Abstract:
In a device for butt joining workpieces by friction stir welding along a connecting area, including a shaft having a driven end and, at the opposite end, a pin-like projection to be disposed in the connecting area of the workpieces, whereby, upon rotation of the shaft, the workpiece material in contact with the pin-like projection is plasticized, first and second stops with first and second shoulders are provided on the device and disposed at opposite sides of the workpieces and at least one of the stops is movable relative to the other and biased toward the other with a controllable force for engaging the workpieces between the shoulders of the stops during the friction stir welding procedure.

Description:
This is a Continuation-In-Part application of international application PCT/DE00/04033 filed Nov. 16, 2000 and claiming the priority of German application 199 57 136.8 filed Nov. 18, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for joining workpieces by the method known as friction stir welding, wherein the workpieces are disposed adjacent one another in the areas in which they are to be joined. The device includes a rotatable shaft with a free end forming a first shoulder and being provided with a pin-like projection which, while being rotated at high speed is moved into contact with the workpieces and plasticizes the workpieces in the contact area. At its free end, the pin-like projection is provided with a second shoulder having a diameter larger than the pin-like projection so as to form between the first and second shoulders a space in which the workpieces are accommodated. 
     A friction stir welding apparatus is known for example from EP-B-0 615 480. The friction stir welding (FSW) has been known basically for several years and has been continually further developed. Originally, the friction stir welding was performed in such a way that two workpieces which were to be joined by friction stir welding were moved relative to each other in the area in which they were to be joined while they were pressed against each other with a predetermined adjustable force. As a result of the heat generated by the friction, the material of the workpieces was finally plasticized in the area of jointure. Upon sufficient plastification, the materials of the two workpieces intermix sufficiently so that, upon cooling, the desired weld connection between the two workpieces is formed. 
     In the apparatus referred to above, a pin-like projection which is rotated by a drive or, respectively, a motor at a suitably high speed, is placed between the adjoining edges of two workpieces disposed adjacent each other. The pin-like projection is suitably guided for example by a special guide structure or a robot and moved, for example, along a butt seam of the workpieces to be joined. When, after the beginning of the welding procedure, the material of the workpieces is sufficiently plasticized by the frictional heat generated by the rotation of the pin-like projection, the pin-like projection is moved along the seam between the adjoining workpieces while the pin-like projection is continued to be rotated whereby an elongated continuous butt weld is formed. 
     With the known apparatus, the workpieces are held in the area of the butt joint in spaced relationship by two stops with a diameter exceeding that of the pin-like projection which is disposed between the two stops. The opposite sides of the two stops form quasi-shoulders, which are disposed at opposite sides of the workpieces in the area of the welding seam to be formed and which rotate adjacent the surfaces of the workpieces to be joined. If, for example, a pressure is applied by the apparatus normal to the surfaces at one side of the two workpieces, the engagement pressure at the opposite side of the workpieces to be joined is correspondingly reduced because the shoulders are disposed at a certain distance from each other. For this reason, a suitable counter support must be provided by separate support means, which can be provided at reasonable expenses for plate-like workpieces such as metal sheets but which is generally impossible to provide for friction stir welding joints of workpieces having a complicated shape. 
     Furthermore, in many manufacturing operation, for example, in motor vehicle or airplane manufacturing, manufacturing robots are used which have multiple functions and relatively large outer supports for establishing the weld joint not only form obstacles but can simply not be provided. In addition, the robots can generally not provide the relatively large engagement forces or they can provide them only with high additional expenses. 
     It is desirable to eliminate the need for a robot to provide large forces and to provide a large operational area for the robot and furthermore to reduce the loads on the workpieces to be joined. 
     It is therefore the object of the present invention to provide a device for joining workpieces by friction stir welding, wherein, for forming a weld seam in accordance with the friction stir welding procedure, no welding seam support or rather no outer support structure is required and whereby also welding seams extending three-dimensionally can be produced in such a way that no plasticized material can escape. With the use of manufacturing robots, no large forces should be required from the robots for forming the weld seam and the workpieces to be joined should be held close together in the area in which they are to be welded together without the need for outer support means. Furthermore, the devices should be inexpensive and simple. 
     SUMMARY OF THE INVENTION 
     In a device for butt joining workpieces by friction stir welding along a connecting area, including a shaft having a driven end and, at the opposite end, a pin-like projection to be disposed in the connecting area of the workpieces, whereby, upon rotation of the shaft the workpiece material in contact with the pin-like projection is plasticized, first and second stops with first and second shoulders are provided on the device and disposed at opposite sides of the workpieces and at least one of the stops is movably relative to the other and biased toward the other with a controllable force for engaging the workpieces therebetween during the friction stir welding procedure. 
     The advantage of the solution according to the invention resides mainly in the fact that the forces required for the engagement of the workpieces to be joined are generated internally in the device so that no outer support structure is required. As a result, welding seams can be made also with complicated workpieces for example welding seams which extend three dimensionally in the space can be made without the need for a support which, so far, was needed to secure the base of the seam and to support the workpieces. Also, the workpieces are not warped during the establishment of the welding seam, or even destroyed. By the engagement of the stops or, respectively, the shoulders of the two stops with the respective sides of the workpieces to be joined, also material plasticized during the welding procedure cannot escape. In summary, it can be said the device according to the invention has the advantage that the engagement forces are provided by the stops disposed at opposite sides of the workpieces and these forces are oppositely directed so that they cancel each other and do not bias the workpieces toward one side or the other. In this way, also thin-walled workpieces can be welded together without being damaged. Also, workpieces in the form of hollow profile structures can be welded in this way and workpieces of complicated shapes which could not be achieved with the known apparatus. Finally, also the flexibility of the friction stir welding is increased with the device according to the invention beyond what has been achievable with prior art equipment. 
     In a preferred embodiment of the invention, a housing is provided for supporting and guiding a shaft, which includes a piston element provided with a pin-like projection. The piston is axially movable back and forth in a cylinder formed in the housing. By suitable actuation of the piston the thickness of the workpieces to be joined can be taken into account and the force desired for the engagement of the workpieces for forming the welding seam can be easily generated. 
     It is basically possible to provide for the back and forth movement of the piston element and consequently, the pressure applied to the workpieces engaged between the two stops in various ways, for example, electromotively or pneumatically. It has been found however, to be particularly advantageous to operate the piston hydraulically especially of the robot is operated by hydraulically since pressurized hydraulic fluid is then available at the location of the device. 
     The pressure applied to the opposite sides of the workpieces in the area in which the welding seam is to be formed may be adjusted as desired, as pointed out already earlier, depending on the type of workpieces or the shape of the workpieces and depending on the workpiece material. However, the device is preferably so designed that it is capable of providing between the shoulders of the two stops a force of up to 1.5 tons and possible even a greater force. 
     In the prior arrangements, both stops and therefore both shoulders at the opposite sides of the workpiece are rotating together with the welding pin. It is however expedient for certain applications if only one of the shoulders disposed adjacent the welding seam surface is rotatable while the other remains stationary. It is then advantageous if the housing of the device is stationary and the stop including the first shoulder is connected to the housing. The application of the suitable engagement force is independent therefrom. The housing is held stationary by suitable means and together therewith the stop connected to the housing in such an embodiment. 
     The shaft is preferably supported in the housing by a guide structure formed with housing so as to be axially movable so that the shaft can be moved axially by the piston for applying the engagement forces to the workpieces. 
     In another advantageous embodiment of the device, a housing is provided with a guide element, which is connected to the shaft and cylinder space, is disposed within the guide element. A piston, which is axially movable in the cylinder space, includes a pin-like projection forming the welding tool. If the stop with the first shoulder is connected to the guide element also the first stop and the shoulder thereof rotate with the shaft in unison with the second shoulder and the second stop. In this embodiment, the stops and the respective shoulders at both sides of the workpieces to be joined are rotating in unison. 
     In an embodiment with an outer stationary housing, the pin-like projections and the second stop connected thereto is to rotate with the shaft of the device. In this connection, it is principally possible to provide in the cylinder axial guide structures by which the piston is engaged so that it rotates together with the cylinder. For example, axial groove and key arrangements could be provided. It is however particularly advantageous if the piston and the cylinder have an essentially elliptical cross-section so that the piston is engaged for rotation with the cylinder. 
     The invention will be described below in greater detail on the basis of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial perspective view showing the friction stir welding procedure using a device as it is used, in principle, for such welding tasks, 
     FIG. 2 is a perspective view of the front end of a device for friction stir welding wherein two spaced shoulders are provided between which the mark pieces are received, 
     FIG. 3 is a cross-sectional side view of a first embodiment of the device according to the invention, and 
     FIG. 4 is a cross-sectional side view of a second embodiment of the device according to the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     On the basis of FIG. 1, which shows the state of the art, the friction stir welding procedure is first shortly explained, since the principle is basically used by the device  10  according to the invention as shown in FIGS. 3 and 4. 
     Two workpieces  11  and  12  which are shown as plates are placed next to each other so as to abut each other along an area where they are to be joined by a butt weld seam  31 . At the ends to be joined, that is, in the connection area  13 , a small space is provided. The two workpieces  11  and  12  are pressed toward each other against support spacers, which are not shown in the Figure so that they cannot move during the welding procedure. 
     A shaft  14 , which can be rotated by drive means  28  as indicated by the arrow  30  is provided at its end  15  remote from the drive means  28  with a first shoulder  18  which forms a first stop  19 , see also FIG.  2 . At the end  17  of the pin-like projection remote from the drive means  28  a second stop  21  is provided which forms a second shoulder  20  directed toward the pin-like projection  16 . The axial distance between the first and the second shoulders  18 ,  20  is fixed in the device of the prior art. By rotating the shaft  14 , and together therewith the pin-like projection  16 , the material of the workpieces  11  and  12  is plasticized between the opposite shoulders  18  and  20  by friction heating when the pin-like projection is moved into the connection area  13  of the two workpieces. The plasticized materials of the two workpieces  11  and  12  intermix and form a weld seam  31 , see the completed weld seam at the left end of the seam shown in FIG.  1 . The rotating pin-like projection  16  is moved in the direction of arrow  29  along the connection area  13  by suitable guide and drive means. This procedure is applicable generally also to the device according to the invention  10  as shown in FIGS. 3 and 4. 
     The device  10  according to the invention, in the embodiment as shown FIG. 3 differs from the state of the art in that the second stop  21  and, together therewith, the shoulder  20  is movable axially back and forth with respect to the shoulder  18  (arrow  32 ) in order to engage the workpieces with a predetermined force between the two shoulders  18  and  20  of the two stops  19  and  21 . For generating this engagement force, a housing  22  is provided in which the shaft  14  is rotatably supported by bearings  33  in an axially movable guide structure  25 . At the end  15  of the shaft  14 , a piston element  23  is provided which is axially movably supported in a cylinder chamber  24 , which is also formed in the housing  22 . The shaft  14  is provided with a pin-like projection  26  forming a welding tool. The piston element  23  is sealed in the walls of cylinder chamber  24  in a suitable manner so that no flow connection exists between the cylinder chambers  24  at opposite sides of the piston or between the chambers  24  and the rest of the housing  22  in which the guide structure  25  is disposed. By way of communication lines which are only indicated in the figures pressurized hydraulic fluid  34  is supplied selectively to the chambers  24  or removed therefrom whereby the housing  22  and together therewith the first stop  19  are moved back and forth, see arrow  32 . In this way, it is made sure that, depending on the pressurization of the hydraulic fluid chambers  24 , a suitable engagement pressure between the first and the second stop  19  and  21  can be established, or respectively, between the first and the second shoulder  18 ,  20 , between which the two workpieces are disposed thereby providing for an engagement force of the two workpieces which is independent of the thickness of the workpieces. 
     By support means which are not shown in the drawings, for example, by stops on a drive motor or on a manufacturing robot on which the device  10  is mounted the housing  22  is retained stationary that is non-rotating relative to the shaft  14 . The stop  19  provided on the housing  22  are then also stationary relative to the shaft  14 . 
     In the embodiment according to FIG. 4, parts essentially identical to those shown in FIG. 3 are indicated by the same reference numerals. Also in this embodiment, the outer housing  22  is stationary with respect to the shaft  14 . Other than in the embodiment shown in FIG. 3, the embodiment of FIG. 4 includes in the housing  22  a guide element  26 , which is not axially movable relative to the housing  22  but which is rotatably supported in the housing  22  by bearings  33  and which is rotatable together with the shaft  14 . The guide element  26  includes a cylinder space  27  in which a piston  23  is disposed. The piston  23  is provided with a pin-like projection  16  and is movable back and forth by the admission or removal of hydraulic fluid to or from the cylinder chambers  27  formed at opposite sides of the piston  23 . The stop  19  forming the first shoulder  18  is provided on the guide element  26 . In the embodiment shown in FIG. 4, the stop  19  is formed integrally with the guide element  26 , but it could be provided by a separate element. The cylinder chambers  27  and the piston  23  have both an essentially elliptical cross-section so that they are rotatable in unison. In this way, rotation of the shaft  14  in the direction as indicated by the arrow  30  is transmitted to the piston  23  and to the pin-like projection  16 , to which the second stop  21  is connected. 
     It is pointed out that the connection between the end  17  of the pin-like projection  16  and the second stop  21  may be releasable. Also, the pin-like projection  16  may be releasably connected to the end  15  of the shaft  14  remote from the drive means so that the pin-like projection  16  can be easily replaced if it is damaged or when it is worn by a new pin-like projection. The removability of the second stop  21  may further be advantageous when, for performing the welding procedure, the pin-like projection is to be moved into welding position in an axial direction of the pin that is when lateral positioning is not possible. It may be necessary to drill a hole into the connection area of the workpieces  11 ,  12  which hole has a diameter corresponding to the diameter of the pin-like projection  16  and to insert the pin-like projection, with the second stop removed, through the hole and then to re-mount the second stop onto the pin-like projection. The second stop may be mounted onto the pin-like projection for example by a thread or by another suitable connection. 
     In the embodiment of the device according to FIG. 4, the first stop  19  with the first shoulder  18  and the second stop  21  with the second shoulder  20  both rotate during the welding procedure. With the embodiment according to FIG. 3, the device may be arranged at an angle to the workpieces  11 ,  12 , which is different from a normal 90° angle. Such an angle may be provided by a corresponding form of the shoulder  18 . 
     In contrast to the arrangements as known in the art, the device according to the invention permits a control of the forces with which the workpieces  11 ,  12  are engaged between the stops  19 ,  21 . 
     During rotation of the shaft  14 , the force can be adjusted by a corresponding control of the hydraulic fluid admitted to the cylinder chambers  24  at opposite sides of the piston  23 . Upon completion of the welding procedure, the two stops  19 ,  21  can be moved apart by an appropriate control of the hydraulic fluid  34  providing for the appropriate movement of the piston  23 . Then the pin-like projection is released or, in connection with the above mentioned example with the hole in the connection area of the workpieces  11 ,  12 , the pin-like projection is pulled out of a target hole at the end of the welding seam  31  after the second stop  21  has been removed. 
     It is finally pointed out that the material of which the pin-like projection  16  or the stops  19 ,  21  consist can be selected depending on the material of which the workpieces  11 ,  12  consist. The selection is to be made by an expert depending on the circumstances.