Abstract:
The invention, in another form thereof, comprises a method of laser shock peening the surface of a solid material with or without the use of a transparent overlay material. An energy absorbing coating is applied to a portion of the surface of a solid material. An ultraviolet-curable resin coating is applied to the energy absorbing coating and the curable resin is exposed to an ultraviolet light and forms a pellicle over the energy absorbing coating. A pulse of coherent energy is directed to the energy absorbing coating of the solid material to create a shock wave.

Description:
This invention was made with United States Government support under Contract No. F33615-00-C-5304 awarded by the Department of the Air Force. The United States Government has certain rights to this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the use of coherent energy pulses, as from high powered pulsed lasers, as well as an ultraviolet curable resin and an ultraviolet light in the shock processing of materials, and, more particularly, to methods and apparatus for improving properties of solid materials by providing shock waves therein. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength. 
     2. Description of the Related Art 
     Laser peening (hereinafter referred to as laser shock processing) utilizes two overlays, a transparent overlay (usually water), and an opaque overlay (previously a black paint). During processing, a laser is directed to pass through the water overlay and is absorbed by the black paint, causing a rapid vaporization of the paint surface and the generation of a high-amplitude shock wave. The shock wave cold works the surface of the part creating compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover fatigue critical zones of the part. 
     The current art utilizing laser shock processing has a problem with the amount of time necessary to dry the black paint. The problem with drying the black paint usually occurs with solvent-based paint but other types of paint may have this problem. Another problem is that the black paint begins eroding as soon as the water layer is applied to the paint. This problem with erosion usually occurs with water-based paint but other types of paint may have this problem. During the laser processing of a workpiece, the black paint must be applied to the workpiece multiple times. Each coat of paint takes approximately ten minutes to dry, which makes the processing time lengthy. Also, once the transparent overlay is applied to the paint, the paint may start to erode immediately. This opaque layer erosion causes a turbulence in the surface of the workpiece and reduces the shock pressure during the laser shock processing cycle. Therefore, the paint must be applied again to the workpiece further increasing the processing time. This problem with erosion of the paint decreases the efficiency and effectiveness during the laser shock processing of the workpiece. The reasons for the efficiency and effectiveness being decreased is because of the amount of time the paint takes to dry and because the paint begins eroding immediately once the transparent overlay is applied which causes the shock pressure to the workpiece to be reduced once the laser is applied to the eroding paint. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of laser shock processing that can be used in a production environment that significantly reduces laser shock processing time and increases the effectiveness of the laser shock processing of the workpiece. The method begins with a step of applying an energy absorbing coating to the surface of the workpiece that is to be laser shock processed. In one form of the invention, an ultraviolet-curable resin, such as an ultraviolet-curing acrylic or urethane resin, is applied to the energy absorbing coating of the workpiece. The present invention is not necessarily limited to ultraviolet-curable resins, any type of curable resin can be used. A resin is cured when the resin becomes chemically inert or upon the polymerization of the resin. In one form, the ultraviolet-curable resin is exposed to an ultraviolet light which causes the curable resin to form a pellicle over the energy absorbing coating of the workpiece. The present invention is not limited to an ultraviolet light to form a pellicle or skin over the resin. The curing effect is determined by the type of resin used and the means of curing that resin. A transparent overlay, such as water, is applied in a thin flowing layer over the pellicular energy absorbing layer. When the transparent overlay has covered the energy absorbing layer of the workpiece, the laser is fired through the transparent overlay and onto the energy absorbing coating. After the laser has been fired, the remaining portion of the coating can be washed off of the surface of the workpiece using a high-speed jet of fluid. The entire sequence and event timing is controlled by an electronic processor. The sequence is repeated for each spot to be processed along the workpiece surface. 
     The invention, in one form thereof, comprises a method of laser shock processing the surface of a solid material. An energy absorbing coating is applied to a portion of the surface of the solid material. An ultraviolet-curable resin is applied to the coated portion of the solid material and the curable resin is exposed to an ultraviolet light and forms a pellicle over the energy absorbing coating. A transparent overlay material is applied to the pellicular portion of the energy absorbing coating. A pulse of coherent energy is directed to the energy absorbing coating of the solid material to create a shockwave. 
     The invention, in another form thereof, comprises a method of laser shock processing the surface of a solid material with or without the use of a transparent overlay material. An energy absorbing coating is applied to a portion of the surface of a solid material. An ultraviolet-curable resin coating is applied to the energy absorbing coating and the curable resin is exposed to an ultraviolet light and forms a pellicle over the energy absorbing coating. A pulse of coherent energy is directed to the energy absorbing coating of the solid material to create a shockwave. 
     The invention, in yet another form thereof, comprises a method of laser shock processing the surface of a solid material. An energy absorbing material is mixed with an ultraviolet-curable resin to form an energy absorbing mixture coating that is applied to the surface of the solid material. The mixture coating is exposed to an ultraviolet light and forms a pellicle over the mixture coating. A transparent overlay material is applied to the pellicular portion of the energy absorbing coating and a pulse of coherent energy is directed to the energy absorbing coated portion of the solid material to create a shockwave. 
     The invention, in still another form thereof, comprises a method of laser shock processing the surface of a solid material without the use of a transparent overlay material and using a mixture of the resin and the energy absorbing material. An energy absorbing material is mixed with an ultraviolet-curable resin to form an energy absorbing mixture coating that is applied to the surface of the solid material. The mixture coating is exposed to an ultraviolet light and forms a pellicle over the mixture coating and a pulse of coherent energy is directed to the energy absorbing coated portion of the solid material to create a shock wave. 
     The invention, in yet another form thereof, comprises a method of laser shock processing the surface of a solid material using a mixture consisting of a wet energy absorbing material and a resin. A wet energy absorbing material is mixed with an ultraviolet-curable resin to form an energy absorbing coating that is applied to the surface of the workpiece. The curable resin causes the wet energy absorbing coating to form a pellicle over once exposed to an ultraviolet light. A pulse of coherent energy is directed to the energy absorbing coated portion of the solid material to create a shockwave. 
     The invention, in still another form thereof, comprises an apparatus for laser shock processing the surface of a solid material. The apparatus includes a material applicator for applying an energy absorbing material onto the workpiece to create a coating on the workpiece. A curable resin applicator applies an ultraviolet curable resin onto the energy absorbing coating of the workpiece and a light applicator applies an ultraviolet light, to the curable resin. A transparent overlay applicator applies a transparent overlay to the energy absorbing coating of the workpiece. A laser provides a laser beam through the transparent overlay and to the energy absorbing coating to create a shock wave on the workpiece. A positioning mechanism is included to selectively position the workpiece relative to the material applicator, the curable resin applicator, the light applicator, the transparent overlay applicator, and the laser. Conversely, a positioning mechanism may position the material applicator, the curable resin applicator, the light applicator, and the transparent overlay applicator correctly over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism to control the operation and the timing of each of the applicators, laser, and the selective operation of the positioning mechanism. 
     The invention, in yet another form thereof, comprises an apparatus for laser shock processing the surface of a solid material. The apparatus includes a material applicator for applying an energy absorbing material onto the workpiece, to create a coating on the workpiece. An ultraviolet curable resin applicator applies a curable resin to the energy absorbing coated portion of the workpiece end an ultraviolet light applicator applies an ultraviolet light to the curable resin. A laser provides a laser beam to the energy absorbing layer to create a shock wave on the workpiece. A positioning mechanism is included to selectively position the workpiece relative to the material applicator, curable resin applicator, light applicator, and the laser. Conversely, the positioning mechanism may position the material applicator, light applicator, and curable resin applicator correctly over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism, to control the operation and the timing of each of the applicators, laser, and the selective operation of the positioning mechanism. 
     The invention, in still another form thereof, comprises an apparatus for laser shock processing the surface of a solid material. The apparatus includes a material applicator for applying a mixture of an energy absorbing material and an ultraviolet curable resin onto the workpiece to create a coated portion and a light applicator applies an ultraviolet light to the mixture. A transparent overlay applicator applies a transparent overlay to the energy absorbing mixture coating of the workpiece. A laser provides a laser beam through the transparent overlay and to the energy absorbing coating to create a shockwave on the workpiece. A positioning mechanism is included to selectively position the workpiece relative to the material applicator, light applicator, the transparent overlay applicator, and the laser. Conversely, a positioning mechanism may position the material applicator, the light applicator, and the transparent overlay applicator directly over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism to control the operation and the timing of each of the applicators, laser, and the selective operation of the positioning mechanism. 
     The invention, in yet another form thereof, comprises an apparatus for laser shock processing the surface of a solid material. The apparatus includes a material applicator for applying a mixture of an energy absorbing material and an ultraviolet curable resin onto the workpiece to create a coated portion and a light applicator applies an ultraviolet light to the mixture. A laser provides a laser beam through the energy absorbing coating to create a shock wave on the workpiece. A positioning mechanism is included to position the workpiece relative to the material applicator, light applicator, and the laser. Conversely, the positioning mechanism may position the material applicator and light applicator over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism to control the operation and the tinting of the applicators, laser, and the selective operation of the positioning mechanism. 
     An advantage of the present invention is that the method assists in erosion resistance of the energy absorbing coating once the transparent overlay is applied because the curable resin forms a pellicle over the energy absorbing coating and therefore delaying the erosion of the coating from the transparent overlay material. Prior laser shock processing methods have a problem in which, once the transparent overlay is applied to the energy absorbing coating, erosion of the energy absorbing coating takes place immediately and the coating has to be applied again. Having to re-apply the coating significantly increases the processing time. 
     Another advantage of the present invention is that the curable resin will form a pellicle over the energy absorbing coating in less than one second. Normally, it takes approximately ten minutes tar the energy absorbing coating to dry so that laser shock processing of the workpiece can occur. Using the fast curing resin, the wet paint is protected in less than one second, which increases the processing time significantly. 
     A further advantage of the present invention is that higher peak pressures of the shock wave to the workpiece can be achieved because the curable resin causes more of its energy absorbing coating to be left intact when the laser beam impacts the energy absorbing layer. Eliminating premature mixing of the energy absorbing coating with the transparent overlay allows for higher peak pressures of shock waves to the workpiece. 
     Yet another advantage of the present invention is that reduced power can now be used to achieve the same residual stress levels because previously, higher power was used to compensate for the erosion of the energy absorbing layer. Due to the fact that the curable resin assists in the resistance of the erosion to the energy absorbing layer, less power can be used to achieve the same results which will increase the life of the laser equipment. 
     Another advantage of the present invention is that by the curable resin forming a pellicle rather than becoming fully cured, the energy absorbing coating is more easily cleaned off the workpiece once the laser beam has been applied. 
     An advantage of the present invention is that the method assists in erosion resistance of the energy absorbing coating once the transparent overlay is applied because the curable resin forms a pellicle over the energy absorbing coating and therefore delaying the erosion of the coating from the transparent overlay material. Prior laser shock processing methods have a problem in which, once the transparent overlay is applied to the energy absorbing coating, erosion of the energy absorbing coating takes place immediately and the coating has to be applied again. Having to re-apply the coating significantly increases the processing time. 
     Another advantage of the present invention is that the curable resin will form a pellicle over the energy absorbing coating in less than one second. Normally, it takes approximately ten minutes for the energy absorbing coating to dry so that laser shock processing of the workpiece can occur. Using the fast curing resin, the wet paint is protected in less than one second, which increases the processing time significantly. 
     A further advantage of the present invention is that higher peak pressures of the shock wave to the workpiece can be achieved because the curable resin causes more of its energy absorbing coating to be left intact when the laser beam impacts the energy absorbing layer. Eliminating premature mixing of the energy absorbing coating with the transparent overlay allows for higher peak pressures of shock waves to the workpiece. 
     Yet another advantage of the present invention is that reduced power can now be used to achieve the same residual stress levels because previously, higher power was used to compensate for the erosion of the energy absorbing layer. Due to the fact that the curable resin assists in the resistance of the erosion to the energy absorbing layer, less power can be used to achieve the same results which will increase the life of the laser equipment. 
     Another advantage of the present invention is that by the curable resin forming a pellicle rather than becoming fully cured, the energy absorbing coating is more easily cleaned off the workpiece once the laser beam has been applied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a flow chart of the method of one form of the present invention; and 
     FIG. 2 is a diagrammatic view of another form of the present invention. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     To laser shock process a material, an energy absorbing coating is applied to the surface of the material. An overlay layer is then applied to the coated surface of the material. As the overlay layer is applied to the energy absorbing coating, the energy absorbing coating starts immediately eroding. 
     This is the problem the present invention solves by applying a curable resin to the energy absorbing coating. There are different ways of curing resins. Light can be used to cure resins. In particular, an ultraviolet light can be used to cure ultraviolet curable resins. Examples of ultraviolet curable resins include LIGHT WELL, MULTI-CURE, SURE CURE, and DARK CURE available from Dymax, Inc. of Torrington, Conn. Other examples of ultraviolet curable resins include a clear ultraviolet coating (product no. 62875) and a black ultraviolet coating (product no. 72256) available from UV Coatings Limited, 140 Sheldon Road, Berea, Ohio 44017. Flash heated (thermally-cured) resin coatings or coatings cured by chemical catalysis (e.g., amine gas-curing, aerobic or anaerobic curing resins) are other ways of curing resins. 
     The curable resin can be mixed with graphite or black iron oxide to create an energy absorbing paint. The curable resin can be a solid polymer as well. Also, the resin can have different viscosities. 
     In the preferred embodiment of the present invention, an ultraviolet-curable resin is applied to the energy absorbing coating and the resin is less than halfway cured to form a pellicle over the energy absorbing coating by exposing the resin to an ultraviolet light source to increase the resistance to erosion of the energy absorbing coating once the transparent overlay material is applied. The resin can be less than halfway cured, halfway cured or more than halfway cured to form a pellicle over the energy absorbing coating. 
     The curable resin can be mixed with graphite or black iron oxide to create an energy absorbing paint. The curable resin can be a solid polymer as well. Also, the resin can have different viscosities. 
     In the preferred embodiment of the present invention, an ultraviolet-curable resin is applied to the energy absorbing coating and the resin is less than halfway cured to form a pellicle over the energy absorbing coating by exposing the resin to an ultraviolet light source to increase the resistance to erosion of the energy absorbing coating once the transparent overlay material is applied. The resin can be less than halfway cured, halfway cured or more than halfway cured to form a pellicle over the energy absorbing coating. 
     Referring now to the drawings, and particularly to FIG. 2, there is shown a preferred embodiment  10  of the present invention including a laser shock processing chamber  12  in which the laser shock process takes place. The laser shock processing chamber  12  includes an opening  14  for a laser beam  16  created by laser  18 , a source of coherent energy. The laser pulse length and focus of the laser beam  16  may be adjusted as known in the art. Shown in FIG. 2, a workpiece  20  is held in position within laser shock processing chamber  12  by means of a positioning mechanism  21 . Positioning mechanism  21  may be of the type of a robotically controlled arm or other apparatus to precisely position workpiece  20  relative to the operational elements of laser shock system  10 . 
     System  10  includes a material applicator  24  for applying an energy absorbing material onto workpiece  20  to create a coated portion. The material utilized by material applicator  24  is an energy absorbing material, preferably that of a black paint such as those sold under the trade names of ZYNOLYTE BAR-B-Q or ZYNOLYTE STOVE COAT BLACK, available from Saria International, Inc. of San Carlos, Calif., but other energy absorbing materials may be used. 
     Another component of system  10  is a curable resin applicator  22  for applying an ultraviolet-curable resin onto the energy absorbing coating applied to workpiece  20 . The curable resin can be applied when the energy absorbing coating is wet or dry. 
     System  10  also includes a light applicator  25  for applying light energy, for example an ultraviolet light to the ultraviolet curable resin. The light applicator may be manufactured using fiberoptic cable. The light emitted from light applicator  25  in one form is an ultraviolet light. The light emitted from light applicator  25  can have a wavelength in the range of 100 nm to 450 nm and an amplitude in the range of 1 W/cm 2  to 60 W/cm 2  if the curable resin can be cured using a light having a wavelength and amplitude in those ranges. These ranges of light parameters are examples of light parameters that can be used to cause the curable resin to become cured although others may be utilized depending on the equipment and the resin. 
     System  10  further includes a transparent overlay applicator  26  that applies a transparent overlay onto the portion of workpiece  20  coated by material applicator  24  and curable resin applicator  22 . The transparent overlay material should be substantially transparent to the radiation from laser beam  16 . 
     As shown in FIG. 2, the material applicator  24 , the curable resin applicator  22 , the light applicator  25 , and the transparent overlay applicator  26  are shown directly located within laser shock processing chamber  12 . In a production operation environment, only the necessary operative portions need to be located through and within laser shock processing chamber  12 , such as the portion through which the materials actually flow through a flow head. The supply tanks for the applicators may be located outside of laser shock processing chamber  12 . 
     A control unit, such as controller  28  is operatively associated with each of the material applicator  24 , curable resin applicator  22 , light applicator  25 , transparent overlay applicator  26 , laser  18 , and positioning mechanism  21 . Controller  28  controls the operation in timing of each of the applicators  24 ,  22 ,  25 ,  26 , laser  18 , and selective operation of positioning mechanism  21  to ensure proper sequence and timing of system  10 . As shown in FIG. 2, controller  28  is connected to laser  18 , positioning mechanism  21 , light applicator  25 , curable resin applicator  22 , material applicator  24 , and transparent overlay applicator  26  via control lines  30 ,  31 ,  32 ,  33 ,  34 , and  36  respectively. Controller  28  may be a programmable personal computer. 
     In operation, controller  28  controls operation of system  10  once initiated. As shown in FIG. 1, the method of the invention is that first, workpiece  20  is located ( 38 ) particularly within targeting chamber  12  by positioning mechanism  21 . Controller  28  activates material applicator  24  to apply an energy absorbing coating ( 40 ) onto a particular location of workpiece  20  to be laser shock processed. The next step of the process is that controller  28  causes the curable resin applicator  22  to apply a curable resin ( 41 ) to the coated portion of the workpiece  20 . A light is applied ( 42 ) to the curable resin to cause the curable resin to form a pellicle over the energy absorbing coated portion of workpiece  20 . The next step of the process is that controller  28  causes the transparent overlay applicator  26  to apply a transparent overlay ( 43 ) to the pellicular energy absorbing coated portion of workpiece  20 . At this point, laser  18  is immediately fired ( 44 ) by controller  28  to initiate a laser beam  16  to impact workpiece  20  through the transparent overlay and pellicular portion of the energy absorbing coating on workpiece  20 . By directing this pulse of coherent energy to the energy absorbing coated portion, a shockwave is created. As the plasma expands from the impact area, it creates a compressional shockwave passing through and against workpiece  20 . 
     The above-described process or portions of the process are repeated to shock process the desired surface area of workpiece  20 . Depending upon the energy levels and the amount of laser shocking desired on workpiece  20 , controller  28  may position or re-index workpiece  20  into another position using positioning mechanism  21  so that system  10  may apply coatings to and laser beam  16  may impact a different portion which can overlap the previous impact area. After laser beam  16  impacts the energy absorbing coated portion ( 44 ) of workpiece  20 , it may be necessary to remove the remaining energy absorbing coating from workpiece  20 . This additional step is referred to as ( 46 ) of FIG.  1 . This step may be accomplished by a high speed jet of fluid to forcibly remove the remaining coating from workpiece  20 . 
     The present invention has three other embodiments. The first embodiment eliminates the need for the traditional flowing transparent overlay. The second embodiment combines the ultraviolet curable resin and the energy absorbing material so that only one applicator is needed to apply the coating to the workpiece. The third embodiment not only eliminates the need for the traditional flowing transparent overlay, but also combines the ultraviolet curable resin and the energy absorbing material so only one applicator is needed to apply the coating to the workpiece. 
     In another form of the present invention, workpiece  20  is located ( 38 ) particularly within targeting chamber  12  by positioning mechanism  21 . Controller  28  activates material applicator  24  to apply an energy absorbing coating ( 40 ) onto a particular location of workpiece  20  to be laser shock processed. The next step of the process is that controller  28  causes the curable resin applicator  22  to apply for example an ultraviolet-curable resin ( 41 ) to the energy absorbing coating applied to workpiece  20 . The light applicator  25  applies an ultraviolet light ( 42 ) to the curable resin in order to form a pellicle over the energy absorbing coating. Laser  18  is immediately fired ( 44 ) by controller  28  to initiate a laser beam  16  to impact workpiece  20  through the pellicular portion of the energy absorbing coating on workpiece  20 . By directing this pulse of coherent energy to the energy absorbing coated portion, a shockwave is created. As the plasma expands from the impact area, it creates a compressionable shockwave passing through and against workpiece  20 . The workpiece can then be cleaned by having a high speed jet of fluid applied to its surface to remove the remaining energy absorbing coating ( 46 ). 
     In yet another form of the present invention, the workpiece  20  is located ( 38 ) particular within targeting chamber  12  by positioning mechanism  21 . The energy absorbing material is mixed with an ultraviolet-curable resin and material applicator  24  applies that mixture ( 40 ) to coat workpiece  20 . The light applicator  25  applies an ultraviolet light ( 42 ) to the coating in order to form a pellicle over the mixture. Controller  28  causes the transparent overlay applicator  26  to apply a transparent overlay ( 43 ) to the pellicular energy absorbing coated portion of the workpiece  20 . Laser  18  is immediately fired ( 44 ) by controller  28  to initiate a laser beam  16  to impact workpiece  20  through the transparent overlay and pellicular portion of the energy absorbing coating on workpiece  20 . By directing this pulse of coherent energy to the energy coated portion, a shockwave is created. As the plasma expands from the impact area, it creates a compressional shockwave passing through and against workpiece  20 . 
     In another form of the present invention, the workpiece  20  is located ( 38 ) particularly within targeting chamber  12  by positioning mechanism  21 . The energy absorbing material is mixed with an ultraviolet-curable resin and material applicator  24  applies that mixture ( 40 ) to coat workpiece  20 . The light applicator  25  applies an ultraviolet light ( 42 ) to the coating in order to form a pellicle over the mixture. Laser  18  is immediately fired ( 44 ) by controller  28  to initiate a laser beam  16  to impact workpiece  20  through the pellicular mixture coating on workpiece  20 . By directing this pulse of coherent energy to the mixture coated portion, a shockwave is created. As the plasma expands from the impact area, it creates a compressionable shockwave passing through and against workpiece  20 . After the pulse of coherent energy, a high speed jet of fluid can be used to clean workpiece  20  by removing the remaining mixture coating ( 46 ). 
     Depending upon the workpiece material, many parameters of the present invention may be selected to control the shock process. For example, the operator controller may select a particular laser, a particular laser pulse energy, laser pulse time, number of laser pulses, focal lens, working distance, thickness of the energy absorbing coating, curable resin, and transparent overlay to control the laser shock process. More particularly, laser pulse energy and laser pulse width directly effect the cycle. The amount of energy placed on the surface of the workpiece and number of laser pulses effects the depth of each shock and the speed of the shocking process. It has been found that the energy of the laser pulse as well as other parameters should be controlled in order to prevent surface irregularities of the workpiece. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.