Abstract:
A knock-down anvil assembly provides a rotary anvil ring inside the cylindrical end portions of tank sections to be stir welded together as a butt joint, and an external reactive roller unit is provided which bears against the outer faces of these end portions at a location diametrically opposite from the stir welding tool. The anvil assembly can be disassembled within the finished tank into components which can be individually removed through an access opening in an end cover of the tank.

Description:
TECHNICAL FIELD 
     The present invention relates to the welding of joints on large tanks, and particularly to stir welding circumferential joints between cylindrical bodies and dome shaped end covers. 
     BACKGROUND OF THE INVENTION 
     Friction stir welding (hereinafter referred to as “stir welding”) is a relatively new joining technology disclosed in U.S. Pat. No. 5,460,317, whereby alloys such as 7075 aluminum and various forms of aluminum/lithium previously considered to be unweldable alloys, can be welded to provide joints having minimal change to the structural properties of the parent materials and extremely low dimensional distortion. Unlike fusion welding where portions of each piece being welded are melted and then resolidified to form a new and different material structure, stir welding is a solid state process. Joining is accomplished by stirring and intermixing plasticized material from each part across the joint boundary. Heating and mixing is accomplished simultaneously in stir welding by employing a stepped cylindrical tool having a head pin or probe surrounded at its root end by an annular shoulder. The tool is rotated at a precise computer controlled speed and pressed against the work piece at the joint with substantial axial force which may be in the range of 12,000 to 20,000 pounds. Friction between the rotating probe and work pieces causes local heating and ultimately plasticizing of the material in close proximity to the probe. Once sufficient heat has been generated the material yields and the probe plunges through most of the thickness of the material until the annual shoulder of the tool contacts the surface of the work pieces. The material adjoining the joint is then blended by moving the spinning tool along the joint centerline at a carefully controlled rate and attack angle between the plane of the annular shoulder and that surface of the work pieces. As the probe and shoulder move relative to the work pieces they simultaneously heat and remove material from the leading surfaces in front of the tool and deposit this material onto the trailing surface. The relative movement of the tool along the joint may be accomplished by moving the tool or by moving the work pieces. 
     Fuel tanks for space vehicles commonly have thin cylindrical body shells with dome-shaped end covers secured together by welding. The thin walls of the tanks may be reinforced by integral internal grids. These grids are machined in flat tank sections which are then bent to provide laterally curved sections. They are then placed side-by-side and connected together by longitudinal butt welds to form cylindrical tank sections. Such sections are in turn welded together at circumferential joints at their ends, and finally, end domes are welded in place at circumferential end joints to complete a tank. Each end cover normally has an access opening which usually is centered, but may not be, and is of a size, about thirty inches in diameter, adequate to permit personnel to enter the tanks during construction and for inspection. Currently, to achieve minimum weight while maintaining adequate strength, materials such as aluminum/lithium alloys are preferred for fabricating the fuel tanks, but this choice requires that stir welding be used. Since the fuel tanks are large, in the range of about 15 to 27 feet in diameter, and commonly 50 feet or so in length, and have relatively thin walls, in the range of about {fraction (3/16)} to ⅝ inches, for example, stir welding of circumferential joints is difficult. The final joint securing the second end cover in place, has been a particularly vexing problem since the tank is then completely closed except for the access openings in the end covers, and hence, passage for removal of fixtures is very limited. The problem is compounded by the fact that considerable pressure is exerted by the stir welding probe on the work, and hence the work must be backed by a suitable anvil opposite the probe. This means that an anvil must be inside the tank. It also means that the probe pressure must be resisted by a reactive force acting on the outside of the tank. However, such a reactive force together with the probe pressure may seriously distort and perhaps even partly crush the tank unless they are somehow adequately resisted by structure other than the tank. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to solve the foregoing problems by providing a rotary ring inside the cylindrical end portions of the tank sections to be stir welded together as a butt joint, and by providing an external reactive roller unit bearing against the outer faces of these end portions at a location diametrically opposite from the stir welding tool. Further, the anvil ring is constructed so that it can be disassembled within the finished tank and removed in pieces through one of the end access openings. 
     A huge lathe-like machine is used to handle and turn the tank sections and the anvil ring relative to the stir welding tool. This machine has a headstock in the form of a long projecting beam truss with a large motor driven head at its outer end, and has a tailstock with a complementing motor driven head. The turning of these heads is synchronously controlled so that the heads turn in perfect unison. The cylindrical tank sections are supported on sets of rollers mounted on rails extending longitudinally between the headstock and tailstock and the end covers are supported by extenders from the headstock and tailstock. The hub of the anvil ring is mounted on a tubular spindle projecting from the headstock. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, partly in phantom, illustrating a tank having a central circumferential weld being performed in accordance with the present invention. 
     FIG. 2A is a side elevational view, partly in phantom, of the reactive roller unit shown in operating position. 
     FIG. 2B shows the head portion of the reactive roller unit as illustrated in FIG. 2A drawn to a larger scale. 
     FIG. 2C is a side view of the rear adjustable wheel assembly in operative position mounted on the rear lower cross-member of the reactive roller unit, the cross-member being shown in transverse section. 
     FIG. 2D is a rear view of the adjustable wheel assembly shown in FIG.  2 C. 
     FIG. 3 illustrates a stir welding operation in accordance with the present invention. 
     FIG. 4A is a perspective view showing the supporting of the dome at one end of a tank by a frusto-conical extender. 
     FIG. 4B is a detail view showing the fit of the flange on the dome with the extender. 
     FIG. 5 is an end view of an anvil assembly made in accordance with the present invention. 
     FIG. 6 is a side elevational view of the anvil assembly mounted in operating position when a dome is being welded onto a cylindrical tank section. 
     FIGS. 7A,  7 B and  7 C are side views of the various rim sections of the anvil as viewed in FIG.  5 . 
     FIGS. 8A and 8B are top and side views of a spoke unit of the anvil assembly when viewed as in FIG.  5 . 
     FIG. 9 is a detail view of the anvil at a joint being stir welded, as indicated by the FIG. 9 notation in FIG.  6 . 
     FIG. 10 is an enlarged detail view showing the connection of two rim sections and a shim as indicated by the FIG. 10 notation in FIG.  5 . 
     FIG. 11 is a detail view showing a spoke adjustment indicator and is taken as indicated by line  11 — 11  in FIG.  8 B. 
     FIG. 12A is an end view of a head plate for a tubular spindle. 
     FIG. 12B is a side elevational view of the tubular spindle. 
     FIG. 12C is a plan view of the hub plate for the anvil assembly. 
     FIG. 13A is a plan view of one of the hub assembly segments of the anvil assembly as viewed in FIG.  5 . 
     FIG. 13B is an end view of the hub assembly segment taken as indicated by line  13 B— 13 B in FIG.  13 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As previously indicated, the present invention provides a special anvil assembly  20  and reactive roller unit  21  used in conjunction with a stir welding machine  22  and lathe-like machine having a synchronized headstock and tailstock on which frusto-conical extenders  24  are mounted. The anvil assembly  20  comprises a hub assembly  26 , adjustable spoke units  27  and a rim assembly  28 , all of which can be disassembled into components which can be handled manually and removed by passing through an access opening  30   a  in one of the cover domes  30  of a tank  31 . The hub assembly  26  has four like segments  26   a  (FIGS. 13A-13B) each having a pair of front and back face plates  28 - 28 ′ separated by an arcuate web  29  and by a pair of radial end plates  30 - 31 ′. Cut from one corner portion of the face plates is a quadrant leaving a curved edge  31  extending between inner ends of the radial end plates  30 - 31 ′. The parts of each hub assembly segment are welded together and nuts  32  are welded to the inner face of the front face plate in alignment with of set a three holes  33  therethrough arranged in an arcuate layout. These holes register with holes  33   a  in a center round hub plate  34  (FIG.  12 C). A second set of three holes  35  in the face plates  28  is radially aligned with the first set and a seventh hole  36  is radially aligned with the center holes of the two sets of three holes each. This hole pattern is provided in both face plates. In addition the two radial end plates  30 - 31 ′ each have a set of four holes  37  arranged in a rectangle so that two holes are inboard and two holes are outboard with respect to the respective end of the arcuate web  29 . The four hub segments  26   a  are bolted together by bolts  38  passing through registering sets of the holes  37  with the end plate  30  of each registering with the end plate  30 ′ of another. 
     Continuing to the spoke units  27 , each has a tubular spoke  40  of rectangular cross-section, and has a length adjustment mechanism comprising a threaded rod  41 , a rectangular rotation stop element  42  fixed on the rod inside the spoke  40 , a clevis  43  fixed on the outer end of the rod, and an adjustment nut  44  threaded on the rod. A rectangular end plate  45  is mounted on the outer end of the tubular spoke  40  and has a center hole for free passage of the rod  41 . As shown in FIG. 11, the end plate  45  also has a decal  46  presenting radiating markings  46   a  every 10 degrees, for example, to register with a pointer in the form of a v-notch  47   a  in the perimeter of a washer  47  attached to the adjustment nut  44 . Clockwise turning of the nut  44  while bearing against the end plate  45  causes the rod  41  and clevis  43  to move outwardly relative to the spoke  40  to make the effective length of the spoke units  27  adjustable. At their inner ends, the spokes  40  have reinforced transverse holes  35   a  to register with one of the holes  35  in the hub segments  26   a,  and four of the spokes  40  also have a second reinforced transverse hole  36   a  to register with one of the holes  36  in the hub segments  26   a.  Bolts  40   a  pass through the registering holes to secure the spokes to the hub assembly. 
     The rim assembly  28  is formed by multiple arcuate segments  48  and shims  49 . In cross-section each arcuate rim segment  48  is an I-beam having inner and outer flanges  50 - 51  separated by a central web  52 . The outer flanges  51  are narrower than the inner flanges and function as an anvil. For purposes of example the rim assembly is shown as having twelve segments  48  and four shims  49  arranged in a symmetrical arrangement. Each segment  48  has a fork unit  53  extending radially therefrom toward a common center and having fork arms welded to the outer edges of the inner flanges  50  of the segment and to the outer edges of transverse supplemental web  54  on the segment located on opposite sides of the central web  52 . Bolts  55  connect the fork units  53  to the spoke clevises  43 . 
     Four of the rim segments designated  48   a  (FIG.  7 B), have like end plates  57  mounted on both ends and these end plates are opposed by like end plates on one of the ends of the remaining eight rim segments. Four bolts  58  passing through registering holes  58 ′ secure each pair of opposing end plates together. The other ends of the remaining eight segments are provided with wider end plates  59  functioning as clamping jaws for gripping the shims  49  at expansion joints. These clamping jaws each have a rectangular pattern of four bolt holes  60  and also have two intermediate guide holes  61  which are preferably spaced apart slightly further than the width of the rectangular pattern of holes  60 . This hole pattern width is such that clamping bolts  58 ′ passing through the holes will closely straddle a shim  49  when it is positioned between two of the clamping jaws. As shown in FIGS. 7C and 10, alternate of the end plates  59  have their intermediate holes backed with nuts  62  to threadably receive dowel functioning bolts  64  for use in initially positioning the rim segments before insertion of the shims. Then the bolts  58  are inserted in the holes  60  and tightened with the outer end of the shims flush with the outer surface of the outer flanges  51 . 
     A relatively large tube  66  functions as a spindle projecting axially from the tailstock and, as shown in FIG. 12A, has an annular head plate  67  providing a border flange with a set of holes  67   a  for receiving bolts to mount the spindle on the tailstock. The projecting outer end of the spindle is closed by an end plate  68  having a ring of studs  68   a  projecting outwardly therefrom to fit into the holes  33   a  in the hub plates  34 . A length of pipe  69  projects from the center of the end plate  67   b  and has a welded collar  69   a.  The pipe  69  is threaded outwardly of the collar. When the spindle  66  is joined to the anvil ring assembly the pipe collar backs against the center hub plate  34  and the outer threaded portion of the pipe  69  projects through a center opening  34   a  in the hub plate and receives an anchoring cap  70 . 
     The anvil ring assembly  20  is initially assembled without the shims  49  in place at the four expansion joints. Then after a cylindrical tank section (shell)  31   a  of the tank and another tank section or a dome-shaped end cover  30  are positioned in butting relation preparatory to being welded together, the adjustment nuts  55  are tightened sequentially one turn each at a time until the anvil ring expands sufficiently to make contact with the shell head. Then expansion of the spokes is continued by turning the adjustment nuts  55  about 60° at a time in sequence. The gaps at the expansion joints are measured and the shims are machined to fit the gaps. The adjustment bolts are turned an additional 120° and the shims are installed. Then the adjustment bolts are backed off 120° and the bolts  58 - 58 ′ at the expansion joints are fully tightened. The outer ends of the shims are prefinished with a curvature to match the curvature of the outer anvil faces of the rim segments. Thus when the expansion joints are tight the rim assembly presents a smooth circular anvil face. 
     Each dome-shaped end cover  30  is provided with an external wrap around flange  30   b  adjacent its mouth for ease of handling. As previously indicated, the headstock and tailstock may each be provided with a frusto-conical extender  24 . Each extender has a trusswork  24   c  extending between front and back mounting rings  24   a - 24   b , with the back ring  24   b  being the larger of the two and may have a T-shape. The front ring  24   a  is bolted to the headstock or tailstock, and the back ring  24   b  is bolted to the wrap around flange  30   b  on the related end dome  30  to thereby hold and position the dome such that it is centered on the rotational axis of the lathe. The spindle  66  of the tailstock extends through the center access opening  30   a  of the respective cover dome when the cover is to be welded in position. A pair of floor-mounted linear bearing rails  76  are mounted in parallel relation to the rotational axis at equal distances therefrom greater than the radius of the tank being fabricated. Sets of roller units  78  are mounted by linear bearings on the rails  76  at intervals there along such that the rollers on the units hold the tank sections and back ring  24   b  of the extenders  24  centered at the rotational axis. 
     The external reactive roller unit  21  has a rigid frame  80  having a pair of rear corner posts  80   a - 80   b  interconnected by a rear tubular cross member  80   c  from which a wheel  82  is suspended. This wheel is adjustable in elevation by the turning of a wing nut  83  on a vertical rod  84  to optionally lower the wheel  82  into ground engagement and thereby raise the rear of the frame  80  such as to lift the rear corner posts  80   a - 80   b  out of the ground engagement. The rod  84  is vertically screwed through the upper wall of the cross member and carries a box frame  85  straddling the cross member  80   c  and carrying the wheel  82 . The box frame  85  is vertically guided by a pair of bolts  86 - 86 ′ passing horizontally through the cross member and vertical slots  87  in opposite sides  85 - 85 ′ of the box frame. 
     A pair of arms  80   d  extend forwardly from the corner posts  80   a - 80   b  and are braced by a pair of sloped braces which together with the outer ends of the arms  80   d  are connected to brackets  80   e  from which a pair of feet  87  are swing mounted. These feet carry linear bearings  88  mounted on a longitudinal rail  90  extended along the floor. 
     At their upper end the corner posts are connected by a second cross member  91  on the center of which is mounted a support unit  92  for a cross pin  93  carrying a vertical swing frame  94 . This swing frame is urged to swing forwardly away from the upper cross member by a sloping pneumatic cylinder unit  95  having its cylinder  95   a  pivotally mounted at the rear on pin  96  and its piston rod  95   b  pivotally connected to the swing frame  94  so that extension of the piston rod swings the swing frame forwardly. This forward swing is limited by a stop bolt  97  anchored to the support unit and passing through a slot in a central part of the swing frame  94  to a stop head bridging the front end of the slot. The swing frame in turn has a cross-pin  98  on which are swing-mounted a pair of triangular cheek plates  99  carrying a pair of rollers  100   a - 100   b  on axles  101  adjacent to the apexes. These rollers are arranged to track on the seam being stir welded at a location whereat the midpoint between the rollers is substantially diametrically opposite from the probe  22   a  of the stir welding machine  22 . This machine is mounted so that the angle of attack of the probe can be adjusted as the seam to be welded moves circumferentially. In other words the tilt axis of the probe and related mechanism is kept parallel to the center axis of the tank sections. 
     As illustrated in FIG. 3, the stir welding machine rotates the probe  22   a  while pressing the probe axially into the material M of the butting parts at the joint. In accordance with the present invention, the axial pressure P of the probe against the material M is resisted by the cooperative resistance R of the anvil assembly  20  and the reactive roller unit  21 . The angle of attack A of the probe is also indicated in FIG.  3 . 
     After the described equipment is used to stir weld the second end cover  31 ′ of a tank in place, the anvil assembly is manually disassembled by a worker or workers entering the tank through the access opening  30   a  in the second end cover after the tailstock has been disconnected and backed away. The construction of the anvil assembly is such that it can be disassembled to parts which all can be lifted and passed outwardly through one of the access openings. Normally this removal from the tank can be done manually, but if necessary, a small gantry crane can be utilized by passing the boom through one of the access openings. 
     If a tank is to be constructed in which the cylindrical portion of the tank consists of multiple cylindrical sections such as in FIG. 1, or in which the access openings in the tank end covers are not centered on the tank axis, to permit entry of a spindle  66 , the anvil ring assembly can be utilized without the benefit of a spindle from the headstock or tailstock. In such an instance rotation of the anvil ring assembly will occur in response to turning of the related end cover of the tank by the lathe via the respective frusto-conical extender  24 . 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.