Abstract:
The present invention relates to a method and apparatus for preparing a metal or metal-alloy product for a casting process—wherein the product is brought into a partly solidified (semi-solidified) state before casting—in which the product contains crystallization nuclei uniformly distributed throughout its volume. The method involves introducing an amount of a chosen alloy (in pulverized form) and an amount of a chosen melt, which is at a temperature above the liquefaction temperature of the alloy, into a crystallization vessel, which is heated to below the liquefaction temperature of the alloy, and mixing the melt and the alloy together in the crystallization vessel by means of electrical and/or magnetic forces to create the desired product.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of German patent application 10212349.7 filed Mar. 13, 2002, incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for preparing a metal or metal-alloy product for a casting process—wherein the product is brought into a partly solidified (semi-solidified) state before casting—in which the product contains crystallization nuclei uniformly distributed throughout its volume. 
     BACKGROUND 
     The production of semi-solidified metal or metal-alloy products is known, for example, from an article by J. -P. Gabathuler and J. Erling, entitled “Thixocasting: ein modernes Verfahren zur Herstellung von Formbauteilen” [Thixocasting: A Modem Method for Producing Molded Components], which was published in the proceedings of “Aluminium als Leichtbaustoff in Transport und Verkehr” [Aluminum as a Light Building Material for Transporting and Traffic], pages 63–77 (ETH Zürich, May 27, 1994). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to prepare a metal or metal-alloy product from a metal or metal alloy carrier material (hereinafter referred to as “melt”) and an alloy, the product having a homogeneous distribution of crystallization nuclei throughout its volume at a point prior to the product being introduced into a mold during the casting process. 
     The present invention achieves this object by introducing an amount of a chosen alloy (in pulverized form) and an amount of a chosen melt, which is at a temperature above the liquefaction temperature of the alloy, into a crystallization vessel, which is heated to below the liquefaction temperature of the alloy, and mixing the melt and the alloy together in the crystallization vessel by means of electrical and/or magnetic forces to create the desired product. 
     During the introduction of the alloy and the melt into the crystallization vessel, the pulverized particles of the alloy, which preferably is in a powdered form, are immediately enclosed by the melt to form crystallization nuclei, which are then homogeneously distributed within the subsequent mixture by means of the electrical and/or magnetic forces to form the product. 
     In another embodiment of the present invention, the melt is introduced into the crystallization vessel in the form of a stream flowing between two electrodes, which are supplied with an electrical voltage. The resulting stream is narrowed, based on the so-called pinch effect, compressed and is already partially split into individual liquid drops as the melt flows into the crystallization vessel. Thus, the crystallization vessel is not filled by means of compact and separate streams (one of melt and one of alloy), but rather by a dispersed stream in which the melt and alloy are partially intermingled. Such a dispersal means that the surface area of the resulting stream is clearly increased, so that degassing also occurs. 
     After the melt has completely flowed into the crystallization vessel, the melt stream disappears so that the flow of the dispersed product stream is also interrupted. For achieving further dispersion, and also for creating an electrical field, an electrical arc is established between the product and an electrode within the crystallization vessel after the introduction of the alloy and the melt into the crystallization vessel. 
     A magnetic field may be generated in the crystallization vessel to promote additional mixing of the product contained therein, and to improve the uniformity of the distribution of the crystallization nuclei therein. The magnetic field and the electrical field act in different ways on the product, and the particles contained therein, so that the mixing effect is enhanced. 
     In another embodiment of the present invention, the melt flows into the crystallization vessel, to which a vacuum has been applied. By creating a vacuum in the crystallization vessel, the dispersed melt stream is further dispersed into individual drops, increasing the mixing of the alloy with the melt and, thus promoting the formation of crystallization nuclei within the product. 
     In a further embodiment of the invention, a protective gas is added to the melt as it is being fed into the crystallization vessel. In particular, the process is further improved if the protective gas is supplied under pressure. The introduction of the protective gas prevents chemical reactions of the alloy with the atmosphere, which could negatively affect any subsequent casting process using the product. 
     In an apparatus for performing the method, a crystallization vessel with an inlet for melt and an inlet for alloy in powder form is provided. The crystallization vessel includes a heating arrangement and is provided in the area of its bottom and its melt inlet with electrodes connected to a voltage source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, embodiments, and advantages of the present invention will become apparent from the following detailed description with reference to the drawings, wherein: 
         FIG. 1  is a cross-sectional view, of a schematic representation of the present invention, illustrating the connection between the crystallization vessel and the furnace; 
         FIG. 2  is a cross-sectional view, of a schematic representation illustrating another embodiment of the present invention; 
         FIG. 3  is a cross-sectional view, of a schematic representation of another embodiment of the present invention illustrating the crystallization vessel with an added arrangement for receiving the processed melt; and 
         FIG. 4  represents a nomograph for predicting the thermo-kinetic progress of a product produced by the method of the present invention, specifically the alloy AISI9Cu 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to  FIG. 1 , in a furnace  10  a melt  11  of a metal alloy, for example AISI 9, is maintained at a temperature greater than the liquefaction temperature of the particular alloy. The furnace  10  is maintained at a vacuum by means of an exhaust device  12 . 
     The furnace  10  is connected to the crystallization vessel  14  by a casting conduit  13 . The crystallization vessel  14  includes a cylinder  15  made of an electrically non-conducting material that has a heat conducting capability between 0.20 and 1.5 W/mk. A cover  16 , made of an electrically nonconductive material, closes the top of the cylinder  15 . The casting conduit  13  is connected to the cover  16 . Preferably, a melt inlet element  17  extends from the casting conduit  13  through the cover  16  to allow the melt  11  to flow into the crystallization vessel  14 . The melt inlet element  17  has a conically widening inlet opening and is made of an electrically conductive material. A vacuum line  18  is connected to the cover  16  to provide communication between the crystallization vessel  14  and a suction removal device  19 , so that a vacuum may be created within the crystallization vessel  14 . The cover  16  is also provided with a filler neck  20 , through which alloy in powder form can be introduced into the crystallization vessel  14 . A piston  21 , also made of an electrically nonconducting material, is movably inserted into a bottom of the cylinder  15  to seal a bottom of the crystallization vessel  14 . The cylinder  15 , the cover  16  and the piston  21  form a chamber for mixing the melt and the alloy into the product. The piston  21  travels within a guide cylinder  22  which is connected to the crystallization vessel  14 . A product outlet port (not shown) is integral to the guide cylinder  22  and is used to affect the removal of the product from the crystallization vessel  14 . 
     A heating device  26  is arranged about the crystallization vessel  14 , to selectively heat and maintain the crystallization vessel  14  at a pre-selected temperature. Preferably, the heating device  26  is electrical and is adjustable. A magnetic coil  27  is arranged about the crystallization vessel  14 . The magnetic coil  27  preferably generates an adjustable magnetic field in the chamber defined by the cylinder  15 , the cover  16  and the piston  21  inside the crystallization vessel  14 . 
     A gate slide  28  is disposed within the casting conduit  13  to regulate flow of the melt from the furnace  10  to the crystallization vessel  14 . A gas supply line  29  is connected to the casting conduit  13 , through which a protective gas, for example argon, can be supplied to a melt stream flowing through the casting conduit  13 . Preferably, the protective gas is supplied under overpressure. 
     In a preferred embodiment, an electrode  23  is disposed on an interior of the cylinder  15 , preferably near the bottom of the cylinder  15  of the crystallization vessel  14 . As already mentioned, the melt inlet element  17  is made of an electrically conducting material. A voltage source  24  is connected to the electrode  23  and the melt inlet element  17  to provide electrical power to both. Preferably, the voltage source  24  is adjustable, in particular its current strength, by an adjustment device  25 . 
     The product is prepared by the method discussed as follows. The furnace  10  is maintained at a vacuum by operation of the exhaust device  12 . Preferably, the furnace  10  is maintained at a vacuum between about 0.5 mbar and 3 mbar. The melt within the furnace  10  is maintained at a temperature greater than the liquefaction temperature of the alloy. 
     The crystallization vessel  14  is heated to a temperature less than the liquefaction temperature of the alloy by selectively controlling the heating device  26  attached thereto. Preferably, the crystallization vessel  14  is maintained at a temperature which is about 3% to 50% lower than the liquefaction temperature of the respective alloy. The suction removal device  19  attached to the crystallization vessel  14  by the vacuum line  18  creates and maintains a vacuum within the crystallization vessel  14 . Preferably, the vacuum in the crystallization vessel  14  is greater than the vacuum maintained in the furnace  10  to promote the flowing of the melt from the furnace  10  into the crystallization vessel  14 . 
     Upon opening of the slide gate  28 , the melt  11  within the furnace  10  flows into the crystallization vessel  14 . Protective gas is supplied to the aspirating melt by the gas supply line  29 . The vacuum created within the crystallization vessel  14  causes the alloy powder to be dispensed into the crystallization vessel  14  through the filler neck  20 . The dispensed alloy powder is thus combined with the melt and is distributed therethrough to form the product. 
     A voltage is applied to the electrode  23  and the inlet element  17  by the voltage source  24  to establish an electrical current through the product within the crystallization vessel  14 . Preferably, the current is less than about 10 A. To promote as homogeneous as possible distribution of the crystallization nuclei within the product, radial movement of the product within the crystallization vessel  14  is created generating a magnetic field within the interior of the crystallization vessel  14  by the magnetic coil  27 . 
     Once the desired amounts of melt and alloy have been introduced into the crystallization vessel  14 , the electric current generated between the electrode  23  and the melt inlet element  17  may be temporarily interrupted. Thereafter an electrical current is established therebetween that preferably has a voltage between about 150 V and 400 V, so that an arc is ignited between the electrode and the product, the arc preferably having a current of up to about 1300 A. To prevent a directional orientation of the crystallization nuclei within the product, the magnetic field generated by the magnetic coil  27  is adjusted accordingly and, for example, is continuously increased in the direction of the fill. 
     After the product has been prepared in this manner, the piston  21  is lowered, so that the product flows out via the guide cylinder  22  and the product outlet port for further processing. The product prepared by the method disclosed herein is suitable for use with all known casting methods. 
     In another preferred embodiment, the electrode  23  is integrated into the piston  21 . 
     In another preferred embodiment, illustrated in  FIG. 2 , the voltage source  24  is connected to two electrodes  30  and  31  arranged, preferably, in a vertically spaced manner along a portion of the cylinder  15  of the crystallization vessel  14 . The voltage source is also connected to a portion of the casting conduit  13 . In this embodiment the piston  21  continuously moves downward while the melt and alloy are fed into the crystallization vessel, so that the electrodes  30  and  31  are sequentially employed and are switched on and off during the piston movement by means of switches  32  and  33 . 
     In another preferred embodiment, as shown in  FIG. 3 , the product prepared in the crystallization vessel  14  is passed on to a storage or transport vessel  34 , in which the product is maintained in its prepared state. The storage vessel  34  is provided with an exhaust device  35 , so that a vacuum may be established therein. A heating device  36  and a magnetic coil  37  are arranged about the storage vessel  34 . An electrode  38  is disposed within the storage vessel  34 . Finally, two opposing walls  39 ,  40  of the storage vessel  34  are comprised of pistons that manipulate the product as it is stored therein. The storage vessel  34  may for forming the product therein into a more desired configuration for continued storage or casting. 
     The thermo-kinetic progress of a particular melt/alloy product can be predicted by means of a nomograph. For example, a nomograph for the melt/alloy product AISI9Cu 3  is represented in  FIG. 4 . The amount of pulverized alloy—added at a grain size of approximately 125 μm to approximately 400 μm—is entered as percentile amounts (see vertical axis). The temperature difference (Delta T) in C.° is the difference between the casting temperature and the liquefaction temperature of the alloy (see horizontal axis). If the percentage amount of pulverized alloy added lies within the nomograph range A, it only causes a reduction in the temperature of the product, i.e., the product is placed into a semi-solidified state without the pulverized particles forming crystallization nuclei. If the percentage amount of pulverized alloy is added so that the nomograph range B is reached, then the pulverized particles act as additional, unmelted crystallization nuclei. Finally, and most desired, if the percentage amount of added pulverized particles lies within the C range of the nomograph, then the two processes will take place side-by-side, i.e. a reduction of the product temperature and formation of crystallization nuclei because of unmelted particles. It is of course necessary to draw different nomographs for different alloys. It is understood that products of different melts and alloys will have their own nomographs. 
     It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof.