GB2455538A

GB2455538A - Laser processing

Info

Publication number: GB2455538A
Application number: GB0724264A
Authority: GB
Inventors: Paul Williams
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2007-12-13
Filing date: 2007-12-13
Publication date: 2009-06-17
Also published as: GB0724264D0

Abstract

A method of detecting breakthrough in a laser drilling operation, the method comprising the steps of directing a laser at a component in which an aperture is to be formed. A stop-off material is provided on the opposing side of the component to which the laser is directed. An optical sensor is provided to generate a spectral wavelength signal for the process light which is reflected when the laser is incident on the component. Breakthrough is determined when a change occurs in the spectral wavelength signal that indicates that process light is being reflected from the stop-off material.

Description

Laser Processing The present invention relates to machining operations and in particular machining operations including laser drilling of holes.

It is known to drill apertures in components using high energy beams such as laser beams. In the aerospace industry typical components that may have holes drilled by laser are, for example: high pressure turbine blades, nozzle guide vanes, combustors. It will be appreciated upon reading this application, however, that the invention is not limited to just these applications but will also be applicable to other industries e.g. inkjet, for example and components where apertures are formed using high energy beams.

As the aperture is formed there is breakthrough to the opposing side of the component. On breakthrough there is a danger that the beam will damage other components in the line of the laser. This is a particular problem with hollow components where the aperture is formed in one wall and it is desirable that there is no damage to an opposing wall.

A sacrificial stop-of f media is often used within a hollow component to absorb the energy of the beam on breakthrough and prevent damage to the rear wall. Without an effective stop-off media, either the internal structures may be damaged through laser beam impingement or, the hole may not be fully opened up, leading to variations in the hole sizes and quality.

A system to detect breakthrough, into the stop-off media is very important as the stop of f media has a finite life in terms of laser beam exposure, and must not be completely decomposed otherwise damage will occur to the rear wall. By accurately detecting the point of breakthrough, the time required to form the holes in the component may be reduced as no unnecessary time is taken during the drilling process.

A number of materials have been proposed to be used as a stop-of f media including: PTFE, wax based medias, salts, granules and ceramics. A number of distinct breakthrough detection methods have also been proposed as certain breakthrough or interface detection techniques are not be effective with particular stop-off media.

Conventional breakthrough techniques of detecting ever decreasing spark intensity, decreasing rate of decay signals or decreasing duration of detector signal at one particular spectral band cannot be used for some stop-off media where it lies immediately adjacent the breakthrough interface.

For example, where an air gap is being used as the stop-off, the laser process light emitted with each pulse of input laser beam energy is of a fairly stable ultraviolet intensity, pulse to pulse till breakthrough into the airgap. The ultraviolet process light is generated by a small ionised plume which is not emitted when the beam travels through the air gap and is therefore extinguished very quickly giving a dramatic and noticeable fall in ultraviolet light intensity. Where a ceramic is used as a stop-off media filling the cavity, or located directly behind the wall to be drilled the ionised plume is not extinguished at breakthrough, but instead there is a very slight increase in the ionisation and a small increase in the reflection. The deviation between the signal observed at the breakthrough pulse is within the deviation (or process noise) observed with prior pulses which means that simply increasing the sensitivity of the detector would increase process noise rather than improving the detection of breakthrough.

It is an object of the present invention to seek to provide an improved breakthrough detection method and apparatus.

According to a first aspect of the invention there is provided a_method of detecting breakthrough in a laser drilling operation, the method comprising the steps: directing a laser at a component in which an aperture is to be formed, the component having a face towards which the laser is directed and an interface with a second, dissimilar component, and using the laser to drill a hole extending between the face and the interface; the method being characterised in that an optical sensor is provided to generate a spectral wavelength signal for the process light which is reflected when the laser is incident on the component and dissimilar component, wherein the spectral wavelength signal for the process light reflected from the component is different from the spectral wavelength signal for the process light reflected from the second component, and determining breakthrough when a change occurs in the spectral wavelength signal that indicates that process light is being reflected from the second component.

Preferably the optical sensor is selected to generate a spectral wavelength signal for a particular wavelength or band of wavelengths.

The particular wavelength or band of wavelengths may be selected from the group IN, near UV, IR, near IR, blue visible and near blue visible.

Preferably further optical sensors are provided with each sensor generating a different spectral wavelength signal for the process light.

Each optical sensor may be selected to generate a spectral wavelength signal for a particular wavelength or band of wavelengths, the particular wavelength or band of wavelengths is selected from the group tJV, near liv, IR, near IR, blue visible and near blue visible.

Preferably breakthrough is determined when a change occurs in the spectral wavelength signal from at least one of the further optical sensors that indicates that process light is being reflected from the second component.

Preferably the optical sensors are selected from the group comprising photodiodes, CCDs or spectrometer.

Process light may pass through a mirror prior to detection by the optical sensor or optical sensors.

The second component may be sacrificial.

The first component may be an aerofoil or hydrofoil or a combustor component such as a casing or tile element. The second component may comprise a ceramic.

Preferably the laser is a pulsed laser and the spectral wavelength signal generated from the reflected process light of a first laser pulse is compared with the spectral wavelength signal generated from the reflected process light of an earlier laser pulse.

Preferably the wavelength signal generated from the reflected process light of the first laser pulse has a pulse width less than the wavelength signal generated from the reflected process light of the earlier laser pulse.

The wavelength signal generated from the reflected process light of the first laser pulse may have an amplitude greater than the wavelength signal generated from the reflected process light of the earlier laser pulse.

According to a second aspect of the invention there is provided a method of drilling an aperture in a component using a pulsed laser comprising the step of detecting breakthrough according to any one of preceding 13 paragraphs and applying a selected number of further pulses from the laser.

Preferably the second component is removed following application of the selected number of further pulses.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 depicts a cross section of a simplified aerofoi 1, Figure 2 depicts a first apparatus for detecting breakthrough of a laser, Figure 3 depicts a signal response detected by the apparatus of Figure 2, Figure 4 depicts a second apparatus for detecting breakthrough of a laser, Figure 5 depicts a third apparatus for detecting breakthrough of a laser.

Referring to Figure 1, a turbine blade 2 is provided and which contains a stop-off material 4. The stop-of f material 4 comprises a ceramic material and protects a rear wall 6 from damage caused by a laser beam 8 which is used to form an aperture in a front wall 10 of the blade. Upon breakthrough of the laser into the interior of the blade the energy of the laser is absorbed by the stop-off material and prevents strike on the rear wall.

The interface of metallic/ceramic cannot reliably be detected using the existing techniques of detecting ever decreasing spark intensity, decreasing rate of decay signals or decreasing duration of detector signal, each relative to breaking through the alloy material as the alloy-ceramic media interface reacts differently. The introduction of this media alters the spectral emission and emission levels to such a point that these prior methods cannot be employed.

The preferred apparatus and method for detecting the breakthrough is shown in Figure 2. A laser beam 8 is directed towards a partially reflective delivery mirror or pinhole mirror 20 which directs the beam to impact the blade 2. Interaction of the laser beam upon the component material generates process light which passes through the pinhole or partially reflective mirror and into a beam splitter 22. This beam splitter splits the process light to a number, typically 1 to 3, of photo sensors 24, 26, 28 such as photodiode or charge coupled device (CCD). Each of these photodiodes or CCDs are selected for their sensitivity to a narrow wavelength band which can further be narrowed if required using optical filters.

The signal from each of the photo sensors passes through an amplifier before being processed. The processing of the signals involves comparing the response from each of the photo sensors against a datum response. This operation is performed after a predetermined number of process laser shots to avoid coupling effects generated when the beam begins to machine the parent component material. With each process laser pulse fired, a response from each amplified photo sensor is captured. A typical response is shown in figure 3.

Figure 3 shows the laser output 3a and the response from each of the photo sensors 24, 26, 28 for 3 pulses from the laser. Plot 3b is the typical response from a sensor detecting in the UV or near TJV wavelength, plot 3c is the typical response from a sensor detecting in the IR or near IR wavelength, plot 3d is the typical response from a sensor detecting in the blue visible or near blue visible range.

The first two responses from each sensor trace are generated from process light emitted where the laser output is incident onto the metal of the turbine blade. The third response is generated from process light emitted where the laser has made breakthrough. It is notable from figure 3 that whilst the hole has not broken through the sensor output is the same, subject to differences generated due to the pulse response natural variance. This is the datum response.

The comparison of the responses from each photo sensor against their datum response compares rate of amplitude change and change in response sustained time duration.

Where breakthrough has been achieved e.g. as can be detected in the third column for the sensor.2 where between the second and third laser output pulse, the amplitude of the sensor response remains relatively constant whilst the time sustained by the pulse is significantly decreased. Providing a tolerance, which covers pulse to pulse response natural variance, is exceeded the change in output will trigger a signal to be raised to the machine control to indicate the material interface has been reached. The system may be configured to wait until more than one spectral wavelength signal reports, i.e. from the sensor 1 or.3, a significant change in process signal response.

In its best mode of operation the system is configured to enable a selected number of the spectral sensors, depending on materials being used and hence breakthrough response generated. Threshold levels for each of these sensors in use are allocated such that, at the point of breakthrough, each of the active sensors produced a change in signal sufficient to trigger the host controller due to reaching or exceeding the threshold value. The host controller waits can either act on the first signal to indicate that a sensor's threshold has been broken, or wait until all or a predetermined number of the active sensors threshold's have been broken before halting further machining or initiating a predetermined number of cleanup shots' Because breakthrough is identif led by the first sensor that produces a change in signal sufficient to trigger the controller and subsequently confirmed by the other sensors the reliability of detection is increased and false triggers, which may occur when a single sensor is used, are reduced. Beneficially, as breakthrough is identified by the first signal which gives a notable trigger, the laser can be promptly controlled following breakthrough. The trigger can also initiate other signal processing steps which can improve detection such as increasing amplification or the frequency of the signal or changing the detection algorithm which can require increased computational power than can be afforded for the whole process time, but which can be enabled for the short period between detecting the first breakthrough trigger and subsequent breakthrough triggers.

Upon the interface signal being generated, the laser machine controller can apply a predetermined number of final process laser shots to ensure the laser machined feature in the component is free from faults and satisfactorily opened. Alternatively, a second signal may be generated by the photo sensor comparator to indicate to the main laser machine controller that the laser machined feature is sufficiently open.

In a second embodiment of the invention a spectrometer with a spectral range which lies within the band of 200nm to 2200nm is used to detect breakthrough. Such an embodiment is depicted in Figure 4. The spectrometer 30 replaces the optical splitter and the discrete photo sensors. The signal from the spectrometer is examined and compared with a datum signal as in the above embodiment.

Whilst a coaxial arrangement to sense the process light is preferred as it is the most practical method, a direct, non-coaxial line of sight method could be implemented without a beam splitter as shown Figure 5.

In this embodiment the sensors are located close to the workpiece to sense the light. A filter may be placed over each of the sensors to select the wavelength of light impinging on the sensor.

It will be appreciated that a fibre beam delivery may be used to bring the laser to the substrate to be drilled.

In this case it may be desirable to locate the photo sensor and beam splitter or spectrometer at the opposing end of the fibre.

Beneficially, the above techniques offer greater reliability in detecting the interface between parent material and stop-off media than prior art methods. The invention finds particular application in situations where there is difficulty in determining laser breakthrough using conventional applications. Ceramic when used as a stop-off media when laser drilling a metal component gives of f a different spectral profile to that of the metal. This invention is not just limited to ceramic stop-off material but can also be used to detect breakthrough when other stop-off materials are used.

The invention can also be used to detect a change between layers in a laminated component. Laser Processing The present invention relates to machining operations and in particular machining operations including laser drilling of holes.

The second component may be sacrificial.

The invention can also be used to detect a change between layers in a laminated component.

Claims

Claims 1. A method of detecting breakthrough in a laser drilling operation, the method comprising the steps: directing a laser at a component in which an aperture is to be formed, the component having a face towards which the laser is directed and an interface with a second, dissimilar component, and using the laser to drill a hole extending between the face and the interface; the method being characterised in that an optical sensor is provided to generate a spectral wavelength signal for the process light which is reflected when the laser is incident on the component and dissimilar component, wherein the spectral wavelength signal for the process light reflected from the component is different from the spectral wavelength signal for the process light reflected from the second component, and determining breakthrough when a change occurs in the spectral wavelength signal that indicates that process light is being reflected from the second component.
2. A method according to claim 1, wherein the optical sensor is selected to generate a spectral wavelength signal for a particular wavelength or band of wavelengths.
3. A method according to claim 2, wherein the particular wavelength or band of wavelengths is selected from the group UV, near UI!, IR, near IR, blue visible and near blue visible.
4. A method according to any preceding claim, wherein further optical sensors are provided with each sensor generating a different spectral wavelength band signal for the process light.
5. A method according to claim 4, wherein each optical sensor is selected to generate a signal for a particular wavelength or band of wavelengths, the particular wavelength or band of wavelengths is selected from the group UV, near UI!, IR, near IR, blue visible and near blue visible.
6. A method according to claim 4 or claim 5, wherein breakthrough is determined when a change occurs in the spectral wavelength signal from at least one of the further optical sensors that indicates that process light is being reflected from the second component.
7. A method according to any preceding claim, wherein the optical sensors are selected from the group comprising photodiodes, CCDs or spectrometer.
8. A method according to any preceding claim, wherein the process light passes through a mirror prior to detection by the optical sensor or optical sensors.
9. A method according to any preceding claim, wherein the second component is sacrificial.
10. A method according to any preceding claim, wherein the first component is an aerof oil, hydrofoil or combustor component.
11. A method according to any preceding claim, wherein the second component comprises a ceramic.
12. A method according to any preceding claim, wherein the laser is a pulsed laser and the spectral wavelength signal generated from the reflected process light of a first laser pulse is compared with the spectral wavelength signal generated from the reflected process light of an earlier laser pulse.
13. A method according to claim 12, wherein the wavelength signal generated from the reflected process light of the first laser pulse has a pulse width less than the wavelength signal generated from the reflected process light of the earlier laser pulse.
14. A method according to claim 12 or claim 13, wherein the wavelength signal generated from the reflected process light of the first laser pulse has an amplitude greater than the wavelength signal generated from the reflected process light of the earlier laser pulse.
15. A method of drilling an aperture in a component using a pulsed laser comprising the step of detecting breakthrough according to any one of claims 1 to 14 and applying a selected number of further pulses from the laser.
16. A method according to claim 15, wherein the second component is removed following application of the selected number of further pulses.
17. A method or apparatus substantially as hereinbefore described with reference to the accompanying drawings.

17. A method or apparatus substantially as hereinbefore described with reference to the accompanying drawings.

Claims 1. A method of detecting breakthrough in a laser drilling operation, the method comprising the steps: directing a laser at a component in which an aperture is to be formed, the component having a face towards which the laser is directed and an interface with a second, dissimilar component, and using the laser to drill a hole extending between the face and the interface; the method being characterised in that an optical sensor is provided to generate a spectral wavelength signal for the process light which is reflected when the laser is incident on the component and dissimilar component, wherein the spectral wavelength signal for the process light reflected from the component is different from the spectral wavelength signal for the process light reflected from the second component, and determining breakthrough when a change occurs in the spectral wavelength signal that indicates that process light is being reflected from the second component.

2. A method according to claim 1, wherein the optical sensor is selected to generate a spectral wavelength signal for a particular wavelength or band of wavelengths.

3. A method according to claim 2, wherein the particular wavelength or band of wavelengths is selected from the group UV, near UI!, IR, near IR, blue visible and near blue visible.

4. A method according to any preceding claim, wherein further optical sensors are provided with each sensor generating a different spectral wavelength band signal for the process light.

5. A method according to claim 4, wherein each optical sensor is selected to generate a signal for a particular wavelength or band of wavelengths, the particular wavelength or band of wavelengths is selected from the group UV, near UI!, IR, near IR, blue visible and near blue visible.

6. A method according to claim 4 or claim 5, wherein breakthrough is determined when a change occurs in the spectral wavelength signal from at least one of the further optical sensors that indicates that process light is being reflected from the second component.

7. A method according to any preceding claim, wherein the optical sensors are selected from the group comprising photodiodes, CCDs or spectrometer.

8. A method according to any preceding claim, wherein the process light passes through a mirror prior to detection by the optical sensor or optical sensors.

9. A method according to any preceding claim, wherein the second component is sacrificial.

10. A method according to any preceding claim, wherein the first component is an aerof oil, hydrofoil or combustor component.

11. A method according to any preceding claim, wherein the second component comprises a ceramic.

12. A method according to any preceding claim, wherein the laser is a pulsed laser and the spectral wavelength signal generated from the reflected process light of a first laser pulse is compared with the spectral wavelength signal generated from the reflected process light of an earlier laser pulse.

13. A method according to claim 12, wherein the wavelength signal generated from the reflected process light of the first laser pulse has a pulse width less than the wavelength signal generated from the reflected process light of the earlier laser pulse.

14. A method according to claim 12 or claim 13, wherein the wavelength signal generated from the reflected process light of the first laser pulse has an amplitude greater than the wavelength signal generated from the reflected process light of the earlier laser pulse.

15. A method of drilling an aperture in a component using a pulsed laser comprising the step of detecting breakthrough according to any one of claims 1 to 14 and applying a selected number of further pulses from the laser.

16. A method according to claim 15, wherein the second component is removed following application of the selected number of further pulses.