cle. Specifically, when the wire with molten droplet is in contact with the weld pool, the current increase that occurs is controlled so that spatter resulting from the clearing of the short circuit is reduced. These advanced short circuit transfer modes are deployed for many applications, including those that have historically used short-circuit transfer GMAW but desire lower spatter levels, as well as root pass welding in the pipeline industry. Both the conventional and advanced short circuit modes are most often deployed semiautomatically. The advanced short circuit modes are insensitive to changes in the contact tip-to-work distance, making them ideally suited for semiautomatic operation (Refs. 2, 3). With RWF-GMAW, the wire is reciprocated in and out of the weld pool in synchronization with the current waveform (Refs. 4–9). This process is an even more controlled variation of GMAW because of the synchronization between the wire feed and current waveforms. Plots of wire feed speed, current, and voltage vs. time for a weld made with Fronius’s Cold Metal Transfer (CMT) process are shown in Fig. 3. High-speed video images of a CMT weld are shown in Fig. 4. During the arcing phase, a droplet is formed on the end of the wire. The wire is retracted and then moved toward the weld pool. The shorting phase begins when the wire and molten droplet come into contact with the weld pool. The current first increases slightly, and then reduces to a minimum value. The retraction of the wire, in combination with the surface tension forces, cause the droplet to detach. After the droplet detaches, an arc is formed and the cycle repeats itself. Similar wire feed speed, current, and voltage waveforms are expected for RWF-GMAW welds made with other manufacturer’s equipment. With RWF-GMAW, there is no current spike when the wire and molten droplet come into contact with the weld pool. With both conventional and advanced short circuit modes, droplet transfer is achieved through the combination of the pinch force imposed by an increased current level and surface tension forces. With the RWF-GMAW mode, the current isn’t used to transfer the droplet, rather the droplet is transferred through the combination of surface tension forces and the mechanical retraction of the wire (Ref. 9). The wire retraction greatly reduces the spatter that’s typically associated with conventional short-circuit GMAW (Refs. 5, 9, 10). The arc is only established for a portion of the cycle and therefore only inputs heat for a portion of the cycle, which contributes to the low heat input levels that are characteristic of RWF-GMAW (Refs. 4, 10). A fast-freezing weld pool is characteristic of welds made with RWFGMAW because the arc is only established for a portion of the cycle, and the current is reduced during the shorting phase. The frequency of RWF-GMAW can range from around 20 up to 90 cycles/s depending on the equipment manufacturer and program parameters used. The RWF-GMAW process has very low spatter levels when set up correctly. For many applications, RWFGMAW will produce no visually observable spatter. It’s capable of heat input levels ranging from less than 1 kJ/in. up to that of the GMAW-P mode. Wire feed speed settings commonly range from 35 up to several hundred in./min, and travel speeds used commonly range from 6 to more than 25 in./min. In addition to its low heat input and low spatter characteristics, RWFGMAW is capable of having low dilution levels and precise bead placement. It’s more complex than the conventional and advanced short-circuit modes. A given wire feed speed setting can have 11 or more parameters associated with it. However, some manufacturers offer precanned programs that enable synergic control of the welding program. These precanned programs can be used in the asreceived condition, or the background parameters can be tailored for the specific application. The RWF-GMAW process is most commonly applied as a mechanized or automated process rather than a semiautomatic process. Also, RWF-GMAW equipment is typically more expensive than the equipment required for conventional and advanced short-circuit modes. Due to the added complexity and cost, RWF-GMAW is most often utilized for applications that warrant its positive attributes. For the majority of applications EWI, Columbus, Ohio, has applied this process for, RWF-GMAW was selected due to one or more of the following process capabilities: 1) low heat input, 2) low and controlled base metal dilution, and/or 3) precise bead placement. The organization has been deploying this process for more than a decade. The vast majority of this work is proprietary to the customers who funded it. Applications that this process has successfully been applied for include APRIL 2015 / WELDING JOURNAL 71 Fig. 3 — Wire feed speed, current, and voltage vs. time for a weld made with a CMT system (numbers in parentheses correspond to images shown in Fig. 4). Fig. 4 — High-speed video images of a T-joint fillet weld made with a CMT system (image numbers correspond to the waveform location in Fig. 3). RWF-GMAW Applications
Welding Journal | April 2015
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