The modes of metal transfer for arc welding processes have been extensively characterized during the literature review (Section 2.1). The recent advances in GMA welding technologies launched new challenges to understand the physical phenomenon and the transfer characteristics resulting from these new waveform designs.
Fronius launched a new concept of transfer with CMT/CMT-P waveforms, where the power source and wire feeder are automatically synchronized and can vary to promote the final arc stability. This mechanical assisted transfer process is a result of a push-pull wire feeder in the torch body assisted by a free loop in the centre of the torch hose, where the wire can advance and retract according to the power source instructions. The wire retracting and advancing system allow much higher stable arc lengths since the wire can quickly move and touch the molten pool right after the arc ignition, and retract again after transfer is completed. This results in a smooth rupture of the molten bridge during the short-circuiting period and is characterized by the reduction of splashing and spatter projection often observed in GMAW-S, with further benefits in aesthetics and bead appearance. This is in agreement with the analysis of CMT/CMT-P by (Pickin and Young 2006).
Within the CMT-P waveform, pulse transfer is synchronized with short-circuiting, increasing significantly the arc energy, and therefore promoting higher dilution ratios and depths of penetration. This is the first time a waveform has been developed as a controlled mixed mode, since previously mixed modes have been observed but under uncontrolled conditions, as a result of instability (Palani and Murugan 2006) (Scotti 2000). Although some results have been shown for welding of aluminium with CMT-P (Pickin and Young 2006), the
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metal transfer mechanism has not been investigated, nor has arc stability control or process characteristics.
Some researchers (Feng, Zhang and He 2008) (Zhang, et al. 2009) (Pickin and Young 2006) have already analysed the mechanism of metal transfer for CMT waveform when applied to aluminium, but with limited information on process characteristics.
2.5.3.1. Classification of Metal Transfer
The first classification of the modes of metal transfer in arc welding with addition of filler metal was proposed in 1976 (Anon (1976)). Three categories were included in this classification, respectively short-circuiting or dip transfer, globular and spray transfer.
Several advances have been developed during the last three decades and new concepts included in this classification. Norrish (2003) extended this classification considering three main groups: natural, controlled and extended operating modes. The natural modes subdivide into the categories defined by Anon (1976) and the controlled modes into both short-circuiting and spray. Iordachescu and Quintino (2007) included a sketch of the modes of transfer into the classification for natural modes.
More recently, Ponomarev et al. (2009) proposed a new classification system based on three classes as follow: natural, controlled and combined metal transfer, but considering a more complex sub-classification scheme where contact mode and free-flight mode groups are established as the natural modes and different controlled and combined sub-categories are also considered. This proposal does not satisfy the initial idea of the classification where modes of transfer are simply classified by the way that metal is transferred, and then the advances in welding controlling systems are variants of the main modes.
Furthermore this classification does not provide a full range of controlled modes classified according with main parameters, instead of the mode of transfer as established in the natural modes, e.g. short-circuiting, spray and globular, the last ones included in the free-flight group. In addition, according with the results obtained for CMT-P the controlled metal transfer classification should include a sub-category for controlled mixed modes. The results of metal transfer classification shows that for the first time a welding process includes two different modes of controlled transfer, short-circuiting and pulse spray transfer.
STT, CMT and FastROOT should be included in the sub-category of short-circuiting controlling metal transfer, as already mentioned by some authors (Iordachescu and Quintino 2007) (Ponomarev, et al. 2009). GMAW-P and RapidArc should be included in the category of pulse spray controlled metal transfer. It was mentioned by Norrish (2003) that RapidArc should be considered in the category of extended stick out GMAW in the group of short-circuiting transfer, but in fact this process works as pulse spray transfer process.
The analysis of the all waveforms showed significant conditions where uncontrolled mixed modes were identified, but those are mainly due to the instability phenomena and result
133 from process setting conditions applied. These phenomena will be discussed in more detail in a further section.
Considering the Tables 2.1 and 2.2, the new classification should respect the main modes defined in the Table 2.1 and an extension should be added to the Table 2.2 in order to include the new waveforms/ processes recently developed, as illustrated in the Table 2.27, as follow.
Table 2.27 – Proposal for classification of controlled transfer modes.
Metal Transfer Mode Commercial name
Controlled Spray Pulse transfer Lincoln RapidArc
Controlled Short-circuiting Dip transfer Lincoln STT, Fronius CMT, Kemppi
FastROOT Controlled mixed modes Alternated dip transfer with pulse
transfer
Fronius CMT-P
2.5.3.2. Analysis of Metal Transfer
The classification of metal transfer for the waveforms evaluated were defined under the previous section, but different process settings may generate arc instability phenomena resulting in variations on the mechanism of metal transfer. The analysis of the processes parameters will be discussed from the results presented in Section 2.4.4.
An analysis of metal transfer for all waveforms investigated would indicate the following sequence according with arc stability: CMT>RapidArc>CMT-P>STT>FastROOT>GMAW-P.
(Note that this sequence is only valid for the experimental conditions used in this study).
Different process parameters have produced unstable disturbances for the all waveforms considered. For high current levels, i.e. high wire feed rate, different settings of arc length adjusting parameter and CTWD, and different shielding gas composition can be associated with disturbances and process instability. However, this varies according to waveform. The disturbances observed are generally reflected by variations in arc voltage, since the arc current is well controlled for most of the waveforms under investigation.
This analysis will be performed for pulse spray transfer processes and dip transfer processes separately, since the balance of forces controlling the metal transfer mechanisms is different.
Pulse Spray Transfer
In GMAW-P the irregular behaviour observed was mainly associated with a low CTWD. Two different reasons justify the application of low CTWD, the fact that the remaining processes worked positively with this condition for bead on pipe welding tests and because low CTWD’s and short arc lengths are necessary in narrow groove pipe welding to eliminate arc repulsion and deflection to the groove side (Modenesi 1990). This will be discussed in detail
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in Chapter 4. However, these unstable conditions obtained with the lower CTWD set for GMAW-P limited the understanding of the effect of other parameters, such as arc current.
In fact it is observed that at the CTWD applied (i.e., 11mm) the power source characteristics result in a very short arc length, increasing the arc pressure and resulting in arc splashing phenomenon. The trials carried out at higher CTWD (13.5 and 16mm) suggested a progressive decrease of the uncontrolled short-circuiting phenomena from 70% (11mm) to 20% (13.5mm) and 10%(16mm) (approximated percentages relative to the ratio of short-circuiting – droplet transfer mechanism). The analysis of the effect of trim at low trim values (below 1.0) shows that arc splashing is more frequent due to the increase of uncontrolled short-circuiting phenomena.
The uncontrolled short-circuiting observed for this process is mainly due to the voltage drop phenomena, which may result from the decrease of the electromagnetic force. This force has been considered the controlling force during the droplet detachment mechanism and its effect increases at high current levels by the effect of a high Lorentz force during a short time period (Palani and Murugan 2006). Furthermore, the decrease of arc voltage is also determined by the short arc length levels observed, which results from the balance of the forces acting in those conditions.
In contrast to what was observed for GMAW-P, for RapidArc good process stability is obtained with most of the welding conditions applied for CTWD as low as 11mm. The most significant unstable mechanisms observed for RapidArc are due to the short arc lengths observed at very high current levels (i.e. high wire feed rates) and when low trim (arc length adjusting parameter) values were set. The uncontrolled short-circuiting phenomenon was mostly identified at low trim values, while at high current levels the increase of voltage during background time generates stubbing phenomena characterized by explosions and spatter projections.
The CMT-P waveform is characterized by a controlled mixed with constant background and peak arc current, but where the number of pulses between each short-circuit increases with the wire feed rate. The results of metal transfer revealed that very good arc stability is achieved within this waveform. The main disturbances are observed at the high wire feed rates and at the limits of arc length correction and pulse control parameters. At very high currents unstable mechanisms are often observed, characterized by a high background voltage level. This may result from a very high arc pressure due to the deviation of burn-off criterion conditions. Rotation transfer is also observed and characterized by arc deflection when peak arc current is very high.
Dip Transfer
In short-circuiting transfer STT, CMT and FastROOT are characterized by a bridge effect developed between the molten wire and the weld pool. However, the wire motion system
135 present in CMT results in much smoother and stable control of the short-circuit promoting a clean rupture of the molten bridge.
Compared to other Lincoln waveforms analysed, i.e. GMAW-P and RapidArc, for STT welding the most stable conditions are identified within nominal trim values. In fact at very low and high trim values different disturbances were often identified, such as arc plashing, spatter and globular formations when trim is too high. This effect of globular droplet transfer is due to the significantly higher current and voltage levels identified, associated with longer arc length.
Furthermore, higher instability phenomena were found at higher current levels, at the limit of wire feed ratio allowed for this process. The increase of CTWD generated longer arc lengths and longer background periods, where short-circuiting frequency is reduced. This effect results in a considerable change of the waveform shape and droplet detachment takes place, sometimes globular and linked to splashing.
CMT is generally more stable than STT, and the waveform is much better controlled at most of the range of conditions used in this project. However, at very high arc length correction parameter arc voltage instability is linked to droplet detachment mechanism of drop spray transfer (drop size similar to wire diameter) and splashing.
At the limit of arc current levels for this process, i.e. when the wire feed rate is maximum (WFS set of 8m/min), uncontrolled mixed modes can be found, from globular, short-circuiting and drop transfer, becoming the overall transfer generally unstable.
Also when CTWD is increased from 11mm, drop spray transfer is observed within CMT. The arc length, in general considerably higher than for the remaining waveforms, becomes even higher and possibly the surface tension forces acting during dip transfer are reduced with the electromagnetic forces becoming more significant. The analysis of FastROOT transfer shows that the process is characterized by significantly higher arc instability and almost all the conditions used in this project were associated with spatter. However, some conditions worked under severe disturbances. In particular at very low base current levels (i.e. -50%) splashing is generally associated with the arc re-ignition. The instability is also significantly higher at the highest current levels (when WFS is in the range of 8-9m/min). In this case, the waveform is significantly uncontrolled, resulting in very high arc splashing. However, the arc length is always kept very short which enables the droplet detachment to occur, even at high current levels. One reason for the unstable arc conditions generated within FastROOT may due to the fact that its intended application is on open gap welding, as was pointed out by Uusitali (2007).
2.5.3.3. Droplet Detachment in Pulse Waveforms
In pulse spray transfer welding the “one droplet per pulse” (ODPP) condition has been extensively discussed in the literature (Amin 1983) (Kim 1989) (Kim and Eagar 1993 b) (Wu,
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Chen and Lu 2005). These authors attempted that to ensure repeatability and controlling of GMAW-P to achieve only one droplet per pulse. Miranda et al. (2007) developed an optical sensor to recognize if less than one, one or more than droplet per pulse during GMAW-P.
The results obtained suggest that for GMAW-P the condition of one drop per pulse is generally obtained when good stability conditions were achieved. However, unstable conditions can result in a droplet diameter greater than the wire diameter, characterized by irregular globular transfer, at relatively low – medium arc currents. At high peak current levels and short pulse periods the energy is too small and the droplet transfer is characterized by large globules (Ueguri, Hara and Komura 1985).
The RapidArc waveform is characterized by more than one drop per pulse; the results suggest that a drop with size similar to the wire diameter is followed by a second small drop.
At significantly high CTWD (i.e., 16mm) more than one small drop can follow the main drop.
These results suggest that arc stability can be achieved with reproducible conditions when more than one droplet per pulse is achieved when this waveform is applied. This behaviour is associated with the significantly higher voltage level and has an interesting effect on bead shape characteristics. These results contradict the published data from Lincoln (Lincoln Electric 2004) (Lincoln Electric 2005), which claims that RapidArc operates at lower arc voltage levels than conventional GMAW-P. Choi et al. (1998 b) considered that when more than one droplet is transferred, higher drop transfer frequency is identified. These authors also pointed out that axial flow and radial pinch force play an important role in the detaching mechanism associated with spray and globular transfer modes. The effect of these two forces increases with arc current level, when axial flow becomes dominant. In spray transfer, the current density has also a significant effect on the free surface profile and drop size. These results are important since they are in agreement with previous work based on pinch instability theory and force balance model, and can provide some information about the forces acting during detachment mechanism in spray transfer.