Gas Metal Arc Welding Metal Transfer Modes The characteristics of the gas metal arc welding (GMAW) process are best described in terms of the basic means by which metal is transferred from the electrode to the workpiece. These modes of metal transfer are short-circuiting transfer, globular transfer, and spray transfer — Fig 1. A number of factors determine the mode of transfer, the most influential of which are the following: 1. Magnitude, type, and polarity of welding current 2. Electrode diameter 3. Electrode composition 4. Electrode extension 5. Shielding gas composition. Short-circuiting transfer. Employed in short-circuit gas metal arc welding (GMAW-S), short-circuiting transfer encompasses the lowest range of welding currents and electrode diameters associated with GMAW. Metal transfer results when the molten metal from a consumable electrode is deposited during repeated short circuits. This mode of transfer produces a small, fast-freezing weld pool generally suited for joining thin sections, for out-of-position welding, and for bridging large root openings. In this mode, metal is transferred from the electrode to the workpiece only during the period in which the electrode is in contact with the weld pool. No metal is transferred across the arc. The electrode contacts the weld pool in a range of 20 to more than 200 times per second. Globular transfer. The globular transfer mode involves the transfer of molten metal in the form of large drops from the consumable electrode across the arc. This transfer mode is characterized by a drop size with a diameter greater than that of the electrode. Gravity easily acts upon this large drop, thereby generally limiting the successful application of this mode of transfer to the flat position. At average current ranges only slightly higher than those in short-circuiting transfer, axially directed globular transfer can be achieved in a substantially inert gas shield. If the arc length is too short (indicating low voltage), the enlarging drop may short to the workpiece, become superheated, and disintegrate, producing considerable spatter. The arc must Fig. 1 — Metal transfer in gas metal arc welding. be long enough to ensure detachment of the drop before it contacts the weld pool. A weld made with higher voltage is likely to be unacceptable because of incomplete fusion, incomplete joint penetration, and excessive weld reinforcement. This characteristic greatly limits use of the globular transfer mode in production applications. Spray transfer. The spray transfer mode occurs when the molten metal from a consumable electrode is propelled axially across the arc in the form of minute droplets. With argon-rich (at least 80%) gas shielding, it is possible to produce a very stable, spatter-free axial spray transfer mode. This mode requires the use of direct current with a positive electrode (DCEP) and a current level above a critical value, termed the spray transition current. Below this current level, transfer occurs in the globular mode at a rate of a few drops per second. Above the transition current, transfer occurs in the form of very small drops that are formed and detached at the rate of hundreds per second. They are accelerated axially across the arc. WELDING WORKBOOK 84 WELDING JOURNAL / APRIL 2015 WJ DATASHEET 355 Excerpted from the Welding Handbook, Vol. 2, 9th Edition, Welding Processes, Part 1, and AWS A3.0, Standard Welding Terms and Definitions.
Welding Journal | April 2015
To see the actual publication please follow the link above