BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved method for joining metals or alloys together. Specifically, the present invention relates to a method comprising the laser alloying of a first metal/alloy piece so that it can be joined with a second metal/alloy piece of a nonidentical composition. 
     2. Description of the Prior Art 
     It is well known in the science of corrosion chemistry that certain metals and/or alloys will corrode if they are joined together. This corrosion results in a deterioration of the joint where the two metals or alloys are joined. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for joining two dissimilar metals or alloy pieces. The invention comprises coating a first surface of a first metal or alloy piece with a precursor layer comprising a binder and elemental/ceramic powder. The invention further comprises irradiating the first surface of the first piece with a laser beam at a sufficient energy and for a sufficient time to produce a surface alloyed layer on the first piece. The invention further comprises allowing the alloyed layer to solidify. The invention further comprises joining a second metal or alloyed piece to the surface alloyed layer of the first piece, wherein the second piece has a composition that is not identical to that of the first piece. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a first method of the present invention. 
     FIG. 2 is a block diagram of a second method of the present invention. 
     FIG. 3 is a side view of a first joining step embodiment of the present invention. 
     FIG. 4 is a side view of a second joining step embodiment of the present invention. 
     FIG. 5 is a block diagram of a third method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first method of the present invention is directed toward joining two dissimilar metal or alloy pieces, as shown in FIG.  1 . This method comprises coating a first surface  32  of a first metal or alloy piece  30  with a precursor layer, comprising a binder and elemental/ceramic powder, as shown in Block  10  of FIG.  1 . In one preferred embodiment, the first piece comprises steel and the precursor comprises chromium. In this embodiment, the second piece may comprise aluminum, and the joining may be accomplished by diffusion bonding. 
     In another embodiment, the first piece comprises titanium-6 aluminum-4 vanadium and the precursor comprises chromium. In another embodiment, the first piece comprises aluminum or an aluminum alloy and the precursor comprises chromium or nickel. 
     The invention further comprises irradiating the first surface  32  of the first piece  30  with a laser beam at a sufficient energy and for a sufficient time to produce a surface alloy layer  36  on the first piece, as shown in Block  12  of FIG.  1 . Irradiation may produce a heat affected zone in the alloy layer. In a preferred embodiment, a heat affected zone does not extend more than 20% of the depth of the surface alloy layer. 
     In a preferred embodiment, the invention further comprises directing a shielding gas at the region of the surface being irradiated as shown in Block  14  of FIG.  1 . The shielding gas may be nitrogen or argon. The invention also comprises allowing the alloy layer to solidify, as shown in Block  16  of FIG.  1 . 
     The invention further comprises joining a second metal or alloy piece  40  to the surface alloy layer of the first piece, wherein the second piece has a composition that is not identical to that of the first piece, as shown in Block  18  of FIG.  1 . The second piece comprises a first surface  42 , as shown in FIG.  3 . 
     In one preferred embodiment, the joining is accomplished by welding or cladding the second piece to the alloy layer of the first piece, as shown in Block  58  of FIG.  5 . In another preferred embodiment, the joining is accomplished by cladding the second piece to the alloy layer of the first piece, as shown in Block  58  of FIG.  5 . 
     In another preferred embodiment, the joining is accomplished by diffusion bonding, the second piece to the alloy layer of the first piece as stated in Block  18  of FIG.  1 . In this embodiment, the diffusion bonding is carried out at a temperature, T, such that ½ T M &lt;T&lt;T M , where T M  is the melting temperature of the metal or alloy piece with the lower melting temperature, as shown in FIG.  3 . The diffusion bonding is further carried out at a load pressure, P, that is less than one third of the yield strength at room temperature, Y, of the metal or alloy piece with the lower melting temperature. 
     In another embodiment, the joining comprises placing the first surfaces of the first and second pieces against each other and pressing the first and second pieces together by applying a compressive force, P, against the second side of each piece as shown in FIG.  4 . In the preferred embodiment showed in FIG. 4, the first and second pieces each have second surfaces  34 ,  44  opposite their first surfaces. This embodiment may be carried out under the temperature and load conditions as shown in FIG.  3 . 
     A second method embodiment of the present invention is depicted in FIG.  2 . This embodiment comprises coating a first surface of a first metal or alloy piece with a precursor layer comprising a binder and elemental/ceramic powder, as shown in Block  20  of FIG.  2 . This embodiment further comprises irradiating the surface of the first piece with a laser beam at a sufficient energy and for a sufficient time to produce a surface alloy layer on the first piece, as shown in Block  21  of FIG.  2 . The alloy layer is then allowed to solidify, as shown in Block  22  of FIG.  2 . 
     The invention further comprises flattening the first surface of the first piece, as shown in Block  23  of FIG.  2 . In a preferred embodiment, the flattening is accomplished by grinding, milling, machining, or hot pressing. 
     This embodiment further comprises flattening a first surface of a second metal or alloy piece, wherein the second piece has a composition that is not identical to that of the first piece, as shown in Block  24  of FIG.  2 . The invention further comprises cleaning the first surfaces of the first and second pieces, as shown in Block  25  of FIG.  2 . In a preferred embodiment, the cleaning is performed using alcohol. 
     The invention further comprises joining a second metal or alloy piece to the surface alloy layer of the first piece, as shown in Block  26  of FIG.  2 . In a preferred embodiment, this joining may be accomplished by diffusion bonding, subject to the temperature and load pressure limitations, discussed above. In another preferred embodiment, the melting points of the first piece and second piece are greater than or equal to 231° C. In another preferred embodiment, the first piece comprises aluminum and the powder comprises chromium or nickel. 
     A third embodiment of the present invention is shown in FIG.  5 . This embodiment comprises the same coating and irradiating steps as shown in FIG.  1 . This embodiment of the invention further comprises allowing the alloy layer to solidify as shown in Block  54  of FIG.  5 . This embodiment of the invention further comprises cleaning the first surface of a second piece, as shown in Block  56  of FIG.  5 . This embodiment also comprises joining the first surface of the second metal or alloy piece to the surface alloy layer of the first piece, wherein the second piece has a composition that is not identical to that of the first piece. The joining is performed by welding or cladding, as shown in Block  58  of FIG.  5 . 
     The foregoing disclosure and description of the invention are illustrative and explanatory. Various changes in the size, shape, and materials, as well as in the details of the illustrative construction may be made without departing from the spirit of the invention.
The present invention relates to an improved method for joining metals or alloys together. Specifically, the present invention relates to a method comprising the laser alloying of a first metal/alloy piece so that it can be joined with a second metal/alloy piece of a nonidentical composition.