Abstract:
A method for hardfacing a surface including: depositing a powder ( 68 ) having alloy particles onto a surface ( 70 ) of a substrate ( 66 ); rastering a laser beam ( 60 ) across the surface to melt the powder and to form a weld pool ( 78 ) having a width ( 64 ); directing particles ( 74 ) of a material exhibiting a different property than the substrate into the weld pool in a spray pattern having a width less than the width of the weld pool; and establishing the rastering and directing steps such that material circulation within the weld pool is effective to distribute the particles in the weld pool into a pattern having a width greater than the width of the spray pattern prior to re-solidification of the weld pool.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to laser cladding of a substrate including hard particle injection. 
       BACKGROUND OF THE INVENTION 
       [0002]    During laser microcladding a layer of material is deposited onto a surface by using a laser beam to melt a flow of powder directed toward a substrate surface to be clad. The powder is propelled toward the surface by a jet of gas, and when the powder is a reactive alloy material, the gas is chosen to be argon or other inert gas which shields the molten alloy from atmospheric oxygen and nitrogen. Laser microcladding is limited by its low deposition rate, such as on the order of 1 to 6 cm 3 /hr. Furthermore, because the protective argon shield tends to dissipate before the clad material is fully cooled, superficial oxidation and nitridation may occur on the surface of the deposit, which is problematic when multiple layers of clad material are necessary to achieve a desired cladding thickness. 
         [0003]    In a variation of conventional laser cladding particles may be injected into the weld pool. The injected particles are captured by the meld pool which then solidifies around the particles, thereby metallurgically bonding the particles to the solidified weld pool material. The injected particles are often selected for properties that are different than that of the substrate. One example includes injecting hard particles into a weld pool formed at a tip of a gas turbine engine blade. The hard particles in the resulting blade tip offer better wear resistance for those instances when the blade tip may rub against an abradable blade ring disposed just outside a sweep of the blade tip. 
         [0004]    U.S. Pat. No. 4,299,860 to Schaefer discloses a technique for injecting particles into a weld pool (melt). Schaefer discloses injecting these particles in a vacuum environment to maintain quality of the weld pool. U.S. Pat. No. 4,981,716 to Sundstrum discloses injecting particles without a vacuum, but instead by carrying the particles with an inert gas, such as argon or helium. Sundstrum further improved the process by introducing surfaces to reflect scattered particles back into the melt and thereby enhance a capture efficiency of the particles. It is also known to pre-place stainless steel powder onto a carbon steel substrate with powdered flux material. In this technique the powdered flux provides shielding of the melt pool instead of the inert shielding gas. 
         [0005]    It is further recognized that superalloy materials, such as those that form a turbine blade, are among the most difficult materials to weld/clad due to their susceptibility to weld solidification cracking and strain age cracking. The term “superalloy” is used herein as it is commonly used in the art; i.e., a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures. Superalloys typically include a high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g. IN 738, IN 792, IN 939), Rene alloys (e.g. Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g. CMSX-4) single crystal alloys. An improved method for welding/cladding superalloys is disclosed in commonly assigned U.S. patent application Ser. No. 13/755,098, filed on 31 Jan. 2013, attorney docket number 2012P28301 US, publication number XXX, which is hereby incorporated by reference in its entirety. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is explained in the following description in view of the drawings that show: 
           [0007]      FIGS. 1 and 2  show a prior art technique of laser cladding with particle injection. 
           [0008]      FIG. 3  shows a side view of an exemplary embodiment of laser cladding with particle injection. 
           [0009]      FIG. 4  shows a top view of the exemplary embodiment of laser cladding with particle injection of  FIG. 3 . 
           [0010]      FIG. 5  shows a side view of an alternate exemplary embodiment of laser cladding with particle injection. 
           [0011]      FIG. 6  shows a top view of yet another exemplary embodiment of laser cladding with particle injection. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The present inventors have developed a laser cladding particle injection process that captures more injected particles in the melt while maintaining sufficient particle distribution within the melt, thereby improving manufacturing efficiency. In an exemplary embodiment the process may use a powdered flux instead of a vacuum environment or inert gas to protect the weld pool and consequently the process is able to clad the most difficult to clad superalloy materials. The powdered flux material of an exemplary embodiment is effective to provide beam energy trapping, impurity cleansing, atmospheric shielding, bead shaping, and cooling temperature control in order to accomplish crack-free joining of a wide variety of superalloy materials. 
         [0013]      FIG. 1  shows a side view of a prior art laser cladding particle injection technique that occurs in a vacuum. A conventional laser beam  10  is directed toward a substrate  12  to form a weld pool (melt)  14  that may include a small portion of the substrate  12  that has been melted. The conventional laser beam  10  travels in a direction of travel  16 , and therefore the weld pool  14  also travels in the same direction of travel  16 . Particles  18  are injected into a target area  20  between a back side  22  of the conventional laser beam  10  and a perimeter  24  of the weld pool  14  behind the back side  22  of the conventional laser beam  10 . Upon solidification of the weld pool  14  a conglomerate deposit  26  is achieved with entrapped particles  18  that remain discrete but which may metallurgically bond to the conglomerate deposit  26 . The conventional laser beam  10  forms a very small interface  28  with the substrate  12 , melts a small amount of the substrate  12 , and therefore forms a relatively small weld pool  14 . Consequently, the target area  20  is also relatively small. Conventional techniques direct the particles into the target area  20  as best as possible, but due to the relatively small target area  20  some particles may not reach the target area and may instead be deflected off the substrate  12  as indicated by deflected particle  30 . Similarly, some particles may strike a surface  32  of the weld pool  14  and ricochet due to an angle  34  of the particle&#39;s trajectory  36  to the surface  32  of the weld pool. 
         [0014]      FIG. 2  shows a top view of the prior art laser cladding particle injection technique of  FIG. 1 . Here it can be seen that the perimeter  24  of the weld pool  14  forms a tear drop shape that narrows. This narrowing toward a back side  38  of the tear drop shape reduces a size of the target area  20 . In this area a greater percentage of directed particles  18  may be deflected, in particular if a cross sectional shape of a stream  40  of particles  18  is round. In such a configuration there may be many deflected particles  30  near the back side  38  of the weld pool  14 . In addition, in conventional configurations a target location  42  ( FIG. 1 ) for the particle injection may be tied to a focal point  44  of the conventional laser beam  10 . Thus, when the focal point  44  is at a certain location in the substrate  12  the target location  42  will coincide approximately with an ideal aiming point in the weld pool  14 . However, in certain instances it may be desirable to defocus the laser beam, moving the focal point  44  up or down. In such instances the target location  42  may also move up and down. This, in turn, moves the target location  42  from the ideal aiming point and may result in a greater amount of deflected particles  30  because the stream  40  of particles is no longer properly aimed. As a result of these limitations, to the inventor&#39;s knowledge a capture efficiency of particles has not been known to exceed about 60% of the particles that are directed toward the weld pool. 
         [0015]    The present inventors have recognized that technology that has been known in the art has been improved to the point where it can be combined in an innovative process that overcomes the long-standing limitations of the prior laser cladding particle injection processes. Specifically, the inventors propose to use laser scanning (rastering) optics to form a weld pool of greater size. This, in turn, allows for a greater capture rate of injected particles by ensuring that dimension of the weld pool formed by laser scanning are greater than a pattern of a stream of injected particles. The action of the laser and of currents within the weld pool (e.g. the Marangoni effect) help to distribute the particles into those peripheral parts of the weld pool into which particles are not directly injected. Together these mechanisms allow for a greater capture rate of the particles in the weld pool and a sufficiently even distribution of particles throughout the entire volume of the weld pool. In addition, when using a powder flux, the process disclosed herein can be applied to a greater variety of superalloys then ever before. The particles may be characterized by a different characteristic than the substrate. For example, the particles may be harder, as may be desired if the cladding is to produce a more wear-resistant surface. Materials characterized by a greater hardness include, but are not limited to: tungsten carbide, titanium nitride, and diamond. Alternately, the particles may have a greater lubricity etc. 
         [0016]      FIG. 3  shows an exemplary embodiment of the laser cladding particle injection processes. A laser beam  60  generated by a laser arrangement (not shown) may be rastered along a length  62  and a width  64  to form a rastered area (not visible) on a substrate  66 . The laser beam may be any sufficient type, including but not limited to CO2, NdYAG, fiber, slab and diode. A powder  68  may optionally be preplaced on a surface  70  of the substrate  66  and may include metal/alloy particles and/or powdered flux. The metal particles may be made of a single alloy (or superalloy) having the same or different composition than that of the substrate  66  such that a subsequently formed conglomerate deposit  72  may have particles  74  trapped in a layer having a same composition as the substrate  66 . Alternately, the metal particles may be composite particles containing more than one alloy (and/or superalloy) each. Thus, the powder  68  may include any combination of particles made of single alloys (or superalloys) and composite particles containing more than one alloy (and/or superalloy) each. The powder  68  may include typical wrought alloys, such as alloy nickel alloy 625 powder. In addition, superalloys of useful application include, but are not limited to: CM-247; Rene® 80, 142, and N5; Inconel® 718, X750, 738, 792, and 939; PWA 1483 and 1484; C263; ECY 768; and CMSX-4, and X45. The particles may be approximately 25-100 microns. 
         [0017]    In an exemplary embodiment the powder  68  includes a powder flux. Flux may be necessary because gas shield may not adequately cover the larger weld pool  78 , and the gas shielding may be more readily displaced and leave the weld pool  78  exposed to the atmosphere. When powder flux is present an overlying layer of protective slag  76  may be formed on the conglomerate deposit  72  upon solidification of the weld pool  78 . Typical fluxes for nickel alloy submerged arc welding and electroslag welding may be used in the process described herein for a nickel based superalloy. Examples include Special Metals NT100, Lincoln P2007, Bohler Soudokay NiCrW-412, ESAB OK 10.90, Sandvik 50SW, Sandvik 59S, Bavaria WP-380, Avesta 805, and Oerlikon OP76. Other fluxes typically used for coated electrodes or flux cored electrodes may also be effective. 
         [0018]    Various exemplary embodiments are envisioned for the process. In one exemplary embodiment no powder is used. Particles  74  are simply injected into the weld pool  78  in a manner effective to create a greater capture ratio than previously available. The rastering of the laser beam  60  itself may be used to help distribute the particles  74  throughout the weld pool  78 . The powder  68  may include alloy particles, composite alloy particles, flux, or any combination thereof. If no flux powder is used the welding operation may occur in a vacuum environment or an inert gas may be supplied to protect the weld pool  78 . Any powder  68  that is used may be preplaced, separately fed using one feed path for the powder  68 , discrete feed paths for each constituent of the powder  68 , and any combination of feed paths and constituents. Further, the powder  68  may be fed through the same feed path used to deliver the particles  74 . 
         [0019]    When the powder  68  is used and if it contains flux, the powder  68  may be placed ahead of the laser beam  60 . In such an exemplary embodiment the powder flux and trailing blanket of slag may provide sufficient shielding. However, it would be necessary to ensure that the particles  74  settle into the weld pool  78  through any floating and likely molten slag that may form. The powder  68  with flux may alternately be fed behind the laser beam  60 . This may provide good shielding, but it would be necessary to ensure that the molten slag be fluid enough to shield the entire melt zone, including any zone astride the laser beam  60 . This exemplary embodiment may require a run-on tab at the beginning. During subsequent steady state operation the flux and slag should be effective to provide the desired effects. Powder flux may alternately be fed as a separate stream of material, ahead of, coincident with, or behind the laser beam  60 . This provides great flexibility in terms of delivery, though it may add some complexity due to the logistics associated with a separate delivery path for the powder flux. In another alternate exemplary embodiment, it is possible to precoat the surface  70  of the substrate  66  with the powder flux. This avoids the need to mix and deliver the powder flux. However, it would be necessary to ensure a sufficient amount of powder flux is delivered. This may require an additional processing step prior to the cladding operation, but the benefits of eliminating the delivery step during the cladding operation may provide other benefits. 
         [0020]    While rastering over both the length  62  and width  64  of the rastered area, the laser beam  60 , and hence the rastered area, both move in a direction of travel  82  along the substrate  66 . As a result, the rastered area has a leading edge  84  and a trailing edge  86 . By virtue of a larger interface  88  between the rastered area and the material to be melted a larger weld pool  78  is formed when compared to the weld pool of the conventional technique of  FIGS. 1-2 . The larger weld pool  78  remains liquid for a longer time, and therefore a target area  90  behind the trailing edge  86  of the rastered area is larger than the target area  20  of  FIGS. 1-2 . This larger target area  90  is more efficient at capturing a stream  92  of particles  74 , resulting in a greater capture ratio of at least 60%. In an exemplary embodiment the length  62  of the rastered area may be 5-10 mm and the width  64  may be 3-10 mm. A perimeter  94  of the weld pool  78  defines a length  96  of the target area  90  that may be about 7-8 mm and a width  98  of the target area that may be about 3-10 mm. The resulting weld pool  78  may then have a length  100  of 12-18 mm and a width  102  of 3-10 mm. These dimensions are indicative of those possible with the process as disclosed herein. However, larger and/or smaller dimensions are also possible. Also, the weld pool  78  may not be exactly the same size as the rastered area due to localized melting and/or solidifying at the edges of the weld pool  78 . Should it be desired that the particles  74  maintain as much of their original shape as possible the particles  74  may be directed toward the rear of the weld pool  78 . 
         [0021]    In  FIG. 4  it can be seen that in an exemplary embodiment a pattern of the stream  110  of particles is characterized by a width  112  transverse to the direction of travel  82  that is smaller than the width  98  of the weld pool  78  where the stream  110  and a surface  80  of the weld pool  78  interface (i.e. where the particles  74  enter the weld pool). Likewise the pattern of the stream  110  is characterized by a length (not visible) that is smaller than the length  96  of the weld pool  78 . As a result there is a peripheral region  114  between the perimeter  94  of the weld pool  78  into which particles  74  are not directly injected. Nonetheless, the rastering motion of the laser beam  60  acts to stir the weld pool  78  and this stirring, together with natural currents (e.g. convective currents) within the weld pool  78 , work together to distribute the particles  74  into the peripheral region  114 . Consequently, particles  74  are distributed sufficiently throughout an entire volume of the weld pool  78 , not just where the stream  110  is aimed. In an exemplary embodiment the width  112  of the stream  110  may be as little as 75%, or even 50% of the width of the target area  90 . In an alternate exemplary embodiment instead of being a fanned-out pattern as shown, the stream  110  may be smaller and may raster with the laser beam  60  in the area surrounded by the peripheral region  114 . Alternately, the narrowed stream  110  may move in its own distinct pattern in the area surrounded by the peripheral region  114 . 
         [0022]    The powder flux and resultant layer of slag  76  provide a number of functions that are beneficial for preventing cracking of the conglomerate deposit  72  and the underlying substrate  66 . First, they function to shield both the region of weld pool  78  and the solidified (but still hot) conglomerate deposit  72  from the atmosphere in the region downstream of the rastered area  116 . The slag  76  floats to the surface  80  of the weld pool  78  to separate the molten or hot metal from the atmosphere, and the flux may be formulated to produce a shielding gas in some embodiments, thereby avoiding or minimizing the use of expensive inert gas. Second, the slag  76  acts as a blanket that allows the solidified conglomerate deposit  72  to cool slowly and evenly, thereby reducing residual stresses that can contribute to post weld reheat or strain age cracking. Third, the slag  76  helps to shape the weld pool  78  to keep it close to a desired height/width ratio. Fourth, the powder flux provides a cleansing effect for removing trace impurities such as sulfur and phosphorous which contribute to weld solidification cracking. Such cleansing includes deoxidation of the metal powder. Because the flux powder is in intimate contact with the metal powder, it is especially effective in accomplishing this function. Finally, the powder flux may provide an energy absorption and trapping function to more effectively convert the laser beam  60  into heat energy, thus facilitating a precise control of heat input, such as within 1-2%, and a resultant tight control of material temperature during the process. Additionally, the flux may be formulated to compensate for loss of volatized elements during processing or to actively contribute elements to the deposit that are not otherwise provided by the metal powder itself. Together, these process steps produce crack-free deposits of superalloy cladding on superalloy substrates at room temperature for materials that heretofore were believed only to be joinable with a hot box process or through the use of a chill plate. 
         [0023]    In an alternate exemplary embodiment the rastered point laser beam of  FIGS. 3 and 4  may instead be a rectangular laser beam emitted from a diode laser. In such an exemplary embodiment the diode laser beam may travel back and forth along a swept area that would be comparable to the rastered area  116 . In another exemplary embodiment the laser beam may be defocused to increase the size of the weld pool  78 . 
         [0024]      FIG. 5  shows an alternate exemplary embodiment of the laser cladding particle injection processes. In this exemplary embodiment the rastered area  116  is characterized not by a constant energy distribution, but instead by an energy gradient from the leading edge  84  to the trailing edge  86  as indicated by the different line thicknesses. For example, the energy density may be greatest at the leading edge  84  and gradually decrease, either linearly or non-linearly, to a lesser energy density at the trailing edge  86 . Such an arrangement would provide the extra energy necessary to change the phase of the substrate  66  from solid to liquid at the leading edge  84 , and would reduce the energy delivered to the already-melted weld pool  78  so that the weld pool would remain melted but not acquire any more energy than necessary to do so. This may reduce the amount of heat delivered to the substrate material surrounding the weld pool  78 , and this may reduce cracking etc. 
         [0025]    In a variation of the of this alternate exemplary embodiment the particles  74  may be directed such that they traverse a column  118  above the rastered area  116  in which the laser beam  60  moves. Due to the speed of the laser beam  60  within the column  118  the particles  74  will inevitably traverse the laser beam  60 . The energy gradient may be set so that a surface  120  of the particles  74  is melted but the bulk of the particles  74  remain unmelted. This may be used for any number of reasons, including to enhance metallurgical bonding with the subsequently formed conglomerate deposit  72 , and to smooth out irregular surfaces if deemed desirable etc. Some or all of the stream  92  may traverse the column  118 . 
         [0026]    At some point within the column  118  the energy density would rise to a point where the particles would likely fully melt due to the increased energy density and the increased length of travel through the column  118 . Before reaching this point the particles may be considered to be within a safe zone  122  within the column  118 . Consequently, in this alternate exemplary embodiment a new target area  124  is created which includes the target area  90  of the exemplary embodiment of  FIG. 3  plus a length  126  of the safe zone  122 . This is much larger than the target area  90  of the exemplary embodiment of  FIG. 3 . Consequently more particles  74  are likely to be captured and this would increase the capture ratio even more. Controlling the capture rate of the particles may allow further control of the solidification of the weld pool  78  because the relatively cool mass of the particles  74  will cool the weld pool  78 . 
         [0027]    In another alternate exemplary embodiment the angle  130  of the particle trajectory with respect to the surface  80  of the weld pool  78  can be increased due to the greater amount of available room in the target zone  90 . The increase in the angle  130  decreases the likelihood of deflection, and hence increases the capture ratio. 
         [0028]    In an alternate exemplary embodiment of the above process shown in  FIG. 6  a first row  150  of cladding may be formed and followed by a second row  152 . The first row may occupy a first footprint  154  and the second row may occupy a second footprint  156 . There may be an overlap  158  of the two footprints  154 ,  156  and the overlap  158  may be arranged so that any lower density of particles in the peripheral region  114  of any row may be corrected when the next row is applied so that a density of particles across a width  160  of both rows  150 ,  152  is made more uniform. 
         [0029]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.