Form Join Cut
Figure 2: CMT Twin process sequence when the pulsed-arc and CMT processes are combined
Figure 1: The CMT Twin tandem welding system has two independently functioning digital power sources
The high welding speed can also cause problems in itself, such as undercutting. To prevent this, the arcs must be set as short as possible. On the one hand, it is not possible to control dip-transfer arcs in double-wire welding. On the other hand, when the arc is longer, there will be more interference. As such, negative effects such as spattering and process interruptions are almost impossible to prevent. Tandem welding is the better alternative here. Tandem Welding Unlike in double-wire welding, tandem welding always uses two mutually insulated power sources and a torch with isolated current conduction. In this way, all common types of arc (diptransfer, spray and pulsed) can be combined in one and the same system. Each arc voltage can be measured separately and used as the control variable. For ma ximum stability, a tandem welding process that (eg: TimeTwin) synchronises two arcs used in pulse welding in a 180 degree phase opposition is used. Although the wirefeed speeds can be diverged here, this is only possible within comparatively narrow boundaries. Due to the phase displacement, the weld-pool stays quieter in TimeTwin — there is less spattering www.equipment-news.com
than in double-wire welding or conventional tandem welding, and the arcs are more stable. Tandem processes are generally characterised by lower thermal input than double-wire processes. With some welding-tasks, even this decreased thermal input may still be a problem. Fillet welds are a good example: if the weld-pool becomes too fluid, the seam will sag when welding is performed on larger seam cross-sections in the PB position or in out-of-position work. In these welding positions, process stability also suffers. Cases such as these call for a more stable process in which the thermal input is even lower and can be precisely adjusted. More Flexibility & Control In Tandem Welding Fo r a p p l i c a t i o n s i n w h i c h greater process stability and uninterrupted control of the weld process across the entire power range are required, a tandem welding system may be more suitable (Figure 1). This welding system is equipped with two digital power sources that work completely separately from one another. This allows the weld processes to be individually a d j u s te d to t h e d i f f e r e n t applicational requirements in each case, and also means that — within the given physical limitations —
any wirefeeder can be chosen. As a result, widely diverging wirefeed-speeds can be set. It is even possible to utilise entirely different weld processes. This twin-wire solution allows users to exploit two Cold-Metal-Transfer (CMT) processes, or combine a GMA pulsed-arc welding process (‘Lead’) with a CMT process (‘Trail’), all within one single system (Figure 2). A characteristic feature of the CMT process is that the electrode is moved rapidly back and forward, in a controlled manner, while welding is in progress. The short circuit (when the electrode touches the weldpool) initiates the reverse motion of the electrode. After a defined arc-burning duration has elapsed, the wire changes direction and is moved back towa rds the workpiece. Unlike a pulsed or spray arc, the droplet is now detached in a controlled manner during the short circuit. In CMT, the arc length is subject to little or no fluctuation, as the wire is retracted by a defined distance independently of the stickout. The heat input is significantly less compared to a conventional diptransfer arc, as the short circuit is not broken under high current-flow (as it would be in a conventional diptransfer arc), but at a low amperage when the wire is retracted. May-Jun 2013 asia pacific metalworking equipment news
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