A new iteration of Fronius International’s CMT welding process has been gaining attention lately, with presentations at EuroBLECH 2012 in October and FABTECH 2012 in November. The developer said the twin-wire version of the concept lets users exploit two cold-metal-transfer (CMT) processes, or combine a single CMT process with a gas-based pulsed-arc welding process.

Fronius developed CMT as package system for joining steel and aluminum, but it also addresses critical mechanical and technological objectives. It is the result of research in MIG/MAG technology that indicated – despite past difficulties and doubts - that arc welding could be a successful approach for joining the two materials.

CMT is a controlled process in which the aluminum base material melts together with the aluminum filler, with the melt wetting the galvanized steel. The filler wire is constantly retracted at very short intervals. The precisely defined retraction of the wire facilitates controlled droplet detachment to give a clean, spatter-free material transfer. The wire movement takes place at a very high frequency and requires a quick-response, gearless wire drive directly on the torch. Because the main wire feeder will not be able to keep up with this movement, the wire-feeding hose provides with a wire buffer that compensates for the forward and backward movement of the wire.

The new CMT Twin package involves two digital power sources that work independently from each other. This allows the welding processes to be adjusted individually according to the different application requirements of the user. Further, it means that, within physical limitations, any wire feeder can be selected.

This makes it possible to set a wide variety of wire-feeding speeds, and it allows significant flexibility among the choice of welding parameters. According to Fronius, users can control the thermal input and deposition rate with precision, and as a result they are able to achieve optimal welding speeds and seam quality with virtually any welding position and combination of materials.

The developer calls CMT Twin an “ideal complement” to its high-performance twin-wire weld process, Time Twin.

The Time Twin uses two arcs (one leading, the other trailing, which may be pulsed if necessary), synchronized in 180° phase opposition. As a result, there is a relatively limited opportunity to vary the wire-feeding speeds. Consequently, TIME Twin is the right choice when the priority is to raise the welding speeds and/or deposition rate.

However, on welds requiring a large seam cross-section, there is a risk that excessive heat input may cause problems. For example, on fillet-welds, the seam may 'sag' if the weld-pool becomes too liquid. These cases call for a process in which the thermal input is lower and can be precisely adjusted.

It’s in these applications that Fronius indicated CMT Twin is especially well suited. Because it uses the CMT process - in which the electrode is moved forward and backward in a controlled manner, to assist droplet detachment – CMT Twin imparts considerably less heat than would a conventional dip-transfer arc. Other advantages are that it promotes very little spattering, and the process is more stable because the arc length is unaffected by the surface of the workpiece or the welding speed. As a result, it is possible to achieve high welding speeds: a fillet weld in the lap joint of two, 2-mm steel sheets can be completed as fast as 4 m/min.

The question of which combination of processes is right for CMT Twin will depend on the users’ requirements for different welding tasks, with respect to welding speed, deposition rate, gap bridging, and frequency of spattering.

For example, for difficult fillet welds it is best to combine a pulsed arc ('lead') with the CMT process ('trail'). While the leading electrode is fed at a high wire-feed speed to achieve the desired deep penetration, the trailing electrode fills the seam at a low wire-feed speed, and with minimized thermal input. Fronius stated this combination would let welders achieve 'textbook' results, even when welding a6 fillet welds in the horizontal-vertical (PB) position.