WO2021260362A1

WO2021260362A1 - Improvements in or relating to laser peening

Info

Publication number: WO2021260362A1
Application number: PCT/GB2021/051577
Authority: WO
Inventors: Michael Fitzpatrick; Niall Smyth; Demosthenes KOUTSOGEORGIS; Nikolaos KALFAGIANNIS; Mitchell LEERING
Original assignee: Coventry University; Nottingham Trent University
Priority date: 2020-06-24
Filing date: 2021-06-22
Publication date: 2021-12-30
Also published as: GB202009657D0

Abstract

A workpiece (1) is subjected to laser shock peening by the application of a series of high power pulses of a laser beam (2). When incident upon a target area (3) of the surface of the workpiece (1), a plasma (4) is generated. The rapid expansion of the plasma (4) applies a pressure (5) to the workpiece (1). According to the present invention, gas is applied to the target area in order to produce a region of gas (7) having a higher pressure than the ambient environment, thereby confining the plasma (4). Advantageously, this may remove the need for a liquid confinement medium (6). In this embodiment, there is an optional ablative layer (15) disposed on the surface of the workpiece (1).

Description

IMPROVEMENTS IN OR RELATING TO LASER PEENING

Technical Field of the Invention

The present invention relates to laser shock peening, in particular to an improved apparatus and method for laser shock peening.

Background to the Invention

Laser shock peening is a known method of surface treatment of a material. Laser shock peening involves applying high power laser pulses to a workpiece. The high power pulses generate a plasma where they strike the workpiece surface. The plasma generated for laser shock peening purposes is intermittent. This prevents the material from being melted, cut or parted during the peening process.

The rapid expansion of the plasma is usually confined by a liquid or solid overlay material, to direct a pressure pulse into the workpiece. The subsequent deformation generates a compressive residual stress in the workpiece surface. This induced stress beneficially increases the resistance of the workpiece to surface-related failures, such as fatigue, fretting fatigue and stress corrosion cracking. The laser shock peening process can also be used to strengthen thin sections, work-harden surfaces, shape or straighten parts (known as laser peen forming), break up hard materials, compact powdered metals or the like.

In order to achieve the benefits of the shock peening process for a given laser pulse energy, it is necessary to generate a high plasma pressure at the workpiece surface.

This can be achieved by confining the plasma generated by the laser pulses. In practice, this can be achieved by applying a transparent confinement overlay over the workpiece surface. A confinement overlay may comprise a solid transparent coating. In most modem techniques the confinement overlay is liquid water, which has the benefit of being readily available and inexpensive.

Increasingly, laser shock peening is used in the in situ treatment of complex assemblies such as aerospace components. Whilst water is a convenient confinement overlay for such a purpose, there is a danger that the water ultimately ingresses into other parts of the structure or assembly. This can provide a corrosion or contamination risk.

It is therefore an object of the present invention to provide a method and apparatus for laser shock peening that at least partly overcomes or alleviates the above problems.

Summary of the Invention

According to the present invention, there is provided a method of laser shock peening comprising the steps of: applying one or more laser pulses to a target area on a workpiece, thereby generating a plasma, and confining the plasma to within the target area by applying gas to the target area.

Advantageously, this allows for confinement of the plasma without necessarily needing a separate confinement medium, which enables a simpler process and associated apparatus. Particularly, the use of a gaseous confinement medium prevents issues which could arise due to application and subsequent removal of a solid confinement medium. The use of a gaseous confinement medium also allows for ready heat dissipation between pulses helping to avoid the possibility of localised melting of the workpiece.

The applied gas may have a high pressure and/or form a high pressure region. The pressure of the gas may be high relative to the ambient environment.

The gas may be applied to the target area by a gas applicator. The gas applicator may apply gas to the target area substantially continuously. Alternatively, the gas applicator may apply gas to the target area intermittently. Intermittent application may comprise application of a series of bursts of gas. The gas may be applied at a substantially constant rate. Alternatively, the rate at which the gas is applied may be varied over time.

The gas may be applied in a direction substantially perpendicular to workpiece. This provides that the gas applies a force on the plasma in the direction of the workpiece. The application of the gas may be synchronised with the one or more laser pulses. In such embodiments, the gas may be applied in pulses at a substantially constant pressure. Alternatively, the gas may be applied in pulses where the pressure varies during the pulse. In such embodiments, the gas pressure may generally rise during each pulse, generally fall during each pulse or rise to one or more peaks during each pulse. Such pressure variation may be achieved by controlling the flow rate of applied gas.

In such embodiments, the application of the gas may commence before, after or substantially simultaneously with the one or more laser pulses. In such embodiments, the application of the gas may cease before, after or substantially simultaneously with the one or more laser pulses. Preferably, but optionally, the application of the gas continues for a set time period after the one or more laser pulses have ceased.

The applied gas may be ambient air. In other embodiments, the applied gas may be a selected gas, including but not limited to nitrogen, argon or carbon dioxide. In such embodiments, the applied gas may be selected so as to minimise adverse reactions with the workpiece, optimise heating or cooling effects and/or to maximise confinement effects. Where the applied gas is ambient air, the gas applicator may draw air from the ambient environment. Where the applied gas is a selected gas, the gas applicator may draw the gas from a local reservoir. The local reservoir may comprise a tank or the like.

The gas applicator may be placed adjacent the workpiece. The gas applicator may comprise one or more nozzles aimed toward the target area. The gas applicator may be connected to an air supply or gas reservoir.

The method may comprise confining the plasma and/or applied gas using a confinement unit. The confinement unit may be placed adjacent the workpiece. The confinement unit may comprise a shield arranged to contain the gas within the vicinity of the target area.

The method may include the step of varying the position and/or orientation of the gas applicator or confinement unit relative to the target area. The method may also comprise varying the position and/or orientation of the gas applicator and confinement unit relative to each other. This variation may include varying the separation between the gas applicator, the confinement unit and the surface of the target area. This can enable further variation in the gas applied to the plasma and/or the space in which the plasma is confined. It may also minimise potential for damage of the gas applicator and confinement unit or for damage to the workpiece surface by other interactions between the workpiece surface, gas applicator, confinement unit and the plasma.

The method may include the further step of moving the target area. This can allow an extended surface of a workpiece to be subject to laser peening. In suitable embodiments, this step may include moving and/or changing the orientation of the gas applicator and/or confinement unit.

The method may include the application of a known confinement technique. The method may include the application of a transparent confinement overlay over the workpiece surface. The confinement overlay may comprise a solid transparent coating.

The method may include the provision of an ablative layer on the workpiece. The ablative layer may protect the workpiece from the effects of the laser pulse. Such ablative layers may comprise metal or polymer tapes, or painted coatings, as appropriate.

According to a second aspect of the present invention, there is provided an apparatus for laser shock peening comprising: a laser emitter arranged to emit one or more laser pulses to a target area on a workpiece, thereby creating a plasma, and a gas applicator operable to apply gas to the target area in order to confine the plasma to within the target area.

The apparatus of the second aspect of the present invention may incorporate any or all features of the method of the first aspect of the present invention, as desired or as required. In particular, the apparatus may be configured to perform the method of the first aspect on a workpiece in situ.

The gas applicator may be operable to apply high pressure gas and/or to form a region of high pressure gas. The gas may be at a high pressure relative to the ambient environment. The gas applicator may be arranged such that the gas is applied in a direction substantially perpendicular to workpiece. This provides that the gas applies a force on the plasma in the direction of the workpiece.

The gas applicator may be operable to apply air. The gas applicator may be operable to a selected gas including but not limited to nitrogen, argon or carbon dioxide. In such embodiments, the applied gas may be selected so as to minimise adverse reactions with the workpiece, optimise heating or cooling effects and/or to maximise confinement effects. Where the applied gas is ambient air, the gas applicator may comprise a pump operable to draw air from the ambient environment. Where the applied gas is a selected gas, the gas applicator may comprise a pump operable to draw the gas from a local reservoir. The local reservoir may comprise a tank or the like.

The gas applicator may be mounted to the laser emitter. This ensures that the gas applicator is suitably positioned with respect to the laser emitter. In such embodiments, the mounting may be adjustable. This allows the separation between the laser emitter and the gas applicator to be varied as required.

Additionally or alternatively, the gas applicator may be mounted on or to the workpiece. This ensures that gas applicator is suitably positioned with respect to the work piece. In such embodiments, the mounting may be adjustable. The adjustment may facilitate variation in positioning parallel to and/or perpendicular to the workpiece surface. This allows the positioning of the gas applicator to be varied as required, for instance as the target area is varied or if a different level of separation between gas applicator and workpiece is required.

The apparatus may comprise a confinement unit operable to confine the plasma and/or applied gas. The confinement unit may be placed adjacent the workpiece. The confinement unit may comprise a shield arranged to contain the gas within the vicinity of the target area. One suitable form of shield may comprise a ring or similar adapted to be placed on the workpiece around the target area. Another form may be that of a dome that surrounds the target area of the workpiece. The dome may comprise one or more apertures in order to allow the one or more nozzles to apply gas to the target area.

The confinement unit may be mounted to the laser emitter. This ensures that the confinement unit is suitably positioned with respect to the laser emitter. In such embodiments, the mounting may be adjustable. This allows the separation between the laser emitter and the confinement unit to be varied as required.

Additionally or alternatively, the confinement unit may be mounted on or to the workpiece. This ensures that confinement unit is suitably positioned with respect to the work piece. In such embodiments, the mounting may be adjustable. The adjustment may facilitate variation in positioning parallel to and/or perpendicular to the workpiece surface. This allows the positioning of the confinement unit to be varied as required, for instance as the target area is varied or if a different level of separation between confinement unit and workpiece is required.

The confinement unit may be mounted within a protective housing. The housing may beneficially protect the confinement unit from the plasma. Additionally or alternatively, the housing may protect the workpiece surface from other interactions between the workpiece surface, confinement unit surface and the plasma.

The apparatus may comprise a control unit operable to control the operation of the laser emitter, gas applicator and confinement unit. The control unit may be operable to vary the intensity, focus, timing or duration of the laser pulses. The control unit may be operable to vary the rate at which gas is applied, the pressure and/or flow rate of the applied gas, and/or the timing of application of the gas.

The control unit may comprise a user interface. The user interface may comprise user actuable control inputs and one or more audio or visual data output units. In some embodiments, the user interface may comprise a touchscreen. The user interface may thus facilitate the provision of data about the operation of the apparatus and the control of the ongoing operation of the apparatus. The control unit may be operable to receive input data from one or more connected sensors. This can aid monitoring of operation.

Detailed Description of the Invention In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic illustration of a state-of-the-art method for laser shock peening.

Figure 2 is a schematic illustration of a method of laser shock peening according to a first aspect of the present invention.

Figure 3 is a schematic diagram showing an apparatus according to the second aspect of the present invention in a first configuration. Figure 4 is a schematic diagram showing an apparatus according to the second aspect of the present invention in a second configuration.

Figure 5 is a schematic illustration of a first embodiment an apparatus of the second aspect of the present invention.

Figure 6 is a schematic illustration of a second embodiment an apparatus of the second aspect of the present invention.

As shown in Figure 1, a known technique for laser shock peening comprises a workpiece 1 is subjected to laser shock peening by the application of a series of high power pulses of a laser beam 2. When incident upon a target area 3 of the surface of the workpiece 1, a plasma 4 is generated. The rapid expansion of the plasma 4 applies a pressure 5 to the workpiece 1. To help confine the plasma 4 and thus increase the pressure 5 applied to the workpiece 1, a transparent confinement overlay 6 is applied over the workpiece 1 before initiating the laser pulses 2. Known choices for the confinement overlay 6 are liquid water or transparent solids placed adjacent the target area 3 of the workpiece 1 in order to confine the plasma 4 to the target area 3. Turning now to Figure 2, there is provided a workpiece 1 is subjected to laser shock peening by the application of a series of high power pulses of a laser beam 2. When incident upon a target area 3 of the surface of the workpiece 1, a plasma 4 is generated. The rapid expansion of the plasma 4 applies a pressure 5 to the workpiece 1. According to the present invention, gas is applied to the target area in order to produce a region of gas 7 having a higher pressure than the ambient environment, thereby confining the plasma 4. Advantageously, this may remove the need for a liquid confinement medium 6. In this embodiment, there is an optional ablative layer 15 disposed on the surface of the workpiece 1.

Figure 3 shows a schematic diagram of an apparatus 10 comprising a laser emitter 11 operable to emit one or more laser pulses 2 onto a target area 3 of a workpiece 1. A gas applicator 12 is placed adjacent the target area 3 of the workpiece 1. In some embodiments, the laser emitter 11 and gas applicator 12 are connected to a control unit 13 operable to control the operation of the laser emitter 11 and gas applicator 12. The control unit 13 is operable to receive input data from one or more linked sensors (not shown) so as to provide accurate feedback on the process. The control unit 13 is also provided with a user interface 14 operable to output data relating to the operation of the process and to enable user control of the ongoing process.

Optionally the laser emitter 11 and gas applicator 12 are movably mounted to the workpiece 1. This allows the target area 3 to be moved upon completion of the peening for a portion of the workpiece 1. As a further option, the gas applicator 12 is independently movable relative to the target area 3 and laser emitter 11. In particular, the gas applicator 12 is movable in a manner that can increase or decrease the distance between the gas applicator 12 and the target area 3, and/or to vary the angle between the gas applicator 12, target area 3 and laser emitter 11. This can vary the size and/or shape of the region of high-pressure gas 7 in the vicinity of the target area 3. Additionally, it may reduce the prospect of adverse reactions between the workpiece 1, plasma 4 and gas applicator 12. In embodiments, where the position of the laser emitter 11 and/or gas applicator 12 is variable, this can be controlled by use of control unit 13.

Figure 4 corresponds to Figure 3, where the distance between the workpiece 1 and gas applicator 12 has been increased in order to vary the size and/or shape of the gas region 7. This allows the pressure 5 at the surface of the workpiece 1 to be altered accordingly, as well as allowing the area of the workpiece 1 exposed to the plasma 4 to be altered.

Figure 5 shows one embodiment of an apparatus 10 according to the present invention. This embodiment differs from the embodiment of figure 4 in that the gas applicator 12 is a pair of nozzles 20A, B. The control unit 13 is operable to control gas flow through the or each nozzle 20A, B . When a suitable gas application rate is reached, a region of gas 7 having a higher pressure than the surrounding environment is produced, which can thereby contain the plasma 4 generated in response to the laser beam 2. In some embodiments, the control unit 13 is operable to vary the gas flow through the or each nozzle 20A, B in order to vary the size and/or shape of the region of gas 7 in the vicinity of the target area 3. The control unit 13 comprises a user interface 14 operable to receive input data from a user, and control the laser emitter 11 and control unit 13 accordingly.

Turning now to Figure 6, there is shown another embodiment of the present invention. This embodiment differs from the embodiments of figure 4 and 5 in that the gas applicator 12 is a singular nozzle 21. The nozzle 21 comprises a hollow, truncated, substantially fmstoconical body arranged such that the laser pulses 2 are aligned with the central axis of the nozzle 21. The nozzle 21 comprises a substantially fmstoconical channel which allows gas to flow out of the narrow end of the nozzle 21, which serves to apply gas to the target area 3 of the workpiece, thereby creating a region of gas 7 having a higher pressure than the surrounding environment, and containing the plasma 4. This arrangement ensures the gas applies a force to the plasma 4 in the direction of the target area 3, thereby confining the plasma 4. The nozzle 21 is connected to the control unit 13 in order to allow the nozzle 21 to be moved in order vary the shape and/or size of the region of gas 7 in the vicinity of the target area 3. The control unit 13 comprises a user interface 14 operable to receive input data from a user, and control the laser emitter 11 and gas applicator 12 accordingly. In this embodiment, the apparatus 10 has a confinement unit 30 comprising a ‘shield’ which serves to confine the plasma 4 and/or gas to a particular region. In this case, the confinement unit 30 comprises a dome placed on the workpiece 1 that surrounds the target area. The dome comprises a port through which the narrow end of the nozzle 21 protrudes, allowing the application of gas to the target area 3. This ensures that the gas and/or plasma 4 does not escape and damage the workpiece 1 and/or apparatus 10, and also to ensure the plasma 4 and/or gas is confined and used efficiently. In some embodiments, the gas applicator 12 is contained within a protective housing (not shown). The prevents any plasma 3 that is not confined by the gas region 7 from interacting with, and potentially damaging the gas applicator 12.

In embodiments comprising a control unit 13, the control unit may be operable to vary the gas application to the target area 3 so as to apply a series of relatively high pressure gas pulses. These applied gas pulses can be synchronised so as to lag, lead or coincide with pulses of the laser beam. This can help confine the plasma generated by each laser beam pulse more effectively, improving performance.

The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

1. A method of laser shock peening comprising the steps of: applying one or more laser pulses to a target area on a workpiece, thereby generating a plasma, and confining the plasma to within the target area by applying gas to the target area.

2. A method according to claim 1 wherein the applied gas has a high pressure and/or forms a high pressure region.

3. A method according to either claim 1 or claim 2 wherein the gas is applied to the target area by a gas applicator.

4. A method according to claim 3 wherein the gas is applied to the target area substantially continuously.

5. A method according to claim 3 wherein the gas is applied to the target area intermittently.

6. A method according to any preceding claim wherein the gas is applied at a substantially constant rate.

7. A method according to any preceding claim wherein the gas is applied with a varying rate.

8. A method as claimed in any preceding claim wherein the application of the gas is synchronised with the one or more laser pulses.

9. A method according to any preceding claim wherein the method comprised confining the plasma and/or applied gas using a confinement unit

10. A method according to claim 9 wherein the method comprises the step of varying the position of the gas applicator and/or confinement unit relative to the target area.

11. A method according to either claim 7 or claim 8 wherein the method comprises the step of moving the target area.

12. An apparatus for laser shock peening comprising: a laser emitter arranged to emit one or more laser pulses to a target area on a workpiece, thereby creating a plasma, and a gas applicator operable to apply gas to the target area in order to confine the plasma to within the target area.

13. An apparatus according to claim 12 wherein the gas applicator is operable to apply gas that has a high pressure and/or forms a high pressure region.

14. An apparatus according to claim 13 wherein the gas is applied to the target area substantially continuously.

15. An apparatus according to claim 13 wherein the gas is applied to the target area intermittently.

16. An apparatus according to any of claims 12 to 15 wherein the gas applicator is operable to apply gas at a substantially constant rate.

17. An apparatus according to any of claims 12 to 15 wherein the gas applicator is operable to apply gas at a varying rate.

18. An apparatus as claimed in any of claims 12 to 17 wherein the gas applicator is operable such that application of the gas is synchronised with the one or more laser pulses.

19. An apparatus as claimed in any of claims 12 to 18 wherein the apparatus comprises a confinement unit operable to confine the plasma and/or applied gas.

20. An apparatus as claimed in claim 19 wherein the confinement unit is placed adjacent the target area of the workpiece.

21. An apparatus according to any of claims 12 to 20 wherein the gas applicator comprises one or more nozzles operable to apply gas to the target area.

22. An apparatus as claimed in any of claims 12 to 21 wherein the gas applicator and, when dependent on claim 19, the confinement unit are movable in order to vary the position and/or orientation of the gas applicator and confinement unit relative to the target area.

23. An apparatus as claimed in any of claims 12 to 21 wherein the laser emitter, gas applicator and, when dependent on claim 19, the confinement unit, are movably mounted to the workpiece so as to move the target area.

24. An apparatus as claimed in any of claims 12 to 23 wherein the apparatus comprises a control unit operable to control the operation of the laser emitter, gas applicator and, when dependent on claim 19, the confinement unit.

25. An apparatus as claimed in any of claims 12 to 24 wherein the apparatus is arranged to operate on the workpiece in situ.