GB1338866A - Method of and device for plasma-mig-welding - Google Patents

Abstract

1338866 Welding by fusion PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 29 March 1972 [1 April 1971] 14811/72 Heading B3R In joining or deposition of metal by plasma- MIG-welding, wherein an arc is maintained in a gas flow between a non-consumable electrode 9 and a workpiece 35, the arc plasma is constricted by a nozzle 5, a current-carrying consumable welding wire 11 is fed axially of and into said arc plasma, and a MIG-arc is maintained between the wire and the workpiece, the electrode 9 and the wire 11 are connected to positive terminals of two independently controllable current supply sources and the forms of the MIG arc and the material transfer from the wire are varied by varying the current in said wire. The current sources preferably form part of a common control unit. At a low welding wire current there is a drop-like (globular) material transfer which changes gradually as the current is increased into an axial transfer of very small droplets (axial spray). Upon reaching a critical value of welding current (referred to as transition current) there is a change to rotating transfer of very small droplets (rotating spray). The wire 11 is fed, by controllable motordriven rollers 15, through a guide and contact tube 13 which is connected to the positive terminal of a flat voltage-current characteristic current supply source 33. The electrode 9 (e.g. tungsten) is carried in a holder connected to the positive terminal of a drooping voltage-current characteristic current supply source 29 via an H.F. generator 31. The negative terminals of the sources 29, 33 are connected to the workpiece 35. The tube 13, the electrode holder 8 and a gun body 3 are water cooled through ducts 23<SP>1</SP>, 23<SP>11</SP>, 25<SP>1</SP>, 25<SP>11</SP>, 27<SP>1</SP>, 27<SP>11</SP>. Plasma gas, e.g. argon, and a shielding gas, e.g. a mixture of argon and CO 2 or argon, CO 2 and O 2 , are fed respectively through ducts 19, 21. As shown in Fig. 3, a low welding wire current produces a MIG-arc in the form of a contracted, non-rotating cylinder within a plasma-arc 37 and provides for a narrow bead with deep penetration. As shown in Fig. 5, at the transition current the MIG-arc suddenly changes into a rotating arc 43 describing a cylindrical surface 45, the end 47 of the welding wire 11 becoming plastic and bent laterally to describe a conical surface 49. This shape of arc is particularly suitable for metal deposition with a wide bead and small penetration. It is found that the transition current increases with increase in welding wire diameter and increase in plasma nozzle aperture diameter, and that deposition rate and arc rotation speed increase with welding wire current increase. The Specification includes examples of butt welds of mild steel plate using 1À6% Mn, 0À8% Si, 0À1% C, balance Fe welding wire: and of butt welding stainless steel plate and deposition of stainless steel on mild steel plate using 20À6% Cr, 9À7% Ni, 1À8% Mn, 0À85% Si, 0À025% C, balance Fe welding wire.