Abstract:
An apparatus that continuously processes a metal workpiece without substantially altering its cross section includes a wheel member having an endless circumferential groove, and a stationary constraint die that surrounds the wheel member, covers most of the length of the groove, and forms a passageway with the groove. The passageway has a rectangular shaped cross section. An abutment member projects from the die into the groove and blocks one end of the passageway. The wheel member rotates relative to the die in the direction toward the abutment member. An output channel in the die adjacent the abutment member has substantially the same cross section as the passageway. A metal workpiece is fed through an input channel into the passageway and carried in the groove by frictional drag in the direction towards the abutment member, and is extruded through the output channel without any substantial change in cross section.

Description:
STATEMENT REGARDING FEDERAL RIGHTS  
       [0001]     This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to extrusion and more particularly to an apparatus and method for continuous equal channel angular pressing a solid workpiece without substantially changing the cross-section of the workpiece.  
       BACKGROUND OF THE INVENTION  
       [0003]     Plastic deformation by rolling, extrusion and drawing often increases the strength of metal alloys, but decreases their ductility [1]. By contrast, processing metals and alloys by severe plastic deformation (SFD) can increase their strength while maintaining good ductility by forming ultrafine grains (UFGs), and subgrains, from smaller than 100 nanometers (nm) to about 1000 nanometers [2]. The combination of high strength and good ductility makes SPD-produced ultrafine-grained (UFG) materials very attractive for medical implants [3], aerospace structures, sporting goods, automobile parts and other devices.  
         [0004]     Among the SPD techniques, “equal channel angular pressing” (ECAP), also known in the art as “equal channel angular extrusion” (ECAE™), has attracted much attention because it is very effective in producing UFG structures and can produce UFG billets that are large enough for practical structural applications [4]. Only High Pressure Torsion (HPT) [5] is more effective in producing UFG structures. However, HPT can only produce small disks with a typical diameter of about 10 millimeters (mm) and a thickness of less than about 1 mm. These dimensions make them unsuitable for most structural applications. By contrast, ECAP has been used to produce billets that are long enough and wide enough for some practical structural applications.  
         [0005]     The original ECAP technique involves pressing a workpiece through a die with two channels that are equal in cross-section and intersect each other at an angle. Sending the workpiece through the die refines the microstructure, and when the die cross-section is circular or square shaped, the workpiece can be turned 90 degrees and extruded again and again because the shape and size of the workpiece does not change substantially during the pressing.  
         [0006]     The ECAP technique in its original design has some limitations: the aspect ratio (i.e. the length to diameter ratio) of the workpiece must be smaller than a critical value so that the workpiece does not bend during the pressing, and the ram that forces the workpiece through the die has a limited travel distance. These aspects of the ECAP technique place limits on the length of the workpiece and make ECAP a discontinuous process with low production efficiency and high cost. In addition, a significant length near each end of a workpiece is usually cracked and has to be removed, wasting a significant portion of the workpiece and further increasing the cost of the product. The discontinuous nature of ECAP and the wasted portions of the processed workpiece make UFG products expensive, which limits their applications to high-valued markets such as medical implants and devices where the cost of the materials is a relatively minor portion of the total cost. A key to commercializing the preparation of UFG materials is to lower their processing cost and minimize waste through continuous processing.  
         [0007]     In the early 1970&#39;s, Green and Etherington developed an effective process, now known as the CONFORM™ process, which is directed to continuous rotary extrusion that converts powder feedstock into a long solid article [6]. Briefly, a CONFORM™ apparatus includes a disk and a shoe that provide frictional force to drive feedstock through the apparatus. Feedstock is sent through a channel formed in between the disk and the shoe. A groove in the disk covered with the stationary shoe forms the channel, and the contact interface between the feedstock and the shoe results in dragging frictional force. The feedstock has three interfaces driving it forward and one interface dragging it backward, with a net forward driving force. An abutment on the inner surface of the shoe stops the feedstock and forces it through an outlet. The outlet cross-section usually has a different shape from the groove because the objective of CONFORM™ is to change the geometry of the feedstock (and consolidate the feedstock if powder feedstock is used), which usually requires only one pass. The deformation of the feedstock during extrusion is similar to a conventional extrusion process.  
         [0008]     Another continuous method called “repetitive corrugation and straightening” (RCS) has been used to process metal sheets and rods in a continuous manner [7]. RCS is less effective at refining grains than ECAP is, and each RCS pass produces non-uniform strain along the length as well as the thickness of the workpiece.  
         [0009]     A coshearing process [8] and a “continuous constrained strip shearing (C2S2) process” [9] were recently reported for continuously processing thin strips and sheets. Both processes use the friction created between the rollers and the workpiece to push the workpiece through a modified ECAP die. The former [8] uses several rollers to increase the frictional force, while the latter uses one set of rollers but employs workpiece thickness reduction to increase the frictional force. Both are limited to processing sheet metals because the frictional force required to push the workpiece through the ECAP die is proportional to the contact area between the workpiece and the rollers, and only a workpiece in sheet form can provide enough frictional force. To process a workpiece in the form of a rectangular bar, more frictional force is needed to push the workpiece through an ECAP die.  
         [0010]     No continuous process or apparatus thus far can refine the grain size of a rectangular bar without significantly affecting the cross section. There remains a need for an apparatus and process for the continuous processing of rectangular bars to refine the grain size without substantially affecting the cross section.  
         [0011]     Therefore, an object of the present invention is to provide an apparatus for the continuous equal channel angular pressing processing of a rectangular bar workpiece without substantially affecting the cross-section.  
         [0012]     Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
       SUMMARY OF THE INVENTION  
       [0013]     In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes an pressing apparatus having a wheel member having an endless circumferential groove therein; a stationary constraint die surrounding the perimeter of said wheel member and covering most of the length of the groove and forming a passageway with the groove having a rectangular shaped cross section; an abutment member projecting from the stationary constraint die into the groove and blocking one end of the passageway; the wheel member being rotatable relative to the stationary constraint die in the direction toward the abutment member; an output orifice in the stationary constraint die adjacent the abutment member and having substantially the same cross section as the cross section of the passageway; and an input orifice for feeding a solid metal workpiece to be extruded into a portion of the passageway remote from the abutment member so that the workpiece is carried in the groove by frictional drag in the direction towards the abutment member and is thereby extruded through the output orifice and without any substantial change in cross section.  
         [0014]     The invention also includes a method for continuously extruding metal. The method includes feeding a solid metal workpiece into one end of a passageway formed between a wheel member having an endless groove and a stationary constraint die that surrounds the wheel member and covers some of the length of the groove. The wheel member has a greater surface area for engaging the metal workpiece than the stationary constraint die. The passageway has a closed end remote from the end of the passageway where the workpiece is fed. An outlet at the closed end of the stationary constraint die has substantially the same rectangular cross section as the cross section of the passageway. During operation, the wheel member moves toward the outlet, and the frictional drag of the passageway-defining surfaces of the second member drags the metal workpiece through the passageway and through the outlet.  
         [0015]     The invention also includes an pressing apparatus. The apparatus includes a first wheel member having an endless circumferential groove therein; a shoe member covering only part of the length of the groove and forming an input orifice with the groove and a passageway with the groove. The passageway has a rectangular cross section. A solid metal workpiece to be extruded is fed into the input orifice and, from the input orifice, into a portion of the passageway remote from the abutment member. The first wheel member has a greater surface area for engaging the metal workpiece than the shoe member. The apparatus also includes an abutment member that projects from the shoe member into the groove and blocks one end of the passageway. The first wheel member is rotatable relative to the shoe member in the direction toward the abutment member. The shoe member includes an output orifice adjacent the abutment member; the output orifice has substantially the same cross section as the cross section of the passageway. The apparatus also includes a second rotatable wheel member remote from the abutment member of the shoe. The second rotatable wheel member is configured to contact a side of the workpiece, and urges the workpiece into the passageway so that the workpiece is carried in the groove by frictional drag in the direction towards the abutment member and is extruded through the output orifice without any substantial change in cross section.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiment(s) of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0017]      FIG. 1  shows a representation of an apparatus of the invention processing a metal workpiece.  
         [0018]      FIG. 2  shows an exploded view of a wheel member used with the apparatus of  FIG. 1 .  
         [0019]      FIG. 3  shows an isometric view of an embodiment wheel member and stationary constraint die of the invention. The stationary constraint die includes an input channel for a metal workpiece, an output channel through which the workpiece is extruded, and an abutment that extends from the stationary constraint die into the groove of the wheel member and diverts the workpiece into the output channel.  
         [0020]      FIG. 4  shows an image of an aluminum bar workpiece during processing using the apparatus of the invention.  
         [0021]      FIG. 5  shows a transmission electron microscopy (TEM) image of a portion of the extruded aluminum bar of  FIG. 4  after 4 passes through the apparatus.  
         [0022]      FIG. 6  shows a side view of an embodiment apparatus of the invention that employs two circular disks, one of which drives the rectangular bar workpiece through the apparatus; and  
         [0023]      FIG. 7  shows an isometric view of a portion of the apparatus of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0024]     The present invention includes an apparatus and method for continuously processing rectangular bar feedstock into ultrafine-grained bars without substantially altering the cross-section. Reference will now be made to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Similar or identical structure is identified using identical callouts.  
         [0025]     Turning now to the figures,  FIG. 1  shows a side view of an embodiment apparatus of the invention. Apparatus  10  includes wheel member  12  and stationary constraint die  14  coaxial with, and configured to fit around, wheel member  12 . An exploded isometric view of wheel member  12  is shown in  FIG. 2 , and an isometric view of the wheel member  12  and stationary constraint die  14  are shown in  FIG. 3 . Wheel member  12  includes first portion  16  and a second portion  18  configured such that when they are joined together, an endless groove  20  about midway along the circumference of wheel  12  is formed. Both first portion  16  and second portion  18  of wheel member  12  are hollow at their respective axes for insertion and attachment of an axle to rotate the wheel. Stationary constraint die  14  includes mounting portion  22  configured for engagement with a workbench (not shown) to prevent stationary constraint die  14  from moving. Stationary constraint die  14  includes an input channel  24  for receiving metal workpiece  26 . Die  14  also includes abutment  28  that protrudes from the inside of die  14  and is configured to fit inside groove  20  of wheel member  12 . When assembled, groove  20  and die  14  form a passageway with a rectangular cross section through which the metal workpiece  26  moves. Die  14  also includes an outlet channel  30  configured with substantially the same cross section as that of the passageway. During operation; as workpiece  26  moves through the passageway, it reaches abutment  28  and the leading end of the workpiece undergoes shear forces and grain refinement as abutment  28  redirects the workpiece as it is forced out of die  14  through outlet channel  30 . This grain refinement results in an improvement in the strength of the workpiece as it is extruded out of the die, and without any significant change in the cross section of the workpiece.  
         [0026]     During operation, rectangular bar workpiece  26  enters apparatus  10  through orifice  24  and moves into groove  20  in wheel member  12 . Wheel member  12  is rotatable and as wheel member  12  is forced to rotate clockwise for the views shown in  FIGS. 1-3 , frictional forces are generated with the workpiece  26  from the surfaces of wheel member  12  that define groove  20 , and also from the inner surface of the stationary constraint die  14 . Groove  20  is slightly wider than workpiece  26  before processing, but after workpiece  26  enters apparatus  10  and starts moving through the passageway, it widens slightly until contacts the surfaces of the wheel that define the groove. The frictional forces exerted by the wheel member  12  and stationary die  14  produce a net force on workpiece  26  that drags it through the passageway in the same direction as wheel member  12 . Die  14  constrains workpiece  26  within groove  20  as it moves along until the leading end of the workpiece contacts abutment  28 , which forces the workpiece through outlet channel  30 . As the workpiece is extruded, it undergoes shear forces that result in grain refinement. In the current set-up, the angle is about 90 degrees, which is the most commonly used channel intersection angle in ECAP. The shear forces are well known and have already been described in the prior art for equal channel angular pressing of metal billets.  
         [0027]     The invention was demonstrated using apparatus  10  and an aluminum rectangular bar workpiece. The diameter of the woripiece was about 3.4 millimeters.  FIG. 4  shows the bar during processing. Progressing from the end portion of the bar that had not yet entered the apparatus to the leading end that had been extruded, the bar was forced to bend within the groove of the wheel until reaching the abutment on the stationary constraint die. This is clearly shown by the abrupt changes in the shape of the bar from a linear shape (prior to entering the apparatus) to a curved shape (inside the apparatus but before reaching the abutment) to the shape resulting from having been forced through the stationary die at an angle of about 90 degrees. The extruded portion of the bar has a linear shape.  
         [0028]     The cross-section of the workpiece after the first pass was 3.78 mm by 2.78 mm. The workpiece was rotated by 180 degrees in between successive passes for a total of 4 passes. The mechanical properties of the aluminum bar were determined after 1 pass, 2 passes, 3 passes, and 4 passes. The data are shown in TABLE 1.  
                                                                 TABLE 1                                   Processing                           state   σ 0.2  (MPa)   σ u  (MPa)   δ (%)   ψ (%)                                        Starting bar   47   71   28   86           1 pass   130   160   13   73           2 passes   140   170   12   72           3 passes   130   160   14   76           4 passes   140   180   14   76                      
 
         [0029]     The symbols σ 0.2  and σ u  relates to the yield strength and ultimate strength of the bar, respectively, in units of megapascals (MPa). The symbol δ relates to the percent elongation to failure for the bar. The symbol Ψ relates to the percent necking cross-section reduction of the bar. As the data of TABLE 1 show, the yield strength and ultimate strength of the bar have improved while maintaining good elongation to failure (i.e. ductility) of about 12-14 percent.  
         [0030]      FIG. 5  shows a transmission electron microscopy (TEM) image of a portion of the extruded aluminum bar after 4 passes through the apparatus. The image clearly shows that ultrafine-grained structures of the bar have grain sizes below 500 nanometers.  
         [0031]     There are differences between the invention and the known CONFORM process. One difference is related to the shear strain in the workpiece generated at the intersection of the die channel and the groove. The invention subjects the workpiece to a pure shear strain that is the same type of strain as in the well-known ECAP process. By contrast, the CONFORM process subjects the workpiece to a more complex strain [10] that is similar to the strain experienced by a workpiece undergoing normal pressing through a narrow opening.  
         [0032]     Another difference is related to changes in the shape of the bar workpiece. The invention does not significantly change the shape or cross section of the workpiece (except during the first pass in some cases). This aspect of the invention enables a single workpiece to be processed repeatedly for multiple passes to further improve its strength. By contrast, CONFORM typically changes the shape and cross-section of a workpiece to the extent that workpieces can be passed through a CONFORM apparatus only once.  
         [0033]     Another difference is related to the presence of inactive zones in a typical CONFORM apparatus that are absent from the invention. The die used with the CONFORM process usually includes an inactive zone where workpiece gets trapped and does not move. No such zone is present with the invention.  
         [0034]      FIG. 6  shows a side view of second embodiment apparatus of the invention, and  FIG. 7  shows an isometric view of a portion of the apparatus. Apparatus  32  includes wheel member  12 , which is configured as previously described for apparatus  10 . Apparatus also includes second wheel member  34 , which differs from wheel member  12  in that wheel member  34  does not include groove  20 , but instead has substantially flat circumferential surface for contacting and driving workpiece  26 , along with wheel member  12 , by supplying frictional force with workpiece  26 . Apparatus  32  also includes die member  36 , which has an inner surface portion similar to that of die  14 . As  FIG. 7  shows, die member  36  also includes an abutment  28  that protrudes from the inside of die member  36  and is configured to fit inside groove  20  of wheel member  12 . When assembled, groove  20  and die  14  form a passageway with a rectangular cross section through which the metal workpiece  26  moves. Die member  36  also includes an outlet channel  30  configured with substantially the same cross section as that of the passageway. During operation, as workpiece  26  moves through the passageway, it reaches abutment  28  and the leading end of the workpiece undergoes shear forces and grain refinement as abutment  28  redirects the workpiece as it is forced out of die  14  through outlet channel  30 , the same way as described for apparatus  10 . Thus, the grain refinement that occurs results in an improvement in the strength of the workpiece as it is extruded out of the die, and without any significant change in the cross section of the workpiece.  
         [0035]     During operation, wheel member rests against surface portion  38  of die member  36  and also against wheel member  32  such that wheel member  32  and wheel member  34  and die member  36  form an entrance through which workpiece enters apparatus  12 . As workpiece  26  enters apparatus  32  through this entrance, it moves into groove  20  in wheel member  12 . Both wheel member  12  and wheel member  34  are rotatable and as wheel member  34  rotates, wheel member  12  is forced to rotate (clockwise for the views shown in  FIG. 6-7 . Frictional forces are generated, first between workpiece  26  and both wheel member  12  and wheel member  34 , and then between the inner surface of die member  36  and the surfaces of wheel member that define groove  20  as the workpiece moves. As described for apparatus  10 , groove  20  is slightly wider than workpiece  26  before processing, but after workpiece  26  enters apparatus  10  and starts moving through the passageway, it widens slightly until contacts the surfaces of the wheel that define the groove. The frictional forces exerted by wheel member  34 , wheel member  12  and die member  36  produce a net force on workpiece  26  that drags it through the passageway in the same direction as wheel member  12 . Die member  34  constrains workpiece  26  within groove  20  as it moves along until the leading end of the workpiece contacts abutment  28 , which forces the workpiece through outlet channel  30 . As the workpiece is extruded, it undergoes shear strain that results in grain refinement. In the current set-up, as described for apparatus  10 , the angle is about 90 degrees, which is the most commonly used channel intersection angle in ECAP. The shear strain is well known and have already been described in the prior art for equal channel angular extrusion of metal billets. Preferably, the second wheel member  34  is as wide as first wheel member  12  and shoe  38 , but it can also be wider or narrower, which is not critical. Second wheel member  34  widens the billet enough so that the widened billet contacts the surfaces of groove  20 .  
         [0036]     Ultrafine-grained (UFG) materials processed by Severe Plastic Deformation (SPD) have attracted attention in the research and development community in recent years. Currently, most SPD techniques produce UFG materials in a costly, batch-processing manner. This invention enables the continuous processing of metal and metal-alloy rectangular bars and wires to produce metal bars and wires with an ultrafine-grained structure and without significant changes in cross-section.  
         [0037]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, while aluminum bar workpieces were used to demonstrate this invention, it should be understood that this invention is not limited to processing only aluminum, and that any metal or metal alloy workpiece could be used instead.  
         [0038]     The embodiment(s) were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.  
         [0039]     The following references are incorporated by reference herein.  
       REFERENCES  
       [0000]    
       
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