Abstract:
A friction stir tool is provided to perform friction stir riveting using a partially consumable pin, wherein the pin includes a cutting edge on a bottom surface thereof, wherein the tool is rotated at a first speed to enable cutting by the pin into a first material that is overlapping a second material, wherein after the pin has cut to a sufficient depth, the rotational speed of the tool is increased to thereby enable plasticization of the consumable pin, the first material, and the second material, wherein the tool is then rapidly decelerated until stopped, enabling diffusion bonding between the pin, the first material and the second material.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to and incorporates by reference all of the subject matter of U.S. Patent Application Ser. No. 60/804,628, filed Jun. 13, 2006, and of U.S. Patent Application Ser. No. 60/816,396, filed Jun. 23, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to friction stir joining methods. More specifically, the present invention is a method of joining metal work pieces together using a consumable friction stir tool that has at least a partially consumable pin, wherein the pin has a cutting edge that cuts through a first work piece material when rotated at a first speed. After at least cutting through the first work piece material to a sufficient depth, the rotational speed of the tool is changed to cause plasticization of the pin itself, as well as the first and second work piece materials being joined. After sufficient heating of the first and second work piece materials and the pin, the rotation of the tool is rapidly decelerated or stopped completely to enable the bonding of the pin and the first and second work piece materials. This process will be referred to throughout this document as friction stir riveting. 
         [0004]    2. Description of Related Art 
         [0005]    There are many methods for joining metal work pieces together; some of which include welding, spot welding, fasteners (such as screws and bolts), friction stir welding, etc. The three fundamental principles that govern all joining methods include mechanical attachment, fusion joining (welding), and solid state joining (friction welding). Each principle technique has advantages; however the method often selected for an application is dictated by the one having the fewest tolerable disadvantages. 
         [0006]    Examples of mechanical work piece joining methods include screws, nuts and bolts, dove tail, swaging, riveting, interference attachment, etc. Many applications cannot use screws or bolts because the threads have limiting load carrying capability, the high cost of multiple components and assembly, the cost of the hole that must be placed in the work pieces and/or the space required for the fasteners. Dove tail and other work piece locking methods lock in specified directions but can slide or rotate apart in other directions. Rivets have perhaps the greatest joining strength per unit area and volume of any mechanical fastener but the mechanical deformation of the rivet head reduces the energy absorbing capability as well as elongation. 
         [0007]    When mechanical methods are not acceptable joining techniques, fusion welding methods are utilized unless the work pieces are not considered weldable. For example, aircraft components made from 7000 series aluminum are not considered weldable because the resulting weld strength is as low as 50% of base metal properties. High melting temperature materials (HMTM) such as steel, stainless steel and nickel base alloys can be welded but the joint strength is limited to problems associated with fusion welding. These problems include, but are not limited to, solidification defects, hard/soft zones within the weld macrostructure, residual stresses resulting from liquid to solid phase transformation, porosity, cracking, non-uniform and unpredictable microstructures, corrosion susceptibility, work piece deformation, and loss of work piece base material properties. Post weld operations are often needed to repair distortion or evaluate the weld nondestructively and add cost to the process. In addition, there are health issues related to hexavalent chromium and manganese exposure as well as potential retina damage to the operator if proper safety procedures are not followed. In many cases, work pieces must be increased in size to use a base material of lower strength that is considered weldable in favor of a higher strength material that is not considered weldable. This is the case with automobile car bodies that are currently manufactured from lower strength steels. Advanced high strength steels (Dual Phase and TRIP steels) could be used in the frame construction to dramatically lower vehicle weight but these materials have not been used because of fusion weldability issues. 
         [0008]    Friction stir welding (FSW) is a solid state welding process that has many advantages over fusion welding methods.  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 work piece  16  until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized work piece material. The work piece  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 work piece  16  at the joint line  18 . Although this tool has been disclosed in the prior art, it will be explained that the tool can be used for a new purpose. It is also noted that the terms “work piece” and “base material” will be used interchangeably throughout this document. 
         [0009]    The frictional heat caused by rotational motion of the pin  14  against the work piece material  16  causes the work piece 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 work piece material  16  itself, in comparison to other welds. 
         [0010]    It is observed that when the shoulder  12  contacts the surface of the work pieces, 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 . 
         [0011]    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, re-crystallized, fine grain microstructure formed in the area of the weld. 
         [0012]    Travel speeds are typically 10 to 500 mm/min with rotation rates of 200 to 2000 rpm. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material&#39;s thermal properties, high temperature flow stress and penetration depth. 
         [0013]    Previous patents by some of the inventors such as U.S. Pat. Nos. 6,648,206 and 6,779,704 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. 
         [0014]    It is also noted that the phrase “friction stir processing” may also be referred to interchangeably with solid state processing. Solid state processing is defined herein as a temporary transformation into a plasticized state that typically does not include a liquid phase. However, it is noted that some embodiments allow one or more elements to pass through a liquid phase, and still obtain the benefits of the present invention. 
         [0015]    In friction stir processing, a tool pin is rotated and plunged into the material to be processed. The tool is moved transversely across a processing area of the material. It is the act of causing the material to undergo plasticization in a solid state process that can result in the material being modified to have properties that are different from the original material. 
         [0016]    The main disadvantage with FSW is a remaining hole left in the work pieces at the end of the weld. In many cases this is not a problem since a run off tab can be used at the end of the weld and later removed. A retractable pin can be used as the weld progresses to eliminate the end hole; however the tool and equipment requirements are extensive and costly. Tool geometries that allow the tool to be extracted from the weld gradually during FSW can also be used but the added process time combined with the added heat cycle over an existing weld increases cost and decreases base metal properties. 
         [0017]    Friction stir spot welding (FSSW) is now being used experimentally to join advanced high strength steels in lap welding configurations. FSSW is being used commercially to lap weld aluminum components as described in US Patent application 20050178817. Two approaches are currently used. 
         [0018]    The first approach involves plunging a pin tool (a FSSW tool comprised of a pin and a shoulder) into work pieces until the work pieces are spot friction welded together. The disadvantage with this method is the hole  26  left behind from the pin as shown in  FIG. 2 . The bond between the work pieces  28  is achieved under the shoulder of the tool while the pin hole reduces the strength of the weld. 
         [0019]    A second method involves the design of equipment to force material back into the pin hole (U.S. Pat. No. 6,722,556). This method is quite cumbersome because of the large spindle head, fixturing requirements, and loads needed to make a spot weld. 
         [0020]    It would be an advantage over the state of the art in the joining of metal work pieces to be able to provide and system and method that uses a partially consumable tool to perform FSSW using a rivet in a rapid and economical manner. 
       BRIEF SUMMARY OF THE INVENTION 
       [0021]    In a first aspect of the invention, a friction stir tool is provided to perform friction stir riveting using a at least a partially consumable pin, wherein the pin includes a cutting edge on a bottom surface thereof, wherein the tool is rotated at a first speed to enable cutting by the pin into a first material that is overlapping a second material, wherein after the pin has cut to a sufficient depth, the rotational speed of the tool is increased to thereby enable plasticization of the consumable pin, the first material, and the second material, wherein the tool is then rapidly decelerated until stopped, enabling diffusion bonding between the pin, the first material and the second material. 
         [0022]    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 
         [0023]      FIG. 1  is a prior art perspective view of an existing friction stir welding tool capable of performing friction stir welding on high melting temperature materials. 
           [0024]      FIG. 2  is a profile view of three welds performed using friction stir spot welding (FSSW) as done in the prior art. 
           [0025]      FIG. 3  is a perspective view of a rotational tool that is constructed in accordance with the principles of one embodiment of the present invention that can perform fiction stir riveting. 
           [0026]      FIG. 4  is a profile view of the tool of  FIG. 3  wherein the consumable pin has fully penetrated two work pieces. 
           [0027]      FIG. 5  is a profile cut-away view of a tool having a central hole for a multi-segmented pin for rapid friction stir riveting. 
           [0028]      FIG. 6  is a perspective view of a multi-segmented consumable pin that is manufactured in accordance with the principles of one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be 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. 
         [0030]    In a first aspect of the invention, a novel approach is used to solve many of the joining problems mentioned above. A rotating friction stir riveting tool having a non-consumable shoulder combined with a detachable and at least partially consumable pin forms the basis of a friction stir riveting joining method of the present invention. The pin may be totally consumable or partially consumable.  FIG. 3  shows an example of how the tool can be constructed. 
         [0031]      FIG. 3  shows a friction stir riveting tool  30  having a shoulder area  32  and a detachable and at least partially consumable pin  34 . In this particular embodiment, the detachable and at least partially consumable pin  34  includes a small gap  36 . The small gap  36  is formed by a much smaller pin diameter portion  42  of the pin  34 . This small pin diameter portion  42  of the pin  34  will be caused to break. The small gap  36  enables the detachable portion  38  of the pin  34  to remain embedded within the work pieces as a rivet. It is also noted that the non-detached portion  40  of the pin  34  might also be the top of another pin segment as will be explained. 
         [0032]    Using  FIG. 4  as an illustration, to friction rivet steel or another metal using a tool of this first embodiment of the present invention, the tool  30  is rotated at a speed that allows the pin  34  of the tool to machine a first work piece material  50  away to form a hole  54  therein. Features can be added to the end of the pin  34  to facilitate machining the desired hole. For example, a cutting feature  44  is shown in this first embodiment. 
         [0033]    It is preferred but not required that the depth  56  of the hole  54  extend completely through the first work piece material  50  and at least partially into the second work piece material  52 . 
         [0034]    It should be understood that depending upon the application, the hole  54  may only extend partially into the first work piece material  50 , completely through the first work piece material but not into the second work piece material  52 , completely through the first work piece material but only partially into the second work piece material, or substantially through both the first and the second work piece materials. One the initial hole  54  has been made, the tool  30  can then have the pin  34  make the desired level of penetration in accordance with understood principles of friction stir riveting. The pin  34  may extend completely through both the first and second work piece materials  50 ,  52 , or it may extend completely through the first work piece material but only partially into the second work piece material. Again, this depends upon the application of the user. 
         [0035]    In this first embodiment, once the depth  56  of the hole  54  has extended into the second work piece  52  as shown in  FIG. 4 , the rotational speed of the tool  30  is slowed down to generate heat between the pin  30  and the two first and second work pieces  50 ,  52  that are being joined together. A spindle (not shown) that is holding and rotating the tool  30  can either be immediately stopped or slowed down until the torque required to rotate the tool exceeds the shear strength of the smaller pin diameter portion  42 . The smaller pin diameter portion  42  is designed to shear the detachable portion  38  of the pin  34  off of the tool  30  at a specified torque. 
         [0036]    In this first embodiment, once the detachable portion  38  of the pin  34  has been sheared off the tool  30 , the tool is retracted and a new pin  34  can be replaced. The detachable portion  38  of the pin  34  or rivet left behind in the first and second work piece materials  50 ,  52  is friction welded into the work pieces. There is a bond not only under the tool shoulder between the first and second work pieces  50 ,  52  but around the pin  34  or rivet. 
         [0037]    In an alternative embodiment of the present invention as shown in  FIG. 5 , a tool  60  has a hole  62  disposed through a central axis. The hole  62  allows a multi-segmented pin  64  (shown here with three segments separated by a smaller diameter pin portion  72 ) to be inserted and pushed through the hole  62  as needed. The multi-segmented pin  64  includes a plurality of gaps  66  having a smaller diameter pin portion  72 . Some type of plunger mechanism  68  would then be used to push the multi-segmented pin  64  through the tool  60  and out a working end  70 . As each segment of the multi-segmented pin  64  is broken off, the plunger mechanism  68  pushes the multi-segmented pin down through the hole  62  until enough of the pin  64  is exposed for the next friction stir riveting process. In this way, multiple rivets can be inserted into work pieces without having to stop and reload a multi-segmented pin  64 . 
         [0038]    The number of segments that can be used in a multi-segmented pin  64  should not be considered to be limited to three.  FIG. 5  is for illustration purposes only. More segments can be disposed on the multi-segmented pin  64 . The number of segments may also depend on the length of the tool  60  and the length of the plunger mechanism  68 . 
         [0039]      FIG. 6  is provided to illustrate a multi-segment pin  64  that can be used for an automatic and rapid friction stir riveting process. The segments of the multi-segment pin  64  are co-axial so that they can be disposed in the hole through the central axis of the friction stir riveting tool  60 . 
         [0040]    The materials used to create a tool having a shoulder that can be used in the present invention can be found from tools created by some of the inventors 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. 
         [0041]    This technology involves using a special friction stir welding tool. The shoulder can be created using materials such as polycrystalline cubic boron nitride (PCBN) and polycrystalline diamond (PCD). Other materials that can be included are refractories such as tungsten, rhenium, iridium, titanium, molybdenum, etc. 
         [0042]    The work pieces that can be joined using the principles of the present invention include materials that have melting temperatures higher than bronze and aluminum. This class of materials includes, but is not limited to, metal matrix composites, ferrous alloys such as steel and stainless steel, non-ferrous materials, superalloys, titanium, cobalt alloys typically used for hard-facing, and air hardened or high speed steels. However, the present invention can also be used on materials that may be considered to be all other lower melting temperature materials that are not included within the definition of the higher melting temperatures described above. 
         [0043]    The shoulder  32  of the tool  30  can be made from polycrystalline cubic boron nitride or similarly described materials that can prevent adhesion of the shoulder to the first work piece  50  and provide superior thermal stability and wear resistance characteristics. Several shoulder configurations can be used to form the shape of the rivet head or even cut away the rivet head after the pin  34  has been friction welded into the work pieces  50 ,  52 . 
         [0044]    The materials used for the pin  34  are generally going to be those that can consumed during the friction stir riveting process. Such materials will preferably enhance the bond between the first and second work piece materials, and are known to those skilled in the art of friction stir welding. 
         [0045]    Alternative embodiments of the present invention include various aspects that should also be considered as important elements. First, a variety of cutting structures or profiles can be used on the end, of the pin  34  that will be inserted as a rivet. A helically notched profile could be used as an alternate cutting structure instead of the feature shown in  FIG. 3 . 
         [0046]    In another alternative embodiment, inert gas such as argon or carbon dioxide can be caused to flow through the center of the tool  30  to prevent oxidation during friction stir riveting. 
         [0047]    In another alternative embodiment, more than two work pieces might be joined using the friction stir riveting process of the present invention. The length of the segments of the pin  34  would therefore be adjusted according. 
         [0048]    In another alternative embodiment, it should be noted that the work pieces that are being joined can be the same or different materials, depending upon the application. 
         [0049]    Similarly, the material used in the pin might be a different material from the work pieces, the same material as at least one of the work pieces, or the same as the material on all the work pieces. 
         [0050]    Pin profiles can be varied greatly. The pin profile can be a taper, hexagonal, or any desired shape that will perform a cutting process and friction stir riveting process. The shape will likely depend on various aspects, such as the desired bonding characteristics or the strength of the various materials being used. 
         [0051]    In another embodiment, the pin could also be hollow. The pin could be in rod or wire form and fed automatically through the center of the tool. When a square shape is used for the pin, this allows for torque from the tool to be transmitted to the pin or rivet. However, other torque transmitting profiles could be used. Even a round shape could be used for the pin as long as a clamping force or clamping mechanism on the outside diameter of the pin material is sufficient to keep the pin from slipping within the tool when rotational forces are applied. 
         [0052]    The pin or rivet can have a variety of hardnesses or hardness profiles to facilitate work piece penetration. 
         [0053]    The tool can run to a specified position or load value at RPMs ranging from 1 to 10,000 RPM. 
         [0054]    The tool could be run in the same configuration as fusion spot welding. For example, rather than using clamping with welding tips in a C clamp configuration, a small diameter rotating tool ( FIG. 3 ) could be placed in a C clamp on the end of a robot. The C clamp configuration could also be used manually. 
         [0055]    The pin can have a fastener on the “head” so mechanical attachment can be used at that location. For example, the end of a friction rivet can have a threaded stub that is left to protrude above the work pieces after they have been joined. A nut could then be used to attach another component to the work pieces. 
         [0056]    Some of the advantages of the friction stir riveting process include, but should not be considered limited to, a solid state joining process that is rapid, low energy input process requirements, low residual stresses because of the solid state process, no predrilled hole is necessary as in conventional riveting, there is reduced or eliminated distortion of the work pieces, no hole is left in the work pieces as in FSSW, the process can be used in confined areas, Z-axis forces are comparable to current forces required to resistance spot weld, the shoulder/pin ratio can be sized to generate a specific heat profile to optimize joint strength, corrosion resistant pin materials can be used, because the process is completed at an elevated temperature the formation of the pin or rivet has not yielded and will have greater energy absorption characteristics, the pin or rivet material can be overmatched to the work piece material for greater strength, and the rivet or pin can be used at the tip of a crack to prevent further crack propagation in a work piece. 
         [0057]    It is generally the case that the pin will be made using a material that is harder than the materials being joined. However, the pin might be softer, but pushed with sufficient force and quickly enough; it can be used to join the harder work piece materials. 
         [0058]    Another aspect of the invention is the option of removing the material being cut from the hole in the work pieces and being formed by the pin. One method of removing the material is to use a pecking motion. A pecking motion of the tool can also be combined with a fluid flow to remove the material. The fluid can be compressible or non-compressible, including gas, air, mist, and water. 
         [0059]    As previously mentioned the present invention can be used to join different materials together, and is not limited to three body (two work pieces and a pin) configurations. Multiple layers of materials can be joined simultaneously. Any number of materials can be bonded so long as the materials are subjected to a temperature gradient that is less than the melting temperature of the materials being bonded. 
         [0060]    The range of surface travel speeds of the tool should be considered to be from 0.1 mm per minute to 10 meters per minute. The rotational speed of the tool can vary from 1 rpm to 100,000 rpm. 
         [0061]    Coatings can be used on the tool, on the work pieces being joined, or on both the tool and the workpieces. 
         [0062]    The tool of the present invention can be a composite tool, such as a tool having a CBN shoulder, or different materials having a higher or lower modulus than the materials being bonded. 
         [0063]    The hardness of the materials being bonded should be considered to include all materials on the Rockwell Scales A, B and C. 
         [0064]    The cutting edge on the pin of the present invention can have any suitable cutting geometry. Thus, any feature can be included on the pin that enables cutting, cutting and heating, and heating with the intent of causing a bond. The pin may also be threaded. Thus, the pin does not have to have a cutting geometry. An alternative embodiment uses heating of the pin to enable creation of a hole or an aperture in or through other work piece materials. 
         [0065]    The present invention enables diffusion bonding on multiple planes, include axially and the sides of the hole that is created. 
         [0066]    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.