GB2342883A

GB2342883A - Improvements in or relating to laser machining of articles

Info

Publication number: GB2342883A
Application number: GB9823210A
Authority: GB
Inventors: Pamela Jean Byrd
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1998-10-23
Filing date: 1998-10-23
Publication date: 2000-04-26
Also published as: GB9823210D0

Abstract

An apparatus for laser machining a metal article includes means for generating a laser beam and directing the beam 12 through a nozzle 14 towards a location on the article, and means for generating a jet stream of assist gas 20 coaxial with the laser beam as it leaves the nozzle and directing said jet stream towards the article. There is provided a conical shroud 24 surrounding the nozzle and extending therefrom to the surface of the article and enclosing said location on the article, whereby debris thrown up by the interaction of the laser beam with the article will be trapped on the inside of the shroud and thereby prevented from failing onto the surface of the article.

Description

IMPROVEMENTS IN OR RELATING TO LASER MACHINING OF ARTICLES This invention concerns improvements in or relating to apparatus and a process for laser machining of a workpiece.

When a laser interacts with an article, such as a metal component or other workpiece, the physical mechanism for removal of material from the article by the laser is melting or vaporisation. An interface region in which the material removal occurs, the erosion front, is formed in front of the laser beam. The temperature field across this interface depends on the thermal properties of the material and the laser machine process parameters. In laser drilling a hole in a metal article some of the metal drilled out may not be vaporised and may be ejected as"spatter"onto the surface of the article immediately surrounding the drilled hole. The ejection of this spatter occurs for a number of reasons including expansion and explosion of the volume of plasma, evaporation of the molten liquid as it boils, or the direct sublimation of some of the solid metal onto the surface of the article.

Aeroengine components such as a turbine blades or combustion rings are drilled to carry internal cooling air and to provide a means by which cooling air can flow over the surface of the component. If ejected spatter comes into contact with the surface of the metal surrounding the holes as they are drille, it cools and welds to the component.

The main problem presented by build-up of spatter is that, if it is not removed, then it will interfere with cooling air flow over the surface of the component and cause undesirable hot spots. Build-up of spatter may also cause an irregular weight distribution in the component which could adversely affect its performance and cause unwanted stresses.

A further problem concerns measurement of the diameter of small holes in an article, such as an aeroengine component, as part of an in-process inspection procedure. One way of measuring the diameter of a small hole is to use a device which directs a jet of air at the hole and converts a resulting change in air flow characteristics into a dimensional parameter characteristic of the diameter of the hole. One such device, for example, measures the back pressure of the jet of air as a function of the area and diameter of the hole. Clearly, in order to obtain accurate readings from the measured device it is essential that the surface of the component adjacent the hole be clean, smooth, and free from spatter.

Hitherto, the removal of spatter has been carried out by a process involving pre-and post-drilling stages. In one such process, before laser drilling commences, a layer of a special paste is applied to areas of the article where it is expected that spatter will deposit. When drilling is completed, the paste together with any adherent spatter is removed. This is a time consuming procedure.

An in-process spatter removal system which prevents the formation of spatter would eliminate the need for the prior application and subsequent removal of spatterremoving paste, thus rendering the laser drilling process more efficient. Further, the absence of spatter during drilling may make possible the measurement of hole size during the drilling process.

Laser drills are usually operated in conjunction with a jet of high pressure assist gas which is directed coaxially with the laser beam at the article being drilled as, for instance, with Nd-Yag lasers. The jet of assist gas through the laser nozzle reduces impingement of debris on the focal lens or exit window of the laser thereby protecting the optical system. It does not, of itself, prevent metal spattering onto the surface of the article or workpiece.

The assist gas can influence the drilling process in several ways: the high pressure flow helps to blast away the molten metal as it forms in the hole ; secondly, the small diameter of the laser nozzle causes a high velocity gas jet which tends to cool the metal ; thirdly, the composition of the assist gas can be used to prevent or bring about, as the case may be, oxidation or other chemical changes in the material of the component during the laser drilling, according to the requirements of the particular process.

Additionally if the assist gas is oxygen, oxidation of the metal being drilled increases and, being an exothermic reaction, generates heat at the surface of the article or workpiece. Also, the absorption of the laser beam by the metal may change due to the build-up of the oxide layer and the absorptivity of the oxide compared with that of unoxidised metal.

It is an objective of the present invention to provide improved apparatus for laser machining an article or workpiece which prevents or substantially reduces deposition of spatter on the workpiece during the laser machining process.

According to one aspect of the present invention there is provided an apparatus laser machining a metal workpiece including a laser, a nozzle for directing a laser beam towards a machining location on the workpiece, hood or shroud means arranged and adapted to be located over or offered up to the workpiece so as to enclose the machining location within a work chamber and which has means for admitting the laser beam into the work chamber to perform the machining operation, and means for directing a stream of assist gas into the enclosed work chamber and towards the machining location on the workpiece, whereby debris thrown up by the interaction of the laser beam with the workpiece is trapped inside the shroud means and prevented from falling onto the surface of the workpiece.

Preferably there is provided a means for applying further assist gas radially inwards at the base or rim of the hood or shroud means.

The further assist gas supply means may be a circumferentially extending tube attached to the base of the shroud, and means may also be provided for supplying pressurised assist gas to the tube. The tube is preferably provided with a series of holes directed radially inwards towards the interior of the shroud.

The shroud is preferably provided with an exit nozzle for exit assist gas, the exit nozzle being located at a level above the assist gas supply and the nozzle supply, thereby to ensure optimal gas flow within the shroud.

The shroud may also be provided with an exit aperture in the vicinity of the laser nozzle for the egress of assist gas into the ambient atmosphere. This exit aperture may be defined by a radial gap between the laser nozzle and the upper part of the shroud.

Also, according to another aspect of the invention there is provided a hood or shroud means arranged and adapted to be located over or offered up to a workpiece so as to enclose the machining location within a work chamber including means for admitting the laser beam into the work chamber to perform the machining operation, and means for directing a stream of assist gas into the enclosed work chamber towards the machining location on the workpiece, whereby debris thrown up by the interaction of the laser beam with the workpiece prevented from falling onto the surface of the workpiece.

According to a further aspect of the invention a process for laser machining a workpiece includes the steps of locating a hood or shroud means over a machining location on a workpiece to form thereby an enclosed work chamber, admitting a laser beam into the work chamber in the direction of the machining location, and directing a stream of assist gas into the work chamber towards the machining location on the workpiece in a direction whereby debris thrown up by the interaction of the laser beam with the workpiece is prevented from falling onto the surface of the workpiece.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a diagrammatic sketch of a perspective view of a basic form of an embodiment of the invention, and Figures 2 and 3 are sketches of perspective views of developments of the invention.

In the drawings like parts carry like reference numerals. Referring firstly to the embodiment of Figure 1, there is shown a portion of a metal article 10 which is to be laser machined, for example drille. It will be appreciated that the exact nature of the machining operation is not of the essence of the invention. The point of initial impingement of the laser beam is denoted by a cross 12. Laser nozzle 14 is located over the drilling point 12, in accordance with normal operation adopted for such operations, the nozzle is positioned immediately above point 12. A laser beam 16, generated by a laser 18, passes axially through the nozzle 14 towards the impingement or drilling point 12. The laser 18 is of a kind, such as a Nd-Yag commonly used for industrial applications.

A jet of assist gas 20 flows through the laser nozzle 14 coaxially with the laser beam 16 to impinge on the workpiece 10 at the drilling point 12 and the area immediately surrounding it. The design and use a such a nozzle utilising a c-axial assist gas jet stream 20 and the manner in which the gas is introduced into the nozzle from an assist gas source 22 is well known in the art.

There is provided a metal hood or shroud 24 is arranged and adapted to surround the laser nozzle 14 and the drilling point 12, effectively enclosing both in a work chamber in isolation from ambient atmosphere. The hood or shroud means 24 is in the shape of a hollow truncated cone 13 mounted on a hollow cylindrical base or annular portion 15.

The shroud is located over, or offered up to, the workpiece so that the lip or rim 17 of the annular portion 15 rests on the surface of the workpiece 10 to form a work chamber. The apex of the conical portion 13 is arranged to admit the laser beam into the machining chamber through an aperture or window in the wall of the shroud 24. In the embodiment illustrated the aperture surrounds the nozzle 14 and is spaced therefrom to leave an annular gap 8 surrounding the nozzle. This gap 8 acts as a means effective for exhausting assist gas from the interior of the work chamber, or shroud 24.

During the laser machining process, any molten debris thrown up by the impingement of the laser beam 16 on the article 10 will strike the inside of the shroud 24 and will solidify there, and so will be prevented from dropping onto the metal surface of the article. The continuously moving stream of assist gas will have a tendency to rapidly cool spatter particles thereby helping to prevent them adhering to the surface of the workpiece. Excess assist gas, and any gaseous products or fumes from the laser machining operation will exit through the radial gap 8 surrounding the nozzle 14.

Without the hood or shroud means 24 the assist gas may tend to dissipate too rapidly to be effective as either a transport medium or to chill spatter particles.

In the second embodiment of Figure 2, the base portion 15 of the shroud 24 is provided with a circumferentially extending conduit in the form of a circular tube or ring 26 running around its rim 17. The tube or ring 26 is provided with an inlet 28 by means of which it may be connected to an external source whereby a further stream 29 of assist gas can be passed into the tube under pressure from an assist gas supply 30. The ring 26 is provided with a series of exit holes 32 facing radially inwards into the interior of the shroud 24. Hence, a further or secondary stream of assist gas is directed radially into the space within the shroud 24 through the holes 32 at or near the surface of the article being laser drille. Since the lip 17 of the shroud, in use, abuts the workpiece this secondary supply of assist gas is directed at the machining location across the face of the workpiece.

The third embodiment illustrated by Figure 3, includes a modification to the embodiment of Figure 2 whereby an exit nozzle 34 and further exhaust aperture is provided through the shroud 24, through which gases are extracted by an extraction fan or pump unit indicated at 36. The extraction point of exit nozzle 34 is chosen to be above, in the orientation normally employed, the levels of both the radial assist gas supply holes 32 and the assist gas nozzle supply 14, in order to providing optimal gas flow through the work chamber within the shroud 24.

The extraction rate of assist gas is at, or near, its total input rate, so that gas pressure inside the shroud 24 is close to atmospheric pressure. It will be understood that pressure and flow rate adjustments of the assist gas may be necessary to accommodate localise heating and expansion of the gas. In this way, the assist gas forms a self-contained circulating system around the drilling point 12. Spatter debris is drawn away from the surface along with metal vapours which tend to collect above the surface of the article 10 being machined.

Thus, some spatter is physically prevented from failing onto the surface of the article being drilled and may is drawn away from the work surface. If the assist gas is oxygen, oxidation of the material is enhanced, increasing the temperature of the melt pool by exothermic reaction. The flow of gas across the surface of the workpiece 10 in the vicinity of the hole being drilled tends to draw out molten material from the hole due to a Bernoulli effect, enhancing removal of material. Metal vapour is drawn away from the surface of the article, allowing unobstructed access of the laser beam to the surface of the article.

It will also be understood that, in the embodiment of Figure 3, it may be appropriate in some circumstances to close off the radial gap 8 between the shroud 24 and the nozzle 14 or to do without it completely.

Therefore, in accordance with the invention it is implicit that there is provided a novel process for laser machining a workpiece which includes the steps of locating a hood or shroud means, of the kind described above, over a machining location on a workpiece to form thereby an enclosed work chamber, admitting a laser beam into the work chamber in the direction of the machining location, and directing a stream of assist gas into the work chamber towards the machining location on the workpiece in a direction whereby debris thrown up by the interaction of the laser beam with the workpiece is prevented from falling onto the surface of the workpiece.

The process may include, in addition to a primary stream of assist gas directed into the work chamber co-axially with the laser nozzle, a secondary stream of assist gas directed towards the laser machining location from adjacent the rim of the hood or shroud means. The assist gas is preferably exhausted form the work chamber through an aperture in the wall of the shroud or hood means and may be positively drawn off by means of an extractor unit, fan or pump.

A hood or shroud means as described above may be constructed as a separate assembly and arranged and adapted to be used in association with an existing laser machining facility. Thus it may be adapted to be located over or offered up to a workpiece clamped in position on the bed of an existing laser welding machine so as to enclose the machining location within a work chamber and to receive an existing laser nozzle.

Claims

CLAIMS 1 Apparatus for laser machining a metal workpiece including a laser, a nozzle for directing a laser beam towards a machining location on the workpiece, hood or shroud means arranged and adapted to be located over or offered up to the workpiece so as to enclose the machining location within a work chamber and which has means for admitting the laser beam into the work chamber to perform the machining operation, and means for directing a stream of assist gas into the enclosed work chamber and towards the machining location on the workpiece, whereby debris thrown up by the interaction of the laser beam with the workpiece is trapped inside the shroud means and prevented from falling onto the surface of the workpiece.
2 An apparatus as claimed in claim 1 wherein the hood or shroud means is adapted to admit the laser beam into the machining chamber through an aperture or window in a wall of the hood or shroud means.
3 An apparatus as claimed in claim 1 or 2 wherein the hood or shroud means is provided with means for directing assist gas inside the work chamber towards the machining location from adjacent the rim of the machining chamber.
4 An apparatus as claimed in any preceding claim wherein the laser nozzle includes a primary assist gas supply, in use, co-axial with the laser beam, and the shroud or hood means is provided with a secondary assist supply means for directing the assist gas inside the work chamber towards the machining location.
5 An apparatus as claimed in claim 4 wherein the secondary assist gas supply means comprises conduit means extending around the interior of a lip of the hood or shroud means which, in use, abuts the workpiece, and means is also provided for connecting the conduit means to a source of assist gas.
6 An apparatus as claimed in claim 5 wherein the conduit means is formed with a series of holes directed inwards towards the workpiece machining location in the interior of the work chamber.
7 An apparatus as claimed in any preceding claim wherein the hood or shroud means is further provided with means for exhausting assist gas from the interior of the work chamber.
8 An apparatus as claimed in claim 7 wherein the means for exhausting assist gas is located, in normal operation, at a level above the secondary assist gas supply.
9 An apparatus as claimed in claim 7 wherein the means for exhausting assist gas comprises an exit aperture located in the vicinity of the laser nozzle.
10 An apparatus as claimed in claim 9 wherein the laser nozzle protrudes into the work chamber through an aperture in the wall of the shroud or hood means and the circumference of the aperture is spaced apart from the nozzle so as to introduce a gap constituting the exit aperture.
11 An apparatus for laser machining a workpiece, substantially as hereinbefore described with reference to the accompanying drawings.
12 A process for laser machining a workpiece includes the steps of locating a hood or shroud means over a machining location on a workpiece to form thereby an enclosed work chamber, admitting a laser beam into the work chamber in the direction of the machining location, and directing a stream of assist gas into the work chamber towards the machining location on the workpiece in a direction whereby debris thrown up by the interaction of the laser beam with the workpiece is prevented from falling onto the surface of the workpiece.
13 A process for laser machining a workpiece as claimed in claim 12 wherein a primary stream of assist gas is directed into the work chamber co-axially with the laser nozzle, and a secondary stream of assist gas is directed towards the laser machining location from adjacent the rim of the machining chamber.
14 A process for laser machining a workpiece as claimed in claim 12 or claim 13 wherein the assist gas is exhausted form the work chamber through an aperture in the wall of the shroud or hood means.
15 A process for laser machining a workpiece, substantially as hereinbefore described with reference to the accompanying drawings.
16 A hood or shroud means arranged and adapted to be located over or offered up to a workpiece so as to enclose the machining location within a work chamber including means for admitting the laser beam into the work chamber to perform the machining operation, and means for directing a stream of assist gas into the enclosed work chamber towards the machining location on the workpiece, whereby debris thrown up by the interaction of the laser beam with the workpiece prevented from falling onto the surface of the workpiece.
17 A hood or shroud means as claimed in claim 16 further comprising an aperture or window in a wall of the hood or shroud means for receiving the laser nozzle whereby to admit the laser beam into the work chamber.
18 A hood or shroud means as claimed in claim 17 wherein the aperture for receiving the laser nozzle is oversize whereby to form around the nozzle an exit aperture for exhausting assist gas from the work chamber.
19 A hood or shroud means as claimed in any of claims 16 to 18 wherein the means for directing a stream of assist gas into the enclosed work chamber towards the machining location on the workpiece includes a secondary assist gas supply means adjacent the rim of the hood or shroud means.
20 A hood or shroud means as claimed in claim 19 wherein the secondary assist gas supply means comprises conduit means extending around the rim of the hood or shroud means which, in use, abuts the workpiece, and includes means for connecting the conduit means to a source of assist gas.
21 A hood or shroud means as claimed in claim 20 wherein the conduit means is formed with a series of holes directed inwards towards the workpiece machining location in the interior of the work chamber.
22 A hood or shroud means as claimed in any of claims 16 to 21 wherein the hood or shroud means is further provided with means for exhausting assist gas from the interior of the work chamber.
23 A hood or shroud means as claimed in claim 22 wherein the means for exhausting assist gas is positioned in a wall of the hood so that, in normal operation, it is located above the secondary assist gas supply.
24 A hood or shroud means for use in laser machining a workpiece, substantially as hereinbefore described with reference to the accompanying drawings.