according to the invention is particularly advantageous for the provision of thixotropic metal bolts made of aluminum or aluminum alloys. Aluminum bolt are very particularly preferred in an induction furnace heated up in a vertical heating room.

According to the invention, the object directed to the method is achieved by that the container is a cylindrical heating tube, the metal bolt during the heating process and the subsequent transport to the casting chamber always remains in the heating tube, and the heating tube containing the metal bolt is positioned in the casting chamber so that during the subsequent Thixoforming process of the casting flask of the thixoforming device the thixotropic Can push metal bolts out of the heating tube.

Preferred developments of the inventive method are in the dependent Claims 13 to 17 described. The for the pretreatment device according to the invention statements made regarding special characteristics and details apply mutatis mutandis to the inventive method.

The heating tube containing the thixotropic metal pin becomes after the heating process and after any draining from during the heating process Metal bolts that have leaked out, for example by a robot Casting chamber transported and in the front, half-open part of the casting chamber inserted. 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 one closed part of the casting chamber; after that the thixotropic metal alloy through a through opening into the pouring channels and then into the mold cavity initiated. After the thixoforming process, the casting piston is pulled back, see above that afterwards a gripper arm of the transport device removes the heating pipe from the Pick up the casting chamber and use it for further pretreatment processes can supply. The return of the heating pipe for further use for further pretreatment procedures expediently take place during the Solidification phase of the thixotropic metal alloy in the mold cavity. That while the solidification of the thixotropic metal alloy in the mold cavity Cast structure essentially determines the properties of the molded parts. The microstructure is characterized by 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.

Preference is given to the heating process, but before the introduction of the heating tube containing thixotropic metal bolts into the casting chamber, the Liquid metal escaping from the metal bolt during the heating process at least partially removed from the heating tube. The one during the heating process Liquid metal portion emerging from the metal bolt 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 using a robot typically takes 5 to 30 s and preferably 8 to 15 s. The length of time that the thixotropic metal bolt remains in the casting chamber is typically between 3 and 5 s. This time is for driving away a robotic gripper arm from the casting chamber and for the electronic readiness control a thixoforming device is required.

• es führt zu einer wesentlichen Reduktion der Wärmeverluste des thixotropen Metalles während dem Transport vom Aufheizofen zur Giesskammer und in der Giesskammer dank des bis auf die gleiche Temperatur wie der Metallbolzen erwärmten Aufheizrohres;
• durch das Absetzen des Metallbolzens innerhalb des Aufheizrohres in die Giesskammer, insbesondere in die Giesskammer einer Horizontal-Thixoformeinrichtung, eliminiert es den Schock des Falls, was die Beibehaltung der Form und der Homogenität des Metallbolzens gewährleistet;
• es vermeidet die Trennung des Flüssiganteils beim Einlegen des thixotropen Metallbolzens in die Giesskammer, so dass damit verbundene Vorerstarrungen von Flüssigmetall in der Giesskammer vermieden werden;
• es verhindert zu einem wesentlichen Teil die Oxidation der Metallbolzenoberfläche, da der Metallbolzen während des Aufheizvorganges und dem Transport in die Giesskammmer und dessen Aufenthalt in der Giesskammer bis zum Beginn des eigentlichen Thixoformprozesses nicht einer freien Atmosphäre ausgesetzt ist;
• es vermindert das Risiko von Lufteinschlüssen und von Oxiden in der Giesskammer während der Füllphase der Formkavität, da einerseits die Oxidbildung vermindert wird und andererseits durch den Formerhalt des Metallbolzens während der Vorbehandlung auch Lufteinschlüsse durch Bolzenverformungen vermieden werden.

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 bolts;
• 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 the thixotropic metal bolt is inserted into 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 inclusions 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 retention due to stud deformation is avoided due to the shape of the metal stud during pretreatment.

The advantages mentioned have a direct influence on the quality of the produced Molded parts; this reduces the scrap.

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

For example :

A circular cylinder is used to verify the heating principle in a heating pipe Aluminum bolts with a diameter of 100 mm and one Length of 200 mm in a vertical position in an oven with a resistance heater heated to a required temperature above the solidus temperature, where the final temperature and the time-dependent temperature profile of the heating furnace be chosen such that at the end of the heating process a thixotropic Metal bolt with a liquid content of about 50 wt .-% is present. The aluminum bolt is located in a heating pipe during the heating process Stainless steel with a wall thickness of 5 mm. The heating pipe and thus the Metal bolts rest on a heat insulation plate at the lower end. At the top The end of the heating pipe protrudes about 5 mm from its circular top edge the upper edge of the bolt. The upper end of the heating pipe is not closed, so that the change in length using a laser interferometer during the entire heating process can be measured.

The 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 - 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 T. _{1 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 ₁ (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 measured during the heating profile shown in FIG. 3 of the metal bolt is shown in Fig. 4. It can be seen from this that the Aluminum bolts until the solidus temperature is reached in the longitudinal direction by approx. 1.5 mm, the thermal linear expansion above the solidus temperature increases rapidly.

An aluminum bolt is used to examine the dimensional stability of the thixotropic bolt According to the invention heated in a vertical position until the thixotropic bolt has a liquid content of about 50% by weight, then taken out of the oven, in transferred to a horizontal position and ejected from the heating tube. The exam The geometric shape of the thixotropic aluminum bolt shows that the Dimensional stability is given, i.e. the thixotropic aluminum bolt essentially has - apart from the thermal expansion - the same shape as the original Aluminum bolt.

It is also found that very little during the heating process liquid metal emerges from the metal bolt. The thixotropic bolt also retains its smooth surface even during the heating process. Traces of oxidation none can be observed on the surface. Testing the liquid metal distribution by means of a cutting test further shows that the homogeneity of the thixotropic condition is also very well met.

Fig. 1

zeigt schematisch die zeitliche Abfolge der wesentlichen Verfahrensschritte für die Bereitstellung eines thixotropen Metallbolzens in der Giesskammer einer Horizontal-Thixoformeinrichtung, wobei der Metallbolzen in einer Horizontallage in den thixotropen Zustand überführt wird;

Fig. 2

zeigt schematisch die zeitliche Abfolge der wesentlichen Verfahrensschritte für die Bereitstellung eines thixotropen Metallbolzens in der Giesskammer einer Horizontal-Thixoformeinrichtung, wobei die Aufheizung des Metallbolzens in einer vertikalen Lage geschieht;

Fig. 3

zeigt beispielhaft eine typische Aufheizkurve;

Fig. 4

zeigt beispielhaft eine typische temperaturabhängige Deformationskurve eines Metallbolzens während eines erfindungsgemässen Aufheizvorganges.

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

Fig. 1

schematically shows the chronological sequence of the essential process 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 process steps for the provision of a thixotropic metal bolt in the casting chamber of a horizontal thixoforming device, the heating of the metal bolt taking place in a vertical position;

Fig. 3

shows an example of a typical heating curve;

Fig. 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 ℓ of a metal bolt 10, respectively. through the Device elements 14, 20, 30, in which the metal bolt 10 during the Pretreatment takes place, the heating process of the metal bolt 10 in one horizontal position happens.

Fig. 1 a) shows the loading of a in a solid physical state Metal bolt 10 in a horizontally lying heating pipe 14. The heating pipe 14 is closed with plug-shaped closure elements 16, 18, the Closure elements 16, 18 on the one hand on the end faces 15 of the heating tube 14 rest and, on the other hand, close flush with the metal bolt 10, i.e. 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 ℓ 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 T _solidus , 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 due to the usually very small radial expansion of the heating pipe 14 limited. Accordingly, the further thermal expansion of the metal bolt 10 after reaching the solidus temperature T _solidus is essentially only possible in the direction of its concentric longitudinal axis ℓ, the closure elements 16, 18 being pushed away from one another in accordance with the thermal expansion of the metal bolt 10, so that the stopper-shaped closure elements 16, 18 no longer rest against the end faces 15 of the heating tube 14.

Fig. 1 b) shows the discharge of the induction furnace 20, i.e. bringing out the the thixotropic metal bolt 10 containing heating pipe 14 from the heating room 21 of the induction furnace 20. The closure elements 16, 18 are after the discharge of the induction furnace 20 separated from the heating tube 14. In the further the metal bolt 10 which emerged during the heating process, liquid molten metal 24 is removed from the heating tube 14 by removing the liquid Metal 24 can be drained from the heating tube 14, the liquid metal for example, is collected in a drip tray (not shown).

1 c) shows this in a casting chamber 30 of a horizontal thixoforming device introduced heating tube 14. In this case, the heating tube 14 is in the casting chamber cavity 32 the casting chamber 30 positioned that during the subsequent Thixoform process of the casting piston 34 the thixotropic metal bolt 10 from the Heating pipe 14 can come across, so that the thixotropic metal alloy subsequently through the passage opening 36 into the sprue channels (not shown) and can then be introduced into the mold cavity (not shown). The casting chamber 30 has a recess for receiving the heating tube. This The recess serves on the one hand for centering the heating tube 14 and on the other hand as a stop for setting the heating pipe 14 during the pushing out 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 ℓ of a metal bolt 10, respectively. through the Device elements 14, 20, 30, in which the metal bolt 10 during the Pretreatment takes place, the heating process of the metal bolt 10 in one vertical bolt position happens.

2 a) shows the insertion of one located in a vertical heating tube 14 Metal bolt 10 in a vertically lying, cylindrical heating chamber 21 of an induction furnace 20. The heating tube 14 is at the lower end of the tube, i.e. at the lower end face 15 of the heating tube 14, with a stopper-shaped closure element 16 closed. The closure element lies on a table top 26. The introduction of the heating tube 14 containing the metal bolt 10 into the induction furnace 20 is done by placing the heating pipe 14 vertically the table top 26, wherein the closure element 16 to lie on the table top 26 comes, and by lifting the table top 26 until the heating tube 14 completely comes to rest in the heating chamber 21 of the induction furnace 20. The heating pipe 14 is located in the middle of the heating space delimited by the induction coils 22 21, i.e. the concentric longitudinal axis of the heating space 21 coincides with the concentric Longitudinal axis ℓ of the metal bolt 10 together.

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 _solidus , the metal bolt 10 abuts the heating tube 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 due to the usually very small radial one Expansion of the heating tube 14 limited. Accordingly, the further thermal expansion of the metal bolt 10 after reaching the solidus temperature T _solidus is essentially only possible in the vertical direction, parallel to its concentric longitudinal axis ℓ. The upper tube end 15 of the heating tube 14 is open, so that the metal bolt 10 can expand thermally without hindrance.

Fig. 2 b) shows the induction furnace 20 after the heating tube 14 with the thixotropic metal bolt 10 is led out of the heating chamber 21. The leading out the thixotropic metal bolt is done by lowering the Table top 26.

Fig. 2 c) shows that led out of the furnace in the vertical direction thixotropic metal bolts 10 containing, fixed vertically on the table top 26 Heating tube 14, which also with the plug-shaped closure element 16 is tightly closed.

2 d) shows that separated from the table top 26 and the closure element 16, the heating tube 14 containing the thixotropic metal bolt 10 in a horizontal manner Location. The heating tube 14 is expediently separated from the Closure element 16 and the transfer of the heating tube into a horizontal Position using a robot. The robot arm is used to lift the To heat up the heating tube 14 from the table top 26 and the closure element 16 Low power.

The closure element 16 closes the heating pipe 14 in a form-fitting manner tight. However, the tightness is only to avoid the outflow of liquid Metal required so that due to the surface tension of the liquid metal the closure element 16 essentially only in a form-fitting manner in the heating tube 14 must grip 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. whose Adhesion is so great that the heating tube in the vertical direction from the closure element can be lifted off without the thixotropic metal bolt 10 the heating tube 14 falls. The vertical lifting of the heating tube 14 from on the Table top 26 fixed closure member 16 also allows dripping of the liquid metal 24 formed during the heating process outside of the heating furnace 20.

2 e) shows this in a casting chamber 30 of a horizontal thixoforming device introduced heating tube 14. In this case, the heating tube 14 is in the casting chamber cavity 32 the casting chamber 30 positioned that during the subsequent Thixoform process of the casting piston 34 the thixotropic metal bolt 10 from the Heating pipe 14 can come across, so that the thixotropic metal alloy subsequently through the passage opening 36 into the sprue channels (not shown) and can then be introduced into the mold cavity (not shown). The casting chamber 30 has a recess for receiving the heating tube. This The recess serves on the one hand to center the heating tube 14 and on the other hand as a stop for setting the heating pipe 14 during the pushing out of the thixotropic metal bolt 10 at the beginning of the thixoforming process.

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

FIG. 3 shows a typical heating curve up to reaching the solidus temperature T _{solidus of} an aluminum bolt 10 located in a heating pipe 14 according to the invention 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 vertical position in a heating tube 14 made of stainless steel, the heating tube 14 having a wall thickness of 5 mm and the lower end face 12 of the aluminum bolt 10 a heat insulation plate 26 rests directly, ie the lower end face 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 ℓ, whereby - with respect to the concentric longitudinal axis ℓ of the aluminum bolt 10 - a first thermocouple for measuring the edge temperature T ₀ (t) in the edge area of Aluminum bolt 10 is inserted, a second thermocouple for measuring the temperature T ₁ (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 T ₀ (t), T ₁ (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, which is represented by the heating curve in FIG. 3. From FIG. 4 it can be seen that the aluminum bolt 10 is at approximately 560 ° until the solidus temperature T _{solidus is reached} C extends in the longitudinal direction essentially linearly, depending on the temperature, by approx. 1.5 mm, the thermal linear expansion ▵L (T) increasing suddenly above the solidus temperature.

Claims (19)

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), an oven (20) for transferring the metal bolt (10) located in the container into a partially liquid, thixotropic State, as well as a transport device for transporting and inserting the thixotropic metal bolt (10) into the casting chamber (30),
characterized in that
the container is 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 Linger in the casting chamber (30) until the start of the thixoforming process, can remain in the heating tube (14). Pretreatment device according to claim 1, 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 metal bolt (10) at its solidus temperature T _{solidus has} essentially the same diameter d _B as the inner diameter d _R of the heating tube (14). Pretreatment device according to claim 1 or 2, characterized in that that the heating tube (14) made of metal, preferably steel, in particular made of stainless steel or tool steel. Pretreatment device according to claim 1 or 2, characterized in that that the heating tube (14) consists of ceramic material. Pretreatment device according to one of claims 1 to 4, characterized characterized that essentially during the heating process horizontal position of the metal bolt (10) the heating tube (14) on both sides is tightly closed by closure elements (16, 18), the closure elements (16, 18) in terms of choice of material and shape are that their friction properties in the heating tube (14) on the one hand due to the thermal expansion of the metal bolt (10) during the heating process conditional shift in the direction of the longitudinal axis ℓ of the heating tube (14) allow and on the other hand a shift by the of Metal bolts (10) exerted pressure on the closure elements (16, 18) after reaching the required for the desired thixotropic state Temperature is avoided. Pretreatment device according to one of claims 1 to 4, characterized characterized that essentially during the heating process vertical metal bolt position the heating pipe (14) on one side, at the bottom Pipe end (15) is closed, with the use of a closure element (16) the closure element (16) with regard to choice of material and shape is designed such that, on the one hand, during the heating process in the furnace (20) no liquid metal (24) can emerge from the heating pipe (14) and on the other hand, the friction of the closure element (16) in the heating tube (14) is less than 10 N. Pretreatment device according to claim 5 or 6, characterized in that that the closure elements (16, 18) consist of ceramic material. Pretreatment device according to one of claims 1 to 7, characterized characterized in that the furnace (20) is an induction furnace. Pretreatment device according to one of claims 1 to 8, characterized characterized in that the transport device represents a robot, wherein the clamping device of the robot for holding the heating tube at least on the surface made of ceramic against the heating pipe Material exists. 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) a horizontal thixoforming device. 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. 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 container in a solid state of aggregation and the metal bolt (10) in the container is heated in an oven (20) for as long as until the metal bolt (10) is in a thixotropic state and the thixotropic metal bolt (10) is inserted into the casting chamber (30) of a thixoforming device by means of a transport device,
characterized in that
the container is a cylindrical heating tube (14), the metal bolt (10) always remains in the heating tube (14) during the heating process and the subsequent transport into the casting chamber (30), and the heating tube (14) containing the metal bolt (10) in the casting chamber (30) is positioned 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 tube (14). A method according to claim 12, characterized in that the inner diameter d _R of the heating tube (14) as a function of the bolt diameter d _B is chosen such that at room temperature the inside diameter _R d is greater than the bolt diameter d _B and the solidus temperature T _solidus of the metal bolt (10) the bolt diameter d _B essentially corresponds to the inner diameter d _{R of} the heating tube (14). A method according to claim 12 or 13, characterized in that after the heating process, but before inserting the thixotropic metal bolt (10) containing heating tube (14) in the casting chamber (30), the Liquid that has escaped from the metal bolt (10) during the heating process Metal (24) is at least partially removed from the heating tube (14). Method according to one of claims 12 to 14, characterized in that that the heating tube (14) during the heating process of the metal bolt (10) is closed at least on one side. Method according to one of claims 12 to 15, characterized in that that one end at the lower tube end (15) with a closure element (16) tightly sealed heating tube containing the metal bolt (10) (14) is placed vertically on a table top (26) in the vertical position, preferably on a table top (26) made of ceramic material, and the Table top (26) in the vertical direction, preferably from below, in the oven (20) and the heating tube (14) containing the metal bolt (10) until the metal bolt (10) is thixotropic State, and subsequently the thixotropic metal bolt (10) containing heating tube (14) in the vertical direction, preferably by a Lowering the table top (26), out of the oven (20) and from Closure element (16) is separated. Method according to one of claims 12 to 15, characterized in that that the heating tube (14) containing the metal bolt (10) on both sides with each one in the direction of the concentric longitudinal axis ℓ of the heating tube (14) sliding closure element (16, 18) with between the heating tube (14) and the closure element (16, 18) predetermined friction properties tightly closed and in a substantially horizontal position is introduced into the oven (20), the during the heating process two closure elements (16, 18) due to the thermal expansion of the bolt material (10) pushed away from each other and after reaching of the thixotropic state, the closure elements (16, 18) due to friction are held in position, and after the metal bolt has been removed (10) containing the heating tube (14) from the oven (20) the two Closure elements (16, 18) are removed from the heating tube (14). Use of the method according to one of claims 12 to 17 for provision a thixotropic metal bolt (10) in a horizontal thixoforming device. Use of the method according to one of claims 12 to 17 for provision a thixotropic metal bolt (10) made of an aluminum alloy.

Images