GB1450912A - Arc welding process - Google Patents

Abstract

1450912 Welding by fusion WELDING INSTITUTE 30 Oct 1974 [31 July 1973] 36375/73 Heading B3R [Also in Division H2] In submerged arc, TIG or, as shown, MIG automatic or semi-automatic arc welding process (including build-up welding), a welding torch or gun moved relatively along a predetermined path comprises at least two electrodes disposed transversely of said path, and each electrode receives a continuous low level background current and pulses of higher level current said pulses being supplied sequentially such that no two electrodes pass a higher level current at the same time. Current level and pulse duration, in respect of different electrodes, and composition, size, feed rate and stick-out of consumable electrodes may be varied. In the MIG welding arrangement of Fig. 2, contact tubes 10a, 10b, in a common shielding gas nozzle 13, are fed with background current from a low level supply P1 and alternately with pulses derived from a single phase A.C. supply via a transformer T1 and diodes D1, D2. Consumable electrode wire 14 is fed by a motor M1, and a workpiece 11 presenting a joint line 12 is traversed by a motor M2. Each pulse detaches a metal droplet from the relevant electrode. The electrodes are shown equally spaced about the joint line but their orientation and height may be varied. In an alternative embodiment, three electrodes may be fed from a three-phase supply. For superimposed thermal pulsing providing periodic reduction of heat to the weld pool, for better control of cooling and solidification of the latter, the diodes D1, D2 may be replaced by silicon controlled rectifiers which are switched on and off by a cyclic timer, the latter also reducing power to the wire feed motor M1 to reduce wire feed rate while background current is being supplied. The background current level may also be reduced by the timer. Current and thermal pulse frequency may be 50 and 5 Hz respectively and the mark-space ratio of the thermal pulsing may be 1 : 1; alternatively, thermal pulsing frequency may be such that alternate current pulses are omitted. In Fig. 3 (not shown) a pair of electrodes are fed alternately with pulses derived from a D.C. supply via a mechanical commutator driven by a variable speed motor, background current being continuously supplied from said D.C. supply via resistors. In Fig. 4 (not shown), a pair of electrodes are fed alternately with pulses derived from a D.C. supply via transistorized switches under logic control background current being continuously supplied from said D.C. supply via resistors. Alternatively for any number of electrodes, e.g. in build-up welding, the transistorized switches may be controlled by a sequence timer. In Fig. 5, pulse supplying transistorized switches TS1, TS2 controlled by a logic SL are fed by power selected from a Programme Bank the output of which also adjusts various welding parameters. Selection from the Bank is by the welder or by a Comparator and Selector Unit. Signals from transducers Z1, Z2 detecting heat penetration along each side of the joint line and from transducers Z3, Z4 detecting weld gap non-uniformities are fed through amplifiers A1, A2 to the Comparator and Selector Unit to select from the Bank respectively to vary heat input to each side and to vary arc power and/or traverse speed. The transducers Z3, Z4 may also detect joint linear variation to act via servomotors (not shown) to alter the electrode positions. In Fig. 6, contact tubes 35a, 35b are threaded at angles into guide tubes 32a, 32b which are held in liquid cooled clamps 31a, 31b and are rotatable, e.g. by manually or servo-actuated gearing, to vary the distance between electrodes. Shielding gas is fed to a common nozzle 30 through a gas lens 33 comprising a pattern of holes of small diameter-to-length ratio providing laminar gas flow. The lens 33 may be of electrically insulating material or metal in which latter case the guide tubes have insulating bushes. In Fig. 8 (not shown), a third electrode is provided for disposition on the joint line behind or in front of an abreast pair. The third electrode may be replaced by a cold or pre-heated areless filler wire. In a combination of MIG electrodes and a TIG electrode, the former may have pulses superimposed on background current and the latter may receive a continuous current. The MIG and TIG ares may be connected in series. For welding thick-to-thin sections, multiple electrodes may be asymmetrically positioned across the joint line. Argon with 5% oxygen is referred to as a shielding gas.