Patent Publication Number: US-6901991-B2

Title: Semi-solid molding apparatus and method

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to semi-solid molding (SSM) of metal alloys and the equipment and methods used for SSM, and which are disclosed in many U.S. and foreign patents, for example, in U.S. Pat. Nos. 3,954,455, 4,434,837, 5,161,601 and 6,165,411. SSM is also discussed in technical publications, for example, in a book entitled  Science and Technology of Semi-Solid Metal Processing , published by North American Die Casting Association in October, 2001. Chapter 4 of this publication was authored by a co-inventor of the present invention. In conventional SSM processes, it is necessary to use either a specially treated, pre-cast billet of appropriate microstructure or a slurry especially prepared from molten alloy in equipment external to a die casting press. The cost premiums associated with either the pre cast specially treated billet that must be sawed to length before using, or the slurry especially prepared in equipment external to the die casting press, have severely limited the commercial applications of the SSM processes. Also, the pre-cast billet is available from a relatively few sources, is currently made only from primary alloys, and process offal cannot be reused unless reprocessed back into a billet. 
   Still, SSM provides some important and highly desirable characteristics. Unlike conventional die castings, die cast parts which are produced using SSM processes can be produced substantially free of porosity, they are able to undergo high temperature thermal processing without blistering, they can be made from premium alloys, and they provide reliable high levels of strength and ductility when made using appropriate alloys and heat treatments. Because of the thixotropic nature of semi-solid slurry and the non-turbulent way that relatively viscous thixotropic slurries flow in die casting dies, the SSM process is capable of producing cast parts having thin sections, great detail and complexity and close dimensional tolerances, without the entrapped porosity and oxides which are commonplace in conventional die casting processes. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a new SSM method and apparatus which significantly reduces the costs of producing parts by the SSM process. The method and apparatus of the invention is ideally suited for producing parts having thin sections, fine detail and complexity and close dimensional tolerances, and which are substantially free of porosity and oxides, can be processed at elevated temperatures without blistering and which can provide high and reliable levels of strength and ductility. The method and apparatus of the invention avoids any need to produce a specially treated, pre-cast billet that must be sawed to length before using or a slurry especially prepared from molten alloy in equipment external to the die casting press. The method and apparatus of the invention is also applicable to a wide variety of alloys, for example, standard A356 alloy and alloys of the Al—Si, Al—Cu, Al—Mg and Al—Zn families, all of which can be acquired in the form of and at prices normal to conventional foundry ingot, including both primary and secondary origin. 
   In accordance with one embodiment of the present invention, an ingot of commercially available solid metal or metal alloy, such as aluminum foundry alloy ingot, is heated to the molten state. If not permanently grain refined, such as by employing a foundry alloy called SiBloy produced by Elkem Aluminum, AS, an α aluminum grain refining material such as 5:1::Ti:B master alloy produced by numerous suppliers, or a product called TiBloy produced by Metallurg, is added to the molten alloy in appropriate quantities to accomplish fine grains in the solidified alloy product. The grain refined molten alloy is poured directly into a shot chamber defined by a large diameter shot sleeve of a vertical die casting machine or press. The shot sleeve or chamber receives a vertically movable shot piston which forms the bottom of the shot chamber, and the diameter of the shot chamber is greater than its depth or axial length. In a preferred embodiment of the present invention, the shot chamber is greater than its depth by a ratio of 2:1 or more. The shot chamber is then shifted or indexed from the initial filling position to a slurry injection position under a die. The molten alloy is permitted to cool within the shot chamber to a predetermined temperature range by means of water cooling passages within the shot sleeve and within a retractable cooling pin extendable into a center portion of the shot chamber. The alloy forms a semi-solid slurry having 30 to 60 percent solid, the solid fraction having a globular, generally non-dendritic microstructure. The portion of the slurry immediately adjacent to the wall of the shot chamber or shot sleeve and the shot piston and cooling pin become colder and more solid. 
   When the semi-solid slurry within a central portion of a first shot chamber, now in the slurry injection position under the die, has cooled to the predetermined temperature range in which it has 30 to 60 percent solids, the shot piston is moved upwardly by a mechanical actuator or a hydraulic shot cylinder to transfer or inject the semi-solid slurry within the central portion of the shot chamber through one or more gate or sprue openings and into one or more cavities in the die above the shot chamber. The more solid portion of the slurry adjacent the shot sleeve is prevented from entering the die cavity or cavities, either by appropriately distancing the gate or sprue openings from the shot sleeve walls or by entrapping the more solid portion within an annular recess in the gate plate through which the gates or sprue openings communicate with the die cavity or cavities. As a result, the more solid portion of the slurry remains in the residual solidified biscuit. After the semi-solid slurry solidifies in the die cavity or cavities, the shot piston retracts to retract the biscuit intact with gates or sprues. The shot chamber is then transferred or shifted laterally or indexed back to its initial filling position where the biscuit with the gates is removed laterally from the shot chamber and piston, and the shot chamber is then ready to repeat the cycle. After the die is opened, the part(s) is ejected and then indexed to a position where it is removed, and the die is ready to repeat the cycle. 
   During the slurry forming, slurry injection and slurry solidification steps described above relative to the first shot chamber while in its shot position, a second shot chamber in the original filling position has similarly been filled with grain refined molten alloy. When the first shot chamber and its piston are transferred or shifted or indexed back to the initial filling position for biscuit removal, the second shot chamber and molten alloy are shifted or indexed to the metal transfer or slurry injection position under the die, and the process of slurry formation, slurry injection and slurry solidification is accomplished just as with the first shot chamber. The process is repeated over and over again. 
   Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a vertical section through a vertical die casting press which is used to perform the method of the invention and with the die set shown in its open position; 
       FIG. 2  is an enlarged fragmentary section of the semi-solid slurry transfer or injection position or station shown in FIG.  1  and with the die set shown in its closed position; 
       FIG. 3  is a diagrammatic illustration of the metal temperature profile of the semi-solid slurry before a center portion of the slurry is transferred or injected into the die cavities shown in  FIG. 2 ; 
       FIG. 4  is a section similar to FIG.  2  and showing a modification of the press with a retractable cooling pin within the shot piston; and 
       FIG. 5  is a section similar to FIG.  4  and showing the cooling pin extended into the molten metal alloy within the shot chamber. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a vertical die cast machine or press  10  is constructed similar to the press disclosed in U.S. Pat. No. 5,660,223 which issued to the assignee of the present invention and the disclosure of which is incorporated by reference. The press  10  includes a frame  12  formed by a pair of parallel spaced vertical side walls or plates  14  rigidly connected by top plate  16  a base or bottom plate  18  and a set of intermediate cross plates or bars  22  and  24  all rigidly secured to the side panels  14 . The top cross plate  16  supports an upper double acting hydraulic clamping cylinder  30  having a piston rod  32  projecting downwardly on a vertical center axis of the press. The piston rod  32  carries an adapter plate  34  which supports a hydraulic ejector cylinder  36  having a piston  37  projecting downwardly to support a plate  38  which carries a set of ejector pins  39 . 
   An upper die or mold section  40  ( FIG. 2 ) is secured to the bottom of the plate  38  by an annular retaining plate  41  and has a pair of recesses  42  which receive corresponding core members  43 . A lower die or mold section  45  is recessed within a circular indexing or transfer table  48  and defines a pair of cavities  50  which cooperate with the core members  43  to define the corresponding metal parts P produced in accordance with the method of the invention. The transfer or indexing table  48  is mounted on a shaft  52  ( FIG. 1 ) supported by a set of bearings  53  retained within the frame member  54 . The table  48  carries a plurality of at least two lower mold sections  45  and is rotated or indexed by a pinion (not shown) engaging periphery teeth  56  on the table  48  and driven by a stepping motor (not shown). A gate plate  60  is positioned under the bottom mold section  45  and defines a pair of slightly tapered gates or sprue openings  62 , one for each of the cavities  50 . The gate plate  60  also defines an annular metal entrapment recess or groove  63 . It is to be understood that the parts P to be die cast within the corresponding mold sections  40  and  45  are shown for illustration only and that the configuration or size of the parts form no part of the present invention. The parts P may be any size or shape, corresponding to the desired die cast article. 
   A cylindrical vertical column or post  66  is secured to a plate  67  mounted on the base plate  18  and projects upwardly to support a rotatable circular table  68  by a set of anti-friction bearings  69  mounted on a top hub of the post  66 . The table  68  supports a plurality or a pair of diametrically opposite cylindrical shot sleeves  70  which have parallel vertical axes. The table  68  is also supported by a set of thrust bearings  72  mounted on the cross bars or plates  22  and  24 . The table  68  also has peripheral gear teeth  74  which engage a pinion (not shown) mounted on a vertical shaft of an electric stepping motor (not shown). Actuation of the stepping motor is effective to index the table  68  in steps or increments of 180° for alternately presenting the pair of shot sleeves  70  between a molten metal receiving or pour station  80  and a metal injecting or transfer station  82  located under the die sections  40  and  45  and in axial alignment with the clamping cylinder  30 . 
   Each of the shot sleeves  70  defines a cylindrical shot chamber  86  which receives a corresponding shot piston  88 . The upper end portion of each shot piston  88  has one or more laterally extending and tapered dovetail slots  92 , and a shot piston rod  94  projects downwardly from each piston  88 . Each of the shot sleeves  70  and each of the piston rods  94  is provided with internal passages  87  ( FIG. 2 ) by which cooling fluid or water is circulated through the sleeves and pistons  88  for cooling the molten metal and to form a metal residue biscuit B having integrally connected and upwardly projecting gate pins formed by the gate openings  62 . 
   A double acting hydraulic shot cylinder  95  is mounted on a spacer plate  96  secured to the base plate  18  under the metal transfer station  82  and in vertical alignment from the axis of the hydraulic clamping cylinder  30 . The shot cylinder  95  includes a piston and piston rod  98  which projects upwardly, and a guide plate  99  is secured to the upper end of the piston rod  98 . Another double acting hydraulic ejection cylinder  110  is substantially smaller than the cylinder  95  and is mounted on the plate  67  by a spacer block  112 . The cylinder  110  includes a piston and piston rod  114  and a guide plate  116  is secured to the upper end of the piston rod  114 . A guide rod  118  projects downwardly from the plate  116  and through a guide block  121  mounted on the cylinder  110  to prevent rotation of the plate  116  and piston rod  114 . The cylinder  110  is located in vertical axial alignment with each shot sleeve  70  when the sleeve is located at the metal receiving or pouring station  80 . 
   A pair of opposing retaining or coupling plates  126  are secured to the upper surface of each of the guide plates  99  and  116 . Each set of coupling plates defines inner and outer opposing undercut slots for slidably receiving an outwardly projecting circular flange  128  formed on the bottom of each shot piston rod  94 . Thus when the table  68  and shot sleeves  70  are indexed in steps of 180°, the shot piston rods  94  are alternately connected or coupled to the piston rods  98  and  114 . 
   In operation of the vertical die cast machine or press  10  to perform a semi-solid molding method, a commercially available permanently grain refined alloy such as SiBloy foundry ingot produced by Elkem Aluminum AS, or a non-permanently grain refined alloy such as standard A356 aluminum foundry ingot or foundry alloy ingot of the Al—Si, Al—Cu, Al—Mg or Al—Zn families, is heated to a molten state. Preferably, when a melt of non-permanently grain refined alloy is at a predetermined temperature, for example 650° C. or higher, an a aluminum grain refining material, for example, a titanium boron master alloy sold under the trademark TiBloy and produced by Metallurg, is added at a preferred melt-to-master alloy ratio according to the manufacturer&#39;s recommendations. The grain refinement step is not necessary when utilizing a permanently grain refined alloy such as SiBloy. After the molten grain refined alloy is lowered to a temperature of about 616° C., or within the range of 612° C. to 620° C., the molten alloy is poured into the vertical shot chamber  86  located at the pour or fill station  80  above the ejection cylinder  110 . Preferably, the shot chamber  86  has a diameter substantially larger than its depth or axial length, for example, a diameter over 6 inches, such as 7½ inches and a depth of less than 6 inches. 
   The shot sleeve  70  confining the molten alloy is then indexed to the transfer or injection station  82  while a cooling period occurs. The molten alloy is allowed to cool in the shot chamber  86  to a temperature range that produces a semi-solid slurry having a range of 40% to 60% solid, such as approximately 50% solid and a globular generally non-dendritic microstructure. For example, the A356 aluminum alloy is allowed to cool to a temperature range between 570° C. and 590° C. for a period of fifteen seconds or within the range of five to twenty five from the time it entered that temperature range to the shot or injection time. When the alloy has cooled to this temperature within the shot chamber  86  at the transfer station  82 , the temperature profile of the alloy is close to that shown in  FIG. 3  wherein a center portion A of the alloy has a substantially uniform temperature, and the peripheral portion of the alloy adjacent the shot sleeve  70  is significantly cooler due to the cooling effect of the shot sleeve. 
   With the mold sections  40  and  45  in their closed position ( FIG. 2 ) by actuation of the cylinder  30 , the injection or shot cylinder  95  is actuated to move the shot piston  88  upwardly. This transfers the semi-solid slurry S 1  within the center portion A ( FIG. 3 ) of the alloy upwardly through the gate or sprue openings  62  and into the corresponding die cavities  50  to form the parts P which have the desired globular, generally non-dendritic microstructure. The more solidified outer portion of the slurry S 2  within the shot chamber adjacent the sleeve  70  is captured or trapped in the annular recess  63  and prevented from entering the sprue openings  62 . 
   While the parts P are solidifying within the cavities  50 , another charge of molten alloy is poured into the second shot chamber  86  located at the pour station  80 . When the parts in the cavities  50  are solidified, the shot cylinder  95  is actuated to retract the piston  88  and the residual solidified alloy material-or biscuit B within the shot chamber  86  and to shear the metal within the gate or sprue openings  62  from the parts P at the interface of the lower mold section  45  and the gate plate  60 . The residual solidified metal or biscuit B, including the sprues, within the shot chamber  86  is then transferred by indexing the table  68  to either a biscuit removal station or to the metal pour station  80 . At this station, the piston  88  is elevated to a level where the biscuit B is ejected laterally by a fluid cylinder (not shown). After the parts P are fully solidified, the upper mold section  40  is retracted upwardly by actuation of the cylinder  30  while the cylinder  36  is actuated to eject or release the parts with the pins  39 . The table  48  is then indexed to transfer the parts P to a part removal station where the parts are lifted and removed, for example, by a robot (not shown). The above method steps for semi-solid molding are then repeated for successively molding another set of parts. 
   Referring to  FIGS. 4 and 5  which show another embodiment of the vertical die cast press described above in connection with  FIGS. 1 and 2 , a shot sleeve  70 ′ is constructed substantially the same as the shot sleeve  70  and includes water cooling passages similar to the passages  87 . The sleeve  70 ′ is ideally suited for use in a vertical die cast press having a reciprocating shuttle table which carries a pair of the shot sleeves  70 ′. A shot piston  88 ′ cooperates with each shot sleeve  70 ′ to define a shot chamber  86 ′. The top surface of the piston  88 ′ has a pair of a dovetail slots  92 ′ and also defines an annular cooling chamber  135  which receives a cooling fluid or water within passages (not shown) extending downwardly within the annular piston rod  94 ′. 
   In accordance with the present invention, an elongated cooling element or pin  140  is supported for vertical movement within a center portion of the shot piston  88 ′ and within a center portion of the piston rod  94 ′. The cooling pin  140  includes an integral piston  142  which is supported for sliding movement within a cylindrical chamber  144 . The piston  142  is retained by an annular cap  146  which is secured to the piston rod  94 ′ by a series of screws  148 . Hydraulic fluid is selectively supplied to the upper end and the lower end of the chamber  144  to provide a double acting hydraulic piston  142  for moving the cooling pin  140  between a lower retracted position ( FIG. 4 ) and an upwardly extended position (FIG.  5 ). The cooling pin  140  has an axially extending bore or passage  152  which receives a tube  154 . A fitting  156  is secured to the lower end of the pin  140  and has passages  157  and  158  connected to the passage  152  and tube  154  for circulating a cooling fluid or water through the tube  154  and the annular passage defined within the chamber  152  around the tube  154 . Another axially extending bore or hole  159  is formed in the cooling pin  140  for receiving a temperature sensing thermocouple. 
   As shown in  FIG. 5 , before the molten metal or alloy is poured into the shot sleeve  70 ′ above the shot piston  88 ′, the cooling pin  140  is extended so that the upper end portion of the cooling pin projects upwardly into a center portion of the molten alloy to produce a semi-solid slurry S 1  with a more consistent temperature profile so that the slurry develops a fine, equiaxed primary grain structure ideally suited for the semi-solid molding process. The upper end portion of the extended water cooled pin  140  removes heat from the center portion of the molten material while heat is removed from the outer portion of the material by the water cooled sleeve  70 ′ to provide a more consistent temperature profile throughout the molton metal. As a result, the time required for ripening or forming the semi-solid slurry S 1  is reduced. The cooling pin  140  also improves the microstructure of the slurry S 1  and provides more control over the cooling rate of the molten metal. As mentioned above, the more solidified outer portion of the slurry S 2  is located adjacent the shot sleeve  70 ′ and the upper surface of the shot piston  88 ′ and the upper end portion of the cooling pin  140 . When the piston  88 ′ is moved upwardly, the more solidified slurry S 2  adjacent the shot sleeve  70 ′ is captured in the annular recess  63  formed in the lower die member or gate plate  60 . The cooling pin  140  is retracted downwardly by the piston  142  to its retracted position ( FIG. 4 ) prior to moving the shot piston  88 ′ upwardly. 
   From the drawings and the above description, it is apparent that a method of semi-solid molding of parts with a vertical die casting press in accordance with the present invention, provides desirable features and advantages. For example, the method of the invention provides for rapidly producing die cast parts free of porosity and which may be heat treated to provide a reliable high level of strength and ductility. As a result, the parts may have thin wall sections and be lighter in weight and/or may be complex die cast parts having close tolerances. The method also extends the service life of the die sections since the die sections receive less sensible heat because the injected slurry is at a lower temperature than fully molten metal and with less heat of fusion since the slurry is already approximately 50 percent solid when injected. Also, since the die is required to absorb much less heat in the process, the overall cycle time may be decreased to obtain more efficient production of parts. 
   The semi-solid molding method of the invention also eliminates the preparation of special billets or special slurries and the substantial cost of the preparation equipment, and enables the reuse of process offal and scrap. That is, by using conventional foundry ingots or ingots of pure metal, which may be grain refined, the method of the invention significantly lowers the cost of input material for semi-solid molding. As another feature, the large diameter to depth ratio of the shot chamber and the controlled cooling of the shot sleeves and shot piston provide for obtaining the desired cooling and temperature profile of the alloy within the semi-solid slurry S 1  in the center portion of the shot chamber. The annular entrapment recess  63  is also effective to prevent the more solidified alloy S 2  adjacent the shot chamber wall or sleeve from entering the sprue openings  62  and flowing into the cavities  50 . 
   The short stroke of the shot piston  88 , which is greater than its diameter, also provides for a broad range of cavity fill rates, for example, when a rapid fill rate is desired for parts having thin wall sections or a slow fill rate is desired for parts having heavy wall sections. The diameter of the shot sleeve and piston are preferably over 6″ and may be substantially more, for example, 24″ in order to die cast a large diameter SSM part such as a motor vehicle wheel or frame member. In addition, the retractable cooling element or pin  140  for the molten metal within a center portion of the shot chamber  86 ′ provides the semi-solid slurry with a more consistent temperature profile. 
   While the methods and forms of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to the precise methods and forms of apparatus described, and that changes may be made therein without departing from the scope and spirit of the invention as defined in the appended claims. For example, while the vertical die cast press  10  incorporates rotary indexing tables  48  and  68 , vertical die cast presses with other forms of transfer means may be used, for example, a reciprocating shuttle table for the bottom die section or for two of the shot sleeves  70 ′, or a tilting mechanism for a single shot sleeve.