WO2004002668A1

WO2004002668A1 - A method of providing burr-free bores

Info

Publication number: WO2004002668A1
Application number: PCT/DK2003/000430
Authority: WO
Inventors: Flemming Ove Olsen
Original assignee: Danmarks Tekniske Universitet
Priority date: 2002-06-28
Filing date: 2003-06-24
Publication date: 2004-01-08
Also published as: AU2003239779A1

Abstract

By thermal-based removal of material, for instance by way of laser drilling or laser ablation, the material is removed from the article (2) in form of steam or drops. The material is removed by being subjected to a short-term intense laser pulse (4) causing a generation of considerable pressure from vaporised and/or ionised material. As a result, vaporised melt material is sprayed to the sides or upwards along the sides of a drilled hole. These squirts can either damage the surface around the processed area or result in an upward burr. The method according to the invention provides an additional local steam/plasma pressure causing the ejected material to change direction in such a manner that the surface of the material is not damaged or a stricking burr is not formed. The local steam/plasma pressure can, for instance, be provided by an intense secondary laser beam being emitted downwards and towards an area around the processing location.

Description

Title: A method of providing burr-free bores

The present invention relates to a method of providing processings of materials, such as removal of material, in particular drillings by means of a laser beam, said method further employing a secondary laser beam.

Technical Field

By thermal-based removal of material, for instance by way of laser drilling or laser ablation, the material is removed from the article in the form of steam or drops. The processings are characterised by a short heating by means of a short intense laser pulse which locally generates a significant pressure from an evaporated and/or ionised material. As a result, evaporated/molten material is either squirted to the sides in processings in which the depth of the hole is less than the width of the hole or along said sides when the depth is comparable with or exceeds the width of the hole. These squirts can damage the surface around the processed area or result in an upward burr, which obviously is undesirable especially in connection with microprocessing where a subsequent mending of the damage is practically impossible.

Background Art

DE 2713904 discloses a method of burr-free laser processing. This method employs a first focusing laser beam, and the resulting drilling burr is molten by means of a second defocusing laser beam and welded together with the material of the proc- essed material. The beams can be generated by means of two lasers or a single laser in connection with an optical system dividing the laser beam into a centrally focusing drilling laser beam and a melting beam surrounding the above drilling laser beam. The melting beam has a focal point which lies deeper in the article than the focal point of the drilling beam. It would, however, be advantageous if it were pos- sible to avoid the formation of the burr altogether. US patent specification No 5,378,582 discloses a method for modifying target surfaces through ablation by means of a number of secondary laser beams.

Brief Description of the Invention

The object of the present invention is to render it possible to completely avoid the undesired damages to the surface and the formation of burrs.

A method of the above type is according to the invention characterised in that the local steam/plasma pressure generated by the secondary beam is utilized for changing the direction of the material ejected in such a manner that the surface of the article is not damaged. In this manner, it is avoided that the molten material settles and forms burrs of the edges of a drilled hole. The formation of burrs is caused by a hardening of the molten material when said material leaves the hole. In this manner, a heating by the secondary beam can counteract the formation of burrs which in combination with the generation of steam/plasma ensures that the formation of burrs is prevented.

Brief Description of the Drawing

The invention is explained in greater details below with reference to the accompanying drawings, in which

Figs, la and lb illustrate a known laser processing method by which burrs are formed along the sides of a bore,

Fig. 2 illustrates a laser processing method according to the invention whereby a secondary laser beam is directed towards the centre line of a bore,

Fig. 3 illustrates an alternative example of the laser processing method illustrated in Fig. 2, Fig. 4 illustrates an alternative example of the laser processing method whereby the secondary laser beam is focused on the surface of an article outside the processing area, said article optionally being applied a layer of easily evaporable and ionisible material,

Figs. 5 and 6 illustrate how a secondary beam surrounding the primary beam can be obtained by means of an axicon mirror,

Fig. 7 illustrates an arrangement for providing a scanning laser beam and which comprises a flat field lens,

Fig. 8 illustrates an arrangement for providing an asymmetrically positioned plasma, and

Fig. 9 illustrates an arrangement also including a tertiary beam in order to ensure that undesired particles do not deposit at a later stage.

Best Mode for Carrying out the Invention

A local steam/plasma pressure is provided by directing an intense secondary laser beam towards an area around a processing spot of an article 2. This secondary beam 6 can be in form of one or more focusing laser beams hitting one or more spots around the processing area. The secondary beam 6 can be a pulsed beam which is caused to successively hit different locations around the processing area so as to provide a suitable plasma/steam pressure.

The secondary beam 6 may alternatively be such that it forms a focusing ring around a processing spot in form of a drilled hole or only part of a ring around said hole during a processing thereof. The formation of plasma can be intensified by a suitable gas, such as argon having a low ionisation potential, being fed to the processing area. However, the ionisation potential depends on wavelength. It is thus possible for a gas to easily transmit laser light at one wavelength while a strong plasma is formed at another preferably long- waved laser beam.

However, the formation of plasma depends also on the intensity of the light hitting the article 2. A plasma formation in gas requires a rather high intensity, whereas metallic vapours, vapours derived from fat, oil and other volatile materials only require a low intensity to ionisise and form plasma. Thus, the plasma is easily formed in metallic vapours or simply fluids, surface films and the like products which, for instance, exist on the surface of an article.

The steam and/or plasma can for instance be formed by irradiating the material re- suiting from the processing in such a manner that the secondary laser beam 6 is for instance directed towards the processing area from outside said area, such as radially towards the centre line of a hole in the article just above said hole and further such that it is focused in order to provide a high intensity at the surface of the ejected material. The position of the secondary laser beam 6 can optionally be adjusted so that both the ejected material and the surface of the article round the hole are subjected to parts of said laser beam.

According to a particularly preferred embodiment, the secondary beam 6 can be focused on the surface of the article 2 next to the processing spot, cf. Fig. 4, which has optionally been applied a layer of volatile and ionisible material, such as fluid/fat, oil or polymers. A suitable selection of laser parameters causes the formation of plasma at the location where the article 2 is subjected to the secondary laser beam 6 without involving serious damages to the surface. Alternatively, the secondary beam can be focused on a material just opposite the article 2. In this case the focusing can be carried out by the laser beam being supplied from the outside and hitting an annular spot in a component in front of the article 2 or by said laser beam being focused in such a manner that the beam path of the sec- ondary beam 6 intersects the beam path of the primary beam, cf. Fig. 3.

Alternatively, the secondary beam can produce a plasma in a gaseous medium and thus produce a pressure in a gas above the article 2 so as to remove squirts/material from said article. As mentioned previously, such a procedure requires a higher inten- sity than the formation of plasma on the surface of the article. By this method the position of the plasma cloud can be controlled by the focusing of the secondary laser beam 6 as plasma is only formed in the location(s) being subjected to the focusing of the secondary laser beam.

Two different laser sources can optionally be used for the primary beam 4, viz. the removal of material, and for the secondary beam 6, viz control of the flow of molten material, respectively. When two laser sources are used, a laser can be used as the secondary laser source, said laser producing a strong plasma. In addition, different pulse forms can be used for the two lasers. It is, however, also possible to use the same laser type or even the same laser source for both radiations.

Processings for removing materials usually aim at preventing steam and gases between the article 2 and the laser beam 4 from absorbing the laser light and forming plasma because such plasma can easily shade the laser beam 4 and thus impede the processing.

When it is desired to remove squirts/steam, it is, however, to be preferred that the absorption of the secondary laser beam 4 in these fluids is strong enough to produce a high pressure in connection with a strong generation of steam or in connection with formation of a plasma. Thus, advantage may be found in choosing two different wavelengths for the primary 4 and the secondary beam 6, respectively. It is for instance relatively easy for long- waved light from CO₂ lasers to generate plasma which is more or less transparent to light from ND-YAG lasers having a first harmonic wavelength of 1.06 μm, said CO₂ lasers emitting light at a wavelength of 10.6 μm.

Another essential process parameter for hole drilling/ablation and other laser processing techniques is the time in which the laser beam irradiates a predetermined area.

At short laser pulse rates, the heating occurs so rapidly that it only allows for a very limited heat transmission into a solid material; even though the irradiated surface is heated to plasma temperature. Short intense pulse rates are therefore primarily used for processing, in particular for precision processing.

The various laser types present different characteristics; some may emit laser pulses in the area 0.01 mseconds for continuous light, others in the μsec.-area and then others in the nano-area and in the picosecond and the femtosecond area.

However, generally it applies that the shorter the pulse rates, the more precise the processing and the longer the processing time.

Lasers emitting short or long pulse strings can also be used. However, for some types of lasers, the pulse rate intervals are critical for the result of the processing. When the interval is too short, the material reacts more or less to the pulse train as one long pulse. The plasma cloud does not leave a hole until the next pulse hits. The use of two laser sources renders it possible to generate a time delay between the emissions of the two lasers in such a manner that the formation of plasma is optimised in relation to the primary laser pulse. The optimum moment in time can vary during the processing of a pulse batch. For instance, molten material resulting from a laser drilling flows out later and later in relation to the primary pulse rates as a hole grows deeper and deeper. The optimum moment in time for the formation of plasma is therefore later and later in the process. It is also possible to select types of pulses for the secondary laser which provides a maximum plasma pressure one the one hand and on the other hand minimizes or completely excludes a removal of ma- terial from the secondary radiation. It is, for instance, possible to use short pulse rates and pulse trains with short pulse rates, respectively.

The primary laser 4 and the secondary laser 6 can be of the same type, e.g. ND- YAG. However, when the primary beam is emitted by an electronically pulsed laser, then the secondary beam is emitted by a Q-switched laser emitting very short and very intense pulses.

A number of embodiments are described below.

One simple solution is to use the same laser source for both the secondary and the primary radiation because then the laser beam is divided into suitable proportions by means of a beam divider, cf. Fig. 5. Then a standard focusing of the primary beam is used while the secondary beam is transformed into a beam of an annular a circular cross section via a suitable optical element, such as a mirror or an axicon 8. This an- nular beam 6 is directed towards the article 2 parallel to the axis of the primary beam. Just above the surface of the article, the annular beam 6 hits an annular focusing mirror 10 which frustoconically focuses the beam 6 downwards towards the processing location with the result that the secondary circular beam 6 hits the processing location from the side. By using separate laser sources for the primary radiation and the secondary radiation, respectively, cf. Fig. 6, it is obviously possible to optimise the individual laser source for removing material and removing squirts/steam, respectively.

Instead of using an annular beam as a secondary beam, it is possible to use a laser beam being round the processing location.

In other applications, such as laser caving, viz. depth engraving or lasersintering, i.e. the structuring of components by welding thin layers of powder thereon, squirt prob- lems can also arise. In these cases, it is possible to scan the primary laser beam across the article at a high speed where obviously the secondary laser beam must to follow.

A scanning beam can be provided in several ways^'.

Fig. 7 illustrates a primary laser 4 emitting a beam via galvanometrically controlled mirrors 14, 15. This beam is directed downwards towards a flat field lens 17, which focuses the beam on the article 2, the position of the focused spot being controlled by the two mirrors 14, 15. Furthermore, a secondary laser beam 6 is shown which is directed inwards parallel to the axis of the path of the primary beam 4 via an optical system comprising galvanometric mirrors and a beam expander 20 with axicon lenses, a focusing lens and a mirror 22 with an aperture yielding room for the primary beam 4.

The axicon expander 20 allows the secondary beam to be transformed into an annular beam. In connection with a pre-focusing lens, a ring of focused secondary radiation is thereby generated at a suitable distance from the primary beam 4 and with a focal point adjusted inwards in such a manner that the secondary beam 6 forms a plasma either on or immediately opposite the article. A suitable control of the two galvanometric mirrors in front of the axicon expander 20 renders it possible to control the distribution of the secondary beam 6 round the primary beam 4 in such a manner that either a symmetrical or an asymmetrical secondary radiation can be obtained.

Furthermore, it is possible to produce a scanning of the secondary beam 6 on the axicon expander 20 in such a manner that the entire beam is concentrated in a spot next to the primary beam 4 at a predetermined moment. A quick scanning of the secondary beam 6 round the axicon expander 20 causes the focusing location to fol- low an annular path round the primary beam. Due to the slow dynamics during the formation of plasma, it is thus possible to provide a plasma ring round the processing spot, said plasma ring following the movements of the primary beam.

It is also possible to change this scanning so as to cover a large or small portion of the circumference round the processing spot where the scanning is controlled by a computer controlling the entire processing assembly in such a manner that protective plasma is, for instance, formed on one side only of a laser beam during a processmg.

The above structuring of the assembly has the limitation that the primary and the secondary laser beams use the same flat field lens 17 which limits the laser selecting options as it is difficult to find lens materials capable of transmitting all wavelengths. Even though an ND-YAG laser is preferred for primary radiation and a C0₂ laser is preferred for secondary radiation due to the plasma formation, such lasers imply that the laser beams can be transmitted only with difficulty through the same lens material . It is possible to establish such a system with two laser beams being transmitted through different lenses where the lens includes a central lens transmitting the primary beam and an annular lens positioned round the first lens and focusing the secondary radiation. A suitable dimensioning as described later renders it possible to di- rect the secondary beam outwards so as to move round the primary beam at such a suitable distance that it always passes through the outer annular-focusing lens, irrespective of the oscillation.

However, adjustment of the pulse types and the laser effect of the primary and sec- ondary radiation, respectively, renders it possible to use the same laser type, e.g. such as an ND-YAG laser.

Alternatively, it is possible to use a processing assembly with separate scanning systems for the primary and the secondary beam, respectively, cf. Fig. 8. This solution allows the use of a primary focusing system and one or more secondary scanning systems 6, each system covering a portion of the circumference round the processing spot.

Where the oscillation areas for the primary beam 4 are small, viz. where the primary processing area is covered by a successive sequential processing, i.e. by an oscillation of the primary beam across a limited area followed by a slow and relative movement between the article and the processing system and then followed by a new processing sequence involving an oscillation of the primary beam; it is possible to provide solutions resembling the processing systems with an annular focusing system shown in Figs. 5 and 6.

It is possible to choose between

- the secondary optical system being stationary and arranged in such a man- ner that the secondary beam can hit ejected material across the entire processing area; or - elements being present in the beam path of the secondary beam, said elements deflecting the secondary beam in such a manner that it is moved around on the annular focusing mirror with the result that the material ejected is only irradiated on one side or asymmetrically.

Alternatively, it is possible to produce an asymmetric pressure formation, viz. plasma formation, by means of the secondary laser beam in combination with an obliquely positioned nozzle emitting a gas beam downwards towards the article and blowing steam and squirts over the plasma cloud.

A further measure of preventing particles from redepositing on the surface of the article is to use a laser beam referred to as a tertiary beam. This tertiary laser beam is focused in a suitable way, for instance, by means^'of a cylindrical lens 25 for obtain- ing a linear focusing which provides a powerful intensity at a level above the article 2. By sweeping this linearly focused beam in such a manner that it moves upwards from the article, it is possible to push the flying particles upwards so as subsequently to be sucked away by an exhaustion system , cf. fig. 9.

It is furthermore possible to combine the above measures so as to have, for instance, both a primary, a secondary and a tertiary laser radiation. The primary radiation carries out a processing, the secondary radiation removes particles and squirts from the surface of the article and the tertiary radiation lifts the removed particles from the article whereby they can be removed by a nozzle.

It should be mentioned that the method can also be used for removing burrs from the back of an article, for instance from the location where a laser beam breaks through the article at the drilling of a hole.

In principle, the method can be used at every laser processing process, including hole drilling, laser engraving, resistance frimming at the manufacture of integrated circuits in the form of chips, laser caving or depth engraving, and lasersintering.

A system for these purposes includes a laser source, viz. a primary laser, being deflected by means of mirrors, which are controlled by galvanometric engines capable of providing sudden angle changes of the beam at one level. These mirrors deflect the laser beam towards a so-called flat field lens which focuses the laser beam to one level, regardless of where it hits the lens and regardless of the entrance angle. The angle changes of the incident beam are thereby transformed into position changes at the subjacent focusing level.

The galvanometric engines are controlled by a computer which can provide fast angle changes at two levels by combining the oscillation of the two mirrors with the result that the beam can be scanned quickly across the surface of the article and, for instance, print signs, such as letters, carry out removals of material by way of resistance welding, make depth engravings because a relative relocation between the op- tical system and the article must occur at regular intervals so that the focusing level of the laser beam is moved downwards into the article concurrently with the penetration of the depth engraving, or said laser beam can carry out successive structurings of an article by welding.

This system can be combined with a secondary beam system to ensure that a secondary laser beam follows the primary beam during a processing and produces a suitable plasma/steam pressure round the processing spot of the primary beam. The secondary beam system comprises a secondary laser beam, which is caused to run substantially parallel to the axis of the primary beam through the entire path of the primary beam. This is achieved by the secondary beam initially being directed by a focusing system towards a mirror with a central aperture for the primary beam. Subsequently, the latter mirror directs the secondary beam so as to run substantially parallel to the axis of the primary beam. The pre-focusing system directs the secondary beam in such a manner that it either forms a ring round the primary beam or is focused next to the primary beam, the focusing spot being moved rapidly round the primary beam.

If the two beams had the same wavelength and were completely axis-parallel, they would be focused in the exact same spot. Since the secondary beam must hit the article exactly next to the primary beam, it is therefore necessary to introduce a small angle difference to cause a slight diversion of the centrelines of the two beams. The angle difference between the two beams is then transformed into a distance between the primary and the secondary beam on the article.

The use of a secondary laser with very short pulse rates and a high repetition frequency enables the secondary laser beam to rapidly emit a number of pulse rates round the primary laser beam by scanning said beam by means of the galvanometri- cally suspended mirrors optionally via a focusing lens and by directing the beam into an axicon telescope including two transmitting axicons.

When hit slightly next to the centreline, viz. the left cross-sectional Figure, the axi- con directs the entire beam to one side. However, when the axicon is hit centrally, the beam is then transformed into an annular beam. The angle changes introduced by an oscillation of the mirrors must, of course, be adjusted in such a manner that a suitable distance is obtained between the primary and the secondary beam when said secondary beam oscillates round the primary beam. This means that regardless of where the secondary beam hits the mirror, its centre- line must have the same angle of divergence relative to the centreline of the primary beam when the beams are deflected from said mirror. Thus, the angle changes of the mirrors 27, 28 thus define the distance between the primary and the secondary beam at the processing spot, said distance being equal to the product of the angle of divergence and the focal distance of the flat field lens 17. The distance between the in- ward optical components as well as the focal distance of possible focusing lenses determines primarily at which distance the secondary beam hits the axicon from its centre line and at which distance it hits the mirror from its centre and thereby the distance between the secondary laser beam and the primary beam while moving towards the flat field lens 17. A large distance between the mirrors 27, 28 and the mir- ror 22 and to the flat field lens 17 means that the secondary laser beam is far from the primary laser beam in the common beam path and consequently that the angle between the two beams is relatively large when they hit the article.

The focusing system can be established in several ways. For instance, the galvano- metric mirrors can be replaced by less complicated beam-deflection systems. When the purpose is to use only an annular secondary radiation, it is not necessary to employ computer-controlled mirrors 27, 28 but merely to use stationary adjustable mirrors. A moving of the secondary beam along a fixed annular path round the primary beam alone can be provided by means of a mirror positioned at the end of a rotating axis, where the mirror is positioned slightly asymmetrically in such a manner that the normal forms a small angle with the rotating axis. The use of lenses and the position thereof in the beam path depend on the prepagation of the beam throughout the system. The use of a computer-controlled focusing system renders it possible to provide scanning patterns where the secondary beam only oscillates along a portion of the circumference of the primary beam. This is especially convenient for making depth engravings and for protecting the surface round the area in which the depth engraving is made provided it is acceptable that squirts are carried into the hollow being provided. This asymmetrical protection of the article must, of course, be continuously adjusted during a depth engraving in such a manner that the protection by the secondary beam is constantly positioned on the side of the primary beam where the non-processed area is.

The system shown in Fig. 7 is suitably used in connection with scanning optics and in the situation where the secondary laser beam hits substantially axis-parallel next to the primary beam. However, the system of Fig. 7 is conditioned by both the primary and the secondary beam being transmitted through the same flat lens 17.

Such a system can be established by means of two laser beams being transmitted through different lenses, said lens including a central lens transmitting the primary beam and an annular lens positioned round the first lens and focusing the secondary radiation. A suitable dimensioning renders it possible to direct the secondary beam outwards so as to move round the primary beam at such a suitable distance that it always passes through the outer annular-focusing lens, irrespective of the oscillation.

When different laser types are used or it is desired to direct the secondary beam towards the side of a burr, it is possible to employ the system shown in Fig. 8 provided an oscillating beam is used. Fig 8 also shows a primary scanning-optics processing system . The secondary laser beam is transmitted through a beam distributor 29 and beam controls, viz. mirrors or optical fibers to one or more focusing and beam-controlling units 28 for the secondary laser beam. These units 28 are each structured like the focusing and beam- controlling unit 21 of the primary beam with computer-controlled galvanometrically suspended mirrors and a focusing lens. The control of all mirrors must obviously be co-ordinated in such a manner that the secondary beams hit the edge of the processing area, as outlined in Fig. 2. The beam distributor 29 can be either a beam divider dividing the laser beam into several concurrent beams, or a beam switch rapidly moving the secondary beam between the secondary focusing systems.

Such a switching between the different secondary focusing systems can either be carried out in such a manner that the protection is made asymmetrically for a long period of time or in such a manner that the scanning is rapidly carried out round the processing spot. In the former case, the beam switch can operate relatively slowly because it, within a predetermined period of time, directs the beam towards a beam focusing and controlling unit and then moves said beam to the next unit, hi this case, long pulse trains are emitted to a beam-focusing and controlling unit before the beam is moved to the next unit, such as for instance by changing the direction of the movement of the primary beam.

In the latter case, the secondary beam is moved relatively quickly between the individual beam-focusing and controlling units 28, optionally to successively emit short pulse trains to the individual beam-focusing and controlling units 28.

Fig. 9 shows a system which serves to prevent the particles from depositing. It comprises a laser source, viz. a primary laser, where the laser beam is deflected via a mirror to a focusing lens. This system is combined with a tertiary beam system which serves to ensure that the tertiary laser beam of a tertiary laser, viz. a secondary laser is focused in a linear focus by means of a cylindrical lens 25 during the processing. The laser beam is deflected and adjusted inwards by means of a pair of mirrors in front of the cylindrical lens 25 with the result that it is focused at a level above the point at which the primary laser beam processes the article.

The cross sections of the two laser beams are outlined in a and b, respectively. The two Figures show two side views of both beams, but at two different angles.

A mirror mounted on a galvanometric engine can cause the beam to move in such a manner that the flat tertiary beam lifts the particles from the surface of the article. The latter procedure necessitates that the emission of the two beams is synchronized by a common control so that the tertiary beam lifts the particles by means of one or more pulse rates generating a local plasma above the surface of the article within a suitable period of time during and after the processing of the article by means of the primary beam.

In order to ensure a sufficiently high intensity of the tertiary radiation, an optical system can be used, for instance with two successively arranged lenses ensuring that the tertiary beam is focused in a more narrow line than the raw beam in combination with a simple cylindrical lens.

Claims

1. A method of providing processings, such as removal of material, in particular drillings in an article (2) by means of a laser beam, said method further employing a secondary laser beam (6), characterisedin that the local steam/plasma pressure generated by the secondary beam (6) is utilized for changing the direction of the material ejected by the processing in such a manner that the surface of the workpiece (2) is not damaged.

2. A method according to claim 1, characterised in that the secondary laser beam (6) is supplied from the side of the processing area.

3. A method according to claim 2, characterised in that the secondary laser beam (6) supplied from the side of the processing area is shaped as an annular beam surrounding the primary beam (4) .

4. A method according to claim 3, characterised in that a suitable optical system, such as an axicon expander (8), is employed for transforming the secondary beam (6) into an annular beam.

5. A method according to claim 1, characterised by adjustable mirrors in front of the axicon expander for controlling the dispersion of the secondary beam so as to provide an asymmetrical secondary beam.

6. A method according to claim 1 or5,characterisedin that the secondary beam is moved partly or completely around the processing area.

7. A method according to one or more of the preceding claims, characterised in that the adjustable mirrors are motor-operated for a scanning of the secondary beam.

8. A method according to one or more of the preceding claims, characterised in that an easily ionisible gas is fed to the processing area.

9. A method according to one or more of the preceding claims, charaterisedin that an ND-YAG-laser is used for the primary beam (4), and that a C0₂ laser is used for the secondary beam (6).

10. A method according to one or more of the preceding claims, charaterised in that a tertiary laser is focused to a linear focus and is moveable in such a manner that the plane tertiary beam can lift particles, if any, from the surface of the article.