Abstract:
A method of forming a material includes the steps of: vibrating a molten material at an ultrasonic frequency while cooling the material to a semi-solid state to form non-dendritic grains therein; forming the semi-solid material into a desired shape; and cooling the material to a solid state. The method makes semi-solid castings directly from molten materials (usually a metal), produces grain size usually in the range of smaller than 50 μm, and can be easily retrofitted into existing conventional forming machine.

Description:
The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 

   FIELD OF THE INVENTION 
   The present invention relates to semi-solid processing of materials, and more particularly to semi-solid processing of materials using ultrasonic vibration to form non-dendritic grains therein. 
   BACKGROUND OF THE INVENTION 
   Thixocasting and rheocasting are widely used industrial process for high volume production of SSM components. Problems associated with such processing include: costly and complex feed (process) material preparation (thixocasting); material loss (thixocasting), agglomeration, and grain coarsening during process material preparation (rheocasting), causing large grain size in the product; costly equipment to hold semi-solid slurry process material at constant temperatures (rheocasting); low solid fractions of process materials (rheocasting); and oxidation of process material during processing. 
   OBJECTS OF THE INVENTION 
   Accordingly, objects of the present invention include: methods of forming a semi-solid structure directly from molten metal prior to metal forming (e.g., casting, forging) with desired fraction solid, producing grain size much smaller than thixocasting and rheocasting, reducing or eliminating process run-around, and reusing process run-around if there is any. Further and other objects of the present invention will become apparent from the description contained herein. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method of forming a material that includes the steps of: vibrating a molten material at an ultrasonic frequency while cooling the material to a semi-solid state to form non-dendritic grains therein; forming the semi-solid material into a desired shape; and cooling the material to a solid state. 
   In accordance with another aspect of the present invention, a machine for forming a material includes means for vibrating a molten material at an ultrasonic frequency while cooling the material to a semi-solid state to form non-dendritic grains therein. 
   In accordance with another aspect of the present invention, a article includes a semi-solid-processed body characterized by globular, non-dendritic grains having an average diameter of no more than 1000 μm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cutaway side view of an ultrasonic processor in accordance with the present invention. 
       FIG. 2  illustrates an embodiment of the present invention using a turntable conveyer. 
       FIG. 3  illustrates an embodiment of the present invention using a chain-type conveyer. 
       FIGS. 4(   a )– 4 ( e ) illustrate an embodiment of the present invention wherein a forming machine (die caster) is modified to incorporate an ultrasonic processor directly into its mechanism. 
       FIG. 5  is a photomicrograph of aluminum A356 alloy cooled in a copper mold with no ultrasonic vibration. 
       FIG. 6  is a photomicrograph of aluminum A356 alloy cooled in a copper mold with ultrasonic vibration in accordance with the present invention. 
   

   Equivalent components are assigned the same reference numerals throughout the drawings. 
   For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is carried out by “ultrasonic processing”, which comprises vibrating molten process material (usually a metal) at an ultrasonic frequency as it cools to a semi-solid state in order to form non-dendritic, (i.e., globular-shaped, rounded), ideally spherical) grains having an average diameter of no more than 1000 μm, preferably no more than 100 μm, more preferably no more than 50 μm, most preferably no more than 1 μm. Such grain structure is most beneficial for semi-solid forming processes. Ultrasonic processing in accordance with the present invention generally avoids formation of large and/or dendritic grains in the process material. 
   In accordance with the present invention, vibration at an ultrasonic frequency is operably applied at a frequency in the range of 1 kHz to 10 6  kHz, preferably in the range of 15 kHz to 25 kHz, and at a power intensity in the range of 1 W to 10 6  W, preferably in the range of 500 to 1000 w. The duration of ultrasonic processing is in the range of 1 millisecond to one hour depending on the type and volume of metal being processed. Once the beneficial results of ultrasonic processing are achieved, continued subjection of the process material is not deleterious; therefore duration is not considered to be a critical parameter. 
   Referring to  FIG. 1 , an example of a basic apparatus for carrying out the present invention comprises an ultrasonic processor  10 . A cylindrical sleeve  12  contains molten and/or semi-solid process material  14 . A ram (piston)  16  is inserted into the lower end  18  of the sleeve  12 . An ultrasonic transducer  20  produces ultrasonic vibration that is transmitted to the process material  14  via an ultrasonic radiator (horn)  22 . Process material  14  is transferred into and out of the sleeve  12  through the upper end  24  thereof. 
   In operation, molten process material  14  is transferred into the ultrasonic processor  10  at a temperature of at least above the liquidus temperature of the process material  14 . The ultrasonic transducer  20  produces ultrasonic vibration that is transmitted to the process material  14  via an ultrasonic radiator (horn)  22 . The process material  14  cools to the semi-solid state while being exposed to ultrasonic vibration. The ultrasonic vibration promotes nucleation and the formation of predominantly non-dendritic, generally globular grains. The ram  16  then pushes the semi-solid process material  14  as a slug (billet) out of the sleeve  12  through the upper end  24  thereof to transfer the semi-solid process material  14  to a forming machine. The non-dendritic, generally spherical grains persist throughout the forming process. 
   Some embodiments of the present invention include a conveyer interposed in the process between a heater that melts the process material and a forming machine that forms the process material. Any conveyer that can support at least one ultrasonic processor  10  is contemplated to be suitable for application to the present invention. It is preferred that a conveyer support a plurality of ultrasonic processors  10 . Examples of conveyers are set forth below to show the general principle of the present invention. 
   Referring to  FIG. 2 , a conveyer  40  comprises a turntable  42  that supports a plurality of ultrasonic processors  10 . The turntable  42  having six positions A–F is indexed so that an ultrasonic processor  10  is aligned with the furnace  44  in position A and another ultrasonic processor  10  is aligned with the forming machine  46  in molten process material  14  is transferred from the furnace  44  to the ultrasonic processors  10  while semi-solid slugs of process material  14  are transferred to the forming machine  46 . As the ultrasonic processors  10  rotate through positions B, C, D, and E, the process material  14  is cooled to a semi-solid state while undergoing exposure to ultrasonic vibration, causing the formation of predominantly non-dendritic, generally spherical grains in the process material  14 , which persist through the forming process. 
     FIG. 3  illustrates an embodiment wherein a conveyer  50  comprises a belt or chain  52  with ultrasonic processors  10 . The furnace  44  and forming machine  46  can be at any desired location, and the belt or chain  52  can be in any desired configuration. 
   In other embodiments of the present invention, the forming machine is modified to incorporate an ultrasonic processor directly into its mechanism. Molten process material is transferred directly to the forming machine and the ultrasonic processing takes place therein. 
     FIGS. 4(   a )– 4 ( e ) illustrate an embodiment of the present invention wherein a die-casting machine  60  is modified to incorporate an ultrasonic processor  10  directly into its shot-sleeve  64 . 
   In  FIG. 4(   a ) an ultrasonic processor  10  is inserted into an opening  68  in the shot-sleeve  64  just ahead of the injection ram  66 . Molten process material  14  is transferred into the ultrasonic processor  10  where it is processed in accordance with the present invention. 
   In  FIG. 4(   b ) the ultrasonic processor  10  retracts downwardly sufficiently to allow the injection ram  66  to pass thereover. In  FIG. 4(   c ) the ultrasonic processor  10  and the injection ram  66  advance toward the casting die  62  sufficiently to close the opening  68 , which has an extension  70  therein to accommodate advance of the ultrasonic processor  10 . 
   In  FIG. 4(   d ), ultrasonic processing having been completed, the ram  16  of the ultrasonic processor  10  advances and forces the process material  14  into the shot-sleeve  64 . In  FIG. 4(   e ) the injection ram  66  advances and forces the process material  14  into the die  62 . 
   Within the scope of the present invention, an ultrasonic processor can be brought into operable communication with process material in any configuration. For example, an ultrasonic processor can be attached to a vessel wall, or can be inserted directly into the process material. 
   EXAMPLE I 
   An acoustic radiator was attached to the bottom of a copper mold. Aluminum alloy A356 was melted and poured into the mold and allowed to cool to a solid state with no ultrasonic vibration. The microstructure of the resultant solid alloy is shown in  FIG. 5 . The grains are observed to be large (1–10 mm) and dendritic. The microstructure is deleterious to semi-solid processing, especially forming. 
   EXAMPLE II 
   An acoustic radiator was attached to the bottom of a copper mold. Aluminum alloy A356 was melted and poured into the mold and allowed to cool to a solid state while being exposed to ultrasonic vibration in accordance with the present invention. The microstructure of the resultant solid alloy is shown in  FIG. 6 . The grains are observed to be smaller than 50 μm in diameter and globular—ideal for semi-solid processing. 
   Utilization of the present invention provides the advantage of resource savings because less capital investment (equipment, etc.) and energy are required to carry out the present invention than that required by conventional technology. Moreover, the present invention allows for the reuse of the process run-around (5% of the feedstock metals). Moreover, less oxide waste is produced because there is less exposure of process material to air. 
   Moreover, the present invention enables a large process window for semi-solid processing because the metal is held in containers throughout the processing shown in  FIG. 4 . The process material can be injected into a forming machine at any desired solid fraction. 
   Although the present invention is generally used to process metallic materials, other materials can be processed in accordance with the present invention, for example, polymers, ceramics, and composite materials. 
   While there has been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the inventions defined by the appended claims.