Abstract:
The present invention is directed toward a method for plasma assisted laser surface alloying. Specifically, the present invention is directed toward a method for surface alloying using a laser beam having a rectangular cross sectional area and a plasma arc, in order to produce an alloyed substrate on the surface of the material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed toward a method for plasma assisted laser surface alloying. Specifically, the present invention is directed toward a method for surface alloying using a laser beam having a rectangular cross sectional area and a plasma arc, in order to produce an alloyed substrate on the surface of the material. 
     2. Description of the Prior Art 
     Laser beams have been used to irradiate the surface of a material coated with preselected alloying elements in order to produce a substrate having enhanced physical and/or metallurgical characteristics. The speed with which the surface of a material may be processed by laser alloying is a function of the thermal energy input produced by the laser. Greater processing speeds require more powerful and more expensive lasers. 
     The processing speed with which a material can be laser alloyed is also a function of the width of the laser beam used for alloying. Producing a laser beam having a greater cross sectional area, while maintaining a constant power density requires a larger and more expensive laser. 
     Plasma arcs have been used in the field of laser welding to provide increased energy input to the weld area without the power expenditure needed to produce an equivalent increase in energy input using only a laser. Prior art plasma assisted welding techniques have used a laser beam having a circular cross sectional area. 
     The present invention provides a plasma arc assisted method of laser alloying in which a plasma arc is used in conjunction with a laser beam having a rectangular cross sectional area. The plasma arc provides additional energy input resulting in an increase in processing speed, without the corresponding increase in cost that would result from using a more powerful laser to produce the increased energy input. Additionally, the plasma arc results in additional heating to the zone surrounding the melt region, thereby reducing the thermal gradient between the melt region and the adjacent regions of the material that is being processed. This reduced thermal gradient results in a reduced cooling rate which is regarded as advantageous. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a method for laser/plasma surface alloying. This method comprises applying a precursor layer comprising a binder and a ceramic or metallic powder comprising chromium or silicon carbide to the surface of a metal substrate. The invention further comprises irradiating the surface of a substrate with a laser beam having a rectangular cross sectional area. The irradiation takes place at a sufficient energy level and for a sufficient time to melt a portion of the substrate such that it forms an alloy with the precursor. 
     The invention further comprises directing an uncollimated plasma arc to the surface of the substrate at the same time and location as the irradiating with the laser beam. While the irradiation takes place, the substrate is moved relative to the laser beam and the plasma arc. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 are block diagrams of the first embodiment of the present invention. 
     FIG. 2 are block diagrams of the second embodiment of the present invention. 
     FIG. 3 is an isometric view of an apparatus for practicing the present invention. 
     FIG. 4 is an enlarged top view of a laser beam cross sectional area for use in practicing the present invention. 
     FIGS. 5 a - 5   d  are side views of four embodiments of the laser beams and plasma arc of the present invention at various angular orientations. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention is directed toward a method for laser/plasma surface alloying. The invention comprises applying a precursor layer comprising a binder and a ceramic or metallic powder comprising chromium or silicon carbide to the surface of the metal substrate  39 , as shown in Block  10  of FIG.  1  and in FIG.  3 . 
     The invention further comprises irradiating the surface of the substrate with a laser beam  30  having an oval/rectangular cross sectional area as shown in FIGS. 3-4. The term “oval/rectangular”, as used herein, refers to a cross sectional area comprising two straight parallel sides and two opposing curved edges connecting the parallel sides, such that if the curved edges were replaced with straight edges, the cross sectional area would be rectangular. The irradiating takes place at a sufficient energy and for a sufficient time to melt a portion of the substrate, such that it forms an alloy with the precursor, as shown in Block  12  of FIG.  1 . 
     In one preferred embodiment, the laser beam is generated by a carbon dioxide laser having a power level of at least 400 watts. In another preferred embodiment, the laser beam is generated by a Nd:YAG laser as shown in FIG.  3 . 
     A preferred embodiment of the laser beam cross sectional area is shown in FIG.  4 . The laser beam  30  has a cross sectional area comprising two curved edges  32  and two straight sides  34 . The distance between the straight sides is referred to as the “beam width”. The distance between opposing curved edges along the axis which bisects the beam width is referred to as “beam length”. In one preferred embodiment, the length of each parallel side is at least four times the beam width. In another preferred embodiment, the length of each parallel side is less than or equal to 10 times the beam width. 
     The invention further comprises directing an uncollimated plasma arc  42  to the surface of the substrate at the same time and location as the irradiating with the laser beam, as shown in Block  14  of FIG.  1 . In a preferred embodiment, a plasma torch  40  is used to generate the plasma arc  42 . In another preferred embodiment, the plasma arc has a current level of at least 25 amperes. 
     The invention further comprises moving the substrate relative to the laser beam and plasma arc, as shown in Block  16  of FIG.  1 . In a preferred embodiment, the substrate is moved relative to the laser beam and plasma arc at a translation rate of at least 100 millimeters per minute, as shown in FIG.  3 . The translation axis along which the laser beam and plasma arc are moved relative to the substrate is labeled  36  in FIG.  3 . 
     The axis  36  on FIG. 3 is labeled with a “+” and a “−” sign to denote movement in either direction along the axis. When the substrate is moved in the − direction, it will be irradiated first by the plasma arc and then be irradiated by the laser. This embodiment of the invention is referred to as “plasma leading”. When the substrate is moved in the + direction, along the translation axis, it will be irradiated first by the laser and then be irradiated by the plasma arc. This embodiment of the invention is referred to as “laser leading”. 
     The translation axis direction depicted in FIG. 3 is referred to herein as the “y axis”. The axis perpendicular to the y axis is referred to as the “x axis”. The distance in the y axis directions between an axis which bisects the laser beam in the width dimension and a parallel axis which bisects the plasma arc is referred to as “Δy”. In the plasma leading environment, the ratio of Δy to laser beam width should be less than or equal to 4.28. In the laser leading embodiment, the ratio of Δy to laser beam width should be less than or equal to 0.89. 
     In FIG. 3, dotted line  50  represents an axis in the y dimension which bisects the laser beam in its length dimension. In FIG. 3, dotted line  52  represents an axis in the y dimension which bisects the plasma arc. As shown in FIG. 3, axis  52  is parallel to axis  50 . The displacement in the x dimension between axis  50  and axis  52  is referred to as “Δx”. The ratio of Δx to laser beam length should be less than or equal to 0.156. 
     Another embodiment of the present invention is depicted in FIG.  2 . As shown in Blocks  18 ,  20 ,  22  and  24  of FIG. 2, this embodiment of the invention is directed toward the use of a laser beam having a rectangular cross sectional area in which the longer sides of the rectangular cross sectional area are at least four times as long as the shorter sides of the rectangular cross sectional area and in which the substrate is moved at a translation rate of at least 100 millimeters per minute relative to the plasma arc and the laser beam. 
     The angular orientations of the laser beam and the plasma arc with respect to the substrate alpha (α) and beta (β), respectively, can be varied, as shown in FIGS. 5 a - 5   d . In one preferred embodiment, α and β are 90° and 45°, respectively, as shown in FIG.  5   b . In another preferred embodiment, α and β are 70° and 70°, respectively, as shown in FIG. 5 c . In a third embodiment, α and β are 45° and 90° respectively. 
     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.