Abstract:
A mandrel that provides a counter-force to the pressure exerted on the outside of a pipe or other arcuate surface by a friction stir welding tool, wherein the mandrel is expandable through the use of a wedge, and wherein the mandrel enables multiple friction stir welding heads to simultaneously perform welding on the arcuate surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to friction stir welding. More specifically, the present invention addresses improvements in the ability to perform friction stir welding of pipe or other arcuate objects, wherein a mandrel is needed to provide a counter-balancing force against the inside of the arcuate surface being welded, to thereby prevent a friction stir welding tool in contact with the outside of the arcuate surface from damaging the workpiece being welded. 
     2. Description of Related Art 
     Friction stir welding (hereinafter “FSW”) is a technology that has been developed for welding metals and metal alloys. The FSW process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin or spindle. Force is exerted to urge the spindle and the workpieces together and frictional heating caused by the interaction between the spindle and the workpieces results in plasticization of the material on either side of the joint. The spindle is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing spindle cools to form a weld. 
       FIG. 1  is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool  10  having 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 workpiece material. The workpiece  16  is often two sheets or plates of material that are butted together at a joint line  18 . The pin  14  is plunged into the workpiece  16  at the joint line  18 . 
     The frictional heat caused by rotational motion of the pin  14  against the 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. The result is a solid phase bond  20  at the joint line  18  that may be generally indistinguishable from the workpiece material  16  itself, in comparison to other welds. 
     It is observed that when the shoulder  12  contacts the surface of the 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 . 
     During FSW, the area to be welded and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint. The rotating FSW tool 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 face of the pin to its trailing edge. As the weld zone cools, there is typically no solidification as no liquid is created as the tool passes. It is often the case, but not always, that the resulting weld is a defect-free, recrystallized, fine grain microstructure formed in the area of the weld. 
     Previous patent documents have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, metal matrix composites, ferrous alloys such as steel and stainless steel, and non-ferrous materials. Another class of materials that were also able to take advantage of friction stir welding is the superalloys. Superalloys can be materials having a higher melting temperature bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium. 
     It is noted that titanium is also a desirable material to friction stir weld. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials. 
     The previous patents teach that a tool is needed that is formed using a material that has a higher melting temperature than the material being friction stir welded. In some embodiments, a superabrasive was used in the tool. 
     The embodiments of the present invention are generally concerned with these functionally unweldable materials, as well as the superalloys, and are hereinafter referred to as “high melting temperature” materials throughout this document. 
     Recent advancements in friction stir welding (FSW) technologies have resulted in tools that can be used to join high melting temperature materials such as steel and stainless steel together during the solid state joining processes of friction stir welding. 
     As explained previously, this technology involves using a special friction stir welding tool.  FIG. 2  shows a polycrystalline cubic boron nitride (PCBN) tip  30 , a locking collar  32 , a thermocouple set screw  34  to prevent movement, and a shank  36 . Other designs of this tool are also shown in the prior art of the inventors, and include monolithic tools and other designs. 
     When this special friction stir welding tool is used, it is effective at friction stir welding of various materials. This tool design is also effective when using a variety of tool tip materials besides PCBN and PCD (polycrystalline diamond). Some of these materials include refractories such as tungsten, rhenium, iridium, titanium, molybdenum, etc. 
     The inventors have been the leader in developing friction stir welding technology for use with high melting temperature alloys such as steel, stainless steel, nickel base alloys, and many other alloys. This technology often requires the use of a Polycrystalline cubic boron nitride tool, a liquid cooled tool holder, a temperature acquisition system, and the proper equipment to have a controlled friction stir welding process. 
     Once the technology had been established (current literature indicates the state of the technology) as a superior method for joining these materials, MegaDiamond and Advanced Metal Products (working together as MegaStir Technologies) began searching for applications that would greatly benefit from this technology. One of the largest applications for friction stir welding (FSW) is joining pipe lines. Joining pipe line is extremely costly because of the manpower and equipment needed to weld and move needed components.  FIG. 3  shows the manpower and equipment needed to fusion weld a typical pipeline. The pipe  40  is shown with a plurality of welding stations  42  (each of the white enclosures) that are needed to lay down progressive layers of welding wire to create a fusion welded joint between segments of pipe. 
     Advanced high strength steels (AHSS) are being implemented into pipe lines because less material is needed, higher strength properties are obtained and the total pipeline cost can be lower. The difficulty with AHSS lies in the conventional fusion welding methods being used. It is accepted in the industry that every pipe line joint contains a defect or crack. These defects are accepted because they cannot be eliminated even with sophisticated automated fusion welding systems. Welding AHSS is far more difficult than existing pipe line steels because the material composition inherently causes more fusion welding defects. 
     FSW has now been established as a viable technology to join pipe segments. A friction stir welding machine  50  to join pipe segments has been developed as shown in  FIG. 4 . A rotating tool plunges into a joint as it creates frictional heat. Once the tool has plunged into the workpiece cross section, the tool is caused to travel circumferentially around the pipes while the joint is “stirred” together. The FSW tool is then retracted and the machine  50  is moved along the pipe to the next pipe joint to be friction stir welded. 
     The friction stir welding machine  50  shown in  FIG. 4  illustrates the machine that operates on the exterior of the pipe being welded. One of the requirements of FSW in any form is to have a counter-balancing force on the back side (opposite the tool) of the workpiece being joined. This need arises from the large forces that are applied by the tool against the workpiece. The nature of friction stir welding requires that some support be provided to prevent the workpiece from bending or otherwise being damaged.  FIG. 5  shows the current design of a rotating mandrel  60  or “pipe pig” that is currently being used when a friction stir welding pipe. 
     The mandrel  60  is hydraulically actuated to follow the tool path on the inside of the pipe as the tool follows circumferentially around the pipe joint on the exterior. When the pipe joint is complete, the mandrel  60  is reconfigured so that it can be moved to the next pipe joint. While this mandrel  60  is an effective means to provide support on the opposite side of the tool, the hydraulics and controls are expensive and the construction of the pipe is therefore also costly. A mandrel  60  for FSW of a 12 inch pipe diameter using this design also weighs about 800 lb. This means that moving the mandrel requires additional equipment and support. A further disadvantage is that this mandrel configuration must also have additional hydraulics and rams added to align two pipe segments, further adding to the weight of the mandrel  60 . While this design is workable in the field, it would be preferable to have a lighter weight and lower cost mandrel design that can add to the speed and reduce the cost of FSW of a pipeline. 
     Accordingly, what is needed is a less expensive, less complex, and lightweight pipe pig that can be more easily deployed on-site. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an expandable mandrel that is less complex than those used in the prior art. 
     It is another object to provide an expandable mandrel that is lighter in weight and therefore easier to use than those used in the prior art. 
     It is another object to provide an expandable mandrel that can easily move along a length of a pipe in order to reposition itself for use in subsequent friction stir welding operations on-site. 
     The present invention is a mandrel that provides a counter-balancing force to the pressure exerted on the outside of a pipe or other arcuate surface by a friction stir welding tool, wherein the mandrel is expandable through the use of a wedge, and wherein the mandrel enables multiple friction stir welding heads to simultaneously perform welding on the arcuate surface. 
     These and other objects, features, advantages and alternative aspects of the present invention 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 
         FIG. 1  is a perspective view of a tool as taught in the prior art for friction stir welding. 
         FIG. 2  is a perspective view of a removable polycrystalline cubic boron nitride (PCBN) tip, a locking collar and a shank. 
         FIG. 3  is a perspective view of a plurality of welding stations that are needed to lay down progressive layers of welding wire to create a fusion welded joint between segments of pipe in the prior art. 
         FIG. 4  is a perspective view of a friction stir welding machine that is capable of joining pipe segments. 
         FIG. 5  is a perspective view of a current design of a rotating mandrel “pipe pig” currently being used when friction stir welding pipe. 
         FIG. 6  is a perspective view of a mandrel shell. 
         FIG. 7  is a perspective view of a mandrel shell having attached lips for expanding a gap. 
         FIG. 8  is a perspective view of a mandrel shell showing the means for expanding the gap in the mandrel shell. 
         FIG. 9  is a cross-sectional perspective view of a mandrel shell and the means for expanding the gap in the mandrel shell. 
         FIG. 10  is a cross-sectional perspective view of a mandrel shell and the means for expanding the gap in the mandrel shell, disposed inside a pipe. 
         FIG. 11  is an end view of a mandrel shell showing a system of cable and pins for closing the gap when the expanding wedge is retracted. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the details of the invention in which the various elements of the present invention will be described and discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. 
     The presently preferred embodiment of the invention is an expandable mandrel for use in friction stir welding operations on arcuate surfaces such as pipe. An expandable mandrel concept was developed that proved to be simple, light weight, and inexpensive. The construction of the mandrel is shown in the following steps. 
       FIG. 6  shows a first embodiment of a mandrel shell  72  that forms an outer shell of the mandrel or “pipe pig”  70  of the present invention. The mandrel shell  72  is a hollow cylinder having an opening or gap  74  along the length thereof. The diameter of the mandrel shell  72  is selected so that the mandrel shell will slide inside of the pipe segments (not shown) that are to be welded, when the gap  74  is allowed to close. 
       FIG. 6  also illustrates relief cuts  76  that are made on the inside diameter  78  of the mandrel shell  72  so that the mandrel shell can spring and flex at the locations of the relief cuts  76 . Once the mandrel shell  72  is machined, lips  80  are welded into place on the inside diameter  78  of the mandrel shell  72  immediately adjacent to the gap  74  as shown in  FIG. 7 . 
     Once the lips  80  have been welded into place, the mandrel shell  72  is further modified so that the gap  74  is naturally in a closed position when there is no external force being applied to the mandrel shell. This closing of the gap  74  is accomplished by running a fusion weld bead, as is known to those skilled in the art, parallel to the length of the relief cuts  76 , and in equiangular positions relative to each other. In other words, enough weld beads are disposed on the inside of the mandrel shell  72  in uniform locations to distort the mandrel shell so that the gap  74  is closed as a result of the residual stresses caused by the solidifying weld beads. Thus, the mandrel shell  72  now springs back to a closed position if the gap  74  is forced apart. 
     In  FIG. 8 , the next component of the pipe pig  70  is to provide a mechanism whereby the mandrel shell  72  can be caused to expand and open the gap  74  when needed. Accordingly, an expanding wedge  82  is provided so that it can be inserted between the lips  80  of the mandrel shell  72 . Note that the angle of the expanding wedge  82  that makes contact with the lips  80  is constructed to easily allow the expanding wedge to move upwards into the gap  74 , and thereby cause the gap to continue to widen as long as the expanding wedge can be pushed against the lips  80 . 
     Expansion of the mandrel shell  72  stops when the expanding wedge  82  makes contact with the inside of a pipe, or when the outside diameter of the mandrel shell  72  can no longer expand outwards against the inside diameter of a pipe. 
       FIG. 8  also illustrates a platform or plate  84 , and a plurality of hydraulic cylinders  86  that are disposed on the plate. The hydraulic cylinders  86  push against the plate  84  and the expandable wedge  82  to cause the expandable wedge to move upwards into the gap  74 . It is envisioned that the bottom of the hydraulic cylinders  86  could also be modified so as to fit the inside of the mandrel shell  72 . However, as the mandrel shall is designed to expand and contract, the bottom of the hydraulic cylinders  86  would need to be able to compensate for the shift in shape. 
     It should be noted that a single hydraulic cylinder  86  could be used in place of the plurality of hydraulic cylinders being shown. Furthermore, the length of the mandrel shell  72 , the lips  80 , the expandable wedge  82 , and the plate  84  can all be modified depending upon the required application. Thus, a system that is smaller in length may be useful in applications where the space or length of horizontal sections within a pipe are restricted. 
     Similarly, the length of the components listed above might be expanded to enable multiple tools to simultaneously be used to perform friction stir welding on a pipe while the pipe is supported by the single pipe pig  70 . 
       FIGS. 9 and 10  show how the expanding wedge  82  is positioned to slide outwards through the lips  80  of the mandrel shell  72  if the gap  74  is large enough to accommodate the expanding wedge when the gap is as wide as it can be.  FIG. 9  is a cross-sectional view of the invention that also shows the hydraulic cylinders  86  in cross-section.  FIG. 10  is a cross-sectional view that shows all the elements shown in  FIG. 9 , but with the addition of a pipe  90 . This figure also shows a joint  96  that is the seam between the pipes being friction stir welded. 
       FIG. 10  shows the expanding wedge  82  fully inserted between the lips  80 . When hydraulic pressure is removed from the hydraulic cylinders  86 , the mandrel shell  72  retracts and the mandrel shell springs closed. The mandrel shell  72  can now be moved to a different location within the pipe  90 . The mandrel shell  72  is moved to the next pipe joint where it is expanded. Hydraulic hoses and fittings that lead to the hydraulic cylinders  86  are not shown. However, these hoses and fittings are disposed on an end of the mandrel shell  72  so that they are coupled to the hydraulic cylinders  86 . 
     It is noted that not only does the pipe pig  70  provide the counter-balancing force necessary for friction stir welding of the pipe  90 , but it can also function to further align the segments of the pipe  90   
     It is also noted that the plate  84  that supports the hydraulic cylinders  86  is coupled to the mandrel shell  72  so the expanding wedge  82  can be retracted from the gap  74  instead of lifting the plate. 
     The following are modifications that can be made to the mandrel shell  72  design above that can enhance the operation of the pipe pig  70 . For example, holes can be machined through the mandrel shell  72  so that air can flow through the holes when the mandrel shell is collapsed. This creates an “air bearing” on the bottom of the mandrel shell  72  so that one person can easily slide the pipe pig  70  to the next pipe joint that is to be friction stir welded. 
     Another aspect of the invention is that quick disconnects can be used on the hydraulic hoses that are coupled to the hydraulic cylinders  86  so that the hoses can be quickly disconnected and reconnected when the pipe pig  70  is re-positioned at a next pipe joint. 
     In another aspect of the invention, a variety of materials can be used to construct the mandrel shell  72 . Spring steel could be used to always maintain the relaxed closed position of the mandrel shell  72 . The material must always be in the elastic region and not be easily stress relieved. This way, the mandrel shell  72  will always keep its shape. If the mandrel shell does start to lose its shape and spring outward when in a relaxed position so that the gap  74  is visible, more welding beads can be run along the length of the inside diameter to restore the residual stresses that cause the mandrel shell  72  to close. 
     Another aspect of the invention is that expanding wedges can be made in different sizes to compensate for different tolerances of pipe segments. 
     Another aspect of the invention is that coatings (TiN, TiCN, etc.) can be used on an outer surface of the mandrel shell  72  to thereby prevent the pipe joint from diffusion welding to the mandrel shell during friction stir welding. 
     Another aspect of the invention is that the invention can be used for any diameter pipe. 
     It is noted that a rod is attached (not shown) that feeds hydraulic hoses through the next section of pipe. 
     Another aspect of the present invention has to do with a means for pulling the mandrel shell  72  closed when in a relaxed position. As shown in  FIG. 11 , the expanding wedge  82  can include posts or pins  92  and a cable  94  disposed therebetween. The cable  94  is run around a pin  92  on both lips  80  of the mandrel shell  72 . When the expanding wedge  82  is retracted, the cable  94  performs the function of pulling on the two lips  80  so that they are forced to come together and close the gap  74 . It is anticipated that this system of pins  92  and cable  94  can be disposed on both ends of the mandrel shell  72  if needed. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.