Achieving Higher Productivity Rates Using Reciprocating Wire Feed Gas Metal Arc Welding Reciprocating wire feed gas metal arc welding (RWF-GMAW) is a variation of GMAW that’s capable of producing virtually spatter-free welds with precise heat input control and bead placement. A feature that distinguishes this process from advanced short-circuit GMAW processes is the mechanical pulsing of the wire into and out of the weld pool. Droplet detachment is achieved via surface tension forces in combination with mechanical retraction of the wire. For many applications, RWF-GMAW has enabled higher productivity rates over legacy processes, and it has enabled welding materials that were previously considered unweldable with conventional arc welding processes. Gas metal arc welding is a fusion welding process that produces coalescence of metal using the heat from an arc between a continuously fed consumable electrode wire and the base metal — Fig. 1. Electrical contact is made to the consumable electrode wire at the contact tip. A stream of shielding gas is fed through the nozzle and displaces the atmosphere in the vicinity of the arc and weld pool. In addition to preventing atmospheric contamination of the weld pool and heated consumable electrode wire, the shielding gas also provides the medium for current flow. For conventional GMAW, there are four primary modes of metal transfer: short-circuit, globular, spray, and pulse spray transfer. Short-circuit transfer has historically been used for applications requiring low heat input, low distortion, and/or small weld size — Fig. 2. With the short-circuit mode, the current level is not sufficient to maintain an arc or to enable transfer of droplets across the arc. The droplet forms on the end of the wire and is transferred to the weld pool when the wire with molten droplet come into contact with the weld pool. When the wire with molten droplet comes into contact with the weld pool, an electrical short is created and the current spikes. The combination of surface tension forces and magnetic pinch force created by the current spike cause the droplet to transfer to the weld pool and the arc is reestablished. The electrode contacts the molten weld pool between approximately 20 and 200 times/s (Ref. 1). The short-circuit mode can be used to produce relatively low heat input welds with lower distortion levels than that achievable with other GMAW modes. The process uses conventional GMAW equipment and has a limited number of controls. However, because of the violent nature of the shortcircuiting event, the process mode is characterized as having high spatter levels. The short-circuit mode also has a tendency for incomplete fusion discontinuities when welding thicker materials and is not allowed by several codes. Several manufacturers have advanced versions of the short-circuit mode that enable more controlled droplet transfer, precise bead placement, and reduced spatter levels. These advanced versions control the current waveform throughout the stages of the short circuit transfer cy- 70 WELDING JOURNAL / APRIL 2015 This process is discussed in comparison with short-circuit gas metal arc welding modes and other welding methods BY NICK KAPUSTKA Fig. 1 — Illustration of GMAW. Fig. 2 — High-speed video image of a short-circuit transfer weld made with conventional GMAW equipment. Low Heat Input GMAW Modes
Welding Journal | April 2015
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