Patent Publication Number: US-7895875-B2

Title: Methods and systems for reducing tensile residual stresses in compressed tubing and metal tubing products produced from same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to U.S. Provisional Patent Application No. 60/824,036, filed Aug. 30, 2006, and entitled, “METHODS AND SYSTEMS FOR REDUCING FORMING INDUCED TENSILE RESIDUE STRESS IN COMPRESSED TUBING”, which is incorporated herein by reference. This patent application is also related to PCT Patent Application No. PCT/US07/77270, filed Aug. 30, 2007, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Substituting large diameter, one-piece aluminum drive shafts for conventional two-piece steel constructions has become a popular means of reducing NVH (Noise, Vibration, and Harshness), cost, and weight in passenger car and light truck drive lines. In order to make use of common yokes or end fitting configurations, tubes ends are generally reduced in size via swaging to an inside diameter that appropriately matches the end fittings&#39; mating surface. 
     A conventional system for reducing a tube end using a swaging process is illustrated in  FIGS. 1(   a )- 1 ( c ). Generally, a tube  10  is interconnected to a clamp  20 , which fixedly positions the tube  10  in a predetermined orientation, as illustrated in  FIG. 1(   a ). A push point swage die  30 , having a swaging diameter  32  less than a diameter  12  of the tube  10 , is pushed onto an end of the tube  10 , thereby compressing that portion of tube  10  to the diameter  32  of the swage die  30 , as illustrated in  FIG. 1(   b ). The swage die  30  is then removed from the tube  10  to provide a compressed end portion  14  of the tube  10  as illustrated in  FIG. 1(   c ). The compressed end portion  14  generally has residual tensile stress, such as a tensile residual axial stress T 1  and/or a tensile residual hoop stress T 2 . 
     In metal forming, “spring back” occurs after process defined deformation. The amount of spring back is a function of, among others, the material&#39;s dimensions, yield strength, tooling design, and degree of deformation as plotted against the respective material&#39;s stress strain curve. In the case of swaged drive shaft tubing, the diameter reduction may create tensile hoop and/or axial residual stress states, as illustrated above. In some cases, the degree of diameter reduction can impart a tensile residual stress state in the swaged portion that adversely effects weld quality, design interference fit, and the fatigue performance of the drive shaft. 
     Thermal techniques are known for treating wrought aluminum products having forming induced stress, such as those induced via swaging processes, but such thermal techniques are generally not effective in relieving stress without substantially reducing the mechanical properties of the material. 
     SUMMARY OF THE INVENTION 
     Broadly, the instant invention relates to methods and systems for reducing the stress state of compressed ends of a compressed aluminum tube with restricted or no loss of mechanical strength. The instant methods and systems may provide a reduced tensile residual stress state, or even a compressive residual stress state, after compression, which may improve weld quality while maintaining design interferences and mechanical transfer of torque. 
     In one aspect, aluminum alloy tubes having compressed, but stress-relieved ends are provided. In one approach, an aluminum alloy tube includes a middle portion and an end portion are disclosed. In one embodiment, the end portion comprises a diameter that is smaller than a diameter of the middle portion, and the end portion has a residual hoop stress of less than about 0 ksi. In one embodiment, the aluminum alloy is a series 6061 alloy. In one embodiment, the aluminum alloy tube is suited for use as a drive shaft in an automotive application. In one embodiment, internal surfaces of the stress-relieved end are substantially free of grain profiling. 
     In another aspect, systems for producing tubes having stress-relieved ends are provided. In one approach, a system includes a tube, a swaging die and an expander. In one embodiment, the tube has a first end portion and the swaging die has an inner portion adapted to compress the first end portion of the tube to a compressed end. In this regard, the compressed end has a diameter that is smaller than a diameter of the first end portion. In one embodiment, the expander has a head and a rod rigidly interconnected with the head, and the head is capable of expanding the compressed end to produce a stress-relieved end (e.g., via extraction from the tube). In one embodiment, the outermost diameter of the head is smaller than the diameter of the first end portion. In one embodiment, the outermost diameter of the head is larger than the diameter of the compressed end. Thus, as the head is extracted, a stress-relieved end may be produced, and the stress-relieved end may have a diameter that is slightly larger than a diameter of the compressed end. In one embodiment, a diameter of the stress-relieved end is at least about 0.04% larger than a diameter of the compressed end. 
     In one approach, the stress-relieved end has a residual stress state that is less than the residual stress state of the compressed end. In one embodiment, the stress-relieved end has a residual stress state that is 25% less than the residual stress state of the compressed end. In one embodiment, the compressed end has a tensile residual hoop stress and the stress-relieved end has a compressive residual hoop stress. In one embodiment, the compressed end has a residual hoop stress of greater than 0 ksi. In one embodiment, the stress-relieved end has a residual hoop stress of less than 0 ksi. In one embodiment, the stress-relieved end has a residual hoop stress of less than about −1.0 ksi. 
     The rod may be used to extract the head from the tube. In one embodiment, a portion of the rod extends out of an end of the tube. In one embodiment, the rod is adapted to protrude through a passageway of the swaging die. 
     The produced tubes having at least one stress-relieved end may be used in a variety of applications. In one embodiment, the tube comprising the stress-relieved end is suited for use as a drive shaft in an automotive application. 
     In another aspect, methods of producing tubes are provided. In one approach, a method includes inserting a portion of an expander into a tube, compressing a first end portion of the tube thereby producing a compressed end of the tube, extracting the expander from the tube through the compressed end, and moving, concomitant to the extracting step, internal surfaces of the compressed end via the expander, thereby creating a stress-relieved end of the tube. In one embodiment, the moving internal surfaces step includes at least one of (i) outwardly stretching internal surfaces of the compressed end of the tube via the expander, and (ii) outwardly expanding internal surfaces of the compressed end of the tube via the expander. In one embodiment, the moving internal surfaces step includes moving at least some of the internal surfaces toward a distal end of the tube, the distal end being associated with the first end portion of the tube. In one embodiment, the moving internal surfaces step comprises moving at least some of the internal surfaces away from a center axis of the tube. In one embodiment, the compressed end comprises a first residual stress after the compressing step. In one embodiment, the stress-relieved end portion comprises a second residual stress after the extracting step, where the first residual stress of the compressive end is greater than the second residual stress of the stress-relieved end. In one embodiment, the compressed end comprises a residual hoop stress of at least 0 ksi. In one embodiment, the stress-relieved end comprises a residual hoop stress of less than 0 ksi. 
     To facilitate the extracting step, the method may include applying force to a rod of the expander, where at least a portion of the rod is accessible after the compressing step. In one embodiment, prior to the applying force step, at least a portion of the rod is located outside the tube. In one embodiment, the expander includes a head, and the head is adapted to complete the inserting a portion of the expander step without restrictively engaging the internal surfaces of the first end portion of the tube. In one embodiment, the methods result in the production of an aluminum alloy tubing product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(   a ) is a schematic view of one prior art system of producing a compressed tubing end. 
         FIG. 1(   b ) is a schematic view of the system of  FIG. 1(   a ) illustrating the production of a compressed end via a die. 
         FIG. 1(   c ) is a schematic view of the system of  FIG. 1(   a ) illustrating the produced tube and stress states after the die has been removed from the end of a tube. 
         FIG. 2(   a ) is a schematic view of one embodiment of a system for producing stress-relieved tubing ends in accordance within the instant application. 
         FIG. 2(   b ) is a schematic view of the system of  FIG. 2(   a ) illustrating the production of a compressed end via a die. 
         FIG. 2(   c ) is a schematic view of the system of  FIG. 2(   a ) illustrating the produced compressed end. 
         FIG. 2(   d ) is a schematic view of the system of  FIG. 2(   a ) illustrating production of a stress-relieved end via removal of an expander. 
         FIG. 2(   e ) is a schematic view of the system of  FIG. 2(   a ) illustrating the produced tube and stress states after removal of the expander. 
         FIG. 3  is a flow chart illustrating one embodiment of a method for producing stress-relieved tubing ends in accordance with the instant application. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the accompanying drawings, which at least assist in illustrating various pertinent features of the instant application. One embodiment of a system for forming a stress-relieved tubing end in accordance with the instant application is illustrated in  FIGS. 2(   a )- 2 ( e ). In  FIG. 2(   a ), an aluminum tube  10  of generally equally cross-section and having an inner diameter  12  is interconnected with clamp  20 . Clamp  20  may fixedly position tube  10  in a predetermined orientation. An expander  40  having a head  42  and a rod  44  may be inserted into and through a first end portion  11  of the tube  10 . The head  42  may have an outermost diameter that is smaller than the inner diameter  12  of the tube  10 . The head  42  may be coaxially aligned with a center axis of the tube  10 . The rod  44  of the expander  40  may extend from the head  42  and through tube  10  and out of first end portion  11 . Thus, after compression of first end portion  11 , as described below, forces may be applied to rod  44  to remove head  42  from the tube  10 . 
     The system also includes a swaging die  30  for producing a compressed end of the tube  10 . The swaging die  30  generally comprises internal surfaces for compressing the first tubing end  11 , such as a passageway  31  extending between a proximal end  38  of the die and a distal end  39  of the die  30 . The passageway  31  generally comprises a proximal end portion  33  and a distal end portion  36  having a diameter  32 . The proximal end portion  33  generally comprises a receiving portion  34  and a tapered portion  35 . The receiving portion  34  is adapted to receive/is capable of receiving the first end portion  11  of the tube  10 . The tapered portion  35  comprises a distal end that has a diameter coincidental to the diameter  32  of the distal end portion  36 . The tapered portion  35  is adapted to compress/is capable of compressing the first end portion  11  of the tube  10  to produce compressed end  14  and transition zone  13  ( FIG. 2(   b )). 
     In particular, and with reference to  FIGS. 2(   a )- 2 ( c ), as swage die  30  is pushed onto and about the first end portion  11  of the tube  10  (e.g., toward middle portion M), tapered portion  35  compresses the first end portion  11  of the tube  10  to a size corresponding with diameter  32  of the swage die  30 . In turn, the swage die  30  is removed from the tube  10 , thereby leaving the compressed end  14  and the transition zone  13 . Compressed end  14  generally comprises a tensile residual stress, which may be a tensile residual axial stress and/or a tensile residual hoop stress. Compressed end portion  14  generally comprises an inner diameter  18 , which is smaller than inner diameter  12  of non-compressed portions of tube  10 . Head  42  is generally located in a middle portion M of tube  10  (or even further away from compressed end) during the compression of first tubing end  11  so as to avoid interference with the production of compressed end  14 . 
     To at least partially relive the tensile residual stress of compressed end  14 , the expander  40  is removed from the tube  10 . In particular, and as illustrated in  FIG. 2(   d ), rod  44  may be pulled in a distal direction to force head  42  through the transition zone  13  and compressed end  14  of tube  10 . As the head  42  passes through the compressed end  14 , it moves (e.g., stretches and/or expands) internal surfaces of the compressed end  14 , thereby creating stress-relieved end  15 . In this regard, head  42  may be sized to have an outermost diameter that is slightly larger than the inner diameter  18  of the compressed end portion  14 . After extraction from the tube  10 , the expander  40  may be removed from the swage die  30 , as illustrated in  FIG. 2(   e ). 
     As illustrated in  FIGS. 2(   c ) and  2 ( d ), the compressed end  14  generally has a diameter  18  that is slightly smaller than the diameter  19  of the stress-relieved end  15 . For example, the diameter  18  of the compressed end  14  may be in the range of from about 0.04% to about 1.4% smaller than the diameter  19  of the stress-relieved end  15 . Thus, use of the expander may result in expansion of the compressed end  14  by from about 0.04% to about 1.4% to produce stress-relieved end  15 . In one embodiment, the diameter  19  is at least about 0.05% larger than diameter  18 . In other embodiments, the diameter  19  is at least about 0.1% larger than diameter  18 , such as at least about 0.2% larger, or even at least about 0.3% larger, or even at least about 0.4% larger, or even at least about 0.5% larger, or even at least about 0.6% larger, or even at least about 0.7% larger than diameter  18  of compressed end  14 . In one embodiment, the diameter  19  is not greater than about 1.4% larger than diameter  18 . In other embodiments, the diameter  19  is not greater than about 1.35% larger than diameter  18 , such as not greater than about 1.3% larger, or even not greater than about 1.2% larger, or even not greater than about 1.1% larger, or even not greater than about 1.0% larger than the diameter  18  of compressed end  14 . 
     As described below, the inner diameter  19  and outer diameter of the stress-relieved end  15  may be selected in accordance with predetermined design criteria. In turn, the inner diameter  18  and outer diameter of the compressed end  14  may be selected in advance and in conjunction with a selected percentage increase between the diameters of the stress-relieved end and the diameters of the compressed end (e.g., the above-described percentage increase). In turn, die  30  and expander  40  may correspondingly be selected. 
     The stress-relieved end  15  generally has reduced tensile stress relative to the compressed end portion  14 , and in some cases has a reversed stress field relative to the compressed end portion  14  (e.g., a compressive axial stress field T 3  and/or a compressive hoop stress field T 4 ). For instance, the stress-relieved end  15  may comprise a residual stress state that is at least about 25% less than the residual stress state of the compressed end  14  as determined using ASTM E1928-99 (hereinafter “the Espey and Sachs method”). In one embodiment, the stress-relieved end  15  comprises a residual stress state that is at least about 35% less than the residual stress state of the compressed end  14  as determined using the Espey and Sachs method. In other embodiments, the stress-relieved end  15  comprises a residual stress state that is at least about 50% less than the residual stress state of the compressed end  14 , such as at least about 60% less, or even at least about 70% less, or even at least about 80% less, or even at least about 90% less, or even at least about 100% less than the residual stress state of the compressed end  14  as determined using the Espey and Sachs method. 
     In some instances, the stress-relieved end  15  may comprise a compressive stress state, as opposed to the tensile stress state of compressed end  14 . For example, the stress-relieved end  15  may comprise a residual stress of less than 0 ksi as measured by the Espey and Sachs method. In one embodiment, the stress-relieved end  15  comprises a residual hoop stress of not greater than about −1.0 ksi as measured by the Espey and Sachs method. In other embodiments, the stress-relieved end  15  comprises a residual hoop stress of not greater than about −1.25 ksi, or not greater than about −1.5 ksi, or not greater than about −1.75 ksi, or not greater than about −1.90 ksi as measured using the Espey and Sachs method. 
     The head  42  of the expander  40  may be of any suitable shape. In general, the shape of the head  42  is generally coincidental to the shape of the tube  10 . In the illustrated embodiments, the head  42  is of a generally torus configuration, but the head  42  may also be of a cylindrical or other configuration. In any event, the outermost diameter/perimeter of the head  42  should be sized such that the head  42  may readily/freely enter tube  10  prior to producing compressed end  14 . Furthermore, the size of head  42  should be such that, as head  42  is extracted from tube  10 , outer surfaces of head  42  engage inner surfaces of compressed end portion  14  so as to move (e.g., expand and/or stretch) at least a portion of the inner surfaces (e.g., expand and/or stretch) of compressed end portion  14 , and create stress-relieved end  15 . 
     The expander  40  may be any suitable apparatus for expanding the compressed end portion  14  of the aluminum tube  10  after swaging. In the illustrated embodiments, the expander  40  comprises the head  42  and the rod  44  rigidly interconnected with the head  42 . In another embodiment, the expander  40  may comprise a bladder, such as those used in conjunction with a hydroforming process. In another embodiment, the expander  40  may comprise plugs (e.g., urethane plugs) adapted to push against separate inner portions of the compressed end  14  as the plugs are removed from the tube  10 . In one embodiment, the expander  40  is an expansion means capable of moving internal surfaces of a compressed end  14  of a tube  10 , thereby reducing the stress state of the compressed end  14 . The expansion means may be any suitable apparatus in this regarding, including any one of a mandrel, a bladder, and a plug. 
     The tube  10 , swage die  30  and expander  40  are generally sized in accordance with the desired final dimensions of the compressed tube end  14 . In one embodiment, the tube  10 , swage die  30  and expander  40  are sized such that the tubing product has outer and/or inner surfaces (e.g., perimeter, diameter, surface area) that are in accordance with a predetermined design parameter. For example, the tube  10  and the swage die  30  may be sized to compress a tube end to an outer and/or inner size that is slightly smaller than the design requirements of the final tubing product. In turn, the expander  40  may be sized to expand and/or stretch this compressed end so that the resultant stress-relieved tube end has an outer and/or inner size that is within tolerable limits of the design requirements of the final tubing product. 
     The tube  10  may consist essentially of aluminum, or may be an aluminum-containing alloy. In particular, the tube  10  may comprise any of the 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX, 7XXX or 8XXX series alloys, as defined by The Aluminum Association, Inc. In one embodiment, the tube  10  comprises a 6061 series alloy. It is anticipated that metals other than aluminum may be used. 
     As noted, the tube  10  may be of any suitable size. In one embodiment, the outer diameter of the stress-relieved end  15  is in the range of from about 4 inches (about 10.2 cm) to about 6 inches (about 15.2 cm), such as in the range of about 4.5 inches (about 11.4 cm) to about 5.8 inches (about 14.7 cm). In a related embodiment, the inner diameter of the stress-relieved end  15  may be in the range of from about 3.5 inches (about 8.9 cm) to about 4.5 inches (11.4 cm), such as in the range of from about 4 inches (about 10.2 cm) to about 4.35 inches (about 11.0 cm). In a related embodiment, the wall thickness of the stress-relieved end  15  may be in the range of from about 0.08 inch (about 0.203 cm) to about 0.1 inch (0.254 cm), such as in the range of from about 0.083 inch (about 0.211 cm) to about 0.098 inch (about 0.249 cm). 
       FIG. 3  illustrates one embodiment of a method for producing a tubing product. In the illustrated embodiment, the method includes the steps of securing a tube ( 310 ), inserting a portion of an expander into the tube ( 320 ), compressing an end of the tube via a compressive apparatus (e.g., a die), thereby producing a compressed end of the tube ( 330 ), extracting the expander from the tube ( 340 ), and, moving, concomitant to the extracting step ( 340 ), internal surfaces of the compressed end via the expander ( 350 ) to produce a tubing product having a stress-relieved end. 
     The step of securing the tube ( 310 ) may be accomplished in any conventional fashion so long as the tube remains substantially stationary during the compressing the tube end step ( 330 ), the extracting the expander step ( 340 ) and the moving internal surfaces step ( 350 ). In one embodiment, clamps are used to secure the tube ( 311 ). In an alternative embodiment (not illustrated), the compressive apparatus and/or the expander may be secured, and the tubing may be moved relative thereto. 
     The step of inserting the expander into the tube ( 320 ) may be accomplished in any conventional fashion. In one embodiment, a head of an expander is placed within the tube ( 321 ). In this regard, the head may have a smaller size (e.g., outermost perimeter) than the inner size (e.g., perimeter) of the tube ( 322 ). After the inserting step ( 320 ), the head is generally located in a middle portion of the tube so as to avoid interfering with the compressing step ( 330 ). In one embodiment, a rod may be fixedly/rigidly interconnected with the head and the rod may have a sufficient length to extend out of the end of the tube ( 323 ). Thus, after or concomitant with the compressing the tube end step ( 330 ), forces may be applied to the rod to accomplish the extracting the expander step ( 340 ). 
     The step of compressing a tube end via a compressive apparatus ( 330 ) is generally accomplished by engaging an end of the tube with a die having a swaging portion. For example, the die may be moved toward a center portion of the tube to engage outer surfaces of the tube with inner surfaces of the die (e.g., a swaging portion of the die) ( 331 ), thereby compressing the end of the tube into a compressed end. After the compressed end portion is produced, the die may be removed from the tube end ( 332 ). In another embodiment (not illustrated), the die is stationary and the tube is moved relative thereto to accomplish the compressing step ( 330 ). As may be appreciated, compressive apparatus other than dies may be used to create the compressed end of the tube. For example, rotary swaging, spin forming and/or electromagnetic pulse forming apparatus/systems may be used to create the compressed end. 
     The step of extracting the expander ( 340 ) may include the step of applying a removal force to a rod of the expander ( 341 ). Thus, the head of the expander may be extracted from the tube. In this regard, after the compressing step ( 330 ), at least a portion of the rod may be located outside of the tube. 
     Concomitant to the extracting step ( 340 ), internal surfaces of the compressed end may be moved via the expander ( 350 ). In this regard, the moving step ( 350 ) may include engaging one or more inner surfaces of the compressed end with one or more outer surfaces of the expander (e.g., the head) ( 351 ) to move the inner surfaces to different positions. For example, the engaging step ( 351 ) may expand the compressed end portion ( 352 ) and/or stretch the compressed end portion ( 353 ) and/or move the inner surfaces toward the distal end of the tube ( 354 ) and/or move the inner surfaces away from a center axis of the tube ( 355 ). In turn, production of a stress-relieved end from the compressed end ( 360 ) may be accomplished. In an alternative embodiment (not illustrated), the expander is stationary and the tube is moved relative thereto to accomplish the expanding step ( 340 ) and/or moving step ( 350 ). 
     In addition to the tensile stress reduction benefits, reduced grain profiling on internal surfaces of the tube may be realized. For example, during the compressing step ( 330 ), internal portions of the tube may develop a surface condition know as grain profiling (e.g., “orange peel”), which may ultimately result in peak to smooth contact between the tube and yoke in a drive shaft application. With the instant methods, grain profiling may be reduced as the expander may smooth the internal surfaces of the tube during the extraction step ( 340 ) and/or moving step ( 350 ). 
     The methods and systems of the instant application may result in a tube product having a tubing end that has a lower stress state than conventionally produced tubing products. The methods and systems are generally useful in conjunction with single-piece aluminum tubes (e.g., single-piece drive shaft tubes). The methods and systems of the instant application are relatively efficient and cost-effective. For example, the additional production time associated with inserting the expander into and extracting the expander from the tubing, relative to conventional swaging processes, is generally only a few seconds (e.g., not greater than 10 seconds). The additional capital cost is also relatively low. Thus, the methods and systems of the instant application are suited for reducing tensile stresses on tubing ends. The methods and systems of the instant application may also be used in drive shaft forming applications, as well as other applications, such as butted bicycle frame tubing, or for aerospace materials, such as torque tubes or control rods, to name a few. 
     EXAMPLES 
     Example 1 
     Production of Tubing End Via Conventional Process 
     A compressed tubing end of a 6061 series aluminum alloy is produced substantially in accordance with  FIGS. 1(   a )- 1 ( c ) and the description associated therewith. The residual axial hoop stress of the tubing end is measured via the Espey and Sachs method. The diameter before testing is about 4.528 inches. The diameter after testing is about 4.728 inches. The residual hoop stress is measured to be about 10.17 ksi. 
     Example 2 
     Production of Tubing End Via Conventional Process with Subsequent Heat Treatment 
     A compressed tubing end of a 6061 series aluminum alloy is produced substantially in accordance with  FIGS. 1(   a )- 1 ( c ) and the description associated therewith. After production, the compressed end is heat treated via conventional processes. The residual axial hoop stress of the tubing end is measured via the Espey and Sachs method. The diameter before testing is about 4.532 inches. The diameter after testing is about 4.532 inches. The residual hoop stress is measured to be about 0 ksi. 
     Example 3 
     Production of Stress-Relieved Tubing End Via Expander 
     A compressed tubing end of a 6061 series aluminum alloy is produced substantially in accordance with  FIGS. 2(   a )- 2 ( d ) and the description associated therewith. After production, the residual axial hoop stress is measured via the Espey and Sachs method. The diameter before testing is about 4.525 inches. The diameter after testing is about 4.492 inches. The residual hoop stress is measured to be about −1.95 ksi. 
     While the instant application has used a cylindrical tube and the term “diameter” in various instance to facilitate ease of description, the term “diameter” should be construed broadly and is meant to include dimensions of non-cylindrical tubes, as well as the dimensions of the cylindrical tubes illustrated in the instant application. Additionally, the term “tube” is intended to include cylindrical as well as non-cylindrical shapes. Moreover, while various embodiments have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.