Presented at EUROJOIN4 - 4th European Conference on Welding, joining and cutting. Development of welding and allied processes at the beginning of the new millennium. Cavtat-Dubrovnik, Croatia, 24-26 May 2001, pp 313-318 (ISBN 953-96454-0-9)
The productivity gains achievable by using the tandem MIG process, have been investigated for butt and fillet welding of 6mm and 10 mm thick steel plates. The commercial tandem MIG system, as described in this paper, is one in which two wires are fed through one torch into a common weldpool. The wires are electrically insulated, fed through two contact tips, and independently controlled by two power sources. Significantly higher travel speeds and deposition rates were achieved compared to a single wire.
In today's economic climate, fabricators are increasingly under pressure to reduce costs and substantial benefit can be achieved by increasing productivity. One of the most versatile arc welding processes is MIG/MAG welding, and in recent years, there have been a number of process developments to enhance deposition rate, and hence productivity. Developments in welding equipment and shielding gases have led to substantial claims for process improvement for a single wire with high wire feed speed, high current and stable metal transfer. This has been achieved by using extended stickout, waveform control and four component shielding gases.
Developments in inverter power sources and electronic control systems have recently led to new process developments with two wires fed through one torch. One variant on these developments, the tandem MIG/MAG process, with independent control over each wire, is the subject of this paper. The main potential advantages of the process are claimed to be increased deposition rate and faster travel speeds.
In the tandem MIG/MAG process, two wires are fed into a single molten pool through the same torch, but they are electrically isolated from each other and connected to two separate power sources, Fig.1. The process is commercially available in Europe and is beginning to be applied in the USA and Japan. At the time of writing it is estimated that between five hundred and one thousand systems have been sold worldwide. The main advantages of the process are claimed to be faster travel speeds and increased deposition rate [1-3] .
Fig. 1. Single torch, tandem wire MIG/MAG system with electrically isolated contact tips
Arc welding with two wires was first developed back in the 1940s for submerged arc welding, to give increased productivity. The technique was later applied to MIG/MAG welding with two torches, but equipment limitations prevented the widespread adoption of the process.
Developments in MIG/MAG welding concentrated on improvements to the single wire process to increase productivity, with extended electrode stickouts and four-component gas mixtures in which arc stability is maintained by electronic control [4,5] .
In more recent developments  , two wires were fed through one torch with a common contact tip, either with one or two power sources. This process is referred to as twin wire or double wire MIG/MAG welding. However, it was not until the process was developed with two electrically isolated contact tips in the same torch that the single torch process has become viable. The tandem process has now achieved widespread recognition, because modern developments in inverter power sources and electronics permit independent, but synchronised, control over each wire resulting in improved process stability over the complete range of operation from dip transfer to spray transfer and pulse.
Equipment continues to be developed and most of the main manufacturers now have commercially available tandem MIG/MAG systems. The manufacturers claim that substantial improvements in travel speed and deposition rate can be achieved. This paper describes an experimental programme of work carried out to assess these claims using a commercial system.
3. Experimental approach
Tests have been carried out with a commercially available tandem MIG/MAG arc welding system to determine the capabilities of the process for high deposition and high productivity. Fillet and butt welds have been made on 10mm-thick mild steel plate with solid EN440 G3 Si1 wires and the results compared to those achieved with a single wire.
4. Results and discussion
The literature review revealed the main process benefits to be increased welding speed, deposition rate and penetration, reduced porosity and better tolerance to variations in fit-up compared with conventional single wire MIG/MAG welding. Travel speeds and deposition rates are typically 2-3 times higher than for conventional single wire MIG/MAG welding. The process has been chiefly applied to steels, and some aluminium alloys with thicknesses in the range 1.5 to 25mm for steel, and 2 to 6mm for aluminium. In the experimental work, a single pass 10mm leg length fillet weld was produced in the PB position on 10mm thick mild steel plate at a travel speed of 600mm/min, twice that at which a single pass, single wire weld was produced. At the same travel speed of 600mm/min, three passes were required to complete the joint using single wire MIG/MAG, Fig.2. Not only was productivity found to be significantly higher with the tandem process, but the bead profile and penetration were also improved. The tandem process gives a deeper and more bowl-shaped penetration profile, which is likely to be beneficial in mechanised welding, where variations in fit up and joint position may occur.
Fig. 2. Macrosections of 10mm leg length fillet welds. From left to right: A single pass tandem weld made at a travel speed of 600mm/min, a single pass single wire weld made at a travel speed of 300mm/min and a three-pass single wire weld made at 600mm/min
The fillet size vs. travel speed data are compared in Fig.3 for single wire and tandem processes operating at the maximum stable deposition rate condition for each process. A 6mm leg length fillet weld was produced using the tandem MIG/MAG process at 3.67 times faster travel speed and 3.5 times the deposition rate of the single wire weld.
Fig. 3. The effect of increasing travel speed on fillet size for single and tandem MIG/MAG processes
A 4mm leg length fillet weld was made on 6mm thick plate at a travel speed of 1800mm/min at a deposition rate of 4.5kg/h and under these conditions pulsed transfer was found to reduce undercut. The authors anticipate that fillet welds could be achieved at travel speeds in excess of 2000mm/min on thinner material.
Similar improvements in deposition rate and travel speed were found for butt welds in 10 mm thick plate. Pulsed transfer was found to be more tolerant to burn-through than spray transfer for the root run, which was completed at a travel speed of 700mm/min. The butt joint was completed in two passes, Fig.5. The same preparation required five single wire passes to fill in this work, although an optimised single wire procedure would typically require three passes, Fig.4.
Fig. 4. A 5-pass single wire 10mm butt weld
Fig. 5. A 2-pass tandem wire 10mm butt weld
5. Main conclusions
A series of bead on plate, butt and fillet welds have been made on 6 and 10mm thick mild steel plate material to assess the capabilities of the tandem MIG/MAG welding process for high productivity welding. Solid EN440 G3Si1 filler wires of 1.2mm diameter were used for both tandem and single wire tests. From these tests, it can be concluded:
- The travel speed was limited by the onset of undercut for both tandem and single wire processes. However, these welding defects did not occur in tandem fillet welds until travel speeds up to 3-4 times faster than those achieved in single wire welds were reached. 6mm leg length fillet welds were made at a travel speed of 1.1m/min using the tandem process and 300mm/min with single wire MIG/MAG welding.
- Deposition rates of 17kg/h can be obtained for butt and fillet welds in 10mm thick mild steel plates. This is a factor of three times higher than can be achieved with a single wire.
- For butt welds in thick plate, the tandem MIG/MAG process can substantially reduce the number of passes to complete the joint compared to using a single wire. In this work, the number of passes was reduced from five to two for a butt weld in 10mm thick mild steel plate. A three pass 10mm leg length fillet weld was reduced to a single pass at the same welding speed of 600mm/min by using tandem MIG/MAG welding instead of single wire MIG/MAG.
- The total cost per metre of a butt weld in 10mm plate was found to be 45% lower for a 2-pass tandem weld compared with a 5-pass single wire weld. For a fillet weld in 10mm thick plate, the total cost per metre of weld is reduced by26% compared with single wire MIG/MAG welding, because of the increase in welding speed.
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