GB2259269A

GB2259269A - Apparatus and method for monitoring laser material processing

Info

Publication number: GB2259269A
Application number: GB9118316A
Authority: GB
Inventors: David Justin Brookfield; William Maxwell Steen; Lin Li
Original assignee: University of Liverpool; CMB Foodcan PLC
Current assignee: University of Liverpool; Crown Packaging UK Ltd
Priority date: 1991-08-24
Filing date: 1991-08-24
Publication date: 1993-03-10
Also published as: GB9118316D0; WO1993003882A1; AU2449192A

Abstract

An apparatus and method for monitoring laser material processing of a workpiece, comprising an acoustic sensor 10 mounted on a laser nozzle support assembly 12 which is thermally insulated from the nozzle itself by a heat insulating member 30. A further heat insulating member in the form of a plate 32, having a hole 34 for the laser beam 16 to pass through it, is disposed between the sensor 10 and workpiece 24. <IMAGE>

Description

DESCRIPTION APPARATUS AND METHOD FOR MONITORING LASER MATERIAL PROCESSING The present invention relates to an apparatus and method for monitoring laser material processing.

The advantages of laser welding over the conventional welding processes in terms of flexibility, speed and weld quality are now well recognised. Indeed many applications of lasers to welding in industry have already been accepted. As more welding systems are being installed by industry, the demand increases for the development of in-process techniques to monitor and control the process quality.

This is necessary since weld quality is often affected by the instability of plasma formation during laser welding and instabilities of laser power density.

A multitude of techniques have already been investigated for detecting the weld quality during processing. These include the use of an acoustic mirror which detects the high frequency component of back reflected laser beam from the beam material interaction zone; an acoustic probe which detects the shock wave generated by the plasma and vapour; an acoustic workpiece which detects the workpiece internal stress waves generated during laser welding; a photo-electric sensor for detecting plasma intensity, a probe laser for detecting melt ripple and plasma diagnosis; a pyrometer for detecting the temperature near the melt pool, and a video camera for monitoring the interaction zone shape.

In this specification, the expression "acoustic sensor" includes sensors sensitive to vibration in the sonic or ultrasonic range of frequency.

Three of these known techniques which use acoustic sensors are now discussed in more detail 1. Acoustic mirror: by mounting an acoustic (piezoelectric) sensor on the back of the laser beam guide mirror (usually gold coated copper with water cooling) nearest to the workpiece, back reflected laser beam fluctuations at high frequency can be detected as they generate thermally induced stress waves in the mirror. This plasma or melt pool modulated laser reflection fluctuation has been found to be indicative of laser processing quality. It is non-intruding and coaxial to the laser beam. The noise level is, however, found to be high with this method due to the effect of strong incoming laser beam fluctuations. The mirror has to be properly cooled, otherwise the sensor signal is affected. Applications to laser welding have been reported.

2. Acoustic workpiece: By mounting one or several acoustic sensors on the workpiece, temperature, phase and structural changes in the workpiece caused by the laser radiation can be monitored since they generate stress waves inside the workpiece. The technique is not practical when the workpiece must move relative to the laser beam since the signal is dependent on the distance between the sensor and the laser/material interaction point.

Also, if the workpiece has an overall temperature elevation, the sensor signal is affected.

Applications to laser hardening and laser welding have been reported.

3. Acoustic Probe: by positioning a metal probe near the melt pool but not in contact with the workpiece and an acoustic sensor in contact with the other end of the probe, laser generated plasma or vapour shock waves can be detected which have been found to be indicative of processing quality. The probe has been reported in three forms: a metal bar, a plate, and attaching an acoustic sensor on the focusing lens holder. In all three forms, the probe is coupled to the sensor directly and the probe is facing the melt pool. Therefore they are subject to laser reflection and melt pool heat radiation, which affect the sensing repeatability and reliability.

Applications to laser welding have been reported.

It is a primary object of the present invention to provide an apparatus and method of in-process, noncontact monitoring of laser material processing (such as welding and cutting) using an acoustic technique which is not affected by thermal effects.

A secondary object is to provide a means of inprocess monitoring of variations in workpiece distance from the laser.

In accordance with a first aspect of the present invention, there is provided an apparatus for monitoring laser material processing of a workpiece, comprising an acoustic sensor which is mounted to a laser nozzle support assembly, the latter assembly being thermally insulated from the nozzle itself.

Preferably, the thermal insulation is achieved by means of an annular member made of heat insulating material and disposed between the nozzle support assembly and the nozzle.

Advantageously, the apparatus further comprises a protective shield, having a hole for the laser beam to pass through, the shield being disposed between the sensor and a workpiece location for shielding the sensor from direct heat radiation.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic illustration of one embodiment of an apparatus in accordance with the present invention for monitoring laser material processing; Figs. 2a - 2c shows typical response patterns for an acoustic nozzle in accordance with the present invention for various high speed laser can weld faults; and Fig. 3 shows typical response patterns for an acoustic nozzle in accordance with the present invention for various laser cut qualities.

The present invention uses a development of the type 3 acoustic probe discussed hereinbefore. Instead of using an additional sensor probe as in the prior art, the sensor 10 (e.g. a piezoelectric detector or some other ultrasonic detector) is attached to the nozzle support assembly 12 which is used to hold and support the processing nozzle 14. The nozzle 14 is mounted in the path of the laser beam 16 downstream of a lens arrangement 18 and is used in most laser processing applications for applying gas to the melt area of the workpiece for either shroud, lens protection, or gas assist for the process. The lens arrangement 18 is mounted in a lens holder 20 to which gas is applied via a gas inlet duct 22. The workpiece is inducted at 24. During laser processing, a plasma and vapour "cloud" is generated in the region marked by the reference numeral 26.A typical melt pool on the workpiece is inducted at 28.

The face of the sensor 10 is arranged to be attached to a flat surface on the nozzle support assembly 12, preferably with vacuum grease between the opposed surfaces. If the nozzle assembly does not already have a suitable flat surface, then the provision of such a surface can be achieved by, for example, machining a suitable surface directly on an existing component, or by fixing a metal collar with a suitable flat surface onto the nozzle support assembly. A mechanical clamping device may be used to fix the sensor 10 onto the flat surface.

The nozzle assembly 12 is thermally insulated from the nozzle 14. The temperature of the nozzle 14 is likely to rise during laser processing since it is directly facing the melt pool and the sensor should be likely to be affected if a large part of this temperature rise was communicated to it. In the present embodiment, the insulation is achieved by the use of an insulating member 30, e.g. a ceramics or PTFE member, disposed between the nozzle 14 and nozzle assembly 12. The plastics member 30 can, for example, be in the form of an annular sleeve or bush.

Further thermal insulation of the sensor 10 is achieved by the provision of a plate 32 of metal or heat insulation material disposed between the sensor 10 and workpiece 24. The plate 32 has an aperture 34 through which the laser beam passes to the workpiece.

The plate 32, which can be flat (as shown) or in the form of a curved enclosure, prevents the radiation of the reflected laser beam and heat from the workpiece from reaching the sensor 10. The plate or curved enclosure 32 is not in contact with the sensor 10 or the nozzle assembly 12 and may or may not be in contact with the nozzle itself.

In the illustrated embodiment, the sensor output is coupled to an r.m.s. converter 36 via a pre-amp 38 and filter 40. The r.m.s. converter output can be fed to a data-logger 42. These particular circuit devices 40,36,42 are optional and others may be used depending on the information format required.

At high nozzle gas flowrates, say 6000 litres/min, the sensor signal source is mainly gas generated impact of the workpiece. The system therefore works on a completely different principle to the previous acoustic sensory units. It can be used as a distance sensor, proximity sensor and edge detector as well as for processing quality monitoring.

Experiments have been performed to evaluate the present device. These experiments have indicated that: 1. The sensing operation is not affected by the heat radiation, so that therefore the device is robust and provides repeatable results.

2. There is very little noise.

3. The output signal is cleaner and stronger than in the case of the known acoustic mirror.

4. The sensing is non-contact and is in real time.

5. The sensing is independent of workpiece moving directions and is therefore omni-directional.

By way of example only, applications for laser welding and curbing are described briefly below with reference to Figs. 2 and 3: Laser welding monitoring: In this application, it is found that the principal source that causes the acoustic signal is the process of vapour generation in the melt pool 28. It could arise from the melt pool temperature gradient generated shock wave transmitted through the air. Therefore, if there is any variation in penetration or beam absorption on the workpiece, the acoustic signal is affected. Fig. 2 summarises typical sensor response patterns to different weld faults in high speed (500mm/sec. traverse) laser butt welding. The signal is obtained by smoothing the acoustic nozzle signal using an RMS circuit. The patterns shown in Fig. 2 are idealized.The effect of operating parameters during laser welding is found to be: Laser power variation effect: at either too high power or too low power the acoustic nozzle signal reduces. There is an optimum power value so that the acoustic nozzle signal is at peak; at the same time the weld quality is optimum.

Traverse speed effect: During, say, laser lap welding, it is required that the workpiece traverses relative to the laser beam. It has been found that there is a peak acoustic signal response at a certain speed. Above or below this speed the acoustic sensor signal reduces. The weld bead appearance at the peak acoustic sensor signal appears optimum for lap welding.

Shroud gas flowrate effect: In laser welding, inert gas (He, Ar, N2) is projected through the nozzle to the work area for the purpose of shroud and prevention of the hot particles reaching the lens in the lens holder. The sensor signal response seems to increase linearly with the gas flowrate at a low flowrate region of, say, less than 200 litre/min.

When gas flowrate is too high the principal signal source changes are described in the following.

Laser cutting monitoring: For laser cutting, high pressure, high flowrate gas is used and injected through the coaxial nozzle to the laser generated melt pool to both blow away the molten material and to assist thermal input by chemical reaction (when 02 is used). The acoustic signal in this case is overwhelmed by the gas impact on the object below the nozzle. This signal is therefore dependent on gas pressure, flowrate and the object distance. The nearer the distance of the object to the workpiece the stronger the signal for the sub-sonic gas flow speed.

In this case, the acoustic nozzle is used as a distance sensor, a proximity sensor and workpiece edge detector. The sensor signal is found to be very sensitive to object distance variations, especially at edges or holes. During laser cutting, if there is a section not cut through, the sensor response can clearly indicate this by a signal rise. If the cut kerf is too wide, the sensor signal response drops lower than for the normal cut. If there is dross or irregular holes, sensor response also fluctuates. A good cut produces a smooth sensor response at a specified value, dependent on gas flowrate and nozzle stand off to the workpiece. Fig. 3 summarizes the sensor response pattern for different cut qualities.

One advantage of the present sensor unit for laser cutting applications, is that the signal is relatively insensitive to the workpiece material type so that, once calibrated, it can be used with different materials without changing the calibration.

Claims

1. An apparatus for monitoring laser material processing of a workpiece, comprising an acoustic sensor which is mounted to a laser nozzle support assembly, the latter assembly being thermally insulated from the nozzle itself.

2. An apparatus as claimed in claim 1, wherein the thermal insulation is achieved by means of an annular member made of heat insulating material and disposed between the nozzle assembly and the nozzle.

3. An apparatus as claimed in claim 1 or 2, further comprising a protective shield, having a hole for the laser beam to pass through, the shield being disposed between the sensor and a workpiece location for shielding the sensor from direct heat radiation.

4. An apparatus as claimed in claim 3, wherein said protective shield is not in contact with the sensor or the nozzle support assembly.

5. An apparatus as claimed in claim 3 or 4, wherein said protective shield is in contact with the nozzle.

6. An apparatus as claimed in claim 3 or 4, wherein said protective shield is not in contact with the nozzle.

7. An apparatus as claimed in claim 3, 4, 5 or 6, wherein said protective shield is in the form of a plate.

8. An apparatus as claimed in any of the preceding claims, wherein the acoustic sensor comprises a piezoelectric sensor.

9. A method for monitoring laser material processing of a workpiece, wherein an acoustic sensor is coupled to a support for the conventional gas nozzle but is thermally insulated from the nozzle itself.

10. The use of the apparatus of any of claims 1 to 8 for monitoring laser materials processing.

11. The use of the apparatus of any of claims 1 to 8 for optimizing laser power during laser processing.

12. The use of the apparatus of any of claims 1 to 8 for optimizing laser traverse speed during laser processing.

13. The use of the apparatus of any of claims 1 to 8 for continuous distance sensing at high gas flow rates.

14. The use of the apparatus of any of claims 1 to 8 for edge detection at high gas flow rates.

15. The use of the apparatus of any of claims 1 to 8 for proximity sensing at high gas flow rates.

16. The use of the apparatus of any of claims 1 to 8 for hole and perforation detection at high gas flow rates.

17. An apparatus for monitoring laser material processing of a workpiece, substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

18. A method for monitoring laser material processing of a workpiece substantially as hereinbefore described with reference to the accompanying drawings.