Abstract:
In a method for plasma-arc welding of metals with a direct or indirect arc, used as a plasma-producing medium is the vapor of a water-containing fluid doped with an organic solvent appearing as oxygen-containing hydrocarbon compounds, in particular, alcohols, and vapors are generated immediately in the plasma-arc torch.

Description:
TECHNICAL FIELD 
     The present invention relates to methods for plasma-arc welding of metals and can find application in mechanical engineering, construction engineering, and in other industries. 
     BACKGROUND ART 
     Methods for plasma-arc Welding of metals using the direct constricted-arc welding technique are in widespread use currently, wherein metal is fused with a plasma jet (cf. U.S. Pat. No. 4,791,268, IPC B23K 9/00, 1988; SU #1,683,927, IPC B23K 10/00, 1991). Extensively used nowadays are also methods for welding metals with a direct constricted arc stricken between the plasmatron electrodes and the workpiece (cf. SU #880,6,54, IPC B23K 10/02, 1981; the textbook &#34;Microplasma-arc welding&#34; ed. by B. Ye. Patton, Ukraine, Kiev, &#34;Naukova dumlka&#34; publishers, 1979, pp.19-21 (in Russian). The methods mentioned above provide for establishing a plasma jet by constricting an arc discharge with an inert gas to obtain quality welded joints. However, provision of an external inert-gas source involves some operational inconveniences in carrying said method into effect, while high production cost of the inert gas and considerable expenses for recharging gas bottles impose substantial limitation on practical applicability of the aforementioned method of plasma-arc welding. In addition, handling easily inflammable metals involves using not only a plasma-producer gas but also a shielding gas (cf. SU #1,680,463, IPC B23J 10/02, 1991). 
     Known in the present-state welding practice are plasma-arc welding torches (plasmatrons), wherein used for establishing a plasma jet is vapor fed from a special vapor generator or generated in the welding torch itself by evaporating the fluid by virtue of the heat released on the electrodes (cf. PCT RU Application #93/00053 published Sep. 1, 1994 under # WO/94/19139). The latter torch is most economic and simple in operation, though its use in welding is difficult due to intense metal oxidizing caused by the presence of vapor and molecular and atomic oxygen in the plasma jet. 
     DISCLOSURE OF THE INVENTION 
     The present invention is aimed at the provision of a method for plasma-arc welding of metals using vapor as a plasma-producer medium ensuring high-quality welded joints due to reduced oxidizing properties of the plasma jet. The essence of the invention resides in that in a method for plasma-arc welding of metals with a direct or indirect constricted arc comprising constricting the arc with a plasma-producer medium to obtain a plasma jet, used as the plasma-producer medium is the vapors of a fluid containing water doped with an organic solvent appearing as oxygen-containing hydrocarbon compounds capable of preventing oxygen atoms and molecules contained in said vapors from entering into oxidation reaction with the metal being handled. 
     It is found experimentally that the best results is obtained when used as the additives are alcohols, their optimum percentage content in the vapor-generating fluid from the viewpoint of energetics and welded joint strength being within 30 and 50. When the percentage of alcohols in the mixture is below the lower limit, a larger proportion of scale occurs in the welded joint which in turn affects adversely its strength characteristics. In case the percentage of alcohols in the mixture exceeds 50, the miscibility of the organic solvent with water is affected with the resultant stratification of the mixture and hence its nonuniform evaporation. Moreover, vapor enthalpy is reduced in this case which in turn reduces the plasma-jet temperature. Best results were obtained when using a plasma-arc welding torch, wherein the plasma-producing vapor is obtained from a fluid most commonly used in every-day life, consisting of 60% water and 40% ethanol. 
     Vapor can be generated immediately in the plasma-arc torch by evaporating the fluid filling a special container, by virtue of heat energy generated by the arc burning between the torch electrodes. 
     The herein-proposed method for plasma-arc welding of metals can be carried out using both indirect plasma-arc torches, wherein heat energy is transferred to the workpiece directly by the plasma jet emerging from the nozzle-anode of the welding torch and direct plasma-arc torches, wherein the workpiece is acted upon by a plasma-jet-stabilized arc. In the latter case used for fusing metals is not only the plasma-jet energy but also the energy of the electric arc which adds substantially to the energy characteristics of the method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents schematically a device for welding metals with an indirect plasma-arc; and 
     FIG. 2 presents schematically a device for welding metals with a direct plasma arc. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Variants of Practical Embodiment of the Invention 
     Given below are some exemplary embodiments of the proposed method. FIG. 1 presents schematically a device for welding metals with an indirect plasma-arc. The device comprises a plasma-arc torch (plasmatron) incorporating a cathode (1), a nozzle-anode (2) provided with passages for a plasma-producing medium, a container (3) filled with a moisture absorbent material and having an inlet (4), an electrically-insulated cover (3) and a power source (6) with a current regulator (7). Used as the moisture absorbent material may be kaolin wool, carbon fabric, or carbon felt, and as the vapor-producer fluid, e.g., a mixture of water with ethanol or acetone. 
     Welding is carried out as follows. 
     The inlet (4) of the container (3) is opened and the aforementioned fluid is filled therein, whereupon the inlet is closed. Then a voltage is applied to the cathode and the nozzle-anode. Next an arc is initiated be, e.g., reciprocating the cathode until it contacts the nozzle-anode. The heat energy released by the burning arc on the nozzle-anode causes the fluid held in the container (3) to evaporate. The resultant vapor enters the discharge chamber via the passages of the nozzle-anode and emerge through the central nozzle opening, thus drawing the arc-gaseous column. The vapor is then heated to high temperatures due to the arc-gaseous column being constricted in the opening of the nozzle-anode, and thus goes into the plasma state to establish a plasma jet at the outlet of the nozzle-anode, whereupon the resultant plasma jet is directed at the welding spot to bring the metal to the fusion point. The arc current is varied by the regulator (7). 
     FIG. 2 presents schematically a device for welding metals with a direct plasma-arc. The device comprises a plasma-arc torch incorporating a cathode (1), a nozzle-anode (2) provided with passages for a plasma-producing medium, a container (3) filled with a moisture absorbent material and having an inlet (4), an electrically-insulated cover (5), a power source (6) with a current regulator (7), as well as a second power source (9) cut in between the cathode (1) and the metal (8) being welded. 
     Welding is carried out as follows. 
     A plasma jet is established at the outlet of the nozzle-anode (2) as described hereinbefore. Once the torch has got steady-state temperature conditions, a voltage is applied from the second power source (9) across the cathode 1 and the metal 8 being welded. Then the plasma jet is directed onto the welding spot, and the space between the nozzle-anode and the metal being welded is reduced until a Direct arc is initiated therebetween and the metal (8) is fused. The depth of fusion are changed by changing the first arc current using the current regulator (7) by so selecting the regulator position as, on the one hand, to prevent molten metal from being blown out of the weld puddle and on the other hand, to provide a steady stabilization of the direct arc position. 
     Whenever a filling material (10) is used, it is electrically connected to the metal being welded and is brought into the plasma jet as necessary, wherein said material is fused under the action of the direct arc and fills the weld puddle formed on the surface of the metal being welded. 
     INDUSTRIAL APPLICABILITY 
     Pilot models of the device the diagrams of which are present in the specification are found to ensure welding of steel structures having the sheet thickness of 3 to 5 mm and the depth of fusion from 3 to 4 mm, when the aforementioned operations making the essence of the proposed method have been carried out in the specified sequence.