Patent Publication Number: US-2011068150-A1

Title: Friction Stir Weld Assembly And Associated Method For Mitigating Distortion In A Friction Stir Weld Joint

Description:
FIELD 
     Embodiments of the present invention relate generally to friction stir weld technology and, more particularly, to the mitigation of distortion in a friction stir weld joint. 
     BACKGROUND 
     Friction stir welding may be utilized in order to join two or more workpieces along a friction stir weld joint. In this regard, two or more workpieces may be abutted or overlapped with the region of contact between the two workpieces being engaged by a friction stir weld tool. A friction stir weld tool generally has a tool shoulder and a probe extending therefrom that is rotated and then plunged or otherwise inserted into the region of contact between the two workpieces and moved along a joint line while the probe continues to be rotated. 
     Frictional heat is generated as a result of the interaction of the rotating friction stir weld tool and the workpieces through which the friction stir weld tool is advanced. The frictional heat causes those portions of the workpieces along the joint line to soften without reaching the melting point, thereby creating plastic deformation with the workpieces being joined along the joint line upon their rehardening. 
     Friction stir welding introduces strain, however, into the resulting welded assembly. The strain may, in turn, induce distortion of the welded assembly including, for example, bowing and complex buckling, e.g., oil-canning. For example, the strain induced by friction stir welded assembly may cause distortion of the welded assembly due to shrinkage of the plasticized material, that is, shrinkage of the weld nugget, in comparison to the surrounding, unwelded material. The shrinkage of the weld nugget may cause the welded assembly to distort in the direction of the friction stir weld joint, which, in turn, may result in out-of-plane bowing. The distortion in the direction of the friction stir weld joint and the resulting out-of-plane bowing may also place the surrounding material in compression which, in turn, may cause the surrounding material to buckle. The potential for distortion of the welded assembly is particularly significant in conjunction with the friction stir welding of relatively thin materials. In this regard, less distortion is generally created by the friction stir welding of more rigid materials than the friction stir welding of less rigid materials. Since rigidity is directly proportional to the thickness of a given material, friction stir welding is accordingly more apt to create distortion in thinner materials than in thicker materials. 
     The distortion of a welded assembly may create difficulty during assembly of the welded component. For example, friction stir welding may be utilized in order to form portions of an aircraft with the assembly of those portions being more difficult in instances in which the friction stir welded portions are distorted. In addition to the potential distortion of the welded assembly and the corresponding difficulties with subsequent assembly of the distorted assembly, the internal residual stress that may be created by a friction stir weld joint may also reduce the fatigue strength and static strength of the resulting welded assembly which, in turn, may adversely affect the performance of the resulting assembly. 
     A number of techniques have been proposed in an effort to eliminate or reduce friction stir weld-induced distortion. With respect to eliminating friction stir weld-induced distortion in the first instance, techniques have been developed to improve the methods by which the workpieces are clamped during the friction stir welding process, to reduce the movement of the pieces of material during or following friction stir welding, to friction stir weld the pieces of material under a compression or tensioning arrangement, and to rapidly quench the welded assembly during or relatively immediately following the friction stir weld process. In addition, techniques have been developed that preheat and expand the workpieces prior to friction stir welding, thereby preforming the parts prior to friction stir welding so that the shape of the welded assembly is balanced between the prestressed state and post-weld stress state. However, techniques that are implemented prior to friction stir welding or during the friction stir welding process that are intended to counterbalance the weld-induced stresses have proven relatively difficult to control with sufficient precision in order to fully eliminate net distortion in the welded assembly. These difficulties are exacerbated and the techniques designed to counterbalance weld-induced stresses generally prove even less effective in instances in which a welded assembly includes multiple friction stir weld joints, each of which may induce strain into the welding assembly. 
     Other techniques have been employed following the formation of a friction stir weld joint in an effort to counterbalance the post-weld stresses induced within the welded assembly. These post-weld techniques include treatments of the entire welded assembly, such as stress-relieving heat treatments, tensioning or stretching of the welded assembly and/or forging of the welded assembly. These post-weld techniques also include treatments that are more localized relative to the friction stir weld joint and the neighboring portions of the workpieces. These more localized techniques include localized thermal-mechanical surface treatments to relieve stress and/or to induce counterbalancing compressive layers, such as peening, burnishing and laser shock peening. The thermal-mechanical treatments that may be employed following formation of a friction stir weld joint generally require relatively high temperatures that are significantly higher than the temperatures employed for an aging process that may otherwise be employed for strength and metallurgical stabilization. These relatively high temperatures that would be required for thermal-based post-weld treatment may be deleterious to coatings, surface treatments and/or other aspects of the welded assembly. In addition, the thermal-mechanical surface treatment techniques that may be employed in an effort to relieve weld-induced stress, such as peening, burnishing and laser shock peening, may require relatively expensive hardware systems that require a trained and experienced technician for proper operation. In addition, such thermal-mechanical surface treatment techniques may require a plurality of passes over the same section of the welded assembly to be effective, thereby increasing the time required for and decreasing the efficiency of these techniques. Moreover, such surface treatment techniques may only plastically deform the material that is relatively near to the surface and, as such, may only correct relatively small amounts of distortion. 
     As such, it may be desirable to provide improved techniques for mitigating the distortion created by a friction stir welding process. In this regard, it may be desirable to provide an improved technique for counteracting the strain introduced into a welded assembly by a friction stir welding process in order to reduce the weld-induced distortion, such as out-of-plane bowing and complex buckling, e.g., oil-canning. 
     BRIEF SUMMARY 
     A method of mitigating distortion in a friction stir weld joint as well as an associated friction stir weld assembly for mitigating distortion in a friction stir weld joint are provided. The method and associated friction stir weld assembly may reduce out-of-plane bowing and complex buckling, such as oil-canning. As such, the method and associated friction stir weld assembly may facilitate the subsequent integration of a welded assembly into a larger structure and may also improve the strength of the welded assembly, such as a fatigue strength and/or the static strength, by reducing the internal residual stresses. 
     In one embodiment, a method of mitigating distortion in a friction stir weld joint is provided that includes the provision of a workpiece having the friction stir weld joint and associated distortion and the subsequent application of force to the workpiece along at least a portion of friction stir weld joint. The application of force induces plastic deformation along at least the portion of the friction stir weld joint in order to reduce the distortion of the workpiece. 
     Force may be applied in accordance with one embodiment by moving a roller, such as a hardened cylindrical roller having chamfered edges, along at least a portion of the friction stir weld joint in order to apply a compressive force. While moving the roller, at least the portion of the friction stir weld joint over which the roller is moved may be supported with a backing member. The application of force to the workpiece may include directing the roller along a predefined weld path in accordance with a common weld path definition to that previously employed in conjunction with a friction stir weld tool. 
     As noted above, the application of force to the workpiece induces plastic deformation which, in one embodiment, elongates the workpiece along at least the portion of the friction stir weld joint. While force is applied to the workpiece, the workpiece may be secured upon a platform with a holding device. In this regard, the platform and the holding device may also be configured to secure the workpiece during the prior formation of the friction stir weld joint. 
     In accordance with another embodiment of the present invention, a method of mitigating distortion in a friction stir weld joint is provided in which a friction stir weld joint is formed in a workpiece and force is then applied to the workpiece following formation of and along at least a portion of the friction stir weld joint in order to reduce the distortion of the workpiece. While forming the friction stir weld joint, the workpiece is secured upon a platform by a holding fixture and a friction stir weld assembly is utilized that is configured to direct a friction stir weld tool along a predefined weld path while applying a predefined force. Following the formation of the friction stir weld joint, force may be applied to the workpiece while the workpiece remains secured upon the platform with the holding fixture. In addition, force may be applied to the workpiece by engaging a roller with the friction stir weld tool that is also configured to apply a compressive force by moving the roller along the predefined weld path while applying a predetermined force. 
     The method of one embodiment also directs the friction stir weld tool and the roller along the predefined weld path that was followed during the formation of the friction stir weld joint in accordance with a common weld path definition. In order to apply force to the workpiece, the method may apply force with a hardened cylindrical roller with chamfered edges. The application of force may induce plastic deformation along at least a portion of the friction stir weld joint. The plastic deformation may elongate the workpiece along at least a portion of the friction stir weld joint. In one embodiment, the workpiece may be supported during movement of both the friction stir weld tool and the roller along the predefined weld path. 
     In accordance with a further embodiment to the present invention, a friction stir weld assembly is provided that includes a platform and a holding fixture configured to secure a workpiece to the platform during friction stir welding operations. The friction stir welding assembly also includes a friction stir weld tool and a controller configured to direct the friction stir weld tool along a predefined weld path while applying a predefined force in order to form a friction stir weld joint. The friction stir weld assembly also includes a roller with the controller being configured to direct the roller along the predefined weld path while the workpiece is secured to the platform by the holding fixture and while applying a predefined force in order to reduce the distortion of the workpiece. 
     The controller of one embodiment is configured to direct movement of both the friction stir weld tool and the roller in accordance with a common weld path definition. The controller may also be configured to apply the predefined force in order to induce plastic deformation along at least a portion of the friction stir weld joint. By inducing plastic deformation, the workpiece may be elongated along at least a portion of the friction stir weld joint. The platform of one embodiment may include a backing member to support the workpiece during movement of both the friction stir weld tool and the roller along the predefined weld path. The roller of one embodiment may include a hardened, cylindrical roller having chamfered edges. 
     The method and friction stir weld assembly may therefore mitigate distortion in a friction stir weld joint, such as by reducing out-of-plane bowing and complex buckling, e.g., oil-canning. As such, the resulting welded assembly may more consistently be fabricated in accordance with the desired specifications and within the acceptable tolerances, thereby facilitating its integration into an overall structure. In addition, the mitigation of distortion may improve the strength of the resulting welded assembly, such as the fatigue strength and/or static strength, by reducing the internal stresses. Additionally, embodiments of the method and friction stir weld assembly of the present invention may utilize a common device both during the formation of the friction stir weld joint and the subsequent application of force to the friction stir weld joint, thereby simplifying the implementation of certain embodiments and ensuring that the force is applied along the friction stir weld joint by directing the roller along the same predefined weld path, such as defined by a common weld path definition. 
     The features, functions and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a perspective view of a friction stir weld assembly in accordance with one embodiment of the present invention; 
         FIG. 2  is a perspective view of welded assembly having a friction stir weld joint and associated out-of-plane bowing and complex buckling; 
         FIG. 3  is a perspective view of a roller assembly in accordance with one embodiment of the present invention; 
         FIG. 4  is a more detailed plan view of a roller in accordance with one embodiment of the present invention; and 
         FIG. 5  is a flowchart of operations performed in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     Referring now to  FIG. 1 , a friction stir weld assembly  10  in accordance with one embodiment of the present invention is depicted. As shown, the friction stir weld assembly includes a platform  12  having a backing member  14  for supporting the workpieces  16 . For example, the backing member may be a table or other support surface that is formed of a material, such as steel in instances in which aluminum alloys are friction stir welded, that is configured to remain rigid while subjected to the forces and other processing conditions that are applied to the workpieces during formation of a friction stir weld joint and thereafter during the application of force along the friction stir weld joint. In the illustrated embodiment, the workpieces include a sheet and a stringer that is disposed upon and extends across the sheet. The sheet and the stringer of this embodiment may be formed of aluminum or titanium alloys, for example. However, the friction stir weld assembly can be utilized to process a wide variety of other workpieces that are formed of a number of different materials and that are abutted or overlapped in a variety of manners. 
     The friction stir weld assembly  10  of  FIG. 1  also includes a holding fixture  18 , such as one or more clamps, configured to releasably secure the workpieces  16  to the platform  12 , such as to the backing member  14 . The friction stir weld assembly of the embodiment of  FIG. 1  also includes a friction stir weld tool  20 . The friction stir weld tool of one embodiment includes a probe  22  that extends at a distal end of the friction stir weld tool beyond a shoulder  24 . However, the friction stir weld tool may have other configurations, if so desired. 
     The friction stir weld assembly  10  may also include a controller  26  configured to direct the friction stir weld  20  tool along a predefined path while applying a predefined force to thereby form a friction stir weld joint in the workpiece  16 . In this regard, the controller may be a processor or other computing device that either includes a memory device or is otherwise in communication with a memory device which stores the definition of the predefined weld path, such as in terms of a series of coordinates, as well as a definition of the predefined force to be applied to the workpieces. The controller of one embodiment is configured to operate in accordance with and under the control of a computer program consisting of a plurality of computer program instructions that, in one embodiment, may also be stored by the memory device. 
     In operation, the controller  26  may position the friction stir weld tool  20  at an initial location and may cause the friction stir weld tool to begin rotating. As indicated by  FIG. 1 , the friction stir weld assembly  10  may also include a plurality of actuators  30  that may be directed by the controller to appropriately move the friction stir weld tool. For example, under the direction of the controller, the actuators may be configured to cause the friction stir weld tool to rotate and/or to move to certain positions while applying predetermined amounts of force. As such, subsequent discussion regarding the controller directing the friction stir weld tool to move in certain manners may, in one embodiment, actually include the controller directing the actuators to cause the friction stir weld tool to move in certain manners. In the illustrated embodiment, once the friction stir weld tool is rotating, the controller may direct the friction stir weld tool to move toward the workpieces  16  such that the probe  22  is plunged into the workpieces. The controller may then direct the friction stir weld tool to move along a predefined weld path  28  (shown in dashed lines in  FIG. 1 ) while continuing to cause the probe to be rotated and to remain in contact with and, indeed, to remain plunged into the workpieces. Upon reaching the final position of the predefined weld path, the controller can direct the friction stir weld tool to withdraw from the workpieces, thereby removing the probe from the workpieces. Upon rehardening of the portions of the workpieces that have been plasticized during the friction stir welding process, the workpieces will be joined along a friction stir weld joint that follows the predefined weld path. 
     As described above, friction stir welding may introduce strain into the resulting welded assembly which, in turn, may induce distortion of the welded assembly including, for example, bowing and complex buckling, e.g., oil canning. For example,  FIG. 2  depicts a welded assembly in which a pair of workpieces  16  have been joined by a friction stir weld joint. As shown, the welded assembly is bowed and also exhibits complex buckling (schematically illustrated by the arcuate lines on either side of the friction stir weld joint) in at least portions of the workpieces proximate friction stir weld joint. While the welded assembly of the illustrated embodiment is bowed in a concave upward manner, the welded assembly may be otherwise bowed, such as in a concave downward manner. 
     In order to reduce the distortion introduced by the friction stir welding process, force may be applied to the workpieces  16  along at least a portion of the friction stir weld joint. In this regard, the force that is applied is sufficient to induce plastic deformation along at least the portion of the friction stir weld joint. By inducing plastic deformation, the distortion of the workpieces is reduced, if not eliminated. Moreover, by inducing the plastic deformation that reduces the distortion of the workpiece, the strength of the welded assembly, such as the fatigue strength and/or the static strength, may also be improved so as to thereby enhance the performance of the resulting welded assembly. 
     In one embodiment, the force is applied to the workpieces  16  by moving a roller  40  along at least a portion of the friction stir weld joint so as to apply a compressive force thereto. In one embodiment, the roller is engaged by and the force is applied by the same friction stir weld assembly  10  that previously formed the friction stir weld joint. As such, the workpieces may remain engaged by the same friction stir weld assembly both during the formation of the friction stir weld joint and the subsequent application of force along the friction stir weld joint so as to reduce any resulting distortion in the welded assembly. By utilizing the same friction stir weld assembly, the overall process of forming the friction stir weld joint and then reducing the distortion of the workpieces may be performed efficiently without much, if any, manual intervention and in a manner that provides accurate results since the alignment of the workpieces to the friction stir weld assembly is maintained and, indeed, may remain the same throughout both the friction stir weld process and the subsequent application of force for the reduction of distortion within the workpieces. 
     The friction stir weld assembly  10  of one embodiment includes force-measurement and force-feedback control systems. For example, the force-feedback control system may be embodied by the controller  26 . These force-measurement and force-feedback control systems permit the force that is applied to the workpieces  16  during the formation of the friction stir weld joint to be measured and then adjustably controlled, such as in a precise manner. By utilizing the same friction stir weld assembly during the movement of the roller along the friction stir weld joint, the friction stir weld assembly including the controller may also measure the force applied by the roller and then adjustably control the force, also such as in a precise manner. 
     In this embodiment, the welded assembly continues to be secured to the platform  12  and supported by the backing member  14  during the application of force along the friction stir weld joint. In this regard, the welded assembly may remain secured to the platform, such as by means of the holding devices  18  both during the application of force to the workpieces and during the prior formation of the friction stir weld joint. In contrast to the friction stir weld tool  20  that is utilized during the formation of the friction stir weld joint, the friction stir weld assembly  10  may engage a roller  40  during the application of force to at least a portion of the friction stir weld joint. In this regard, the friction stir weld assembly may include an end effector that engages and holds the friction stir weld tool during the formation of the friction stir weld joint, such as shown in  FIG. 1 . Following formation of the friction stir weld joint, however, the end effector may be disengaged from the friction stir weld tool and may, instead, engage a roller tool, such as shown in  FIG. 3 . Thus, the friction stir weld assembly of one embodiment of the present invention may alternately engage a friction stir weld tool during formation of the friction stir weld joint and a roller tool during application of force to the friction stir weld joint. 
     The roller tool of one embodiment depicted in  FIG. 3  may include a handle  42  or shaft and a roller  40  rotatably mounted at one end thereof, such as by means of a pair of arms  44 . The end effector may therefore grasp and engage the handle or shaft while permitting the roller to be disposed in contact with the welded assembly. As shown in  FIGS. 3 and 4 , the roller of one embodiment includes a generally cylindrical body  46  and is mounted at its opposed ends to the handle or shaft such that the roller may rotate about its longitudinal axis. As also depicted in  FIG. 4 , the roller may include chamfered end portions  48  and may be formed of a hardened material, such as steel. Although the roller may be fabricated in various manners, the roller of one embodiment has a diameter that is greater than its length, such as more than 2 times greater than its length and, in one embodiment, between 2 and 3 times greater than its length. For example, the roller of one embodiment may be 1.5 inches in diameter and may have a length of 0.6 inches as measured along its longitudinal axis. 
     In operation, the workpieces  16  are secured to the platform  12  and a friction stir weld tool  20  is directed by the controller  26  along a predefined weld path  28  to form a friction stir weld joint, as shown in  FIG. 1  and in operations  50  and  52  of  FIG. 5 . In order to mitigate the distortion that may have been created during the formation of the friction stir weld joint, the welded assembly is positioned upon the platform and force is applied along at least a portion of the friction stir weld joint. As described above and as noted in operation  54  of  FIG. 5 , the welded assembly may remain secured in the same configuration to the same platform as in the friction stir welding process. However, other factors may impact the manner in which the welded assembly is secured upon the platform. For example, as a result of the distortion that may occur following formation of the friction stir weld joint, such as the out-of-plane bowing and complex buckling, the resulting welded assembly may be concave downward or concave upward relative to the platform. While the force may be applied, such as by means of a roller  40 , to either side of the welded assembly along the friction stir weld joint, the method and friction stir weld assembly of one embodiment of the present invention is configured to apply force to the workpiece along the friction stir weld joint while the workpiece is supported upon the platform in a concave downward configuration. The distortion of the workpiece and, consequently, the manner in which the force is most effectively applied to the workpiece to reduce the distortion of the workpiece may also be partially dependent upon the configuration of the welded assembly. In one embodiment in which the welded assembly includes a stringer on one surface of the welded assembly as shown, for example, in  FIG. 2 , the method and friction stir weld assembly  10  of one embodiment may apply the force along the friction stir weld joint on the stringer side of the welded assembly in order to remove a greater degree of the distortion of the workpiece. 
     Once the welded assembly has been secured to the platform  12 , the controller  26  may be configured to position the roller  40  at an initial position, such as the same initial position that was utilized for the friction stir weld tool  20  during the formation of the friction stir weld joint. The controller may then move the roller into contact with the workpieces  16 . In embodiments in which the initial position of the roller and the initial position of the friction stir weld tool are identical, the controller will therefore have positioned the roller in contact with the welded assembly at one end of the friction stir weld joint. The controller may then be configured to move the roller along a predefined path. In this regard, the controller of one embodiment is configured to move the roller along the same predefined weld path  28  as that utilized during the formation at the friction stir weld joint. See operation  56  of  FIG. 5 . As such, the controller may move both the friction stir weld tool and the roller along the same predefined weld path that is defined by a common weld path definition, such as the same series of coordinate points stored, for example, by the memory device. By utilizing a common weld path definition, the controller may therefore ensure that the roller moves along the friction stir weld joint and does not deviate therefrom. 
     In addition to moving the roller  40  along the friction stir weld joint, the controller  26  is also configured to apply a predefined amount of compressive force to the welded assembly as the roller moves along the friction stir weld joint, generally at a predefined velocity. Upon the roller reaching a final point, such as the same final point that defines an end of the friction stir weld joint, the controller may lift or otherwise remove the roller from contract with the welded assembly. 
     As a result of the application of compressive force applied by the roller  40  along at least a portion of the friction stir weld joint, plastic deformation is induced along at least that portion of the friction stir weld joint. The plastic deformation, in turn, causes the distortion of the workpieces  16  including both the out-of-plane bowing and the complex buckling, e.g., oil canning, to be reduced, if not eliminated. Indeed, the plastic deformation that is induced by the roller within at least that portion of the friction stir weld joint may cause the workpieces to elongate along at least that portion of the friction stir weld joint, thereby reducing the distortion of the workpieces. 
     The amount of force to be applied to the workpieces  16  may vary depending upon a number of factors including the material properties of the workpieces that are joined by the friction stir weld joint, the thickness of the friction stir weld joint, the extent of the distortion of the workpieces including out-of-plane bowing and complex buckling, and the like. In addition, the amount of force to be applied by the roller  40  and the other related parameters, such as the speed at which the roller traverses the friction stir weld joint, may be determined by conducting a series of tests on sample workpieces and then identifying the most appropriate combination of force and speed for a sample workpieces that is distorted in the same fashion and that has the same material properties and is of the same thickness as the workpieces to be joined by a friction stir weld joint. Although the friction stir weld assembly may be configured to apply a wide variety of forces to the friction stir weld joint, the friction stir weld assembly  10  of one embodiment applies between about 1500 pounds and about 4500 pounds to the workpieces. By controlling the force, the force applied to the workpiece may remain within a predefined range, such as within 5% and, in one embodiment, within 1%, of a predefined force level along the length of the friction stir weld joint. Also, although the friction stir weld assembly may be configured to move the roller along the friction stir weld joint at any of a variety of speeds, the friction stir weld assembly of one embodiment is configured to move the roller at 8 inches per minute along the friction stir weld joint. 
     Following the application of the force along the friction stir weld joint, and the removal of the roller  40  from the welded assembly, the holding devices  18  may release the welded assembly and the welded assembly may be removed from the platform  12 . Since the distortion of the welded assembly has been reduced, if not eliminated, the welded assembly can be more readily assembled or integrated with other substructures since the welded assembly will now have been fabricated with improved tolerances. Additionally, the resulting welded assembly may have improved strength, such as improved fatigue strength and/or static strength, as a result of the reduction in the internal stresses occasioned by the reduction in the distortion of the workpiece. Further, in conventional applications, friction stir welding conditions and parameters may be established as a compromise between joint properties and distortion. In accordance with embodiments of the present invention, the friction stir welding conditions and parameters may be selected so as to produce stronger welds but larger distortion with the subsequent application of force, such as by means of a roller moved along the friction stir weld joint, reducing or eliminating the distortion, but maintaining the strength. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.