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Welding

Tandem welding

By Franz J. Rossmann

Above: By permitting various process combinations and providing specially prepared characteristics, CMT Twin supports a broad spectrum of standard applications.

Multiwire weld processes offer more economical welding of plates and sheets

April 2014 - Simultaneous welding with several electrodes has long been an economically efficient method of high-performance welding. In practice, however, users can have difficulty identifying the correct parameters. The reason lies in the complexity of the welding systems and processes that have been available until now. 

The development of new weld processes for mechanical engineering, shipbuilding and vehicle manufacturing has always sought to boost productivity. In the past, particular attention has been paid to shortening welding times by increasing the welding speed, minimizing post-weld machining and straightening work needed to weld large cross-sections of thick-walled plates. Alongside single-wire processes with higher deposition rates, such as submerged-arc and electroslag welding, efforts have focused on developing systems for welding with several electrodes simultaneously.

A newer process, CMT Twin, from Austria-based Fronius, which has U.S. headquarters in Portage, Ind., aims to make matters easier. Right from welding start-up, the process is more controlled, and the characteristics provided by the manufacturer for various welding tasks make it easier to work with. Better results are possible with this tandem process, as well as efficient welding of light-gauge sheets.

There’s a distinction here between double-wire and tandem welding processes. The advantage of both two-wire processes lies in their higher deposition rate, welding speed and resultant lower thermal input. Welding with two wires in a shared weld-pool offers the added advantage of a larger weld-pool that solidifies. Also, two-wire processes make it possible to increase gap-bridging by turning the welding torch sideways, allowing broad wetting to sidewalls in the top pass with no need for oscillation.

In double-wire welding, otherwise known as parallel-wire welding, two wire electrodes are melted off using a shared torch in which both wires are at the same electrical potential. Depending on the power requirements, welding is carried out either with a shared power source or with two separate ones. Separate control of the weld processes is not possible in either case, however, because only one common voltage is available for use as a control variable. The two electrodes are melted off at an approximately equal wirefeed speed. This high deposition rate is not needed in all welding applications, such as on lap-welds with small seam cross-sections. 

Moreover, thermal input is still high despite the faster welding speed. This can make work more difficult. To ameliorate this, the arcs must be set as short as possible. On one hand, it is not possible to control dip-transfer arcs in double-wire welding, which means these arcs cannot be used. On the other hand, the longer the arc, the greater the interference, with the result that in double-wire welding, negative effects such as spattering and process interruptions are almost impossible to prevent. In many cases, then, tandem welding is the better alternative.

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Tandem welding

Unlike 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 (dip-transfer, spray and pulsed) can be combined in the same system. Each arc voltage can be measured separately and used as the control variable.

For stability, the two arcs used in pulse welding are synchronized in 180 degrees of phase opposition. Such is the case in Fronius’ advanced TimeTwin tandem welding process. Although the wirefeed speeds can diverge here, this is only possible within comparatively narrow boundaries. Due to the phase displacement, arcs are synchronized to eliminate issues with arc blow—there is less spattering than in double-wire welding or conventional tandem welding, and the arcs are more stable. Tandem processes are generally characterized by lower thermal input than double-wire processes.

With some welding tasks, however, 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 horizontal-vertical PB position or in out-of-position work. In these welding positions, process stability also suffers. Such cases call for a more stable process with lower, more adjustable thermal input.

CMT Twin control

For applications where greater process stability and uninterrupted control across the entire power range are required, Fronius has developed CMT Twin. This welding system is equipped with two digital power sources that work separately. This lets weld processes be individually adjusted to different application requirements in each case, and also means that—within the given physical limitations—any wirefeed speed can be chosen. As a result, widely diverging wirefeed speeds can be set. It is even possible to use entirely different weld processes. This novel twin-wire solution lets users exploit two cold-metal-transfer processes, or combine a GMA pulsed-arc welding process (‘lead’) with a CMT process (‘trail’), all within one single system. 

With the CMT process, the electrode is moved rapidly back and forth, in a controlled manner, while welding is in progress. The short circuit (when the electrode touches the weld-pool) initiates the reverse motion of the electrode. After a defined arc-burning duration has elapsed, the wire changes direction and is moved back towards the workpiece. 

Unlike with a pulsed or spray arc, the droplet is thus detached in a controlled manner during the short circuit. In CMT, moreover, 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 less than from a conventional dip-transfer arc, as the short circuit is not broken under high current-flow (as it would be in a conventional dip-transfer arc), but at a low amperage when the wire is retracted. This controlled shedding of the droplet into the weld-pool means little or no spattering occurs, making the process even more stable in comparison with conventional (pulsed) arc processes. 

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In CMT Twin, these properties can be combined with the advantages of a GMA pulsed-arc welding process. For example, the first (‘lead’) wire electrode can achieve deep penetration with a pulsed arc and/or a high welding amperage, while the following (‘trail’) electrode works in CMT mode and fills the seam as needed, with reduced wirefeed speed and heat input.

The possible welding speeds are high; in the case of a fillet weld welded at the lap joint of two 2 mm steel sheets, they can be as fast as 4 m/min. Due to its lower thermal input, CMT Twin performs better than conventional tandem processes in terms of gap-bridging ability as well.

The question of which combination of processes should be chosen for CMT Twin will depend on the requirements applying to each welding task in terms of welding speed, deposition rate and gap-bridging.

To make fillet welds in the PB position or out-of-position, it is best to combine a pulsed arc (‘lead’) with the CMT process (‘trail’). This combination lets welders achieve ‘textbook’ results, even when welding a6 fillet welds in the PB position.

Compared to conventional tandem processes, combining the pulsed-arc and CMT processes in this way, with all other parameters remaining the same, decreases the thermal input by between 10 and 20 percent. The thermal input from a CMT-CMT combination is even lower. 

To give one example, CMT Twin allows out-of-position welding of root passes that would either not be possible with other welding processes, or not economically feasible. 

Easy to handle

To ensure a perfect weld right from the very beginning, in CMT Twin the trail electrode does not start work until the patented-lead electrode process has stabilized and the power source for the trail electrode has received a start-up signal. The high process stability and lower level of reciprocal influencing have made it possible for Fronius to draw up optimized characteristics for various applications. These coordinate the processes for users, making it easier to identify the correct parameters. Among the characteristics offered by this welding-technology specialist are ones for welding at high speed (‘speed’), for joining thick plates (‘heavy duty’), for root welding (‘root’) and for overlay welding with two CMT arcs (‘cladding’). Fronius also provides characteristics for extra stable, low-spatter welding under pure CO2 atmospheres. This differentiates CMT Twin from conventional two-wire processes, in which welding with low-cost carbon dioxide shielding gas either leads to poor welding results or else is not possible at all.

CMT Twin combines the advantages of TimeTwin and CMT in one system. Compared to conventional two-wire processes, the TimeTwin and CMT combination is ideal for high process reliability, seam quality, gap-bridging ability and speed. This requires minimal post-weld machining. Thanks to these special performance attributes, CMT Twin delivers better welding results than conventional double-wire or tandem processes. CMT Twin also stands out for its handling ease. All these many capabilities make CMT Twin ideal for use in the automotive and supplier industries, in shipbuilding and mechanical engineering, and power generation. FFJ

Franz J. Rossmann is a freelancer writer.

 

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