Abstract:
A clad billet  120  has a core material  122  of which outer surface is coated by a coating material  124.  A circular front plate  126  is provided at the head of the clad billet  120.  The front plate  126  is made of the same material as the coating material  124.  As the clad billet  120  is extruded, the front plate  126  first flows out. Accordingly, instead of the core material  122,  the front plate  126  forms dead metal. Moreover, since this front plate  126  is made of the same material as the coating material  124,  a defective clad such as a three layer clad are not formed. Also, since a billet thrusting face  102   a  is tapered toward an axis A of a die at an angle of 55-85 degrees, the volume of dead metal itself is reduced, and therefore it is possible to flow out the defective clad, even if it is generated, at an early stage of extrusion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    i) Field of the Invention  
           [0002]    This invention relates to a method for manufacturing a clad material by means of indirect extrusion.  
           [0003]    ii) Description of the Related Art  
           [0004]    As shown in FIG. 7, for example, when conventional indirect extrusion processing is performed, an extrusion tool comprised of a die  3  for defining an outer shape of a product and a mandrel  4  for defining an inner shape of the product is installed inside a container  1 , and a billet  2  is set inside the container  1  and thrust against a loose dummy  6 . This conventional method of indirect extrusion is disclosed in the Unexamined Japanese Patent Publication No. 9-201618.  
           [0005]    In this case, the loose dummy  6  is stationary, and the die  3  and the mandrel  4  are forced together to move relatively toward the billet  2  (more particularly, toward the loose dummy  6 ) set inside the container  1 . Then, the billet  2  is extruded through a die opening  11  into a product shape to form an extruded material  10 .  
           [0006]    In such a processing method by means of indirect extrusion, the billet  2  is not moved with regard to the container  1 , and no friction is generated between an inner wall of the container  1  and the billet  2 . Accordingly, less formation of dead metal is achieved, and thus the method has been in the limelight in the field of manufacturing of extruded products with high precision.  
           [0007]    Although the above example is for indirect extrusion of a tube, indirect extrusion of a stick (solid material) can be also performed in the same manner only by removing the mandrel  4 .  
           [0008]    When a clad billet composed of a core material and a coating material undergoes indirect extrusion according to the aforementioned prior art technique, however, dead metal composed of the core material is formed, though it is little, and the core material in the dead metal is extruded as a surface layer of the product at an early stage of extrusion. Accordingly, a defective clad called three-layer clad is formed which is composed of the core material, coating material and core material in layers.  
           [0009]    In other words, referring first to FIG. 8A, a die  3  is forced to move relatively toward a billet  50  which is set inside the container  1  and composed of a core material  52  and a coating material  54  coating the outer surface of the core material  52 . Then, referring to FIG. 8B, a dead metal  52   a  area composed of the core material  52  which fails to flow into the die opening  11  is formed in the vicinity of the die  3 , although the area is narrow. As the die  3  is further forced to move, referring to FIG. 8C, the core material  52  in the vicinity of the dead metal  52   a  area is extruded through the die opening  11  as a product surface layer  62 . As a result, an extruded material  60  makes a defective clad (three-layer clad) comprising a first layer composed of the core material  52  (product surface layer  62 ), a second layer  64  composed of the coating material  54  and a third layer  66  composed of the core material  52  (refer to FIGS. 8C and 8D which is a cross sectional view taken along a line  8 D- 8 D of FIG. 8C).  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the present invention is to provide an indirect extrusion method which can substantially reduce a cut-off ratio of a defective clad and improve the product yield.  
           [0011]    In order to attain the above object, the present invention provides an indirect extrusion method for manufacturing a clad material by indirect extrusion in which a die is forced to move relatively toward a billet set inside a container. The billet is composed of a cylindrical or tubular core material and a coating material coating the outer surface of the core material. A billet thrusting face of the die is tapered at an angle of 55-85 degrees with regard to the axis of the die. To an end of the billet, a circular or annular front plate made of the same material as the coating material is attached, and the plate is extruded together with the billet.  
           [0012]    According to the above indirect extrusion method for a clad material (hereinafter, referred to merely as indirect extrusion method), as extrusion processing is performed, the front plate provided at the head of the billet flows out first. Therefore, dead metal composed of the front plate instead of the core material is formed. Since the front plate is made of the same material as the coating material, generation of a three-layer clad is avoided. Additionally, since the billet thrusting face is tapered at an angle of 55-85 degrees with regard to the axis of the die, the volume of the dead metal itself is reduced. Therefore, a defective clad, even if it is generated, can be driven out at an early stage of extrusion. If the angle of the billet thrusting face with regard to the axis of the die (hereinafter, referred to as taper angle) is more than 85 degrees, there is no improvement in reduction of the dead metal volume. If the taper angle is less than 55 degrees, part of the billet is adhered to the billet thrusting face of the die upon cutting off the extruded remainder. Therefore, removal of the adhesion part is necessary at completion of extrusion, and this substantially decreases the workability.  
           [0013]    In order to prevent a phenomenon (blister) in which a space is generated between the core material and the coating material in the extruded material manufactured according to the indirect extrusion method of the present invention and to improve the product yield, the indirect extrusion method can be performed under the following condition.  
           [0014]    The second aspect of the indirect extrusion method for a clad material according to the present invention is that a diameter of the front plate is 90-100% of a diameter of the billet.  
           [0015]    According to the above indirect extrusion method, a space between a peripheral corner of the front plate and the container is minimized. Additionally, since deformation of the coating material is prevented by the front plate, less air is caught at the time of extrusion and generation of a blister is thus avoided. In case that the front plate diameter is less than 90% of the billet diameter, the space between the peripheral corner of the front plate and the container is enlarged and the air is easily caught. This thus causes a blister. In case that the front plate diameter is more than 100% of the billet diameter, the front plate diameter is then larger than a diameter of the container, and there would be a trouble in fitting the front plate into the container.  
           [0016]    The third aspect of the present invention is, in the indirect extrusion method for a clad material, that a thickness of the front plate is 5-20% of the billet diameter.  
           [0017]    According to the above indirect extrusion method, generation of a defective clad (which is necessary to be removed from the product) at an early stage of extrusion is further avoided. If the front plate thickness is less than 5% of the billet diameter, dead metal composed of the core material is not effectively reduced, and it is likely that a four-layer clad (defective clad portion) composed of the front plate, core material, coating material and core material in layers is formed at an early stage of extrusion. If the front plate thickness is more than 20% of the billet diameter, a cladding ratio (i.e. a coating material thickness of the extruded material divided by a radius of the extruded material) at an early stage of extrusion becomes too high. As a result, an elongate portion with heavy coating is generated and the portion to be cut off is increased.  
           [0018]    A mechanism in which a four-layer clad is generated at an early stage of extrusion is described hereafter. It should be noted that, in FIGS.  9 A- 9 C used in the following description, the angle of a billet thrusting face  3   a  of the die  3  is not 55-85 degrees but 90 degrees with regard to an axis P of the die  3  (refer to FIG. 9A). Therefore, the description using these figures is not within a scope of the present invention. However, the similar mechanism applies to a case in which a four-layer clad is generated at an early stage of extrusion when the front plate thickness is set to less than 5% of the billet diameter as above.  
           [0019]    As shown in FIG. 9A, a clad billet  70  comprises a core material  72 , a coating material  74  coating the outer surface of the core material  72 , and a front plate  76  provided at the head of the core material  72  and made of the same material as the coating material  74 . In the clad billet  70 , a thickness of the front plate  76  is small with regard to a diameter of the billet  70  (less than 5% of the diameter, for example).  
           [0020]    As the die  3  is forced to move relatively toward the clad billet  70  constituted as above, the clad billet  70  is extruded through the die opening  11  into an extruded material  80  comprising a product surface  82  composed of the front plate  76  and a layer  84  composed of the core material  72  arranged inside the product surface  82 , as shown in FIG. 9B. In this case, a dead metal  78  area including not only the front plate  76  but also the core material  72  is formed in the vicinity of the die  3  due to the thin front plate  76 . As the die  3  is further forced to move, the core material  72  and the coating material  74  in the vicinity of the dead metal  78  area are respectively extruded in layers into the product surface  82  composed of the front plate  82 , as shown in FIG. 9C. In other words, the product makes a defective clad (four-layer clad) comprising a first layer composed of the front plate  76  (product surface  82 ), a second layer  86  composed of the core material  72 , a third layer  88  composed of the coating material  74  and a fourth layer  90  composed of the core material  72  (also refer to FIG. 9D which is a cross sectional view taken along with a line  9 D- 9 D of FIG. 9C).  
           [0021]    A mechanism in which the aforementioned elongate portion with heavy coating is generated is explained by way of FIGS. 10A and 10B. It should be noted that in FIGS. 10A and 10B, the angle of the billet thrusting face  3   a  of the die  3  is not 55-85 degrees but 90 degrees with regard to the axis P of the die  3  (refer to FIG. 10A). Therefore, the description using these figures is not within a scope of the present invention. However, the similar mechanism applies to a case in which the elongate portion with heavy coating is generated at an early stage of extrusion when the front plate thickness is set to more than 20% of the billet diameter as above.  
           [0022]    [0022]FIG. 10A shows a clad billet having the same constitution with the clad billet of FIG. 9A (the same components as those in FIG. 9A are shown with the same reference numbers, and the descriptions are omitted). In the billet  70 , the thickness of the front plate  76  is large with regard to the diameter of the billet  70  (more than 20% of the diameter, for example).  
           [0023]    As the die  3  is forced to move toward the clad billet  70  constituted as above, a dead metal  79  area comprising essentially the front plate  76  is formed in the vicinity of the die  3 , as shown in FIG. 10B, due to the considerably thick front plate  76 . An extruded material  81  comprises a product surface layer  83  composed of the front plate  76  or the coating material  74 , and a layer  85  composed of the core material  72  arranged inside of the product surface player  83 . When the thickness of the front plate  76  is large with regard to the diameter of the billet  70 , however, a cladding ratio of the extruded material  81  becomes higher than the desired value at an early stage of extrusion. Therefore, an elongate portion  81   a  with heavy coating, which is to be cut off, is generated.  
           [0024]    In the fourth aspect of the indirect extrusion method for a clad material according to the present invention, the outer surface of the front plate is in the form of a cone which is fitted along the taper of the die.  
           [0025]    According to the indirect extrusion method as above, the space between the front plate and the die is reduced. Therefore, it is effective to prevent generation of a blister. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The invention will now be described, by way of an example, with reference to the accompanying drawings, in which:  
         [0027]    [0027]FIG. 1 is an explanatory view illustrating a schematic constitution of an indirect extrusion mechanism used in an indirect extrusion method according to the present invention;  
         [0028]    [0028]FIGS. 2A and 2B are explanatory views of a clad billet used in the indirect extrusion method according to the present invention;  
         [0029]    FIGS.  3 A- 3 C are explanatory views of a die used for evaluating the indirect extrusion method according to the present invention;  
         [0030]    [0030]FIG. 4 is an explanatory view illustrating an example in which a diameter of a front plate is less than a diameter of the clad billet;  
         [0031]    [0031]FIG. 5 is an explanatory view illustrating an example in which an outer surface of the front plate is in the form of a cone;  
         [0032]    [0032]FIG. 6 is an explanatory view illustrating a schematic constitution of the indirect extrusion mechanism in which a mandrel is arranged inside a container;  
         [0033]    [0033]FIG. 7 is an explanatory view illustrating a schematic constitution of a conventional indirect extrusion mechanism;  
         [0034]    FIGS.  8 A- 8 D are explanatory views illustrating a mechanism on how a defective clad called three-layered clad is formed;  
         [0035]    FIGS.  9 A- 9 D are explanatory views illustrating a mechanism on how a defective clad called four-layered clad is formed; and  
         [0036]    [0036]FIGS. 10A and 10B are explanatory views illustrating a mechanism on how a defective clad called a portion with heavy coating is formed. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]    [0037]FIG. 1 is an explanatory view showing a schematic constitution of an indirect extrusion mechanism  100  used in an indirect extrusion method according to an embodiment of the present invention.  
         [0038]    In this mechanism  100 , a die  102  which defines an outer shape of an extruded material is fitted inside a container  108 , being thrust by a platen  106  via a die-stem  104 . A closing plate  110  facing the die  102  is attached to an end of the container  108 . A clad billet  120  is arranged inside the container  108  between the die  102  and the closing plate  110 , to be served in indirect extrusion processing.  
         [0039]    In this case, the die  104  is forced to move relatively toward the clad billet  120  set inside the container  108  so that the billet  120  inside the container  108  is pressurized by a billet thrusting face  102   a  to be extruded through a die opening  102   b  as an extruded material.  
         [0040]    In the present embodiment, the indirect extrusion processing is performed by moving a set of container  108 , closing plate  110  and clad billet  120  together toward a direction of the die  104  (direction of an arrow α in FIG. 1).  
         [0041]    The clad billet  120  is now described by way of FIGS. 2A and 2B. FIGS. 2A and 2B are explanatory views of the clad billet  120 . FIG. 2A is the cross sectional view and FIG. 2B is the side view. As can be seen from the figures, an outer surface of a cylindrical core material  122  of the clad billet  120  is coated with a tubular coating material  124 . A circular front plate  126  is provided at the head of the clad billet  120  (with which the billet thrusting face  102   a  is in first contact upon extrusion). The front plate  126  is made of the same material as the coating material  124 .  
         [0042]    [0042]FIG. 3A shows an example of the die  102  used in the present indirect extrusion method. As shown in the figure, the billet thrusting face  102   a  of the die  102  is tapered at an angle of 70 degrees with regard to an axis A of the die  102 .  
         [0043]    Extrusion processing using the above described clad billet  120  and die  102  was performed and the results of the processing are described below. For comparison, a number of extrusion processing were also performed, varying a taper angle of the thrusting face  102   a  of the die  102 , and a thickness as well as a diameter of the front plate  126 . FIGS. 3B and 3C show examples of dies used in the comparative examples, and the taper angles of the dies are 90 degrees and 110 degrees, respectively. The core material  122  used is made of JISA3003 and is cylindrical, having a diameter of 92 mm and a length of 300 mm. The coating material  124  used is made of JISA1070, and an outer diameter, an inner diameter and a length of the coating material  124  are 100 mm, 92 mm and 295 mm, respectively. FIG. 2A shows the front plate  126  of which diameter is equal to a diameter of the billet  120 . FIG. 4 shows the front plate  126  of which diameter is less than the diameter of the billet  120 . In both cases shown in FIGS. 2A and 4, a peripheral corner  126   a  of the front plate  126  and a front end  124   a  of the coating material  124  are firmly stuck, ex. welded, to each other for convenience of positioning, before the billet  120  is set inside the container.  
         [0044]    Visual observation on a blister was conducted throughout the length of the product after extrusion, and a length of a defective clad portion (portion made of the front plate only, portion having a too much cladding ratio due to the front plate, and a portion comprising four layers composed of the front plate, core material, coating material and core material) was measured. Table 1 shows results of evaluation along with parameters of the prepared examples and comparative examples.  
                                                                                                   TABLE 1                                           front plate   front       bad           taper   diameter/   plate       clad           angle   billet diameter   thickness   blister   lengh                (°)   (%)   (mm)   Y/N   level   (m)                        Comp. Ex. 1   50   100   10   Y   L   0.8       Example 1   55   100   10   N   —   0.8       Example 2   60   100   10   N   —   0.7       Example 3   60   80   10   Y   L   1.0       Example 4   70   90   4   N   —   3.0       Example 5   70   90   5   N   —   1.0       Example 6   70   95   10   N   —   0.5       Example 7   70   100   15   N   —   0.9       Example 8   80   100   18   N   —   1.0       Example 9   80   100   20   N   —   1.0       Example 10   80   100   23   Y   S   4.2       Example 11   85   100   10   N   —   1.0       Comp. Ex. 2   90   100   4   Y   S   5.5       Comp. Ex. 3   90   100   10   Y   S   5.1       Comp. Ex. 4   100   100   10   Y   S   7.7                  
 
         [0045]    As can be seen from the table, the length of the defective clad portion was equal to or less than 5 m in the first example in which the taper angle is 55 degrees, the second and third examples in which the taper angle is 60 degrees, the fourth to seventh examples in which the taper angle is 70 degrees, the eighth to tenth examples in which the taper angle is 80 degrees, and the eleventh example in which the taper angle is 85 degrees. The length of the defective clad portion is favorably shortened.  
         [0046]    On the other hand, although the length of the defective clad portion was 0.8 m in the first comparative example in which the taper angle is 50 degrees, there was a conspicuous blister (level: large) which cannot be corrected. Furthermore, part of the billet  120  was adhered to the billet thrusting face  102   a  of the die  102  when the extruded remainder of the billet  120  was cut off. It took more time than expected to remove the adhesion part after extrusion, and therefore, the workability was determined poor.  
         [0047]    The length of the defective clad portion was more than 5 m in the second and third comparative examples in which the taper angle is 90 degrees and in the fourth comparative example in which the taper angle is 110 degrees. These examples failed to shorten the length of the defective clad portion.  
         [0048]    Among the first to eleventh examples, no blister is generated in the first, second, seventh to ninth and eleventh examples in which a diameter of the front plate  126  is equal to a diameter of the clad billet  120 , in the fourth and fifth examples in which the diameter of the front plate  126  is 90% of the diameter of the clad billet  120 , and in the sixth example in which the diameter of the front plate  126  is 95% of the diameter of the clad billet  120 . In the tenth example in which the diameter of the front plate  126  is equal to the diameter of the clad billet  120 , a blister was generated but small enough to be corrected.  
         [0049]    In the third example in which the diameter of the front plate  126  is 80% of the diameter of the clad billet  120 , however, a blister so large as cannot be corrected was generated. This is because there was a large space left between the container  108  and the front plate  126  in the third example. In other words, the air was caught between the coating material  124  and the core material  122  during extrusion due to the large space, and it resulted in generation of a large blister.  
         [0050]    Among the first to eleventh examples, the length of the defective clad portion was equal to or less than 1 m in the first to third, sixth and eleventh examples in which the thickness of the front plate  126  is 10 mm (10% of the 100 mm diameter of the clad billet  120 ), in the fifth example in which the thickness is 5 mm, in the seventh example in which the thickness is 15 mm, in the eighth example in which the thickness is 18 mm, and in the ninth example in which the thickness is 20 mm. The length of the defective clad portion was further shortened.  
         [0051]    In the fourth example in which the thickness of the front plate  126  is 4 mm, however, the length of the defective clad portion was more than 1 m, that is, 3.0 m. This is because the front plate  126  was too thin. The thin front plate  126  failed to reduce the volume of dead metal composed of the core material  122 , and thus a four-layer clad was formed at an early stage of extrusion.  
         [0052]    In the tenth example in which the thickness of the front plate  126  is increased to 23 mm, a cladding ratio at an early stage of extrusion was too high, and a portion with heavy coating (defective clad portion) was generated.  
         [0053]    From the above, it was found that, in order to favorably shorten the length of the defective clad portion upon manufacturing a clad material, the taper angle is preferably set to 55-85 degrees. In addition to setting the taper angle to 55-85 degrees, in order to further prevent generation of a blister, it was found that the diameter of the front plate  126  is preferably set to 90-100% of the diameter of the billet  120 . In addition to setting the taper angle to 55-85 degrees, in order to further shorten the length of the defective clad portion, it was found that the thickness of the front plate  126  is preferably set to 5-20% of the diameter of the billet  120 .  
         [0054]    Although a preferred embodiment of the present invention has been described, it is to be clearly understood that the invention may be embodied in a variety of ways.  
         [0055]    For instance, the outer surface of the front plate  126  may be in the form of a cone which is fitted along the taper of the thrusting face  102   a  of the die  102 . Since the space between the front plate  126  and the die  102  is reduced, it is effective in avoiding generation of a blister.  
         [0056]    [0056]FIG. 5 shows an example in which the outer surface  126   b  of the front plate  126  is in the form of a cone. In FIG. 5, the angle of the outer surface  126   b  with regard to the axis A is set to 70 degrees so that it fits to a case that the thrusting face  102   a  of the die  102  is tapered at an angle of about 70 degrees with regard to the axis A of the die  102 .  
         [0057]    As shown in FIG. 6, the clad billet  120  may be hollowed. By arranging the mandrel  112  piercing the billet  120  inside the container  108 , and moving the die  102  and the mandrel  112  relatively toward the billet  120  upon indirect extrusion, a clad material (extruded material) in the form of a pipe may be obtained from the die opening  102   b.    
         [0058]    In this case, the core material  122  is in the form of a tubular member having a piercing hole  122   a  extending toward the axial direction. In addition, the front plate  126  is in the form of a ring having a piercing hole  126   c  extending toward the axial direction. As shown in FIG. 6, a diameter of the piercing holes  122   a  and  126   c  corresponds to a diameter of the mandrel  112 .