Abstract:
A hot chamber method of die casting material in a semisolid state. The semisolid material has a high viscosity, which can be controlled by controlling the fraction of solid phase and the morphology of the solid phase. By controlling the viscosity of the melt, turbulence and consequent gas entrapment can be minimized or eliminated. Further, shrinkage is substantially reduced, thereby reducing porosity and hot tearing to form stronger, more reliable castings.

Description:
[0001]    This application is a continuation of and claims the priority of U.S. patent application Ser. No. 09/228,965, filed Jan. 12, 1999, the entire contents of which are incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This application relates to methods and apparatus for hot chamber die casting of semisolid materials.  
         BACKGROUND OF THE INVENTION  
         [0003]    Die casting has traditionally been divided into cold chamber processes and hot chamber processes. Hot chamber processes are distinguished by the fact that the injection cylinder is at least partially immersed in the molten metal, and thus is at the same temperature as the molten metal. Hot chamber die casting is widely used for light alloys such as magnesium- and zinc-based alloys, but has not been found to be commercially viable for casting aluminum alloys. These alloys generally have a higher melting temperature, and thus tend to rapidly degrade steel die casters using a hot chamber process.  
           [0004]    Advantages of the hot chamber casting process include higher productivity, reduced scrap and metal losses, reduced die closing pressures, and reduced die wear. Both hot and cold chamber processes, however, suffer from the disadvantage that it is difficult to produce fully sound castings. Liquid metal generally enters the die in a turbulent fashion, entrapping mold gases and forming oxide inclusions in the finished part. Further, solidification shrinkage produces porosity and sometimes tears in the finished casting. It is an object of the present invention to provide a hot-chamber die casting system which minimizes or eliminates these disadvantages.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention supplies a hot chamber method of die casting material in a semisolid state. The semisolid material has a high viscosity, which can be controlled by controlling the fraction of solid phase and the morphology of the solid phase. By controlling the viscosity of the melt, turbulence and consequent gas entrapment can be minimized or eliminated. Further, shrinkage is substantially reduced, thereby reducing porosity and hot tearing to form stronger, more reliable castings.  
           [0006]    In one aspect, the invention provides a method of die casting, in which a semisolid composition is held between its liquidus and solidus temperatures, and agitated to prevent the formation of interconnected dendritic networks. The composition forms a slurry of solid particles in liquid, which is pumped into a die by an immersed pump. The material is then cooled to cast it in the die. The material may be, for example, a light alloy such as a magnesium, zinc, or aluminum alloy.  
           [0007]    In another aspect, the invention includes a hot chamber die caster adapted to cast semisolid materials. The die caster includes a container for holding a composition in the semisolid state, and a pump for pumping the semisolid material into a die. Agitation means prevent the formation of dendrites, holding the material in a semisolid slurry state. The agitation means may be, for example, mechanical or electromagnetic. The caster may be used to cast a variety of light alloys, such as magnesium, zinc, and aluminum alloys. The pump may comprise ferrous materials such as stainless steel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    The invention is described with reference to the several figures of the drawing, in which,  
         [0009]    [0009]FIG. 1 is an illustration of a typical hot chamber die casting machine;  
         [0010]    [0010]FIG. 2 is an illustration of one embodiment of a hot chamber die caster according to the invention; and  
         [0011]    [0011]FIG. 3 is an illustration of another embodiment of a hot chamber die caster according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 1 shows a typical hot chamber die caster  10 , such as is commonly used for casting of magnesium and zinc alloys. The caster works on a “sump pump” principle, using an immersed piston  12  to force molten metal into the casting chamber  14 .  
         [0013]    A hydraulic cylinder  16  reciprocates the piston  12 , within a piston chamber  17  whose end is connected to a gooseneck chamber  18  leading to the casting chamber  14 . As the piston  12  reaches the top of its stroke, molten metal  20  flows into the piston chamber  17  and the gooseneck chamber  18  through an aperture  22 . When the piston  12  then moves down into the chamber  18 , it seals the aperture  22  and forces molten metal into the casting chamber  14 . The casting chamber  14  is defined by two mold halves  24  and  26 . Once the molten metal  20  in the casting chamber  14  has solidified, mold half  26  is moved to release the cast part. The mold is then closed and another cycle of the system can be performed. The gooseneck  18  and cylinder head  16  are thus continuously exposed to molten metal in this process.  
         [0014]    The semisolid (or rheocasting) process was discovered about twenty years ago in the laboratory of one of the present inventors. It was found that mechanical stirring of a material between the liquidus and solidus temperatures could break up dendrites, forming a slurry of spheroidized solid particles in liquid. The viscosity of the material can be set to a value in the range of 10 −1 -10 8  poise, simply by controlling the stirring rate. Detailed descriptions of semisolid processing techniques can be found, for example, in U.S. Pat. Nos. 3,954,455 and 3,948,650 to Flemings, et al., both of which are incorporated herein by reference. Rheocast castings are generally of more uniform strength and of lower porosity than conventional castings.  
         [0015]    The present invention uses semisolid processing to die cast materials using a hot chamber process. FIG. 2 shows a die caster designed to carry out this process. It is similar to the die caster shown in FIG. 1, but includes a mechanical stirrer  28  for agitating semimolten metal  21 . In the embodiment shown, the furnace is provided with a cover  29  and a pressure inlet  30  to aid in forcing semimolten metal  21  through the aperture  22  into the piston chamber  17 . Added pressure is not necessary in standard hot-chamber casting processes, because of the very low viscosity of fully molten metal (typically on the order of 10 −2  poise). The higher viscosity of the semisolid compositions of the present invention may make applied pressure preferable or even essential, depending on the properties of the semisolid composition and of the caster material.  
         [0016]    The optimum applied pressure for any given embodiment depends on the solid fraction of the semisolid metal and the speed with which it is desired to fully fill the piston chamber  17 . It is preferred that die casters according to the invention be able to apply a pressure of at least 30 psi gauge (i.e., 30 psi above atmospheric pressure). If desired, applied pressure and the viscosity of the semisolid metal can be adjusted to provide a relatively high fill rate while minimizing the turbulence of flow into the casting chamber  14 .  
         [0017]    A temperature controller maintains the melt  20  within a relatively narrow temperature range, in order to ensure that it stays between the liquidus and solidus temperatures. For example, the liquidus and solidus temperatures differ by about 120° C. for Mg-8%Al-1%Zn, a common magnesium casting alloy. Known process-control techniques can be used to ensure that the metal temperature and viscosity are kept within acceptable limits.  
         [0018]    [0018]FIG. 3 depicts an embodiment of the die caster related to that of FIG. 2, but using electromagnetic, rather than mechanical, stirring means. A set of coils  32  is provided for heating and stirring the semimolten metal  20 . The use of electromagnetic stirring and heating may simplify the application of pressure, since the coils  32  do not need to be placed within the semimolten metal  21 .  
         [0019]    Hot chamber die casting of semisolid materials offers several advantages. The lower temperatures required may provide reduced energy costs and reduced wear rates for casters, and may expand the list of materials which can be inexpensively die cast by the hot chamber method. Further, the increased viscosity of the melt reduces turbulence as the melt enters the die. Reduced turbulence leads to minimal gas entrapment and thus to a reduced concentration of oxide inclusions. In addition, the shrinkage from the semisolid to the solid state is substantially less than that from the fully liquid to the solid state. Thus, shrinkage porosity and hot tearing are reduced in the present process, allowing simpler and less expensive mold designs to be used.  
         [0020]    Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.