MIG/MAG – developments in low heat input transfer modes

Introduction

Since its initial development in the late 1940s, the MIG/MAG process has seen several iterations in equipment development; which have included control of the way in which metal is transferred from the wire to the weld pool. This mode determines the operating features of the process. Historically, it is considered that there are three principal metal transfer modes:

• Short circuiting/ dip
• Droplet / spray
• Pulsed

Dip transfer combines low current/heat input and a small wire diameter with repeated short-circuiting between the wire and the weld pool ⁽¹⁾, making the process suitable for joining thin sheet and/or positional welding, where precise control of the weld pool is required. Recent developments in equipment, associated with advances in inverter technology and electronic control have resulted in greater refinements to the process, including improvements in the control and stability of short circuiting or dip transfer.

Dip transfer

Also known as short circuit metal transfer, this provides the lowest heat input of all transfer modes. The metal transfer occurs when the wire is in contact with the weld pool during the short circuit ⁽²⁾. The current delivered by the power source heats the wire until it begins to melt, during which time the electromagnetic field surrounding the wire increases in strength and creates a force (the pinch effect) which separates the molten part from the rest of the electrode (Figure 1). After this, the wire melts into the weld pool and the cycle begins again.

The steps in the sequence for conventional dip transfer are illustrated in Figure 2. When the electrode touches the weld pool (A) a short circuit is created, the arc extinguishes, the voltage decreases and the current increases into the short circuit. This causes the drop to be released (B). The arc reignites when the contact between the wire and the weld pool is broken (C). The cycle then starts again with the arc reignition (D) and re-melting and wire contact (E) .The frequency of the cycle is typically 50 to 150Hz.

Dip transfer, 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 and/or positional welding, where precise control of the weld pool is required. The main disadvantage associated with conventional dip transfer is the high spatter level associated with the fluctuations of the current and voltage cycle. In order to improve the quality of the weld and the efficiency of the process it is necessary to increase the control on the current cycle applied. This improvement has come about with the development of the most recent power sources.

Equipment developments

There have been a large number of developments by welding equipment manufacturers to improve process stability and reduce spatter, giving a large choice of potential systems that fabricators can adopt.

Spatter is associated with molten metal being squeezed by a pinch force during the current rise–droplet detachment phases 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. To this end, different equipment manufacturers have come up with a number of solutions; all of which relate to improving the control and stability of the current profile, with an aim to reducing spatter and improving heat input control. To do this, all of the systems rely on development of digitally controlled power sources, and a more precise control of the waveform. Such systems include the Fronius Cold Metal Transfer (CMT), EWM ColdArc®, Lincoln Electric Surface Tension Transfer (STT), Miller Regulated Metal Deposition (RMD™), Kemppi FastROOT, Jetline Controlled Short Circuit (CSC™) MIG, Daihen Corp. Controlled Bridge Transfer (CBT), Merkle ColdMIG  and ESAB QSet™ processes.

The majority of these systems control the process by electronic regulation within the power supply. Whilst each has its own particular current profile characteristics, all rely on a rapid reduction of the welding current immediately prior to arc re-ignition ^(4-8) which can take place in a more controlled manner compared with conventional dip transfer (see Figure 3). As a result, manufacturers claim significant reductions in spatter, with a 5-30% reduction in heat input, permitting joining of material thicknesses as low as 0.3mm ^(4,5), and high gap bridging ability (up to 4.8mm, ⁽⁹⁾).

The only major difference between most of the systems relates to whether they are regulated by software or hardware. The exception is the Fronius CMT system, which integrates control of the motion of the wire into the welding process control to support droplet formation and detachment ^(10,11); see Figure 4. Wire feed towards the workpiece (A) is reversed when short circuiting occurs (B), at which point the wire is retracted (C). Metal transfer supported by surface tension in the melt means that the current can be maintained at a very low level; with reduced heat input and spatter ⁽¹²⁾. After opening of the short circuit, the wire speed is changed back to feed into the weld pool (D). In this case, the process cycle is random, with the oscillation frequency varying with time; but typically around 70 Hz ⁽⁵⁾. In addition to the power source, there is the additional requirement of a special wire feed unit and torch. Whilst it has been reported that the system can be adopted for manual welding ⁽⁵⁾, the majority of applications are mechanised or automated, partly as a consequence of the need to manipulate a larger, heavier torch ⁽⁴⁾.

Summary

When asked to state their needs for further developments for MIG/MAG processes, fabricators expressed the following as critical: reduction of spatter (on dip transfer), improved arc stability, lower non-fusion defects, increased tolerance to wide gaps and positional welding ability. As such, these have featured heavily as the primary advantages of by most recent short-arc process variants. There are now several MIG/MAG short-arc welding manufacturer specific variants available and well known by the users in industry which offer users some if not all of these capabilities.

References

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