GB2161409A

GB2161409A - Inert gas arc welding

Info

Publication number: GB2161409A
Application number: GB08514147A
Authority: GB
Inventors: George Goh; Bal Bhajn; Michael Simpson
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-06-06
Filing date: 1985-06-05
Publication date: 1986-01-15
Also published as: GB8414403D0; GB8514147D0

Abstract

In a method of striking a welding arc in e.g a TIG (tungsten inert-gas) welding process it is essential that the workpiece shall not be contaminated by sputtering of the electrode which would happen in conventional TIG welding in which a welding arc is struck by touching the electrode on the workpiece. A low-current discharge is first initiated and only converted to a high welding current when the electrode is withdrawn a sufficient distance from the workpiece. This is detected by a detector 11 sensing a voltage drop of a given amount at the low current source 7, and which switches in the arc power source 9. <IMAGE>

Description

SPECIFICATION Arc welding method & apparatus This invention relates to a method-and apparatus for inert gas arc welding of the kind known as for inert gas are welding of the kind known as "TIG" welding, i.e. "tungsten inert-gas" welding. In this form of welding an arc is struck between a tungsten electrode and the workpiece of mild steel or other metal. In a basic such arrangement the electrode would be placed in contact with the workpiece and an arc struck as the electrode is withdrawn. Very large voltages and currents are in volved in such welding arcs and despite the fact that the tungsten electrode has a very high melting point and is not intended to be consumed in the welding process, some particles of tungsten inevit ably form inclusions in the metal of the workpiece.

In high quality welding processes such as, for example, in the construction of nuclear reactors and submarine structures the quality of the weld is of vital importance and the formation of inclusions in the manner just described is not permissible, since they lead to fractures and failure of the weld.

Another means of striking the arc in TIG welding processes has therefore been devised to avoid so far as possible the inclusion of electrode material in the workpiece. This method involves keeping the electrode spaced from the workpiece by the nor may, arc gap and striking an arc by means of a high voltage, high frequency discharge between electrode and workpiece. The voltage is typically of the order of 1015 kilovolts and the frequency of the order of 2 kHz. The high voltage effectively causes a discharge over the substantial gap and ionises the surrounding inert gas (usually Argon).

A high current welding arc is then easily initiated.

A DC or low-frequency high voltage ionising dis charge would be dangerous to personnel con cerned in the operation and is not therefore permissible. The use of high-frequency discharge removes this dangerous effect since any fault or leakage currents pass over rather than through the skin without harm to the operative.

While this would appear to be a solution to the difficulty a further problem is unfortunately intro duced. The use of high voltage high-frequency dis charges causes very extensive problems with radiated and line-fed high-frequency currents and transients. Such currents tend to cause widespread damage in any electrode equipment that may be in the vicinity. The precautions necessary to prevent such damage are commonly very inconvenient and often expensive.

An object of the present invention is therefore to provide a method and apparatus for arc welding which does not suffer from the above disadvan tages.

According to one aspect of the present invention, in a method of inert-gas arc welding a discharge of limited low current-value is initiated between a workpiece and a non-consumable electrode, the electrode is withdrawn from the workpiece and the low current discharge replaced by a welding arc when the electrode is sufficiently spaced from the workpiece to obviate contamination of the workpiece by electrode material under the effect of the welding arc.

The electrode may be driven into contact with the workpiece, contact with the workpiece closing a limited low current circuit and closure of the circuit causing immediate withdrawal of the electrode to initiate the low current discharge.

The welding arc is preferably initiated when the voltage drop across the low current discharge reaches a predetermined value.

According to another aspect of the invention, a circuit including a non-consumable electrode, means for initiating a low current discharge between the electrode and the workpiece and means for replacing the low current discharge by a welding arc when the low current discharge extends over a gap of sufficient length to obviate adulteration of the workpiece by electrode material under the effect of the welding arc.

There is preferably included means responsive to the voltage drop across the low current discharge to initiate the welding arc at a predetermined value of the voltage drop.

The equipment preferably includes servo-controlled driving means for driving the electrode into contact with a workpiece and immediately withdrawing the electrode when the contact between electrode and workpiece closes the circuit supplied by the low current source.

A TIG welding process and apparatus will now be described by way of example with reference to the accompanying drawing which is a block diagram of welding apparatus employing a 'touch start' in accordance with the invention.

Referring to the drawing, a welding torch 1 is mounted adjacent a workpiece 3, the welding torch incorporating a tungsten electrode and an argon supply to sheath the weld in known manner. In addition to any facility for moving the torch over the workpiece, for seam welding for example, the torch 1 is mounted on a slide which allows the torch to approach and withdraw from the workpiece 3. This slide mount is driven by a servo motor 5 having very fast response.

The torch 1 is supplied from two sources, a con trolledsource 7 of limited, low current-value, and a transistorised power source 9. The low-current source 7 is limited to a supply of, suitably, four amps, at a voltage (subject to a limit of about 80v) necessary to achieve this current. The high current power source 9 is a transistorised supply which can very rapidly produce a current of 300 amps suitable for a welding arc.

Detector circuit 11 is responsive to the closure of the low current circuit, i.e. to contact between the torch electrode and the workpiece, and the resulting current arising on such contact, to reverse the drive to the servo motor 5 and cause immediate withdrawal of the electrode from the workpiece. The detector circuit 11 includes logic circuitry triggered by detection of the low current supply.

The detector circuit 11 also includes a start switch for initiating the forward drive of the torch toward the workpiece.

In operation, the torch is driven slowly towards the workpiece and, on contact, the current is detected by detector 11 and the servo motor reversed. The power source is held off at this time.

As the electrode is withdrawn from the workpiece the limited current is maintained with increasing voltage drop across the discharge. The diode 13 permits the flow of the limited current to the torch, while the diode 15 prevents reverse flow into the power source 9. As the voltage across the dis charge increases with increasing gap dimension, a predetermined level, of about 60 volts, is detected by the detector 11. It is found that this voltage cor responds to a gap sufficient to prevent electrode contamination of the workpiece.

On detection of the predetermined voltage level the detector 11 switches off the low current source and switches on the power source. The diode 13 now prevents the reverse flow of current into the low current source. Current to the torch rises rap idly to a high level sufficient to produce a welding arc. The electrode is however, now too remote from the workpiece (at a normal welding distance) to cause contamination and welding can proceed with no fear of inclusions of tungsten in the weld.

The use of transistorised power source permits very rapid transition from a low current discharge to a welding arc.

While the invention is particularly applicable to TIG welding it would be equally advantageous in any welding process using a non-consumable elec trode where it was required to avoid contamination of the workpiece by the electrode-material.

Claims

1. A method of inert-gas arc welding in which a discharge of limited low current value is initiated between a workpiece and a non-consumable elec trode, the electrode is withdrawn from the work piece and the low current discharge replaced by a welding arc when the electrode is sufficiently spaced from the workpiece to obviate adulteration of the workpiece by electrode material under the effect of the wedding arc.

2. A method of welding according to Claim 1, wherein said electrode is driven into contact with the workpiece, contact with the workpiece closes a limited low current circuit and closure of said cir cuit causes immediate withdrawal of the electrode to initiate the low current discharge

3. A method of welding according to Claim 2, wherein said welding arc is preferably initiated when the voltage drop across the low current dis charge reaches a predetermined value.

4. A method of welding a workpiece by means of tungsten inert-gas (TIG) equipment and in ac cordance with any of Claims 1 to 3.

5. Inert-gas arc welding equipment comprising a circuit including a non-consumable electrode, means for initiating a low current discharge between said electrode and the workpiece and means for replacing the low current discharge by a welding arc when the low current discharge extends over a gap of sufficient length to obviate adulteration of the workpiece by electrode material under the effect of the welding arc.

6. Inert-gas arc welding-equipment according to Claim 5, including means responsive to the voltage drop across the low current discharge to initiate the welding arc at a predetermined value of the voltage drop.

7. Inert-gas arc welding equipment according to Claim 5 or Claim 6, including servo-controlled driving means for driving the electrode into contact with a workpiece and immediately withdrawing the electrode when said contact between electrode and workpiece closes the circuit supplied by the low current source.

8. Inert-gas arc welding equipment according to Claim 7, wherein said driving means is controlled by a power transistor convertor.

9. Inert-gas arc welding equipment according to any of Claims 5 to 8, wherein said electrode is of tungsten as used in tungsten inert-gas (TIG) weld-ing.

10. A method of inert-gas arc welding substatially as herein before described with reference to the accompanying drawing.

11. Inert-gas arc welding equipment substantially as hereinbefore described with reference to the accompanying drawing.