WELDING WORKBOOK Metal Transfer Modes in Gas Metal Arc Welding Fig. 1 — Schematic representation of short circuiting metal transfer. Fig. 2 — Schematic representation of globular transfer. 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. The modes of metal transfer for GMAW are short-circuiting transfer, globular transfer, and spray transfer. A number of factors determine the mode of transfer. Following are the most influential factors: 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 (Fig. 1) encompasses the lowest range of welding currents and electrode diameters associated with the process. 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 that is generally suited for the joining of thin sections, for outof position welding, and for bridging large root openings. In short-circuiting transfer, metal is transferred from the electrode to the workpiece only when the electrode is in contact with the weld pool. No metal is transferred across the arc. The electrode contacts the weld pool from 20 to more than 200 times per second. Globular Transfer In this mode, molten metal in the form of large drops transfers from the consumable electrode across the arc — Fig. 2. This Datasheet 346 transfer mode is characterized by a drop size with a diameter greater than that of the electrode. Gravity easily acts upon this large drop, generally limiting the successful application of this mode of transfer to the flat position. At average current rates that are only slightly higher than those used 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 therefore be long enough to ensure detachment of the drop before it contacts the weld pool. A weld made with a higher voltage is likely to be unacceptable because of incomplete fusion, incomplete joint penetration, and excessive weld reinforcement. This characteristic greatly limits the 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 — Fig. 3. 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 the 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 that are accelerated axially across the arc.® Excerpted from the Welding Handbook, Vol. 2, Part 1, ninth edition. Fig. 3 — Schematic showing the spray transfer mode. 70 FEBRUARY 2014
Welding Journal | February 2014
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