Abstract:
This invention relates to a method of using a laser to produce a decorative appearance on the surface of a bumper. More specifically, the present invention relates to a laser alloying method to create a decorative alloyed layer on the surface of a bumper.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of using a laser to produce a decorative appearance on the surface of a bumper. More specifically, the present invention relates to a laser alloying method to create a decorative alloyed layer on the surface of a bumper. 
     2. Description of the Prior Art 
     Automotive bumpers are often chrome plated in order to give them a shiny appearance. Such chrome plating is subject to corrosion and/or pitting. The present invention provides a method for producing a bumper with an alloyed layer that has an appearance equivalent to that of chrome and resistance to environmental conditions equivalent to that of stainless steel. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a process or method for producing a decorative appearing bumper surface. The present invention comprises applying a layer of precursor comprising chromium or nickel to a metallic bumper surface. The precursor layer is applied to have a thickness in the range of 50-75 microns. 
     The present invention further comprises irradiating the surface of a bumper with a laser beam while the bumper is moved relative to the laser beam in a preselected pattern. The irradiation occurs at a sufficient energy level and for a sufficient time to produce an alloyed surface layer on the bumper. The alloyed surface layer has an environmental resistance equivalent to that of stainless steel and a shininess equivalent to that of chrome. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a block diagram depicting the method of the present invention. 
     FIG. 2 is an isometric view of an apparatus suitable for practicing the present invention. 
     FIG. 3 is an enlarged top view of a laser beam cross section for use in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed toward a method for producing a decorative appearing bumper surface. This method comprises applying a layer of precursor  21  comprising chromium or nickel to a metallic bumper surface  26 , as shown in FIG.  2  and in Block  10  of FIG.  1 . The precursor has a thickness in the range of 50-75 microns. 
     The invention further comprises irradiating the surface of the bumper with the laser beam  28  while the bumper surface is moved relative to the laser beam, as shown in FIG.  2  and in Block  12  of FIG.  1 . In a preferred embodiment, the bumper is moved relative to the laser at a translation rate of 4500-9000 millimeters per minute. In another preferred embodiment, the bumper is moved relative to the laser beam along a linear track  20 , as shown in FIG.  2 . 
     In a preferred embodiment, the laser beam  22  has a rectangular cross sectional area comprising two shorter sides  25  and two longer sides  23 , as shown in FIG.  3 . 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. In another preferred embodiment, the direction of laser beam translation relative to the bumper surface is perpendicular to the larger sides of the rectangular beam cross section. 
     The term “track index”, as used herein, refers to the center to center distance between adjacent laser beam irradiation tracks. In a preferred embodiment, the track index, x, is less than or equal to the width of the laser beam, as shown in FIG.  2 . This ensures that there are no nonirradiated regions between adjacent tracks. 
     The irradiating takes place at a sufficient energy level and for a sufficient time to produce a surface alloy layer having an environmental resistance equivalent to the environmental resistance of stainless steel. The irradiation also takes place at a sufficient energy level and for a sufficient time to produce a surface alloy layer having a shininess equivalent to the shininess of chrome, as shown in Block  12  of FIG.  1 . In a preferred embodiment, the irradiating is performed at a laser power density in a range of 45-55 kilowatts/cm 2 . In a preferred embodiment, the irradiating step is repeated along at least one parallel track  20  adjacent to the most recently irradiated track, as shown in FIG.  2 . 
     In a preferred embodiment, gas  24  is directed at the region of the surface being irradiated by the laser beam, as shown in FIG. 2, and in Block  14  of FIG.  1 . In a preferred embodiment, the gas is nitrogen or argon. In a preferred embodiment, the irradiating step and the directing gas step are repeated along at least one parallel track adjacent to the most recently irradiated track, as shown in FIG. 2, and in Block  16  of FIG.  1 . 
     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.