GB1600796A

GB1600796A - Methods and apparatus for cutting welding and surface treating

Info

Publication number: GB1600796A
Application number: GB6008/78A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1978-02-15
Filing date: 1978-02-15
Publication date: 1981-10-21

Description

(54) IMPROVEMENTS IN METHODS AND APPARATUS FOR CUTTING, WELDING AND SURFACE TREATING (71) We, NATIONAL RESEARCH DE VELOPMENT CORPORATION, a British Corporation established by Statute of Kingsgate House, 66-74 Victoria Street, London SW1, 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 an improvement in or modification of the methods and apparatus for cutting, welding and surface treating a workpiece using a laser disclosed in Patent Specification No.

1,547,172 (Application No. 12681/75).

According to a first aspect of the present invention there is provided a method for treating a workpiece including projecting a beam of light from a laser on to a workpiece, striking an arc between an end portion of an electrode and the heat affected zone created by the laser beam in the workpiece, moving the laser beam and the workpiece relatively to one another while maintaining the arc at the heat affected zone, and positioning and maintaining the end portion of the electrode nearest to the workpiece at a point in the workpiece where treatment is just about to take place.

The term "treating" in this specification includes cutting, welding, surface hardening, surface alloying, and surface coating, and the word "workpiece" particularly in welding, includes the plural.

The heat affected zone is that zone of the workpiece which is heated by the laser beam and wherein the temperature is 100"C or more above the temperature of the material of the workpiece which is not heated by the laser beam. The heat affected zone is preferably regarded as that zone having a temperature of 300"C or more above the material not heated by the laser beam.

Where cutting or welding is being carried out the end portion of the electrode is positioned nearest to the workpiece at a point in the workpiece where cutting or welding is just about to take place.

The arc is on the same side of the workpiece as the laser beam unless the workpiece is sufficiently thin to allow the heat affected zone to extend to an opposite surface of the workpiece, when the arc may be struck to the zone on this surface. In these circumstances two arcs on opposite sides of the workpiece may be operated at the same time.

The method of the invention may additionally be considered as a method of mani- pulating one or more arcs since as is mentioned below an arc root tends to follow the point of incidence of the laser beam on the workpiece. However, in some cases, notably at high currents (for example 130 Amps) the arc may root anywhere in the said heat affected zone.

A main advantage of the invention is that the arc roots more surely in the heat affected zone and not on any hot material, for example waste material, which may be present after the laser and arc have recently passed by.

Preferably a jet of gas from a nozzle is also projected at the said zone. The gas may be inert or may react with either the workpiece, the electrode (for welding) or a flux. However it is important that the heat affected zone remains substantially as caused by the laser alone. Significant spreading of the zone may occur ifajet of very hot gas impinges on an area which is larger than and includes the point of incidence of the laser beam on the workpiece and for this reason when a gas jet is used the gas temperature at the nozzle exit must be less than 1500to and preferably less than 1 230'C, that is the transition point for steels where changes in the nature of steel occur. Where two arcs are used two gas jets may be provided.

According to a second aspect of the present invention there is provided apparatus for treating a workpiece including means for projecting a laser beam on to a workpiece.

means for striking an arc between an end portion of an electrode and the heat affected zone created, in operation, by the laser beam in the workiece, and means for moving the laser beam and the workpiece relatively to one another while maintaining the arc at the heat affected zone, the electrode being so arranged that the end portion of the elec trode can be maintained, while treatment takes place, in a position nearest the workpiece at a point in the workpiece where treatment is just about to take place.

The means for striking the arc is positioned to strike the arc on the same side of the workpiece as the laser beam unless the expected workpiece is thin enough to allow the heat affected zone to extend to an opposite surface, when the means for striking an arc may be positioned to strike the arc to the zone on this opposite surface. Two electrodes may then be used to strike arcs to both sides of the workpiece.

The apparatus may be used for manipulating an arc.

Preferably means are also provided for projecting a jet of gas from a nozzle at the said point. The gas may be inert or may react with the workpiece, or, for welding, the electrode or a flux but the temperature of the gas must be below 1500 C at the nozzle exit.

As before where two arcs are used two gas jets may be provided.

Magnetic means or the gas jet, when present, may be used to deflect the plasma away from the laser beam except at or near the arc root, so that masking of the point of incidence of the laser beam on the workpiece by the plasma is reduced. Where magnetic means are used a magnetic field is set up which reacts with the current in the arc exerting a force which moves the arc and plasma away from the laser beam except at the said point of incidence. Secondary gas jets may also be used to blow the plasma to one side.

When cutting additional penetration is possible if the laser beam strikes at a slight angle to the normal to the workpiece, thereby reducing reflection or absorption in the arc plasma.

When the arc is on the same side of the workpiece as the laser beam the heat affected zone created in the workpiece by the laser beam naturally provides a "hot spot" which often acts as an arc root so that when a suitable voltage relative to the workpiece is applied to the electrode the arc tends to be between the said point and the electrode. The laser induced plasma acts as a guide for the direction and shape of the arc discharge. The arc geometry and stability is changed by following the plasma, the size of the arc root being affected by the heat affected zone, and in particular being narrower than in the absence of laser induced heat affected zone.

When the heat affected zone penetrates the workpiece and the arc is on the other side of the workpiece from the laser, the "hot spot" provided again acts as an arc root. Hence, in either configuration when cutting, the quality of the cut is improved as compared with conventional arc cutting. The arc follows the heat affected zone at high speed and avoids the usual intermittent jumps which occur in the absence of the laser beam. In fact the root of an arc can be guided, over a limited range, by movement of the laser beam. It is expected that the method will prove useful in manufacture and in equipment operation where it is desirable for arc roots to follow a certain path.

The method and apparatus of the invention may be used for surface hardening of metals such as steel where that part of the workpiece surface on which the laser beam and the arc are incident are temperature cycled to produce hardening.

In addition the method and apparatus of the invention may be used for surface alloying, or surface coating. In the former an alloy powder such as stellite (Registered Trade Mark) is sprinkled over the surface. and the laser beam and arc are applied to melt the workpiece surface where they are incident to form a surface alloy in this region. In one example of surface coating one or more powders are sprinkled on to the surface and are melted by the laser beam and the arc; however, the melted material does not alloy with the workpiece material significantly.

Gas jets are not usually used in surface hardening or alloying according to the invention except for deflecting the plasma, shielding the electrode and protecting the laser lens.

Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a cross-section of an embodiment of the invention in which a laser beam and an arc are on opposite sides of a workpiece, and Figure 2 shows a view of the workpiece of Figure 1 from below while cutting is in progress.

An arrangement for welding or cutting using an inert gas, or a reactive gas, such as oxygen or chlorine, which takes part in an exothermic reaction is shown in Figure 1. A laser beam 10 is focussed at a point 16 on a workpiece 15 by means of a lens 22 which may, for example, be made of potassium chloride, germanium or gallium arsenide, zinc selenide or cadmium telluride. Since Figure 1 is intended to show a general arrangement, the workpiece 15 is not shown, for example as being partially cut or as made up of two portions being welded, rather its position and general form only are shown.

The inert gas used for welding or the reactive gas used for cutting passes through an inlet 23 into a housing 24 having a nozzle 25 which is, in operation, just above the point 16.

An electrode 26 is positioned to strike an arc on the opposite side of the workpiece 15 from the laser beam 10, the electrode 26 being shielded by an inert gas directed by a gas shield 27 having an inlet 28.

In the arrangement of Figure 1 there are thus three sources of heat for cutting: the laser beam, the reaction between the gas the workpiece and the arc struck between the workpiece and the electrode 26. The arrangement may also be used for welding where additional heat is required. However more usually an inert gas is substituted for the reactive gas and the shield 27 is omitted.

Where workpieces are being welded a consumable electrode may be used as the electrode 26. The whole electrode system may be similar to that used in MIG or TIG.

The arc is struck to a point 33 where the heat affected zone created by the laser beam appears on the lower side of the workpiece.

The additional heat from the arc at the point 33 keeps the workpiece slag (that is molten metal or metal compound for a metal workpiece) in a state which allows it to be blown clear using the gas from the nozzle 25 or ajet of inert gas. Thus access for the arc is achieved in cutting or full penetration welding. In addition there is no blocking of the laser beam by the arc induced plasma but a nozzle may be provided to allow the plume produced by the laser to be blown from the upper surface.

The workpiece 15 may be either positive or negative with respect to the electrode 26. As has been mentioned, any arc struck between the electrode 26 and the workpiece 15 will naturally tend to have its root on the workpiece at the point 33.

The workpiece 15 moves towards the left of Figure 1 with respect to the laser beam 10 and the electrode 26 when welding or cutting are taking place, and the electrode 26 is positioned with its tip slightly ahead of the point at which the heat affected zone created by the laser beam in the workpiece appears on the lower side of the workpiece. Further the axis of the electrode 26 is in the plane of the cut or weld at the said point on the lower side of the workpiece, with the axis of the electrode making an acute angle with that part of the lower surface of the workpiece which has not yet been welded or cut. Figure 2 shows the arrangement of Figure 1 from below the workpiece when cutting is in progress, a cut or kerf 35 being shown in relation to the electrode 26. The direction of movement of the workpiece 15 relative to the other components is shown by the arrow 29.

In the electrode position shown, the arc roots on the said point on the lower surface of the workpiece and does not lock on to any molten waste material or slag from the cut or weld.

In general the desired arc rooting can be obtained when the tip of the electrode is just ahead of, but spaced from, the current point of cutting or welding, and preferably when the axis of the electrode makes an acute angle with that part of the upper or lower surface of the workpiece which is on that side of a line at right angles to a tangent to the cut or weld at the current point of cutting or welding which is remote from the completed cut or weld.

A further gas shielded electrode (not shown) but as shown in Figure 3 of the above mentioned Application No. 12681/75 Serial No 1,547,172 may be used with the apparatus of Figure 1 to strike a further arc on the upper surface of the workpiece 15 at the point 16. A nozzle (not shown) for supplying reactive or inert gas and/or blowing away the plume may also be provided.

In one mode of operation of the arrangements of Figure 1, a fluid flow balance is maintained to ensure that the electrode is shrouded in an inert gas while the workpiece is covered by a separate gas stream which is a slow moving inert gas for use, for example, in welding, surface alloying or surface hardening and which is fast flowing oxygen, possibly supersonic, for cutting.

Any of the many known arrangements for providing relative movement between the workpiece and the laser beam and arc may be used. For example in making a cut the laser and electrode may be kept stationary and the workpiece moved under the electrode on a table perhaps under the control of a lead screw. Where welding is carried out the two workpieces are clamped side by side along the projected weld on the table.

For surface hardening, surface alloying and surface coating the relative movement must be such that the point of incidence visits every part of the surface portion to be treated.

WHAT WE CLAIM IS: 1. A method for treating a workpiece including projecting a beam of light from a laser on to a workpiece, striking an arc between an end portion of an electrode and a point within the heat affected zone created by the laser beam in the workpiece, moving the laser beam and the workpiece relatively to one another while maintaining the arc at the heat affected zone, and positioning and maintaining the end portion of the electrode nearest to the workpiece at a point in the workpiece where treatment is just about to take place.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. up of two portions being welded, rather its position and general form only are shown. The inert gas used for welding or the reactive gas used for cutting passes through an inlet 23 into a housing 24 having a nozzle 25 which is, in operation, just above the point 16. An electrode 26 is positioned to strike an arc on the opposite side of the workpiece 15 from the laser beam 10, the electrode 26 being shielded by an inert gas directed by a gas shield 27 having an inlet 28. In the arrangement of Figure 1 there are thus three sources of heat for cutting: the laser beam, the reaction between the gas the workpiece and the arc struck between the workpiece and the electrode 26. The arrangement may also be used for welding where additional heat is required. However more usually an inert gas is substituted for the reactive gas and the shield 27 is omitted. Where workpieces are being welded a consumable electrode may be used as the electrode 26. The whole electrode system may be similar to that used in MIG or TIG. The arc is struck to a point 33 where the heat affected zone created by the laser beam appears on the lower side of the workpiece. The additional heat from the arc at the point 33 keeps the workpiece slag (that is molten metal or metal compound for a metal workpiece) in a state which allows it to be blown clear using the gas from the nozzle 25 or ajet of inert gas. Thus access for the arc is achieved in cutting or full penetration welding. In addition there is no blocking of the laser beam by the arc induced plasma but a nozzle may be provided to allow the plume produced by the laser to be blown from the upper surface. The workpiece 15 may be either positive or negative with respect to the electrode 26. As has been mentioned, any arc struck between the electrode 26 and the workpiece 15 will naturally tend to have its root on the workpiece at the point 33. The workpiece 15 moves towards the left of Figure 1 with respect to the laser beam 10 and the electrode 26 when welding or cutting are taking place, and the electrode 26 is positioned with its tip slightly ahead of the point at which the heat affected zone created by the laser beam in the workpiece appears on the lower side of the workpiece. Further the axis of the electrode 26 is in the plane of the cut or weld at the said point on the lower side of the workpiece, with the axis of the electrode making an acute angle with that part of the lower surface of the workpiece which has not yet been welded or cut. Figure 2 shows the arrangement of Figure 1 from below the workpiece when cutting is in progress, a cut or kerf 35 being shown in relation to the electrode 26. The direction of movement of the workpiece 15 relative to the other components is shown by the arrow 29. In the electrode position shown, the arc roots on the said point on the lower surface of the workpiece and does not lock on to any molten waste material or slag from the cut or weld. In general the desired arc rooting can be obtained when the tip of the electrode is just ahead of, but spaced from, the current point of cutting or welding, and preferably when the axis of the electrode makes an acute angle with that part of the upper or lower surface of the workpiece which is on that side of a line at right angles to a tangent to the cut or weld at the current point of cutting or welding which is remote from the completed cut or weld. A further gas shielded electrode (not shown) but as shown in Figure 3 of the above mentioned Application No. 12681/75 Serial No 1,547,172 may be used with the apparatus of Figure 1 to strike a further arc on the upper surface of the workpiece 15 at the point 16. A nozzle (not shown) for supplying reactive or inert gas and/or blowing away the plume may also be provided. In one mode of operation of the arrangements of Figure 1, a fluid flow balance is maintained to ensure that the electrode is shrouded in an inert gas while the workpiece is covered by a separate gas stream which is a slow moving inert gas for use, for example, in welding, surface alloying or surface hardening and which is fast flowing oxygen, possibly supersonic, for cutting. Any of the many known arrangements for providing relative movement between the workpiece and the laser beam and arc may be used. For example in making a cut the laser and electrode may be kept stationary and the workpiece moved under the electrode on a table perhaps under the control of a lead screw. Where welding is carried out the two workpieces are clamped side by side along the projected weld on the table. For surface hardening, surface alloying and surface coating the relative movement must be such that the point of incidence visits every part of the surface portion to be treated. WHAT WE CLAIM IS:

1. A method for treating a workpiece including projecting a beam of light from a laser on to a workpiece, striking an arc between an end portion of an electrode and a point within the heat affected zone created by the laser beam in the workpiece, moving the laser beam and the workpiece relatively to one another while maintaining the arc at the heat affected zone, and positioning and maintaining the end portion of the electrode nearest to the workpiece at a point in the workpiece where treatment is just about to take place.

2. A method according to claim I

wherein the heat affected zone extends through the workpiece to an opposite surface thereof from the surface on which the laser beam is incident, and the arc is struck to the said zone on the said opposite surface.

3. A method according to claim 2 wherein arcs are struck to the heat affected zone on the two said surfaces.

4. A method of surface alloying according to any preceding claim including applying a material, to be alloyed with a portion of the surface of the workpiece, to the said portion, and at the same time, or after, so causing the said relative movement between the said zone and the workpiece that the said zone visits every point in the area to be surface alloyed.

5. A method according to any preceding claim wherein either a jet of gas is directed by a nozzle towards the heat affected zone from a position opposite that workpiece surface which receives the laser beam or from a position opposite another workpiece surface; or gas jets from first and second nozzles are directed towards the heat affected zone from positions opposite first and second workpiece surfaces, respectively, the temperature of the gas at the nozzle exit or exits being less than 1500'C.

6. A method of cutting or welding according to claim 5 wherein an exothermic reaction takes place between the gas and the workpiece and/or a flux, if any, applied to the workpiece.

7. A method of welding according to claim 5 using a consumable electrode, wherein an exothermic reaction takes place between the electrode and the gas.

8. A method according to any preceding claim including deflecting away from the laser beam at least part of the plume of gaseous materials and/or particles which is emitted from the point of incidence of the laser beam on the workpiece.

9. A method of cutting or welding according to any preceding claim wherein the electrode used is elongated including so positioning the electrode that its axis makes an acute angle with a major surface of the workpiece on that side of a line at right angles to a tangent to the weld or cut at the current point of cutting or welding, which is remote from the currently completed part of the cut or weld.

10. Apparatus for treating a workpiece including means for projecting a laser beam on to a workpiece, means for striking an arc between an end portion of an electrode and a point within the heat affected zone created, in operation, by the laser beam in the workpiece, and means for moving the laser beam and the workpiece relatively to one another while maintaining the arc at the heat affected zone, the electrode being so arranged that the end portion of the electrode can be maintained, while treatment takes place, in a position nearest to the workpiece at a point in the workpiece where treatment is about to take place.

I I. Apparatus according to claim JO for treating workpieces in which the heat affected zone extends, in operation, through the workpiece to an opposite surface thereof from the surface at which the laser beam is directed, wherein the means for striking an arc is positioned to strike an arc to the said zone in the said opposite surface.

12. Apparatus according to claim 11 including a further electrode and means for striking an arc between the further electrode and the heat affected zone.

13. Apparatus according to any of claims 10 12 including means for projecting either a jet of gas from a nozzle towards the heat affected zone from a position opposite that workpiece surface which receives the laser beam or from a position opposite another workpiece surface; or jets of gas from first and second nozzles, towards the heat affected zone from positions opposite first and second workpiece surfaces, respectively, the temperature of the gas at the nozzle exit or exits being less than 1500 C.

14. Apparatus according to claim 13 wherein the gas is exothermically reactive with the expected workpiece and/or a flux expected to be applied to the workpiece.

15. Apparatus according to any of claims 10 to 14 including an enclosure for the or each electrode, the or each enclosure having an inlet port for an inert gas and an aperture for allowing an arc to be struck to the workpiece.

16. Apparatus according to any of claims 10 to 15 including means for deflecting at least part of the plume of gaseous materials and/or particles which is emitted from the point of incidence of the laser beam on the workpiece away from the laser beam.

17. Apparatus according to claim 16 wherein in operation, an arc is struck on the same side of the workpiece as the laser beam, and the deflection means includes magnetic means for applying a magnetic field to deflect the arc away from the laser beam except at the said point of incidence.

18. Apparatus according to claim 16 insofar as dependent on claim 13 or 14 wherein the deflection means includes the means for projecting a jet of gas or further such means, the means for projecting ajet of gas which forms part of the deflection means being positioned to deflect the plume.

19. Apparatus for cutting or welding according to any of claims 10 to 18, wherein the apparatus is constructed to receive the workpiece in a predetermined position, and the electrode is elongated and so positioned that, in operation, its longitudinal axis can make an acute angle with a major surface of the workpiece on that side of a line at right angles to a tangent to the weld or cut at the current point of cutting or welding, which is remote from the currently completed part of the cut or weld.

20. A method of treating a workpiece substantially as hereinbefore described.

21. Apparatus for treating a workpiece substantially as hereinbefore described with reference to, and as shown in, Figure 1 of the accompanying drawings.

22. Apparatus for treating a workpiece or substantially as hereinbefore described with reference to, and as shown in Figures 1 and 2 of the accompanying drawings.