Abstract:
A system and method for joining curved surfaces such as pipes by obtaining pipes having additional rough stock material on the pipe ends, the rough stock material being precision machine processed to prepare complementary face profiles on each of the curved surfaces and then performing friction stir joining of the pipes to obtain a joint that has fewer defects than joints created from conventional welding.

Description:
BACKGROUND 
       [0001]    Friction stir joining is a technology that has been developed for welding metals and metal alloys. Friction stir welding is generally a solid state process that has been researched, developed and commercialized over the past 20 years. Solid state processing is defined herein as a temporary transformation into a plasticized state that may not include a liquid phase. However, it is noted that some embodiments allow one or more elements or materials to pass through a liquid phase, and still obtain the benefits of the present. 
         [0002]    Friction stir joining began with the joining of aluminum materials because friction stir joining tools may be made from tool steel which may adequately tolerate the loads and temperatures desired to join aluminum. Friction stir joining has continued to progress into higher melting temperature materials such as steels, nickel base alloys and other specialty materials because of the development of superabrasive tool materials and tool designs that may withstand the forces and temperatures that may be used to flow these higher melting temperature materials. 
         [0003]    Even though there are several publications including patents that may describe the process of friction stir joining, there are several elements of the process that may be improved for friction stir joining to become a large-scale production process rather than a small-scale research project. 
         [0004]    The friction stir joining process often involves engaging the material of two adjoining planar workpieces on either side of a joint by a rotating stir pin. Force is exerted to urge the pin and the workpieces together and frictional heating caused by the interaction between the pin, shoulder and the workpieces results in plasticization of the material on either side of the joint. The pin and shoulder combination or “FSW tip” is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing FSW tip cools to form a weld. The FSW tip may also be a tool without a pin so that the shoulder is processing another material through FSP. 
         [0005]      FIG. 1  is a perspective view of a tool being used for friction stir joining that is characterized by a generally cylindrical tool  10  having a shank  8 , a shoulder  12  and a pin  14  extending outward from the shoulder. The pin  14  is rotated against a workpiece  16  until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized planar workpiece material. The pin  14  is plunged into the planar workpiece  16  until reaching the shoulder  12  which prevents further penetration into the workpiece. The planar workpiece  16  is often two sheets or plates of material that are butted together at a joint line  18 . In this example, the pin  14  is plunged into the planar workpiece  16  at the joint line  18 . 
         [0006]    Referring to  FIG. 1 , the frictional heat caused by rotational motion of the pin  14  against the planar workpiece material  16  causes the workpiece material to soften without reaching a melting point. The tool  10  is moved transversely along the joint line  18 , thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge along a tool path  20 . The result is a solid phase bond at the joint line  18  along the tool path  20  that may be generally indistinguishable from the workpiece material  16 , in contrast to the welds produced when using conventional non-FSW welding technologies. 
         [0007]    It is observed that when the shoulder  12  contacts the surface of the planar workpieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin  14 . The shoulder  12  provides a forging force that contains the upward metal flow caused by the tool pin  14 . 
         [0008]    During friction stir joining, the area to be joined and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint at a tool/workpiece interface. The rotating friction stir welding tool  10  provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the base metal, while transporting metal from the leading edge of the pin  14  to its trailing edge. As the weld zone cools, there is desirably no solidification as no liquid is created as the tool  10  passes suitably resulting weld is a defect-free, re-crystallized, fine grain microstructure formed in the area of the weld. 
         [0009]    In the present state of the art, arcuate or curved surfaces such as pipes or tubes are joined together by butting the ends of the tubing together, inserting a support mandrel from an open end of the tubing under the joint, and then performing friction stir joining of the tubing. This concept has already been disclosed in patents and publications and is widely accepted as an effective means of joining curved surfaces together. 
         [0010]    The terms “tubular”, “coiled tubing”, “tube”, “tubing”, “drillpipe”, “casing”, and “pipe” and other like terms for a curved surface may be used interchangeably. The terms may be used in combination with “joint”, “segment”, “section”, “string” and other like terms referencing a length of tubular. 
         [0011]    Pipelines, tubulars and the like are widely used in many industries throughout the world and in many applications. Construction and manufacturing methods may be regulated by governments and industry standards organizations. Such oversight is considered desirable because any failure may be a risk for loss of life and limb. There have been several cases, for example, where numerous persons have been killed by natural gas line explosions that were caused by a faulty fusion weld. Decades of analyzing and documenting field failures have been the foundation of code cases currently followed for new construction of pipelines and other structures. 
         [0012]    Even though friction stir joining is a newer joining technology, the process may still meet existing applicable government and industry standards as well as have new code cases written and approved for friction stir joining specific defects. While the concept of friction stir joining is relatively simple, there appears to be a lack of information in patents and literature that provides information for performing friction stir joining as a manufacturing process for curved surfaces that is free from defects. 
         [0013]    For example, one of the defects found in friction stir joining is the root defect. A root defect may result when the material being stirred adjacent or nearly adjacent to a support mandrel experiences little or no flow from stirring. 
         [0014]      FIG. 2  illustrates a cross section of two pipes  30  being joined together at a pipe joint  32  and a friction stir joining tool  34  performing the joining. A mandrel  36  provides support along the pipe joint  32 . This figure shows that a friction stir joining root defect  38  is caused by a lack of penetration of the friction stir joining tool  34  into the pipe  30 . The tip of the tool  34  is shown at what is likely an exaggerated distance from the mandrel  36  in order to illustrate the cause of the root defect  38 . 
         [0015]    The material being stirred at the pipe joint  32  by the friction stir joining tool  34  may need to flow completely from the bottom to the top of the pipe joint in order to create a solid state bond between the pipes  30 . As shown, this defect is often the result of a lack of tool penetration and/or an oxide layer on the surfaces of the pipes  30  at the pipe joint  32  that has not been broken and consumed by the friction stir joining tool  34 . Even careful microstructure evaluation of the pipe joint  32  after friction stir joining may not reveal the presence of the root defect  38 . In most cases, the root defect  38  may be found by performing a bend test that opens the underside of the pipe joint  32  using stress and plastic strain. 
         [0016]    The lack of friction stir joining tool penetration may often be a result of varying pipe wall thickness, or an “oval” shape of the pipe. The wall thickness variation may be common for the pipe manufacturing process and may occur from pipe section to pipe section as well as mill run to mill run. Pipe manufacturers dramatically raise the price of their products if tighter material and geometric tolerance specifications are requested because of the difficulties in ensuring consistent quality pipe manufacturing. 
         [0017]    Furthermore, there is a belief in the industry that any variation in pipe dimensions may be compensated for with the fusion welding process because overmatched filler metal is used to join the pipes together. This is because conventional welding processes have the ability to compensate for broad geometric variances in pipe joints. However, compensation comes at the expense of consistency due to the broad range of solidification defects, residual stresses and cross section hardness variation at the fusion welding joint. Friction stir joining will have the same degree of variation in joint quality between pipes if new and innovative approaches are not implemented to take advantage of the benefits offered by a solid state joining process. 
       SUMMARY 
       [0018]    A system and method for joining curved surfaces such as pipes by obtaining pipes having additional rough stock material on the pipe ends, the rough stock material being precision machine processed to prepare complementary face profiles on each of the curved surfaces and then performing friction stir joining of the pipes to obtain a joint that has fewer defects than joints created from conventional welding. 
         [0019]    These and other objects, features, advantages and alternative aspects of the present will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0020]      FIG. 1  is an illustration of the prior at showing friction stir welding of planar workpieces. 
           [0021]      FIG. 2  is an illustration of the prior art showing a perspective view of a root defect being caused by lack of penetration of a friction stir joining tool along a pipe joint. 
           [0022]      FIG. 3  is a perspective view of a pipe with an end having additional rough stock material that may be precision machine processed to create a face profile. 
           [0023]      FIGS. 4A and 4B  are cut-away side and perspective views of pipe ends prepared for friction stir joining, including a standard butt joint and a mandrel underneath the joint. 
           [0024]      FIGS. 5A and 5B  show cut-away side and perspective views of a first embodiment showing complementary face profiles on pipe ends, where the pipes are precision machine processed to provide a thread or groove profile. 
           [0025]      FIGS. 6A and 6B  show cut-away side and perspective views of an alternative embodiment showing complementary face profiles on pipe ends, where the pipes are precision machine processed to provide a chamfer taper or bevel at the root or ID of the pipes. 
           [0026]      FIGS. 7A and 7B  show cut-away side and perspective views of an alternative embodiment showing complementary face profiles on pipe ends, where the pipes are precision machine processed to provide a chamfer taper or bevel at the root of the pipes and at the OD corner of the pipes. 
           [0027]      FIGS. 8A and 8B  show cut-away side and perspective views of an alternative embodiment showing complementary face profiles on pipe ends, where the pipes are precision machine processed to provide a curved profile. 
           [0028]      FIGS. 9A and 9B  show cut-away side and perspective views of an alternative embodiment showing face profiles on pipe ends, where the pipes are precision machine processed to provide a profile that combines different profiles on each of the pipe ends. 
           [0029]      FIGS. 10A and 10B  show cut-away side and perspective views of an alternative embodiment showing complementary face profiles on pipe ends, where the pipes are precision machine processed to provide a partial thread, groove or other profile. 
           [0030]      FIGS. 11A and 11B  show cut-away side and perspective views of an alternative embodiment showing complementary face profiles on pipe ends, where the pipes are precision machine processed to provide a single or multiple different profiles. 
           [0031]      FIGS. 12A and 12B  show cut-away side and perspective views of an alternative embodiment showing face profiles on pipe ends, where the pipes are precision machine processed to provide a single or multiple different profiles. 
           [0032]      FIGS. 13A and 13B  show cut-away side and perspective views of an alternative embodiment illustrating that the mandrel is machined to include a profile that will alter flow of the pipe material. 
           [0033]      FIGS. 14A and 14B  show cut-away side and perspective views of an alternative embodiment showing face profiles and possibly the mandrel are precision machine processed in order to allow a second material to be joined to the pipes during friction stir joining. 
           [0034]      FIG. 15  is a cut-away perspective view of an alternative embodiment that shows a second material disposed between the pipe ends, the second material standing proud relative to the pipes. 
           [0035]      FIG. 16A  is a cut-away perspective view of a filler material that may be substituted for the filler material of  FIG. 15 . 
           [0036]      FIG. 16B  is a cut-away perspective view of an alternative embodiment of the filler material of  FIG. 16A . 
           [0037]      FIG. 16C  is a cut-away perspective view of an alternative embodiment of the filler material of  FIG. 16A . 
           [0038]      FIG. 17  is a perspective view of a stationary shoulder tool configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Reference will now be made to the drawings in which the various embodiments will be given numerical designations and in which the embodiments will be discussed so as to enable one skilled in the art to make and use the embodiments of the invention. It is to be understood that the following description illustrates embodiments of the present invention, and should not be viewed as narrowing the claims which follow. 
         [0040]    The first embodiment begins with the preparation of the pipes to be joined. In order to achieve the desired consistency, a precision machining process for preparing the ends of the pipes to be joined may be introduced as a prelude to friction stir joining. The precision machine processing may be unlike a conventional welding process that does not use precision machine processing to prepare the pipe ends for welding. Therefore, it is desired that all pipes to be joined may first be precision machine processed in order to have a higher degree of geometric precision, as compared to pipes that are conventionally welded, that is a precision machining process performed prior to joining. 
         [0041]    Accordingly, the pipes may need to have sufficient extra material or rough stock material on the portion of the pipes where they are to form a joint. The rough stock material may then be removed in a pre-joining precision machining process in order to achieve the desired geometric specifications of pipes that are ready to be joined using friction stir joining. The desired level of ovality, concentricity, wall thickness and diameter specifications for the pre-joined pipes may be known and compared to the capabilities of the pipe manufacturing process. The rough stock that is desirable in order to consistently maintain these final specifications may be supplied with the pipe from the mill. 
         [0042]      FIG. 3  shows a cross section of a pipe  30 . The pipe  30  may be considered to be a curved surface within the definition of this document. The pipe  30  shows an example of how a pipe end  40  may be supplied for precision machine processing in order to meet desired dimensional specifications. The pipe end  40  may be formed, for example, by a swaging process, a hot upset process or any hot or cold forming process that may generate the desired pipe end. 
         [0043]    The inside diameter of one or both of the pipes  30  to be joined may be machined such that the inside diameters are substantially concentric, having the same inside diameter within a specified tolerance. The face planes, mating surfaces or face profiles  42  of the pipe joint may be precision machine processed such that they are parallel or non-parallel and coincident (unless otherwise specified). The outside diameters of each pipe  30  may be machined such that the outside diameters are substantially concentric and having the same outside diameter within a specified tolerance. The pre-joining precision machine processing may include one or more pre-joining processes that include but are not limited to turning, milling, reaming, facing, etc. as known to those skilled in the art. Precision machine processing of the pipes  30  may occur immediately prior to friction stir joining. 
         [0044]    Machining equipment is currently used to prepare pipe joints for conventional fusion welding using stationary machining equipment as well as portable machining equipment in the field. However, this machining equipment described above typically may only machine the face of each pipe by cutting a bevel on an outside corner of each pipe end. 
         [0045]    In contrast, the first embodiment may use stationary precision machine processing equipment and portable precision machine processing equipment that may be operated in the field or at a work site, but with the capability of performing precision machine processing of the pipe ends  40 . 
         [0046]    More specifically, the machining equipment may be capable of modifying curved surfaces of the pipe ends  40 . The curved surfaces include any part of the pipe ends  40 , whether or not the surface being machines is actually curved or not. Thus, modifying the curved surfaces includes but is not limited to modifying a face profile  42  of each pipe end  40 , modifying an ID, modifying an OD, modifying concentricity of the curved surfaces of the pipe ends, modifying coincidence of the face profile, modifying the face profile to include a non-linear feature, modifying the face profile to include at least one thread, at least one groove, at least one chamfer, at least one mating spline, at least one non-mating spline, and reaming. 
         [0047]    The machining equipment may also be capable of forming a face profile that may be non-planar and coincident. Non-planar features of a path along the pipe joint  32  may include one or more of the following: a bias, an elliptical configuration and an arcuate configuration on the face profile. 
         [0048]    In addition, the machining equipment may be capable of machining specific geometries on the pipe end  40  at the face profile  42  in order to manage heat and material flow during the friction stir joining process.  FIGS. 4A through 14B  show various embodiments of geometries and configurations on the curved surfaces at the pipe ends  40  that are representative of, but should be considered as limited to, some of the modifications to the curved surfaces for enhancing friction stir joining capability and consistency. 
         [0049]      FIGS. 4A and 4B  are perspective cut-away views of pipe ends  40  prepared for friction stir joining including a standard butt joint  44  and a mandrel  36  underneath the pipe joint  32 . In one or more embodiments, the mandrel  36  may expand or otherwise provide a force that counters the force of a friction stir joining tool that is pressing on the pipes  30  at the joint during friction stir joining processing. 
         [0050]    For  FIGS. 5A through 15 , the pipes  30 , the pipe ends  40  and the mandrel  36  are the same, while the face profile  42  may be modified. Accordingly, only the changes to the face profile will be labeled and numbered. 
         [0051]      FIGS. 5A and 5B  show a perspective cut-away view of an embodiment of face profile  42 , where the pipes  30  may be machined to provide a thread or groove profile  46 . The thread or groove profile  46  may enable the pipes  30  to more precisely align and avoid any offset. 
         [0052]      FIGS. 6A and 6B  show a perspective cut-away view of another embodiment where the face profile  42  of the pipes  30  may be machined to include a chamfer  48 , taper or bevel at the root  50  or ID. 
         [0053]      FIGS. 7A and 7B  show a perspective cut-away view of another embodiment where the face profile  42  of the pipes  30  may be machined to include a chamfer  48 , taper or bevel at the root  50  of the pipes and at the OD corner  52  of the pipes. 
         [0054]      FIGS. 8A and 8B  show a perspective cut-away view of another embodiment where the face profile  42  of the pipes  30  may be machined to have a curved profile  54 . The curved profiles  54  of the two pipes  30  may be complementary, thereby enabling precise alignment of the pipes. 
         [0055]      FIGS. 9A and 9B  show a perspective cut-away view of another embodiment where the face profile  42  of the pipes  30  may be machined to have a profile that combines different profiles on each of the pipes. The face profiles  42  may not necessarily be complimentary to each other. For example, in this embodiment, a first face profile  42  includes a chamfer  48 , bevel or taper at the root  50 , while the second face profile  42  includes an end profile including a groove  58  that does not extend to the ID (root)  50  or OD corner  52 . Grooves in this or other embodiments disclosed herein may be continuous or interrupted. Any combination of face profiles  42  may be provided on the profiles of the pipes  30 , as long as the profiles do not prevent precise alignment of the pipes. 
         [0056]      FIGS. 10A and 10B  show a perspective cut-away view of another embodiment where the mating surface  42  of the pipes  30  may be machined to include a partial thread  60 , groove or other profile, extending a selected distance from the root  50  of the pipes  30 . 
         [0057]      FIGS. 11A and 11B  show a perspective view of another embodiment where the face profile  42  of the pipes  30  may be machined to include single profiles  62  (e.g., grooves) located interior of the ID (root)  50  and OD corner  52  and aligned with each other. In this and other embodiments, the face profile  42  may have multiple different profiles  62  which may or may not be aligned with each other, and which may or may not extend to the root  50  or the OD corner  52 , and do not prevent pipe alignment. 
         [0058]      FIGS. 12A and 12B  show a perspective view of another embodiment where the face profile  42  of the pipes  30  may be machined to include single profiles (e.g., grooves)  62  at the ID (root)  50 . 
         [0059]      FIGS. 13A and 13B  show a perspective view of another embodiment where the face profiles  42  of the pipes  30  do not include profiles, but the mandrel  36  may be machined to include a profile that may alter flow of the pipe material. For example, a dimple  64  is shown in the mandrel  36 . In additional embodiments, one or both face profiles  42  of the pipes  30  may have a profile machined thereon. 
         [0060]      FIGS. 14A and 14B  show a perspective cut-away view of another embodiment where the face profiles  42  and the mandrel  36  may be machined and configured to allow a filler material  66  to be joined to the pipes  30  during friction stir joining to thereby alter mechanical flow, and/or temperature and/or mechanical properties of the pipe joint  32 . In this figure, the filler material may be disposed as a ring at the root  50  of the pipes  30 . The filler material may be pushed farther up the pipe joint  32 . 
         [0061]      FIG. 15  is a perspective view of another embodiment that shows a filler material  68  disposed between the face profiles  42 . The filler material  68  may have the same face profile  42  as those mentioned above for the pipe ends  40 , or it may something different such as a fusion weld bead. The filler material  68  may have a larger OD than the pipe so that it functions as rough stock that can be removed or for strengthening the pipe joint  32 . 
         [0062]    The filler material  68  is not required but is an optional component that may be selected in some embodiments for enhancing corrosion resistance properties of the pipe joint  32 , improving pipe joint strength, providing material for friction stir joining, standing proud of the two curved pipe surfaces, and/or enabling conventional welding or tacking of the pipe joint before friction stir joining. 
         [0063]      FIG. 16A  is a perspective view of another embodiment that shows filler material  80  that may be disposed between the face profiles  42 . The filler material  80  includes a rounded head  82  that may fit above the OD of the pipes  30  and a rounded head  84  that may fit below the ID of the pipes. The filler material  80  may have a larger OD than the pipe so that it functions as rough stock that can be removed or for strengthening the pipe joint  32 . It should also be understood that the rounded head may be replaced by another shape. What is important is that additional material is found on the filler material  80  both above the OD of the pipes  30 , and below the ID of the pipes. 
         [0064]      FIG. 16B  is an alternative embodiment of  FIG. 16A , where the filler material  80  may only have the rounded head  82  above the OD of the pipes  30 . 
         [0065]      FIG. 16C  is an alternative embodiment of  FIG. 16A , where the filler material  80  may only have the rounded head  82  below the ID of the pipes  30 . 
         [0066]    Once the face profile  42  is complete on the pipe ends  40 , any oxides present may be removed. Oxides may be removed from the end surface(s) of the pipes to be joined, as well as the surface of the mandrel  36  if a mandrel is being used, and any other surface that is exposed to and therefore may affect the friction stir joining process. In working environments where there is high humidity, careful attention should be paid to assure oxide does not reform on surfaces before initiating the friction stir joining process. If any oxide does reform, it may be removed just before joining. Oxide may be removed by mechanical abrasion such as sanding, grit blasting, etc. Oxide may also be removed by oxide reducing materials which include liquids and jells. 
         [0067]    When a mandrel  36  is being used, the mandrel may be positioned to align the pipes  30  and position the pipe faces together for friction stir joining. Once positioned, the mandrel  36  may be expanded into position against the inside diameter of the pipes  30 . 
         [0068]    The friction stir joining process may be performed with or without a shielding gas. Possible shield gases that may be used include argon and other inert gases that inhibit corrosion or explosions. The friction stir joining process is well known to those skilled in the art. Tool geometries, offset tool position, traverse speed and other parameters may be set and maintained for desired mechanical properties of the joint. 
         [0069]    Another aspect of this and other embodiments may be the use of a stationary shoulder and a rotating pin on a curved surface. 
         [0070]      FIG. 17  is a perspective view of a stationary shoulder tool configuration. This configuration may or may not use a mandrel. The configuration shown in  FIG. 16  allows for the pin of the friction stir joining tool to be retracted during friction stir joining to thereby avoid using a run-off tab. 
         [0071]    The stationary shoulder friction stir joining tool  72  may be used in a manner such that it is not normal to the pipes  30 . The stationary shoulder friction stir joining tool  72  may be operated such that it may rotate at greater than 10 revolutions per minute, it may have a Z-axis load on the pin that may be greater than 10 lbf, it may have a clearance between the pin and the stationary shoulder  74  that may be greater than 0.0001 inches, and it may provide a channel for the stationary shoulder around the pin for flash control. 
         [0072]    Liquid cooling may be provided to the pin and/or the stationary shoulder  74 , or cooling may be used that includes using a heat transfer material, radiative cooling, conductive cooling, and/or convective cooling. 
         [0073]    The friction stir joining process may benefit from making the stationary shoulder friction stir joining tool  72  or the friction stir joining tool  34  traverse a path that is non-linear along the pipe joint  32 . These non-linear paths include an arc path, a helical path, an elliptical path, a sinusoidal path and an oval path. 
         [0074]    Post joining processes may be performed such as run-off tab removal, flash removal and/or post weld heat treatment in order to alter the mechanical properties of the pipe joint  32  after friction stir joining processing. 
         [0075]    In another embodiment, a first pipe includes rough stock material, and a second pipe does not. However, both the first pipe and the second pipe may still be precision machine processed. For example, a face profile of the second pipe may be precision machine processed in order to have a face profile that is complimentary to the face profile of the first pipe. 
         [0076]    Similarly, in another embodiment, neither the first pipe nor the second pipe includes rough stock material. However, both the first pipe and the second pipe may still be precision machine processed in order to have face profiles that are complementary. 
         [0077]    There are many configurations of the embodiments described above that may be used independently or jointly to enhance the capability and consistency of the friction stir joining process. 
         [0078]    Although a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.