Patent Publication Number: US-2003222059-A1

Title: High energy beam cladding

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
     [0001] This application claims priority from provisional application serial No. 60/386,009 filed on Jun. 4, 2002, and entitled “High Energy Beam Cladding.” 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] The present invention relates to a system and method for area treatment or area improvement of a surface using a high energy beam. More specifically, the present invention relates to a system and method for heating a substrate surface with a high energy beam while simultaneously spray depositing clad material onto the substrate to achieve a fusion or metallurgical bond between the clad material and the substrate.  
       [0003] Large complex machinery is expensive to produce. To control costs, such machines are often built using low-grade or inexpensive materials, such as carbon steel. While such low-grade materials are subject to corrosion, producing the same machine using corrosion resistant materials would be cost prohibitive. However, it is possible to treat the surface of such low-grade materials with corrosion resistant materials in order to prevent corrosion and to extend the usable life of the machine. This type of treatment is referred to as “area treatment”, and it can be performed with corrosion resistant materials or with materials having other material characteristics, such as hardness, conductivity and the like.  
       [0004] Generally, crevices tend to trap moisture, which leads to oxidation and corrosion. With respect to large machinery, typically crevices exist wherever parts come together. For example, hinged joints, bolts and other fasteners all provide crevices of a sort which can trap moisture. In particular, crevice corrosion in areas near seals can result in costly repairs, both because such parts can be difficult and time consuming to replace and because deterioration of a part adjacent a seal can lead to larger problems if the problem is not detected at an early stage.  
       [0005] Cladding is a technique for area treating a workpiece with a deposit of material having desired characteristics (such as corrosion resistance). Cladding involves bonding a deposit of material onto a substrate. Typically, the cladding deposit is relatively thin, as compared with the thickness of the substrate. The goal of most cladding operations is to form a sound interfacial bond between the deposit and the substrate without diluting the cladding deposit with substrate material and without altering the material properties (such as corrosion resistance, electron mobility, and the like) of the cladding deposit. Ideally, the cladding material establishes a fusion bond with the substrate.  
       [0006] There are many known cladding processes, including laser, chemical (electrolysis), welding, spraying, plasma arc, chemical vapor deposition, physical vapor deposition, mechanical plating, and electrochemical deposition. In the laser cladding art, two common methods of supplying the cladding material are pre-placement of cladding material powder onto the substrate and inert gas propulsion of cladding material powder into the path of the laser beam (off-axis delivery or coaxial delivery).  
       [0007] Cladding with a powder involves scanning a laser beam over the pre-placed or propulsed powder. The energy of the laser beam melts the powder onto the substrate, thereby cladding the substrate. However, cladding with powder is susceptible to contamination and porosity (voids), resulting in incomplete fusion of the cladding to the substrate or inclusions and voids within the deposit. Moreover, propulsed powder placement has a low clad capture rate caused by divergence by the powder stream and incomplete interaction as it is exposed to the beam.  
       [0008] Porosity, uneven deposition, and undercut contribute to failures in the cladding deposit, such as voids, cracking, corrosion, separation, and the like. Where cladding is added to prevent corrosion, or where it is located near a seal, a defective clad deposit will result in product failure.  
       [0009] In the welding arts, it is known to transfer a deposit onto a substrate via a process called “short circuit transfer” or via a spray or globular transfer. In the traditional Metal Inert Gas (MIG) short circuit transfer process, a consumable wire is held on a spool and fed automatically to a torch or gun through a nozzle and into the weld arc. The short circuit transfer gets its name from the welding wire actually “short circuiting” (touching) the base metal or substrate many times per second. When the welding gun trigger is pressed, the electrode wire feeds continuously from the wire feeder, through the gun, and to the weld arc, repeatedly touching the substrate or base metal. Each time the short circuit is completed, a piece of the wire melts off onto the substrate.  
       [0010] Generally, in the traditional MIG short circuit transfer process, a current is applied to the wire. When the wire touches the base metal, there is no arc and the current flows through the wire and the base metal. The current flow causes a magnetic field to develop around the wire. Typically, the wire has difficulty supporting all the current flowing through the wire. As resistance builds up in the wire, the wire heats, and the tip of the wire begins to melt. The magnetic field squeezes the melting wire, assisting the wire in separating from the molten tip. As the wire separates, the current often continues to rise, arcing across the small gap between the wire and the separated tip. The arc across the small gap causes current surges during and immediately after each short circuit contact, resulting in arc instability and spatter. Specifically, after separation, the arc is “on” and the heat of the arc causes the weld puddle to flatten out (in a stable arc), but when the arc surges across the small gap, the weld puddle can vaporize (explode) so as to cause uneven cladding.  
       [0011] In the spray or globular transfer method, the wire does not short circuit to the substrate by touching the substrate surface. Instead, the spray arc transfer uses relatively high voltage, wire feed speed and amperage as compared with the short circuit transfer, resulting in a high current density through the wire. This high current density produces a high degree of heat, melting the wire into droplets. The high heat in the spray generally leads to a larger, more fluid weld puddle (“improved wetting”) than that produced by the short circuit transfer process. While the resulting improved wetting of the weld puddle leads to better flow from the high heat, the higher heat input of the spray arc transfer can cause excessive melting of the substrate.  
       [0012] The gun position or technique refers to the way in which the gun (and weld wire) are situated relative to the workpiece. The gun position with respect to the direction of travel is said to be progressing with either a push technique (nozzle pointed opposite the direction of motion of the advancing workpiece, i.e. toward the advancing workpiece), a pull or drag technique (nozzle pointed in the direction of motion, i.e. the torch is “dragged” away from the deposited weld), or a perpendicular technique. A perpendicular technique means that the wire is fed straight into the weld at an angle of 90 degrees relative to the surface of the substrate.  
       [0013] The travel angle is the angle of the torch and wire relative to the perpendicular, in other words, zero degrees from a vertical line drawn upward from the surface of the workpiece. The travel angle is defined as the angle relative to this perpendicular value of zero and in the direction of the weld. A product manual for a Gas Metal Arc Weld (GMAW) system produced by Miller Electric Manufacturing Company of Appleton, Wis. (such as that used in an embodiment of the present invention) indicates that “normal welding conditions in all positions call for a travel angle of 5 to 15 degrees [from the perpendicular] for good weld puddle control.” The manual also states that “travel angles beyond 20 to 25 degrees lead to more spatter, less penetration and general arc instability.” See Miller Electric Manufacturing Company GMAW Product Manual, page 47.  
       [0014] In addition to limiting the travel angle of the torch, to aid in the wire transfer process, a shielding gas is fed through the gun to provide a “blanket” over the weld to protect the weld from oxidation and contaminants. Argon is a common shielding gas, in part, because it is heavier than air and therefore provides a fairly stable coverage. Although argon is suitable for non-ferrous metal and alloys, if it is used for welding steel, the process becomes unstable and the weld profile uneven. Carbon dioxide shielding gas, while currently one of the least expensive shielding gases, is not suitable for use with stainless steel because it reduces the corrosive resistance of the weld. Generally, the flow of gas provides a cleaning action over the weld and helps to provide a more stable arc between the welder and the workpiece. The flow necessary for good welding depends primarily on the thickness of the material, the welding current, the size of the nozzle, the joint design, the specific gas coverage, and the type of gas used.  
       [0015] Typically, it is very difficult with the short circuit process to maintain a stable process at high deposition rates. In general, the short circuit transfer process results in a rough deposition deposit with a relatively significant crown height. For work pieces to be used with seals, the deposition surface must be machined smooth to facilitate a proper seal; however, the clad surface of the short circuit transfer deposition (because of the crown height of the clad and the roughness of the deposition) requires multiple passes of the machining process in order to achieve the requisite surface. The multiple passes significantly slow the production time. Moreover, the wasted material that is ground away from the work piece adds significantly to the production costs and the overall waste.  
       [0016] Thus, the traditional short circuit transfer process suffers from arc instability and uneven cladding, requiring costly downstream machining and polishing. The spray technique provides a more stable transfer, but the high heat required to generate the spray can cause excessive melting.  
       BRIEF SUMMARY OF THE INVENTION  
       [0017] A system and method for cladding utilizes a gas metal arc welding system and a high energy beam. The gas metal arc welding system has a torch and a wire fed by a wire feeder for depositing cladding material onto a target on a substrate. The high energy beam is focused on the substrate to assist fusion and wetting of the clad material to the substrate. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018]FIG. 1 is a block diagram of the laser cladding system of the present invention.  
     [0019]FIG. 2 is a block diagram illustrating the angle of the torch from normal relative to a workpiece.  
     [0020]FIG. 3 is a front plan view of the system of the present invention.  
     [0021]FIG. 4 is a block diagram of the laser cladding system of the present invention from a profile view.  
     [0022]FIG. 5 is a top plan view of a workpiece being processed with the laser cladding system of the present invention.  
     [0023]FIG. 6 is a top plan view of a cylindrical workpiece being processed with the laser cladding system of the present invention with the differentially focused beam at an angle relative to normal.  
     [0024]FIG. 7 is a graph illustrating an example of power delivered by the differentially focused beam relative to the width of a workpiece.  
     [0025]FIG. 8 is a top plan view of the system of the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0026] As shown in FIG. 1, the cladding system  10  includes a controller  12  for controlling a laser system  14  and a gas metal arc welder (GMAW) or a gas metal arc welder pulsed (GMAW-P) system  16 . The laser system  14  focuses a beam  18  onto a surface  20  of a substrate  32 . The GMAW  16  generally supplies a torch  22  with a wire  24  fed by a wire feeder (not shown). The GMAW  16  generally includes the torch  22 , a power supply (not shown), a cover gas and supply line (not shown), and the wire feeder. The wire  24  extends out of the end of the torch  22  toward the beam  18  and angled toward a target location  28  on the surface  20 . The target location  28  may be coincident with the focal area  30  of the laser beam  18  or the target location  28  may lead the focal area  30 .  
     [0027] As the tip of the wire  24  melts, a spray or arc cone  26  of the molten wire material is directed toward the surface  20  of the substrate  32 . The term “arc cone” generally refers to the conical shape of the spray (continuous or pulsed) of molten material generated by the torch  22 . The continuous or pulsed spray of molten material does not extend in a stream or straight line, but rather in a cone-shape toward the surface  20 . The arc cone  26  therefore refers to the cone shaped spray area produced by the torch  22 . Generally, the GMAW  16  controls a current to the wire  24 , which melts the wire  24  into the arc cone  26  aimed at a target  28  on the surface  20 . The GMAW  16  also controls delivery of a shielding gas (not shown) to the torch  22  to cover the arc cone  26  to prevent contaminants from interfering with the process.  
     [0028] The laser beam  18  is focused differentially at a focal area  30  on the surface  20  of the substrate  32 , as the substrate  32  is advanced in the direction of motion (A) by a delivery system  34 , such as a conveyor. Generally, the laser beam  18  is focused at a focal area  30  between the torch  22  and the target location  28 ; however, sometimes the beam  18  may be focused at a focal area  30  that is coincident with the target location  28 , depending on the particular application. Ordinarily, laser beams are focused to a focal point or spot on a surface. This type of focus may be viewed as a Gaussian-type energy distribution. For area cladding, the traditional focused spot or even a defocused spot centers the energy toward the beam center, therefore failing to heat the substrate evenly, which may lead to uneven flow of the cladding material and uneven bonding to the substrate.  
     [0029] In this instance, the beam  18  is differentially focused. The phrase “differentially focused” refers to a technique for focusing the beam  18  into a substantially linear or slit-like shape (similar to a mostly closed cat&#39;s eye). More specifically, “differentially focused” means creating a substantially linear focus for the laser beam, allowing for adjustment of the “line” length and of the energy distribution alone the so-called “line”. The term “line” is provided in quotations to illustrate that the focal area of the beam  18  is typically not reduced to a line, but rather to an elongated elliptical shape, a slit-like shape, or an elongated rectangular shape. However, the ideal focus would be a very narrow rectangle, which would appear more or less as a line.  
     [0030] One technique for providing the differential focus of the beam  18  uses a segmented mirror that breaks the incoming beam into small squares or rectangles and distributes the energy appropriately along the focal line. A second technique is to sweep the beam  18  back and forth across the surface with a galvanometer-driven mirror at a high rate of speed, typically 50 to 200 Hz. If the laser is varied relative to the beam position, the average energy distribution along the line can be controlled. Another technique involves two “crossed” cylindrical optics to create a line focus.  
     [0031] In a preferred embodiment, the laser beam  18  strikes the surface  20  of the substrate  32  at an angle that is normal to the surface  32 . Off normal angles may be used to avoid reflections or for other process related issues, depending on the particular application.  
     [0032] As shown in FIG. 1, the tip of the wire  24  may extend partially into the differentially focused beam  18 , allowing the differentially focused beam  18  to assist in the transfer. Specifically, the arc cone  26  is directed to a target location  28  in advance of the focal line  30  of the beam  18 , relative to the direction of travel of the substrate  32  (as indicated by the arrow labeled with the letter A). The shadow cast by the wire  24  in the differentially focused beam  18  does not significantly effect the bonding of the clad deposit to the substrate.  
     [0033] In this non-contact “spray” mode with the wire  24  extending into the beam  18 , the laser beam  18  assists in heating the wire  24 , in addition to heating the substrate  32 . In current controlled GMAW and GMAW-P systems  16 , the heating of the wire  24  provided by the beam  18  reduces the power consumption, as less current from the GMAW  16  is required to melt the clad wire  24 . The arc cone  26  is directed toward the target location  28  (shown in phantom), and the droplets tend to spray onto the surface  20  in a cone-shaped spray arc  26 . The droplets land on the surface  20  in the vicinity of the target location. Thus, the differentially focused beam  18  heats the melt puddle of the clad material, thereby assisting in wetting out the clad and producing a smoothly finished clad deposit  36 .  
     [0034] In one embodiment, the distance between the contact tip of the torch  22  and the target location  28  along a target line (shown in phantom) is between 0.75 to 1.00 inches. The relative distance between the focal area  30  of the beam  18  and the target location  28  for the arc cone  26  is determined by the position of the torch  22  and the laser  14 . However, in some instances, it may be desirable to extend the distance between the target location  28  and the focal area  30  of the beam  18 , in which case the torch  22  or the laser  14  may be moved so that the distances may be further adjusted.  
     [0035] The differentially focused beam  18  is directed onto the substrate at an angle that is approximately normal to the surface  20 , so as to maximize absorption of laser energy by the substrate  32 . The beam  18  heats the substrate  32  so as to assist in wetting of the molten clad material to the substrate. This deposition technique results in a clad deposit  36  with minimal porosity (minimal voids) and a relatively smooth profile as compared with “short circuit” clads.  
     [0036] Generally, the cladding system  10  of the present invention utilizes a non-contact transfer technique (meaning that the wire does not contact the surface of the substrate) for depositing the clad deposit  36  as a spray-type deposition onto the surface  20  of a substrate  32 . The droplet sizes of the spray of the cladding material vary. Droplets vary in size from very small droplets relative to the diameter of the wire  24  (spray transfer) to larger droplets comparable to or larger than the diameter of the wire  24  (globular transfer). In one embodiment, the spray droplets are of approximately equal diameter. Typically, a non-contact transfer may be either a spray or a globular transfer. In the present invention, the arc cone  26  refers to either the globular or the spray deposition technique.  
     [0037] The substrate  32  (or workpiece) shown is one type of substrate that can be treated with the system  10 ; however, other shapes may be treated with the system  10  by advancing the particular workpiece under the beam  18  and torch  22  on a delivery system  34 . Depending on the particular application and type of workpiece, the delivery system  34  may be a conveyor belt, a spindle with a mounting apparatus, or any other mechanism for positioning and/or rotating a workpiece in the path of the laser beam. FIG. 1, the delivery system  34  is shown as a flat surface.  
     [0038] Additionally, though the present invention is shown as depositing a clad deposit  36  across an entire surface of a substrate  32 , the system  10  may be used to clad an area of a substrate  32 . For example, the substrate  32  could be provided with a groove or pattern carved in a surface  20 , and the system  10  can be adjusted to area clad the groove or pattern.  
     [0039] Generally, the laser  12  may be an Nd:YAG laser, a Direct Diode laser, a CO 2  laser, a fiber laser, or any other type of laser. There may be a marginal cost valuation between power consumption and workpiece production speed depending on the cladding and workpiece material properties. For fastest production without concern for power consumption or system costs, the Nd:YAG laser may a good choice. By contrast, for improved power consumption efficiency and slightly slower production speeds, the more economical CO 2  laser may be a better choice. In one embodiment, the CO 2  laser was made by Trumpf Inc. of Farmington, Conn., and the gas metal arc welder system was produced by Miller Electric Manufacturing Company of Appleton, Wis. The Miller system is a pulsed Gas Metal Arc Welding (GMAW) system, operated in spray transfer mode. However, other gas metal arc welding systems may be used.  
     [0040] In a preferred embodiment, the GMAW welder  26  is run in a pulsed mode, spray transfer mode of operation. Since pulsed-spray transfer uses two heat levels, the system runs at a lower average heat level than standard spray transfer, which is continuous. A system operator selects the frequency of pulse (pulses per second), the peak power (a peak pulse amplitude), the background power, and the wire feed speed on the wire feeder. Alternatively, the system operator can use a controller that automatically sets peak, background and wire feed speed for a given wire diameter and metal type.  
     [0041] In the pulse spray transfer, the GMAW  16  and/or a pulse controller pulses the welding output with high current peaks that are set by the operator at a selected amperage level, which will cause the wire to melt and transfer to the substrate as a spray. Generally, the pulsed GMAW system  16  operating in spray transfer mode melts the cladding material delivered as a cold wire  24  into a spray of droplets. A control unit  12  allows the user to manipulate the GMAW  16  settings (including the spray area and the coverage) to control the spray for area treatment of a substrate  32 . The background current is set at a level sufficient to maintain the arc between the current peaks, but at a much lower level than the pulses. When pulsing, the peak current (or spikes) is superimposed upon the background current. The metal from the wire melts and transfers at the pulse current level and is not transferred at the background current level. When properly adjusted, small molten drops of metal are transferred during each pulse. Though the metal is transferred only during the pulse cycle, the wire feed speed is constant from a constant speed feeder.  
     [0042] Generally, the pulsed mode spray transfer offers several advantages over continuous spray transfer. Specifically, by operating in the pulsed mode, the GMAW  16  reduces energy consumption, reduces dilution, reduces heat, and improves process stability.  
     [0043] The wire  24  may be a standard, off-the-shelf solid or cored wire having a selected chemical composition. If tolerance is fairly high for mixing with the underlying substrate, the off-the-shelf wire may be more than adequate for meeting the project specifications. Alternatively, the clad wire  24  may be a custom solid or a metal cored wire of a particular chemistry. A metal cored wire has a conductive sheath wrapped around a lower-conductivity metal alloy powder core. The metal cored wire offers two advantages over off the shelf wire: 1) the chemical content of the cladding deposit must be tightly controlled, the custom solid or metal cored wire allows custom compositions so that the clad deposit will meet the product requirements, and  2 ) the metal cored wire requires less current to melt the wire because the current density is higher in the metallic sheath than if the entire wire were conductive.  
     [0044] The metal alloy powder contained within the metal cored wire or the metal of the solid wire provides the clad material for the deposit. Clad material is generally selected according to its material properties, such as corrosion resistance, wear resistance, conductivity, temperature resistance, strength, and the like. The predominant physical characteristics selected for cladding are typically wear resistance and corrosion resistance. Typical cladding materials include stainless steels, cobalt alloys, nickel alloys, aluminum alloys, and other metals with desirable material properties.  
     [0045] As previously discussed, traditional laser systems focus the laser beam to a focal point or spot on the surface of the workpiece. A defocused spot provides most of the energy down the center of the beam. In the case of a grooved surface, the defocused spot would focus most of the beam energy down the middle of the groove. In the present invention, the beam  18  is focused differentially using two cylindrical lenses to produce a narrow slit or “cat&#39;s eye” shape in order to more uniformly heat the substrate  32 . The differentially focused beam  18  is focused to provide uniform energy distribution across the surface  20  of the area of the substrate  32  to be clad. Ultimately, the shape of the beam  18  can vary according to the surface  20  of the substrate  32  in order to control the energy distribution. In the embodiment shown, the focused beam  18  has a spot shape that is almost linear to facilitate even energy distribution. The differentially focused beam  18  thus uniformly heats the surface  20  of the substrate  32  across the entire width of the surface  20  to be clad.  
     [0046] Once the cladding process is completed, further processing of the cladded substrate  32  may be performed, such as machining and polishing to provide a uniformly smooth surface. However, the relative height of the cladding crown is 1.5 to 2 times smaller than the crown of a short-circuit clad deposit. Thus, significantly less downstream processing is required to produce a finished workpiece.  
     [0047] As shown in FIG. 2, the torch  22  is progressing in a push technique relative to the direction of motion of the workpiece (indicated by A). The push technique helps to provide a flatter surface of the clad deposit and a high quality deposit (with minimal porosity) at high feed rates.  
     [0048] The angle of the torch  22  relative to a perpendicular line extending from the surface  20  of the substrate  32  is referred to as the “travel angle”. Generally, the perpendicular line is referred to as a zero angle, such that the travel angle is the angular difference from the perpendicular. Unlike traditional welding techniques, the travel angle of the present invention is generally greater than 15 degrees. The travel angle of the torch  22  may vary according to the specific implementation. As the travel angle increases, the level of dilution of the cladding deposit  36  from the underlying substrate  32  decreases. Preferably, the travel angle is between 30 and 45 degrees. However, depending on the angle of the surface  20  to be clad, the travel angle may be adjusted to exceed 45 degrees in certain circumstances, such as where the surface  20  is at an angle relative to the horizontal plane of motion. Generally, the travel angle is greater than 15 degrees.  
     [0049]FIG. 3 illustrates a work angle of the torch  22  and the laser  14 . As shown, both the torch  22  and the laser  14  are oriented at a work angle of 90 degrees relative to the surface  20  of the substrate  32  (from the position of the workpiece advancing into the page away from the viewer). Tilting of the torch  22  and/or the laser  14  may be desirable under certain circumstances, such as when the laser  14  or the torch  22  have limited access to the area to be clad. In general, the work angle is 90 degrees relative to the surface  20  of the substrate  32 .  
     [0050] The torch  22  is preferably positioned in plane with the laser beam  18  and the direction of travel (e.g. the workpiece  32  advancing directly into the page). The phrase “in plane” is intended to indicate that the laser  14  and the torch  22  are aligned (one behind the other) in the direction of motion.  
     [0051] As shown, the differentially focused beam  18  converges to the width of the area to be clad as it travels from the laser  14  to the substrate  32 . As previously discussed, the focal area  30  is designed to cover the entire area to be clad. The focal area  30  may be made wider or narrower as needed. While the clad area has been shown to be the entire width of a workpiece surface  20 , it should be understood that the clad area may be less than the width of a workpiece surface. For example, the clad area may be a groove or a pattern traced on the surface, and the focal line  30  may be sized to cover just the groove or the traced pattern in order to area clad the surface  20 .  
     [0052] As shown in FIG. 4, the target location  28  of the arc cone  26  from the wire  24  leads the focal line  30  of the differentially focused beam  18 ; however, as previously discussed, the molten droplets of the clad wire  24  spray from the end of the wire  24  in an arc cone  26  that is directed toward the target location  28 . Typically, the droplets of the arc cone  26  fall on the substrate  20  within the arc cone  26 . For example, some of the droplets fall at the target location  28  and some fall slightly short or slightly long of the target location  28 . Depending on the spacing between the focal area  30  and the target location  28 , some of the droplets may fall in the focal area  30  and some ahead of the focal area  30 . As shown, the focal line or area is an elongated elliptical shape or “cat&#39;s eye”. In some instances, the ideal focal area  30  may be a line or narrow rectangle.  
     [0053] Generally, the laser system  14  differentially focuses a beam  18  onto the surface  20  of the substrate  32 . The GMAW system  22  in pulsed spray transfer mode controls a current to a wire  24  to produce an arc cone  26  aimed at a target location  28  coincident with or in advance of the focal area of the beam  18 . The beam  18  is focused along a line  30  on the surface  20  of the substrate  32 . Generally, the focal line  30  is oriented at an angle perpendicular to the direction of travel of the substrate  32  in the plane of the workpiece surface  20 .  
     [0054]FIG. 5 shows a top plan view of the substrate  32  to show the orientation of the focal line  30  relative to the direction of motion A. As shown, in this embodiment, the target location  28  (shown as a cross-hair) and the projection  38  of the arc cone  26  on the surface  20  of the substrate  32  is shown as a semi-elliptical shape, which is intended to illustrate a deposit area of droplets at approximately the target location  28 . The projection  38  of the arc cone  26  is shown in phantom to illustrate that the projection  38  may vary as the spray droplets of molten clad material land at slightly varying locations near the target location  28 . Additionally, the beam  18  is oriented at an angle normal to the direction of travel (A). The normal orientation of the focal line  30  ensures uniform heating of the substrate  32  across the entire area to be clad. However, in some instances, it may be desirable to change the angle of the focal line  30 .  
     [0055] In FIG. 6, for illustration purposes, a different workpiece  32  and a different beam orientation are shown. Specifically, the workpiece  32  is cylindrical workpiece with a “concaved” or “grooved” end surface. While the previous illustrations have shown the workpiece  32  to be flat, neither the workpiece  32  nor the surface  20  need to be flat to perform the cladding process of the present invention.  
     [0056] With respect to the orientation of the focal line  30 , it is possible to orient the focal line  30  at an angle other than normal relative to the axis of the workpiece. As shown, the focal shape of the beam  18  at the focal area  30  is elliptical and at an angle relative to the direction of travel (circle).  
     [0057]FIG. 7 illustrates the power distribution of a differentially focused beam relative to the width of a clad area. While ideally the power distribution would be uniform, some non-uniformity of the beam  18  is expected. Generally, the power is evenly distributed over the width of the clad  36 . As shown, the energy distribution is at its lowest point near the edges of the area to be clad. Specifically, the graph illustrates that the power distribution of the beam  18  can be differentially focused to more uniformly heat the substrate  20 , unlike a traditional de-focused laser spot.  
     [0058]FIG. 8 is an overhead view which illustrates the focal area  30  (such as line, thin rectangle, or ellipse) at a 90° degree angle relative to the direction of motion (A). As shown, the torch  22  and the laser system  14  are positioned in-plane (in the same plane along the axis of the substrate  32 ) relative to the direction of motion (A). The wire target location  28  is in advance of the beam and the laser  14 , such that the torch  22  directs the arc cone  26  from the wire  24  toward the target location  28 . The projection  38  of the arc cone  26  is shown overlapping the target location  28 . As shown, the beam  18  converges to a focal line  30  from the laser  14  to the substrate  32 . The rate of convergence of the beam  18  may be varied according to the size of the area to be clad, and according to the particular application.  
     [0059] While the position of the torch  22  relative to the laser  14  have been consistently shown with the torch  22  trailing the laser  14  relative to the direction of motion, the torch  22  may not always trail the laser  14 . In some instances, it may be desirable to reposition the torch  22  relative to the laser  14  in order to direct the arc cone  26  further in front of the focal area  30  of the beam  18 .  
     [0060] The combination of the laser with the GMAW or GMAW-P system provides a number of advantages over traditional cladding systems. First, by spraying liquid droplets using the GMAW or GMAW-P system, the shielding gas shields the surface from contaminants, and any contaminants contained in the droplets are allowed to burn off prior to reaching the substrate. Second, the spray droplets are deposited in liquid form (as opposed to pre-placed or propulsed powder deposition), allowing the droplets to flow evenly on the surface, and providing a clad deposit with minimal porosity (minimal voids). The GMAW-P system provides the additional advantage of operating in a pulsed mode, allowing for lower average power consumption.  
     [0061] In addition to the laser beam  18  being used to heat the surface  20  of the substrate  32 , the laser beam  18  is used sometimes to heat the wire  24  or the spray droplets  26 . By concurrently or simultaneously heating the wire  24  and the substrate  32 , the laser beam  18  can improve wetting and reduce the average power required by the GMAW system  16 . Conversely, by concurrently or simultaneously heating the spray droplets  26  and the substrate  32 , the liquid flow of the droplets or the liquid clad deposit is improved, and complete fusion between the substrate  32  and the clad deposit  36  is achieved.  
     [0062] By heating the substrate  32  with a beam  18  from a laser  14  and by spraying the molten clad material in an arc cone  26  from the wire  24  of a GMAW system  16  toward a target location  28 , the liquid flow of the droplets is improved and complete fusion between the substrate  32  and the clad deposit  36  is achieved over the width of the clad area. Heating the substrate  32  coupled with spray depositing the clad deposit assists the melt puddle flow, allowing the sprayed droplets to cool more slowly for better droplet flow. The heated substrate  32  allows the melt puddle of clad material to flow more evenly to fill minute unevenness in the surface  20  of the substrate  32 , thereby ensuring a better interface between the substrate  32  and the clad deposit  36 .  
     [0063] Heating of the substrate  32  with the laser beam  18  ensures that the interface between the clad deposit  36  and the substrate  32  is a fusion bond. Specifically, the bond is metallurgical in the sense that the heated substrate surface  20  and the clad deposit  36  bond at a molecular level, rather than just mechanically, as would be the case with a plasma spray depositing the clad deposit without substrate heating.  
     [0064] Tests performed using lasers to clad carbon steel with stainless steel powder (using the prior art technique of pre-placement of the powder) showed that an Nd:YAG laser was capable of completing the cladding operation at a linear feed rate of 15 inches per minute. Attempts to use the CO 2  laser to clad the carbon steel workpiece using the prior art pre-placement method were unsuccessful.  
     [0065] With the GMAW system in conjunction with the CO 2  laser, specifically using a Gas Metal Arc Welding system produced by Miller® and a CO 2  laser system produced by Trumpf, the present invention successfully clad a 420 grade stainless steal wire of 0.062 inch diameter onto a carbon steel workpiece at a linear feed rate of greater than 40 inches per minute.  
     [0066] In this embodiment, the GMAW system was operated using an Argon/CO 2  cover gas mixture of 95/5 ratio at a flow-rate of 50 cubic feet per hour. Other mixtures have also been tested. With stainless steels cladding onto steel substrates, other mixtures of Argon/CO 2 , as well as mixtures of Argon/O 2  and Argon/CO 2 /NL were found to be effective. Generally, the specific aplication dictates which cover gas is most suitable.  
     [0067] Generally, the best incidence angle for absorption of the laser beam  18  is at an angle normal to the surface of the workpiece. The travel angle and position of the torch  22  may be varied according to the specific application. In particular, the torch angle and position may be adjusted using the control unit  12 , or by manually adjusting the welder. The torch angle may be adjusted to control the placement of the clad deposit relative to the laser beam  18  in relation to the motion of the substrate  32 .  
     [0068] In the present invention, the controller is an AccuNav™ controller, produced by Preco Laser Systems of Somerset, Wis. The AccuNaV™ controller allows for computer control of large, complex operations by storing set-up and process details in the database for reliable and reproducible results. Other control units may also be capable of such control functions.  
     [0069] As described above, by combining laser and gas metal arc welding systems, a selected material can be deposited on the surface of a substrate in a productive, efficient, low-heat input process. By utilizing gas metal arc systems, area improvements and repairs can be made faster and with complete fusion, minimizing contamination and porosity (void) problems.  
     [0070] Finally, as previously mentioned, when dilution between the cladding deposit  36  and the underlying substrate  32  must be tightly controlled, a special composition wire  24  (solid or cored) may be utilized to adjust the chemistry of the clad deposit so as to account for some mixing. Additionally, the travel angle of the torch  22  may be adjusted to improve process results.  
     [0071] Generally, the present invention assumes that there will be some dilution. The reduction in downstream processes required to finish the substrate  32  justifies the dilution relative to the higher crown of the short-circuit arc welding process. Moreover, the speed and quality of finish (including minimal porosity) produced by the system of the present invention justifies the dilution as well.  
     [0072] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.