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Shielding Gas Selection for Controlled Dip Transfer (Short Arc) Welding

   
R Wiktorowicz (Air Products) and G Melton (TWI Ltd)

TWI Bulletin May/June 2013

Summary

Recent developments in welding power source electronic control technology for Metal Active Gas (MAG) welding has seen the emergence of a number of controlled dip transfer  arc welding power sources. These process control technologies are aimed at welding of thin sheet materials and open root welding. Traditionally, a shielding gas with a low percentage of carbon dioxide in argon would be used for such applications but no clear recommendations are made for these new process variants. In this study, the effect of different shielding gases on spatter level, gap bridging ability, and bead/weld geometry was assessed and contrary to what was expected, Ferromaxx Plus, a hotter gas normally recommended for thicker sections, was found to give the best overall results.

Introduction

The MIG/MAG welding process remains the most widely adopted in industry (1). Dip-transfer MIG/MAG, combines low current/heat input and a small wire diameter with repeated short-circuiting between the wire and the weld pool (2), making the process suitable for joining thin sheet  materials, open root welding  and/or positional welding, where precise control of the weld pool is required. However, the main disadvantage of the dip transfer mode is spatter, which is associated with molten metal being squeezed by a pinch force during the current rise–droplet detachment phase of the process cycle. The rate of current rise is critical, in order to balance maintaining a molten wire for metal transfer against excess current/pinch force, and subsequent spatter.

As a result there have been a number of recent developments in control technology to improve both the stability of the current profile and the heat input (3). These new power sources are collectively referred to as controlled dip transfer (short arc) processes. They control the dip process by monitoring the voltage and modulating the welding current waveform. This reduces spatter and reduces the  amount of residual heat to the work piece. Essentially there is a rapid reduction in the welding current immediately prior to arc re-ignition to control spatter. However, as these techniques reduce the current in the wire towards the end of the short circuit phase, less resistance heating of the wire occurs and the   weld pool is colder and less fluid which often results in a peaky weld bead and can lead to poorer bridge ability or tolerance fit-up. It was felt that by choosing the correct shielding gas, the bead shape the bridging capability and fit-up could be improved.

In the conventional dip transfer process the current rises and falls exponentially at a rate determined by the wire feed speed, wire diameter, inductance and voltage. A stable condition with regular short circuits is achieved by tuning the voltage. A typical waveform is shown in Figure 1.

Figure 1 - Schematic of the dip transfer MAG process
Figure 1 - Schematic of the dip transfer MAG process

The mode of operation consists of a cycle of arc burning, wire contact/arc collapse, short circuiting/current rise, droplet detachment/arc re-ignition as shown in Figure 2.

Figure 2 - A typical voltage/current waveform and cyclogram can be seen above
Figure 2 - A typical voltage/current waveform and cyclogram can be seen above

However, with electronically controlled dip transfer the current is lowered just before short circuit ruptures so spatter is reduced. There are many designs of such power sources but the end result is very similar, as can be seen in Figure 3.

Figure 3 - A typical voltage/current waveform and cyclogram for controlled dip transfer
Figure 3 - A typical voltage/current waveform and cyclogram for controlled dip transfer

There are many companies that produce variants of the short arc processes. Each company has developed their own control algorithms and given the process a name. A short market survey by TWI indicated that whilst in general there is high awareness of these processes, few companies have first hand experience of them, Figure 4. So, there is clearly an opportunity for growth in applications.

Figure 4 - A survey on process variants
Figure 4 - A survey on process variants

Manufacturers claim that these controlled dip transfer processes offer:

  • a more stable arc

  • reduced spatter

  • better heat control

  • reduced distortion

  • potentially a reduction in welding fume

These processes are aimed at the following applications;

  • thin sheet welding

  • poor fit-up

  • automated applications

  • MIG brazing of coated steels, with minimum damage to the coating. 

However, recent work on lap joints at TWI ( Melton, 2012) has shown that the weld bead  suffer from poor wettablity , are often peaky and that the gap bridgeability between sheets was not improved compared to conventional dip transfer, see Figure 5. It is believed that these processes would be more widely adopted with process improvements and increased education on process optimisation by for example selection of the best shielding gas.

Figure 5 -Typical weld bead with a controlled short circuit welding process
Figure 5 -Typical weld bead with a controlled short circuit welding process

There has been little attention given to the selection of the best choice of shielding gas. Generally, a mixture low in CO2 would be chosen, whilst some manufacturers specify an argon 20%CO2 mixture.

Objective

The objective of this project was to evaluate  a range of shielding gases in controlled dip transfer welding of thin sheet materials to support recommendations for gas selection.

Equipment and Consumables

The following shielding gases were evaluated for controlled dip transfer welding;

  • Ferromaxx7 (7%CO2, 2.5%O2 balance Argon)

  • Ferromaxx15 (15%CO2, 2.5%O2 balance Argon)

  • Ferromaxx Plus (12%CO2, 20%Helium balance Argon)

  • Argon – 8% CO2

  • Argon - 20% CO2

  • CO2

Steel sheet to EN 10025-2 S235, of 1.2mm thickness was clamped in a lap joint configuration. Prior to welding the plate edges were ground to remove surface scale and degreased.

A 1.0mm  diameter copper coated  solid wire to BSEN ISO 14341:2011 classificationG3Si1 was used for the trials.

A EWM alphaQ power source was chosen for this work. This is an inverter based multi-process machine. The controlled dip transfer mode  on this power source is called coldArc™. The power source is pre-programmed for different consumables and gases. A standard programme was selected and the voltage trimmed to optimise welding conditions. Two programmes are available for welding with a mild steel consumable, Programme 182 for CO2 and programme 191 for mixed gas.

All welding was carried out  with the power source connected to a Kawasaki JS6 articulated robot and the plates held in the Fixture shown in Figure 6. This fixture allows the plates either to be clamped together  or with a fixed gap. For these trials no gap was set.

Figure 6 - Welding fixture
Figure 6 - Welding fixture

Experimental programme

A set of trials was carried out for lap joints without a gap using the fixture in Figure 7. These trials were conducted with a wire feed speed of 7.5m/min; the highest wire feed speed that could be obtained from the power source. This corresponded to a welding current of about 100-120A. The voltage off- set was adjusted over a range of -2 to +3V, to optimise conditions for the different shielding gases. The travel speed was increased incrementally, from 700mm/min up to the maximum speed that gave an acceptable weld, for a set of welds using a given shielding gas.

Assessment

Welds were assessed in accordance with BSEN ISO 5817:2007 for visual appearance, geometry, penetration. Macrosections of welds for each shielding gas made at a travel speed of 1m/min are shown in Figure 7.

Figure 7 - Macro-sections of lap welds for each gas at a travel speed of 1 m/min
Figure 7 - Macro-sections of lap welds for each gas at a travel speed of 1 m/min

For a CO2 shielding gas the welds made at a travel speed of 0.75 and 1 m/min without voltage correction (programme 182 for CO2) had an irregular bead shape and a peaky profile with excess reinforcement. Penetration into the bottom plate was low.

Argon - 20% CO2  produced welds with greater penetration and a flatter profile but there was a tendency to undercut on the top edge for most welds. Also, although the
profile of the welds appears good in section and similar to those welds produced using Ferromax Plus, the weld bead is very irregular compared to that achieved with Ferromax Plus

Argon – 8% CO2  produced cold peaky weld beads which for some settings were quite irregular . For a travel speed of 1.0m/min, voltage adjustment resulted in either a peak bead or undercut with low penetration. An improvement in bead shape was obtained by reducing the travel speed to 0.75m/min, with no voltage trim but the bead was still slightly peaky  although smooth and consistent with good wetting at the toes.
With Ferromaxx 7 shielding gas (using programme 191) the best results were obtained with a voltage correction of +1V. The weld bead was observed to be cold and peaky, but less so than with CO2. In places, some undercut was observed, although the bead was generally regular.

Improvements were obtained with Ferromax 15  which at a voltage trim of +1V produced a weld bead with a flatter profile and good wetting at the toes. Occasional undercut was noted on the top edge, but generally weld profile and consistency was improved.

With Ferromaxx Plus the bead shape improved dramatically with an increase in voltage trim to +3V. These welds exhibit a wider and smoother profile with good wetting at the toes. The best results were obtained at a travel speed of 1.0m/min at the higher travel speed of 1.25 m/min slight undercut was observed and the appearance of small root imperfections. Reducing the voltage to +2V resulted in a smoother weld profile with less ripple.

Overall the best results were achieved with Ferromax Plus with a voltage trim of +2 to +3 volts.

Discussion

The aim of this project was to investigate the effect that the shielding gas has on the quality of welds produced using a controlled dip transfer  MAG welding process. These results have shown that the shielding gas does influence the weld shape and penetration and regardless of shielding gas, spatter levels are low provided that a stable operating condition is achieved.

Overall, the best results were achieved with Ferromax Plus, with a voltage trim of between 2 and 3V on the standard programme. This produced flat weld beads, with good wetting at the weld toes and good penetration into the bottom plate of a lap weld without burn through at high travel speeds, Figure 8. However, if the voltage is too high or the travel speed too fast, a tendency for root defects was observed.

Figure 8 - Macro-section of a weld made with Ferromax Plus at 1m/min
Figure 8 - Macro-section of a weld made with Ferromax Plus at 1m/min

Further optimisation

All welds were made using the standard programmes available with ther EWM ColdArc® power source. It is recommended that further trials are carried out using modified programmes, which can be achieved using the manufacturer’s software. Specifically, the welding current profile has a high current pulse after each arc re-ignition. Changes in pulse parameters may enable programmes to be tailored to a specific gas mix.

References

  • Website: http://www.twi.co.uk/content/arcwelding_index.html#, date: 02/09/2010

  • Raj B, Shankar V, Bhaduri A K: ‘Welding Technology for Engineers’, Alpha Science, Oxford, 2006

  • Woloszyn A C, Melton G, Sende: ‘Comparison of advanced MIG/MAG and TIG welding processes’,  IIW Doc. XII-2076-12

  • Goecke S F,: ‘Low Energy Arc Joining Process for Materials Sensitive to Heat’,
    EWM HIGHTEC WELDING GmbH, Mündersbach, 2005

Acknowledgments

This work was carried out by TWI on behalf of Air Products. The assistance of EWM who provided the power source and process expertise is gratefully acknowledged.

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