Abstract:
The present invention relates to a method for marking metallic alloys using laser alloying. Specifically, the present invention is directed toward the use of laser alloying steel or aluminum alloys with a mark that provides protection against wear and corrosion and greater permanency.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for marking metallic alloys using laser alloying. Specifically, the present invention is directed toward the use of laser alloying steel or aluminum alloys with a mark that provides protection against wear and corrosion and greater permanency. 
     2 . Description of the Prior Art 
     Items made of steel or aluminum have been marked for a variety of reasons including identifying one or more characteristics of the item. One method of marking employs the use of a visible marking substance, such as ink or paint. Such marking substances are subject to fading and/or wear, particularly in abrasive environmental conditions such as wind, rain, and/or direct sunlight. In many cases it is desirable to mark a component prior to the completion of the manufacturing process. It is desirable to have a mark that will not deteriorate during the manufacturing process, which may include heating, bending, abrasion machining and/or welding. 
     Other methods of marking involve imprinting a visible mark into the surface of an item, such as by etching, engraving, or punching. Methods such as etching or engraving result in some loss of material from the substance to be marked. Methods such as punching result in compression of material, sometimes resulting in a change in the mechanical properties of the material. 
     The present invention overcomes the drawbacks of the prior art by providing a method of marking steel or aluminum alloys which is far more resistant to wear, abrasion, and fading, and which does not result in a loss of material or in compression of the material to be marked. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a method for laser marking of metallic alloys. This method comprises applying a layer of precursor comprising a binder and metallic or ceramic powder to a steel or aluminum substrate. The layer has a thickness in the range of 50-250 microns. The method further comprises irradiating the surface of the substrate with a laser beam at a sufficient energy level and for a sufficient time to produce a corrosion and/or wear resistant alloyed apron on the substrate. The laser beam used to perform this step of producing an alloyed apron is referred to herein as the “alloying laser beam.” The irradiation is performed while the substrate and the laser beam are moved relative to each other. The method further comprises marking on the apron with the laser beam to produce a pre-selected marking on the apron. The laser beam used to perform the step of marking the apron is referred to herein as the “marking the laser beam.” 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a block diagram of a first embodiment of the present invention. 
     FIG. 2 is a block diagram of the second embodiment of the present invention. 
     FIG. 3 is a top view of a substrate being marked by the method of the present invention. 
     FIG. 4 is an enlarged top view of a laser beam cross section for use in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention is shown in FIG.  1 . This method embodiment for laser marking of metallic alloys comprises applying a layer of precursor comprising a binder and metallic or ceramic powder to a metallic substrate, as described in Block  10  of FIG.  1 . In a preferred embodiment, the metallic substrate is made from steel or aluminum. The layer has a thickness in the range of 50-250 microns. In a preferred embodiment, the powder comprises chromium. In another preferred embodiment, the powder comprises silicon carbide. 
     The invention further comprises irradiating the surface of the substrate with an alloying laser beam at a sufficient energy level and for a sufficient time to produce a corrosion resistant alloyed apron on the substrate, as shown in Block  12  of FIG.  1 . The irradiation takes place while the substrate  20  undergoes relative movement with respect to the laser beam  22  as shown in FIG.  3 . 
     In a preferred embodiment, the substrate  20  is moved relative to the alloying laser beam  22  at a translation rate in the range of 500-9000 millimeters per minute. The direction of movement of the laser beam relative to the substrate is known as the “translation axis”  24 , as shown in FIG.  3 . 
     In a preferred embodiment, the irradiating by the alloying laser beam is performed at a laser power density in the range of 45-55 kilowatts/cm 2  where the powder comprises chromium and the metallic substrate comprises steel. In another preferred embodiment, where the binder comprises chromium and the substrate comprises steel, the alloying laser beam power density is 50 kilowatts/cm 2 . In another preferred embodiment, the irradiating is performed with a pulsed laser. 
     In a preferred embodiment, the alloying laser beam  22  has a rectangular cross sectional area comprising two shorter sides  25  and two longer sides  23 , as shown in FIG.  4 . In another preferred embodiment, the longer sides of the rectangular cross sectional area have a length of at least four millimeters and the shorter sides of the rectangular cross sectional area have a length of at least 0.6 millimeters. A rectangular beam profile having the dimensions described above can be achieved by aligning a spherical lens closest to the beam, a second cylindrical lens closest to the substrate and a first cylindrical lens between the spherical lens and the second cylindrical lens. The spherical lens should have a focal length of 101.6 millimeters and the first cylindrical lens should have a focal length of 203.2 millimeters. The second cylindrical lens should have a focal length of 152.4 millimeters. The spherical lens and the first cylindrical lens should be spaced apart by five millimeters. The first cylindrical lens and second cylindrical lens should be spaced apart 15 millimeters. 
     The invention further comprises marking the aprona marking laser beam to produce a preselected marking on the apron, as shown in Block  14  of FIG.  1 . In a preferred embodiment, the marking comprises laser etching, as shown in FIG.  3 . The present invention encompasses both positive and negative marking. The term “positive marking,” as used herein, refers to the laser alloyed region constituting the preselected mark. The term “negative marking,” as used herein refers to the unalloyed region constituting the mark. In either case, the pattern traced out by the laser on the substrate defines the preselected pattern of the mark. 
     In a preferred embodiment, the invention further comprises directing a shielding gas at the region of the substrate being irradiated by the alloying laser beam, as shown in Block  15  of FIG.  1 . In a preferred embodiment, the shielding gas is nitrogen or argon. 
     Another embodiment of the present invention is depicted in FIG.  2 . This embodiment comprises applying a layer of precursor comprising a binder in metallic or ceramic powder to a steel or aluminum or substrate, as shown in Block  16  of FIG.  2 . The precursor layer has a thickness in the range of 50-75 microns. 
     This embodiment further comprises irradiating the surface of the substrate with a laser beam in a preselected pattern at a sufficient energy level and for a sufficient time to produce a preselected alloyed marking on the substrate, as described in Block  18  of FIG.  2  and as shown in FIG.  3 . The laser alloyed region of the substrate will have increased corrosion and/or wear resistance. In a preferred embodiment, the irradiating is performed with a laser beam having a rectangular cross sectional area comprising two shorter sides and two longer sides, as shown in FIG.  4 . 
     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.