Method and apparatus for laser ablation

Abstract: The tendency for sheet-like material such as a metal panel (2) to bend and buckle towards a laser beam (4) applied to a surface (3) to ablate material therefrom such as by drilling is counteracted by applying a further laser beam (6) to a surface (5) of the panel (2) opposite to the surface (3) so as to heat the material and thereby stress relieve the material without ablating material.

Description: This invention relates to a method and apparatus for laser ablating sheet-like material suitable particularly, but not exclusively, for drilling small holes in thin metal sheets. One of the main problems associated with processing materials by laser can be distortion of the component being processed due to the thermal fields present. This problem is particularly acute when processing thin sheet-like materials or when using large amounts of heat. A specific example where there is a major problem is in drilling small holes in titanium sheet. Typically the material is lmm thick and with a hole size of 50µm and a pitch of 500µm the required laser energy input is in the range of 500 to 4000 J/m. Using a pulsed laser with a repetition rate of 50Hz this is equivalent to scanning a 50W power beam at 1.5 metres per minute. Scanning such a beam on a thin sheet of metal causes the sheet to bend towards the laser beam with the angle of bend depending on the laser power, scan speed and spot size as well as on the material thickness and thermal properties. The bend angle varies from fractions of a degree up to about one degree per laser pass. For example Figure 1 of the accompanying drawings shows a panel of sheet-like material, namely titanium metal, having dimensions of 0.5m in the X direction and lm in the Y direction. The central area referenced 1 represents the area which has been drilled with the drilling being carried out by scanning the pulsed laser beam in the Y direction and then incrementing in the X direction. The drilled area 1 is 400mm wide. The result of the drilling process is illustrated by profile shown in the Z direction (not to scale) in Figure la of the accompanying drawings. The drilling process in the area 1 of the panel 2 has resulted in the 0.9mm thick titanium sheet bowing with a height of 30mm. This corresponds to a bend angle of about 0.017 degrees per laser scan along the Y direction. Such bending is a major problem for the drilling example discussed because the depth of field for the optimum drilling position is extremely small, typically less than 100µm for best quality holes. The panels that are to be drilled can be up to 2 metres square so that the amount of distortion produced by these small bend angles is very significant. Conventionally it is necessary to prevent such bending by the use of strong restraining tooling which is difficult to apply over a large area. JP-A-62 238 088 discloses an example of a restraining tooling used when drilling a plate. After drilling, the perforated plate is put into an annealing furnace to relieve the internal strain. Thus the quality of the holes produced in large panels in this way is poorer than is desirable. There is thus a need for a generally improved method and apparatus for laser ablating sheet-like material which at least minimises the foregoing distortion problems. According to a first aspect of the present invention there is provided a method for laser ablating sheet-like material including the steps of applying to one surface of the sheet-like material a first beam of laser energy such as to ablate material therefrom, and applying to a surface of the sheet material opposite to said one surface, at a suitable position thereon, a second beam of laser energy such as to heat the material of the opposite surface sufficiently to counteract bending of the sheet-like material in response to the first laser beam, substantially without ablating the sheet-like material. Preferably the first beam of laser energy is a pulsed beam. Conveniently the second beam of laser energy is a substantially continuous wave beam having a power in the range of from 100 to 200 Watts. According to a second aspect of the present invention there is provided apparatus for laser ablating sheet-like material, including means for applying, to one surface of the sheet-like material, a first beam of laser energy for ablating material therefrom, and means for applying to a surface of the sheet-like material opposite to said one surface, at a suitable position on the opposite surface, a second beam of laser energy for heating the material of the opposite surface sufficiently to counteract bending of the sheet-like material in response to the first laser beam, substantiating without ablating the sheet-like material. Preferably the means for applying the first beam of laser energy includes a pulsed laser source. Conveniently the pulsed laser source is a Nd-YAG laser. Advantageously the means for applying the second beam of laser energy includes a substantially continuous wave laser source. Preferably the substantially continuous wave laser source is a CO 2 laser or a Nd-YAG laser or a diode laser. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which; Figure 1 is a plan view of a titanium sheet treated according to a method and using apparatus not according to the present invention. Figure 1a is an edge view of the panel of Figure 1 showing the distortion shape produced by treatment with a method and apparatus not according to the invention, Figure 2 is a diagrammatic view of apparatus according to a first embodiment of the present invention for use with the method of the present invention, and Figure 3 is a diagrammatic view of a second embodiment of apparatus according to the present invention for use with the method of the present invention. The present invention provides a method for laser ablating sheet-like material such as sheet titanium panels previously described in respect of Figure 1 of the accompanying drawings. The panel may be substantially planar as illustrated or may be preformed into a shape. The method includes the steps of applying to one surface 3 of the panel 2 a first beam 4 of laser energy such as to ablate material therefrom and applying to a surface 5 opposite to the surface 3 at a suitable position thereon a second beam 6 of laser energy such as to heat the material of the surface 5 sufficiently to counteract bending of the panel 2 in response to the first laser beam 4, substantially without ablating the surface 5. Preferably the first beam 4 of laser energy is a pulsed beam and the second beam 6 of laser energy is a substantially continuous wave beam having a power in the range of from 100 to 200 Watts. The above method may be applied using apparatus according to the present invention such as the embodiments illustrated in Figures 2 and 3 of the accompanying drawings. In the first embodiment of the present invention illustrated in Figure 2 of the accompanying drawings, the first beam 4 is provided by a laser source 7 which preferably is a pulsed Nd-YAG laser suitable for drilling holes in 1mm thick titanium panels 2. Typical operating parameters for such a laser are a pulse energy of one Joule, a pulse width of 200 µs and a repetition rate of 50Hz. The beam 4 from laser 7 is directed to the surface 3 of the panel 2 by a turning mirror or mirrors 8. The beam reflected by the mirror 8 passes through a focusing lens 9 and preferably via an assist gassed nozzle 10 to the surface 3 of the panel 2. The panel 2 is mounted in an open frame xy table 11 in any convenient manner to index the panel 2 as required. The apparatus of Figure 2 also includes a second laser source 12 operable to provide a substantially continuous wave beam 6 and conveniently in the form of a substantially continuous wave CO 2 laser. This provides a beam 6 suitable for providing heating effect to the surface 5 of the panel 2 opposite to the point of drilling of the laser beam 4 on the surface 3 to give an equivalent bending effect to that of the laser 7 but in the opposite direction and without actually ablating or melting any material from the panel 2. In this way the bending action typically produced by the laser 7 is counteracted and a typical output power which is suitable for the laser 12 is in the range of from 100 to 200 Watts. In a similar manner to the laser 7 the beam 6 is directed to the surface 5 of the panel 2 by apparatus components including a further turning mirror 13 and a focusing lens 14. To achieve the same, but opposite, bending effect as the drilling laser 7, but without effecting the panel 2, the bending laser 12 has different operating parameters to that of the drilling laser 7. As previously stated the bend angle depends on the laser power, spot size and scan speed and possibly also the laser pulse shape. It is therefore relatively simple to arrange for the drilling laser 7 and the bending laser 12 to have different parameters but the same size effect as far as bend angle is concerned. For example the two lasers may have different average powers, different spot sizes and different pulse shapes as the only requirement for the laser 12 is to provide heating on the surface 5 of the panel 2 and thereby an equivalent but opposite bending effect. This enables the panel 2 to be held in a simple fixture that stays in focus throughout the drilling so improving the whole quality across the panel 2. The embodiment of Figure 3 is basically similar to that of Figure 2 and like components have been given like reference numerals and will not be further described in detail. However, in this embodiment the laser beam 6 is provided from the laser 12 via an optical fibre 15 and the laser 12 is a substantially continuous wave Nd-YAG laser with an average power in the range of from 100 to 200 Watts. Alternatively the laser 12 could be a substantially continuous wave diode laser.

Claims: A method for laser ablating sheet-like material including the steps of applying to one surface (3) of the sheet-like material (2) a first beam (4) of laser energy such as to ablate material therefrom, and applying to a surface (5) of the sheet material opposite to said one surface (3), at a suitable position thereon, a second beam (6) of laser energy such as to heat the material of the opposite surface (5) sufficiently to counteract bending of the sheet-like material in response to the first laser beam, substantially without ablating the sheet-like material. A method according to claim 1, in which the first beam (4) of laser energy is a pulsed beam. A method according to claim 1, in which the second beam (5) of laser energy is a substantially continuous wave beam having a power in the range of from 100 to 200 Watts. Apparatus for laser ablating sheet-like material, including means for applying to one surface (3) of the sheet-like material (2), a first beam (4) of laser energy for ablating material therefrom, and means for applying to a surface (5) of the sheet-like material opposite to said one surface, at a suitable position on the opposite surface, a second beam (6) of laser energy for heating the material of the opposite surface (5) sufficiently to counteract bending of the sheet-like material in response to the first laser beam (4), substantially without ablating the sheet-like material. Apparatus according to claim 4, wherein the means for applying the first beam (4) of laser energy includes a pulsed laser source (7). Apparatus according to claim 5, wherein the pulsed laser is a Nd-YAG laser. Apparatus according to any one of claims 4 to 6, wherein the means for applying the second beam (6) of laser energy includes a substantially continuous wave laser source. Apparatus according to claim 7, wherein the substantially continuous wave laser source is a CO 2 laser or a Nd-YAG laser or a diode laser.

Inventors: WILIAMS STEWART WYNN, WILIAMS, STEWART WYNN, Wiliams, Stewart Wynn, British Aerospace plc
Applicants: BRITISH AEROSPACE, BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1997
Main classification: B23K26
IPC codes: B23K26