Abstract:
Disclosed herein is an ultrasonic extrusion apparatus which causes resonance of an extrusion die using ultrasonic vibrations, thus reducing friction between the extrusion die and an extrusion material. The ultrasonic extrusion apparatus has an increased number of ultrasonic vibrators provided around the extrusion die in the circumferential direction to increase the vibration output, whereby when extruding a metal material such as a magnesium material, friction between the metal material and the extrusion die can be reduced, thus improving the performance of the extrusion process (preventing a reduction in an extrusion rate, generation of heat, cracking of a product, errors in measurements, etc.).

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to ultrasonic extrusion apparatuses which cause resonance of extrusion dies using ultrasonic vibrations, thus reducing friction between the extrusion dies and extrusion materials and, more particularly, to an ultrasonic extrusion apparatus which has an increased number of ultrasonic vibrators provided around the extrusion die in a circumferential direction to increase the vibration output, whereby when extruding a metal material such as a magnesium material, friction between the metal material and the extrusion die can be reduced, thus improving the performance of the extrusion process (preventing a reduction in an extrusion rate, generation of heat, cracking of a product, errors in measurements, etc.). 
         [0003]    2. Description of the Related Art 
         [0004]    Generally, it is difficult to shape a metal material such as a magnesium material because of friction between tools and the material to be shaped. 
         [0005]    For this reason, an extrusion method is mainly used to shape a metal material. Typically, as shown in  FIG. 1A , the method of extruding a metal material includes passing a molten metal material  20  through an extrusion die  10 , thus forming a metal product  22  having a desired shape. 
         [0006]    This metal extrusion method is a method which is used to manufacture high-density and high-quality products at a high production rate. However, the conventional extrusion method has many problems which occur because of friction between the extrusion die  10  and the metal material  20  to be extruded. 
         [0007]    To overcome these problems, as shown in  FIG. 1B , a technique was proposed, in which an ultrasonic vibrator  10  applies ultrasonic resonance to the extrusion die  10  which shapes a metal material, thus minimizing friction between the extrusion die  10  and the metal material, thereby enhancing the effectiveness of the extrusion process. 
         [0008]      FIG. 2  shoos a conventional ultrasonic resonance system  50  in which ultrasonic resonance is applied to the extrusion die  10  to reduce friction between an extrusion material  20  and an extrusion die  10 , thus improving the process of forming a rod-shaped product. 
         [0009]    In the conventional ultrasonic resonance system  50 , two ultrasonic vibrators  40  are respectively provided on upper and lower portions of too extrusion die  10 . The ultrasonic vibrators  40  generate ultrasonic waves. For this, an ultrasonic wave generator  60 , first and second amplifiers  62  and  64 , a pressure controller  66 , an oscilloscope  68 , a heater controller  70 , etc. are connected to the ultrasonic vibrators  40 . 
         [0010]    In detail, the ultrasonic wave generator  60  outputs a voltage at 10V in a from of sine wave to operate the ultrasonic vibrators  40 . Each of the first and second amplifiers  62  and  64  amplifies the voltage output from the ultrasonic wave generator  60  to from 10 to 100 times and supplies it to the corresponding ultrasonic vibrator  40 . 
         [0011]    Furthermore, the pressure controller  66  controls a piston  72  of a double-acting extruder with a pressure ranging from 0 ton to 500 tons. The oscilloscope  68  measures the voltage and current applied to the ultrasonic vibrators  40 . The heater controller  70  maintains the temperature of extrusion material (magnesium)  20 , which is approximately 200° C., so that the extrusion material  20  is prevented from being cooled. 
         [0012]    As shown in  FIG. 3 , compared to the metal extrusion method using no ultrasonic vibrator, the conventional ultrasonic resonance system  50  having the above-mentioned construction can markedly reduce friction between the extrusion die  10  and the extrusion material  20 , thus making it possible to manufacture higher-density and higher-quality metal products at a high production rate. 
         [0013]    However, as shown in  FIG. 4 , in the case of the extrusion die  10  provided in the conventional ultrasonic resonance system  50 , opposite surfaces of a die body  12  which is fixed between an extruder heater  82  and a die holder  34  are planar. An extrusion hole which has a cylindrical or polygonal shape having a constant cross-sectional area is transversely formed through a central portion of the extrusion die  10   
         [0014]    Furthermore, a plurality of mounting holes  16  are formed in a circumferential outer surface of the die body  12  at diametrically opposite positions. The two ultrasonic vibrators  40  are fastened to two portions of the die body  12  through the corresponding amounting holes  16  end apply vibrations to the extrusion die  10 . 
         [0015]    However, in the conventional extrusion die  10 , a front surface of the die body  12  is a planar surface and is brought into direct contact with the extrusion material  20 . Thus, extrusion pressure, ranging from about 90 tons to about 150 tons, for extruding the extrusion material  20  is directly transmitted to the front surface of the die body  12 . A rear surface of the die body  12  is also planar, is brought into close contact with a front surface of the die holder  84 , and is reliably integrally fixed to the die holder  84 . 
         [0016]    Therefore, in the case of the conventional extrusion die  10 , even if vibrations are applied from the ultrasonic vibrators  40  to the extrusion die  10 , they are disturbed by high extrusion pressure of the extrusion material  20  and dispersed to the outside through the die holder  84 . Thus, the vibrations cannot be effectively transmitted to the extrusion die  10 , so it is difficult to obtain a satisfactory vibration effect. Moreover, a phenomenon in which a vibration mode deviates from normal conditions is also caused. 
         [0017]    In addition, because only the two ultrasonic vibrators  40  that are disposed on the upper and lower portions of the die body  12  apply vibrations to the die body  12  in the directions facing each other, vibrations cannot be evenly applied to the overall shaping space in the extrusion die  10 , but vibrations are partially applied to only the upper and lower portions of the die body  12 , thus greatly reducing substantial ultrasonic vibration effect. 
       SUMMARY OF THE INVENTION  
       [0018]    Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an ultrasonic extrusion apparatus for metal material which is configured such that vibrations generated from ultrasonic vibrators mounted to an extrusion die are effectively transmitted to the extrusion die, whereby when an extrusion process is conducted, friction between the extrusion die and the metal material is minimized so that high-density and high-quality metal products can be manufactured at a high production rate. 
         [0019]    Another object of the present invention is to provide an ultrasonic extrusion apparatus for metal material in which pressure required to shape the metal material that is applied to a front surface of the extrusion die can be minimized, and outward dispersion of vibrations transmitted from the ultrasonic vibrator to the extrusion die can be minimized, so that vibrations generated from the ultrasonic vibrators can be more effectively transmitted to the extrusion die while the extrusion process is being carried out. 
         [0020]    In order to accomplish the above object, the present invention provides an ultrasonic extrusion apparatus for extruding a metal material in such a way that an extrusion die is resonated by ultrasonic vibrations, the ultrasonic extrusion apparatus including: a die body disposed between an extrusion piston end a die holder; front and rear conical surfaces respectively formed on front and rear surfaces of the die body, each of the front and rear conical surfaces being concave; an extrusion hole passing through central portions of the front and rear conical surfaces, the extrusion hole being defined by a first inner diameter part through which the metal material is actually extruded and a second inner diameter part having a diameter larger than the first inner diameter part so that the metal material that has passed through the first inner diameter part is prevented from coding into contact with the second inner diameter part; and a plurality of ultrasonic vibrators mounted to a circumferential outer surface of the die body, the ultrasonic vibrators applying the ultrasonic vibrations to the die body, wherein the ultrasonic vibrators are arranged around the circumferential outer surface of the die body to be symmetrical based on the extrusion hole so that the ultrasonic vibrations are evenly applied to the die body. 
         [0021]    Furthermore, a plurality of chamfering surfaces may be formed on the circumferential outer surface of the die body at positions spaced apart from each other at regular intervals with respect to a circumferential direction, a mounting hole may be formed in a central portion of each of the chamfering surfaces, and the ultrasonic vibrators may be mounted to the respective chamfering surfaces through the corresponding mounting holes. 
         [0022]    The chamfering surfaces may comprise six chamfering surfaces formed on the circumferential outer surface of the die body, and the ultrasonic vibratos may comprise six ultrasonic vibrators mounted to the respective six chamfering surfaces through the corresponding six mounting holes. 
         [0023]    The ultrasonic extrusion apparatus may further include a cylindrical jig disposed on the front conical surface of the die body, wherein the extrusion piston may be disposed in the jig so that a shaping pressure is formed in the jig, thus reducing an extruding pressure applied to the front conical surface of the die body. 
         [0024]    The ultrasonic extrusion apparatus may further include a vibration isolation unit disposed between the rear conical surface of the die body and the die holder to reduce a vibration transmitted from the die body to the die holder, the vibration isolation unit including: front and rear circular plates disposed facing each other; and a connector disposed inside the front and rear circular plates, wherein front surfaces of the front circular plate and connector may be closely fixed to a rear surface of the die body, and rear surfaces of the rear circular plate and connector may be closely fixed to a front surface of the die holder, and a through hole may be formed in a central portion of the connector so that the extrusion hole communicates with an internal space of the die holder through the through hole. 
         [0025]    The through hole of the connector may have a diameter larger than the diameter of the extrusion hole of the die body. 
         [0026]    In an ultrasonic extrusion apparatus according to the present invention, front and rear conical surfaces that are concave are respectively formed on front and rear surfaces of a die body which is disposed between an extrusion piston and a die holder. In addition, a cylindrical jig is provided on the front conical surface. Thereby, extrusion pressure which is transmitted to the front surface of the die body can foe markedly reduced. 
         [0027]    Furthermore, a vibration isolation unit is installed between the rear conical our face of the die body and a die holder so that vibrations which are transmitted from the die body to the die holder can be markedly reduced. 
         [0028]    Moreover, a plurality of ultrasonic vibrators which apply vibrations to the extrusion die are arranged around the extrusion die in the circumferential direction so that vibrations can be evenly applied to the entirety of extrusion die. Therefore, vibrations which are generated from the ultrasonic vibrators mounted to the extrusion die can be effectively transmitted to the extrusion die so that friction between the metal material and the extrusion die can be minimized during the extrusion process. 
         [0029]    Ultimately, compared to the conventional ultrasonic extrusion apparatus, the ultrasonic extrusion apparatus of the present invention can produce high-density and high-quality metal products at a high production rate. 
     
    
     
       BRIEF DESCRIPTION OP THE DRAWINGS  
         [0030]    The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0031]      FIG. 1A  is a sectional view showing a conventional typical extrusion method; 
           [0032]      FIG. 1B  is a sectional view shoeing a conventional extrusion method using an ultrasonic vibrator for applying vibrations to a target; 
           [0033]      FIG. 2  is a vice showing the general construction of a typical ultrasonic resonance system; 
           [0034]      FIG. 3  is of graphs comparing the conventional typical extrusion method and the conventional extrusion method using the ultrasonic vibrator; 
           [0035]      FIG. 4  is a sectional view shooing a conventional ultrasonic extrusion apparatus; 
           [0036]      FIG. 5  is a sectional view illustrating an ultrasonic extrusion apparatus for metal material according to the present invention; 
           [0037]      FIG. 6A  is a front view illustrating an extrusion die provided in the ultrasonic extrusion apparatus according to the present invention; 
           [0038]      FIG. 6B  is a sectional view of the extrusion die of  FIG. 6A ; 
           [0039]      FIG. 7  is a perspective view shooing the structure of the ultrasonic extrusion apparatus to which a plurality of ultrasonic vibrators are mounted in radial directions according to the present invention; 
           [0040]      FIGS. 8A and 8B  are views illustrating the result of observation of variation in displacement of a vibration mode depending on variation in the radius of the extrusion die of the ultrasonic extrusion apparatus according to the present invention; 
           [0041]      FIGS. 9A and 9B  are views illustrating the result of observation of variation in the vibration mode depending on variation in the height of the extrusion die of the ultrasonic extrusion apparatus according to the present invention; 
           [0042]      FIGS. 10A and 10B  are views or illustrating the result of observation of variation in the vibration mode depending on variation in the size of an extrusion hole of the extrusion die of the ultrasonic extrusion apparatus according to the present invention; 
           [0043]      FIGS. 11A and 11B  are views illustrating the result of observation of variation in the vibration mode depending on variation in the size of a conical surface of the extrusion die of the ultrasonic extrusion apparatus according to the present invention; 
           [0044]      FIGS. 12A and 12B  are views illustrating the result of simulation of a series of vibration transmission in a conventional extrusion die; and 
           [0045]      FIGS. 13A and 13B  are views illustrating the result of simulation of a series of vibration transmission in the extrusion die of the ultrasonic extrusion apparatus according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the attached drawings. 
         [0047]    Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. 
         [0048]    An ultrasonic extrusion apparatus  100  for metal material according to the present invention is used in an ultrasonic resonance system which shapes metal extrusion material while resonance of the extrusion die  110  is caused by ultrasonic vibrations. As shown in  FIG. 5 , the ultrasonic extrusion apparatus  100  includes a die body  130  which is disposed between an extrusion piston  120  and a die holder  122 . 
         [0049]    The die body  130  forms a main body of the extrusion die  110 . Front and rear conical surfaces  132  and  134  which are concave are respectively formed in front and rear surfaces of the die body  130 . 
         [0050]    In detail, as shown in  FIGS. 6A and 6B , the die body  130  has a disk structure. The front and rear conical surfaces  132  and  134  are respectively formed in the front and rear surfaces of the die body  130  in the opposite directions. An extrusion hole  140  is formed in the die body  130  such that it passes through the centers of the front and rear conical surfaces  132  and  134 . 
         [0051]    The extrusion hole  140  is defined both by a first inner diameter part  142  which actually extrudes material, and by a second inner diameter part  144  which has a diameter larger than that of the first inner diameter part  142  so that the extrusion material that has passed through the first inner diameter part  142  does not make contact with the second inner diameter part  144 . It can be understood that the extrusion hole  140  is reduced in size compared to the conventional structure shown in  FIG. 4 . 
         [0052]    Furthermore, the ultrasonic extrusion apparatus  100  according to the present invention includes a plurality of ultrasonic vibrators  150  which are provided on a circumferential outer surface of the die body  130  to apply ultrasonic vibrations to the die body  130 . 
         [0053]    The ultrasonic vibrators  150  are respectively fastened into mounting holes  130  which are formed in the circumferential outer surface of the die body  130  at positions spaced apart from each other at regular intervals with respect to the circumferential direction. In an embodiment, preferably, six mounting holes  138  are forced at regular circumferential intervals, and six ultrasonic vibrators  150  are fastened into the respective mounting holes  138  so that ultrasonic vibrations are applied to the die body  130 . 
         [0054]    To install the ultrasonic vibrators  150  on the die body  130 , the circumferential outer surface of the die body  130  is diametrically symmetrical based on the extrusion hole  140 . In addition, chamfering surfaces  136  are formed on the circumferential outer surface of the die body  130  and spaced apart from each other at regular intervals in the circumferential direction. 
         [0055]    The mounting holes  138  are respectively formed in central portions of the chamfering surfaces  136 . The ultrasonic vibrators  150  are mounted to the respective chamfering surfaces  136  by the corresponding mounting holes  138 . 
         [0056]    The mounting structure or the ultrasonic vibrators  150  is illustrated in detail in  FIG. 7 . 
         [0057]    As such, in the ultrasonic extrusion apparatus  100  for metal material according to the present invention, the sin ultrasonic vibrators  150  are arranged around the circumferential outer surface of the die body  130  of the extrusion die  110 , whereby vibrations can be uniformly applied to the overall portion of the extrusion die  110 . 
         [0058]    The ultrasonic extrusion apparatus  100  according to the present invention further includes a cylindrical jig  160  which is disposed on the front conical surface  132  of the die body  130 . 
         [0059]    As shown in  FIG. 5 , the jig  160  has a cylindrical structure. A rear surface of the jig  100  is brought into line contact with the front conical surface  132  of the die body  130 . As such, unlike the conventional technique of  FIG. 4  in which the overall front surface of the die body  130  is brought into direct surface contact with extrusion material, the present invention can markedly reduce the contact area between the die body  130  and extrusion material. 
         [0060]    Furthermore, an extrusion piston  120  is disposed in the jig  160 . A heater  162  is provided around a circumferential outer surface of the jib  160 . 
         [0061]    When the extrusion operation is conducted, the extrusion piston  120  is operated in the jig  160  that has the cylindrical structure, so that shaping pressure is formed in the jib  160 , and extrusion pressure transmitted to the front conical surface  132  of the die body  130  can be markedly reduced. 
         [0062]    The ultrasonic extrusion apparatus  100  according to the present invention further includes a vibration isolation unit  170  which is disposed between the rear conical surface  134  of the die body  130  and the die holder  122 . 
         [0063]    The vibration isolation unit  170  includes front and rear circular plates  172   a  and  172   b  which face each other, a connector  176  which is disposed inside the front and rear circular plates  172   a  and  172   b,  and a plurality of springs  178  which are provided between the front and rear circular plates  172   a  and  172   b.    
         [0064]    Front surfaces of the front circular plate  172   a  and connector  176  of the vibration isolation unit  170  are closely fixed to a rear surface of the die body  130 . Rear surfaces of the rear circular plate  172   b  and connector  176  are closely fixed to a front surface of the die holder  122 . 
         [0065]    Furthermore, a through hole  176   a  is formed in a central portion of the connector  176  so that the extrusion hole  140  communicates with the internal space  122   a  of the die holder  122  through the through hole  176   a.  The diameter of the through hole  176   a  of the connecter  176  is larger than that of the extrusion hole  140  of the die body  130 . 
         [0066]    The vibration isolation unit  170  having the above-mentioned construction functions to reduce vibrations transmitted from the die body  130  to the die holder  122 . 
         [0067]    In the ultrasonic extrusion apparatus  100  according to the present invention, when the structure thereof is designed, the optimal vibration mode can be determined in such a way that simulations are conducted in consideration of a variety of factors. 
         [0068]    In detail, as shown in  FIGS. 8A and 8B , variation in displacement of the vibration mode depending on variation in the radius r of the extrusion die  110  was observed. A target value of displacement (hereinafter, referred to as a frequency) of the vibration mode is 20.5 kHz. 
         [0069]    As shown in the graph of  FIG. 8B , when the radius r of the extrusion die  110  is 70 mm and the height thereof is 50 mm, the desired displacement could be obtained. However, when the radius r increased to 80 mm or 90 mm, the vibration frequency reduced to 18.3 kHz or 16.6 kHz so that the vibration mode departed from the target value. Therefore, it could be appreciated that variation of the vibration mode depends on the radius r of the extrusion die  110 . 
         [0070]    Meanwhile, as shown in  FIGS. 9A and 9B , with regard to the ultrasonic extrusion apparatus  100  according to the present invention, it was observed that variation in the vibration mode is affected by variation in the height h of the extrusion die  110 . 
         [0071]    A target value of the vibration mode is the same 20.5 kHz. The simulation analysis was carried out to observe the variation in the vibration mode as the height h is increased to 50 mm, 60 mm and 70 mm. 
         [0072]    When the height h of the extrusion die  110  was 50 mm, the value of the vibration mode was 20.5 kHz, which is the target value, but at 60 mm it was 19.8 kHz, and at 70 mm it was 18.7 kHz. 
         [0073]    Furthermore, when the height h increased to 60 mm, the vibration mode was varied in such a way that the extrusion die shrunk inwards. When the height h was 70 mm, the vibration mode was varied in such a way that the extrusion die turned inside out. 
         [0074]    As shown in  FIGS. 10A and 10B , with regard to the ultrasonic extrusion apparatus  100  according to the present invention, variation in the vibration mode depending on variation in the size of the extrusion hole  140  of the extrusion die  110  was observed. The extrusion die  110  of  FIG. 10A  has a double-stepped hole structure which includes the first inner diameter part  142  which has a radius of 10 mm and a height of 5 mm, and a second inner diameter part  144  which has a radius of 11.5 mm and a height of 15 mm. 
         [0075]    Here, a first-step hole of the first inner diameter part  142  that has a radius of 10 mm is fixed in size for extrusion. In this experiment, the simulation analysis was performed to observe variation in the vibration mode as the size of the hole of the second inner diameter part  144  varies. 
         [0076]    According to the result of the simulations performed while the radius of the second inner diameter part  144  varies to 11.5 mm, 15.5 mm and 19.5 mm, the frequency of the vibration mode varied to 20.5 kHz, 19.8 kHz and 19.2 kHz. It can be understood that, compared to variation in the size of the hole, the variation in the vibration mode is less. 
         [0077]    Furthermore, as shown in  FIGS. 11A and 11B , with regard to the ultrasonic extrusion apparatus  100  according to the present invention, a simulation was conducted when the front and rear conical surfaces  132  and  134  based on the extrusion hole  140  of the extrusion die  110  are different from each other, being respectively 70 mm and 100 mm. 
         [0078]    Although it was expected that variation in the vibration mode would be comparatively large depending on the size of the front or rear conical surface  132  or  134 , it was 20.7 kHz, that is, it was cot largely changed. Therefore, it was confirmed that the size of the front or rear conical surface  132  or  134  did not largely affect variation in the vibration mode. 
         [0079]    As such, according to the result of simulations for the ultrasonic extrusion apparatus  100  of the present invention that were carried out in consideration of different kinds of factors, the vibration mode varied most significantly in accordance with the radius r of the extrusion die  110 , and, to a lesser extent, in accordance with the height h of the extrusion die  110 . It could be under stood that the radius of the extrusion hole  140  can be used to finely control the vibration mode. 
         [0080]    As shown in  FIGS. 12A and 12B , a simulation of a series of vibration transmission with regard to the convention extrusion die  10  was carried out, and the result of the simulation was analyzed. 
         [0081]    As can be understood from the result of the analysis, although vibrations are biased towards the center of the extrusion die  10 , they are concentrated on opposite corners and a lower portion of the extrusion die  10 . This means that vibrations generated by the ultrasonic vibrators  40  spread downward and sideways so that the vibrations are not reliably transmitted towards the center of the extrusion die  10 . 
         [0082]    Furthermore, in the conventional extrusion die  10 , when vibrations are applied thereto, they must be concentrated on small holes formed on an upper end of the extrusion die  10  so as to reduce friction, thus reducing pressure by which an extrusion material must be pushed forwards. However, vibrations are concentrated just on the center of the extrusion die  10 . Thus, when an actual extrusion process is conducted, the effect of the ultrasonic vibrators is reduced. 
         [0083]    However, as shown in  FIGS. 13A and 13B , according to the result of the analysis of the ultrasonic extrusion apparatus  100  of the present invention, it was found that vibrations are concentrated on the center of the extrusion die  110  and transmitted to the first inner diameter part  142  that defines the actual extrusion size of the extrusion hole  140 . It was analyzed that this result is possible because the present invention is configured such that the upper and lower portions of the extrusion die  110  are in balance, and vibrations can be concentrated on the center of the extrusion die  110  through the front and rear conical surfaces  132  and  134 . 
         [0084]    Therefore, according to the result of the analysis of the ultrasonic extrusion apparatus  100  of the present invention, it is most important that the extrusion die  110  is designed such that the shape thereof is as balanced as possible. Furthermore, the size of the extrusion die  110  must be determined depending on the size of the extrusion die  110  and the number of vibrators. 
         [0085]    In the ultrasonic extrusion apparatus  100  of the present invention having the above-mentioned construction, the front and rear conical surfaces  132  and  134  that are concave are respectively formed on the front and rear surfaces of the die body  130  which is disposed between the extrusion piston  120  and the die holder  122 . In addition, the cylindrical jig  160  is provided on the front conical surface  132 . Thereby, the extrusion pressure which is transmitted to the front surface of the die body  130  can be markedly reduced. 
         [0086]    Furthermore, in the present invention, the vibration isolation unit  170  is installed between the rear conical surface  134  of the die body  130  and the die holder  122  so that vibrations which are transmitted from the die body  130  to the die holder  122  can be markedly reduced. Moreover, the six ultrasonic vibrators  150  which apply vibrations to the extrusion die  110  are arranged around the extrusion die  110  in the circumferential direction so that vibrations can be evenly applied to the entirety of extrusion die  110 . 
         [0087]    Therefore, vibrations which are generated from the ultrasonic vibrators  150  mounted to the extrusion die  110  can be effectively transmitted to the extrusion die  110 . Thereby, when the extrusion process is conducted, friction between the extrusion die  110  and extrusion material can be minimized. As a result, compared to the conventional ultrasonic extrusion apparatus, high-density and high-quality metal products can be manufactured at a high production rate. 
         [0088]    Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, the present invention is not limited to such a special structure. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. For example, although the number of chamfering surfaces  136  formed on the circumferential outer surface of the die body  130  has been illustrated as being six so that the six ultrasonic vibrators  150  are mounted to the respective chamfering surfaces  136  through the six corresponding mounting holes  138 , the number of chamfering surfaces  136  or ultrasonic vibrators  150  may be changed, e.g., within a range from four to twelve. It should be understood that such simple design modifications or changes fall within the bounds of the present invention.