Laser welding apparatus

Abstract

A laser welding apparatus comprising an evacuatable chamber 12, with a transparent region 16 through which laser radiation 14 is transmissible to weld a workpiece 32, 30. An ionisation means 33 and a

Description

Laser welding apparatus 10 as shown schematically in Figure 1 comprises an evacuatable welding chamber 12 into which a beam of laser radiation 14 is directed through protective window 16. Adjoining chamber 12 adjacent protective window 16 is an additional vacuum chamber 20 to ensure the integrity of the vacuum within main chamber 12. Vacuum window 22 is provided in a wall of subsidiary chamber 20 and overlies the position of protective window 16 so that laser beam 14 can be focussed to a fine image using a convex lens (not shown) outside chambers 12 and 20 and proximal vacuum window 22. Baffle 26 defines an aperture 28 within chamber 12 through which beam 14 passes to reach a part 30 to be welded. One or more pumps 31 are provided to evacuate chambers 12 and 20 and to maintain a high vacuum. In use, laser beam 14 is focussed to a fine spot 32 on part 30 so as to create a keyhole weld. Within main chamber 12 a pair of ionisation electrodes 33, typically made of a refractory material such as Molybdenum, are positioned between welded part 30 and protective window 16 so as to create an ionisation discharge arc, typically greater than 1000V. Charged positive and negative deflection plates 34, 34’ are disposed between ionisation electrodes 33 and protective window 16, such that ionisation electrodes 33 are closer to weld 32 than deflection plates 34, 34’ and deflection plates 34, 34’ are closer to protective window 16 than ionisation electrodes 33. An optional ring magnet 40 can be disposed between baffle 26 and electrodes 33. Electrodes 33 and deflection plates 34, 34’ are maintained at a high voltage, typically around + / -7kV, using high voltage supply units 44, 46 and if desired an optional additional high voltage supply unit 48 can be connected to protective window 16 to maintain window 16 at a positive voltage and further enhance deflection of ionised vapour away from window 16. Deflection plates 34, 34’ are arranged as positive 34 and negative 34’ plates on opposing sides of the pathway of laser beam 14 and so create an electrostatic potential gradient across chamber 12 in a direction substantially perpendicular to the direction of travel of laser beam 14. During welding, particles 50 known as weld vapour will be emitted from part 30 during the weld process and in prior art apparatus would gradually contaminate protective window 16. In apparatus 10, weld vapour 50 undergoes an ionisation stage to remove some electrons and so become ionised with a small positive charge and then a deflection stage to substantially prevent vapour 50 reaching window 16. The ionised vapour can then be deflected using magnetic or electrostatic fields to hit the wall of chamber 12 or a collector before the vapour reaches protective window 16. As vapour 50 travels from part 30 towards protective window 16 it becomes ionised due to electrons 52 from a high voltage arc passing between the pair of electrodes 33 interacting with vapour 50 to produce ionised vapour 54. Ionised vapour 54 is then deflectable under the influence of electrostatic deflection plates 34, 34’ towards chamber sidewalls 60, see arrow 56, where the vapour can be deposited without impairing visibility through window 16. Ionisation electrodes 33 and deflection plates 34, 34’ are positioned within chamber 12 such that ionised vapour 54 cannot recombine with other particles and so lose its charge before deflection has taken place. Figure 2 is an alternative embodiment where one or more deflector magnets 70 are used instead of electrostatic deflection plates 34, 34’ with ionised vapour 54 again deflected away from window 16 with the intensity of the magnetic fields sufficient to deflect the ionised vapour to hit the wall of chamber 12 or a collector instead of reaching window 16. The deflector magnets 70 typically generate a magnetic field greater than 0.5 Tesla. Providing a separate ionisation stage and deflection stage internally within chamber 12 avoids the need for a gas to be fed into a vacuum chamber to protect window 16. Any weld vapour travelling towards protective window 16 becomes ionised vapour and then is trapped or deflected using magnets 70 and / or electrostatic deflectors 34, 34’. A third embodiment shown in Figure 3 uses an RF cavity 72 with associated RF amplifier 73 where electrons 52 from rod electrode 74, or alternatively electrons from a source of electron emission such as a heated filament, are circulated within the RF cavity to ionise any vapour emitted from the weld. Magnets 70 are used to deflect the ionised vapour 54 to impinge on the walls of chambers 12 or into a collector. The ionisation stage can consist of one of or a combination of the following configurations: 1. A high voltage arc that can be discharged across the path between the weld keyhole 32 and protective window 16 as shown in Figure 1. Such an arc can be generated by an electrode arrangement 33 and / or a filament held at high voltage and heated to the point of thermionic electron emission. 2. A high voltage arc that is discharged from an electrode to part 30 being welded. This discharge electrode can be a rod 74 as shown in Figure 3 or a ring-type electrode. This arc will take the path of least resistance into the keyhole of the weld and the power of the arc can be sufficient to be used for hybrid laser-arc welding if required. Again a filament held at high voltage and so heated to the point of thermionic electron emission can be used instead of or as well as such an electrode. 3. A microwave or radio cavity 72 to assist with the ionisation of vapour 50, as shown in the third embodiment shown in Figure 3. As shown in Figures 1 to 3, additional ring magnets 40 can be placed in the ionisation stage to assist in intensifying the ionisation to ensure all weld vapour is ionised. The deflection stage for the ionised vapour can consist of one of or a combination of the following configurations: 1. One or multiple plates 34 held at a high positive voltage proximal to the protective window to deflect vapour away from the plates 34, see Figure 1. 2. One or multiple plates 34’ held at a high negative voltage proximal to protective window 16 to attract the ionised vapour towards the plates 34’, these can be arranged to create slotted or gridded collectors, see Figure 1. 3. An electromagnet or permanent magnet 70 placed adjacent or surrounding the vapour path with the field lines orientated such that the ionised vapour interacts and is deflected away from protective window 16, see Figures 2 and 3. 4. The protective window electrically isolated from the chamber held at high positive voltage to deflect vapour away from the window, as shown in Figures 1 to 3. 08 01 25

Claims

1. Laser welding apparatus comprising an evacuatable chamber incorporating a transparent region through which laser radiation is transmissible to heat a weld region on a workpiece, wherein ionisation means is disposed within the chamber to ionise 5 any particles emitted from the weld region and deflection means is disposed within the chamber and is configured to deflect ionised particles away from the transparent region and further comprising one or more microwave or RF cavities disposed between the weld region and the transparent region..io 2. Laser welding apparatus according to claim 1, wherein the ionisation means comprises at least one electron discharge means.

3. Laser welding apparatus according to claim 1, wherein the ionisation means comprises one or more discharge electrodes.

154. Laser welding apparatus according to claim 2, wherein the ionisation means comprises first and second electrodes arranged to generate a high voltage arc across a laser beam.20 5. Laser welding apparatus according to any of the preceding claims, wherein theionisation means and deflection means are positioned between the weld region and the transparent region, the ionisation means positioned closer to the weld region than the deflection means.25 6. Laser welding apparatus according to any of the preceding claims, furthercomprising at least one ring magnet disposed between the weld region and the ionisation means.

7. Laser welding apparatus according to any of the preceding claims, wherein the 30 deflection means comprises electrostatic field generation means.

8. Laser welding apparatus according to claim 7, wherein the electrostatic field generation means comprises one or more electrically conductive plates at a positive voltage.5 9. Laser welding apparatus according to claim 7, wherein the electrostatic fieldgeneration means comprises one or more electrically conductive plates at a negative voltage.

10. Laser welding apparatus according to any of claims 1 to 6, wherein the deflectionio means comprises magnetic field generation apparatus.LDCM11. Laser welding apparatus according to any of the preceding claims, further comprising a voltage supply attached to the transparent region to maintain the transparent region at a positive voltage.15

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