Patent Publication Number: US-2003226247-A1

Title: Metal bellows manufacturing method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is related to and based on U.S. Provision Patent Application No. 60/386,860 filed Jun. 6, 2002. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] This invention relates generally to metal bellows and more particularly to methods and apparatus for forming metal bellows.  
       [0003] It is well known to weld a series sheets in the form of annular rings and disks to form an expandable bellows using lasers. A major problem is reliably to achieve a hermetic seal at the inner diameter and outer diameter edges of each annular sheet, particularly with metal foils. When used in this document, the term “metal foil(s)” is intended to indicate metal sheets having a thickness of less than about 0.2 mm.  
       [0004] Typically, the prior art has focused the laser on the line generated by the physical junction of two contiguous foils, the line being situated at either the inner diameter edge or outer diameter edge. The work piece including the two contiguous foils was then rotated relative to the laser to form a seam joining the contiguous edges together. The edges of any adjacent foils, which are not to be joined together, must be held apart by a suitable jig. Further, this edge seam welding process using layer separation jigs can easily produce a thermally induced stress that causes a deformation in the foils that makes the production of fluid-tight bellows very difficult, thus resulting in a high rate of product failure.  
       [0005] It is known to scan and focus the output beam of lasers through computer-controlled mirrors and lenses. This technique has been used, for example, in laser machining and engraving. Computers having ever increasing speeds are available at lower cost thus making possible some solutions that were not previously considered viable or practical. Since there is only a limited area covered by a laser focal beam at the working surface, some amount of beam scanning, or work piece movement, is required to cover a typical working area for manufacturing a bellows.  
       [0006] It is further known to control a laser beam output through pulse shaping so that an optimum pulse can be delivered to a work piece to perform the desired task. This has largely been used in circumstances where the work piece has special thermal characteristics. It has further been recognized that the absorption and reflection characteristics of metals changes significantly from the solid phase to the melt phase of the metal.  
       [0007] There is, however, still a need for a bellows forming operation using a laser that will reliably and repeatedly generate, from a series of ring shaped metal foil sheets, bellows that are hermetically sealed yet avoid the use of overly complex layer separation jigs during any welding process.  
       SUMMARY OF THE INVENTION  
       [0008] The formation of a bellows in accordance with the present invention is achieved by starting with a supply of ring shaped metal foil sheets that have similarly dimensioned outer perimeters and similarly dimensioned inner edges. The supply can be in the form of an automated supply that can feed the metal foil rings one at a time into a jig to position the rings coaxially. A mandrel can be coupled to the jig to apply a pressure normal to the surface of the foil sheets to insure an intimate contiguous relationship between at least the top pair of foil rings.  
       [0009] The output of a laser is focused on an area of the top metal foil ring adjacent to but spaced from either the outer perimeter or the inner edge of the top metal foil ring. This focusing arrangement can be accomplished using computer controlled mirrors and lenses situated as necessary between a laser source and the metal foil rings held in the jig.  
       [0010] In one preferred arrangement at least one of the mirrors or lenses is situated on the common axis of the metal foil rings held in the jig, and can be directed as necessary to any desired position on the top surface of the top foil ring. In another preferred arrangement at least one of the mirrors or lenses is situated in an annulus surrounding the common axis of the foil rings.  
       [0011] The output of the laser source is in the form of an energy pulse, which can be digitally shaped, that has sufficient energy to weld the top pair of metal foil rings together within the area of focus. The energy of the laser pulse is controlled so as to be insufficient to penetrate the second metal foil ring of the top pair of foil rings, the second foil ring being positioned farther from the laser than the top foil ring. In a preferred embodiment, the energy of the pulse is digitally programmed and focused to achieve a modified conduction weld that exhibits greater depth than ordinarily achieved with conventional laser conduction welding. The digital programming of the present invention generally includes an initial energy burst followed by a rest phase. Preferably, more intense energy pulse is delivered after the rest phase followed by a stepped reduction in energy as a function of time until the desired metal penetration is achieved.  
       [0012] Desirably, the energy delivered by the initial energy burst is sufficient to cause a slight melting of the top surface of the top metal foil ring, thus significantly increasing the absorption characteristics of the top metal foil. The rest phase is included to allow for the melting to achieve the maximum extent, thereby effecting a large beneficial reduction in reflection characteristics and enhancement of absorption characteristics of the metal surface prior to any further laser energy delivery. The laser can continue to deliver some energy during the rest phase to off set any tendency of the work piece to cool, but the rate of energy delivery during the rest phase is much less than either the initial burst or the subsequent intense energy pulse and stepped reduction phases.  
       [0013] Following the development of the desired amount of surface melt phase change, the laser is caused to deliver a significant pattern of laser energy focused deeply into the top foil layer, preferably at the interface between the top two foil layers. This focused delivery of laser energy onto a surface that has been modified to enhance the metal absorption of energy results in a deeply penetrating delivery of energy causing melting in the vicinity of the interface between the top two layers, and a liquid metal pool that is as much as twice as deep as it is wide so as to appear in cross-section as elliptical or parabolic instead of hemispherical, which is the typical cross-section achieved by ordinary laser conduction welding.  
       [0014] An infrared or other sensor can be coupled to the output optics of the laser to receive a signal indicative of the temperature achieved in the weld puddle at the top ring pair within the focus area to serve as an indicator of the weld function. A suitable feedback can be coupled to the sensor and to the laser source controls for supplying a corrective signal to the laser source.  
       [0015] In the embodiments wherein the laser is directed to a discrete position on the top metal foil, it will be appreciated that each energy pulse forms a welded spot in the top pair of foil rings. It is then necessary to move the focus area of the laser to another location before initiating a subsequent pulse. The moving can be of the laser as a whole, an element of the output optics of the laser, or the jig holding the pair of foil rings. The preferred method of the present invention is to merely move the output optics so that the focus area is moved to a next location. The next location can be the adjacent area, which is separated from the first area by a distance sufficient to cause the area of laser focus to overlap by between about 20 and 80 percent. While this overlapping area is required, the overlapping focus areas do not have to be welded sequentially.  
       [0016] In one preferred embodiment, the areas that are sequentially subjected to a laser pulse are separated sufficiently that there is very little, if any, residual thermal energy present in the ring at the second location due to the prior activity of the laser. In this way, each area can be supplied with about the same amount of energy without any significant risk of delivering too much energy, which would cause a possible welding to a third contiguous ring. The sequentially welded areas can be adjacent to each other, however such positioning can, in certain circumstances, tend to induce thermal warps in the foil discs that are not desirable. The welding of areas continues until a complete circumferential weld line is formed entirely around the ring pair adjacent either the inner margin or the outer margin with the individual areas overlapping by the previously mentioned margin of about 20 to 80 percent, thus forming a hermetic seal.  
       [0017] When a complete weld ring is completed, another of the plurality of metal rings is deposited on top of the existing pair, thus forming a new top pair of rings proximal to the laser. The process is then repeated, however the laser is directed adjacent to an opposite one of the inner and outer margins. That is, if a first weld line was created adjacent to the outer margin of the first top pair of rings, then the second weld line must be created adjacent to the inner margin of the new top pair of rings. Thus the weld lines alternate between the radii R i  and R o  adjacent the inner and outer margins of each of the succeeding top pairs of rings to form a bellows structure. It will, of course, be appreciated by those skilled in the art that at least one end element included in the bellows construction will take the form of a full disk to form a sealed interior for the bellows.  
       [0018] Each of the layers of the bellows formed according to this invention is secured to the adjacent layer by a weld that is placed in tension as the bellows expands. Since the material forming the weld line will usually have negligible elasticity, any expansion of the bellows will be reflected in a bending force being applied to the metal foil forming each of the rings of the bellows. Thus the elastic memory present in the bellows can be specified by the selection of suitable materials for forming the rings rather than by any characteristics of the weld itself.  
       [0019] In one preferred embodiment of the present invention, the output beam of the laser is aligned with the common axis of the rings to be welded. A mirror is situated on the axis that can be rotated about the axis to redirect the laser beam outward in any selected direction. Two ring mirrors are provided that are situated above the locus of the weld lines adjacent to the inner and outer margins of the rings. Either of the two ring mirrors can be moved into a position to intercept the outwardly directed beam so that the beam is redirected toward one of the weld lines along a line normal to the top metal ring surface. Once a first weld line is completed, the mirror associated with the first weld line can be moved to a non-intercepting position while another metal foil ring is inserted into the jig. The mirror associated with the second weld line is then moved into position to intercept the laser beam as it is reflected from the rotated mirror to effect the welding of the second weld line. Once the second weld line is completed, the second ring mirror is replaced by the first as yet another metal foil ring is added to the jig. The process can be repeated as often as necessary until a bellows of sufficient axial length is achieved.  
       [0020] The movement of the mirrors can be avoided by adopting an alternative embodiment of the present invention in which the supply of ring shaped metal foils sheets takes the form of two linear feed mechanisms supplying a metal foil rings to two positions of a six-position jig, the rings being maintained in a coaxial relation at each of the six positions of the jig. A single pressure bar applies a suitable pressure normal to the surface of the top foil at two other positions of the six position jig for sufficient time to permit a laser to join the top two foil rings together. The ends of the pressure bar bearing on the top foil at the two positions are designed to allow a welding operation. The six position jig is caused to index between welding operations to bring a next set of jigged foil rings into position for a welding operation and simultaneously to allow the addition of a next ring to the stack of previously welded foil rings.  
       [0021] The invention has as objects, features and advantages the accommodation of any number of weld area overlap regimens and laser pulse shapes to achieve the seam connection needed for a particular application. The bellows forming methods and apparatus of the present invention are reliable, durable, and permit real-time sensing of the quality of the connection between sequential layers forming the bellows. The bellows forming methods of the present invention also reduce rejected product output, reduce down time of the manufacturing facility, can be automated, and can be readily adapted for use with a variety of metal foils, although the process and apparatus is not limited to merely bellows constructed from metal foils. It will be apparent to those skilled in the art that the methods and apparatus of the present invention can be adapted for use on a wide variety of products in addition to bellows.  
       [0022] One feature of the present invention is the utilization of a digitally programmed laser to achieved a modified conduction weld that exhibits greater depth than ordinarily achieved with conventional laser conduction welding. This has the advantage of achieving the desired foil penetration depth to join two contiguous foils together directing the laser at the top surface of the pair of metal foil elements.  
       [0023] Another feature of the present invention is the utilization of ring-shaped optical elements to focus the output of a laser on a ring-shaped line of suitable location and so that a weld can be formed entirely around the top foil ring pair in a very short time. This has the advantage of reducing thermal distortion and speeding the process so that the reliable manufacture of metal bellows can be quickly accomplished.  
       [0024] Further features and advantages of the invention are discussed below in conjunction with the preferred embodiments exemplifying the best mode know by the inventor at the time of filing. The description makes reference to the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0025]FIG. 1 is a schematic view, partially in section illustrating an apparatus of the present invention.  
     [0026]FIG. 2 is a plan view of the weld spot patterns according to an embodiment of the present invention.  
     [0027]FIG. 3 is a sectional detail view of the weld pattern achieved by the present invention.  
     [0028]FIG. 4 is a plan view of another apparatus according to the present invention.  
     [0029]FIG. 5 is a sectional view of a first welding station in the apparatus shown in FIG. 4.  
     [0030]FIG. 6 is a sectional view of a second welding station in the apparatus shown in FIG. 4.  
     [0031]FIG. 7 is an exploded perspective view of a beam director that can be employed in the apparatus of both FIGS. 5 and 6.  
     [0032]FIG. 8 is a graph of a typical power v. time program curve for a laser developing a modified conduction weld according to the present invention. 
    
    
     DESCRIPTION OF THE ILLUSTRATED PREFERRED EMBODIMENTS  
     [0033] An apparatus  10  for forming a bellows from a supply of ring shaped metal foil sheets  12  is schematically illustrated in FIG. 1. The apparatus includes a dispenser  14  for dispensing the metal foil sheets one at a time into a jig  16 . The dispenser  14  includes an escapement mechanism  18  holding a stack  20  of the metal rings  12 . The escapement mechanism  18  allows one metal ring  12  from the stack  20  to fall onto a shuttle  22 . The presence of a metal ring  12  on the shuttle  22  can be detected by a sensor  24  that senses, for example, an eddy current, which is induced into the metal ring  12 . The shuttle  22  is powered by shuttle motor  25  to reciprocate in the direction of arrow A between a position below the escapement mechanism  18  and a position located above jig  16 . When the shuttle  22  is located over the jig  16 , the shuttle can dispense any metal ring  12  carried by the shuttle into the jig.  
     [0034] The jig  16  includes a base  26 . A plurality of standards  28  and  30  are spaced around and project upward from the base  26  that cause the metal rings  12  to become coaxially aligned with each other and with axis Y as they descend into the jig  16 . A mandrel  32  is coupled to the jig  16  through a pressure mechanism  34  that can mover the mandrel  32  in the direction of arrow B to a lowered position shown in phantom to apply a pressure normal to the top surface  36  of the uppermost of the foil rings  12  to insure an intimate contiguous relationship between at least the top pair of the rings  12 . The apparatus  10  also includes a laser  38  that can supply a pulse of energy along an optical path  40 . Any suitable laser can be used, with the currently preferred lasers being Nd:YAG and, to a lesser extent, CO 2  lasers. However, laser technology is rapidly evolving and it is anticipated that the desired laser can change and improve over current lasers. One current laser which performs satisfactorily is a Nd:YAG laser, Model No. GSI/Lumonics 702D, from GSI/Lumonics of Northville, Mich. This laser is rated at 4.5 kW, peak power, producing a pulsed beam that can be shaped or modulated to achieve the desired thermal characteristics. The optical path  40  can be defined at least in part by an optical fiber or even an optical fiber cable having a number of optical fibers than can be directed to different portions of a work piece defined by the metal rings  12  in jig  16 .  
     [0035] A lens system  42  is included in the optical path  40  for focusing the laser output on the top surface  36  of the uppermost of the foil rings  12 . One or more fixed mirrors  44  can be included in the system for redirecting the laser output toward a desired location. One or more mirrors  46  are coupled to mirror support  48 , which is in turn coupled to motor  50 , that can be rotated or otherwise moved to redirect the laser output to a set of positions as determined, for example, by computer  52 . The computer  52  can control the output of the laser  38 , the focal length of the lens system  42 , the position defined by the motor  50  and other aspects of the present apparatus  10 . The computer  52  can be a general purpose computer or can be a specialized programmable logic controller.  
     [0036] The apparatus  10  of the present system preferably directs the laser output along an optical path  40  that is at least in part coincident with the axis of symmetry Y of the rings  12  as they are held in the jig  16 . In this embodiment, a mirror  46  capable of being rotationally positioned redirects the laser output beam outward away from the axis Y toward a ring reflector  54 . The ring reflector  54  is positioned above the outer margin  56  of the rings  12  as they are held in the jig  16  so that the laser output beam directed outward by mirror  46  is redirected downward by ring reflector  54  perpendicularly with respect to the top surface  36  of the rings  12  held in the jig  16 . By rotating the mirror  46  to a different position, the output beam  40  of the laser  38  is directed to a different location adjacent to the outer perimeter  56  of the rings  12 .  
     [0037] The apparatus  10  of the present system also includes a second ring reflector  58  that is movable vertically, in the direction of arrow C, to the position shown in phantom in FIG. 1 to intercept the laser output beam  40  directed outward away from the axis Y by the mirror  46 . When repositioned to the location shown in phantom, the second ring reflector  58  redirects the intercepted laser output  40  downward toward the inner margin  60  perpendicularly with respect to the top surface  36  of the rings  12  as they are held in the jig  16 . Again, by rotating the mirror  46  to a different angular position around axis Y, the output beam of the laser  38  is directed to a different location adjacent to the inner margin  60  of the rings  12 .  
     [0038] A source of gas  62  can be employed to provide a shield gas for the laser welding process. The preferred gas is pure argon, which is a relatively heavy, inert shield gas that enables a smoother finished weld with less roughness or jagged edges in any weld seam or puddle area. The jig  16  can be surrounded by a gas containment wall that is spaced from, or contiguous to, the rings  12 . For example, a suitable gas containment wall can be formed around the outer perimeter of the plurality of standards  28 . Accordingly, the relatively heavy argon shield gas can fill the area within the containment wall, to the extent not already filled by the rings  12  and mandrel  32 , to provide an improved environment in the area of the weld during the welding process. This can further improve the quality and integrity of the weld.  
     [0039] An infrared sensor  64  can be included in the system  10  that receives a return signal from a half-silvered mirror  66  indicative of the energy actually delivered to the to surface  36  of the stack of rings  12  held within the jig  16 . This return signal can be delivered from the sensor  64  to the computer  52  to provide enhanced control though the measurement of the thermal characteristics of the welding process achieve by the apparatus  10 .  
     [0040]FIGS. 2 and 3 illustrate weld patterns that can be achieved by using the apparatus of the present invention. A first pair  62  of the metal rings  12 , consisting of rings  64  and  66 , are situated in contiguous relation to each other within the jig  16  so as to be coaxially aligned with respect to their mutual axis of rotation Y. The output of the laser  38  is focused, with the aid of the lens system  42 , on an area  68  adjacent to but spaced from the outer perimeter  56  situated at outer radius R 2  of the pair  62  of metal rings  12 , the area being selected by controlled movement of mirror  46 . The laser  38  is then caused, by computer  52 , to emit an energy pulse of sufficient energy to weld the first pair  62  of metal rings by forming a precise weld nugget  70  as shown in FIG. 3. The energy pulse delivered by the laser  38  is insufficient to penetrate metal ring  64 , which is positioned farther from the laser  38  than is metal ring  66 . The computer  52  then causes the motor  50  to turn mirror  46  to a new selected area  72 , and the laser  38  is again caused to emit an energy pulse. This process is repeated a sufficient number of times until a complete weld line is formed around the circumference of the first pair  62  of metal rings, with the areas of each laser pulse overlapping by between about 20 and 80 percent. This overlap of areas can be accomplished by sequentially stepping the motor  50  around one complete turn by sufficiently small incremental steps to form the desired overlap. Of course, the series of overlapping areas need not be generated in a linear process, and can be generated through a spaced series of welds as shown in FIG. 2, which when completed will still form the continuous ring.  
     [0041] Following formation of the first ring adjacent the outer perimeter  56 , the computer  52  causes the shuttle motor  25  of the dispenser  14  to be activated so that a next metal foil ring  74  is transferred into the jig  16  on top of the existing metal ring  66 . The ring mirror  58  is then lowered until intercepting the optical path between the mirror  46  and the ring reflector  54 . The laser  38  is then caused, by the computer  52 , to initiate another series of energy pulses interspaced by controlled movements of the mirror  46  to form a series of weld nuggets  76  spaced from the inner margin  60  situated at radius R 1  of the new pair  78  of metal rings, consisting of rings  66  and  74 . Again, the welding within the discrete areas is repeated a sufficient number of times until a complete weld line is formed around the inner circumference of the second pair  78  of metal rings  12 , with the areas of each laser pulse again overlapping by between about 20 and 80 percent to form a continuous ring  77 . As before, this overlap of areas can be accomplished by sequentially stepping the motor  50  around one complete turn by sufficiently small incremental steps to form the desired overlap or by a pattern of spaced welds that allow some dispersion of any accumulated heat, thereby reducing the tendency for thermal warping of the metal rings  12 . This process can be repeated as many times as is necessary with as many foil sheets  12  as is necessary to form a bellows of the desired dimensions.  
     [0042] Another apparatus  100  for forming a bellows from a supply of ring shaped metal foil sheets  12  is schematically illustrated in FIGS.  4 - 7 . The apparatus includes a dial plate  102  that is rotated in step wise fashion in the direction of arrows D around a rotation axis  104  by a suitable motor  105 . A plurality of disk holding pots  106  are carried by the dial plate  102 , and each of the disk holding pots  106  is intended to carry a plurality of ring shaped metal foil sheets  12  that are being assembled into a bellows. While FIG. 4 shows there to be six pots  106  carried by the dial plate  102 , it will be appreciated that the number of pots is a matter of choice of design. The disk holding pots  106  are carried by the dial plate  102  past a number of stations that perform a variety of functional steps in the manufacture of a bellows. Two supply stations  108  and  110  are positioned adjacent to the dial plate  102  to supply one ring shaped metal foil sheet  12  to each pot  106  as each pot becomes suitably positioned adjacent to the supply station. The structure and operational mechanisms of each supply station  108 ,  110  can be the same as that described in connection with the dispenser  14  shown in FIG. 1.  
     [0043] Two sensor stations  112  and  114  are provided to detect the presence of an added ring shaped metal foil sheet  12  lying freely on top of any preexisting foil sheets. In the event that a supply station has malfunctioned, the laser welding operation needs to be suspended so that suitable correction of the supply process can occur. The sensor stations  112  and  114  can be employed to detect other conditions as well, for example, the proximity of the top ring to a prescribed datum indicating the progress of the bellows forming process, and the temperature of the work piece so that the laser energy input can be modified to compensate for thermal variations in the materials supplied. One or both of the sensor stations  112  and  114  can be used with a suitable robotic apparatus (not shown) to remove a finished bellows from the apparatus  100 . One or both of the sensor stations  112  and  114  can also be used as insertion locations for inserting end plates that can include couplings for coupling the finished bellows to other apparatus at the completion of the manufacturing process. Two welding stations  116  and  118  are provided for performing the laser welding operation. The two stations  116  and  118  differ from each other in that station  116  is dedicated to welding the outer margin  56  of the ring shaped metal foil sheets  12  while station  118  is dedicated to welding the inner margin  60  of the ring shaped metal foil sheets. A form of station  118  is shown in greater detail in FIG. 5 while a form of station  116  is shown in greater detail in FIG. 6.  
     [0044] As seen in FIG. 5, the dial plate  102  includes openings  120  sized to receive the holding pots  106 . The inside diameter of the openings  120  closely approximates the outside diameter of a lower portion  122  of the holding pots  106  so that the permitted relative motion between the dial plate  102  and the pot  106  is merely vertically in the direction of arrows E. Each holding pot  106  includes an upper portion  123  of somewhat larger diameter than lower portion  122  having a lower edge surface  121  adapted to rest on an upper surface  101  or dial plate  102  when the holding pot  106  is lowered to a lowermost position. Each holding pot  106  includes an upper surface  107  intended to support a lowermost of the plurality of the ring shaped metal foil sheets  12 . The upper perimeter surface  107  includes a plurality of perimeter guide rods  109  that project upwardly from surface  107  and assist in centering the foil sheets  12  on the surface  107  of the holding pot  106 . An upward motion of the pot  106  is achieved by a power lift mechanism  124  connected to three vertically movable rods  126 ,  128  and  130 , which are located at the station  118  and controlled by power lift control  125 . The power lift mechanism  124  can cause the rods  126 - 130  to move the pot  106  upward relative to the dial plate  102  until the stack of ring shaped metal foil sheets  12  carried by pot surface  107  comes into contact with a ring anvil  132  positioned at the station  118  above the power lift mechanism  124 . An upper end  111  of the perimeter guide rods  109  is received in locating openings  134  in a lower surface of the anvil  132  as the holding pot  106  reaches a full upward extent of motion. A welding operation is then performed on the top pair of ring shaped metal foil sheets  12  to form a circular weld line spaced from the inner margin  60  of the metal rings  12  carried by the holding pot  106 .  
     [0045] In order to perform the welding operation, the station  118  includes an axial laser delivery mechanism  136  fixed to an overhead support  138  so as to be aligned with a center  105  of the holding pot  106  when the dial plate  102  is properly positioned at station  118 . A preferred form of the axial laser deliver mechanism  136  is shown in FIG. 5 to include a tubular support  139  fixed to the bottom of the overhead support  138 . A first mirror  140  is fixed to receive a laser beam through opening or window  142  in the tubular support  138  from a laser source  144  shown in FIG. 4. The first mirror  140  is situated to reflect the laser beam received from the laser source  144  downward to a lens system  146  positioned directly above the center  105  of the holding pot  106 . The lens system  146  is designed to spread, and preferably re-collimate, the laser beam that proceeds downwardly. A conical reflector  148  is positioned at the bottom of the tubular support  138  which will direct any impinging laser energy radially outward through a cylindrical window  150 , preferably formed by a dielectric substance that is transparent at the wavelength of the laser, surrounding the conical reflector  148 . A ring reflector  152  is fixed to the anvil  132  so as to intercept the laser energy traveling radially outward from the conical reflector  148  and focus it toward the circular weld line  77  spaced from the inner margin  60  of the metal rings  12  carried by the holding pot  106 . Any movement of the first mirror  140  can be controlled by a suitable mirror controller  141  so as to form the series of modified conduction welds as disclosed in connection with FIGS.  1 - 3 . Alternatively, the mirror  140  can be fixed in position so that a continuous disk of laser energy proceeds outwardly from the conical reflector  148  to the ring reflector  152  and downwardly through a ring-shaped lens  153  to the circular weld line  77  adjacent the inner margin  60  of the metal rings  12  to simultaneously form the desired weld line  77  around the entire circumference adjacent the inner margin  60 .  
     [0046] Each holding pot  106  returns to an initial rest positioning the dial plate  102  upon completion of each welding operation. The return can merely be gravitationally although it is preferable that the power lift mechanism  124  is powered to return to a lowermost position so that the lift rods  126 - 130  are quickly freed from contact with the lower surface of the holding pot  106 . The time required to move the holding pot  106  both upward and downward, plus the time used during the actual welding operation defines most of machine cycle. A remaining portion of the machine cycle is the time required for the holding pot  106  to be transported to the next station by lateral movement of the dial plate  102 . The lateral movement of the dial plate not only moves one holding pot  106  from station  118  to the next station, the movement simultaneously moves another holding pot  106  from a preceding station to station  118  so that the welding step can again be performed on the top pair of sheets in the next stack of ring shaped sheets  12 .  
     [0047] A preferred embodiment of welding station  116  of the present invention is shown in FIG. 6 to include an anvil  160  that is of considerably different character than anvil  132  of welding station  118 . The dial plate  102 , holding pots  106 , perimeter guide rods  109 , and related structure are, of course, the same as shown in FIG. 5 since these elements of the apparatus  100  are common to both welding stations  116  and  118 . The upward motion of the pot  106  is again achieved by a another power lift mechanism  124  connected to three vertically movable rods  126 ,  128  and  130 , which are located at the station  118  and controlled by another power lift control  125  as shown in FIG. 4. It will be appreciated that both power lift controls  125  could be merely portions of the same control or commonly controlled by a computer such as computer  52  shown in connection with FIG. 1. The power lift mechanism  124  can cause the rods  126 - 130  to move the pot  106  upward relative to the dial plate  102  until the stack of ring shaped metal foil sheets  12  carried by pot surface  107  comes into contact with a ring anvil  160  with the upper end  111  of the perimeter guide rods  109  is received in locating openings  162  in a lower surface of the anvil  160  as the holding pot  106  reaches a full upward extent of motion. A welding operation is then performed on the top pair of ring shaped metal foil sheets  12  to form a circular weld line  68  spaced from the outer margin  56  of the metal rings  12  carried by the holding pot  106 .  
     [0048] The anvil  160  can be formed entirely of a dielectric substance that is transparent at the wavelength of the laser  162 . A suitable substance is a borosilicate crown glass such as Schott&#39;s DURAN® Code #  8330 . An axial laser delivery mechanism  136 , similar to that shown in FIG. 5, is fixed to an overhead support  164  so as to be aligned with a center  105  of the holding pot  106  when the dial plate  102  is properly positioned at station  116 . A preferred form of the axial laser deliver mechanism  136  is shown in FIG. 6 to include a tubular support  139  received in an opening  166  of the overhead support  164 . A first mirror  168  is fixed to receive a laser beam from the laser source  162  shown in FIG. 4. The first mirror  168  is situated to reflect the laser beam received from the laser source  162  downward to a lens system  170  positioned directly above the center  105  of the holding pot  106 . The lens system  170  is designed to spread, and preferably re-collimate, the laser beam that proceeds downwardly. A conical reflector  172  is positioned at the bottom of the tubular support  138  which will direct any impinging laser energy radially outward through a cylindrical window  174  surrounding the conical reflector  172 . The laser energy traveling radially outwardly form the conical reflector  172  passes through a ring-shaped lens  176 , which is fixed to support  164 . A ring reflector  178  is fixed to the anvil support  164  so as to redirect the laser energy passing through lens  176  downwardly toward the circular weld line  68  spaced from the outer margin  56  of the metal rings  12  carried by the holding pot  106 .  
     [0049] Any movement of the first mirror  168 , like mirror  140 , can be controlled by a suitable mirror controller  180  so as to form the series of modified conduction welds as disclosed in connection with FIGS.  1 - 3 . Alternatively, the mirror  168  can be fixed in position so that a continuous disk of laser energy proceeds outwardly from the conical reflector  172  to the ring reflector  178  and downwardly to the circular weld line adjacent the outer margin  56  of the metal rings  12  to simultaneously form the desired weld line. The laser  162 , mirror controller  180 , as well as laser  144  and mirror controller  141 , can be controlled by a common computer such as computer  52  as shown in FIG. 1. An exploded view of the axial delivery mechanism  136  is shown in FIG. 7. The first lens system  146 ,  170  is shown to include an outwardly extending flange  182  that can be sized to rest on the upper lip  184  of tubular support  139  as shown in FIG. 6. Alternatively the outwardly extending flange  182  can be sized to rest on internal step  186  of tubular support  139  as shown in FIG. 5.  
     [0050] In applications of the present invention for forming a bellows of foils of less than 0.2 mm thickness made of a non-ferrous material such as 3000 series aluminum, the lasers will likely be operated at between 700 watts and 2 kW, peak power. When used with a ferrous material such as 304L stainless steel foils of a similar thickness, the lasers will likely be operated at between 250 watts and 1 kW, peak power. Further, the power output of the laser is preferably variable in proportion to the processing speed of the assembly apparatus  100 , as well as other parameters. Of particular interest is the profiling of the duration of the laser  144 ,  162  as a function of time such as that shown graphically in connection with FIG. 8. The energy of the pulse is digitally programmed and focused to achieve a modified conduction weld that exhibits greater depth than ordinarily achieved with conventional laser conduction welding as shown by the elongated nuggets forming the weld lines  70  and  76  in FIG. 3. The digital programming of the present invention generally includes an initial energy burst  190  followed by a rest phase  192 . Preferably, a more intense energy pulse  194  is delivered after the rest phase  192  followed by a stepped reduction phase  196  in which the energy is reduced in a controlled fashion as a function of time until the desired metal penetration is achieved.  
     [0051] Desirably, the energy delivered by the initial energy burst  190  is sufficient to cause a slight melting of the top surface of the top metal foil ring  12 , thus significantly increasing the absorption characteristics of the top metal foil. The rest phase  192  is included to allow for the melting to achieve the maximum extent, thereby effecting a large beneficial reduction in reflection characteristics and enhancement of absorption characteristics of the metal surface prior to any further laser energy delivery. The laser  144 ,  162  can continue to deliver some energy  191  during the rest phase  192  to off set any tendency of the work piece  12  to cool. The rate of energy delivery during the rest phase  192  is much less than either the initial burst  190  or the subsequent intense energy pulse  194  and stepped reduction phase  196 .  
     [0052] Following the development of the desired amount of surface melt phase change, the laser  144 ,  162  is caused to deliver a significant pattern  194  of laser energy focused deeply into the top foil layer, preferably at the interface between the top two foil layers  62  or  78  as discussed in connection with FIG. 3. This focused delivery of laser energy onto a surface that has been modified to enhance the metal absorption of energy by the prior initial pulse  190  and soaking rest  192  results in a deeply penetrating delivery of energy during phase  194  causing melting in the vicinity of the interface between the top two layers  62  or  78 , and a liquid metal pool that is as much as twice as deep as it is wide so as to appear in cross-section as elliptical or parabolic instead of hemispherical, which is the typical cross-section achieved by ordinary laser conduction welding.  
     [0053] While the illustrated embodiment of FIG. 1 illustrates laser welding generally perpendicular to the top surface of the metal rings  12 , it will be readily appreciated by those skilled in the art that an alternative apparatus could be constructed by the replacement of mirror  46  with a gimbaled mirror such as mirror  140  and mirror controller  141  as shown in FIG. 5 that could direct the laser pulses directly to the same locus of points as shown in FIG. 2 by omitting one or both of the ring reflectors  54  and  58  of FIG. 1. In this alternative embodiment the laser pulses are necessarily angled with respect to the top surface of the pair of metal rings to be welded, which generally increases the reflection experienced and can represent a problem if the angle is too great.  
     [0054] The embodiments shown in the Figures are merely illustrative of the broad aspects of the invention, and optical and mechanical arrangements other than that illustrated can be employed that will incorporate the basic features and advantages of the present invention. For example, a fiber optic delivery of the power directly to the axis of the bellows being formed permits the adaptation of this system to a further variety of layouts. Further, while FIG. 4 shows a dial plate  102  carrying the pots  106  around a circle, it will be appreciated that the pots  106  could be carried by other functionally equivalent apparatus in a closed loop to accomplish the same function.  
     [0055] From the forgoing description of the structure and operation of a preferred embodiment of the present invention, it will be apparent to those skilled in the art that the present invention is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without exercise of the inventive facility. Accordingly, the scope of the present invention is defined as set forth of the following claims.