GB1593268A

GB1593268A - Methods of and apparatus for effecting localised heating of a workpiece

Info

Publication number: GB1593268A
Application number: GB4519276A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1977-10-03
Filing date: 1977-10-03
Publication date: 1981-07-15

Description

(54) METHODS OF AND APPARATUS FOR EFFECTING LOCALISED HEATING OF A WORKPIECE (71) We, BOC LIMITED, of Hammersmith House, London W6 9DX, England, an English company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to methods of and apparatus for effecting localised heating of work pieces.

It is known to effect simultaneously two welds by splitting the beam of a laser into two beams using for example, partly reflective mirrors. Such beam splitting techniques have the disadvantage that the two beams produced are of reduced intensity compared with the original beam.

According to one aspect of the present invention, an apparatus for effecting localised heating of a workpiece comprises a source of high energy beam and means for deflecting intermittently the beam from a flint path to a second path, when emanating from the source, to effect pulsing of the beam along the first path and pulsing of the beam along the second path.

Preferably, the source of high energy beam is a continuous output laser and the deflecting means is a rotatable member having a quadrant so positioned with respect to the laser that rotation of the member causes the quadrant to interrupt intermittently the beam when emanating from the laser from the first path to deflect same along the second path.

The quadrant may have a reflecting surface for reflecting the laser beam to effect pulsing of the beam along the second path.

According to a further aspect of the present invention, a method of effecting localised heating of a workpiece comprises the steps of directing a high energy beam along a first path onto the workpiece and deflecting intermittently the beam from the first path to a second path to effect pulsing of the beam along the first path, and pulsing of the beam along the second path.

Preferably, the high energy beam is a laser beam and the intermittent deflection of the beam is effected by a rotatable member having a quadrant so positioned with respect to the laser that rotation of the member causes the quadrant to interrupt intermittently the beam when emanating from the laser from the first path to deflect same along the second path.

An embodiment of the invention will now be described by way of example, reference being made to the Figures of the accompanying diagramatic drawings in which: Figure I is a diagramatic representation of an apparatus for effecting localised heating of a workpiece; Figure 2 is a detail of the apparatus of Figure 1; and Figure 3 is a graph showing the energy profile of a pulsed beam produced by the apparatus of Figure 1.

As shown, an apparatus for effecting localised heating of a workpiece comprises a source of high energy beam in the form of 1.8 kilowatt continuous output carbon dioxide laser 10 and deflecting means in the form of a rotatable chopper member 13 for interrupting and deflecting intermittently the beam from the laser 10 to effect pulsing of the beam along a first path 30. The chopper member 13 has two diametrically opposed quadrants (see Figure 2) and is so positioned with respect to the laser that rotation of the member 13 by a motor 15 causes the quadrants to interrupt intermittently and deflect the beam when emanating from the laser 10 by reflect from a reflective surface 14. The beam is pulsed along the first path 30 towards a focussing device 11.

The quadrants are made from or plated with a material which is a good reflector at the laser wavelength of 10.6 u. in the case of a carbon dioxide laser. The quadrants are optically flat. The quadrants may, for example, be made of copper or possibly plated with gold.

The member 13 is set at an angle to the first path 30 of the laser beam so that, when each quadrant interrupts the beam, the beam is reflected along a second path 32 onto a reflective surface 16 and hence along a third path 33 to a focussing device 12. As shown, two identical pulsed beams extend along parallel paths 30 and 33 out of phase with one another to pass respectively into the focussing devices 11 and 12 each having the pulse shape shown by the graph of Figure 3. The focussing devices, 11, 12 focus the pulsed beams onto two respective pairs of abutting edges of workpieces 17, 18 and 19 to be welded together. The workpieces are clamped onto a table 20 which is moved lineally past the focussing devices by a lead screw (not shown).

The following is a specific example of the welding of three abutting stainless steel workpieces by an apparatus as shown in Figure 1: chopper device 13 - 4 inch diameter giving a one-to-one pass/reflect ratio.

chopper rotational speed - 2400 rpm pulse repetition frequency - 8() pulses per second per pulsed beam laser power - 1800 watts continuous output passed to chopper member which provides two 900 watt root mean square (assuming no losses) pulsed beams.

welding speed - 2 inches per second Resultant weld - overlapping spot welds at 0.0125 inches centres. Identical welds were made at the two welding stations.

It will be appreciated that other shapes of workpiece can be welded, for example, flanges can be welded onto tubes.

The number of quadrants of the chopper member 13 and its rotational speed can be altered in order to achieve faster chopping frequencies or different energy profiles, for example, altered rise time, of the pulsed beams in order to provide further parameters which govern weld performance.

Although, in the above described embodiment, the interrupted beam is focussed by the focussing device 12 and is used to carry out work on a workpiece, if necessary the energy of the deflected and interrupted parts of the laser output forming the second beam may be dumped. In such cases, a single welding, cutting or heat treatment operation is carried out.

The apparatus of the present invention may be used to produce heating at a single local zone for example, a single spot weld with the workpiece and laser beam stationary with respect to one another. However, by using the movable table 20 a line of spot welds can be produced which for some applications may be made to overlap one another to produce a substantially continuous weld.

An advantage of the apparatus described above and the method performed with the apparatus is that better control of the energy input to the workpiece can be achieved, improving welding, cutting or heat treating for different applications. Factors such as weld appearance, namely producing a smooth profile to avoid stress raisers and the extent of the heat affected zone can be controlled by suitable adjustment of the relative speed between the workpiece and the beam, the power of the beam and the focussing of the beam. Close control of weld penetration can also be achieved. Pulse frequency, pulse shape can also be carefully controlled with the apparatus described above in order to improve further the quality of effectiveness of the particular localised heating being carried out.

The apparatus described above provides a relatively low cost means of producing a pulsed beam along two paths which permits control of the pulsed shape of the supply simply by altering the shape of the chopper member used for interrupting the beam.

The chopper member can be applied to a standard continuous output laser without internal modifications as well as to a switch laser with a high pulse repetition frequency (prf), the chopper device running at a low prf.

A particular advantage of the pulse beam over a continuous beam in addition to better control of weld profile and heat affected zone, is that the likelihood of a plasma plume of vaporised metal and ionised shielding gas being formed over the heated area is reduced. Such plasma tends to absorb part of the energy of the laser beam, reducing the energy thereof available for heating the workpieces.

WHAT WE CLAIM IS: 1. An apparatus for effecting localised heating of a workpiece, comprising a source of high energy beam and means for deflecting intermittently the beam from a first path to a second path, when emanating from the source, to effect pulsing of the beam along the first path and pulsing of the beam along the second path.

2. An apparatus as claimed in Claim 1, in which the source of high energy beam is a continuous output laser and the deflecting means is a rotatable member having a quadrant so positioned with respect to the laser that rotation of the member causes the quadrant to interrupt intermittently the beam when emanating from the laser from the first path to deflect same along the second path.

3. An apparatus as claimed in Claim 2, in which the quadrant has a reflecting

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. a carbon dioxide laser. The quadrants are optically flat. The quadrants may, for example, be made of copper or possibly plated with gold. The member 13 is set at an angle to the first path 30 of the laser beam so that, when each quadrant interrupts the beam, the beam is reflected along a second path 32 onto a reflective surface 16 and hence along a third path 33 to a focussing device 12. As shown, two identical pulsed beams extend along parallel paths 30 and 33 out of phase with one another to pass respectively into the focussing devices 11 and 12 each having the pulse shape shown by the graph of Figure 3. The focussing devices, 11, 12 focus the pulsed beams onto two respective pairs of abutting edges of workpieces 17, 18 and 19 to be welded together. The workpieces are clamped onto a table 20 which is moved lineally past the focussing devices by a lead screw (not shown). The following is a specific example of the welding of three abutting stainless steel workpieces by an apparatus as shown in Figure 1: chopper device 13 - 4 inch diameter giving a one-to-one pass/reflect ratio. chopper rotational speed - 2400 rpm pulse repetition frequency - 8() pulses per second per pulsed beam laser power - 1800 watts continuous output passed to chopper member which provides two 900 watt root mean square (assuming no losses) pulsed beams. welding speed - 2 inches per second Resultant weld - overlapping spot welds at 0.0125 inches centres. Identical welds were made at the two welding stations. It will be appreciated that other shapes of workpiece can be welded, for example, flanges can be welded onto tubes. The number of quadrants of the chopper member 13 and its rotational speed can be altered in order to achieve faster chopping frequencies or different energy profiles, for example, altered rise time, of the pulsed beams in order to provide further parameters which govern weld performance. Although, in the above described embodiment, the interrupted beam is focussed by the focussing device 12 and is used to carry out work on a workpiece, if necessary the energy of the deflected and interrupted parts of the laser output forming the second beam may be dumped. In such cases, a single welding, cutting or heat treatment operation is carried out. The apparatus of the present invention may be used to produce heating at a single local zone for example, a single spot weld with the workpiece and laser beam stationary with respect to one another. However, by using the movable table 20 a line of spot welds can be produced which for some applications may be made to overlap one another to produce a substantially continuous weld. An advantage of the apparatus described above and the method performed with the apparatus is that better control of the energy input to the workpiece can be achieved, improving welding, cutting or heat treating for different applications. Factors such as weld appearance, namely producing a smooth profile to avoid stress raisers and the extent of the heat affected zone can be controlled by suitable adjustment of the relative speed between the workpiece and the beam, the power of the beam and the focussing of the beam. Close control of weld penetration can also be achieved. Pulse frequency, pulse shape can also be carefully controlled with the apparatus described above in order to improve further the quality of effectiveness of the particular localised heating being carried out. The apparatus described above provides a relatively low cost means of producing a pulsed beam along two paths which permits control of the pulsed shape of the supply simply by altering the shape of the chopper member used for interrupting the beam. The chopper member can be applied to a standard continuous output laser without internal modifications as well as to a switch laser with a high pulse repetition frequency (prf), the chopper device running at a low prf. A particular advantage of the pulse beam over a continuous beam in addition to better control of weld profile and heat affected zone, is that the likelihood of a plasma plume of vaporised metal and ionised shielding gas being formed over the heated area is reduced. Such plasma tends to absorb part of the energy of the laser beam, reducing the energy thereof available for heating the workpieces. WHAT WE CLAIM IS:

1. An apparatus for effecting localised heating of a workpiece, comprising a source of high energy beam and means for deflecting intermittently the beam from a first path to a second path, when emanating from the source, to effect pulsing of the beam along the first path and pulsing of the beam along the second path.

3. An apparatus as claimed in Claim 2, in which the quadrant has a reflecting

surface for reflecting the laser beam to effect pulsing of the beam along the second path.

4. An apparatus as claimed in Claim 3, in which the reflecting surface is gold plated.

5. An apparatus as claimed in Claim 4, in which a reflective surface is provided for reflecting the laser beam from its second path along a third path parallel to but spaced from the first path.

6. An apparatus as claimed in Claim 5, in which two focussing devices are provided, one in alignment with the first path taken by the laser beam and the second in alignment with the third path taken by the laser beam.

7. An apparatus as claimed in Claim 6, in which a device is provided for supporting the workpiece, the device and the focussing devices being movable relative to each other.

8. A method of effecting localised heating of a workpiece comprising the steps of directing a high energy beam along a first path onto the workpiece and deflecting intermittently the beam from the first path to a second path to effect pulsing of the beam along the first path and pulsing of the beam along the second path.

9. A method as claimed in Claim 8, in which the source of high energy beam is a continuous high output laser and the deflection is effected by a rotatable member having a quadrant so positioned with respect to the laser that rotation of the member causes the quadrant to interrupt intermittently the beam when emanating from the laser from the first path to deflect same along the second path.

10. A method as claimed in Claim 9, in which the quadrant has a reflecting surface which reflects the laser beam to effect pulsing of the beam along the second path.

11. An apparatus for effecting localised heating of a workpiece constructed and arranged substantially as herein before described with reference to and as illustrated in, Figures 1 and 2 of the accompanying drawings.

12. A method of effecting localised heating of a workpiece substantially as herein before described.