Abstract:
A remote laser shock processing system for improving the properties of a solid workpiece by providing shock waves therein. The system includes a remote output end and a laser beam delivery arrangement for directing a beam of coherent energy to a specific location along a workpiece. In addition, a method of utilizing the remote laser shock processing system is included.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention. 
     The present invention relates to the use of coherent energy pulses to improve the properties of a solid material by providing shock waves therein and in particular methods and apparatus for remotely directing the pulse of coherent energy. 
     2. Description of the related art. 
     Laser shock processing involves directing a pulse of coherent radiation to a piece of solid material to produce shock waves therein. The produced shock wave causes compressive residual stresses to form within the solid material. These compressive residual stresses improve the fatigue properties of the solid material. 
     Currently, laser shock processing utilizes two overlays: a transparent overlay (usually water) and an opaque overlay (usually an oil based paint or black plastic tape). During processing, a laser beam is directed to pass through the water overlay and is absorbed by the black paint, causing a rapid vaporization of the paint surface (plasma creation) and a generation of a high-amplitude shock wave. The shock wave cold works the surface of the part and creates compressive residual stresses, which provide an increase in fatigue properties of the workpiece. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part. 
     Currently, laser shock processing apparatus are mounted in a fixed or stationary location. The laser and the laser beam pathway are held in a fixed position directed toward a workpiece located in an immobile or stationary processing station. In order to laser shock process a workpiece, the workpiece must be placed within the stationary processing station and aligned within the laser beam pathway. As a result of the workpiece having to be located within the processing station, the size or dimension of the workpiece to be processed is thereby limited; the workpiece must fit within the finite space of the stationary processing station. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for laser shock peening large or difficult to move workpieces, such as aircraft structures, stationary machines, building structures, and other large, similar workpieces. The apparatus includes a laser system and a laser beam delivery means for delivering a laser beam to a moveable, remote, and relocatable remote output end. The remote output end is first aligned to the workpiece to be laser peened. Then, a pulse of laser energy is transmitted from the laser system, through the laser beam delivery means, through the remote output end, and onto the large workpiece. With the present invention, the part-size constraint imposed by typical laser peening cells is eliminated. The laser beam is brought to the area of the workpiece, rather than manipulating the workpiece in the laser beam. 
     The invention in one form thereof, is an apparatus for improving the properties of a workpiece by providing shock waves by remote processing therein. The apparatus includes a remote output end and a laser operatively associated with the remote output end. There is means for aligning the remote output end. The laser beam delivery means delivers a beam of coherent energy from the laser to the remote output end. 
     In a further embodiment, the apparatus includes a power unit and the apparatus is mobile. In various alternate embodiments, the laser beam delivery means includes fiber optics, a light pipe, an articulated arm with pivotable mirrors, or a laser beam delivery service integrated within a building, factory, or structure. Alternate embodiments include a combination of the aforementioned laser delivery systems. The apparatus may also include an overlay system for applying an opaque and transparent overlay to the workpiece to be laser shock processed. 
     The invention in yet another form thereof is a method of improving the properties of a workpiece by providing shock waves therein. The method includes applying an overlay to the workpiece. A remote output end is aligned to the overlay portion of the workpiece. A pulse of coherent energy is directed to the overlay portion of the workpiece. In a further embodiment, the step of directing a pulse of coherent energy to the overlay portion of the workpiece occurs through various means such as a fiber optic bundle, a light pipe, an articulated arm comprising a plurality of pivotable mirrors, a laser beam delivery service integrated within a structure, and/or a combination thereof. 
     An advantage of the present invention is that large workpieces, which can not fit into a conventional laser shock processing station, can now be laser shock peened. For example, air craft structures, such as large wing sections and the fuselage, ships, and large earth moving equipment may now be laser shock processed. Moreover, these large structures may be laser shock processed in their assembled position or condition. The present invention&#39;s remote output end can be aligned to a desired location on the workpiece. In one embodiment of the invention, the laser system is mobile, allowing the laser system to be moved near the workpiece; thereby reducing the size and complexity of the laser beam delivery means. The workpiece no longer has to be moved to and manipulated within a stationary processing station. 
     A laser beam delivery means delivers a pulse of coherent energy from a laser to the remote output end which allows the remote output end to be mobile and relocatable along the workpiece. In other words, the workpiece stays stationary while the remote output end moves as required for laser shock processing. In addition, a laser beam delivery system may be incorporated within a building layout to deliver a pulse of coherent energy to the remote output end. As a result, processing of large material workpieces is achieved. 
     An additional advantage of the present invention is the ability to move the laser processing apparatus from place to place. One embodiment of the present invention is a self-contained or mobile laser shock processing apparatus. The entire apparatus may be moved from one location to another in order to process a workpiece, as opposed to the current practice of sending the material to be laser peened to an offsite processor. For example, the mobile laser peening apparatus may be transported to a manufacturer&#39;s assembly factory where the apparatus could be used to laser shock process a workpiece. Therefore, a workpiece can be assembled and processed within the manufacturer&#39;s own factory. As a result, there may be a decrease in manufacturing costs associated with the increase in convenience of being able to laser process a workpiece within a manufacturer&#39;s own factory. 
     Another advantage of the present invention is an alignment device, such as a bore-sighting device, with a cross hair or other guidelines for properly aiming the pulse of coherent energy to a specific location on the workpiece. In one embodiment, the remote output end contains a window or opening whereby an operator may view the location of where the beam of coherent energy will be placed. As a result, the operator is able to precisely direct the beam of coherent energy to a specific location along the workpiece. 
     In an additional embodiment, a camera is used to remotely align or guide the remote output end to a specific location along the workpiece. An operator may remotely control the remote output end by viewing the processing location on the workpiece via the camera. If the remote output end is located on the end of a robot arm, an operator will be able to process very large or cumbersome workpieces, for example, an airplane fuselage or large wing section, with ease. Consequently, once unprocessable workpieces may be simply and easily laser shock processed. 
     Another advantage of the present invention is that the remote output end may include a light safe seal or shield. This seal or shield may be adaptable to fit the varying geometries of the peened surface. As a result, the seal will prevent potentially dangerous pulses of coherent energy from being emitted from the remote output end into the ambient environment. Therefore, the seal helps protect both workers and equipment within the vicinity of the laser shock processing site from being injured from the laser shock processing laser pulse. 
    
    
     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 cross sectional view of a remote output end for laser processing around a rivet hole; 
     FIG. 2 is a cross sectional view of a remote output end of the present invention including overlay system; 
     FIG. 3 is a top view of a laser beam delivery service integrated within a building; 
     FIG. 4 is a cross sectional view of an articulated arm; 
     FIG. 5 is a diagrammatic view of a mobile laser shock processing unit; 
     FIG. 6 is a flow chart depicting one method of the present invention; and 
     FIG. 7 is a prospective view of an aircraft workpiece to be laser shock processed. 
    
    
     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 
     The improvements in fatigue life produced by laser shock processing are the result of residual compressive stresses developed in an irradiated surface retarding fatigue crack initiation and/or slowing the crack propagation rate. A crack front is the leading edge of a crack as it propagates through a solid material. Changes in the shape of a crack front and slowing of the crack growth rate when the crack front encounters the laser shock zone in a laser shock processing condition have been shown. Laser shock processing is an effective method of increasing fatigue life in metals by treating fatigue critical regions. The effect of tensile residual stresses surrounding the laser shocked region would have on crack initiation has previously been described in “Shock Waves in High Strained Rate Phenomenon in Metals” by A. H. Clauer, J. H. Holbrook, and B. P. Fairand, ed. by M. S. Meyers and L. E. Murr, Plenum Press, New York (1981), pp. 675-702. 
     Overlays are applied to the surface of the target workpiece being laser shock processed. These overlay materials may be of two types, one transparent to the laser radiation, and the other opaque to the laser radiation. They may be used either alone or in combination with each other, but it is preferable that they be used in combination with the opaque overlay adjacent the workpiece and the outer transparent layer being adjacent the opaque layer. 
     Referring to FIG. 1, there is shown remote output end  10 . Remote output end  10  consists of shield or skirt  12 , handle  14  with trigger  16 . Window  18  provides visual access to the workpiece being processed  20 . 
     Remote output end  10  is aligned or positioned to workpiece  20  at a desired location. Window  18  allows an operator to accurately aim and align the remote output end  10 . Window  18  may include a bore-sight, cross hairs or other means for accurately locating processing area  30 , the area to be laser shock processed. Other means for accurately locating processing area  30  include the use of a light beam (alignment beam) shined on a workpiece, depicting where a pulse of coherent energy will be sent. 
     During the operation of the present invention, a means such as shutter or cover  22  may slide over the opening of window  18 . Shutter  22  prevents scattered energy from the beam or pulse from being emitted through window  18 . 
     In addition to window  18 , remote output end  10  may include a camera (not shown). The camera allows an operator to direct the positioning of remote output end  10  along workpiece  20  by viewing the workpiece on a monitor (not shown) rather than directly through window  18 . Alignment or positioning could be done manually or remotely such as by the aid of a robot arm (not shown). Lighting for the camera may be required. If remote output end  10  is attached to the end of a robot arm (not shown), an operator may direct remote output end  10  to workpiece  20  by viewing the specific area to be laser shock processed on a monitor screen in communication with the camera. 
     Skirt  12  is composed of an opaque, flexible, or compressible material which conforms to the varying contours of workpiece  20 . Skirt  12  may be composed of durable foam or rubber. When an operator presses remote output end  10  to workpiece  20 , skirt  12  makes a seal with workpiece  20 . During the operation of the laser, skirt  12  prevents scattered emission of the laser beam and contains any material or debris within remote output end  10  which may have been dislodged from workpiece  20  during the laser&#39;s operation. 
     Overlay system  24  is applied over the area on workpiece  20  to be laser shock processed. Overlay system  24  comprises two layers, an opaque layer  26  and a transparent layer  28 . Opaque layer  26  absorbs the pulse of coherent energy which is delivered to its surface. Transparent layer  28  does not absorb the energy from the laser and allows the pulse of coherent energy to pass through transparent layer  28 . When overlay system  24  is subject to a pulse of coherent energy from a laser, the beam passes through transparent layer  28  and is absorbed by opaque layer  26 . As a result of opaque layer  26  absorbing the beam of coherent energy, a portion of opaque layer  26  is quickly vaporized and turned into plasma at the location where it was struck by the beam of coherent energy. Transparent layer  28  traps the rapidly expanding vaporized opaque layer  26  and directs the corresponding energy into workpiece  20 . Consequently, a shock wave is transmitted into workpiece  20 . 
     The pulse of coherent energy is sent from the laser and delivered to remote output end  10  by laser beam delivery means  34 . The embodiment in FIG. 1 depicts laser delivery means  34  as a fiber optic bundle  36  connected to a laser  37 . Fiber optic bundle  36  comprises a bundle of fibers. The individual fibers are grouped together to form a fiber optic bundle  36 . Other laser delivery means may also be utilized in transferring a pulse of coherent energy from laser  37  to remote output end  10  which include a light pipe, and an articulated arm with pivotable mirrors (described below and depicted in FIG.  4 ). 
     Trigger  16 , when depressed, activates laser  37  to send a beam of coherent energy through laser beam delivery means  34  for processing workpiece  20 . During triggering, a beam of coherent energy proceeds in the direction of the arrow  38  through fiber optic bundle  36 . The beam of coherent energy traveling through fiber optic bundle  36  is transferred to location  30  by lens  40 . Lens  40  transfers the pulse of coherent energy to create the desired spot size at the processing area  30 . 
     As depicted in FIG. 1, a compressive residual stress may be introduced at location  30  on workpiece  20  around rivet  32 . Fatigue or corrosion zones tend to form around rivet holes. Therefore, it is advantageous to introduce compressive residual stresses around a rivet hole to reduce fatigue and increase corrosion resistance. 
     Referring now to FIG. 2, there is shown remote output end  10  with overlay system  42  to supply the opaque and transparent layers  50 , 52  for laser shock processing. Opaque overlay  50  is composed of energy absorbing paint. Transparent overlay  52  is composed of water. 
     Overlay system  42  includes opaque layer applicator  44  with paint spray nozzle  46 . Opaque layer  50  is composed of paint which is applied through opaque layer applicator  44  to the area to be processed  30  on workpiece  20 . 
     Transparent layer applicator  53  contains water nozzle  54  and applies transparent overlay  52  to the area to be processed  30 . Water is removed from remote output end  10  through outlet  55 . 
     During the operation of the present invention, an operator depresses trigger  16  which sends a pulse of coherent energy through laser beam delivery means  34 . The pulse of coherent energy is transferred to the area to be processed  30  by lens  40 . Transferred beam  41  is directed at opaque layer  50  which has been applied to workpiece  20 . Water is applied to the area to be processed  30  by transparent layer applicator  53 . Opaque layer  50  absorbs the energy from the pulse whereby vaporizing the area of opaque layer subject to the pulse of coherent energy. Transparent layer  52  contains the expanding plasma resulting from the vaporization of opaque layer  50  and directs the energy to processing spot  30  whereby creating a shock wave within workpiece  20 . Opaque layer applicator  44  may be used to apply fresh paint to the area to be processed  30  in subsequent laser shock processing cycles. 
     FIG. 3 depicts a laser beam delivery service  56  of the present invention. Laser beam delivery service  56  may be retrofitted into an existing building layout or may be incorporated into a new building design. In addition, laser delivery service  56  may be incorporated into other structures such as overhead scaffolding structure or other structures capable of containing laser delivery service  56 . Laser  58 , which may be located on the floor of the building generates the pulse of coherent energy. The beam of coherent energy is directed from laser  58  up to the ceiling of the room incorporating laser beam delivery service  56 . The building ceiling contains a plurality of mirrors  60 ,  64 ,  68  to direct a beam of coherent energy anywhere throughout a building layout (as depicted in FIG.  3 ). Mirror  60  directs the beam toward the direction of arrow  62 . Mirror  64  redirects the beam in direction  66 . The beam travels along until it reaches mirror  68 . Mirror  68  directs the beam of coherent energy down from the ceiling and into fiber optic bundle  36 . The pulse of coherent energy travels through fiber optic bundle  36  and is directed through remote output end  10  to workpiece  20  for generating a shock wave therein. Trigger  16  of remote output end  10  is in communication with laser  58  for initiating a pulse of coherent energy through laser beam delivery service  56  for processing workpiece  20 . 
     The beam pathway through laser beam delivery service  56  may be modified. Laser beam delivery service  56  contains a plurality of moveable mirrors, for example  60 ,  64 ,  66 ,  70 . Mirrors  60 ,  64 ,  68 , and  70  are maneuverable such that they may be used to direct or redirect the beam of coherent energy to any location within the grid work of laser beam delivery service  56 . For instance, pivoting or relocating mirror  60  permits the beam of coherent energy to travel straight past mirror  60  to a mirror  70 . Alternatively, mirrors  60 ,  54 ,  66 ,  70  may be removed completely from the laser beam pathway. 
     While FIG.  3 . depicts the layout of laser beam delivery service  56  as being a Cartesian coordinate grid network, the present invention is not limited to such a grid. Moreover, it is preferable to limit the number of mirrors to decrease laser system losses. 
     Once a beam of coherent energy is directed down to the floor of the layout, fiber optic bundle  36 , in conjunction with remote output end  10 , permit unlimited directioning of the pulse of coherent energy to any area of the building. 
     Referring now to FIG. 4, laser beam delivery means  34  may include articulated arm  72 . Articulated arm  72  consists of swivels or gimbals  74  which allow articulated arm  72  to move about in three dimensions. Contained within articulated arm  72  are a plurality of mirrors  76 , which are pivotably mounted and direct a beam of coherent energy properly through articulated arm  72  in direction  78 . A beam of coherent energy is directed through articulated arm  72  and passes through light pipe  80  before being focused by lens  40 . Light pipe  80  may be composed of solid glass, plastic, or other materials suitable for carrying a pulse of coherent energy. 
     Referring now to FIG. 5, the present invention, in one embodiment, is a completely mobile unit  82 . Mobile unit contains remote output end  10 , laser beam delivery means  34 , a laser  37 , and power unit  84  for providing power to the laser. Mobile unit  82  is compact enough to be easily transported around a manufacturer&#39;s factory, as well as from building to building. A high repetition rate laser is not required for operation of the present invention. Since remote output end  10  needs to be relocated between each laser peening cycle, the repetition rate of the laser does not need to exceed the time it takes for relocation. 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. 
     Referring to FIG. 7, one area where remote output end  10  of the present invention may be utilized is in laser shock processing of large parts or workpieces which would not fit into a customary stationary processing station. The present invention, via remote output end  10 , allows these fatigue or corrosion sensitive zones to be processed without having to dissemble the workpiece, such as components of an airplane  86  such as wing  88  or fuselage  90 , in order to fit the workpiece within a stationary laser shock processing station of finite size and space. Remote output end  10  may be aligned anywhere to a workpiece, e.g., rivit holes or weld, to be processed. Remote output end  10  may be used to introduced compressive residual stresses within a fully assembled large wing section  88  or fuselage  90 . Mobile Unit  82  can be located in proximity of airplane  86 . A beam of coherent energy may be directed through laser beam delivery means  34  to remote output end  10 . 
     Referring to FIG. 6, during the operation of the invention, an overlay consisting of an opaque and a transparent layer may be applied to a workpiece to be laser shock processed ( 92 ) and a remote output end is located on a workpiece ( 94 ). Either step, applying an overlay ( 92 ) or locating a remote output end on a workpiece ( 94 ), may be done first. 
     The step of applying an overlay ( 92 ) may consist of applying a plastic tape or film containing the opaque and transparent overlay to the workpiece at the area to be laser shock processed. Alternatively, the overlay system could include applying an energy absorbing paint overlay and an energy transparent water overlay. 
     The remote output end may be aligned to the workpiece by visually aligning the remote output end to the workpiece at the area to be laser shock processed ( 94 ). The remote output end contains either a window or a camera for an operator to gain visual access to the processing area. With the aid of cross-hairs, a shined light beam, or other means, the operator is able to accurately align the remote output end at a specific or desired area to be processed ( 94 ). 
     After the overlay has been applied ( 92 ) and the remote output end has been aligned to the workpiece ( 94 ), a pulse of coherent energy is directed to the workpiece at the processing area ( 96 ). The pulse of coherent energy is sent from a laser, through a laser beam delivery means to the remote output end ( 96 ). In the particular embodiment where the laser beam delivery means contains a laser beam delivery system, the pulse of coherent energy is sent from the laser, up to the ceiling, across the ceiling, back down, again, and to the remote output end ( 96 ). The energy from the pulse of coherent energy, then passes through the transparent overlay layer and is absorbed by the opaque overlay layer.