Abstract:
A method of fabrication is adapted to weld paired metal plates, particularly fuel cell bipolar plates, using a partial vacuum to hold the paired metal plates together during the welding process. Each bipolar plate has a plurality of contact surfaces for joining the paired metal plates. The contact surfaces of a first plate are co-aligned with contact surfaces of a second plate. An outer perimeter and a plurality of reactant gas channels and ports of each plate pair are sealed to form a sealable interior volume. A partial vacuum is drawn in the interior volume to clamp each plate pair together at the contact surfaces and a weld joint is made between at least a plurality of the contact surfaces. Laser welding is preferably used for its ability to fuse the contact surfaces together without perforating the plates and thereby losing the partial vacuum.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and system to clamp and weld a bipolar plate assembly and the product thereof. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been proposed as a power source in a variety of vehicular applications, as well as other devices. One example of a fuel cell is the proton exchange membrane (PEM) fuel cell. PEM fuel cells include membrane-electrode-assemblies (MEAs) having a thin, proton-conductive, membrane-electrolyte with an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof. The MEA may also include a diffusion media for dispersing the reactant gases across the catalytic faces. The membrane-electrode assembly is sandwiched between a pair of electrically-conductive bipolar plate elements which serve as a current collector for the anode/cathode of the fuel cell and contain a plurality of lands and channels in the faces thereof for distributing the fuel cell&#39;s gaseous reactants (e.g., hydrogen and oxygen/air). 
     Each bipolar plate serves as an electrical conductor between adjacent fuel cells and is provided with a coolant flowing within a plurality of internal heat exchange passages to remove heat from the fuel cell. The common bipolar plate is an assembly constructed by joining two separate metal sheets or plates each having external facing reactant gas channels and internal facing coolant channels. In order to conduct electrical current between the anode portion of one cell and the cathode portion of the next adjacent cell in the fuel cell, the paired plates forming each bipolar plate assembly are mechanically and electrically joined. 
     Several methods to join bipolar plates are well known. In an exemplary application, the U.S. Pat. No. 5,776,624 issued to Neutzler provides a plurality of lands, which are mechanical connection points between plate pairs forming a bipolar plate. The plurality of lands in the Neutzler device are joined by a brazing process wherein the material used for brazing is carefully controlled to limit the insoluble metal which can leach from the brazed joints. The Neutzler brazing technique is effective at electrically joining adjacent plates of a bipolar plate assembly, but, difficult and costly to ensure a sufficient bond between the plates. Thus, an improvement providing a less expensive and less material critical joining method is desirable. 
     To limit the leaching problem identified above, brazed joints between the plates of a bipolar plate assembly are replaced by welded joints. In order to maintain the necessary metal-to-metal contact for welding, and, to ensure that the proper welding gap is provided, external pressure plates are commonly used to clamp the plates together and physically hold them during the period of time when welding takes place. Several drawbacks to the pressure plate welding method exist. First, a plurality of apertures or access holes must be included within the pressure plate(s) to provide access for the welding torch and welding beam (e.g., laser welding) to contact the desired surfaces of the plates. These apertures increase the cost and complexity for welding bipolar plates, particularly for complex bipolar plate channel and land geometries. A pressure plate prepared for a complex geometry of channels and lands generally can only be used for that design alone, which requires multiple pressure plate designs to accommodate various bipolar plate designs. This decreases the opportunity to use a particular set of pressure plates for welding more than one bipolar plate design because the arrangement of apertures in a pressure plate is highly dependent on the configuration of channels and lands on the individual plates forming each bipolar plate. 
     Another drawback of the pressure plate welding method results from the contact pressure adjacent to the individual welding sites which is lost by providing the welding apertures themselves. The pressure required to maintain clearance for welding is not significant; however, localized gaps between the paired plates forming the bipolar plates can occur where the apertures for welding do not provide sufficient force to maintain the paired plates in contact for welding. 
     A further drawback of the pressure plate welding method results because the plate thickness of the pressure plate increases the welding head separation from the welded surfaces. In a laser welding application, increasing this distance normally requires the addition of a special lens having a longer focal length and a smaller working angle which increases the cost of such a system. Also, a reduced percentage of acceptable weld joints can result. 
     It is therefore desirable to provide a method and system for clamping and welding pairs of plates to form bipolar plates which eliminates the need for pressure plates and therefore the expense and limitations of the pressure plate design. It is also desirable to provide a method and system for clamping and welding bipolar plates which reduces yet accommodates the occurrence of localized gaps between plate pairs to improve bipolar plate joining. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method for joining fuel cell plates is provided. A pair of plates, forming a bipolar plate, each having a plurality of contact surfaces, are aligned and sealed about a circumference of the pair of plates. A partial vacuum is induced between the pair of plates which draws the two plates together in metal-to-metal contact at the contact surfaces. The plates are then welded together at the contact surfaces. A laser welding process is preferred to weld the bipolar plate contact surfaces together because the laser process permits fusing the contact surfaces without perforating the plate(s) and thus losing the partial vacuum. 
     In another aspect of the present invention, a method to clamp and weld fuel cell plates provides pairs of plates having contact surfaces and outer perimeters which are sealed to form an interior volume therebetween. A vacuum pump is then operated to draw a partial vacuum in the interior volume. A welder is operated to join the contact surfaces together. The partial vacuum is then released. 
     In yet another aspect of the present invention, a bipolar plate connection system includes plates having a plurality of coolant apertures, coolant channels, and lands. A vacuum attachment point is made at one of the coolant apertures for each plate of a plate pair to provide for attachment of a vacuum pump. A partial vacuum drawn by the vacuum pump between the plates draws the plates together for welding at contact surfaces defined by the intersection of adjacent land pairs between the plates. A weld joint then joins the plates at the desired contact surfaces. 
     In still another preferred embodiment of the present invention, a vacuum assisted welding system includes the outer perimeters of each of a pair of plates being sealable using a temporary seal. A partial vacuum is drawn in the interior volume between the plates. A plurality of contact surfaces are welded together. The temporary seal is removed following the initial welding of the contact surfaces between each of the plates. The outer perimeter is then permanently sealed by welding. 
     There are several benefits of the present invention to providing a partial vacuum to temporarily hold plates forming a bipolar plate assembly for welding. Coolant channels are generally available across a major portion of the surface of each plate forming a plate pair. Each of the coolant channels are generally joined such that a partial vacuum drawn at one aperture or section of a coolant channel will create a partial vacuum across the entire internal surfaces of the plate pair. The partial vacuum draws the entire facing surfaces of each of the plates together such that a mechanical exterior pressing plate is not required. By using the partial vacuum, any geometry of coolant channels, reactant gas transfer channels, or welding lands/contact surfaces can be accommodated. A simple connection point between a vacuum pump and the pairs of plates is all that is required. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a perspective view showing a preferred embodiment of the vacuum-assisted bipolar plate welding system of the present invention; 
         FIG. 2  is an exploded perspective view of the bipolar plate assembly of the present invention; 
         FIG. 3  is a partial section view taken at Section 3 of  FIG. 1 , showing a laser welding beam fusing adjacent contact surfaces between paired plates to form a bipolar plate assembly; and 
         FIG. 4  is a partial section view taken at Section 4 of  FIG. 2 , identifying a separator plate positioned between plate pairs and an allowable welding gap between the contact surfaces of each plate and the separator plate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , a preferred embodiment of a bipolar plate assembly  10  of the present invention is shown. The bipolar plate assembly  10  includes a first plate  12  and a second plate  14 . Visible on the first plate  12  are a plurality of lands  16 . The plurality of lands is separated by a plurality of channels  18 . The second plate  14  also includes both a plurality of lands and channels (not shown). The channels  18  form a flow field to transport a reactant gas across the face of the first plate  12 . A plurality of coolant flow ports  20  provide either an inlet or an outlet for coolant traversing individual plates of the bipolar plate assembly  10 . The two plates  12 ,  14  of the bipolar plate assembly  10  each join at a flanged area  22  which is defined about a perimeter of the individual first plate  12  and the second plate  14 , respectively. The geometry of the channels  18  are shown as serpentine flow channels arranged in a mirrored configuration. However, one skilled in the art will recognize that the present invention is not limited to a specific flow field design but has application to bipolar plates of similar design independent of the flow field geometry. 
     In the exemplary embodiment shown, to join the first plate  12  to the second plate  14 , the flanged area  22  is preferably permanently sealed using a weld joint. The flanged area  22  can also be temporarily sealed as will be discussed below with reference to  FIG. 2 . The remaining coolant flow ports  20  are temporarily covered to seal the interior volume defined between the plates  12 ,  14 . One of the coolant flow ports  20  is used as a vacuum attachment point. A vacuum pump  24  is connected by a vacuum hose  26  to one of the coolant flow ports  20  previously discussed. Ports  28  which provide entrance or exit flow for reactant gas(es) are also sealed for this welding procedure if the ports  28  form a boundary of the coolant area. In this exemplary embodiment, the plates  12 ,  14  are sealed/joined at the flanged area  22  and at the ports  28 , both to enable formation of a partial vacuum between the plates  12 ,  14 , and to prevent subsequent intermixing of hydrogen and air (i.e., normal reactant gases). 
     The vacuum pump  24  is operated to create a partial vacuum in an interior coolant area between the first plate  12  and the second plate  14  in order to draw the first plate  12  and the second plate  14  into metal-to-metal contact at a plurality of contact surfaces used for fusing the two plates  12 ,  14 . As best seen in  FIG. 3 , the contact surfaces are defined on the interior faces of the plates  12 ,  14 . After a partial vacuum is formed between the first plate  12  and the second plate  14 , a welding apparatus  30  fuses the first plate  12  and the second plate  14  together at a plurality of the contact surfaces. If a temporary seal is used at the perimeter flanged area  22 , the temporary seal is then removed and a permanent weld joint is made about the flanged area  22 . 
     With reference to  FIG. 2 , a bipolar plate assembly  34  further includes an optional spacer plate  32  disposed between a first plate  36  and a second plate  38  with additional flow paths for coolant there-between. When the spacer plate  32  or similar element is positioned between plates  36 ,  38 , each plate of the pair must be welded to the spacer plate  32  as best seen in reference to  FIG. 4 . A plurality of coolant apertures  40  are disposed in the spacer plate  32  to ensure that a fully turbulent flow of coolant exists between the two plates  36 ,  38 . A plurality of reactant gas ports  42  are also identified, which provide inlets or outlets for reactant gas flow to each of a plurality of channels  44  formed in the face of the plates  36 ,  38 . 
     In the embodiment shown in  FIG. 2 , to form the bipolar plate assembly  34 , a first plate flange  46  is aligned with a second plate flange  48  and the two flanges are preferably welded. All interface areas between reactant gas channels or ports and the coolant volume are similarly welded. The welded joints can be formed by any welding method, provided the permanent seal surrounds the coolant volume as well as the plate perimeters. If a temporary joint is formed between the first plate flange  46  and the second plate flange  48 , a seal  50  is disposed between the first plate flange  46  and the second plate flange  48  and the two plates  36 ,  38  are butted. All interface areas between reactant gas channels or ports and the coolant volume are similarly sealed. The seal  50  and the interface seals can be a gasket or a suitable removable sealant or sealant bead (not shown). Exemplary removable sealants include epoxies and similar adhesives. A partial vacuum is formed as discussed in reference to  FIG. 1  and the contact surfaces of the bipolar plate assembly  34  are fused. Similar to the bipolar plate assembly  10 ,  FIG. 2  shows a plurality of lands  52  between the channels  44 , and a plurality of coolant flow ports  54 . 
     Referring to  FIG. 3 , the partial cross section  3  of  FIG. 1  is detailed. The first plate  12  includes the plurality of channels  18  on an outer face  56  and a plurality of coolant channels  58 . The second plate  14  is similarly formed. Each groove  18  and each coolant channel  58  is generally disposed sequentially across the section of each plate  12  and  14 . The partial vacuum formed in the coolant channels  58  draws the first plate  12  into physical contact with the second plate  14 . A laser beam  60  is shown aligned with a base of one of the channels  18 . The laser beam  60  forms a weld zone or fusion area  62  between abutting contact surfaces  64  of both plates  12 ,  14 . The plurality of coolant channels  58  thus form a coolant flow field between the first plate  12  and the second plate  14 . 
     Referring to  FIG. 4 , the partial cross section  4  of  FIG. 2  is detailed. The spacer plate  32  separates the first plate  36  from the second plate  38 .  FIG. 4  further identifies an allowable clearance gap “A” between a contact surface  66  of the first plate  36  and a spacer plate first surface  68 . The allowable clearance gap “A” is also shown between the second plate  38  and the spacer plate  32 . The partial vacuum formed in each of a plurality of coolant channels  70  draws the first plate  36  into physical contact with the spacer plate  32  at the contact surfaces. A laser beam  72  functions similar to the laser beam  60  (shown in  FIG. 3 ), forming a plurality of fusion areas  74 . Similarly, the partial vacuum formed in a plurality of coolant channels  76  of the second plate  38  similarly draws the second plate  38  into physical contact along a spacer plate second surface  78  and a plurality of contact surfaces  80  of the second plate  38 . A laser beam  82  creates each of a plurality of fusion areas  84 . Both fusion areas  74 ,  84  differ from the fusion area  62  shown in  FIG. 3  by the volume of fused material between the plates due primarily to the allowable clearance gap “A”. 
     The allowable clearance gap “A” is shown in  FIG. 4 , but can also occur between the plates  12 ,  14  of  FIG. 3 . The allowable clearance gap “A” will vary depending upon a variety of conditions. These conditions include the thickness of the plates  12 , 14  and  36 , 38 , the thickness and flatness of the spacer plate  32 , the material of the plates  12 ,  14  and  36 ,  38  and the spacer plate  32 , the type and energy of the welding equipment used, and the welding speed employed. An exemplary range of dimensions for the allowable clearance gap “A” ranges between 0 to approximately 20% of the material thickness, meaning 0 to approximately 0.02 mm for an exemplary 0.1 mm material thickness. 
     In accordance with the present invention, the complexity and geometry of various flow fields and thus contact surfaces can be accommodated with laser and other welding methods using a partial vacuum formed by the systems and methods of the present invention. The welding system employed can be readily programmed to form an intricate pattern of weld zones which may be repeatedly reproduced in an automated process. 
     In a preferred embodiment, the partial vacuum between plates  12 ,  14  and  36 ,  38  is formed over a range of pressures between approximately 200 grams/square centimeter (g/cm 2 ) up to approximately 800 g/cm 2 . In another preferred embodiment, any partial vacuum (defined as an absolute pressure below atmospheric pressure) can be used depending on the geometry of the plates. In an exemplary case, as a result of partially evacuating the interior volume to approximately 200 g/cm 2 , an external clamping force of approximately 800 g/cm 2  is generated—the difference between atmospheric pressure (approximately 1000 g/cm 2 ) acting on the exterior surface of the plates and 200 g/cm 2  acting on the interior surface. This will generate a force of approximately 800 grams per square centimeter or greater to hold the plates  12 ,  14  together. For an exemplary bipolar plate assembly  10  having a surface area of approximately 800 cm 2 , the clamping force exerted on the plates  12 ,  14  at 200 g/cm 2  absolute is approximately 640 kg. 
     By monitoring the partial vacuum, the time required to form the initial contact between a pair of plates indicates the viability of the perimeter seal. The partial vacuum is sustained until an engagement condition is achieved between the two plates. The engagement condition is defined as contact or acceptable welding clearance between at least one contact surface of each of the two plates in an adjoining arrangement for welding. The engagement condition is sustained by the partial vacuum for a time period sufficient to form at least one weld joint. It is expected that an absolute pressure of approximately 500 gm/cm 2  is sufficient to produce the force necessary to seal common plate pairs. Thereafter, vacuum changes can be monitored as an indication of perforations in the plate(s). 
     The partial vacuum formed between the plates is normally capable of pulling the plates into intimate contact, including areas having slight defects in the plates (e.g., flatness or incompletely/improperly formed lands or channels). In another preferred embodiment, the partial vacuum can be changed to suit plate geometry. A loss of the partial vacuum during the welding process indicates a hole is formed in the plate to which the laser beam is applied. This indication provides the benefit of a quality check, as a hole destroys the function of the bipolar plate in the fuel cell. 100% welded contact surfaces are desirable where possible, however, particularly complex plate/groove geometry makes 100% welded contact surfaces impractical from a production standpoint. Therefore, 100% of the contact surfaces between joined plates do not have to be fused to form bipolar plates using the systems and methods of the present invention. 
     In still another preferred embodiment of the present invention, in place of the partial vacuum drawn within the coolant volume between the two plates forming a bipolar plate assembly, an external pressure can be used to bring the two plates into intimate contact for welding/sealing. In an exemplary application, the external pressure can be generated by placing the sealed bipolar plate assembly in a pressure chamber to provide an elevated external pressure. A pressure bleed line (in place of the vacuum hose  26  of  FIG. 1 ) is connected to bleed the interior volume of gas (e.g., air or pressure chamber welding gas) to outside the pressure chamber, such that the external pressure forces the two plates into intimate contact for welding the contact surfaces. 
     The pressure-assisted bipolar plate welding assembly systems and methods of the present invention offer several advantages. A complex variety of contact surfaces between adjacent plates can be welded. A partial vacuum created between the plates sufficiently clamps the plates in a metal-to-metal contact position (or within an acceptable clearance dimension) for welding. In one preferred embodiment, a laser welding head has full access to each of the contact surfaces for welding because a pressure plate previously known in the art is not required when the partial vacuum is used to draw the individual plates into contact. By using an existing coolant port, the partial vacuum connection point for a selected plate of the bipolar plate assembly can be easily positioned. The coolant port therefore provides ready access to the interior volume such that the partial vacuum ensures sufficient clamping over a majority of the contact surfaces for welding between the plates. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, laser welding is identified as one preferred welding method. Other welding methods such as spot welding can be used. For a welding method which requires an inert gas atmosphere, the system and method of the present invention can be used, provided the vacuum source (e.g., vacuum pump) is connected remotely from the inert gas working envelope. The flanged perimeter of the pair of plates and the ports of each of the pair of plates can also be crimped prior to applying the partial vacuum. Coolant flow ports are identified as the connection point for vacuum pump connection. Other geometries of bipolar plates having alternate ports for connection of a vacuum pump are also possible.