Methods for the treatment of superalloy articles and associated repair methods

Abstract

Method for treating a superalloy substrate (32) containing at least one cavity (33) containing adhering metal oxide material on its surface (31), the method comprising the steps of contacting the cavity surface (31) with a forming gas and directing a laser beam (46) with short pulses and a high repetition rate against the cavity surface (31) for a time period sufficient to remove all the adhering metal oxide material, wherein the laser beam (46) is characterized by a peak power density in the range of about 100 megawatts / cm² to about 10 gigawatts / cm² sufficient to cause laser ablation of the metal oxide material.

Description

BACKGROUND OF THE INVENTIONThe invention relates generally to metals and metal alloys used in high temperature applications. In some specific embodiments, the invention relates to methods for treating superalloy components to prepare them for further repair methods.Superalloys are often the first choice for component materials intended for high temperature environments. For example, turbine buckets and other parts of turbines (e.g., gas turbines) are often made of nickel-base superalloys because they must withstand temperatures of at least about 1000° C.-1150° C. intact. Protective coatings, often referred to as thermal barrier coatings or "TBC" coatings, effectively increase the operating temperature of turbine components by maintaining or reducing the surface temperature of the alloys used to make the various engine components.Gas turbine components tend to crack over the duration of their operation. In many cases, the cracks begin at the surface of the component and extend into the component. These cracks will appear over time mainly due to high temperatures and pressures to which the gas turbines are exposed. Especially problematic are cracks that arise in turbine blades, e.g., the high pressure turbine (HPT) blades, that are located in close proximity to the engine combustor. These blades are exposed to the hotter combustion gases to which energy is extracted and are mostly made of the nickel (Ni) superalloys. (These specialized alloys tend to produce equiaxed, directionally solidified, and single crystal alloy structures.).FIG. 1 illustrates an exemplary turbine blade 10 for use in power plants turbines, e.g., in the first blade row of a gas or combustion turbine. The turbine blade 10 includes a blade root 12, an airfoil portion 14, and a tip portion 16. The blade root 12 is configured to be inserted into and supported by a disk on a rotating shaft (not shown) of the turbine. The blade portion 14 is configured to draw energy from combustion gases flowing over the blade portion 14, thereby imparting rotational mechanical energy to the turbine shaft. In the case of modern gas turbines, the airfoil section 14 is designed to have one or more cooling channels formed under the surface of the airfoil for the passage of cooling air required to ensure the durability of the airfoil material in the environment of hot combustion gases.FIG. 1 illustrates such a crack 24 that may occur near the tip 16 of the blade 10. As mentioned above, the cracks may arise due to low cycle fatigue stresses imposed on the blade tips 16 during operation of the turbine. If the crack 24 extends beyond a critical dimension, the turbine blade 10 must be taken out of service and / or repaired to avoid failure of the blade and turbine.In many cases, the crack may be repaired by removing the material adjacent the crack 24 to form a crack repair volume, and then filling the crack repair volume with weld metal. Generally, a number of techniques are used to repair cracks. Some important examples include welding, diffusion brazing, Activated Diffusion Healing (ADH), and thermal spraying techniques, e.g., using high velocity oxy-fuel (HVOF).For most of these techniques, initial preparation of the repair surface is critical. The undesirable presence of chemically stable oxides, as well as any metallic bond coats or ceramic thermal barrier coating materials present, can significantly complicate welding or brazing of a repair surface. In order to enable treatment, for example successful turbine repair, such substances must therefore be removed without residue.Various methods have been used in the past to clean the repair surfaces of metal components such as turbine blades. In some cases, manual grinding is performed prior to repair by welding techniques. While the grinding techniques are useful in many cases, they are time consuming. Moreover, the efficiency of grinding is highly dependent on the experience of the user. In addition, grinding processes in the case of cracks in turbine airfoil surfaces are often limited to a crack depth of about 0.35 inch (8.9 mm). Therefore, deep cracks in components such as HPT vanes may not always be successfully "prime" for repair; and attempts to perform the pretreatment processes may occasionally result in "weld drops.".Various fluoride ion cleaning (FIC) techniques have also been used to clean cracks and other cavity surfaces. (Some of the related techniques are referred to as "dayton methods"). The FIC processes are usually based on the thermal decomposition of a fluorine-based polymer, for example polytetrafluoroethylene (PTFE). According to some mechanisms, the resulting carbon fluoride monomers combine with hydrogen and these products contact a variety of oxide deposits, converting the deposits to fluoride compounds. The fluoride compounds are volatile and leave the void region in a gas stream. Heat treatment steps are frequently used to convert any remaining surface deposits and diffused deposits into volatile products which can be discharged in gaseous form.While fluoride ion cleaning and etching processes are effective in some situations, they may also have significant disadvantages. For example, these techniques may require relatively long process times if the workpiece surface is to be fully cleaned. They may also require high cleaning temperatures of, for example, about 1900° F. (1038° C.). Moreover, FIC processes are often based on the use of compounds such as hydrogen fluoride. These materials are corrosive and toxic and require special handling and disposal procedures. Some of the compounds are also classified as objectionable air pollutants. In addition, the FIC methods can attack the materials that form protective layers on superalloy articles, e.g., nickel aluminide or platinum aluminide materials. Thus, certain provisions must be made to mask or otherwise protect substrate areas where these protective layers are to be maintained, for example, at areas remote from the particular repair area.DE 10 2004 001 315 A1 describes a method for repairing a superalloy component having a defect on its surface, the method comprising treating the superalloy substrate in preparation for a welding operation by machining the surface with a cutting tool to form a cavity, in order to remove oxides and other contaminants which could be detrimental to the welding operation. A suitable filler material insert is inserted into the cavity formed in this way, which is then welded to the substrate material by heating the filler material insert.WO 96 / 09 128 A1 describes an apparatus and method for removing an unwanted material from the surface of a substrate by irradiating the unwanted material with high energy photons, for example a short pulse length laser beam of e.g. 34 ns and a repetition rate of e.g. 30 Hz for a period of time and with a peak power density sufficient to remove the metal oxide material but insufficient to alter the physical properties of the underlying substrate. In an exemplary embodiment, the peak power density is about 12 MW / cm 2.DE 10 2006 052 824 A1 describes a method for laser cutting a metallic component, wherein a cutting line in the component is scanned multiple times with a laser beam having high power, short pulse length and high repetition rate in order to successively increase the depth of the cutting gap until the metallic component is completely separated. As a result of a high power density, the material of the metallic component is ablated or sublimated, wherein the laser cutting process is carried out without protective gas, since reduced oxidations of the material are expected due to the low energy input.Accordingly, new methods for cleaning voids and other regions in superalloy substrates would be readily appreciated in the art. The techniques should be efficient in preparing for repair processes in removing oxides and other contaminants from the cavity region as well as eliminating any protective layer materials. Moreover, the techniques should be capable of being carried out in a relatively short time and at ambient temperature. They should also minimize or eliminate the need for hazardous and special safety precautions requiring chemicals.BRIEF DESCRIPTION OF THE INVENTIONOne embodiment of the present invention is directed to a method of treating a superalloy substrate having at least one cavity (e.g., a crack) having adherent metal oxide material on its surface. The method includes the step of contacting the cavity surface with a forming gas. The method further includes the step of directing a short pulse, high repetition rate laser beam against the cavity surface for a time sufficient to remove all of the adherent metal oxide material, the laser beam being characterized by having a peak power density in the range of about 100 megawatts / cm 2 to about 10 gigawatts / cm 2 sufficient to cause laser ablation of the metal oxide material.A further embodiment relates to a method for treating a superalloy substrate having at least one cavity. This embodiment includes the steps of contacting the cavity surface with a forming gas and directing a high power (medium power) laser beam, short pulses, and high repetition rate laser beam to a region on the substrate having the cavity under laser operating conditions capable of cutting into the superalloy material so that cutting into the superalloy substrate forms a boundary region in the substrate enclosing the cavity. The laser beam is characterized by a peak power density in the range of about 100 megawatts / cm 2 to about 10 gigawatts / cm 2 sufficient to cause laser ablation of the metal oxide material. The cavity has walls defining its shape, the boundary area being relatively uniform and smooth compared to the walls of the cavity.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 illustrates, in a perspective view, a turbine blade having the dovetail root, the airfoil portion, and the tip portion. FIG. 2 shows a schematic illustration of a laser device for treating a component according to some exemplary embodiments of the invention. FIG. 3 illustrates a crack in a turbine blade and a surrounding boundary area.DETAILED DESCRIPTION OF THE INVENTIONThe various compositional and process ranges described herein are inclusive and combinatable (e.g., ranges from "up to about 25 wt %", or more specifically, "about 5 wt % to about 20 wt %", include the endpoints and all intermediate values of the ranges). Moreover, the term "combination" includes blends, mixtures, alloys, reaction products and the like. Moreover, the terms "first," "second," and the like herein do not specify an order, quantity, or ranking, but rather are used to distinguish one element from another. The articles "a" and "an" as used herein do not limit the quantity, but merely the presence of at least one of the elements in question. The modifying term "about" used in connection with a quantity includes the stated value and has the meaning dictated by the context (e.g., includes the error deviation in connection with a measurement of the particular quantity). The term "(s)" as used herein is intended to include both the singular and the plural term that it modifies to thereby include one or more elements of that term (e.g., the term "the compound" may include one or more compounds unless otherwise specified). Throughout the specification, reference to "one embodiment," "one embodiment," "another embodiment," and so forth, means that a particular element (e.g., feature, construction, and / or characteristic) described in connection with the embodiment is included in at least one embodiment described herein and may or may not be present in other embodiments. In addition, it is understood that the described features according to the invention can be combined with one another in any desired manner in the various exemplary embodiments.As mentioned above, the treated substrate is at least partially formed from a superalloy material. These materials are well known in the art for use in gas turbine assemblies and other high temperature components. In general, the term "superalloy" is intended to broadly encompass iron, cobalt or nickel-based alloys containing one or more other elements, non-limiting examples of which include aluminum, tungsten, molybdenum, titanium, and iron. Superalloys have desirable chemical and physical properties under the conditions of high temperatures (e.g., up to about 1500°C), high mechanical stresses, and high pressures to which they are usually exposed during operation of a turbine.Components based on superalloys are often provided with indispensable protective coatings. Examples include thermal barrier coatings (TBC layers). Most TBC layers are ceramic based, for example, yttria stabilized zirconia. Usually, the TBC ceramics are applied to an intermediate layer which is directly applied to the surface of the metal component. The intermediate layer is often a bonding layer. Bond layers are usually formed of a material such as "MCrAlX", where "M" represents a metal such as iron, nickel, cobalt, or combinations thereof, and "X" is an element selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof.In most cases, the protective coatings and any byproducts formed when the coatings are exposed to a variety of environmental conditions must be removed before the component is repaired, e.g., by welding. Some of the specific examples of materials to be removed are metal oxides such as alumina, zirconia, nickel oxide, chromium oxide, molybdenum oxide, and calcium magnesium aluminum silicon oxide (CMAS). (It should be understood that terms such as "nickel oxide" and "chromium oxide" are intended to include one or more oxide compounds, e.g., chromium (II) oxide, chromium (III) oxide, or chromium dioxide.).As described above, the substrate has at least one cavity. As used herein, the term "cavity" is intended to include any type of hole, depression, notch, channel, or groove. The term may also refer to a variety of defects or irregularities on the surface of the substrate, e.g., rough surfaces or foraminous surfaces. In the case of turbine components such as turbine blades, the cavity or "defect" is often a crack, such as is shown in FIG. 1. The size of a repairable crack may vary considerably, e.g., from about 25 μm depth to about 10 mm depth; and from about 5 μm width to about 4 mm width (measured in the radial direction, e.g., through the wall thickness of a turbine blade). (The wall thickness of a typical HPT blade may range from about 0.5 mm to about 2 mm.).As mentioned above, a laser is used to treat the cavity surface to remove substantially all of the adherent oxide material, in accordance with embodiments of this invention. A number of different types of lasers can be used for this method. Non-limiting examples include semiconductor lasers (e.g., diode lasers), fiber lasers, and excimer lasers. (Fiber lasers may sometimes be considered a type of semiconductor laser.). The skilled person is familiar with the operation of all these types of lasers. Semiconductor lasers with relatively short pulses are often preferred. Specific examples include 1-micron lasers or green lasers (0.532 μm), although the wavelength may vary from near infrared to ultraviolet. Techniques for adjusting the wavelength of the laser, e.g. by means of "frequency doublers" or doubling crystals, are also known from the art.In preferred embodiments, the laser must be capable of delivering a beam with short pulses and high repetition rate against the surface of the cavity. The pulse length is usually in the range of about 1 nanosecond to about 1 microsecond. However, lasers may occasionally be used in the pulse ranges of femtoseconds and picoseconds. (Those skilled in the art will understand that a reduction in pulse length at about the same energy level will usually result in a higher laser power level.). Generally, use of the shorter pulse lengths results in less melting and evaporation of the substrate surface material and often a smoother treated surface. However, the short pulse length may in some cases slow the overall cleaning process. In some preferred embodiments, the pulse length in treating cracks in turbine blade tips is often in the range of about 1 nanosecond to about 20 nanoseconds.The repetition rate of the laser beam (i.e. the number of pulses emitted per second) can be adjusted as a function of the mean power for a specific laser device. Generally, the repetition rate is at least about 100 Hz and is often in the range of about 1 kHz to about 500 kHz. If much material (e.g., oxide material) is to be removed from the cavity region, or if the material adheres stubbornly to the cavity surface, the repetition rate is occasionally selected in view of a change in the value of the energy / pulse ratio. Those skilled in the art will be able to select the most suitable repetition rate or average / peak laser power based on a variety of factors, e.g., the type of laser system used, the laser pulse energy, the material to be removed from the substrate, and the overall dimensions of the cavity.As mentioned above, the laser beam is characterized by a peak power density that is in the range of about 100 megawatts / cm 2 to about 10 gigawatts / cm 2. The average power value in this case is usually in the range of about 1 watt to about 15 watts. Many of the aforementioned factors are also important herein, such as the type and amount of material to be removed from the cavity region; the depth of the cavity; and the type of laser system used. The skilled person will be able to select the most suitable power density for a given situation.As mentioned above, the laser beam is directed against the cavity surface of the substrate for a period of time sufficient to remove substantially all of the adherent metal oxide material. The time required to reach this goal will depend on the various laser operating parameters mentioned above, as well as on the type and depth of material to be removed from the cavity. A non-limiting illustration may be provided in the case of a nickel-based turbine blade having a V-shaped crack in its tip with a crack depth of about 1 mm (measured through the wall thickness of a substrate) and a crack width in the range of about 0.5 mm to about 4 mm. In this illustration, the surface of the crack is believed to be covered with a layer of metal oxide material having a depth of about 0.3 mm. When using a Nd:YAG green semiconductor laser which produces an energy density of about 8 gigawatts / cm 2, a pulse length of about 10 nanoseconds, and a repetition rate of about 10 kHz, the time required to remove substantially all of the metal oxide material, in this case, is usually in the range of about 30 seconds to about 600 seconds.In many preferred embodiments, the laser beam used for the embodiments of this invention is focused by a scanning module onto the cavity surface. Scan modules (scan modules) are known in the art and are described, for example, in U.S. Patents 7,528,342 (Deshi) and 7,529,010 (Scaggs et al.), both of which are incorporated herein by reference. Generally, the "scanners" can be described as electromagnetic devices that move mirrors according to preselected (usually microprocessor controlled) schemes to reflect the laser beam and direct it to a desired point on a substrate. A variety of sensing modules may be used. Non-limiting examples include galvanometer scanners and piezo scanners (sometimes referred to as "galvanometers" or "galvos"). Those skilled in the art will be familiar with the operation of these scanners and their incorporation into a laser scanning system.Figure 2 schematically illustrates a laser apparatus 30 suitable for embodiments of this invention. The support 34 is an extension of the laser support platform 36 and the platform 36 usually comprises a five axis stage 38 and not all of the details of such a stage need be shown. The five-axis stage is capable of supporting the workpiece 32 to translate it precisely along three axes (X,Y,Z) and to rotate it precisely (in two directions, e.g., "A" and "B") with respect to these axes. (Note that the dimensions of the workpiece are exaggerated for clarity, and the relative size and position of the remaining components may vary considerably in this figure.).This laser support platform may advantageously be part of a complex, multi-axis numerically controlled (CNC) machine. These machines are known from the art and are commercially available. The use of such a substrate handling machine is described in U.S. Patent No. 7,351,290 to S. Rutkowski et al., which is incorporated herein by reference. As described in the Rutkowski patent, the use of such a machine enables movement of the substrate along one or more axes of rotation with respect to linear axes X and Y. CNC machines of this type are commercially available from many companies, e.g., Haas Automation, Inc., Oxnard, Calif. The skilled person can adapt such a machine for use in the present invention without great effort. In addition, industrial robots can be used to coordinate the movement of the laser and the support platform.With further reference to the illustrative apparatus of Figure 2, the underlying machine is usually equipped with a 5-axis controller (not shown). The platform 38 may further include a conventional lift 40 to which a conventional rotating spindle may be attached. The elevator 40 establishes a vertical axis (Z) of translation relative to the table 38. In addition, another spindle (not shown) or similar rotating device may be attached to the lift 40 to provide a second axis of rotation as described in the Rutkowski patent.Additionally, the entire laser apparatus 30 of FIG. 2 may further include at least one conventional digitally programmable computer or controller 42. The computer 42 may provide a variety of functions associated with the laser and scanning units, but may also be configured to control all axes of movement of the support platform 36 using suitable software. In this way, a five-axis coordination between the laser device and the workpiece can be achieved. This capability, in turn, provides accurate scanning, cleaning, and / or cutting of a desired portion of the workpiece, as further described herein.A laser source 44 emits the desired laser beam 46, which is usually collimated and has a linear or circular polarization state. In some cases, the laser beam is incident substantially perpendicularly on a wave plate (not shown) to change the polarization state of the beam. Moreover, the laser beam (i.e., the laser pulse) may be modulated by a suitable beam modulation means (also not shown). In some embodiments, one or more optical lenses may be used to expand or reduce the beam diameter.The laser beam 46 is then scanned by a scanning module 48, as previously described. The scanning module scans the beam onto the preselected surface 31 of the workpiece 32 (a crack 33 present in the surface 31 is shown in simplified form). Moreover, a camera 50, e.g., a CCD camera, captures the workpiece (usually through a polarizer, not shown herein) to aid in alignment of the workpiece with respect to the laser beam and to monitor the cleaning and / or cutting process. During the treatment, the workpiece 32 is usually sealed in a shielded space 52 which is filled with inert or semi-inert gases (or forming gases such as hydrogen or hydrogen containing mixtures) derived from a gas source 53. Such a space is commonly used in the welding industry. In addition, as shown in simplified form, computer 42 communicates with the laser unit, scanner, camera and CNC platform via lines 54, 56, 58 and 59, respectively, by means of known means.A number of other details and optional devices may be used in conjunction with a laser system, such as that described above, or with other laser systems that may be used for this invention. Some of these details are set forth in the above-mentioned Deshi and Scaggs et al. patents and in U.S. Patents 5,419,971 (Skelly et al.); 6,491,207 (Smashey et al.); and 6,759,627 (Kilburn); and in WO 2007 / 096480 A1 and EP 1,247,003 B1, all of which are incorporated herein by reference. Further information can be found in the publications "Laser Beam Machining (LBM), State of the Art and New Opportunistics", J. Meijer, Journal of Materials Processing Technology 149 (2004), pp. 2-17 ff.; and "Modelling and Diagnostic of Pulsed Laser Cleaning of Oxidized Metallic Surfaces, R. Oltra et al, Applied Surface Science 96-98 (1996), pp. 484-490 ff.The method described herein is different from methods previously used in the art using laser generated shock waves. These methods are occasionally used to remove crust-like deposits from turbine components. In contrast to these, the cleaning method described herein is considered a laser ablation method, also referred to as laser ablation, in which the relatively high power of the laser results in a direct evaporation of the solid material / contaminants from the substrate surface.In some embodiments of this invention, the cavity (e.g., a crack or other defect) is repaired by filling it with a replacement material. The filler material may be modified for processing similarly or identically in composition to the original material constituting the substrate. However, depending on factors such as the intended final use of the article, other materials may also be used to fill the cavity.Non-limiting examples of void repair techniques include activated diffusion healing (ADH), diffusion brazing, welding, and thermal spraying techniques, e.g., high speed flame spraying (HVOF); and combinations including one or more of these methods. Any technique, such as the details of using any technique to repair defects such as airfoil cracks, are known in the art.Welding techniques are often preferred for the repair of superalloy materials. Non-limiting examples of welding techniques include arc welding (e.g., sigma welding, gas-metal arc welding (GMAW), UP arc welding; metal shield gas (GMAW) welding, or tungsten shield gas (WIG) welding); and gas welding (e.g., oxyfuel welding). (In some cases, MSG welding and GMAW are considered consistent techniques.). One skilled in the art will be able to select the most suitable welding technique for a particular situation based in part on the teaching provided herein.Tungsten Protective Gas (TIG) welding is the preferred repair technique in some embodiments of this invention. The technique is sometimes referred to by a number of other terms, e.g., Gas Tungsten Arc Welding (GTAW) or, occasionally, Helium Protective Gas Welding. TIG welding usually involves the use of a non-consumable tungsten electrode to produce the weld. Typically, a constant current power source for welding provides power which is passed through a column of highly ionized gas and metal vapors, i.e., the plasma, via the tungsten arc. Usually, the welding area is protected from atmospheric contamination by a shielding gas (e.g., an inert gas such as argon). WIG processes often use a filler metal.In some embodiments of this invention autogenous welding may be performed that does not require a filler material. This welding technique is usually useful if the cavity, e.g., a crack, has a width that does not exceed about 15% of the surrounding wall thickness. For the autogenous methods, electron beam welding or laser welding is usually used.One type of welding technique is particularly suitable for the materials described herein and is known as a "SWEET, Superalloy Welding at Elevated Temperatures" method (superalloy welding at relatively high temperatures). This technique, often considered a subclass of WIG, is described in U.S. Patents 6,996,906, 6,020,511, 6,124,568 and 6,297,474, which are incorporated herein by reference. Typically, SWEET processes are performed in a room in which a controlled atmosphere and temperature are maintained to prevent cracking and oxidation of a superalloy component being repaired. Those skilled in the art will be familiar with further details of the SWEET process which would be important to the present description.In further embodiments of this invention, a high energy laser beam is used to cut a boundary region in the superalloy substrate. FIG. 3 is a simplified representation of a relatively deep crack in a turbine blade that originates from the tip of the blade and continues downward in the radial direction of the blade. As will be understood by those skilled in the art, the walls and overall shape of the crack may be very irregular or jagged. The crack 60 is formed on the surface 62 of a substrate 64. In the figure, a border area or "cut line" 66 is shown representing the area being cut from the substrate. (It should be understood that although the representation of the figure is two-dimensional, the crack and surrounding area are actually three-dimensional, so that the area is sometimes referred to herein as a "volume").In many exemplary embodiments, cutting out the boundary region is advantageous for a number of reasons. First, this allows to fill the cavity, e.g. by welding, according to the exact dimensions. In this manner, the repaired cavity is characterized by a "net near shape" in terms of the original shape and size of the substrate area. Additional application and processing steps can thus be reduced to a minimum.Second, cutting the boundary region can remove regions of the superalloy substrate that are adjacent to the cavity itself. Removal of this region is sometimes desirable because surface oxide material and other contaminants diffused into the region (usually under the action of high temperature conditions over time). The diffused region could otherwise not be an ideal surface for subsequent repair by welding or other techniques.The high-energy laser beam used to cut out the boundary region may be the same laser used for a subsequent cleaning step. Alternatively, it may be another laser, e.g. one of the other types of lasers described above. The choice of laser equipment of a particular type will depend in large part on the particular material from which the substrate is made.In some preferred embodiments, a continuous wave (CW) laser is used. The CW laser should have good "beam quality", e.g. low beam spread, and preferably should be highly collimated. In some preferred embodiments, a fiber laser, a CO 2- laser or a semiconductor laser is used.In other cases, a pulsed laser beam may be used, e.g., a short pulse and high repetition rate laser beam having a pulse frequency ("repetition rate") in the range of about 10 kHz to about 100 kHz and an average power value in the range of about 5 watts to about 30 watts. In this case, the peak power density would usually range from 0.1 gigawatts / cm 2 to about 10 gigawatts / cm 2. Moreover, as in the case of the laser cleaning process, the high-energy laser can be focused on the substrate by a scanning module for the cutting process.As mentioned previously, the use of the laser to cut a boundary area around a crack or other defect or cavity has several advantages. For example, the laser may be adjusted to accurately cut a selected region of the substrate. The area is sufficiently large to ensure removal of all oxide and diffused materials in the adjacent substrate area, but sufficiently small to minimize the risk of damage to the bulk of the substrate or consequent severe change in its shape. In many preferred embodiments, and with reference to Figure 3, the boundary area 66 formed by the cut line is relatively uniform and smooth compared to the interior surfaces or "walls" 67 of the cavity itself. (The boundary region may therefore be referred to as a planar "control volume").After the cutting step, the cavity is usually prepared for additional repair processes, e.g., welding. It should be noted that oxide-like material or other contaminants that may be present on the cavity surface are already removed at this stage, as the cutting step has removed a portion of the underlying substrate. However, in some cases it may be desirable to remove any remaining free-standing superalloy material(s), burrs and "spatter". A number of techniques may be used for this purpose, e.g., water rinsing, sandblasting, fluidized bed cleaning, and the like, or any combination of such techniques.In some embodiments, a laser cleaning step as described above is used at this stage. This step may be particularly useful to remove any other oxidants that may have been formed. For example, during the cutting operation, ambient air may have entered the cavity and treatment area, which reacts with substrate components to form the undesirable oxides. The parameters for this laser step are substantially the same as described above, usually using a pulse laser device. Moreover, this "re-cutting" step can also be accompanied by other cleaning steps, for example by the water flushing or sandblasting techniques mentioned above.At this stage, the crack or other defect is cleaned and prepared for further repair steps as described above. Moreover, the repair region is physically very precisely defined, so that after completion of the further processing steps, almost the net shape is achieved. In a typical scenario, the crack may now be filled with a desired replacement material by a suitable technique, such as welding, as described above. The welding process leads, for example, to the boundary region comprising the crack being completely filled, so that the crack is repaired as a result.Although the invention has been described with reference to specific embodiments, it is intended that these be illustrative only and not be interpreted as limiting in any way. Thus, it should be understood that changes may be made thereto which fall within the scope of the invention and the appended claims. All of the above-mentioned patents, patent applications, articles and texts are incorporated herein by reference.A method of treating a superalloy substrate 32 having at least one cavity 33 comprising adherent metal oxide material on its surface 31 is described. A short pulse, high repetition rate laser beam 46 is directed against the cavity surface for a sufficient period of time to remove substantially all of the adherent metal oxide material. The laser beam 46 is characterized by a peak power density in the range of about 10 megawatts / cm 2 to about 10 gigawatts / cm 2. In another embodiment, a high power, short pulses, and high repetition rate laser beam is directed to a region 62 on the substrate 64 having the cavity 60 under laser operating conditions capable of cutting into the superalloy material, such that a boundary region 66 is formed in the substrate enclosing the cavity. The cavity may be a crack in a turbine blade, and the crack 60 may be repaired after treatment by welding or by any other suitable technique.Parts List:10 Turbine blade 12 blade root 14 airfoil portion 16 tip portion 24 crack 30 laser device 31 workpiece surface 32 workpiece 33 crack 34 fixture 36 five axis controller 38 platform 40 lift 42 computer / controller 44 laser source 46 laser beam 48 scan module 50 camera 52 shielded space 53 gas source 54 conduit 56 conduit 58 conduit 59 conduit 60 crack 62 substrate surface 64 substrate 66 cut line / boundary region 67 cavity wall

Claims

A method of treating a superalloy substrate (32) including at least one cavity (33) containing adherent metal oxide material on its surface (31), the method comprising the steps of contacting the cavity surface (31) with a forming gas and directing a laser beam (46) at short pulses and high repetition rate against the cavity surface (31) for a time sufficient to remove all of the adherent metal oxide material, the laser beam (46) being characterized by a peak power density in the range of about 100 megawatts / cm 2 to about 10 gigawatts / cm 2 sufficient to cause laser ablation of the metal oxide material.The method of claim 1, wherein the laser beam pulses have a pulse length of about 1 nanosecond to about 1 microsecond.The method of claim 1, wherein the source (44) of the laser beam (46) is a device selected from the group consisting of a semiconductor laser, a fiber laser, and an excimer laser.The method of claim 1, wherein the laser beam (46) is focused by a scanning module (48) onto the cavity surface (31).The method of claim 1, wherein the cavity is a crack (33) in the superalloy substrate (32).The method of claim 1, wherein the adherent metal oxide on the cavity surface (31) comprises at least one material selected from the group consisting of alumina, zirconia, nickel oxide, chromium oxide, molybdenum oxide, and calcium magnesium aluminum silicon oxide (CMAS).The method of claim 1, wherein the superalloy substrate (32) is a turbine engine component.The method of claim 1, wherein the cavity (33) is a crack, and wherein the method further includes the step of repairing the crack.A method of treating a superalloy substrate (64) containing at least one cavity (60) having an adherent metal oxide material on its surface (62), comprising the steps of contacting the cavity surface (62) with a forming gas and directing a high power, short pulses, and high repetition rate laser beam (46) onto a region on the substrate (64) having the cavity (60) under laser operating conditions capable of cutting into the superalloy material such that cutting into the superalloy substrate (64) cuts a boundary region (66) in the substrate (64) that encloses the cavity (60), wherein the laser beam (46) is characterized by a peak power density in the range of about 100 megawatts / cm 2 to about 10 gigawatts / cm 2 sufficient to effect laser ablation of the metal oxide material, wherein the cavity (60) has walls (67) defining its shape, and the boundary region (66) is relatively uniform and smooth compared to the walls (67) of the cavity (60).The method of claim 9, wherein at least one additional treatment step is performed on the substrate (64) prior to each cavity repair step, and the additional treatment step comprises a laser cleaning step.

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