Patent Publication Number: US-9427794-B2

Title: Method and apparatus for forging

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional application of U.S. patent application Ser. No. 11/817,779, having a 35 U.S.C. §371(c) date of Feb. 22, 2011, entitled METHOD AND APPARATUS FOR FORGING, which is a U.S. National Phase application under 35 U.S.C. §371 of international Application No. PCT/SG2005/000200, filed on Jun. 20, 2005, entitled METHOD AND APPARATUS FOR FORGING, which claims priority to U.S. provisional patent application No. 60/658,171, filed Mar. 4, 2005. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to forging, in particular to a method and apparatus for forming articles having complex shapes and homogenous structures. 
     BACKGROUND OF THE INVENTION 
     Traditional liquid forging or squeeze casting produces metallic articles with a fine-grained microstructure by exerting a high pressure on at least partially molten metal during solidification. The articles formed have a high density and are ideally porosity-free. For example, on a punch the that moves into a female die to fully enclose a charge of at least partially molten metal, the at least partially molten metal solidifies into an article under external pressure continuously exerted by a hydraulic press on the punch die. Squeeze casting or liquid forging also reduces hot tearing or cracking in an article, effects which arise due to shrinkage during cooling. Such melt shrinkage can be compensated for by applying an oscillating squeeze pressure during solidification or by means of double acting pressure to improve article strength and toughness. 
     The actual volume of the article obtained depends on the quantity of at least partially molten metal supplied. To obtain articles having predetermined dimensions regardless of varying amounts of access of at least partially molten metal, a compensating pressure can be exerted through the cavity base. This compensating pressure allows excess material, which varies in volume from article to article, to extend beyond actual article dimensions. After ejection of the article, solidified excess material is trimmed off. 
     Current techniques are appropriately suited to the production of strong and tough articles having relatively simple shapes. However, more complex shapes require tighter dimensioning tolerances, which existing techniques are unable to achieve. For example, thinner sections tend to prematurely solidify, resulting in greater porosities in the thinner sections compared to the rest of the article, which leads to a non-homogenous structure. At the same time, current liquid forging or squeeze casting techniques as yet have no capability to form parts having relatively small through holes of fine tolerances. However, complex articles also often require homogeneously high strength and toughness, properties which are in themselves readily achieved by forging. 
     Also, when dealing with thin wail sections, quite often the grain size may be close to the same order of magnitude as the wall thickness, In that case a loss of one grain may cause a significant reduction in wall thickness and wall strength. For example, the grain size may be 100 microns and the wall may be 300 microns thick. This is important in small products such as, for example, the casings for small disk drives. 
     SUMMARY OF THE INVENTION 
     According to a first preferred aspect there is provided an apparatus for forming an article. The apparatus comprises at least one die having a die cavity to receive material, said material being at least partially molten and at least one punch adapted to slidably engage the die cavity and to exert a forming pressure on material disposed in the die cavity. There is also at least one pin for forming an article feature. The pin is adapted to slidably engage the die and to contact the material, the pin being further adapted to recede upon exertion of the forming pressure and to exert a feature forming pressure when receded and thereby form the article having the article feature when the material solidifies under the forming pressure. 
     According to a second aspect there is provided a method of forming an article having an article feature. The method comprises:
         (a) locating at least one pin at a first position relative to a die cavity;   (b) inserting into the die cavity a molten material to be used to form the article;   (c) moving a punch relative to the die cavity to contact the molten material to form the article; and   (d) moving the at least one pin to a second position relative to the die cavity, the movement of the at least one pin being prior to solidification of the molten material.       

     The pin may be in contact with the punch when the material solidifies; or may be receded out of the die cavity when the material solidifies. 
     The feature forming pressure may be less than the forming pressure; and the movement of the at least one pin may be in consequence of the movement of the punch. The punch may contact the at least one pin prior to the punch contacting the molten material. When the punch contacts the at least one pin, the punch may force the pin from a first position to a second position. Alternatively, or additionally, the pin may be moved from the first position to the second position by the molten material in consequence of a compressive force applied to the molten material by the punch. 
     When the at least one pin is in the first position, an upper end of the at least one pin may be substantially co-planar with a lower surface of the die cavity. When the at least one pin is in the second position, the upper end of the pin may be recessed below the lower surface. 
     Alternatively or additionally, when the at least one pin is in the first position and the molten material is inserted in the die cavity, an upper end of the at least one pin may be located above a top surface of the molten material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings. 
       In the drawings: 
         FIG. 1  is a sectional view of a first embodiment prier to exertion of a forming pressure; 
         FIG. 2  is a vertical cross-sectional view corresponding to  FIG. 1  of the first embodiment during exertion of the forming pressure; 
         FIG. 3  is a side view of an article formed according to the first embodiment; 
         FIG. 4  is a vertical cross-sectional view of a second embodiment prior to exertion of a forming pressure; 
         FIG. 5  is a view corresponding to  FIG. 4  of the second embodiment during exertion of the forming pressure; 
         FIG. 6  is a side view of an article formed according to the second embodiment; 
         FIG. 7  is a vertical cross-sectional view of a third embodiment prior to exertion of a forming pressure; 
         FIG. 8  is a view corresponding to  FIG. 7  of the third embodiment during exertion of the forming pressure; 
         FIG. 9  is a side view an article formed according to the third embodiment; and 
         FIG. 10  is a flow chart of the process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to one aspect, there is provided a forging apparatus for forming at least one article. The article formed by this apparatus can comprise through holes and thin sections while having a homogenous structure. 
     Referring to the drawings, the apparatus  10  comprises a the  12  having a the cavity  14  to receive material  16  that is at least partially molten. For example, the material  16  can be a fully molten metal, a semi-solid metal, or a composite material having a liquid or a semi-solid metallic phase. 
     The apparatus includes a punch  20  that is adapted to engage the die cavity  14  with a sliding but generally sealing fit, and to exert a forming pressure on material  16  disposed in the die cavity  14 . For example, the forming pressure can be provided by means of a hydraulic press. 
     The apparatus  10  also comprises a pin  24  for forming an article feature. The apparatus  10  is such that the pin  24  engages the die  12  with a sliding fit. The pin  24  can extend into the die cavity  14  to contact the material  16 , as well as recede outside of the die cavity  14 . The pin  24  is also configured to exert a feature forming pressure, and to be able for form a feature in the moulded product. 
     In a first embodiment, as shown in  FIG. 1 , material  16  is disposed in the die cavity  14 . Material  16  is inserted into the die cavity  14  after pin  24  has been extended until it contacts punch  20 , or is at least above the anticipated surface level  26  of the material  16 . This is a first position of the pin  24 . Therefore, the pin  24  initially extends above the surface level  26  of the material  16  when material  16  is inserted into cavity  14 . As the punch  20  approaches the material  16  and exerts the forming pressure, the punch  20  contact and applies a force to the pin  24  before contacting the material  16 . That force is sufficient to overcome the force being applied to pin  24  to move and retain it in the extended or first position shown. The pin  24  correspondingly recedes while exerting the feature forming pressure. The feature forming pressure Is typically less than the forming pressure, and can be provided by the same hydraulic press, for example. Alternatively, it may be substantially the same as the forming pressure. When punch  20  contacts the material  16  it applies the forming pressure to the material to compress the material to form the article and, in combination with the pin  24 , to form the feature. When the punch  20  ceases movement, the pin  24  is in a receded or second position. 
     Solidification of the material  16 , as shown in  FIG. 2 , results in an article  30 , as shown in  FIG. 3 . In the first embodiment, the article feature is a through hole  32 . For example, the pin  24  can have a circular cross-section, such that the through hole  32  is circular. The pin  24  can have a different cross-section, or a plurality of different cross-sections along its length, depending on the desired shape of the through hole  32 . 
     In this way the material  16  remains liquid for a sufficient period to enable the article features to be formed before solidification. It also minimizes the risk of premature solidification—i.e. before punch  20  applies its force to material  16 . This assists in allowing the material  16  to adopt a more uniform and homogenous structure, and to reduce grain size. 
     Furthermore, if pin  24  were moved after material  16  was inserted into die cavity  14 , it may have to penetrate a skin formed on material  16  around cavity  14 . Penetrating the skin may affect pin  24 , the article feature, and the structure of the material  16  after solidification. After punch  20  contacts material  16 , the material  16  solidifies quite rapidly due to thermal transfer to and through punch  26 . 
     In a second embodiment, as shown in  FIG. 4 , material  16  is disposed in the die cavity  14 . Before material  16  is inserted into die cavity  14 , the pin  24  is initially flush with a lower surface  36  of the die cavity  14  and does not extend into the die cavity  14 . This is the first position of the pin  24 . When material  16  is inserted into die cavity  14 , and referring to  FIG. 5 , as the punch  20  exerts a forming pressure on the material  16 , pressurized material  16  causes the pin  24  to recede out of the die cavity  14  in bore  34 . At the same time, the pin  24  exerts a feature forming pressure on the material  16 . As the pin  24  recedes, the material  16  extrudes into the space  38  previously occupied by the pin  24  before receding. When the pin  24  has moved or receded sufficiently, it stops at its second, or receded, position ( FIG. 5 ). By maintaining the pressure on material  16  between pin  24  and punch  20 , the feature Is created by material  16  filling bore  34  to the top of pin  24 , and the material filling bore  34  is homogenous with the remainder of material  16 . 
     Solidification of the material  16 , as shown in  FIG. 5 , results in formation of an article  40 , as shown in  FIG. 6 . In the second embodiment, the article feature is neck  44 . For example, the pin  24  can have a thin cross-section such that the neck  44  is a thin section. Problems associated with premature solidification of the neck  44  are reduced, since solidification takes piece under pressure provided by both the punch  20  and the pin  24 . The structure of the neck  44  is thus homogenous with the rest of the article  40 . The pin  24  can have a different cross-section, or a plurality of different cross-sections along its length, depending on the desired shape of the neck  44 . 
     In a third embodiment, the apparatus  10  includes two pins  46 ,  48 , as shown in  FIG. 7 . In the third embodiment, one of the pins  46  is configured and operates according to the pin  24  of the first embodiment, while the other pin  48  is configured and operates according to the pin  24  of the second embodiment. Upon exertion of the forming pressure by the punch  20  on material  16  disposed in the die cavity  14 , solidification of the material  16 , as shown in  FIG. 8 , results in the formation of an article  50 , as shown in  FIG. 9 . In the third embodiment, two article features, a through hole  52  and neck  54 , are formed. In the third embodiment, the article  50  combines the features and processes of the first embodiment and the second embodiment. 
     Although the embodiments described depict one punch and one die, other embodiments which combine a varied plurality of dies, punches and pins configured similarly to the embodiments earlier described can be provided to form a plurality of articles having multiple article features. These features may be holes, protrusions, necks, recesses, or a combination of them. Such articles and article features can be of various shapes and sires. Articles of complex shapes and homogenous structures can thus be formed. Furthermore, the surfaces of pins  24 ,  46 ,  48  and/or punch  20  may have surface treatments to from further, complex or compound features. 
       FIG. 10  illustrates the processes described above. The pin  24  is in a first position at the start of the process ( 101 ). This will be the position shown in  FIGS. 1, 4 and 7 . The material  16  is then inserted ( 102 ) and the punch  20  moved ( 103 ). If the punch  20  contacts pin  24  before material  16  (i.e.  FIGS. 1 to 3, 7 to 9 ) ( 104 ), the punch  20  moves pin  16  to the second position ( 105 ) and compresses the material  16  ( 106 ) to form the article and the article feature ( 107 ). if the punch  20  contacts material  16  first ( 108 ) ( FIGS. 4 to 6, 7 to 9 ) it compresses the material ( 109 ) to force the pin  24  to the second position ( 110 ) to form the article and the article feature ( 107 ). 
     Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.