GB1405415A - Plasma arc welding - Google Patents

Abstract

1405415 Welding by fusion ROLLS ROYCE (1971) Ltd 1 Nov 1972 [6 Nov 1971] 51659/71 Heading B3R In plasma arc welding, means are provided for controlling the flows of plasma-forming gas (e.g. argon), welding current and filler metal and for co-ordination of these flows so as to provide a gradual lead-in to an even constant weld and a gradual fade-out therefrom. The invention is particularly applicable to circumferential seam butt welding of cylinders (e.g. of nickel alloy) to achieve satisfactory overlap. Either the work or the torch is traversed. The welding may be effected in a single run (" keyhole " technique) or several runs. Fig. 2, shows a typical relationship between plasma-forming gas, welding current and filler metal flows shown respectively by the lines 29, 30, 31. The gas flow 29 is started at 17 at say 50% and finished at 22 level of say 30% of steady maximum requirement 19 to 25. The point 19 may lie to the right or left of the start 26 of the steady maximum current 26 to 27. The commencement of current decrease 27 occurs when gas flow has been reduced to a minimum at 22. Wire feed commences at the point 21 of the start of increase in gas flow and finished when current falls to the point 23. The current point 18, coinciding with gas point 21, and current point 23 correspond respectively to current for " keyhole " establishment and 20% steady maximum. Athough the current is shown starting from and finishing at zero, in practice it may be started and finished at about 30% and below 50% steady maximum current. HF may be used for arc initiation. The apparatus comprises a torch 3 having a central non-consumable electrode, e.g. tungsten, and an arc-constructing nozzle 6. Filler wire 9 is fed by a motor 7 to the arc via a cooler 10, e.g. water-cooled. Oxidation preventing gas is supplied to the torch and the underside of the workpiece, such supply commencing before and continuing until after the welding. Plasmaforming gas flow is adjusted by a valve 39 which is actuated via a gear-box 38 by an electric motor 37 powered via a servo-amplifier 36. The actual position of the valve is detected by a feedback potentiometer 41 driven by the gear-box to provide a signal 42 which together with a desired position signal 43 is fed to the amplifier 36 the latter subtracting 42 from 43. A damping feedback loop 40 is triggered by a gear-box lay shaft. In operation, the operator presses a start button 44 to energize a relay 45 which energizes a relay 46 and hence changes over contacts 47, 48. The contacts 47 enable a block 49 to send to the amplifier 36 a signal at 43 corresponding to an operator pre-set gas flow point 17. The relay 45 also closes a switch 32 to initiate current supply to the torch 3. The contact 48 connects in a block 52 to provide a signal 35 to a switching amplifier 53. When a signal 34 corresponding to an arc current sensed at 33 equals the signal 35, realys 54, 55 are energized. Relay 54 starts the wire feed motor 7 which is then driven at a pre-set rate. Relay 55 changes over contacts 57 to connect a gas increase rate block 58 to the signal input 43, a block 59 determining pre-set maximum gas flow. Blocks 51, 62, 69 determine current rate increase, maximum current, and current rate decrease respectively and may be features of the welding power supply or separate units. At point 25, the operator presses button 63 (or a time switch may be used) to de-energize relays 46, 55 and change over contacts 47, 48, 57. Gas flow decrease rate is controlled by a block 64. A feedback signal from the potentiometer 41 is compared with a signal from a final gas flow block 65 by a switching amplifier 66 which when the two signals are equal energizes a relay 67 to enable the block 69 to start the current decrease. On decrease of current so that the signal 34 equals the signal 35 from a block 71, the amplifier 53 switches off to release the relay 54 and stop wire feed. A re-set button (not shown) may be provided to prepare for a second run, by returning all two-way switches to a null position and disconnecting all one-way switches. Workpiece traverse may be controlled manually or by switches, e.g. starting and finishing may be by the switch 44 and a time switch.