Abstract:
A method and system is provided to join workpieces where a high energy heat source is used to create discrete holes in the workpieces and a filler material is deposited in the discrete holes to create separate fasteners that join the workpieces together.

Description:
PRIORITY 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 61/668,808, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to a systems and methods for hot wire processing. More specifically, the subject invention relates to systems and methods for forming a specialized joint using a hot-wire process to create discrete joining portions. 
       BACKGROUND 
       [0003]    Unlike an arc welding method, hot wire processes do not use an arc between a consumable wire and a workpiece to transfer filler material to a molten puddle. More specifically, in a hot wire or filler wire process between a wire and workpiece, a laser (or other high heat source) heats and melts a workpiece to form a molten puddle. A filler wire is advanced towards a workpiece and the molten puddle. The wire is resistance-heated by a separate energy source, for example, a welder such that the wire approaches or reaches its melting point and contacts the molten puddle. The heated wire is fed into the molten puddle for carrying out the hot wire process. Accordingly, transfer of the filler wire to the workpiece occurs by simply melting the filler wire into the molten puddle. This process is known in the making of continuous welding/coating beads. 
       SUMMARY 
       [0004]    Embodiments of the present invention provide for systems and methods of forming a joint between two or more workpiece members. In one embodiment, a method is provided for forming a lap weld between a first workpiece at least partially overlapping a second workpiece. The method includes forming a first portion of a keyhole in the first workpiece; forming a second portion of said keyhole in the second workpiece; and performing a hot wire process with a filler wire disposed in the keyhole to form a rivet within the keyhole. The hot wire process does not generate an arc within the keyhole between the filler wire and at least one of the first workpiece, second workpiece and a molten puddle of the hot wire process. In an alternate embodiment, the hot wire process uses a laser beam in combination with a controlled arc at the filler wire. However, unlike prior methods, no continuous bead is created. 
         [0005]    Another embodiment provides a lap joint between a first workpiece at least partially overlapping a second workpiece. The joint includes a keyhole extending through said first and second workpieces. The keyhole has a first portion in the first workpiece and a second portion in the second workpiece. In one aspect, the first portion is preformed and defined by an inner surface of said first workpiece. A rivet is formed in the workpiece; the rivet is formed by a hot wire process within the keyhole such that the rivet is a solid combination of a filler wire material and base material of each of the first workpiece and second workpiece. In another particular embodiment, the first and second workpieces are of dissimilar materials. 
         [0006]    These and other features of the claimed invention, as well as details of illustrated embodiments thereof, will be more fully understood from the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The above and/or other aspects of the invention will be more apparent by describing in detail exemplary embodiments of the invention with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is an illustrative view of a hot wire processing system forming an exemplary lap weld joint; 
           [0009]      FIG. 2  is a detailed view of the lap weld joint formation with the system of  FIG. 1 ; 
           [0010]      FIG. 3A  is a schematic view of a laser beam in an embodiment of the subject hot wire process; 
           [0011]      FIG. 3B  is a cross-sectional view of an illustrative rivet formed in a lap weld joint between two workpieces using the system of  FIG. 1 ; 
           [0012]      FIG. 4A  is a cross-sectional view of a partially formed rivet with a preformed portion of a keyhole using the system of  FIG. 1 ; 
           [0013]      FIG. 4B  is a cross-sectional view of a rivet formed within another partially preformed keyhole to form a lap weld between two workpieces of dissimilar materials using the system of  FIG. 1 . 
           [0014]      FIG. 5A  is a plan illustrative embodiment of a lap weld joint between two work pieces having multiple rivets using the system of  FIG. 1 ; 
           [0015]      FIG. 5B  is a cross-sectional view of the lap weld joint along line VB-VB. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Exemplary embodiments of the invention will now be described below by reference to the attached Figures. The described exemplary embodiments are intended to assist the understanding of the invention, and are not intended to limit the scope of the invention in any way. Like reference numerals refer to like elements throughout. 
         [0017]    Shown in  FIG. 1  is a representative system  100  for performing a weld or joining operation using hot wire process. The system shown is using a laser as a heat source, but embodiments are not limited to the use of a laser an other high energy heat sources can be used, consistent with the descriptions herein. Further details of the system  100  are shown and described in U.S. Patent Publication No. 2011/0297658 which is attached as Exhibit A and incorporated by reference herein in its entirety. 
         [0018]    Shown in  FIG. 2  is a detailed view of the hot wire system  100  forming a lap joint  200  between a first workpiece  205  and a second workpiece  210 . In the subject lap joint  200 , a portion of the first workpiece  205  overlaps and engages a portion of the second workpiece  210  to define an overlap interface  215 . Extending through the overlapping regions of the workpieces and the interface  215  is a keyhole  220 . The keyhole is defined by a first portion  220   a  extending through the first workpiece  205  and a second portion  220   b  extending through the second workpiece  210 . As used herein, the term “keyhole” is intended to mean extending through the entirety of the thickness of the workpieces. 
         [0019]    In one embodiment, the keyhole  220  is formed by the laser beam  110  melting the base material in each of the first and second workpieces  205 ,  210 . More specifically, the laser beam  110  delivers a first density of energy to the first workpiece  205 , measured for example in power per area, e.g., (Watts/square in-W/sq. in.), to melt the base material and form the aperture or opening in the first workpiece  205  to define the first portion of the keyhole  220   a.  The laser beam  110  delivers a second density of energy to the second workpiece  210  to melt the base material and form the aperture or opening in the second workpiece  210  to define the second portion of the keyhole  220   b.  The first and second densities of energy delivered by the laser beam  110 , in one aspect may be function of the base materials to be melted. That is, if the materials are the same the energy densities can be the same. However, if the materials to be joined are different, or have a different geometry, the energy densities can be different to effect proper melting of the respective workpieces. Accordingly, in one aspect of forming the keyhole  220  in the process of lap joint formation may be equal or different depending upon the energy density required to melt the base materials. As shown in the particular embodiment of  FIG. 2 , the laser beam  110  can be delivered to the workpiece via appropriate collimating/focusing optics  115  coupled to a fiber laser delivery subsystem  112 . 
         [0020]    In a first embodiment of the formation of joint  200  and in the formation of the keyhole  220 , the laser beam generates a molten puddle  116  within the keyhole  220 . With the formation of the molten puddle  116 , the filler wire  120  is fed by a wire feeder  150 , as seen in  FIG. 1  and heated via a contact tube  160  coupled to a power supply, such as for example, the power supply  150 . The heating can be via resistance heating. Referring again to  FIG. 2 , as the distal end of the filler wire  120  is melted or nearly melted, the distal end of the filler wire  120  is placed in contact with the molten puddle  116  to transfer filer wire material to the molten puddle  116  within the keyhole  220 . Because the melting distal end of the filler wire  120  is continuously in contact with the molten puddle  116 , the location and current and/or voltage to the filler wire  120  is controlled so as to prevent formation of an arc between the wire  120  and the workpieces  205 ,  210 . Accordingly one particular embodiment of lap weld joint formation provides for forming the joint without an arc generated between the wire  120  and the workpieces  205 ,  210 . 
         [0021]    In exemplary embodiments of the present invention, the energy density is varied, as schematically shown in  FIG. 3A , to alter the depth of the laser energy delivery and more particularly reduce the depth at which the laser maintains the molten puddle  116 . Accordingly as the laser depth is reduced, the base material of the workpieces  205 ,  210  and the filler material deposited in the keyhole  220  mix and solidify to form a continuous rivet  230  as shown in  FIG. 3B . The rivet  230  in one embodiment is a substantially frustro-conical formation extending axially to define a rivet axis Y-Y through the workpieces  205 ,  210 . Accordingly in one aspect, the rivet  230  tapers narrowly in the proximal to the distal direction from the upper surface of the first workpiece  205  towards the bottom surface of the second workpiece  210 . However, other shapes for the rivet  230  can be utilized. For example, the rivet  230  can have a cylindrical shape such that there is no appreciable taper along its length. Furthermore, the rivet  230  can have an elongated shape such that its cross-section (when looking down at the top or bottom or the rivet  230 ) is elongated. Such shapes can include ellipses, ovals, etc. The cross-section of the rivets created should be such that they create the desired mechanical strength for the specific application. 
         [0022]    In exemplary embodiments of the present invention, the first workpiece  205  and the second workpiece are made of the same material material. However, in other embodiments they can be a different material. In the embodiments shown, a laser beam  120  is generated from a laser source and power supply  130  and delivered to the joint formation site at the workpiece. A first energy density (W/sq. in.) is delivered to form the first portion of the keyhole in the first workpiece. A second energy density (W/sq. in.) is delivered within the aperture and to the second workpiece  210  to form the second portion of the keyhole. A filler wire material is extended within the aperture. The filler wire is coupled to a the power source  170  and resistance heated to or near to its melting temperature by a pulsed or AC waveform The filler wire can be fed at either a constant or varied wire feed speed rate. 
         [0023]    In a second alternate embodiment, the hot wire process is substantially similar to that previously described except this second embodiment provides for an arc generated between the filler wire  120  and the workpieces  205 ,  210 . More specifically, the power supply  170  delivers a signal to the filler wire  120  sufficient to form an arc between the wire  120  and the workpiece  205 . Accordingly, an arc formed at wire  120  can be used in combination with the laser beam  110  to form the keyhole  220  and/or within the keyhole control the depth and/or width or diameter of the keyhole  220 . In one aspect and with reference to  FIG. 1 , the feeder  150  is coordinated with the power supply  170  to locate the distal end of the filler wire  120  at a distance from the molten puddle  116  within the keyhole  220  with a desired voltage or current carried in the filler wire to generate an arc within the keyhole  220 . 
         [0024]    Alternate embodiments are provided where one or more of the key hole portions  220   a,    220   b  is preformed prior to application of the laser beam  110 . For example, shown in  FIG. 4A  is a preformed aperture defined by an in inner surface  222  of the workpiece  205  to predefine the first keyhole portion  220   a.  The aperture may be preformed by drilling, punching or any other known form of material removal. Shown is the laser beam  110  extending through the first keyhole portion  220   a  to impact the upper surface second workpiece  210 . The laser beam alone or in combination with the filler wire  120  supply an energy density to define the second portion  220   b  of the keyhole in a manner as described above. The rivet  230  is initially formed within the second keyhole portion  220   b  by the mix and solidification of the base material of the second workpiece  210  and the filler material  120 . The rivet  230  is continuously built by mixing the melting or nearly melting filler material into the molten puddle  116  to complete formation of the weld joint  200   a.  The height of the molten puddle  116  varies with the change in the energy density of the laser beam  110 . Moreover as the molten puddle  116  mixes with filler material of the wire  120 , the inner surface  222  may melt to mix and solidify with the molten puddle  116  to form the rivet  230 . 
         [0025]    One particular embodiment of lap weld joint  200   b  is shown in  FIG. 4B  in which workpieces  205 ,  210  are made of dissimilar materials. For example, the bottom or second workpiece  210  may be made of steel and the first workpiece  205  may be made of Aluminum (Al), Manganese (Mn), Copper (Cu), Ceramic or other material. In one exemplary embodiment, a preformed aperture may be formed in the first workpiece  205  and defined by an inner surface  222 ′. The inner surface  222 ′ includes a first portion  222   a′  to define a first angle θ1 with respect to a vertical parallel to axis Y-Y and a second portion  222   b′  to define a second angle θ2 with respect to a vertical parallel to axis Y-Y. Using the hot wire process previously described, a rivet  230  is formed. The proximal portion  230   a  forms an enlarged head  230   a  to engage and meld with the first portion  222   a′  of the inner surface  222 ′. Accordingly, the rivet  230  and rivet head  230   a  facilitates a mechanical joint between the rivet  230  and the workpieces  205 ,  210 . As shown in the embodiment in  FIG. 4B , the laser does not fully keyhole the workpiece  210  but stops short of fully penetrating. While in other embodiments, the laser beam  110  can fully keyhole, thus causing another head portion to form opposite of the head  230   a.  Various shapes and materials for the rivet  230  can be utilized to achieve the desired strength for the joint. 
         [0026]    It should be noted that although the figures described herein depict a lap joint, embodiments of the present invention can be utilized in other joints. It should also be noted that because of the advantages of the present invention, dissimilar metals can be joined that otherwise react chemically with each other. That is, embodiments of the present invention can use a neutral material layer or spacer between the workpieces  205  and  210  and the material for the rivet  230  can be a neutral material such that dissimilar materials that could not otherwise be joined can be joined by embodiments of the present invention. 
         [0027]    It should be noted that if the workpieces  205  and  210  are of the same or similar materials, in addition to using the strength of the rivet  230  to joint the pieces, embodiments of the present invention can also weld the pieces together using the described hot-wire process. This will increase the mechanical bond of the joint. 
         [0028]    In exemplary embodiments, where the workpieces  205  and  210  are dissimilar the material for the rivet  230  should be selected such that it provides the desired strength and is chemically and metallurgically compatible with the workpieces to be joined. In some exemplary embodiments of the present invention, the rivet  230  is to be formed of a material which is comparable in composition to the material used for the workpieces  205 / 210  having the lowest melting temperature. For example, if aluminum is to be joined with steel the rivet  230  can be formed from an aluminum composition. This ensures that the heat input needed to properly melt the material for the rivet  230  will not causes unwanted melting of the any of the workpieces. For example, if a high melting temp material is used for the rivet  230  (e.g., steel) then its melting may cause unwanted melting of lower temp workpiece component (aluminum). The rivet  230  can also be made of a composition which is different from both of the workpieces as desired. For example, the rivet  230  can be aluminum while the workpieces are steel and ceramic, respectively. 
         [0029]    In one aspect of each of the above described joint formations, the laser does not impact the filler wire throughout the hot wire process. In an alternate aspect the laser does impact the filler wire. To the extent the filler wire  120  is impacted by the laser, the heating signal to the filler wire  120  and feed rate of the filler wire are controlled in a desired manner to ensure proper melting of the wire. Depending upon the width of the workpieces, multiple rivets  230  may be spaced apart to form the complete lap weld joint  200  between workpieces  205 ,  210 . Shown in  FIGS. 5A and 5B , are multiple rivets  230   a,    230   b,    230   c  which can be formed by any one of the embodiments described above to form the lap weld between the workpieces  205 ,  210 . 
         [0030]    While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.