Abstract:
A clamp apparatus clamps a circular cover in joining together a base having a circular recess and the cover fitted into the recess of the base, at and around an entire annular joint interface between a circumferential wall of the cover and a circumferential wall of the recess of the base. The clamp apparatus has a clamp body and a presser member attached to the clamp body in a manner rotatable about the axis of the recess of the base and about the axis of the cover. The clamp apparatus enables joining of the base and the cover at and around the annular joint interface therebetween without need to provisionally join them at and around the interface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a clamp apparatus for use in joining two members together at and around an annular joint interface therebetween, to a joining apparatus, and to a joining method. More particularly, the invention relates to a clamp apparatus used when a first member having a circular hole and a second member having a circular portion are joined together with the circular portion being fitted into the circular hole, at and around an entire annular joint interface between a circumferential wall of the circular portion of the second member and a circumferential wall of the circular hole of the first member, the camp apparatus being adapted to clamp the second member so as to prevent movement of the second member in relation to the first member, as well as to a joining apparatus and a joining method.  
         [0002]     Manufacture of industrial metal products for use in, for example, the automobile industry and the shipbuilding industry, may involve a step of joining together a first member having a circular hole and a second member having a circular portion fitted into the circular hole, at and around an entire annular joint interface between a circumferential wall of the circular portion of the second member and a circumferential wall of the circular hole of the first member.  
         [0003]     Specifically, a known method of manufacturing a metal hollow member consisting of a base having a circular recess and a circular cover fitted into the recess and closing the opening of the recess is as follows. There are prepared the base having a circular recess and the cover to be fitted into the recess and to close the opening of the recess. A stepped portion is formed on the circumferential wall of the recess at an intermediate depth for allowing a circumferential portion of the cover to rest thereon. After the cover is fitted into the recess such that the circumferential portion thereof rests on the stepped portion of the recess, the base and the cover are joined together at and around an annular joint interface between the circumferential wall of the recess of the base and the circumferential wall of the cover by a fusion welding process, such as MIG (metal inert gas) welding, TIG (tungsten inert gas) welding, laser beam welding, or electron beam welding, or by a friction stir welding process.  
         [0004]     In the fusion welding process, workpieces must be fixed against movement thereof in the course of joining, since the workpieces are subjected to pressure which is exerted on a molten pool at the time of melting or shield gas pressure, or to shrinkage force induced by welding heat. In the friction stir welding process, large force is exerted on workpieces when a probe is plunged into the interface between the workpieces and when the probe and the workpieces undergo relative movement; thus, the workpieces must be fixed more firmly than in the fusion welding process against movement thereof in the course of joining. In either case of the fusion welding process and the friction stir welding process, movement of workpieces in the course of joining causes impairment in dimensional accuracy of a product obtained by joining, or defective join.  
         [0005]     Conventionally, in order to cope with the above problems, when a cover is to be friction-stir-welded to a base for forming a hollow member, the cover is provisionally and partially joined to the base by a general fusion welding process or a friction stir welding process (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-248584).  
         [0006]     However, since the method disclosed in the above publication includes an additional step for provisional joining, joining the cover to the base is rather troublesome. In the case where the cover is provisionally joined to the base at several points by a fusion welding process, a blowhole, cracks, lack of fusion, or a like defect is likely to arise. In order to avoid occurrence of such defect, preheating, treatment of craters, and the like must be sufficiently performed before main joining. This leads to an increase in the number of steps, thus impairing mass productivity.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to solve the above problems and to provide a clamp apparatus which enables joining of two members without need to provisionally join the members at and around an annular joint interface therebetween, as well as a joining apparatus and a joining method.  
         [0008]     To achieve the above object, the present invention comprises the following modes.  
         [0009]     1) A clamp apparatus used when a first member having a circular hole and a second member having a circular portion are joined together with the circular portion being fitted into the circular hole, at and around an entire annular joint interface between a circumferential wall of the circular portion of the second member and a circumferential wall of the circular hole of the first member, the camp apparatus being adapted to clamp the second member so as to prevent movement of the second member in relation to the first member and comprising:  
         [0010]     a clamp body; and  
         [0011]     a presser member attached to the clamp body in a manner rotatable about an axis of the circular hole of the first member.  
         [0012]     2) A clamp apparatus according to par. 1), wherein a medium supply channel for supplying a cooling medium or a heating medium to the second member is formed in the presser member.  
         [0013]     3) A clamp apparatus according to par. 2), wherein the medium supply channel assumes the form of a through-hole formed on an axis of rotation of the presser member.  
         [0014]     4) A clamp apparatus according to par. 1), wherein the circular portion of the second member has a coaxially formed projection or recess, and a pressing face of the presser member has a recess or a projection which is to mate with the projection or the recess of the second member and is formed such that an axis thereof coincides with an axis of rotation of the presser member.  
         [0015]     5) A joining apparatus comprising:  
         [0016]     a rotary table;  
         [0017]     a bed fixedly provided on the rotary table;  
         [0018]     a first-member clamp device for fixing the first member on the bed such that the axis of the circular hole of the first member coincides with an axis of rotation of the rotary table; and  
         [0019]     a clamp apparatus according to any one of pars. 1) to 4).  
         [0020]     6) A joining method for joining together a first member having a circular hole and a second member having a circular portion with the circular portion being fitted into the circular hole, at and around an entire annular joint interface between a circumferential wall of the circular portion of the second member and a circumferential wall of the circular hole of the first member, comprising the steps of:  
         [0021]     preparing a clamp apparatus having a clamp body and a presser member attached to the clamp body in a manner rotatable about an axis of the circular hole of the first member; and  
         [0022]     joining the first member and the second member together at and around the annular joint interface therebetween while clamping the second member by the presser member of the clamp apparatus so as to prevent movement of the second member in relation to the first member and rotating the first and second members about the axis of the circular hole of the first member.  
         [0023]     7) A joining method according to par. 6), wherein a medium supply channel for supplying a cooling medium or a heating medium to the second member is formed in the presser member, and, while the cooling medium or the heating medium is supplied to the second member through the medium supply channel of the presser member, the first member and the second member are joined together at and around the annular joint interface therebetween.  
         [0024]     8) A joining method according to par. 6), wherein the circular portion of the second member has a coaxially formed projection or recess; the presser member of the clamp apparatus has a recess or a projection which is to mate with the projection or the recess of the second member and is formed such that an axis thereof coincides with an axis of rotation of the presser member; and while the projection or the recess of the second member and the recess or the projection of the presser member are mated with each other, the second member is clamped by the presser member of the clamp apparatus.  
         [0025]     9) A joining method according to par. 6), wherein the first member and the second member are joined together by a friction stir welding process.  
         [0026]     10) A joining method according to par. 6), wherein the first member and the second member are joined together by a fusion welding process.  
         [0027]     11) A method of manufacturing a hollow member in which a base having a circular recess and a cover to be fitted into the recess and to close an opening of the recess are prepared, a stepped portion is formed on a circumferential wall of the recess at an intermediate depth for allowing a circumferential portion of the cover to rest thereon, and, after the cover is fitted into the recess such that the circumferential portion thereof rests on the stepped portion of the recess, the base and the cover are joined together at and around an annular joint interface between the circumferential wall of the recess of the base and a circumferential wall of the cover, comprising the steps of:  
         [0028]     preparing a clamp apparatus having a clamp body and a presser member attached to the clamp body in a manner rotatable about an axis of the recess of the base; and  
         [0029]     joining the base and the cover together at and around the annular joint interface therebetween while clamping the cover by the presser member of the clamp apparatus so as to prevent movement of the cover in relation to the base and rotating the base and the cover about the axis of the recess of the base.  
         [0030]     12) A method of manufacturing a hollow member according to par. 11), wherein a medium supply channel for supplying a cooling medium or a heating medium to the cover is formed in the presser member, and, while the cooling medium or the heating medium is supplied to the cover through the medium supply channel of the presser member, the base and the cover are joined together at and around the annular joint interface therebetween.  
         [0031]     13) A method of manufacturing a hollow member according to par. 11), wherein the cover has a projection or recess coaxially formed on an outer surface thereof; the presser member of the clamp apparatus has a recess or a projection which is to mate with the projection or the recess of the cover and is formed such that an axis thereof coincides with an axis of rotation of the presser member; and while the projection or the recess of the cover and the recess or the projection of the presser member are mated with each other, the cover is clamped in relation to the base by the presser member of the clamp apparatus.  
         [0032]     14) A method of manufacturing a hollow member according to par. 11), wherein the base and the cover are joined together by a friction stir welding process.  
         [0033]     15) A method of manufacturing a hollow member according to par. 11), wherein the base and the cover are joined together by a fusion welding process.  
         [0034]     The clamp apparatus of par. 1) can clamp a second member having a circular portion by means of the presser member so as to prevent movement of the second member in relation to a first member having a circular hole at the time of joining the first member and the second member together with the circular portion being fitted into the circular hole, at and around an entire annular joint interface between a circumferential wall of the circular portion of the second member and a circumferential wall of the circular hole of the first member. In this clamped condition, the first and second members are joined together at and around the annular joint interface therebetween by fusion welding or friction stir welding while being rotated about the axis of the circular hole of the first member. Since the presser member is rotatable about the axis of the center hole of the first member, rotation of the first and second members is not hindered.  
         [0035]     Thus, there is no need to provisionally join the first and second members at and around the annular joint interface therebetween, whereby joining can be performed in fewer steps. In contrast to the case where two members are provisionally joined at several points by a fusion welding process, a blowhole, cracks, lack of fusion, or a like defect does not arise. Therefore, a step of eliminating such defect becomes unnecessary, whereby joining can be performed in fewer steps. As a result, this joining work is excellently suited for mass production.  
         [0036]     The clamp apparatus of par. 2) allows supply of a cooling medium to the second member through the medium supply channel in the course of friction-stir-welding the first member and the second member together at and around the annular joint interface therebetween. The cooling medium removes frictional heat induced by friction stir welding. Accordingly, even when joining consumes a long time, when members having high resistance to deformation are joined, or when members whose shapes are susceptible to accumulation of heat are joined, an increase in temperature difference between a joining start zone and a joining end zone is suppressed, and also occurrence of great variation in joining quality along the longitudinal direction of a joint zone is suppressed. Since an excessive increase in temperature at a joining end zone is suppressed, occurrence of defect at the joining end zone is prevented. Thus, an entire joint zone can exhibit a predetermined joint strength, and appearance of a weld bead is consistent.  
         [0037]     In the course of fusion welding the first member and the second member together at and around the annular joint interface therebetween, the first and second members can be preheated by means of supplying a heating medium to the second member through the medium supply channel. For example, in the case of fusion welding members made of material having higher cold cracking susceptibility, such as high-tensile-strength steel, preheating lowers cooling speed during welding, thereby preventing hardening of a heat affected zone. Thus, cold crack can be suppressed.  
         [0038]     The clamp apparatus of par. 3) allows simplification of the medium supply channel.  
         [0039]     The clamp apparatus of par. 4) can prevent deviation in center alignment between the second member and the presser member during joining, thereby providing a stable clamping condition at all times.  
         [0040]     According to the joining apparatus of par. 5), the first-member clamp device fixes the first member on the bed such that the axis of the circular hole of the first member coincides with an axis of rotation of the rotary table, and the presser member of the clamp apparatus of any one of pars. 1) to 4) clamps the second member on the bed with the circular portion of the second member fitted into the circular hole of the first member. In this condition, while being rotated through rotation of the rotary table, the first member and the second member are fusion-welded or friction-stir-welded at and around the annular joint interface therebetween. Since the presser member is rotatable about the axis of the circular hole of the first member, rotation of the first and second members is not hindered.  
         [0041]     Thus, the first member and the second member can be joined together without need to be provisionally joined; i.e., a step of provisional joining is unnecessary. Therefore, this joining work is excellently suited for mass production.  
         [0042]     According to the joining method of par. 6), a clamp apparatus having a clamp body and a presser member attached to the clamp body in a manner rotatable about an axis of a circular hole of a first member is prepared, and the first member and a second member are joined together, while being rotated about the axis of the circular hole of the first member, in a state in which the second member is clamped by the presser member of the clamp apparatus so as to prevent movement of the second member in relation to the first member. Thus, the first member and the second member can be joined together without need to be provisionally joined; i.e., a step of provisional joining is unnecessary. Therefore, this joining work is excellently suited for mass production. Furthermore, since the presser member is rotatable about the axis of the circular hole of the first member, rotation of the first and second members is not hindered.  
         [0043]     The joining method of par. 7) allows supply of a cooling medium to the second member through the medium supply channel in the course of friction-stir-welding the first member and the second member together at and around the annular joint interface therebetween. The cooling medium removes frictional heat induced by friction stir welding. Accordingly, even when joining consumes a long time, when members having high resistance to deformation are joined, or when members whose shapes are susceptible to accumulation of heat are joined, an increase in temperature difference between a joining start zone and a joining end zone is suppressed, and also occurrence of great variation in joining quality along the longitudinal direction of a joint zone is suppressed. Since an excessive increase in temperature at a joining end zone is suppressed, occurrence of defect at the joining end zone is prevented. Thus, an entire joint zone can exhibit a predetermined joint strength, and appearance of a weld bead is consistent.  
         [0044]     In the course of fusion welding the first member and the second member together at and around the annular joint interface therebetween, the first and second members can be preheated by means of supplying a heating medium to the second member through the medium supply channel. For example, in the case of fusion welding members made of material having higher cold cracking susceptibility, such as high-tensile-strength steel, preheating lowers cooling speed during welding, thereby preventing hardening of a heat affected zone. Thus, cold crack can be suppressed.  
         [0045]     The joining method of par. 8) can prevent deviation in center alignment between the second member and the presser member during joining, thereby providing a stable clamping condition at all times.  
         [0046]     According to the method of manufacturing a hollow member of par. 11), a clamp apparatus having a clamp body and a presser member attached to the clamp body in a manner rotatable about the axis of the recess of the base is prepared, and the base and the cover are joined together at and around the annular joint interface therebetween, while being rotated about the axis of the recess of the base, in a state in which the cover is clamped by the presser member of the clamp apparatus so as to prevent movement of the cover in relation to the base. Thus, the cover and the base can be joined together without need to provisionally join the cover to the base; i.e., a step of provisional joining is unnecessary. Therefore, mass productivity of hollow members becomes excellent. Furthermore, since the presser member is rotatable about the axis of the recess of the base, rotation of the cover and the base is not hindered.  
         [0047]     The method of manufacturing a hollow member of par. 12) allows supply of a cooling medium to the cover through the medium supply channel in the course of friction-stir-welding the base and the cover together at and around the annular joint interface therebetween. The cooling medium removes frictional heat induced by friction stir welding. Accordingly, even when joining consumes a long time, when members having high resistance to deformation are joined, or when members whose shapes are susceptible to accumulation of heat are joined, an increase in temperature difference between a joining start zone and a joining end zone is suppressed, and also occurrence of great variation in joining quality along the longitudinal direction of a joint zone is suppressed. Since an excessive increase in temperature at a joining end zone is suppressed, occurrence of defect at the joining end zone is prevented. Thus, an entire joint zone can exhibit a predetermined joint strength, and appearance of a weld bead is consistent.  
         [0048]     In the course of fusion welding the base and the cover together at and around the annular joint interface therebetween, the base and the cover can be preheated by means of supplying a heating medium to the cover through the medium supply channel. For example, in the case of fusion welding the base and the cover which are made of material having higher cold cracking susceptibility, such as high-tensile-strength steel, preheating lowers cooling speed during welding, thereby preventing hardening of a heat affected zone. Thus, cold crack can be suppressed.  
         [0049]     The method of manufacturing a hollow member of par. 13) can prevent deviation in center alignment between the cover and the presser member during joining, thereby providing a stable clamping condition at all times. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]      FIG. 1  is a vertical, sectional view showing a method of Embodiment 1 using a clamp apparatus according to the present invention as viewed before starting joining;  
         [0051]      FIG. 2  is a perspective view showing the state before starting joining of  FIG. 1 ;  
         [0052]      FIG. 3  is a vertical, sectional view showing the method of Embodiment 1 using the clamp apparatus according to the present invention as viewed after starting joining;  
         [0053]      FIG. 4  is a perspective view showing the method of Embodiment 1 using the clamp apparatus according to the present invention as viewed after starting joining;  
         [0054]      FIG. 5  is a vertical, sectional view showing a method of Embodiment 2 using the clamp apparatus according to the present invention as viewed before starting joining;  
         [0055]      FIG. 6  is a vertical, sectional view showing a method of Embodiment 3 using the clamp apparatus according to the present invention as viewed before starting joining; and  
         [0056]      FIG. 7  is a vertical, sectional view showing a method of Embodiment 4 using the clamp apparatus according to the present invention as viewed before starting joining. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]     Embodiments of the present invention will next be described in detail with reference to the drawings. In the drawings, like sections or components throughout the several views are denoted by like reference numerals, and repeated description thereof is omitted.  
         [0058]     In the following description, the upper and lower sides of the drawings will be referred to as “upper” and “lower,” respectively.  
       Embodiment 1  
       [0059]     The present embodiment is shown in FIGS.  1  to  4  and is implemented by applying a clamp apparatus according to the present invention to manufacture of a hollow member which consists of a base having a recess, and a cover fitted into the recess and closing the opening of the recess.  
         [0060]     A clamp apparatus  1  has an arm-like clamp body  2  and a presser member  3 , which is attached to a distal end portion of the clamp body  2  in a rotatable manner. The presser member  3  includes a columnar basal portion  3   a  and a disklike presser portion  3   b  formed concentrically and integrally with the basal portion  3   a.  The basal portion  3   a  of the presser member  3  is inserted into an attachment hole  2   a  formed in and extending through the distal end portion of the clamp body  2  and is rotatably supported by the clamp body  2  via a roller bearing  4  which intervenes between the clamp body  2  and the basal portion  3   a.  Preferably, the roller bearing  4  bears a radial load and a thrust load. Components of the roller bearing  4  are formed from, usually, high-carbon chromium bearing steel, preferably, heat-resisting steel. A recess  5  which is circular as viewed from underneath is formed on the lower surface of the presser member  3  such that the axis thereof coincides with the axis of rotation of the presser member  3 .  
         [0061]     The presser member  3  of the clamp apparatus  1  has a medium supply channel  6  formed therein for supplying a cooling medium into the interior of the recess  5 . The medium supply channel  6  is a through-hole which extends vertically through the presser member  3  on the axis of rotation of the presser member  3 . A cooling-medium supply pipe  7  for feeding a cooling medium into the medium supply channel  6  is connected to the presser member  3  in such a manner as to communicate with an upper end portion of the medium supply channel  6 . Although unillustrated, preferably, the cooling-medium supply pipe  7  is connected to the presser member  3  in such a manner as to be rotatable in relation to the presser member  3  by use of, for example, a rotary joint.  
         [0062]     The clamp apparatus  1 , a rotary table  10 , a bed  11  fixedly provided on the rotary table  10 , and first-member clamp devices  12  provided on the bed  11  constitute a joining apparatus. In this joining apparatus, the clamp body  2  of the clamp apparatus  1  can move vertically toward and away from the bed  11  and can move horizontally in a horizontal plane parallel with the upper surface of the bed  11 .  
         [0063]     A base  15  used to manufacture a hollow member has a recess  16  which has a circular shape as viewed in plane. The base  15  has a stepped portion  17  which projects radially inward from the circumferential wall of the recess  16  at an intermediate depth.  
         [0064]     A cover  18  used to manufacture the hollow member has a thickness smaller than the depth of the recess  16  and equal to a depth to the stepped portion  17  of the recess  16 . The cover  18  has a diameter equal to or slightly smaller than a diameter of the recess  16  as measured above the stepped portion  17 . A projection  14  which has a circular shape as viewed in plane is formed coaxially on the upper surface of the cover  18 . The projection  14  has such a height and diameter as to be substantially fitted into the recess  5  of the presser member  3  of the clamp apparatus  1 .  
         [0065]     The base  15  and the cover  18  are formed from, for example, any one of JIS 2000 aluminum alloys, JIS 5000 aluminum alloys, JIS 6000 aluminum alloys, and JIS 7000 aluminum alloys. The base  15  and the cover  18  may be formed from the same material or from different materials.  
         [0066]     By use of the base  15  and the cover  18 , a hollow member is manufactured as follows. First, the base  15  is placed on the bed  11  of the rotary table  10  and is then clamped on the bed  11  by means of the first-member clamp devices  12 .  
         [0067]     Next, the cover  18  is fitted into the recess  16  of the base  15  such that a circumferential portion thereof rests on the stepped portion  17 , thereby making the upper surface of a portion of the base  15  around the recess  16  and an upper surface of the cover  18  flush with each other.  
         [0068]     Next, the cover  18  is clamped on the base  15  by means of the clamp apparatus  1 . Specifically, the clamp body  2  is moved horizontally in a horizontal plane parallel with the surface of the bed  11  and is moved toward the bed  11  so as to fit the projection  14  of the cover  18  into the recess  5  of the presser member  3 . Additionally, force directed toward the bed  11  is applied to the clamp body  2 , thereby clamping the cover  18  on the base  15  by means of the presser member  3 . The circumferential wall of the recess  16  of the base  15  and the circumferential wall of the cover  18  form an annular joint interface  19  therebetween.  
         [0069]     Next, by use of a friction-stir-welding tool  20 , the base  15  and the cover  18  are friction-stir-welded together at and around the annular joint interface  19 .  
         [0070]     The friction-stir-welding tool  20  includes a columnar rotor  21  and a pin-like probe  22 . The columnar rotor  21  has a small-diameter portion  21   a  which is coaxially and integrally formed at a distal end portion thereof via a taper portion. The probe  22  is coaxially and integrally formed on the end surface of the small-diameter portion  21   a  of the columnar rotor  21  and has a diameter smaller than that of the small-diameter portion  21   a.  The rotor  21  and the probe  22  are formed from a material which is harder than the base  15  and the cover  18  and is resistant to frictional heat generated in the course of joining.  
         [0071]     While the rotor  21  and the probe  22  of the friction-stir-welding tool  20  are rotated, the probe  22  is plunged from the outside into the annular joint interface  19  at a certain circumferential position so as to be plunged into portions of the base  15  and the cover  18  which are located on opposite sides of the annular joint interface  19 . Also, a shoulder portion  21   b  of the tool  20  located between the small-diameter portion  21   a  and the probe  22  is pressed against the upper surfaces of the base  15  and the cover  18  (see  FIG. 3 ).  
         [0072]     Next, while the rotor  21  and the probe  22  are rotated, the base  15  and the cover  18 , and the friction-stir-welding tool  20  are caused to undergo relative movement so as to move the probe  22  along the annular joint interface  19 .  
         [0073]     By this procedure, frictional heat generated by rotation of the probe  22  and frictional heat generated by rubbing of the shoulder portion  21   b  against the base  15  and the cover  18  soften the base metal(s) of the base  15  and the cover  18  in the vicinity of the annular joint interface  19 . The thus-plasticized metal is stirred and mixed through subjection to the rotative force of the probe  22  and is transferred in such a manner as to fill a groove that is formed through passage of the probe  22 . Subsequently, the plasticized metal quickly loses frictional heat to thereby be cooled and solidified. This phenomenon repeatedly occurs as the probe  22  moves, whereby joining between the base  15  and the cover  18  progresses (see  FIG. 4 ).  
         [0074]     In the course of friction-stir-welding the base  15  and the cover  18  together, a cooling medium is continuously fed into the medium supply channel  6  through the cooling-medium supply pipe  7 . Preferably, the cooling medium is of gas phase. The cooling medium removes the above-mentioned frictional heat, thereby suppressing a rise in temperature of the base  15  and the cover  18 , particularly an excessive rise in temperature of the cover  18  whose thermal capacity is smaller than that of the base  15 .  
         [0075]     When the probe  22  moves along the entire annular joint interface  19 , the base  15  and the cover  18  are joined together at and around the entire annular joint interface  19 . Preferably, after the probe  22  returns to the position where the probe  22  was plunged in, the probe  22  is caused to pass the position and to move straight in a direction tangent to the annular joint interface up to a stopper member (not shown) disposed at the periphery of the base  15 . Then, the probe  22  is drawn out. The base  15  and the cover  18  are thus friction-stir-welded together, thereby yielding a hollow member.  
       Embodiment 2  
       [0076]     The present embodiment is shown in  FIG. 5  and is implemented by applying the clamp apparatus  1  according to the present invention to joining of a first member having a circular hole and a second member having a circular portion to be fitted into the circular hole.  
         [0077]     A first member  25  has a plate-like shape and has a circular hole  26  extending therethrough. A second member  27  consists of a cylindrical portion  27   a  and an outward flange  27   b  (circular portion), which is formed integrally with the lower end of the cylindrical portion  27   a  and projects radially outward and whose periphery has a circular shape. The cylindrical portion  27   a  and the outward flange  27   b  are coaxial with each other. The outward flange  27   b  has such an outside diameter as to be exactly fitted into the circular hole  26  of the first member  25 . The thickness of the outward flange  27   b  is equal to that of the first member  25 .  
         [0078]     The first and second members  25  and  27  are formed from, for example, any one of JIS2000 aluminum alloys, JIS 5000 aluminum alloys, JIS6000 aluminum alloys, and JIS 7000 aluminum alloys. The first and second members  25  and  27  may be formed from the same material or from different materials.  
         [0079]     Although unillustrated fully, the joining apparatus used in Embodiment 1 is used to join the first member  25  and the second member  27  together.  
         [0080]     First, the first member  25  is placed on the bed  11  of the rotary table  10  and is then clamped on the bed  11  by means of the first-member clamp devices  12 .  
         [0081]     Next, the outward flange  27   b  of the second member  27  is fitted into the circular hole  26  of the first member  25 , thereby making the upper surface of a portion of the first member  25  around the circular hole  26  and the upper surface of the outward flange  27   b  of the second member  27  flush with each other.  
         [0082]     Then, the second member  27  is clamped on the bed  11  by means of the clamp apparatus  1  such that the axis of rotation of the presser member  3  coincides with the axis of the cylindrical portion  27   a  and the outward flange  27   b  of the second member  27 , thereby making the second member  27  immobile in relation to the first member  25 . Specifically, the clamp body  2  is moved horizontally in a horizontal plane parallel with the surface of the bed  11  and is moved toward the bed  11  so as to insert an upper end portion of the cylindrical portion  27   a  of the second member  27  into the recess  5  of the presser member  3  in such a manner that the axis of rotation of the presser member  3  coincides with the axis of the cylindrical portion  27   a  of the second member  27 . Additionally, force directed toward the bed  11  is applied to the clamp body  2 , thereby clamping the second member  27  on the bed  11  by means of the presser member  3  and thus making the second member  27  immobile in relation to the first member  25 .  
         [0083]     The circumferential wall of the circular hole  26  of the first member  25  and the circumferential wall of the outward flange  27   b  of the second member  27  form an annular joint interface  28  therebetween.  
         [0084]     As in the case of Embodiment 1, by use of the friction-stir-welding tool  20 , the first and second members  26  and  27  are friction-stir-welded together at and around the annular joint interface  28 . In the course of friction stir welding, a cooling medium is continuously fed into the medium supply channel  6  through the cooling-medium supply pipe  7 . Preferably, the cooling medium is of gas phase. The cooling medium removes frictional heat, thereby suppressing a rise in temperature of the first and second members  25  and  27 .  
       Embodiment 3  
       [0085]     The present embodiment is shown in  FIG. 6  and is implemented by applying the clamp apparatus  1  according to the present invention to joining of a first member having a circular hole and a second member having a circular portion to be fitted into the circular hole.  
         [0086]     A first member  30  has a plate-like shape and has a circular hole  31  extending therethrough. A second member  32  has a circular plate-like shape having the same thickness as the first member  30  and has such an outside diameter as to be exactly fitted into the circular hole  31  of the first member  30 . The second member  32  is circular and is entirely fitted into the circular hole  31  of the first member  30 .  
         [0087]     The first and second members  30  and  32  are formed from, for example, any one of JIS2000 aluminum alloys, JIS 5000 aluminum alloys, JIS6000 aluminum alloys, and JIS 7000 aluminum alloys. The first and second members  30  and  32  may be formed from different materials.  
         [0088]     Although unillustrated fully, the joining apparatus used in Embodiment 1 is used to join the first member  30  and the second member  32  together.  
         [0089]     First, the first member  30  is placed on the bed  11  of the rotary table  10  and is then clamped on the bed  11  by means of the first-member clamp devices  12 .  
         [0090]     Next, the second member  32  is fitted into the circular hole  31  of the first member  30 , thereby making the upper surface of a portion of the first member  30  around the circular hole  31  and the upper surface of the second member  32  flush with each other.  
         [0091]     Then, the second member  32  is clamped on the bed  11  by means of the clamp apparatus  1  such that the axis of rotation of the presser member  3  coincides with the axis of the circular hole  31  of the first member  30  and the axis of the second member  32 , thereby making the second member  32  immobile in relation to the first member  30 . Specifically, the clamp body  2  is moved horizontally in a horizontal plane parallel with the surface of the bed  11  and is moved toward the bed  11  so as to place the presser member  3  on the second member  32  in such a manner that the axis of rotation of the presser member  3  coincides with the axis of the second member  32 . Additionally, force directed toward the bed  11  is applied to the clamp body  2 , thereby clamping the second member  32  on the bed  11  by means of the presser member  3  and thus making the second member  32  immobile in relation to the first member  30 .  
         [0092]     The circumferential wall of the circular hole  31  of the first member  30  and the circumferential wall of the second member  32  form an annular joint interface  33  therebetween.  
         [0093]     As in the case of Embodiment 1, by use of the friction-stir-welding tool  20 , the first and second members  30  and  31  are friction-stir-welded together at and around the annular joint interface  33 . In the course of friction stir welding, a cooling medium is continuously fed into the medium supply channel  6  through the cooling-medium supply pipe  7 . Preferably, the cooling medium is of gas phase. The cooling medium removes frictional heat, thereby suppressing a rise in temperature of the first and second members  30  and  31 .  
         [0094]     In the case where the entire upper surface of the second member  32  is flat as in the case of the present embodiment, the clamp apparatus  1  does not necessarily has the recess  5 .  
       Embodiment 4  
       [0095]     The present embodiment is shown in  FIG. 7  and is implemented by applying the clamp apparatus  1  according to the present invention to joining of a first member having a circular hole and a second member having a circular portion to be fitted into the circular hole.  
         [0096]     A first member  35  has a plate-like shape and has a circular hole  36  extending therethrough. A second member  37  has a cylindrical shape and has such an outside diameter as to be exactly fitted into the circular hole  36  of the first member  35 . A lower end portion of the second member  37  is fitted into the circular hole  36  of the first member  35 .  
         [0097]     The first and second members  35  and  37  are formed from, for example, any one of JIS2000 aluminum alloys, JIS 5000 aluminum alloys, JIS6000 aluminum alloys, and JIS 7000 aluminum alloys. The first and second members  35  and  37  may be formed from the same material or from different materials.  
         [0098]     Although unillustrated fully, the joining apparatus used in Embodiment 1 is used to join the first member  35  and the second member  37  together.  
         [0099]     In the present embodiment, a heating-medium supply pipe  38  for feeding a heating medium into the medium supply channel  6  is rotatably connected to the presser member  3  in such a manner as to communicate with an upper end portion of the medium supply channel  6 .  
         [0100]     First, the first member  35  is placed on the bed  11  of the rotary table  10  and is then clamped on the bed  11  by means of the first-member clamp devices  12 .  
         [0101]     Next, a circular portion  37   a  of the second member  37  is fitted into the circular hole  36  of the first member  35 . Then, the second member  37  is clamped on the bed  11  by means of the clamp apparatus  1  such that the axis of rotation of the presser member  3  coincides with the axis of the circular hole  36  of the first member  35  and the axis of the second member  37 . Specifically, the clamp body  2  is moved horizontally in a horizontal plane parallel with the surface of the bed  11  and is moved toward the bed  11  so as to place the presser member  3  on the second member  37  in such a manner that the axis of rotation of the presser member  3  coincides with the axis of the circular hole  36  of the first member  35  and the axis of the second member  37 . Additionally, force directed toward the bed  11  is applied to the clamp body  2 , thereby clamping the second member  37  on the bed  11  by means of the presser member  3  and thus making the second member  37  immobile in relation to the first member  35 .  
         [0102]     The circumferential wall of the circular hole  36  of the first member  35  and the circumferential wall of the circular portion  37   a  of the second member  37  form an annular joint interface  39  therebetween.  
         [0103]     Subsequently, by use of a fusion welding tool  40 , the first member  35  and the second member  37  are welded together at and around the annular joint interface  39  between the circumferential wall of the circular hole  36  of the first member  35  and the circumferential wall of the circular portion  37   a  of the second member  37  by, for example, an arc welding process, a laser beam welding process, or an electron beam welding process.  
         [0104]     Before this fusion welding is started, a heating medium is fed into the medium supply channel  6  through the heating-medium supply pipe  38 . Preferably, the heating medium is of gas phase. By this procedure, the first and second members  35  and  37  can be preheated.