GB2402904A

GB2402904A - A plasma torch

Info

Publication number: GB2402904A
Application number: GB0314083A
Authority: GB
Inventors: Michael Stewart Hodgson
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2003-06-17
Filing date: 2003-06-17
Publication date: 2004-12-22
Also published as: GB0314083D0

Abstract

A plasma torch for keyhole plasma welding comprises a nozzle 4 in which an electrode 8 is provided. An adjustment device, preferably in the form of a split cap 28,surrounds the nozzle 4 and is displaceable between an operative position and an inoperative position. In the operative position (figure 2), the cap 28 defines a secondary orifice 36 which is smaller than the orifice 26 of the nozzle 4. In the inoperative position (figure 3), the sections of the cap 28 are displaced outwardly of the orifice 26. Consequently, the size of the plasma column 16 issuing from the torch can be adjusted by appropriate displacement of the cap 28, so that the torch may be used to weld material of varying thickness.

Description

A PLASMA TORCH

This invention relates to a plasma torch, and is particularly, although not exclusively, concerned with a plasma torch for use in keyhole plasma welding.

Keyhole plasma welding is a known technique for joining metallic workpieces, and in particular workplaces made from alloys used in the aerospace industry. Plasma is formed from a gas, such as an argon/helium mixture, which is heated to a very high temperature and becomes ionised so that the plasma is electrically conductive. In the welding process, the plasma is expelled through a nozle, and is used to transfer an electric arc to the workpiece. The heat of the arc melts the material of the workpiece to allow welding to take place.

In keyhole plasma welding, the plasma is forced through a small orifice in the nozzle to form a stiff constricted column of plasma which can deliver a high concentration of heat to a very small area. The plasma column pierces the workpiece, so that, as the torch is traversed along the workpiece, a weld is formed through the full thickness of the The heat input to the workpiece must be sufficient to penetrate the thickness of the workpiece. Consequently, the heat input required depends on the thickness of the workpiece. When welding a thicker material, the power supply to the torch is increased, and, to accommodate the increased power, the nozle of the torch has a relatively large orifice. For thinner materials, lower power is required and a nozle having a relatively small orifice is used.

In some circumstances, the thickness of material to be welded varies along the weld.

This occurs, for example, in the automated construction of outlet guide vane ring assemblies for gas turbine engines. Such assemblies are made up from arcuate cast vane assemblies which are formed into an annular array and welded together to form a ring. At some regions of the individual sections, for example, at the feet of the outlet - 2 guide vanes, the thickness of the material can vary significantly, for example from 2 mm to 11 mm. Relatively large orifices are required to perform keyhole plasma welding on 11 mm thick material, but such large orifices would not achieve optimum results in the regions of thinner material.

According to the present invention, there is provided a plasma torch comprising a nozle element having a nozzle orifice, and adjustment means which is displaceable between an operative position, in which it overlaps the nozle orifice to reduce the flow cross-section of the nozzle orifice, and an inoperative position in which the adjustment means is displaced from the nodule orifice.

The adjustment means may comprise a plurality of sections which, in the operative position of the adjustment means, cooperate to reduce the flow cross-section of the nozle orifice. In the operative position, the sections cooperate to define a secondary orifice which has a flow crosssection smaller than that of the nozle orifice. In a preferred embodiment, the adjustment means comprises two sections which are displaceable in opposite directions transversely of the nozle orifice and which, in the operative position, define a circular secondary orifice, which is preferably concentric with the nozzle orifice.

The sections of the adjustment means are preferably disposed outside the nozle element for displacement between the operative position, in which the sections contact the nozzle element, and the inoperative position, in which the sections are spaced from the nozzle element. The sections may comprise part-cylindrical portions which surround the nozle element.

In a preferred embodiment, the nozzle orifice is circular with a diameter not less than 4 mm, preferably not less than 5 mm, and the adjustment means, when in the operative position, provides a secondary orifice having a diameter of not more than 4 mm, for example not more than 3 mm. In this context, "diameter" is used to signify not only the true diameter of a circular orifice, but also the equivalent transverse dimension of an orifice of non-circular shape. - 3

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a plasma welding torch; Figure 2 shows the torch of Figure 1 in one operative condition; and Figure 3 shows the torch of Figure 1 in another operative condition.

Figure 1 shows the nozzle region of a plasma welding torch during a welding operation on a workplace 2. The torch comprises a nozle 4 having a nozle orifice 6. An electrode 8 is disposed within the nozle 4 and the nozle is surrounded by a gas cup 10.

In operation, a DC power supply 12 is connected between the electrode 8 and the workplace 2, so that the electrode 8 serves as an anode and the workplace 2 as a cathode. A suitable gas, such as an argon/helium mixture, is supplied, as indicated by arrows 14, to the interior of the nozzle 4 and, as indicated by arrows 15, to the space between the nozle 4 and the gas cup 10. The gas 14 entering the nozle 4 is heated and an arc is struck between the electrode 8 and the workpiece 2. This causes ionisation of the gas 14 which is thus heated to a high temperature and forms a stiff, high velocity column 16 which penetrates the workplace forming a weld pool 18.

The torch is traversed along the workpiece in the direction of the arrow 20 so that the weld pool 18 moves along the junction 21 to be welded to form a weld 22 following a region of cooling molten material 24. During the welding operation, the gas passing between the nozle 4 and the gas cup 10 serves to shield the plasma column 16 and the molten metal 18 and 24 from the ambient surroundings. - 4

As shown in Figures 2 and 3, the nozzle 4 comprises a fixed component having a primary nozzle orifice 26. Adjustment means in the form of a diametrically split cap 28 is disposed over the nozzle 4. Each section of the cap 28 comprises a semi cylindrical portion 30 which has, at its lower end, an inwardly extending portion 32 terminating at a semicircular edge 34.

In the configuration shown in Figure 2, the cap 28 is shown in an operative position in which it engages the nozzle 4. The two portions of the cap abut each other so that the semicircular edges 34 form, together, a single secondary orifice 36. The orifice 36 has a diameter 38 which is smaller than that of the diameter 40 (see Figure 3) of the nozzle orifice 26.

Figure 3 shows an inoperative condition of the cap 28. In this condition, the two sections of the cap 28 are moved apart from each other transversely of the nozzle orifice 26, i.e. perpendicular to the direction of the plasma column 16, so as to clear the nozzle orifice 26.

It will be appreciated that, in the operative condition shown in Figure 2, the size of the plasma column 16 is determined by the secondary orifice 36 defined by the inner edges 34 of the cap 28. The cap 28 thus serves to reduce the size of the orifice available for the passage of the plasma column 16.

In the condition shown in Figure 3, the sections of the cap 28 are moved out of overlapping relationship with the nozzle orifice 26, so that they play no part in the control of the plasma column 16. Instead, the size of the plasma column 16 is determined by the nozzle orifice 26.

The diameter 38 of the secondary orifice 36 may, for example, be approximately 2 mm so as to provide a plasma column 16 suitable for welding relatively thin material at relatively low power, while the diameter 40 of the nozzle orifice 26 may be approximately 5 mm so as to produce a plasma column 16 suitable for welding thicker material at relatively high power.

The operation of the torch may be under automatic control so as to follow a predetermined weld path. The cap 28 may also be under automatic control so as to move between the position shown in Figures 2 and 3 in accordance with the thickness of material on which the plasma column 16 impinges. At the same time, the electrical power supplied to the torch is varied in accordance with the thickness of material.

Operation of the cap 28 may be achieved by any suitable means, for example by means of electric or pneumatic actuators. - 6

Claims

1 A plasma torch comprising a nozle element having a nozle orifice, and adjustment means which is displaceable between an operative position, in which it overlaps the nozzle aperture to reduce the flow cross-section of the nozle aperture, and an inoperative position in which the adjustment means is displaced from the nozle aperture.

2 A plasma torch as claimed in claim 1, in which the adjustment means comprises a plurality of sections which, in the operative position, abut one another to form a secondary orifice having a smaller flow crosssection than the nozle orifice.

3 A plasma torch as claimed in claim 2, in which the adjustment means comprises two sections.

4 A plasma torch as claimed in claim 3, in which the sections are displaceable towards and away from each other to move between the operative position and the inoperative position.

A plasma torch as claimed in any one of claims 2 to 4, in which the adjustment device is disposed outside the nozle element.

6 A plasma torch as claimed in claim 5, in which the adjustment device contacts the nozzle element in the operative position.

7 A plasma torch as claimed in claim 5 or 6, in which the sections of the adjustment device comprise part cylindrical portions which surround the nozle element, the sections comprising free edges which, in the operative position of the adjustment device, adjoin one another to provide the secondary orifice. - 7

8 A plasma torch as claimed in any one of the preceding claims, in which the adjustment device, in the operative position, provides a circular secondary orifice.

9 A plasma torch as claimed in claim 8, in which the secondary orifice, in the operative position of the adjustment device, is concentric with the nozzle orifice.

A plasma torch as claimed in any one of the preceding claims, in which the adjustment device is under automatic control.

11 A plasma torch as claimed in any one of the preceding claims, in which the adjustment device is movable between the operative position and the inoperative position during the operation of the torch.

12 A plasma torch substantially as described herein with reference to, and as shown in, Figures 2 and 3 of the accompanying drawings.