WO2016106515A1

WO2016106515A1 - Method for machining holes with pulsed laser beams, method and system for detecting breakthrough

Info

Publication number: WO2016106515A1
Application number: PCT/CN2014/095377
Authority: WO
Inventors: Hongtao Li; Yucheng Tang; Chao REN
Original assignee: Siemens Aktiengesellschaft
Priority date: 2014-12-29
Filing date: 2014-12-29
Publication date: 2016-07-07

Abstract

A method for machining at least one hole (14) through a first wall (11) of a component (10) with pulsed laser beams is provided. The component (10) has a second wall (13) with a hollow space (12) between the first wall (11) and the second wall (13). The cycle of each laser pulse comprises a pulse-on period and a pulse-off period. The method comprises: providing light-sensitive or heat-sensitive material at the hollow space (12) of the component (10); directing pulsed laser beams to the first wall (11) for machining a hole (14); acquiring a series of images in the incident direction of the pulsed laser beams; extracting features of images which are acquired during pulse-off periods; stopping pulsed laser beams when an extracted feature of at least one of the images fulfills a predetermined criterion. The breakthrough detection accuracy could be improved.

Description

METHOD FOR MACHINING HOLES WITH PULSED LASER BEAMS, METHOD AND SYSTEM FOR DETECTING BREAKTHROUGH

FIELD

Embodiments of the present invention relate to component machining operations, such as drilling or cutting operations, using a pulsed laser beam. Particularly, this invention relates to a method for machining holes through a wall of a hollow component, a method and a system for breakthrough detection during such operations, i.e. for detecting when the laser has penetrated through the thickness of the wall of the component.

BACKGROUND

Laser machining systems are often used for component processing operations such as drilling and cutting. In the operations, it is common to use a pulsed laser directed at a target area in the wall of a hollow component such as a vane, a blade or a fuel injection nozzle, to produce through holes in the target area of the component wall. The component can, for example, be embodied as a blade for use in a gas turbine shown in FIG. 1. The blade has a first wall 11 and a second wall 13 with a hollow space 12 between them. A plurality of holes 14 are machined to serve as air film holes for cooling the blade which is subject to high temperature when the gas turbine is in operation. When a hole 14 is machined, each time the laser is pulsed from a laser source, it creates and then deepens the hole 14 at the target area in a wall, for example, the first wall 11, until the hole 14 breaks through the wall thickness after a sufficient number of pulsed laser beams. After the hole is formed, the laser and the component are then moved relative to each other to begin to machine another hole in the wall.

The thickness and hardness of the component wall being processed can vary somewhat between different components or between different locations on the same component. In addition, the power output of the laser beams can vary over time. Consequently, the number of pulsed laser beams required to achieve breakthrough can vary from hole to hole. For instance, one hole could be formed in as few as three or four pulsed laser beams. Another hole might require six or seven pulsed laser beams. As a result, there is need to detect when a wall of a component is broken through.

SUMMARY

One aspect of this invention relates to a method for machining at least one hole through a first wall of a component with pulsed laser beams. The component has a second wall with a hollow space between the first wall and the second wall. The cycle of each laser pulse comprises a pulse-on period and a pulse-off period. The method comprises S1: providing light-sensitive or heat-sensitive material at the hollow space of the component. S2: direct pulsed laser beams to the first wall for machining a hole. S3: acquire a series of images in the incident direction of the pulsed laser beams after the pulsed laser beams strike the first wall. S4: extract features of images which are acquired during pulse-off periods. And S5: stop pulsed laser beams when an extracted feature of at least one of the images fulfills a predetermined criterion.

Another aspect of this invention relates to method for detecting breakthrough during hole machining operation with pulsed laser beams in a first wall of a component. The component has a second wall with a hollow space between the first wall and the second wall . The cycle of each laser pulse comprising a pulse-on period and a pulse-off period. And the hollow space of the component is provided with light-sensitive or heat-sensitive material, the method comprises S6: acquire a series of images in the incident direction of pulsed laser beams after the pulsed laser beams strike the first wall for machining a hole. S7: extract features of light spots in the images which are acquired during pulse-off periods； and S8: determine that breakthrough has occurred when an extracted feature of at least one of the images fulfills a predetermined criterion.

Another aspect of this invention relates to a detection system for detecting breakthrough during hole machining operation in a first wall of a component with pulsed laser beams, the component has a second wall with a hollow space between the first wall and the second wall. The cycle of each pulse of the laser beams comprises pulse-on period and pulse-off period. And the hollow space of the component is provided with light-sensitive or heat-sensitive material. The system comprises a digital camera and a processor. The digital camera acquires a series of images in the incident direction of pulsed laser beams after the pulsed laser beams strike the first wall of the component for machining a hole. The processor is programmed to extract features of light spots in the images which are acquired by the digital camera during pulse-off periods and determine that breakthrough has occurred when an extracted feature of at least one of the images fulfill a predetermined criterion.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described more completely in the following Detailed Description, when taken in conjunction with the following drawings, in which like reference numerals refer to like elements throughout.

Fig. 1 is a schematic view illustrating an exemplary component；

Fig. 2 is a schematic view illustrating an exemplary detection system for detecting breakthrough during hole machining operation in a first wall of a component with pulsed laser beams ；

Fig. 3 is a flow chart illustrating an exemplary method for machining at least one hole through a first wall of a component with pulsed laser beams；

Fig. 4A shows an image acquired in the incident direction of pulsed laser beams during pulse-off periods before a hole is not broken through；

FIG. 4B shows another image acquired in the incident direction of pulsed laser beams during pulse-off periods after the hole is broken through；

Fig. 5 is a schematic view illustrating exemplary pulse durations；

Fig. 6 is a flow chart illustrating another exemplary method for machining at least one hole through a first wall of a component with pulsed laser beams；

Fig. 7 is a flow chart illustrating an exemplary method for detecting breakthrough during hole machining operation with pulse laser beams in a wall of a component.

DETAILED DESCRIPTION

The figures and the description represent merely exemplary embodiments of the invention.

Fig. 1 is a schematic view illustrating an exemplary component which may be made of any suitable material. The component has a first wall 11 and a second wall 13 with a hollow space 12 between them. The terms “first” and “second” in the “first wall” and the “second wall” are used to differentiate the two walls of the component 10. The first wall could be a upper wall, a lower wall, a front wall and a back wall of a hollow component, in which at least one hole is needed to machine. A plurality of holes 14 are formed in the first wall 11 with pulsed laser beams. As an example, four laser pulses in a row are shown in FIG. 1 for machining a hole.

Fig. 2 is a schematic view illustrating an exemplary detection system for detecting breakthrough during hole machining operation in a first wall of a component with pulsed laser beams. The detection system includes a digital camera 20 for acquiring a series of images in the incident direction of the pulsed laser beams, and a processor 30 which is programmed to implement a breakthrough detection method, which will be explained in detail below. The digital camera 20 may be a CCD(Charge Coupled Device) camera or a CMOS (Complementary Metal-Oxide Semiconductor) camera. It captures a series of images in the incident direction of the pulsed laser beams when the laser beams are fired by the laser source 40 and directed to the first wall of the component 10. The digital camera 20 may be positioned may be fixed on the nozzle 70 so as to capture images in the incident direction of the pulsed laser beams. Alternatively, mirrors 50 shown in FIG. 2 may be used to reflect the light in the incident direction of the pulsed laser beams to the digital camera 20. The digital camera 20 is connected in the wired or wireless manner with the processor 30, which may be a Programmable Logic Controller (PLC) .

Major devices for machining holes including a laser source 40, a lens 60 for guiding laser beams and a nozzle 70 for injecting laser beams at the first wall of the component 10 (i.e. the upper wall of the component shown in FIG. 2) are also illustrated in this figure to facilitate understanding of the invention.

Fig. 3 is a flow chart illustrating an exemplary method for machining at least one hole through a first wall of the component 10 with pulsed laser beams. The cycle of each laser pulse comprises a pulse-on period and a pulse-off period. The method comprises S1: providing light-sensitive or heat-sensitive material at the hollow space of the component 10.

In S1, light-sensitive or heat-sensitive material is provided at the hollow space 12 before the holes are machined with pulsed laser beams. A light-sensitive material can emit light when a laser beam contacts it. A heat-sensitive material can emit light when a laser beam strikes it and heats it. One example of a light-sensitive material is fluorescent material. As soon as a laser beam breaks through the first wall 11 and strikes the light-sensitive material or heat-sensitive material, the light emitted by the light-sensitive material or heat-sensitive material is detected by a detection system.

When the component 10 filled with light-sensitive or heat-sensitive material at it hollow space 12 is ready for machining holes, then S2: direct pulsed laser beams to the first wall 11. The pulsed laser beams focus on and strike a target area in the first wall 11of the component. Pulsed laser beams heat the component material in the target area and melt some of the material. The process of heating the material of the component 10 to vaporization temperature removes the material of the component 10 at the target area to gradually. When a sufficient number of laser pulses strike the same target area of the first wall 11, the thickness of the first wall 11 is penetrated and a hole is formed at the target area of the first wall 11.

After the pulsed laser beams strike a target area of the first wall 11, S3: acquire a series of images in the incident direction of the pulsed laser beams. Some images are images of the target area of the first wall 11 which is stricken by laser pulses, when the thickness of the first wall 11 is not penetrated. One or more images are formed by light emitted by the light-sensitive or heat-sensitive material which is stricken by one or more pulses, when the laser pulses penetrate the thickness of the first wall and contact the light-sensitive or heat-sensitive material in the hollow space 12. The images may be captured by the digital camera 20.

The frequency of capturing images mainly depends on the parameters of the laser pulses, including pulse frequency and peak power, and machining accuracy requirements. Multiple images within one pulse duration may be acquired in order to achieve high accurate pulse control. At least one image may be acquired during each pulse to achieve reasonable accuracy.

The cycle of each laser pulse comprises a pulse-on period and a pulse-off period. In one embodiment of the method for machining at least one hole with pulsed laser beams, images during pulse-on periods and pulse-off periods are captured so that the digital camera 20 can capture images continuously for a while. No communication between the digital camera 20 and the laser source 40 is needed. Thus, the digital camera 20 and the laser source 40 do not need to coordinate with each other. Images during pulse-on periods and pulse-off periods are acquired, although only images only during pulse-off periods are used in this method for detecting breakthrough of the hole.

In another embodiment of the method for machining at least one hole with pulsed laser beams, the series of images are acquired only during pulse-off periods. In this embodiment, the laser source 40 signals to the digital camera 20 when a pulse-off period begins. Then the digital camera 20 starts to acquire images in the incident direction of the pulsed laser beams.

After a series of images in the incident direction of the pulsed laser beams are acquired, S4: then extract features of images which are acquired during pulse-off periods. And S5: stop pulsed laser beams when an extracted feature of at least one of the images fulfills a predetermined criterion. The provision of the light-sensitive or heat-sensitive material at the hollow space 12 enables the images acquired during pulse-off periods to be used for determining whether a hole is broken through or not.

In one embodiment, the image features may indicate the size of light spots in the acquired images. A light spot in an image is a light area which can be distinguished from other areas in the image. Fig. 4A shows an image acquired in the incident direction of pulsed laser beams during pulse-off periods before a hole is not broken through. The image is dark, because no light exists in the incident direction of the pulsed laser beams. To ensure accurate determination on whether a hole is broken through, images can be captured after a pulse-off period has lasted for a while so that heated material of the component 10 cools down and stops to emit light. Fig. 4B shows another image acquired in the incident direction of pulsed laser beams during pulse-off periods after the hole is broken through. There is a light spot in the image which is formed by the light emitted by the light-sensitive or heat-sensitive material at the hollow space 12. Thus, as soon as one or more pulsed laser beams break through the first wall 11 of the component, and strike the light-sensitive or heat-sensitive material, the light emitted by the material is detected. As a result, at least one image can be captured and acquired of the size of the light spot when a hole is broken through. Image intensifying devices may be employed before the features are extracted if desired. The processor 30 is programmed to calculate the size of the light spots of the images acquired during pulse-off periods. The size of the light spots may be represented by the number of pixels whose grey values are greater than a predefined threshold. The predetermined size, which works as a criterion for determining when to stop pulsed laser beams, may be set by simulation or according to machining experiences.

The processor 30 may also be programmed to respond to a predetermined size of the light spots to stop the pulsed laser beams. The processor 30 signals to the laser source 40 to switch off when the size of a light spot of one image acquired during a pulse-off period is greater than the predetermined size. The processor 30 signals to the laser source 40 to continue to direct more pulsed laser beams to the same target area when the size of a light spot of one image acquired during a pulse-off period is smaller than the predetermined size. Then, the operation process continues with the followings operations. S3: acquire a series of images in the incident direction of the pulsed laser beams. S4: extract features of images which are acquired during pulse-off periods And S5: stop pulsed laser beams when an extracted feature of at least one of the images fulfills a predetermined criterion.

The pulsed laser beams may be stopped when the light spots of two or three images which are acquired in a row during pulse-off periods are bigger than the predetermined size. This alternative manner may further improve the breakthrough detection accuracy.

The nozzle 70 and the component 10 are then moved relative to each other to begin to machine another hole at another target area in the first wal1 11 after the pulsed laser beams are stopped i.e. a through hole at a target area in the first wall 11 of the component is formed.

In another embodiment, the image features may indicate the spectrum of light spots in the images acquired during pulse-off periods. There may be light spots in the images acquired when a hole is not broken through even during pulse-off periods, because the material of the component 10 at the target area which is heated by the pulsed laser beams may emit light even during pulse-off periods. However, the spectrum of the light emitted by the light-sensitive or heat-sensitive material differs from that of the heated component material. Hence, the spectrum of light spots in the acquired images may be extracted and compared with a predetermined spectrum. The predetermined spectrum, which works as a criterion for determining when to stop pulsed laser beams, may be acquired in simulation tests by recording the spectrum of the light emitted by the light-sensitive or heat-sensitive material when a pulsed laser beam contacts the material.

The predetermined criterion such as the predetermined size and spectrum of the light spots may be updated. A size of the spot and spectrum determined before a machining operation begins may be replaced with a size, spectrum and threshold of grey value variation of a light spot which are acquired during pulse-off periods when a hole is broken through. The updated criterion is also the so-called predetermined criterion.

The ratio of the pulse-on period to the pulse-off period is less than 1: 10. Fig. 5 is a schematic view illustrating exemplary pulse durations. The horizontal coordinate denotes time and the vertical coordinate denotes power of the pulsed laser beams. The laser machining process needs high peak power to melt the component material. Therefore, the laser source 40 needs to accumulate enough energy to pump the laser pulse, which means that the pulse-off period is much longer than the pulse-on period, as illustrated in Fig. 5 which shows two pulse-on periods and one pulse-off period. The higher peak power is needed, the shorter the pulse-on period is in a cycle of one pulse. For instance, the ratio of the pulse-off period and the pulse-on period is 1: 40. The ratio of the pulse-on period to the pulse-off period may be adjustable for machining components with different parameters.

The provision of the light-sensitive or heat-sensitive material at the hollow space 12 enables the images acquired during pulse-off periods, instead of pulse-on periods, to be used for determining whether a through hole is formed or not in the first wall of the component. The requirements for image acquisition frequency, data processing speed, and signal transfer speed, etc., can be reduced largely when the through hole machining method is developed based on images during longer pulse-off periods, instead of shorter pulse-on periods. This reduces the costs of the imaging and control devices. Moreover, the images during pulse-off periods are with less interference from plasma produced by molten material, molten particles and vaporization of the material of the component 10 than the images during pulse-on periods. Therefore, using the images during pulse-off periods improves breakthrough detection accuracy.

Fig. 6 is a flow chart illustrating another exemplary method for machining at least one hole through a first wall of a component with pulsed laser beams. Such components as blades and vanes are often filled with protective agents 20 in their hollow space 12. Suitable protective agents may be wax and Teflon. In the exemplary method, S11: a protective agent 20 for the purpose of protecting the second wall 13 from being damaged by laser beams is mixed with a light-sensitive material or heat-sensitive material. Then, S12: the mixture is introduced into the hollow space 12. For example, the mixture of a melted wax and fluorescent material is injected through a nozzle into the hollow space 12. Another manner for introducing the mixture is dipping the component into the solution of the mixture till the inner surfaces of the component 10 are deposited with the mixture. When the component 10 with light-sensitive material or heat-sensitive material is ready for machining holes, Then S2: direct pulsed laser beams on the first wall, S3： acquire a series of images into incident direction of the pulsed laser beams, S4： extract features of light spots in the images which are acquired during pulse-off periods, and S5： stop the pulse laser beams when an extracted feature of at least one of the images fulfills a predetermined criterion.

Fig. 7 is a flow chart illustrating an exemplary method for detecting breakthrough during hole machining operation with pulse laser beams in a wall of a component 10.The hollow space 12 of the component is provided with light-sensitive or heat-sensitive material. The method is as follows. S6: acquire a series of images in the incident direction of pulsed laser beams after the pulsed laser beams strike the first wall 11 for machining a hole 14. Then, S7: extract features of the images which are acquired during pulse-off time. And then, S8: determine that breakthrough has occurred when an extracted feature of an image fulfills a predetermined criterion. For details of these operations, reference should be made to the above description of the method for machining at least one hole.

A detection system for detecting breakthrough during hole machining operation in a first wall 11 of a component 10 with pulsed laser beams is provided in another embodiment. The component 10 has a second wall 13 with a hollow space 12 between the first wall 11 and the second wall 13. The cycle of each laser pulse comprises a pulse-on period and pulse-off period. The hollow space 12 of the component is provided with light-sensitive or heat-sensitive material. The system comprises a digital camera 20 and a processor 30. The digital camera 20 acquires a series of images in the incident direction of pulsed laser beams after the pulsed laser beams strike the first wall 11 of the component for machining a hole 14. The processor 30 is programmed extract features of light spots in the images which are acquired by the digital camera 20 during pulse-off periods, and determine that breakthrough has occurred when an extracted feature of an image fulfill a predetermined criterion. The predetermined criterion is adjustable according to the operation conditions under which the system detection works. The processor 30 is embodied in any suitable circuit contained on a circuit board or the like. In addition, it could be a portion of the hardware and software in a system controller such as a remote control center in a power plant.

The provision of the light-sensitive or heat-sensitive material at the hollow space enables the images acquired during pulse-off periods, instead of pulse-on periods, to be used for detecting breakthrough. The requirements for the imaging and control devices can be reduced largely when the detection method is developed based on images during longer pulse-off periods, instead of shorter pulse-on periods. This reduces the costs of the imaging and control devices. Moreover, the images during pulse-off periods are with less interference than the images during pulse-on periods. Therefore, using the images during pulse-off periods improves breakthrough detection accuracy.

While embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.

Claims

A method for machining at least one hole (14) through a first wall (11) of a component (10) with pulsed laser beams, the component (10) having a second wall (13) with a hollow space (12) between the first wall (11) and the second wall (13) , and the cycle of each laser pulse comprising a pulse-on period and a pulse-off period, the method comprising:

providing light-sensitive or heat-sensitive material at the hollow space (12) of the component；

directing pulsed laser beams to the first wall (11) for machining a hole (14) ；

acquiring a series of images in the incident direction of the pulsed laser beams after the pulsed laser beams strike the first wall (11) ；

extracting features of images which are acquired during pulse-off periods； and

stopping pulsed laser beams when an extracted feature of at least one of the images fulfills a predetermined criterion.
The method of claim 1, wherein providing light-sensitive or heat-sensitive material at the hollow space (12) of the component further comprises:

mixing a protective agent with the light-sensitive or heat-sensitive material, wherein the protective agent is for protecting the second wall (13) of the component from being damaged by the laser beams；

introducing the mixture of the protective agent and the light-sensitive or heat-sensitive material into the hollow space (12) .
The method of claim 1, wherein the ratio of pulse-on period to pulse-off period is less than 1： 10.
A method for detecting breakthrough during hole machining operation with pulsed laser beams in a first wall (11) of a component (10) ， the component (10) having a second wall (13) with a hollow space (12) between the first wall (11) and the second wall (13) , the cycle of each laser pulse comprising a pulse-on period and a pulse-off period, and the hollow space (12) of the component being provided with light-sensitive or heat-sensitive material, the method comprising:

acquiring a series of images in the incident direction of pulsed laser beams after the pulsed laser beams strike the first wall (11) for machining a hole (14) ；

extracting features of light spots in the images which are acquired during pulse-off periods； and

determining that breakthrough has occurred when an extracted feature of at least one of the images fulfills a predetermined criterion.
The method of claim 4, wherein acquiring a series of images in the incident direction of the pulsed laser beams is performed only during pulse-off periods.
The method of claim 4, wherein the ratio of the pulse-on period to the pulse-off period is less than 1: 10.
A detection system for detecting breakthrough during hole machining operation in a first wall (11) of a component (10) with pulsed laser beams, the component (10) having a second wall (13) with a hollow space (12) between the first wall (11) and the second wall (13) , the cycle of each pulse of the laser beams comprising pulse-on period and pulse-off period, and the hollow space (12) of the component being provided with light-sensitive or heat-sensitive material, the system comprising:

a digital camera (20) for acquiring a series of images in the incident direction of pulsed laser beams after the pulsed laser beams strike the first wall (11) of the component for machining a hole (14) ； and

a processor (30) which is programmed to implement:

extracting features of light spots in the images which are acquired by the digital camera (20) during pulse-off periods； and

determining that breakthrough has occurred when an extracted feature of at least one of the images fulfill a predetermined criterion.
The detection system of claim 7, wherein the ratio of pulse-on period to pulse-off period is less than 1:10.