Abstract:
The invention utilizes vibration control, precision manufacturing tolerances for certain components of the system, and techniques for limiting thermal expansion of components, including passive insulation and active cooling techniques, in order to enable high speed tool rotation for Friction Stir Spot Joining of Advanced High Strength Steels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This document claims priority to and incorporates by reference all of the subject matter included in the provisional patent application docket number 4862.MMAT.PR, having Ser. No. 61/385,884. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to tools used for friction stir welding (FSW). Specifically, the invention relates to a system for holding a friction stir spot joining tool that is rotated at a high rate of speed. 
         [0004]    2. Background of the Problems Being Solved 
         [0005]    Friction stir welding 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. 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 can also be a tool without a pin but only a shoulder for processing of another material through FSP. 
         [0006]      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 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 workpiece material. Typically, the pin  14  is plunged into the workpiece  16  until reaching the shoulder  12  which prevents further penetration into the workpiece. The 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 workpiece  16  at the joint line  18 . 
         [0007]    As the tool  10  is rotated, torque is transmitted from the rotating shank  8  to the FSW tip  24 . When the tool  10  is being used on a workpiece that is a high melting temperature material such as steel, the FSW tip  24  is in many situations exposed to temperatures in excess of 1000 degrees C. as it is rotated while traversing steel softened by frictional heating. 
         [0008]    Referring to  FIG. 1 , 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  14  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 contrast to welds using other conventional technologies. It is also possible that the sold phase bond  20  is superior to the original workpiece material  16  because of the mixing that occurs. Furthermore, if the workpiece material is comprised of different materials, the resulting mixed material may also be superior to either of the two original workpiece materials. 
         [0009]    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 rotating tool pin  14 . 
         [0010]    During friction stir welding, the area to be welded and the tool  10  are moved relative to each other such that the tool traverses a desired length of the weld joint. 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 workpiece materials  16 , while transporting metal from the leading edge of the pin  14  to its trailing edge. As a weld zone cools, there is typically no solidification as no liquid is created as the tool  10  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. 
         [0011]    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. 
         [0012]    Previous patent documents have taught that a tool for friction stir welding of high temperature materials is made of a material or materials that have a higher melting temperature than the material being friction stir welded. In some embodiments, a superabrasive was used in the tool, sometimes as a coating. 
         [0013]    Friction Stir Spot Joining This document pertains more specifically to the problems that are unique to friction stir tools that are being rotated at a high rate of speed compared to other conventional friction stir tools. 
         [0014]    Friction Stir Spot Joining (FSSJ) is a solid state joining method typically used to create a lap joint. FSSJ utilizes a rotating tool to plunge into two materials, one material on top of the other, to generate heat and material flow around the tool to allow the two materials to be joined using a diffusion bond. The process steps are to 1) rotate the tool bit, 2) plunge the tool bit through the surface of the top material and into the second material, and 3) retract the tool bit after a period of time. 
         [0015]    The FSSJ spot welding process described above has already been implemented in the automotive industry on a production basis on rear doors of a Mazda RX7®. The manufacturer needed to lower the weight of its premier sports car and was able to do so by making many components out of aluminum. FSSJ was the method of choice for joining the aluminum parts together because resistance welding of aluminum is difficult if not impossible to achieve. Other methods that melt the aluminum components to be joined have the same fatal flaw and prevent aluminum from being joined to it to provide any level of joint strength. This is one of the reasons that commercial aircraft are joined together with rivets instead of being fusion welded. 
         [0016]    With the high cost of energy it is evident that large corporations using energy intensive manufacturing methods need to implement energy saving plans in order to remain competitive. For example, the automotive industry is facing pressure from the public and new government regulations to improve energy efficiency of all vehicles produced. The most significant economic advantage can be achieved by reducing the weight of vehicles so smaller engines and alternate energy systems to power the vehicles can be employed. 
         [0017]    Reducing weight is accomplished through the use of Advanced High Strength Steels (AHSS). For example, by using higher strength steels, thinner steel cross sections can be used to reduce the weight of the car body. The car body or body in white is the heaviest part of the car, except for the engine. 
       STATE OF THE ART 
       [0018]    Aluminum parts are no longer a problem to join together since FSSJ has been implemented and is the best joining method choice available. Unfortunately, aluminum is expensive and its use in the automotive sector is limited to expensive cars that make up only a small percentage of total vehicle sales. 
         [0019]    AHSS make up the majority of newer materials being implemented in vehicle construction and are made in a range of strengths. The lower the strength, the thicker the cross section must be to carry the load requirements established by the car designer. The higher the strength of the AHSS, cross sections can be thinner to bear the same load requirements. The lower strength AHSS are being joined at this time using resistance spot welding. There continues to be extreme pressure on the automotive industry to further reduce the weight of the vehicle and implement the latest AHSS that have the greatest strength and lowest weight. 
       PROBLEMS WITH EXISTING ART 
       [0020]    The latest AHSS have been developed and are ready to implement into production. However, there is a serious problem surrounding the ability to join these steels. These steels have been strengthened by using special chemistries to maximize the strength in combination with “working” or mechanically strengthening the steel. An example of the result of using this work-hardened material is seen when bending a coat hanger to a ninety degree angle. It is extremely difficult to bend the hangar back to a straight position because the location of the bend has been mechanically strengthened and can no longer return to its original position. 
         [0021]    This work-hardening method is used with the AHSS to elevate the strength of the material. Unfortunately, when the steel is melted during the conventional resistance spot welding process, all of the benefits of the work-hardening process are lost since the material at the spot joint has been melted and re-solidified. In addition, as the spot joint solidifies, it has a lower strength at the solidification point, while the surrounding material is the higher strength work-hardened material. This results in cracking and joint failure at the spot joint. Several attempts have been made on the higher strength AHSS to reduce or eliminate this problem. However, the root cause of failure cannot be eliminated since the resistance spot joint creates a melted and re-solidified joint. 
         [0022]    The advantages of using FSSJ of steels using a Polycrystalline Cubic Boron Nitride (PCBN) tool materials and tip geometries are well known in the industry. Furthermore, FSSJ becomes more feasible by increasing the rotational speed of the tool tip by increasing heat input and reducing axial load requirements of the tip that engages with the materials to be joined together. 
         [0023]    However, there are many practical problems associated with implementing high rotational spindle speed tools into an FSSJ manufacturing operation. Premature PCBN tool failure, spindle bearing failure, and safety issues are problems associated with using high rotational speed bits for FSSJ of AHSS. These problems are due to the vibration that occurs if a high rotational speed component is not properly configured and balanced and held in place. In addition, in many cases the rotating tool tip must be extended far away from the spindle bearings making it even more difficult to properly hold and operate the tip. The cutting tool industry has dealt with the vibration issues. However, the resulting high temperatures, thermal expansion of components and high rotation speeds make it extremely difficult for a practical FSSJ application in any industry to succeed. 
         [0024]    It would be an advantage over the prior art to provide a system for performing FSSJ of AHSS while overcoming the problems of vibration and thermal expansion of components. 
       BRIEF SUMMARY OF THE INVENTION 
       [0025]    In one embodiment of the present invention, a system and method is taught that enables high rotational speeds to be applied from the tool holding machine to the FSSJ tip. The invention utilizes vibration control, precision manufacturing tolerances for components of the system, and techniques for limiting thermal expansion of components, including passive insulation and active cooling techniques, in order to enable high speed tool rotation for Friction Stir Spot Joining of Advanced High Strength Steels. 
         [0026]    These and other aspects, features, advantages 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 
         [0027]      FIG. 1  is a perspective view of a tool as taught in the prior art for friction stir welding. 
           [0028]      FIG. 2  is a cross-sectional profile view of the components of a high speed friction stir spot joining machine and a FSSJ tip being held in the machine. 
           [0029]      FIG. 3  is a close-up view of the shank holder of the present invention. 
           [0030]      FIG. 4  is a first cooling configuration in the extended shank with a first FSSJ tip configuration and an end view showing in and out coolant ports. 
           [0031]      FIG. 5  is a second cooling configuration in the extended shank with a second FSSJ tip configuration and an end view showing in and out coolant ports. 
           [0032]      FIG. 6  shows a first extended shank configuration that has external cooling grooves and/or fins for air cooling. Also shown is a locking flat on the extended shank to prevent extended shank rotation in the shank holder. 
           [0033]      FIG. 7  shows a second extended shank configuration that has external cooling grooves and/or fins for air cooling. Also shown is a locking flat on the shank to prevent extended shank rotation in the shank holder. 
           [0034]      FIG. 8  shows a collet holding system with a collet disposed around the extended shank to prevent extended shank rotation in the holding shank. 
           [0035]      FIG. 9  shows a cross section of the collet holding system of  FIG. 8 . 
           [0036]      FIG. 10  shows another embodiment where ball bearings are used in combination with the collet holding system such that accuracy of extended shank position is maintained during tightening. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Reference will now be made to the drawings in which the various elements of embodiments 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 embodiments. 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. 
         [0038]      FIG. 2  shows the elements of a first embodiment of the present invention. It should be understood that following figures snow a single implementation of the principles of the present invention, and thus the invention should not be considered to be limited to the specific embodiment being shown. This embodiment is for illustration purposes only and can be modified and still be considered to be within the scope of the present invention. 
         [0039]    The components of the present invention are shown in  FIG. 2  and include a FSSJ tip or tool  30 , a set screw  32 , an extended shank  34 , a shank holder  36  and a spindle  38 . Each of these components will now be described in more detail in order to understand the modifications that have been made thereto. 
         [0040]    Between the FSSJ tip  30  and the extended shank  34  is a FSSJ tip cavity  40  (cavity  40  hereinafter) in a working end of the extended shaft. The FSSJ tip cavity  40  surrounding the FSSJ tip  30  is precisely and accurately made as either a cylindrical or a multifaceted configuration. What is important is that the FSSJ tip  30  is unable to move within the cavity  40 . 
         [0041]    The set screw  32  is disposed at an angle through the extended shank  34  and is used to retain the FSSJ tip  30 . The concentricity and accuracy of the FSSJ tip  30  and cavity  40  are critical for preventing vibration and maintaining smooth performance of the FSSJ system  28 . 
         [0042]    Not shown in  FIG. 2  but what should be considered another element of the present invention is a thermal insulating material that can be applied to the inside surface of the cavity  40  either by coating the cavity or through the use of a sleeve between the FSSJ tip  30  and the cavity in order to maintain steady state thermal conditions of the FSSJ tip  30 . 
         [0043]    A significant difference in the first embodiment as compared to the prior art is that the extended shank  34  has a greater length when compared to typical rotating tools used for machining and as such must have extreme accuracy. The extended shank  34  includes at least one internal cooling channel  50  which is completely enclosed within the body of the extended shank to form a closed loop cooling system. The internal cooling channel  50  is necessary to prevent thermal expansion along the axis of the extended shank  34  which would change the length of the FSSJ system  28 . If thermal expansion were not addressed, a repeatable position of the FSSJ tip  30  relative to the work piece could not be established. The cooling channel  50  can be created by making the extended shaft  34  in two parts that are joined together. 
         [0044]    The final component of the FSSJ system  28  shown in  FIG. 2  is the shank holder  36  that attaches to the machine spindle  38 .  FIG. 3  shows a typical standard  30  taper that allows attachment of this shank holder  36  to a standard spindle nose configuration of the machine spindle  38 . Shank holder  36  includes a shank holder cavity  54  (cavity  54  hereinafter) for the extended shank  34 . Also not shown is an adjustment stop located within the shank holder  36  for making adjustments to the position of the extended shank  34 . 
         [0045]    Set screws  52  are shown in the wall of the shank holder  36  to prevent rotation of the extended shank  34  during FSSJ. However other means for preventing rotation of the extended shank  34  can be used such as collets, taper locking mechanisms, etc. without departing from the scope of the present invention. 
         [0046]    Another feature of the FSSJ system is the inclusion of an accelerometer or other vibration measurement device that is coupled to the shank holder  3   b  or to the extended shank  34  to enable vibration measurements. Vibration measurements enable the user or the FSSJ system to stop the FSSJ tip rotation if an unsafe or ill advised vibration mode is detected during the FSSJ process. 
         [0047]    As shown in  FIG. 2 , coolant is ported through channel  50  through the shank holder  36  from the machine spindle  38  in order to have closed loop coolant flow that returns coolant back through the FSSJ system  28 . It is important to note that the coolant can be either a liquid or a gas and that it can be ported from the spindle as shown or from another location such as the outside diameter of the tool holding system. 
         [0048]      FIGS. 4 and 5  are provided as cross-sectional views of the extended shank  34 . These figures show two different cooling configurations in the extended shank  34  as well as two different FSSJ tip configurations. What is important to show is that more cooling channels  50  may be required for some applications than others, and the addition of and the path of the cooling channels is not a limitation of the present invention. 
         [0049]      FIGS. 6 and 7  show two different cross-sectional views of extended shank  34  configurations that have external cooling grooves  60  and/or fins instead of the internal liquid or gas cooling. Also shown is a locking flat  62  on the extended shank  34  to prevent extended shank rotation in the shank holder  36 . 
         [0050]      FIG. 8  is a profile and partial cross-sectional view of an alternative embodiment of the FSSJ system  28  which shows a collet holding system having a collet  70  being used for holding the extended shank  34  to prevent extended shank rotation in the shank holder  36 . 
         [0051]      FIG. 9  is a cross-sectional vies of the FSSJ system  28  that shows the collet  70  shown in  FIG. 8 . This figure shows an example of an M10 bolt  72  that retracts the collet  70  to thereby apply holding pressure on the extended shank  34 . Using a clockwise motion on the M10 bolt  72  adjusts the extended shank  34  to a stop position while tightening the collet  70 . Likewise, counterclockwise loosening of the M10 bolt  72  expels the extended shank  34 . 
         [0052]      FIG. 10  is a cross sectional view of another embodiment of the present invention, which shows where ball bearings  74  are used in combination with the collet  70  such that accuracy of extended shank  34  position is maintained during tightening. Ball bearing support of the tightening mechanism is used to maintain extended shank  34  concentricity during tightening. 
         [0053]    These illustrations are only a few configurations possible within the scope of this invention. It is important to note that the FSSJ tip holding, extended shank design and construction and shank holder that mounts to a machine spindle, are designed and made as a unit to manage load, vibration, and thermal cycles experienced during FSSJ. 
         [0054]    In another aspect of the present invention, it is noted that by following the principles of design and operation, common computer numerical control (CNC) machining centers can be converted into precision FSSJ machines, thus enabling the machine to provide dual capability. 
         [0055]    It should be understood that this also implies that the features of an automatic tool changer would be implemented into the design of the FSSJ system being employed. 
         [0056]    An important aspect of the invention is the application of this FSSJ process in situations where the work pieces to be joined cannot be brought immediately adjacent to the FSSJ tip. In other words, when manufacturing certain items, such as vehicles, the components of a vehicle have to remain on an assembly line. Therefore the FSSJ machine must be manufactured such that it can reach the objects to be joined. 
         [0057]    In this example, robots are often used in vehicle construction. Robots have failed to perform adequate FSSJ because of the reasons cited in the prior art. However, the present invention enables a robot to extend the FSSJ tip a substantially greater distance from the spindle that is causing the rotation of the FSSJ tip. Using the present invention, an FSSJ tip can now be extended further from an FSSJ machine and perform FSSJ of the components of an automobile that are manufactured using AHSS. 
         [0058]    Operation of the FSSJ system  28  includes understanding the typical operating parameters. For example, the design of the FSSJ system  28  is particularly robust in order to support rotation of the FSSJ tip  30  at greater than 4000 RPM. The outside diameter of the FSSJ tip  30  is configured as a cylinder, a cylinder with a flat, or a series of flats that form a square, hexagon, octagon or other deviation from a cylindrical shape. The FSSJ tip  30  configuration may also be tapered from end to end with these same geometric configurations. However, it is preferable that at least one flat on the diameter or outer portion of the FSSJ tip  30  forms a drive flat. At least a portion of the side opposite the FSSJ tip  30  side engaged with the work piece contacts the extended shank  34 . A heat shrink configuration can be used between the extended shank  34  and the FSSJ tip  30 . 
         [0059]    As explained previously, there exists an insulation or thermal barrier between the FSSJ tip  30  and the extended shank  34 . The insulation preferably has but is not limited to a thermal conductivity of greater than 0.000018 W/mK. The clearance gap between the FSSJ tip  30  and the cavity  40  of the extended shank  34  is preferably no greater than 0.001″ (0.025 mm). The run-out or out-of-concentricity is preferably no greater than 0.005″ (0.127 mm). 
         [0060]    For high speed operation of the present invention, is it preferred but not required that the concentricity of the FSSJ tip be within 0.0004″ T.I.R., that the tip diameter be within +/−0.00008, that the extended shank length stay within 150-160 mm, and that cooling be performed because not only is cooling vital for keeping the extended shank axis length stable, but it also prevents spindle heat coming from the spotting operation. 
         [0061]    In another aspect of the invention, the extended shank  34  has internal cooling channels  58  that are closed to the outside environment. The cooling may be a liquid or gas. Features such as fins or grooves  60  or any other non-smooth surface utilize air for cooling. The extended shank  34  is made with a two piece construction to allow for cooling channel  50  construction. The extended shank is preferably made from any metallic or ceramic material as is known to those skilled in the art of high FSW. 
         [0062]    In another aspect of the invention, the extended shank  34  end opposite the FSSJ holding tip end mechanically attaches to a shank holder  36 . The shank holder  36  attaches to a machine spindle  38  having a standard industry taper (30, 35, 40, 45, 50 NMTB or 15) or gas cooling is ported through the shank holder  36  in any longitudinal or transverse attachment of flow pattern. The shank holder  36  has at least one set screw  52  to prevent rotation of the extended shank  34 . The extended shank  34  has at least one flat for set screw engagement. A collet  70  holding method is employed in the shank holder  36  to hold the extended shank  34 . Ball bearings  74  may be used in the shank holder  36  around the tightening mechanism to maintain accuracy. An adjustment stop is located in the shank holder  36  to adjust extended shank  34  position in the shank holder. An accelerometer or other vibration measurement device is coupled with the shank holder  36  or extended shank  34  to measure vibration and stop the FSSJ tip  30  rotation if an unsafe or ill advised vibration mode is created during the FSSJ process. 
         [0063]    In another aspect of the invention, the principles of the FSSJ system described above are applied to and used to modify an existing machining center to convert it to a precision. FSSJ machine. 
         [0064]    In another aspect of the invention, a FSSJ system provides any component of the FSSJ system described above as a tool and has features necessary to be used in an automatic tool changing system. Finally, a FSSJ system is a purpose-built machine for FSSJ with an automatic tool changer. 
         [0065]    In a final aspect of the present invention, it is known in the industry that a robot that is performing spot welding cannot apply sufficient Z force to compress two materials for better bonding. Thus, another aspect of the invention is that the same robots can continue to be used with the present invention. This is possible because a FSSJ tool that is rotating at higher speeds than state of the art FSSJ machines requires a lower Z force to obtain proper bonding. 
         [0066]    It is to be understood that the above-described arrangements and embodiments 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.