Pre-treatment of a thixotropic metal bolt

Abstract

The invention relates to a pre-treatment device and method for production of a thixotropic metal bolt (10), in a casting chamber (30) of a thixo-moulding unit. The pre-treatment device comprises a container (14), for accommodating a metal bolt (10), an oven (20), for converting the metal bolt (10) in the container (14) into a partly fluid thixotropic state and a transport unit for transporting and feeding the thixotropic metal bolt (10), into the casting chamber (30). The container (14) is a cylinder-shape heating tube (14), with closable sides. Furthermore, the pre-treatment device is so arranged that, during the entire pre-treatment, namely the heating process in the oven (20), the transport into the casting chamber (30) and the period in the casting chamber (30), the metal bolt (10) remains in the heating tube (14).

Description

[0001] The invention concerns a pretreatment device for provision of a thixotropic metal bolt in a casting chamber of a thixoforming device, containing a container to hold a metal bolt, an oven to transfer the metal bolt in the container into a part-liquid thixotropic state, and a transport device for transporting and introducing the thixotropic metal bolt into the casting chamber, and the use of the pretreatment device. The invention also concerns a corresponding process according to the features of the preamble of claim 12 and the use of the process.

[0002] Thixoforming concerns the production of mouldings from thixotropic metal bolts. The metal bolts can be all bolts of a metal which can be transferred to a thixotropic state. In particular the metal bolts can consist of aluminium, magnesium or zinc or alloys of these metals.

[0003] Thixoforming utilises the thixotropic properties of part-fluid and part-solid metal alloys. The thixotropic properties of a metal alloy mean that a correspondingly prepared metal behaves as a solid when not stressed, however under shear stress its viscosity is reduced so much that it behaves similar to a metal melt. For this, heating of the alloy to the solidification range between liquidus and solidus temperature is required. The temperature must be set so that for example a structural part of 20 to 80 w. % is melted, but the rest remains in solid form.

[0004] In thixoforming, part-solid/part-liquid metal is processed into mouldings in a modified diecasting machine, known as a thixoforming device. The diecasting machine used for thixoforming differs from diecasting machines for diecasting metal melts by for example a longer design of casting chamber to hold the thixotropic metal bolt and the greater piston stroke required as a result, and for example a mechanically reinforced design of the parts of the diecasting machine carrying the thixotropic metal alloy due to the higher pressure loading on these parts during thixoforming.

[0005] The metal bolts are usually heated in a separate oven. The oven can be heated with fuel, for example gas or oil, or electrical energy such as for example by resistance heating or by means of inductive energy supply.

[0006] The heating of the metal bolt has great significance in relation to the great influence exerted by the state of the bolt introduced into the casting chamber on the quality of the product as:

[0007] the bolt state i.e. its part-solidity is normally only present in small temperature ranges,

[0008] long heating times, for example to form a thick oxide skin or avoid possible grain coarsening, must be avoided,

[0009] and to achieve a homogeneous end product, the temperature distribution in the thixotropic metal bolt, the thixoblank, must be as homogeneous as possible.

[0010] Therefore the metal bolt is suitably transferred to the thixotropic state i.e. heated until the required alloy proportion has melted, by an oven temperature regulated with sensors.

[0011] To heat the metal bolt this is usually placed in a dish-like container, for example a metal dish of stainless steel or a crucible of clay-graphite or clay-SiC, and brought into the thixotropic state in a horizontal position.

[0012] The thixotropic metal bolt can then be transferred, for example in the same container, by means of a gripper for example to the casting chamber of a horizontal thixoforming device and by tipping the container transferred to the casting chamber. In this case the metal bolt remains in the same container during the heating process and transport to the casting chamber.

[0013] EP-A-0 645 206 describes a device for production of mechanically highly stressed parts by thixoforming which contains a pretreatment device initially described. For this the metal bolts are transferred in cup-like containers into a tubular passage oven in the thixotropic state, and the containers containing the thixotropic bolts are transported by means of a robot to the casting chamber, and by tipping the container the thixotropic bolt is transferred to the casting chamber.

[0014] EP-B-0 713 736 describes a holder device for inductive heating of bolts of metal alloys with thixotropic properties and for holding and transporting the bolts until casting. The holding device is a specially designed trough-like dish.

[0015] The technical solutions applied in practice with regard to heating and transport of thixotropic metal bolts often guarantee neither adequate thermal homogeneity of the bolt introduced into the casting chamber nor the repeatability of the heating process necessary to achieve stable quality. The lack of thermal homogeneity is manifested for example in a distribution of the liquid part which is irregular in relation to the bolt cross-section, where this often leads to local melting of the metal bolt, in particular at the metal bolt ends. Consequently, the geometry of the bolt can change substantially (ovalisation, craters) which in the initial phase of the thixoforming process usually leads to the inclusion of air and/or aluminium oxides in the mould cavity and consequently an increase in the rejection rate.

[0016] Another problem during the heating process can be caused by the emergence of liquid metal from the bolt as such liquid metal can seep outwards during transport of the container, for example while still within the oven, which frequently damages the inside of the oven, in particular when an induction oven is used. It must be noted that the volume of liquid metal emerging from the metal bolt during the bolt heating process can typically amount to up to 10% of the bolt volume.

[0017] A further problem from the emergence of liquid metal during the heating process can arise when liquid metal is poured into the casting chamber as this can lead to premature solidification in the casting chamber which can then cause a multiplicity of defects such as air inclusions or non-homogenous structure in the thixomoulding.

[0018] The purpose of the present invention is to avoid these disadvantages in the state of the art and specify a pretreatment device and process for reproducible provision of thixotropic metal bolts in a casting chamber of a thixoforming device, where the thixotropic metal bolt has a homogeneous temperature distribution and a liquid part distributed homogeneously over the entire bolt cross-section and entire bolt length, and avoids the emergence of liquid metal in the heating oven and the introduction of liquid metal into the casting chamber. A further task of the present invention is to maintain the bolt shape during the heating process and transport of the thixotropic bolt to the casting chamber, and during its introduction into the casting chamber. Another task of the present invention is the possibility of increasing the transport speed as this will reduce the cooling occurring during transport of the bolt from the oven to the casting chamber.

[0019] According to the invention this is achieved in that the container has a cylindrical heating tube which can be closed at the sides, and the pretreatment device is formed such that the metal bolt can remain in the heating tube throughout the entire pretreatment, namely the heating process in the oven and transport to the casting chamber and its stay in the casting chamber until the start of the thixoforming process.

[0020] Further advantageous designs of the pretreatment device according to the invention are described in claims 2 to 9.

[0021] The new concept according to the invention, according to which the metal bolt remains in the hollow cylindrical container known as the heating tube during the entire pretreatment process:

[0022] guarantees a homogeneous and symmetrical heating in the axial and radial direction of the bolt;

[0023] minimises the non-homogeneity of the magnetic field at the edge of the bolt during the heating process in an induction oven;

[0024] prevents excess melting of the bolt surface;

[0025] guarantees retention of the bolt shape during the heating process, transport to the casting chamber and in the casting chamber;

[0026] eliminates the effect of any structural non-homogeneities present and the chemical composition of the metal bolt on the heating process;

[0027] allows the use of cheaper and more efficient heating by means of an induction field with higher frequency than the state of the art;

[0028] apart from metal bolts and the heating tubes, avoids the requirement for and presence of other metal elements within an induction oven, which minimises the disruption to homogeneity of the magnetic field within an induction oven;

[0029] allows more precise control of the heating process than the state of the art by direct measurement of the temperature of the heating tube on the thermal length expansion of the metal bolt occurring during the heating process, where the length expansion of the metal bolt during the heating process above the solidus temperature of the bolt material is proportional to the liquid part;

[0030] eliminates the need to use compensation plates at the bolt ends within an induction oven to homogenise the magnetic field, which lowers the costs of the pretreatment device and increases its functional reliability; and

[0031] allows faster transport than the known state of the art of the thixotropic metal bolt from the heating oven to the casting chamber.

[0032] The pretreatment device according to the invention is suitable for all metal bolts of commercial alloys which can be transferred to a thixotropic state. Particularly suitable metal bolt materials are alloys of aluminium, magnesium or zinc. In particular, aluminium casting and aluminium wrought alloys are preferred. The pretreatment device according to the invention is advantageously also suitable for processing of particle-reinforced aluminium alloys which contain for example evenly distributed SiC or Al2O3 particles. In particular the pretreatment device according to the invention is suitable for aluminium alloys—which have a pronounced solidification interval such as for example AlSi7Mg.

[0033] The metal bolts suitably contain evenly distributed, primary solidified particles which consist of individual degenerated dendrites. The proportion of primary solidified particles is preferably between 40 and 80 w. %. To achieve a good thixotropic behaviour, for example with aluminium alloys the alpha mixing crystals must be present in globulistic form in order to achieve an even flow of the liquids and solids.

[0034] The degenerated dendrites of the metal bolts generally preferably have a globulistic structure whereby an even homogeneous flow of melt and solid can be achieved without separation of mixture. Metal bolts with a structure with globulistic dendrites are produced inter alia by an extrusion process combined with an intensive electromagnetic agitation also during the solidification phase. This leads to a melting and break-up of dendrite arms which form close to the solidus temperature and form the globulistic structure.

[0035] Before the thixoforming process, the metal bolts required for thixoforming are heated by means of the pretreatment device according to the invention to a temperature above the solidus temperature and below the liquidus temperature, i.e. until they reach a part-solid thixotropic state.

[0036] In the part-solid state the thixotropic alloy, known as the thixotropic alloy pulp, contains the back-developed dendritic primary solid particles in a surrounding matrix of liquid metal. Preferably, the thixotropic alloy pulp contains a liquid part of 40 w. % to 50 w. % and in particular between 43 w. % and 48 w. %.

[0037] During the heating process the metal bolts can be in the vertical or horizontal position in relation to their linear axis.

[0038] To transfer the metal bolt to a thixotropic state, preferably an induction oven is used. For example, a primary coil is arranged around the container where the alternating current flowing in the primary coil induces in the container and/or the metal bolt an alternating magnetic field. The heating tube or metal bolt is heated by the alternating magnetic field which provokes powerful eddy currents and consequently heating in the heating tube or metal bolt. The penetration depth of the eddy currents and hence the depth of the heated layer is frequency-dependent; when high frequencies are used, rapid heating occurs mainly of the layers close to the surface.

[0039] In an advantageous design of the heating tube, this consists of a metal. Preferred are metals of the series of the iron-carbon-containing metals such as steel, stainless steel, thermax steel, hot work steel, or from the series of the metals tantalum, niobium, vanadium, tungsten or titanium or alloys thereof. Also, preferably heating tubes are used which are made of copper or its alloys. Particularly preferably, heating tubes are made of steel and in particular stainless steel or tool steel.

[0040] If a metal heating tube, in particular a steel heating tube is used for the heating process in an induction oven, the magnetic field in essence penetrates only the heating tube so that the induction oven in essence only heats the heating tube directly and the heating of the metal bolt takes place almost exclusively by heat conductance from the heating tube to the metal bolt. The bolt material is thus heated by the heating of the heating tube and by heat conductance from the heating tube to the bolt material. Because of the high heat conductivity of a metal heating tube a radially symmetrical heat conductance occurs from the heating tube to the metal bolt insofar as the metal bolt is in thermal contact with the heating tube over its entire periphery. The radially symmetrical heat conductance then also causes a radially symmetrical temperature distribution in the metal bolt whereby, due to the high heat conductance of the metal bolt, a low radial temperature gradient is rapidly established.

[0041] Due to the heating device according to the invention with a metal heating tube, a very homogeneous temperature and hence very homogeneous liquid metal distribution is achieved in the entire metal bolt. This applies in particular also because the indirect energy transfer takes place to the heating tube and the bolt material is only heated indirectly by heat conductance so that any local differences in the structure or chemical composition of the bolt material have no effect on the direct energy application.

[0042] According to a further preferred embodiment of the heating tube this can be made of ceramic material. When such heating tubes are used in an induction oven, the magnetic field penetrates the heating tube i.e. the ceramic material of the heating tube is transparent to the magnetic field. The metal bolt is then heated by direct interaction with the magnetic field of the induction oven.

[0043] Suitable ceramic materials are for example Al2O3, Al3O4, BN, SiC, Si3N4, MgO, TiO, ZrO2, stabilised such as yttrium-stabilised ZrO2 glasses or refractory cements or mixtures which contain the said materials. Equally preferably, the heating tube can consist of fibre-reinforced ceramic material or contain such materials, and the fibres of the fibre-reinforced ceramic material can for example be made of SiC, Al2O3, glass or carbon.

[0044] To guarantee the insertion of the metal bolt into the heating tube, the internal diameter dR of the heating tube in the cold state, in particular at room temperature i.e. a temperature from 15° C. to 30° C., is suitably slightly larger than the bolt diameter dB of the cold metal bolt.

[0045] The internal diameter dR of the heating tube as a function of the bolt diameter dB is preferably selected such that the metal bolt in the cold state, in particular at room temperature i.e. a temperature from 15° C. to 30° C., is smaller by a dimension &Dgr;d of around 0.5 mm, preferably 0.5±0.3 mm, in particular 0.5±0.1 mm, than the internal diameter dR of the heating tube.

[0046] During the heating process the metal bolt expands radially and axially so that at a particular time the bolt diameter dB corresponds to the internal diameter dR of the heating tube, where it should be taken into account that the internal diameter dR is also temperature-dependent.

[0047] When a metal heating tube, in particular when a steel heating tube is used, during the heating process of the metal bolt—as soon as, due to thermal expansion, the bolt diameter dB corresponds to the internal diameter dR of the heating tube—a very even radially symmetrical heat transfer is guaranteed from the heating tube to the metal bolt. An even radially symmetrical heat transfer is of great importance, in particular after reaching the solidus temperature of the metal bolt, as a local excess temperature in a temperature range above the solidus temperature causes a corresponding local melting of the bolt material and consequently must be avoided.

[0048] As part of the inventive activity it has been shown that the use of a thin lubricant layer to avoid adhesion of the metal bolt to the heating tube has practically no effect on the heat transmission between the bolt and the heating tube. It must be noted that the lubricant used as a separating agent—where necessary at all—is usually only used for heating tubes of metal, i.e. when heating tubes of ceramic material are used no separating agent is normally required.

[0049] In a preferred embodiment of the pretreatment device according to the invention the material of the heating tube and the internal diameter dR of the heating tube are selected such that the metal bolt at a solidus temperature Tsolidus has substantially the same diameter dB as the internal diameter dR of the heating tube. Furthermore, preferably the bolt diameter dB at Tsolidus corresponds to the relationship 0.996 dR≦dB≦dR, particularly preferably 0.998 dR≦dB≦dR and in particular 0.999 dR≦dB≦dR.

[0050] The metal bolts are cylindrical and usually have a round or oval cross-section but can however also have a polygonal cross-section. The diameter of the metal bolt in the cold state is for example 50 to 180 mm, suitably 75 to 150 mm and preferably 100 to 150 mm. The length of the metal bolt in cold state is for example 80 to 500 mm.

[0051] When the metal bolt is heated the metal bolt expands as does the heating tube. Different materials have different thermal expansion co-efficients. The thermal expansion co-efficient of steel or ceramic is substantially less than that of aluminium or aluminium alloys for example. Consequently, a metal bolt of an aluminium alloy for example expands more than a heating tube of steel or ceramic material for example, so that starting from a metal bolt diameter which in the cold state is smaller than the internal diameter of the heating tube, at a particular temperature the metal bolt has the same diameter as the heating tube.

[0052] According to the invention it is now preferred that at the solidus temperature Tsolidus of the metal bolt material, the diameter of the metal bolt substantially corresponds to the diameter of the heating tube, so that in the temperature range in which the thixotropic properties of the metal bolt are substantially set, an optimum thermal contact is formed between the heating tube and the metal bolt. When the heating tube containing the metal bolt is heated above the solidus temperature Tsolidus, a melting of the eutectic occurs associated with a volume expansion of the metal bolt. The eutectic of the aluminium alloy suitable for thixoforming is typically formed at around 550 to 570° C. The volume expansion in the range between the solidus and the liquidus temperature for thixotropic aluminium alloys is typically around 1.0 to 5.5% and in particular 1.5 to 3%. At the heating or warming phase above the solidus temperature Tsolidus, which for aluminium alloys suitable for thixoforming is typically around 520 to 550° C., the bolt material can expand only in the longitudinal direction of the heating tube provided that the condition is fulfilled whereby at Tsolidus the bolt diameter dB corresponds to diameter dR of the heating tube. For aluminium alloys the volume expansion between the solidus and liquidus states—depending on bolt length—is manifested in an increase in bolt length of typically 3 to 16 mm and in particular 3 to 6 mm. The length increase of an aluminium bolt below the solidus temperature—depending on bolt length—is typically between 1 and 2 mm.

[0053] Because of the length change of the metal bolt during its transition to the thixotropic state and at least partial insertion of sealing elements into the cavity of the heating tube, the length of the heating tube must be greater than the bolt length. Preferably, the length of the heating tube is selected so that the heating tube in total is around 5 to 30 mm, in particular 10 to 20 mm longer than the metal bolt to be heated therein. Thus, when the metal bolt is heated in a horizontal position on each side of the heating tube the surface of the heating tube preferably projects beyond the surface of the metal bolt by around 2.5 to 15 mm and in particular by 5 to 10 mm. When the metal bolt is heated in the vertical position the surface of the heating tube at the top tube end projects over the surface of the metal bolt preferably by around 5 to 30 mm and in particular 10 to 20 mm.

[0054] The wall thickness of the heating tube for steel tubes is preferably 1 to 5 mm, for copper tubes preferably 4 to 10 mm and for ceramic tubes 8 to 15 mm.

[0055] The heating tube according to the invention can be closed on both sides. For this, sealing elements of ceramic material are preferably used. Suitable ceramic materials for the sealing elements are the same materials as described above for a preferred embodiment of the heating tube of ceramic material. Ceramic material has a low heat conductance towards the metal bolt so that the radial temperature distribution at the surface edge areas of the metal bolt is only slightly influenced by such sealing elements.

[0056] When the metal bolt is in a horizontal position during the heating process, the heating tube is suitably tightly sealed on both surfaces by means of sealing elements, preferably by stopper- or peg-like sealing elements. The seal here must prevent the emergence of liquid metal from the heating tube during the heating process.

[0057] Also, preferably the stopper- or peg-like sealing elements are selected with regard to material and shape such that their friction properties in the heating tube firstly allow a shift in the direction of the longitudinal axis of the heating tube due to thermal expansion of the metal bolt during the heating process, and secondly prevent a shift due to the pressure exerted by the metal bolt on the sealing element after reaching the temperature necessary for the required thixotropic state. The sealing elements are preferably pushed into the heating tube before the heating process so far that they stand in direct mechanical contact with the surfaces of the metal bolts. This causes the sealing elements to move during the heating process in the heating tube due to the thermal length expansion of the metal bolt.

[0058] When the metal bolt is in a substantially vertical position during the heating process, the heating tube is tightly closed on one side at the lower tube end. Here too, the seal must merely prevent the emergence of liquid metals from the heating tube. The heating tube can for this be placed directly on a preferably height-adjustable table plate, preferably a table plate of ceramic material, or the heating tube can be tightly sealed by a sealing element, preferably a stopper- or peg-like sealing element, and by means of this sealing element placed in a vertical position on a preferably height-adjustable table plate made of any heat-resistant material. In particular, when using a stopper- or peg-like sealing element, the sealing element is designed with regard to material choice and shape preferably such that firstly during the heating process in the oven no liquid metal can emerge from the heating tube and secondly the friction of the sealing element in the heating tube is less than 10 N.

[0059] The friction of the sealing element must be slight so that a gripper arm of a transport device, in particular a robot gripper arm, can raise the heating tube from the sealing element on the table plate without great force, i.e. without the gripper arm needing to be exposed to great mechanical stress. Secondly, a high friction is not necessary to achieve a high seal as the sealing element need merely prevent the emergence of liquid metal during the heating process, and because of the cohesion of metal melts in particular aluminium melts, this does not require a high degree of seal. Suitably, the friction of the sealing element in the heating tube is less than 30 N, preferably between 2 and 20 N and in particular between 5 and 10 N.

[0060] The pretreatment device according to the invention is suitable for the provision of a thixotropic metal bolt in a casting chamber of a vertical or horizontal thixoforming device. Particularly advantageously this pretreatment device is however used to provide a thixotropic metal bolt in a horizontal casting chamber as here the form retention can be guaranteed particularly well with the device according to the invention. In a horizontal thixoforming device the casting chamber which holds the thixotropic metal bolt is horizontal.

[0061] The device according to the invention is particularly advantageous for the provision of thixotropic metal bolts of aluminium or aluminium alloys. Aluminium bolts are quite particularly preferably heated in an induction oven with a vertical heating chamber.

[0062] The task for the process is solved in that the container constitutes a cylindrical heating tube, the metal bolt always remains in the heating tube during the heating process and subsequent transport to the casting chamber, and the heating tube containing the metal bolt is positioned in the casting chamber such that during the subsequent thixoforming process the plunger of the thixoforming device can push the thixotropic metal bolt out of the heating tube.

[0063] Preferred embodiments of the process according to the invention are described in the dependent claims 13 to 17. The statements concerning the pretreatment device according to the invention also apply accordingly, with regard to special features and details, to the process according to the invention.

[0064] The heating tube containing the thixotropic metal bolt is transported after the heating process and any dripping of the metal melt emerging from the metal bolt during the heating process, by for example a robot to a casting chamber and placed in the front half-open part of the casting chamber. After the pretreatment i.e. at the start of the actual thixoforming process, the plunger pushes the metal bolt from the heating tube into a closed part of the casting chamber; the thixotropic metal alloy is then passed through a passage opening into sprues and hence into the moulding cavity. After the thixoforming process the plunger is retracted so that then a gripper arm of the transport device collects the heating tube from the casting chamber and returns it for use in the subsequent pretreatment process. The return of the heating tube for re-use in the subsequent treatment process suitably takes place during the solidification phase of the thixotropic metal alloy in the moulding cavity. The casting structure which has formed during the solidification of the thixotropic metal alloy in the moulding cavity in essence determines the properties of the moulding. This structure is formed characterised by phases such as mixed crystal and eutectic phases, the casting granulation such as globulites and dendrites, segregations and structure faults such as porosity (gas pores, micro-pinholes) and contaminants for example oxides.

[0065] Preferably, after the heating process but before insertion of the heating tube containing the thixotropic metal bolt into the casting chamber, the liquid metal which emerged from the metal bolt during the heating process is at least partly removed from the heating tube. The liquid metal part emerging from the metal bolt during the heating process is typically less than 1 w. % of the bolt material.

[0066] The transport of the thixotropic metal bolt from the heating oven to the casting chamber by means of robots typically takes between 5 and 30 s and preferably 8 to 15 s. The time during which the thixotropic metal bolt remains in the casting chamber is typically between 3 and 5 s. This time is required to remove a robot with the arm away from the casting chamber and for electronic readiness checking of a thixoforming device.

[0067] The pretreatment process according to the invention brings substantial advantages, in particular:

[0068] it leads to a substantial reduction in heat loss of the thixotropic metal during the transport from the heating oven to the casting chamber and in the casting chamber thanks to the heating tube heated to the same temperature as the metal bolt;

[0069] by the settling of the metal bolt within the heating tube in the casting chamber, in particular in the casting chamber of a horizontal thixoforming device, it eliminates the shock of dropping, guaranteeing retention of the form and homogeneity of the metal bolt;

[0070] it avoids the separation of the liquid part on insertion of the thixotropic metal bolt into the casting chamber, so that the associated premature solidification of liquid metal in the casting chamber is avoided;

[0071] to a considerable extent it prevents oxidation of the metal bolt surface, as during the heating process and transport through the casting chamber and during its stay in the casting chamber until the start of the actual thixoforming process, the metal bolt is not exposed to free atmosphere;

[0072] it reduces the risk of air inclusions and oxides in the casting chamber during the filling phase of the moulding cavity, as firstly the oxide formation is reduced and secondly by retention of form of the metal bolt during pretreatment, air inclusions through bolt deformations are avoided.

[0073] The said advantages have a direct influence on the quality of the moulded parts produced; this reduces the rejection rate.

[0074] The pretreatment device according to the invention and the process according to the invention are suitable for provision of thixotropic metal bolts in vertical or horizontal casting chambers. Preferred applications of the process according to the invention are described in the application claims 18 and 19.

EXAMPLE

[0075] To verify the heating principle in a heating tube a circular cylindrical aluminium bolt of diameter 100 mm and length 200 mm is heated in the vertical position in an oven with resistance heating to the necessary temperature above the solidus temperature, where the final temperature and the time-dependent temperature profile of the heating oven are selected such that at the end of the heating process a thixotropic metal bolt is present with a liquid part of around 50 w. %. During the heating process the aluminium bolt is in a heating tube made of special stainless steel with a wall thickness of 5 mm. The heating tube and hence also the metal bolt lie at the lower end on a heat insulation plate. At the upper end of the heating tube its circular upper edge projects around 5 mm above the top bolt edge. The upper end of the heating tube is not closed so that the length change can be measured by means of a laser interferometer throughout the heating process.

[0076] The bolt temperature is assessed continuously during the heating process by means of thermo-elements lying parallel to the bolt longitudinal axis, where—in relation to the concentric longitudinal axis of the aluminium bolt—a first thermo-element to measure the edge temperature T0 is introduced in the edge area of the aluminium bolt, a second thermo-element to measure temperature T1 is positioned in the centre between the bolt centre and bolt edge, and a third thermo-element is arranged around 5 mm from the bolt centre to measure temperature T2. The thermo-elements are inserted around 50 mm deep into the bolt. The time-dependent temperature profiles T0(t), T1(t) and T2(t) measured with the said three thermo-elements are shown in FIG. 3 and within a measurement accuracy of around 1% substantially all show the same temperature development.

[0077] The length change in the metal bolt measured during the heating profile shown in FIG. 3 is shown in FIG. 4. This shows that the aluminium bolt until reaching the solidus temperature expands in the linear direction by around 1.5 mm, where above the solidus temperature the thermal length expansion increases greatly.

[0078] To study the dimensional stability of the thixotropic bolt an aluminium bolt according to the invention was heated in the vertical position until the thixotropic bolt had a liquid part of around 50 w. %, then removed from the oven, transferred to a horizontal position and pushed out of the heating tube. The test of the geometric shape of the thixotropic aluminium bolt shows that dimensional stability exists, i.e. the thixotropic bolt has in essence, apart from the thermal expansion, the same shape as the original aluminium bolt.

[0079] It is also found that during the heating process only very little liquid metal emerges from the metal bolt. Also, the thixotropic bolt retains its smooth surface even during the heating process. No oxidation traces can be detected on the surface. The test of the liquid metal distribution by means of a cutting test also shows that the homogeneity of the thixotropic state is also very well achieved.

[0080] Further advantages, features and details of the invention arise from the following description of FIGS. 1 to 4 and the drawings.

[0081] FIG. 1 shows diagrammatically the temporal sequence of the substantial process steps for provision of a thixotropic metal bolt in the casting chamber of a horizontal thixoforming device, where the metal bolt is transferred to a thixotropic state in the horizontal position;

[0082] FIG. 2 shows diagrammatically the temporal sequence of the substantial process steps for provision of a thixotropic metal bolt in the casting chamber of a horizontal thixoforming device, where the metal bolt is heated in a vertical position;

[0083] FIG. 3 shows as an example a typical heating curve;

[0084] FIG. 4 shows as an example a typical temperature-dependent deformation curve of a metal bolt during the heating process according to the invention.

[0085] Drawings a) to c) of FIG. 1 each show a vertical linear section along the concentric longitudinal axis l of a metal bolt 10, through the device elements 14, 20, 30 respectively, in which the metal bolt 10 lies during pretreatment, where the heating process of metal bolt 10 takes place in a horizontal position.

[0086] FIG. 1a) shows the loading of a metal bolt 10 in a solid state of aggregation into a horizontal heating tube 14. The heating tube 14 is closed with stopper-like sealing elements 16, 18, where the sealing elements 16, 18 lie on one side on the surfaces 15 of the heating tube 14 and on the other side close flush with the metal bolt 10 i.e. the sealing elements 16, 18 lie within the heating tube 14 on the surfaces 12 of the metal bolt 10.

[0087] The heating tube 14 containing the metal bolt 10 and closed with the heating elements 16, 18 is inserted horizontally in the heating chamber 21 of an induction oven 20. The heating tube 14 is here in the centre of the heating chamber 21 surrounded by induction coils 22, i.e. the concentric longitudinal axis of the heating chamber 21 on the concentric longitudinal axis l of the metal bolt 10 coincide. During the heating process the metal bolt expands initially in all directions. When the metal bolt reaches its solidus temperature Tsolidus, the metal bolt 10 presses against the heating tube 14 so that the metal bolt 10 in essence cannot expand further radially i.e. further radial expansion of the metal bolt 10 is restricted to the normally very low radial expansion of the heating tube 14. Thereafter, the further thermal expansion of the metal bolt 10 after reaching the solidus temperature Tsolidus is possible in essence only in the direction of its concentric longitudinal axis l, where the sealing elements 16, 18 are pushed apart corresponding to the thermal expansion of the metal bolt 10 so that the stopper-like sealing elements 16, 18 no longer lie on the surfaces 15 of the heating tube 14.

[0088] FIG. 1b) shows the discharging of the induction oven 20 i.e. extraction of the heating tube 14 containing the thixotropic metal bolt 10 from the heating chamber 21 of the induction oven 20. The sealing elements 16, 18 are separated from the heating tube 14 after discharging from the induction oven 20. The liquid metal melt 24 which emerged from the metal bolt 10 during the heating process is then removed from the heating tube 14 by allowing the liquid metal 24 to drip from the heating tube 14, where the liquid metal is caught for example in a catchment dish (not shown).

[0089] FIG. 1c) shows the heating tube 14 introduced in a casting chamber 30 of a horizontal thixoforming device. The heating tube 14 is positioned in the casting chamber cavity 32 of the casting chamber 30 such that during the subsequent thixoforming process, the plunger 34 can push the thixotropic metal bolt 10 out of the heating tube 14 so that the thixotropic metal alloy is then introduced through the passage opening 36 into the sprues (not shown) and thence into the moulding cavity (not shown). The casting chamber 30 has a recess to hold the heating tube. This recess serves firstly to centre the heating tube 14 and secondly as a stop for confining the heating tube 14 during the emergence of the thixotropic metal bolt 10 at the start of the thixoforming process.

[0090] Drawings a) to e) in FIG. 2 each show a vertical linear section along the concentric longitudinal axis/of a metal bolt 10, through device elements 14, 20, 30 respectively, in which the metal bolt 10 is in the pretreatment phase, where the heating process of the metal bolt 10 takes place in a vertical bolt position.

[0091] FIG. 2a) shows the introduction of a metal bolt 10 in a vertical heating tube 14 into a vertical cylindrical heating chamber 21 of an induction oven 20. The heating tube 14 is closed at its lower tube end i.e. at the bottom surface 15 of the heating tube 14 with a stopper-like sealing element 16. The sealing element lies on a table plate 26. The heating tube 14 containing the metal bolt 10 is inserted in the induction oven 20 through vertical placement of the heating tube 14 on the table plate 26, where the sealing element 16 comes to lie on the table plate 26, and by raising the table plate 26 until the heating tube 14 comes to lie fully in the heating chamber 21 of the induction oven 20. The heating tube 14 is centrally positioned in the heating chamber 21 bordered by the induction coils 22 i.e. the concentric longitudinal axis of the heating chamber 21 coincides with the concentric longitudinal axis l of the metal bolt 10.

[0092] During the heating process the metal bolt initially expands both in the radial and vertical directions. When the metal bolt reaches its solidus temperature Tsolidus, the metal bolt 10 presses radially against the heating tube 14 so that the metal bolt 10 can in essence not expand further radially i.e. the further radial expansion of the metal bolt 10 is restricted to the normally very low radial expansion of the heating tube 14. Then the further thermal expansion of the metal bolt 10 after reaching the solidus temperature Tsolidus is possible in essence only in the vertical direction parallel to its concentric longitudinal axis l. The upper tube end 15 of the heating tube 14 is open so the metal bolt 10 can expand thermally upwards unhindered.

[0093] FIG. 2b) shows the induction oven 20 after the heating tube 14 with thixotropic metal bolt 10 has been removed from the heating chamber 21. The thixotropic metal bolt is here withdrawn by lowering the table plate 26.

[0094] FIG. 2c) shows the heating tube 14 withdrawn vertically from the oven and containing the thixotropic metal bolt 10, standing vertically on the table plate 26 and still tightly sealed with the stopper-like sealing element 16.

[0095] FIG. 2d) shows the heating tube 14 containing the thixotropic metal bolt 10 separated from the table plate 26 and the sealing element 16, in a horizontal position. Suitably, the heating tube 14 is separated from the sealing element 16 and transferred to a horizontal position by means of a robot. For this, the force required by the robot arm for raising the heating tube 14 from the table plate 26 and the sealing element 16 is very low.

[0096] The sealing element 16 closes the heating tube 14 as a tight form fit. The seal however need merely prevent the emergence of liquid metal so that, because of the surface tension of a liquid metal, the sealing element 14 in essence need only engage as a form-fit in the heating tube 14 and therefore no high friction is required between the heating tube 14 and the sealing element 16.

[0097] The thixotropic metal bolt is clamped in the heating tube i.e. its adhesion is sufficiently large that the heating tube can be raised vertically from the sealing element without the thixotropic metal bolt 10 falling out of the heating tube 14. The vertical lifting of the heating tube 14 from the sealing element 16 defined on the table plate 26 also allows the dripping of the liquid metal 24 formed during the heating process outside the heating oven 20.

[0098] FIG. 2e) shows the heating tube 14 introduced into a casting chamber 30 of a horizontal thixoforming device. Here the heating tube 14 is positioned in the casting chamber cavity 32 of the casting chamber 30 such that during the subsequent thixoforming process the plunger 34 can expel the thixotropic metal bolt 10 from the heating tube so that the thixotropic metal alloy can then be introduced through the passage opening 36 into the sprues (not shown) and thence into the moulding cavity (not shown). The casting chamber 30 has a recess to hold the heating tube. This recess serves firstly to centre the heating tube 14 and secondly as a stop to confine the heating tube 14 during expulsion of the thixotropic metal bolt 10 at the start of the thixoforming process.

[0099] The thixotropic metal bolt 10 is laid in the casting chamber cavity 32 of the casting chamber 30 suitably by means of a robot. The thixotropic metal bolt must be inserted so gently that the form of the metal bolt 10 is retained after insertion in the casting chamber 30.

[0100] FIG. 3 shows a typical heating curve, until the solidus temperature Tsolidus is achieved, of an aluminium bolt 10 in a heating tube 14 according to the invention in a resistance oven. The heating curve concerns a circular cylindrical aluminium bolt 10 in a heating tube 14 of stainless steel, with a diameter of 100 mm and a length of 200 mm, in a vertical position, where the heating tube 14 has a wall thickness of 5 mm and the lower surface 12 of the aluminium bolt 10 lies directly on a heat insulation plate 26 i.e. the lower surface 12 of the aluminium bolt 10 and the lower surface 15 of the heating tube 14 lie in the same plane.

[0101] The heating curve, i.e. the time-dependent bolt temperature, is determined continuously by means of thermo-elements lying parallel to the bolt longitudinal axis l during the heating process, where—in relation to the concentric longitudinal axis l of the aluminium bolt 10—a first thermo-element to measure the edge temperature T0(t) is introduced into the edge area of the aluminium bolt 10, a second thermo-element to measure temperature T1(t) is positioned in the centre between the bolt centre and the bolt edge, and a third thermo-element to measure the temperature T2(t) is arranged at a distance of around 5 mm from the bolt centre. The thermo-elements are inserted around 50 mm deep in the bolt 10. The time-dependent temperature profiles T0(t), T1(t) and T2(t) measured with the said three thermo-elements are drawn in FIG. 3 and—within a measurement accuracy of ±1%—substantially all show the same temperature development.

[0102] FIG. 4 shows as an example a typical temperature-dependent deformation curve during the heating process according to the invention of the aluminium bolt 10 shown by the heating curve in FIG. 3. FIG. 4 shows that the aluminium bolt 10 expands substantially linearly, temperature-dependent, by approximately 1.5 mm in the longitudinal direction until the solidus temperature Tsolidus is reached at around 560° C., where above the solidus temperature the thermal linear expansion &Dgr;L(T) increases suddenly.

Claims

1. Pretreatment device for provision of a thixotropic metal bolt (10) in a casting chamber (30) of a thixoforming device, containing a container to hold a metal bolt (10), an oven (20) to transfer the metal bolt (10) in the container into a part-liquid thixotropic state, and a transport device for transporting and introducing the thixotropic metal bolt (10) into the casting chamber (30), characterised in that

the container is a cylindrical heating tube (14) which can be closed at the sides, and the pretreatment device is formed such that the metal bolt (10) can remain in the heating tube (14) throughout the entire pretreatment, namely the heating process in the oven (20) and transport to the casting chamber (30) and its stay in the casting chamber (30) until the start of the thixoforming process.

2. Pretreatment device according to claim 1, characterised in that the internal diameter dR of the heating tube (14) as a function of the bolt diameter dB is selected such that the metal bolt (10) at its solidus temperature Tsolid substantially has the same diameter dB as the internal diameter dR of the heating tube (14).

3. Pretreatment device according to claim 1 or 2, characterised in that the heating tube (14) consists of metal, preferably steel, in particular stainless steel or tool steel.

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

5. Pretreatment device according to any of claims 1 to 4, characterised in that when the metal bolt (10) is in a substantially horizontal position during the heating process, the heating tube (14) is tightly closed on both sides by sealing elements (16, 18), where the sealing elements (16, 18) are designed with regard to material choice and form such that their friction properties in the heating tube (14) firstly allow a shift due to thermal expansion of the metal bolt (10) during the heating process in the direction of the longitudinal axis l of the heating tube (14), and secondly prevent a shift due to the pressure exerted by the metal bolt (10) on the sealing elements (16, 18) after reaching the temperature necessary for the desired thixotropic state.

6. Pretreatment device according to any of claims 1 to 4, characterised in that for a substantially vertical metal bolt position during the heating process, the heating tube (14) is closed on one side at the lower tube end (15), where on use of a sealing element (16) the sealing element (16) is designed with regard to material choice and form such that firstly during the heating process in the oven (20) no liquid metal (24) can emerge from the heating tube (14) and secondly the friction of the sealing element (16) in the heating tube (14) is less than 10 N.

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

8. Pretreatment device according to any of claims 1 to 7, characterised in that the oven (20) is an induction oven.

9. Pretreatment device according to any of claims 1 to 8, characterised in that the transport device is a robot where the clamping device of the robot to hold the heating tube at least on the surface directed towards the heating tube consists of ceramic material.

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

11. Use of a pretreatment device according to any of claims 1 to 9, for provision of a thixotropic metal bolt (10) from an aluminium alloy.

12. Process for provision of a thixotropic metal bolt (10) in a casting chamber (30) of a thixoforming device, where a metal bolt (10) in a solid state of aggregation is placed in a container, and the metal bolt (10) in the container is heated in an oven (20) until the metal bolt (10) is in a thixotropic state, and the thixotropic metal bolt (10) is transported by means of a transport device into the casting chamber (30) of a thixoforming device, characterised in that

the container is a cylindrical heating tube (14), the metal bolt (10) remains in the heating tube (14) throughout the heating process and subsequent transport into the casting chamber (30), and the heating tube (14) containing the metal bolt (10) is positioned in the casting chamber (30) such that during the subsequent thixoforming process the plunger (34) of the thixoforming device can push the thixotropic metal bolt (10) out of the heating tube (14).

13. Process according to claim 12, characterised in that the internal diameter dR of the heating tube (14) as a function of the bolt diameter dB is selected such that at room temperature the internal diameter dR is greater than the bolt diameter dB and at the solidus temperature Tsolidus of the metal bolt (10) the bolt diameter dB substantially corresponds to the internal diameter dR of the heating tube (14).

14. Process according to claim 12 or 13, characterised in that after the heating process but before insertion of the heating tube (14) containing the thixotropic metal bolt (10) into the casting chamber (30), the liquid metal (24) which emerged during the heating process from the metal bolt (10) is at least partly removed from the heating tube (14).

15. Process according to any of claims 12 to 14, characterised in that the heating tube (14) is sealed on at least one side during the heating process of the metal bolt (10).

16. Process according to any of claims 12 to 15, characterised in that a heating tube (14) which is tightly closed at one end, at the lower tube end (15), with a sealing element (16) and containing the metal bolt (10), is placed in a vertical position vertically onto the table plate (26), preferably onto a table plate (26) of ceramic material, and the table plate (26) is introduced vertically, preferably from below, into the oven (20), and the heating tube (14) containing the metal bolt (10) is heated until the metal bolt (10) is a thixotropic state, and subsequently the heating tube (14) containing the thixotropic metal bolt (10) is removed vertically, preferably by lowering the table plate (26), from the oven (20) and separated from the sealing element (16).

17. Process according to any of claims 12 to 15, characterised in that the heating tube (14) containing the metal bolt (10) is tightly sealed on both sides with a sealing element (16, 18) which can be moved in the direction of the concentric longitudinal axis l of the heating tube (14) and with preset friction properties between the heating tube (14) and the sealing element (16, 18), and is introduced into the oven (20) in a substantially horizontal position, where during the heating process the two sealing elements (16, 18) due to thermal expansion of the bolt material (10) are pushed apart and after reaching the thixotropic state the sealing elements (16, 18) are held in their position due to friction, and after removal of the heating tube (14) containing the metal bolt (10) from the oven (20) the two sealing elements (16, 18) are removed from the heating tube (14).

18. Use of the process according to any of claims 12 to 17 to provide a thixotropic metal bolt (10) in a horizontal thixoforming device.

19. Use of the process according to any of claims 12 to 17 to provide a thixotropic metal bolt (10) from an aluminium alloy.