Abstract:
The casting process of this invention focuses on the existing die-casting applications while offering benefits heretofore not available in die-casting. The benefits that the users receive from this process include longer tool life (up to 5 times). The other major benefit is better part quality (i.e., less internal porosity and better dimensional capability). The process utilizes molten metal  20  at the die cast machine  10.  This invention is distinguishable from semi-solid molding or forming due in large part to the use of molten or fully liquid metal  20.  The ladle  22  operation that delivers the molten metal  20  to the injection chamber  34  is another primary feature of this invention. A controller  28  is utilized that communicates with the furnace  30  and the ladle delivery system  22  to regulate the temperature of the molten metal  20  and the pour speed.

Description:
[0001]    This claims the benefit of U.S. Provisional Patent Application Serial No. 60/330,651 filed Oct. 26, 2001 and hereby incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to systems and methods for die-casting and, more particularly, to a system and method for low-velocity die-casting.  
           [0003]    Many industries, such as the automotive industry worldwide, have recently concentrated on downsizing to meet governmental, market and performance pressures. To accomplish target weight reductions, the car makers and others have increasingly looked to the use of aluminum and magnesium alloys for many parts and components. Additionally, they are looking at new developing processes for casting aluminum and magnesium components. These processes offer net shape, appropriate strength and economic potential. The two processes which have already met some target production needs and, as their technologies develop, will continue to expand their applications are squeeze casting and semi-solid metal casting (SSM).  
           [0004]    Each of these two processes has had a traditional history in the marketplace which included an original thinking initiative, the investigative theorizing by the university community, the normal trial and error of the first entrepreneurs and, finally, the risk acceptance by the first consumers. Each process has now progressed beyond this point to be classed “production ready” and a number of parts produced by each process are incorporated in current automotive models around the world. Processes of this type are disclosed in U.S. Pat. Nos. 4,771,818; 4,712,413; 4,709,746; 4,687,042; 4,607,682; and 4,569,218, each of which are hereby incorporated by reference in its entirety.  
           [0005]    While the majority of the current applications utilize aluminum alloys, the two processes are not raw material restrictive. Magnesium, copper, zinc, and to a limited degree, ferrous alloys have been cast in each of these processes.  
           [0006]    Squeeze and semi-solid metal casting can each be described as a casting process employing relatively slow ingate velocities, minimum turbulence and high pressure throughout solidification to consistently produce high integrity casting capable of solution heat treatment.  
           [0007]    While there are fundamental differences between squeeze casting and SSM casting, key aspects which distinguish these two processes from all other casting processes include:  
           [0008]    FILL—the actual filling of the cavity is accomplished at a wide range of speeds; however, tight control to avoid turbulence is essential.  
           [0009]    TURBULENCE—Both processes are designed to achieve minimum porosity and oxides; therefore, turbulence is to be avoided at all costs.  
           [0010]    PRESSURE—Through the use of hydraulic mechanisms, pressures in excess of 20 MPA and as high as 175 MPA (equipment dependent) are applied to the material in the die cavity.  
           [0011]    INTEGRITY—It is an acknowledged fact that a certain amount of porosity, both gas and shrinkage, degrades or reduces the integrity of castings. By eliminating these anomalies, or at least minimizing them, the integral quality of these two processes compares favorably with other high integrity forming processes, e.g., forging, extruding.  
           [0012]    CONSISTENCY—It has been demonstrated over a great many applications that there is minimum variability from part to part and from run to run.  
           [0013]    SOLUTION TREATMENT—The majority of parts produced by these two processes are heat treated to maximize their mechanical properties and fatigue life. This secondary processing requires the highest internal integrity in the high pressure casting.  
           [0014]    In both squeeze and semi-solid metal casting, the major process variants are:  
           [0015]    DIRECT—the pressure is applied directly to the metal (liquid or semi-solid from a hydraulically activated source).  
           [0016]    INDIRECT—Pressure is transmitted from the hydraulic source to the metal being held in a die cavity through a runner system.  
           [0017]    Nevertheless, the two processes are distinguishable. For example, squeeze casting begins with molten metal being positioned into a metallic container (die) and held there, under pressure, until solidification occurs.  
           [0018]    Squeeze casting is a term commonly used today to refer to any process in which liquid alloy is cast without turbulence and gas entrapment and subsequently held at high pressure throughout the freezing cycle to yield high quality heat treatable components. Squeeze casting originally was developed as a liquid forging process in which liquid metal was poured into the lower half of a vertically oriented die set and subsequently closed die forged (now termed “direct squeeze casting&#39;). There are now several “indirect” approaches involving injection of metal into a cavity via massive gates, which allow adequate feeding of solidification shrinkage while the casting freezes.  
           [0019]    In current literature, the term squeeze casting almost universally relates to a process utilizing a runner and gating system. Squeeze castings are made on machines and in steel tooling that are, in many respects, like those employed in conventional die casting. Machines and dies are very robust and capable of containing very high molten metal pressures without deflecting or losing dimensional control.  
           [0020]    Squeeze casting machines and tools are designed to introduce clean molten metal into the tool in a precise, repeatable, controlled flow pattern, filling the cavity quickly but without turbulence. In commercial practices today, there are systems that use either vertical or horizontal injection systems, with the parting line of the die oriented either horizontally or vertically.  
           [0021]    Semi-solid metal casting as we know it today evolved out of a series of studies performed in the late 1960&#39;s and early 1970&#39;s. It was noted that when the normal dendritic microstructure was modified to a non-dendritic, spheroidal microstructure, then the resulting material had a remarkably low shear strength even at relatively high solid fractions-it became thixotropic.  
           [0022]    Semi-Solid Metal (SSM) casting differs from squeeze casting in that it uses a unique “semi-solid” material. SSM casting begins with a semi-solid mass of metal processed in such a way that the solid portion is in the form of “globules” allowing a free flowing but viscous fluid behavior. The principal difference is that the higher viscosity semi-solid metal allows higher metal velocities to be implemented before the onset of turbulence and, of course, the metal is already partially solidified at the time of casting. These attributes allow SSM casting to achieve remarkably high production rates.  
           [0023]    The SSM microstructure has far superior flow characteristics when compared to a particularly solidified dendritic structure. Specially cast billets produced on continuous casting systems, equipped with one of several types of electromagnetic stirrers, are currently the most common solid feedstock processing a very uniform fine grained but essentially equiaxed dendritic microstructure. After cutting to length for a prescribed shot weight (slug), these slugs are then heated to the semi-solid temperature range and cast. During heating, the fine grained billet microstructure becomes globular. When semi-solid (typically 50-60% solid, 40-50% liquid) slugs are stiff enough to retain their shape, and yet the globular microstructure comprised of solid spheroids suspended in the melted portion allows that the slugs can be cut like butter with a knife.  
           [0024]    The basis for SSM casting centers around the fact that aluminum flows in a semi-solid state if the alpha grains rounded (globular) as opposed to a dendritic or tree-like structure. SSM manufacturing process apply either a self-supporting slug that is 50-75% solid (globular structure) in the injection chamber of the die cast machine or a slurry that is 40-60% solid having the approximate consistency of peanut butter. Additionally, liquid metal can be poured into the injection chamber (shot sleeve) and electrical energy used to transform the dendritic structure into a semi-solid globular structure. A mechanical robot can be used to transfer the slug to a slot in the shot sleeve. At this point, a plunger advances to inject the semi-solid alloy into the die cavity and the material flows uniformly as a semi-solid mixture.  
           [0025]    Billets continuously cast, using electromagnetic stirrers, are the popular feedstock for the SSM casting process. They are produced using all the standard techniques to control and monitor metal chemistry, cleanliness, grain size, gas content and microstructure, such that they represent a high quality incoming raw material. Other methods for continuously casting feed stock utilize artificial grain refinement instead of electromagnetic stirring to create very fine grains and are suitable for certain alloys, especially those having low silicon contents.  
           [0026]    For SSM casting, slugs must be heated to the semi-solid condition. For AlSi7Mg (A356) a temperature around 580° C. (1080° F.) corresponds to approximately 55 percent solid and 45 percent liquid. At this point, essentially all of the eutectic portion of the alloy is liquid.  
           [0027]    The most common SSM heating method in use today utilizes induction heating which is a clean controllable approach capable of rapid responses to changes in demand.  
           [0028]    Commercial SSM casters are utilizing both horizontal and vertical injection systems, although horizontal injection is the more common. SSM casters often, in fact, use horizontal die casting machines fitted with precisely controlled injection units, which provide the control necessary to avoid turbulence during injection of the semi-solid metal into the cavity. As with squeeze casting, the metal is fed to the cavity through relatively massive runners and gates, which provide paths for the liquid fraction to be fed into the cavity to feed solidification shrinkage.  
           [0029]    Both processes, squeeze casting and semi-solid metal casting, have established their identity, qualifications and specificity as far as the production of automotive parts is concerned. However, there are still some economic hurdles to jump before either of these processes have universal acceptance. Significant strides in cost reduction must be made to further benefit from these tendencies in the marketplace.  
           [0030]    To date, SSM processing techniques have typically focused on the more difficult structural components or markets that were either produced by permanent mold process or components that were made from other materials such as iron and steel. Conventional die-casting cannot be utilized with semi-solid processes because it would require completely new part designs and, as a result, new tooling.  
         SUMMARY OF THE INVENTION  
         [0031]    These and other shortcomings in the prior art have been addressed by the present invention which in one embodiment involves a system and a method for low-velocity die-casting.  
           [0032]    The casting process of this invention focuses on the existing die-casting applications while offering benefits heretofore not available in die-casting. The benefits that the users receive from this process include longer tool life (up to 5 times). The tool cost for an aluminum component is very substantial from a cost break down analysis; therefore, extending the tool life is a significant advantage. The other major benefit is better part quality (i.e., less internal porosity and better dimensional capability).  
           [0033]    According to one embodiment of this invention, the process utilizes molten metal at the die cast machine. This invention is distinguishable from semi-solid molding or forming due in large part to the use of molten metal. SSM is universally understood as being a casting process which begins with a semi-solid mass of material, typically 50%-60% solid and 40%-50% liquid slugs which are self-supporting and stiff enough to retain their shape. In contrast, the die casting technology according to this invention utilizes molten (100% liquid) metal as input to the die casting machine. The molten metal is not self-supporting nor is it a semi-solid material.  
           [0034]    The ladle operation that delivers the molten metal to the injection chamber is another primary feature of this invention. Known furnaces that hold the molten metal at the die cast machine typically have an operating range of +/−10 degrees F. According to this invention, a controller is utilized that communicates with the furnace thermocouple and the ladle (delivery system). As a result, the temperature of the molten metal is controlled within +/−1 degree F. This is very important for the next step in the operation. Introducing the molten metal to the injection chamber of the die cast machine at a controlled temperature allows the next step of the operation to be fixed.  
           [0035]    In addition to controlling the temperature, the ladle cup pour speed is also important. When the molten metal is poured from the ladle cup to the injection chamber, there is a temperature loss. By controlling the temperature curve of the metal during pouring, the alpha grains form very small crystals, which will grow over time. This keeps the dendritic formation of the microstructure from being created and allows the metal to easily flow into the tool. Since the small grains have been formed from the pouring operation to solidification of the molten metal in the tool, a very homogenous microstructure is produced and higher mechanical properties than dendritic microstructure are provided. This formation of the grains allows for the use of many different alloys as well as a metal matrix composite (MMC) to be introduced into the ladle cup during the temperature control wait time.  
           [0036]    Therefore, lower injection velocities are utilized for the molten metal which yields significantly longer tool life while still providing better part quality with less porosity and better dimensional capability. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    The objectives and features of the invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0038]    [0038]FIG. 1 is a front elevational view, partly in cross section, of the shot mechanism for an exemplary side gate version of a die casting machine according to one embodiment of the present invention;  
         [0039]    [0039]FIG. 2 is an enlarged elevational, sectional view showing the shot injector of an exemplary die casting machine according to one embodiment of this invention with the shot sleeve partially filled with molten metal and the shot plunger moving forward on its injection stroke; and  
         [0040]    FIGS.  3 - 4  are elevational and plan views, respectively, of an example of a part, such as an automobile wheel, which is produced in the die-casting machine of FIG. 1 according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    Die casting machines generally include stationary front and back plates and a movable or traveling plate which is reciprocally mounted between the two stationary plates. The relative positions of the stationary plates are maintained by a number of tie bars which extend between the two stationary plates. Die halves are fastened to the front plate and the traveling plate, respectively, and the traveling plate is extended and retracted to respectively close and open the die. After the die is closed, molten metal is injected into the die to form a die cast part. After a part is thus formed, the die is opened by retracting the traveling die and, after the traveling plate has moved a predetermined distance, bumper pins are commonly used which are slidably mounted in apertures located both in the die and the traveling plate engage a bumper plate which is located behind the traveling plate. These bumper pins engage and eject the die cast part from the portion of the die which is attached to the traveling plate. After the die cast part is removed from the die casting machine, the excess metal, generally referred to as a sprue or runner system, is removed from the die cast part in a separate pressing machine called a trim press.  
         [0042]    Referring now to FIG. 1 there is shown a perspective view of the die casting system  8  according to the present invention. Examples of die-casting machines which could be utilized with this invention are shown in U.S. Pat. Nos. 4,362,205 and 4,886,106, each of which are hereby incorporated by reference. While the disclosed embodiment is shown on a side gate casting machine  10  for exemplary purposes, it should be understood that the machine  10  could also be shown as a pin gate or any other version. Machine  10  includes a frame  12  on which are mounted stationary plates  14  and  18  with a movable plate  16  therebetween. Plates  14  and  18  are secured together by tie rods  24  and plate  16  is movable between plates  14 ,  18  on the tie rods  24 . A metal melter or furnace  30  is provided for melting the die cast metal  20 . The die casting metal  20  may be any suitable metal such as high purity aluminum but may also be other suitable alloys. The molten metal  20  is transferred by a ladle  22  or other appropriate mechanism/system from the melter  30  over a travel path shown by arrow A in FIG. 1 and into an inlet port  32  in the cold chamber sleeve  34 . The ladle  22  according to one embodiment has the capability of regulating, maintaining and/or adjusting the temperature of the metal  20  contained therein. Ladle systems compatible with this invention are commercially available from E-Jay Thermo Products of South Haven, Mich. The ladle  22  may include a thermocouple for temperature control and/or to wick heat away at a pre-determined rate.  
         [0043]    A shot cylinder  36  is provided to move the charge of molten metal  20  from the cold chamber  34  into the die cavity. A hydraulic power unit  38  provides the movement for the shot cylinder  36  as is understood in this art. A runner cavity or sprue cavity (not shown) is provided into which the hot molten metal  20  is transferred by the shot cylinder  36  from the cold chamber  34 . The hot molten metal  20  then is forced into the die cavity as is common for die-casting operations.  
         [0044]    In FIGS. 1 and 2 it can further be seen that the cold chamber sleeve  34  which extends through aperture  38  in stationery plate  18  has been supplied with hot molten metal  20  from the ladle  22  so that the stationary shot cylinder  36  is now in position to move shot rod  40  toward the left whereby the hot molten metal  20  is forced through the runner system in die plate  42  and into the die cavity (not shown).  
         [0045]    The ladle  22  pours the molten metal  20  downwardly through fill passage  32  into shot sleeve  34  wherein it forms a pool  44 . Fill passage  32  is formed adjacent stationary plate  18  and shot sleeve  34 . The shot is injected by plunger  36  received within shot sleeve  34  and actuated by a cylinder rod  40 . Plunger  36  moves forward at slow speed to gradually fill the shot sleeve  34  with the molten aluminum  20 , which will occur sometime after plunger  36  clears the opening  46  in fill passage  32 . The plunger  36  moves at a rate of about 10 inches/second until it passes the opening  46  according to this invention. When shot sleeve  34  is full, plunger  36  will continue to be advanced at a relatively low speed to inject the molten aluminum through the passage into the die chambers. In one embodiment, the plunger  36  moves at a rate of about 15 to about 30 inches/second after it passes the opening  46 . This rate is significantly slower than the 80 to 130 inches/second typical of the prior art. Once the metal  20  is forced into the die, the plunger  36  holds this position for a predetermined length of time, after which die plate  16  retracts.  
         [0046]    The operation of the ladle  22  that delivers the molten metal  20  to the shot sleeve  34  is another primary feature of this invention. Known furnaces that hold the molten metal  20  at the die cast machine  10  typically have an operating range of +/−10 degrees F. For aluminum, this is typically 1200 degrees F. +/−10 degrees F. According to this invention, a controller  28  is utilized that communicates with the furnace  30  thermocouple and the ladle delivery system  22 . As a result, the temperature of the molten metal  20  is controlled within +/−1 degree F. at the furnace  30 , through the delivery by the ladle  22  to the shot sleeve  34 . This is very important for the next step in the operation. Introducing the molten metal  20  to the shot sleeve  34  of the die cast machine  10  at a controlled temperature allows the next step of the operation to be fixed.  
         [0047]    In addition to controlling the temperature, the ladle  22  pour speed of th metal  20  is also important. When the molten metal  20  is poured from the ladle  22  to the shot sleeve  34 , there is a temperature loss. By controlling the temperature curve of the metal  20  during pouring, the alpha grains form very small crystals, which will grow over time. This keeps the dendritic formation of the microstructure from being created and allows the metal  20  to easily flow into the die chamber. Since the small grains have been formed from the pouring operation to solidification of the molten metal, a very homogenous microstructure is produced and higher mechanical properties than dendritic microstructure are provided. This formation of the grains allows for the use of many different alloys as well as a metal matrix composite (MMC) to be introduced into the ladle  22  during the temperature control wait time. Therefore, the pour rate of metal  20  into the shot sleeve  34  is a function of the temperature of the metal  20  in the ladle  22 . In this way, very small alpha grain crystals are formed. The pour rate of the metal  20  is adjusted by the controller  28  as a function of the metal temperature.  
         [0048]    Referring to FIGS.  3 - 4 , an example of a finished part which can be die-cast in accordance with this invention is a wheel generally identified by the numeral  110  is shown in FIGS. 3 and 4. The wheel  110  contains a number of roughly rectangular contours  111  around the periphery, each of the contours containing a punched or machined hole  112  therethrough. A hub area  113  contains four cored and tapped holes  114  and four larger punched or machined holes  115 . A wheel configuration of this complexity is normally readily produced by die casting techniques and is accordingly appropriate for manufacture according to this invention. The wheels  110  made according to this invention have the very important capability of being lighter in weight than comparable wheels of the prior art.  
         [0049]    Representative alloys useful in this die-casting process are, in addition to aluminum alloys, ferrous alloys such as the stainless steels, tool steels, low alloy steels and irons and copper alloys of the type normally used in castings and forgings.  
         [0050]    From the above disclosure of the general principles of the present invention and the preceding detailed description of at least one preferred embodiment, those skilled in the art will readily comprehend the various modifications to which this invention is susceptible. Therefore, I desire to be limited only by the scope of the following claims and equivalents thereof.