CA2317926C - Process and device for laser treatments of inside surfaces - Google Patents

Abstract

A process and device for the laser treatment of the internal surfaces of hollow bodies such as engine blocks with cylinder bores which consist of the matrix alloys such as a light-metal or aluminum. wherein a wear-resistant coating is applied to the working surfaces of the pistons within the cylinder. A probe enters the cylinder and includes the laser beam which produces a beam spot on the internal surface of the cylinder and applies an alloying powder at the focus of the spot using a continuously conveying protective gas. The probe is displaced along the axis of the cylinder and simultaneously rotated while the alloying powder is melted in the spot of the laser beam to a depth sufficient to form a wear-resistant coating which will not break loose during the normal operation and life of the engine block. The engine block can be fixed in a stationary position while the probe or probes move simultaneously within the cylinders to apply the hardened surface coating.

Description

BACKGROUND OF THE INVENTION
The invention relates to a process for the laser treatment of the inside surfaces of hollow pluralities with rotation-symmetric axes. which consist of a matrix alloy.
More specifically. this invention relates to the laser treatment of light metal engine cylinder blocks to provide a wear-resistant inner piston working surfaces. Furthermore. the invention relates to a device for carrying out such a process. --It is known. for example from German Patent DE 3910098 A1 to weld pipes by means of a laser. whereby a rod-shaped probe with a lens system is disposed in a workpiece. In laser surface treatments. it is advantageous to also treat marginal surface coatings. especially on the inside of the workpiece, by the application of laser energy over wide surface areas.
Thus. it is an object of the invention to obtain increased amounts of alloying components in marginal coatings. as well as intermetallic compounds in these coatings, and to obtain a finer grain coating in the marginal layer. particularly in connection with cast aluminum or wrought alloys.

It is a further object of the invention to provide an increased resistance to abrasion in metallic materials. such as in engine block components and also in tools subject to wear. as well as in tubes and guide bushings.
In the prior art. wear-resistant workpieces were produced by alloying the entire material of the workpiece with an additional substance. such as, adding up to 17i silicon to an aluminum cast alloy.
Where the surface is to be provided with higher wear resistance. the silicon coating provides a wear hardened surface. However. this makes the overall workpiece more brittle. which leads to substantial problems in the casting process.
SUMMARY OF THE INVENTION
Therefore. the present invention provides a treatment on workpieces for incorporating alloying components at a later time, that make a surface more resistant to wear. as well as providing a device for carrying out such a process.
The invention provides a process for alloying is substances. wherein the marginal layer is influenced by mixing the alloying components in the marginal zone by 1001. T'he process is preferably carried out using pure silicon powder for the alloying process with aluminum. With depths of penetration of from 0.2 mm to 2 mm. the separated silicon particles. with particle sizes of from 3 to 25 Vin. make up a proportion by volume of from 17 to 50i in the marginal layer. This provides an increased resistance to wear of the aluminum matrix alloy.
It is advantageous if the laser light is directed to the surface with a distal energy of 20 to 800 Jan, and an intensity of 0.5 to 4.0 kWimmZ. The laser light has to be shaped so that a 1 tophatl distribution takes place.
Laser light wattages of 1..3 kW to 3 kW have been successfully employed in the past. whereby both Nd-YAG lasers and high-power diode lasers were employed. Laser light with wavelengths of 1064. 808 and 940 nm is advantageous for mixing up.in the marginal zone. whereby the respective process parameters have to be coordinated with each other.
This particularly applies to the feed rate to be adjusted. which may be in the range of 300 to 4000 mmimin. The feed rate is. in each case, dependent upon the intensity. i.e.. the wattage of the laser light, the way is which the focus is shaped, and on the thermal conditions at the melting site. These conditions are determined by the heat source, the volume of the melt. and the cooling rate. The process parameters have to be coordinated with each other so that the process of separation of hard-substance particles takes place with the desired concentration and particle size.
Thus. by employing a laser light beam which. at the site of impact.
produces a material plasma. it is possible to incorporate powder in the locally produced plasma. By introducing an alloying powder with a grain size of preferably 45 a to 150 u. it is possible to obtain with a laser light beam of 2 kW, a penetration depth of 1 mm over a spot diameter of .05 to 2 mm. This beam produces a sufficiently thick wear-resistant marginal layer. so that the latter will not detach itself from the workpiece under mechanical stress. At the same time, the selected conveying gas, for example a rare (or inert) gas, assures a separation of the plasma from reactive, oxygen-containing atmosphere. The wattage of the laser light is controlled so as to obtain a suitable distribution of primary and secondary hardened phases.
A silicon component of between 17i to 50i by volume so incorporated in the surface, advantageously assures that the elastic properties continue to be present as before in the depth of the workpiece. These properties permit absorption of the mechanical stresses. such as. for example, the working surfaces of the cylinders of an engine.
Considerable thermal problems have to be solved when an energy beam probe is disposed in a cylindrical segment of a workpiece. Apart from the heat reflected by the workpiece. the geometrical conditions lead to considerable heat at the head of the probe. which. according to the invention, is counteracted by water cooling.
Furthermore. the probe head of the invention is rotatable. so that apart from prior art equipment. where the workpiece has to be turned. the inventive probe can now remain stationary. This leads to an easier handling of the engine blocks that have to be treated because the laser light and alloying powder can be supplied in their conveying media to the probe head via a rotating passage.
Moreover. a pore-free result is possible through a suitable feed of the powder and gas when treating interior spaces. In this connection. it is necessary to make sure that with the proper guidance of the protective gas and design measures. that the applied powder will not deposit on the optics of the probe. If need be. a separate protective gas can be fed to the probe nest to the beam of energy in addition to the conveying gas.
The probe is capable of treating the workpiece provided a few conditions are met. First. there should be a laser light wattage of approximately 2 kW on a beam spot having a diameter of about 0.5 to 2 mm, a feed rate of 300 to 1500 mm per minute. and a gas conveying rate of about 10 to 20 liters per minute. In addition, a powder feed of up to 10 grams per minute has to be added. For example. if the probe is focused on a surface site defined by spiral migration. then upon completion of one turn (or rotation) of the probe. the surface site being treated may migrate over the entire surface area to be treated by simultaneously lowering the probe. Thus, from 17i to 50i by volume of silicon is produced on the surface layer through alloying. whereby excess silicon powder is removed from the interior of the workpiece by transporting the excess powder away. especially together with the conveying gas.
The probe can then be focused in the space provided on a spirally migrating surface site. This occurs with an increased laser light wattage of 4 kW focused on a beam spot with a diameter of between 2 to 4 mm, and with a feed rate of between 1500 to 4000 mm per minute. and having a conveying gas feed of about 30 liters per minute. and wherein a powder feed rate of up to 20 grams per minute is added.
The invention is primarily suitable for treating internal spaces.
for example. cylinders and tubes having a shaft ratio in excess of 1 = 10 (measured as the diameter to depth ratio), whereby the rod-shaped probe can be employed in a particularly favorable manner starting with diameters of 50 mm and more. Both the feed means for feeding the silicon powder in a conveying and protecting gas. and means for guiding the laser beam are arranged in the probe. The means for guiding the laser beam projects a beam of laser light via a collimating lens system onto a reversing mirror formed in the head of the probe. The focus can be formed with the reversing mirror, the latter being made of copper and cooled with water due to the high process temperatures. Furthermore, the mirror is preferably provided with an HR-coating. The mirror is a multifaceted mirror having from 5 to 50 facets with sonically. thyroidal or parabolically shaped surfaces. which are arranged on conical segments.
This device is suitable particularly for carrying out surface treatments of the inner space of light metal engine cylinder blocks.
Here, the probe providing the means for guiding the laser beam and for feeding silicon powder in a conveying and protecting gas can be lowered into the engine cylinders without rotating the block, a step that was formally necessary using prior art equipment.

Hy providing a turning drive in the probe in the end segment of the powder-feeding nozzle. and providing a device for projecting the beam of energy and with the help of a telescopic drive which lowers the probe's end segment, a multitude of cylinders can be treated simultaneously by providing a plurality of probes for the cylinders.
In this connection. the rod-shaped probes of the invention provide a variable lens system. collimating a parallel beam of laser light over an extended distance. the lens system being fonaed upstream of a reversing mirror formed in the head of the probe. Moreover, there is provided a rotationally-decoupled through-passage for three process media-conducting means. for a cooling water feed,- and return lines as well as for gas conveying the powder, the through-passage being disposed in the extended, freely rotatable head of the probe in the marginal zone.
Finally, in the zone of the laser light outlet. there is provided a crossjet outlet disposed above the laser beam outlet. the crossjet outlet being connected with the protective gas feed line. and being directed dowawardly. It is possible to select suitable geometry for the powder feed nozzle. depending upon the desired depth of penetration. and rate of progression. by simply changing the exchangeable end components.
The crossjet provided within the zone of the laser outlet is equipped with protective gas feed and cooling. With the small space available for the treatment. flow conditions exist in the interior spaces that greatly effect the melting of the marginal layer. and the powder w CA 02317926 2000-08-31 feed. The crossjet is required as defined by the invention. in order to obtain a pore-free marginal layer.
It is possible with the laser optics. to produce a depth of focus of at least 11100th of the work space. This describes the tolerance width of the tool in that additional alloying components can be alloyed therein.
without readjusting the treatment distance. It is possible to produce both traces disposed next to one another with overlapping zones. and individually displaced traces andior spiral-shaped traces resulting in higher economy. If the zones of added alloying are produced by the laser overlap, the cylinder working surface is fully covered. Partial covering is achieved with displaced or spiral-shaped laser guidance. which provides the same good protection against wear as full coverage. if the piston with piston rings moving in the cylinder are wider than the spacing between the individual traces produced by the laser.
Furthermore. it is possible to obtain full coverage on the cylinder wall working surface at the point of reversal of the piston and a partial coverage on the remainder of the working surface. thus providing a combination of the both types of coverage. Moreover, differently shaped laser traces such as. for example, meander or net-shaped structures are possible with the device of the invention. With partial coverage.
depressions can be formed to create lubricating oil pockets in a targeted manner. to allow the lubricant to collect.
Moreover. with the device of invention. the probe can be very closely applied to the workpiece with a distance of only 70 mm. which is i .."~ni. ~ ~. ii i~,~"..r.. , substantially less than the known distance of conventional powder-feeding devices, which is usually greater than 80 mm.
Also, it is possible to eliminate a heat treatment step that would otherwise be required.
In addition to the laser light, a cooling medium (e.g. H20) and, if necessary, an additional protective gas for the cross jet - which gas may be air - are admitted to the probe head in addition to the process gases carrying the powder. This medium and additional gas are fed into the continuously revolving probe head via its rotary guidance system.
This is accomplished according to the invention by means of a rotationally-decoupled through-passage for the feed and return of cooling water. Within this zone, the laser beam is freely extending.
Thus, in a broad aspect the invention provides a process of laser-treating an inner face of light metal engine blocks consisting of a matrix alloy and having wear-resistant inner piston running faces, wherein, with the engine block being held in a fixed position, a beam spot is generated by a laser light beam on the inner face on to which alloying powder is applied in a continuous conveying and protective gas flow, characterised in that in the point of impact, the laser beam produces a plasma from the matrix alloy, in the course of which process use is made of at least one rotatable probe which, while being rotated, is simultaneously displaced along the cylinder axis while an alloying powder is being added; that in a rotatable probe head, four process medium guiding lines are used to supply cooling water for at least one cooling water supply and return pipe, protective gas for a cross jet and a powder guiding conveying and protective gas; that in the conveying ~ ~i ~ . ~~iwl~ ~ ~ II I~ri-i ~a, gas, via rotary guiding means, the probe head is supplied with alloying powder which is continuously applied to the inner face, and that the alloying powder is melted in the beam spot and alloyed in at a depth of 0.2 to 2 mm.
In another aspect, the invention provides a device for laser-treating an inner face of light metal engine blocks consisting of a matrix alloy and having wear-resistant inner piston running faces, wherein, with the engine block being held in a fixed position, a beam spot is generated by a laser light beam on the inner face on to which alloying powder is applied in a continuous conveying and protective gas flow, comprising: a bar-shaped probe in which there are arranged laser beam guiding means which project a laser light beam via a collimating lens system on to a deflecting mirror provided in the probe head, an axial movement device of the probe, characterised by supplying means for alloying powder in a conveying and protective gas, which supplying means, together with the laser beam guiding means, are arranged in the probe; a rotatable probe head by means of which cooling water for at least one cooling water supply and return pipe, protective gas for a cross jet and powder guiding conveying and protective gas are supplied in 4 process medium guiding lines; a rotary drive arranged at the probe for the portion of a powder ejection nozzle and a laser beam emission device arranged in the region of the reflection angle of the deflecting mirror; a telescopic drive for the portion of a powder ejection nozzle and for the laser beam emission device.
BRIEF DESCRITPION OF THE DRAWINGS
Other objects, and features will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose the 9a ~i .~linl~ ~ iII I~ri-I iii I

embodiments of the invention. It should be understood however, that the drawings are designed as an illustration not as the definition of the limits of the invention.
In the drawings wherein similar reference characters denote similar elements throughout several views:
9b FIG. 1 shows a schematic cross sectional view showing the simplest foxin of a probe in operation;
FIG. 2 shows an enlarged cross sectional view of the lower part of FIG. 1;
FIG. 3a shows the details of the reversing mirror in operation, FIG. 3b shows the details of the reversing mirror in operation;
FIG. 4 shows four different working surfaces of a cylinder treated with the inventive process; and FIG. 5 shows a 4 cylinder engine and four probes.
DETAILED DESCRIP?ION OF THE INVENTION
Fig. 1 schematically shows the device of the invention for the laser treatment of an inside surface. The device has a rod-shaped probe 1, a laser beam guide 2 for a laser light beam 3. n collimating lens system 4, and a reversing mirror S. The latter is arranged in a rotatable probe head 6, which accommodates conduits 7 and 8 for guiding a protective gas.
The protective gas entering conduit 7 is reversed in the lower part of probe head 6. and exits via conduit 8. In this process. the gas intersects laser light beam 3, reflected by reversing mirror S. and forms a crossjet 9. Due to the high energy of the powdered material in the site of impact on workpiece surface 10. a portion of the powdered w CA 02317926 2000-08-31 material is thrown back. Crossjet 9 prevents that portion of the powder material from dropping back onto reversing mirror 5.
Laser beam 3 is directed to the internal cylinder of the engine block 11 having a surface 10. Probe 1 can be rotated along a vertical axis 12 by motor 13 and driving 14. Probe 1 can be further moved into the vertical direction by a guide member 15a. 15b of traverse 16a forming a telescopic drive 16b.
FIG. 2 is an enlarged cross-sectional view of the lower part of FIG.
1. It shows the laser beam 3 being reflected by mirror 5 onto the surface 10 of the internal cylinder of the engine block 11. A beam spot is produced by the laser beam 3 on the internal cylinder surface of the engine block 11. Since the rotation of the probe 1 and the displacement of probe 1 along the cylinder axis will be superimposed, a spiral-shaped windiag 17, 18 . 19. and 20 will be formed on the surface of the cylinder block.
A powder stream is led to the beam spot via pressure line 9a which has a passage onto the probe 1 and connection 7c at the top FIG. 1. Line 7b ends in a powder feeding nozzle 9b.
From FIG. 2 can also be seen that the protective gas flowing through conduit 7 will cross the reflected laser beam 3 when the laser beam 3 reaches an opening 21 in the probe 1. Since the protective gas is under pressure it prevents powder particles from being reflected from the surface 10 from passing through opening 21 and thereby preventing a w CA 02317926 2000-08-31 protecting glass 21a from being contaminated and hereby protecting the mirror 5.
Particles which are reflected from the surface will be carried within the protective gas away from the alloying zone 22. The derivation of the reflected particles stream is schematically demonstrated by arrows 23 in FIG. 2.
FIGS. 3a and 3b show details of reversing mirror 5 in operation.
When reflected at the thyroidal or parabolically shaped facettes 5a the laser beam 3 is concentrated to a line shaped spot 3a. By movement of the line-shaped laser beam a spiral-shaped winding will be produced on the entire surface of the cylinder if the mirror 5 is rotated together with probe 1 along the cylinder axis.
FIG. 9 schematically shows four different working surfaces of a cylinder treated with the inventive process. In all four examples 4a. 4b 4c. 4d the top zone a is 1001 alloying area being produced by an overlapping layer of spiral windiags. This can be continued for the whole area of internal cylinder shown in FIG. 4a.
To reduce the production time. it is preferable to enlarge the distances H between the spiral shaped winding 25. 26 shown in FIG. 4b and FIG. 4c. FIG. 4d shows a net-shaped structure 27 of the windings creating depressions for a series of lubricating oil pockets 28. The net-shaped structure can be produced by shifting the depositing device from an upward to a downward movement.

w CA 02317926 2000-08-31 FIG. 5 shows a 4 cylinder engine and four probes 30. 31, 32, 33, being simultaneously disposed into bores 34, 35, 36, and 37 of each cylinder of the 4 cycle engine. As explained with reference to FIG. 1, the probes 30-33 are simultaneously rotated to apply the spiral treatment to each inner surface of cylinders 34- 37.
The process is preferably carried out using pure silicon powder for the alloying process with aluminum. With depths of penetration of from 0.2 mm to 2 mm, the separated silicon particles. with particle sizes of from 3 to 25 mm, make up a proportion by volume of from 17 to 50i in the marginal layer. This provides an increased resistance to wear of the aluminum matrix alloy.
It is advantageous if the laser light is directed to the surface with a distance energy of 20 to 800 J~, and an intensity of 0.5 to 4.0 kW~2. The laser light has to be shaped so that a 1 tophatl distribution takes place.
Laser light wattages of 1.3 kW to 3 kW have been successfully employed in the past. whereby both Nd-YAG lasers and high-power diode lasers were employed. Laser light with wavelengths of 1064, 808 and 940 mm are advantageous for mixing up is the marginal zone. whereby the respective process parameters have to be coordinated with each other.
This particularly applies to the feed rate to be adjusted. which may be in the range of 300 to 4000 mmimin. The feed rate is in each case, dependent upon the intensity. i.e.. the wattage of the laser light. the way in which the focus is shaped. and on the thermal conditions at the melting site. These conditions are determined hV +ho ~,o.r ~"",.,.... .L_ volume of the melt. and the cooling rate. The process parameters have to be coordinated with each other so that the process of separation of hard-substance particles takes place with the desired concentration and particle size.
Thus. by employing a laser light beam which. at the site of impact.
praduces a material plasma, it is possible to incorporate powder into the locally produced plasma. Hy introducing an alloying powder with a grain size of preferably 45 a to 150 fit, it is possible to obtain with a laser light beam of 2 kW. a penetration depth of 1 mm over a spot diameter of .05 to 2 mm. This produces a sufficiently thick wear-resistant. marginal layer that will not detach itself from the workpiece under mechanical stress. At the same time. the selected conveying gas. for example a rare (or inert) gas. assures separation of the plasma from reactive, oaygen-containing atmosphere. The wattage of the laser light is controlled so as to obtain a suitable distribution of primary and secondary hardened phases.
A silicon component of between 17i to 50i by volume incorporated in the surface. assures that the elastic properties continue to be present in the depth of the workpiece. These properties permit absorption of the mechanical stresses, such as, for example, the working surfaces of the cylinders of an engine.

With a laser light wattage of approximately 2 kW on a beam spot having a diameter of about 0.5 to 2 mm, a feed rate of 300 to 1500 mm per minute. and a gas conveying rate of about 10 to 20 liters per minute.
wherein a powder feed of up to 10 grams per minute. the probe is capable of treating the workpiece in the space allotted is a focused manner. For example. if the probe is focused on a surface site defined by spiral migration, upon completion of one turn (or rotation) of the probe, the surface site being treated may migrate over the entire surface area to be treated by simultaneously lowering the probe.
Thus, from 17i to 50i by volume of silicon is produced on the surface layer through alloying. whereby excess silicon powder is removed from the interior of the workpiece by transporting the excess powder away. especially together with the conveying gas.
With an increased laser light wattage of 4 kW focused on a beam spot with a diameter of 2 to 4 mm, and with a feed rate of 1500 to 4000 mm per minute, and having a conveying gas feed of about 30 liters per minute. and wherein a powder feed rate of up to 20 grams per minute is added. the probe can then be focused in the space provided on a spirally migrating surface site.
Laser beam wattage of between 1.3 to 6 kW can be used with spot diameters of between .2 to 4mm, and feed rates of between 300 to 4000mmimin.
The means for guiding the laser beam. projects a beam of laser light via a collimating lens system onto a reversing mirror formed in the head of the probe. T'he focus can be formed with the reversing mirror, the latter being made of copper and cooled with water due to the high process temperatures. Furthermore, the mirror is preferably provided with an HR-coating. ?he mirror is a multifaceted mirror having frown S to 50 facets with conically. thyroidal or parabolically shaped surfaces. which are arranged on conical segments.
While several embodiments of the invention have been shown and described. it is obvious that many changes may be made thereunto without departing from the spirit and scope of the invention.

Claims (14)

1. A process of laser-treating an inner face of light metal engine blocks consisting of a matrix alloy and having wear-resistant inner piston running faces, wherein, with the engine block being held in a fixed position, a beam spot is generated by a laser light beam on the inner face on to which alloying powder is applied in a continuous conveying and protective gas flow, characterised in that in the point of impact, the laser beam produces a plasma from the matrix alloy, in the course of which process use is made of at least one rotatable probe which, while being rotated, is simultaneously displaced along the cylinder axis while an alloying powder is being added;
that in a rotatable probe head, four process medium guiding lines are used to supply cooling water for at least one cooling water supply and return pipe, protective gas for a cross jet and a powder guiding conveying and protective gas;
that in the conveying gas, via rotary guiding means, the probe head is supplied with alloying powder which is continuously applied to the inner face, and that the alloying powder is melted in the beam spot and alloyed in at a depth of 0.2 to 2 mm.

2. A process according to claim 1, characterised in that the protective gas is deflected in the lower part of the probe and, in the process, crosses the laser beam in the form of a cross jet.

3. A process according to claim 1 or 2, characterised in that under the effect of the beam spot, there is produced an alloyed-on zone which, in accordance with the rotation of the probe, extends in spiral-like windings across the entire surface to be treated.

4. A process according to any one of claims 1 to 3, characterised in that the adjoining windings of the alloyed-on zone overlap one another.

5. A process according to claim 1 or 2, characterised in that the windings are spaced when being applied to the inner piston running faces, with the distance between same being smaller than the width of the piston rings.

6. A process according to claim 1 or 2, characterised in that the cylinder running face, in the point of reversal of the piston, is fully covered and in the remaining part of the running face partially covered by alloyed-in alloy particles.

7. A process according to anyone of claims 1 to 6, characterised in that with a laser light power of 1.3 - 4 kW with a beam spot diameter of approx. 0.5 to 4 mm with a feed of 300 to 4000 mm/min, with a process gas supply of 10 to 20 1/min and a powder supply of 1 to 10 g/min and, respectively, of 1500 to 4000 mm/min feed, with a process gas supply of > 30 1/min, 17 to 50 % of silicon is alloyed into the surface, with the silicon being separated in the form of primary silicon in particle sizes of 3 to 25 µm.

8. A device for laser-treating an inner face of light metal engine blocks consisting of a matrix alloy and having wear-resistant inner piston running faces, wherein, with the engine block being held in a fixed position, a beam spot is generated by a laser light beam on the inner face on to which alloying powder is applied in a continuous conveying and protective gas flow, comprising:
- a bar-shaped probe (1) in which there are arranged laser beam guiding means (2) which project a laser light beam (3) via a collimating lens system (4) on to a deflecting mirror (5) provided in the probe head, - an axial movement device of the probe (1), characterised by - supplying means for alloying powder in a conveying and protective gas, which supplying means, together with the laser beam guiding means (2), are arranged in the probe;
- a rotatable probe head by means of which cooling water for at least one cooling water supply and return pipe, protective gas for a cross jet and powder guiding conveying and protective gas are supplied in 4 process medium guiding lines;
- a rotary drive arranged at the probe for the portion of a powder ejection nozzle and a laser beam emission device arranged in the region of the reflection angle of the deflecting mirror;
- a telescopic drive for the portion of a powder ejection nozzle and for the laser beam emission device.

9. A device according to claim 8, characterised in that in the region of the laser beam exit, there is provided a cross jet which is provided with protective gas guiding means and cooling means.

10. A device according to any one of claims 8 and 9, characterised in that the deflecting mirror is provided in the form of a facetted mirror with conically, torically or parabolically shaped facettes which are arranged on conical segment faces and cooled from inside.

11. A device according to any one of claims 8 to 10, characterised in that the deflecting mirror is produced from copper and provided with a HR coating.

12. A device according to any one of claims 8 to 11, characterised in that a variable lens system permits a depth of sharpness of at least 1/100 of the operating distance, measured from the axis centre to the inner face of the hollow member, and that the variable lens system permits adaptation to different diameters and divergencies of the laser beam.

13. A device according to any one of claims 8 to 12, characterised in that the cross jet nozzle is provided with a cooling system.

14. A device according to any one of claims 8 to 13, characterised in that an excess pressure in the encapsulated probe protects the optical components from becoming dirty.