Metal Transfer Mode in GMAW, FCAW & SMAW


Metal Transfer Mode in GMAW

In GMAW, the mechanism by which the molten metal at the end of the wire electrode is transferred to the workpiece has a significant effect on the weld characteristics. During welding, different forces are applied to the filler wire end, the weld pool and in the arc, which influences the metal transfer from the electrode end into the weld pool and the final deposit weld profile. Figure 1 shows the most important forces.

mode of metal transfer in mig mag welding
Figure 1. Forces in a Welding arc

The pinch force is an electromagnetic force which applies on every current carrying conductor and grow proportionately to the square of the welding current and decrease proportionately to the square of the cross-sectional area of the welding filler wire. However, this force is not large and can only become effective if the wire end is semi-solid or liquid. When welding with low current, the pinch force is not large enough to significantly affect the drop formation but affect large drop transferred to the weld pool.

Different modes of metal transfer in GMAW or MIG MAG

Summary of metal transfer mode in GMAW

Summary of metal transfer mode in GMAW

Short Circuit (Dip) Transfer

Metal is transferred from the electrode to the work only during a period when the electrode is in contact with the weld pool. There is no metal transfer across the arc gap during welding. This mode occurs with voltage of 16-24V, shielding gas with less than 80% argon and current below 200A, the wire feed can be set so that the end of the wire touches the weld pool and short-circuits the system, dip transfer. These short-circuits can take place 20-200 times per second. During the short, the wire heats rapidly and fuses so that molten metal is transferred to the pool after which the arc is re-established. This re-ignition is accompanied by spatter but adjusting the inductance of the system can give a degree of control over this.

Short Circuit (Dip) Transfer
Figure 2. Short Circuit (Dip) Transfer, Figure 3 Below

Characteristics (Advantages & disadvantages)

  1. Short-circuiting transfer occurs at the lowest current and voltage ranges which results in low weld heat input.
  2. It is typically used with smaller diameter filler wire and produces a relatively small and easily controlled weld pool that is well-suited for out-of-position welding and joining thin sections (In the range of 0.8mm to 3.2 mm).
  3. Suitable for welding root runs in butt joints and open root for full penetration T joints, filling large root openings.
  4. Small plate distortion.
  5. Low weld fluidity.
  6. However, the low heat input makes short-circuiting transfer susceptible to incomplete fusion (cold lap) defects, especially when welding thick sections or during multipass welds.
  7. This mode is excluded in AWS D1.1 when using a pre-qualified WPS with GMAW-S.

Globular Transfer

In GMAW, A variation in electrode extension may cause a spray transfer to change to globular. Globular transfer occurs at higher current and voltage levels than short-circuiting, and is characterized by large, irregular drops of molten metal. It is seldom used because it creates inconsistent penetration and uneven weld bead contour that promotes the formation of defects.  Since the force of gravity is critical for drop detachment and transfer, globular transfer is generally limited to flat position welding.

When helium, CO2, or argon mixtures of these gases (CO2 levels higher than 20%) are used as shielding gases, spray transfer does not occur. The anode spot does not grow so remains a small area on the wire end. Melting of the wire commences but, with the small anode spot remaining beneath the droplet, there is no direct impingement of electrons on the outside of the wire. The droplet therefore grows by conduction until its size dictates that it detaches and drops to the weld pool primarily under the action of gravity. The severe disturbance to the arc during this process and fall of a large globule into the weld pool causes very considerable spatter.

Globular Transfer mode
Figure 4. Globular Transfer mode

Characteristics (Advantages & disadvantages)

  1. Irregular metal transfer.
  2. Medium heat input.
  3. Medium deposition rate.
  4. Risk of spatter.
  5. Not suitable for positional welding

Spray Transfer

In argon when the voltage is sufficiently high, >25V for a 1mm diameter wire and the wire feed speed is adjusted to give more than 250A, the welding arc burns continuously, metal melts from the wire and passes across the arc in a series of small droplets, called spray transfer. The droplet size is typically around 0.5-1 times the wire diameter and the arc burns in a stable manner while metal transfer, becomes almost continuous. 2% oxygen is sometimes added to the argon shielding gas for spray transfer. This diatomic gas dissociates then recombines at the anode creating more heat and giving arc stability at lower currents. 5% CO2 also has a similar effect but if CO2 greater than 20% CO2 is used spray conditions cannot be established.

Spray transfer mode is characterized by a highly directed stream of small metal droplets.  It is a high heat input process with relatively high deposition rates that is most effective for welding thick sections of material.  However, it is mainly useful only in the flat position (1G, 1F, 2F or PA & PB), and its high heat input promotes weld hot-cracking and the formation of secondary phases in the microstructure that can compromise service performance.  water-cooled welding gun is always recommended for spray transfer welding and anytime higher welding currents are being utilized.

spray Transfer mode

Characteristics (Advantages & disadvantages)

  1. Free-flight metal transfer & smooth weld surface appearance.
  2. High heat input.
  3. High deposition rate.
  4. Smooth, stable arc.
  5. Suitable for welding fill passes & final passes.
  6. Used on steels above 6mm thickness and aluminum alloys above 3mm thickness.

Pulsed Mode

Pulsed spray transfer is a highly controlled variant of spray transfer, in which the welding current alternates between a high peak current, where spray transfer occurs, and a lower background current.  This results in a stable, low-spatter process at an average welding current significantly below that for spray transfer.  Pulsed spray offers lower heat input compared to spray transfer but is less susceptible to the incomplete fusion defects that are common to short-circuiting transfer.  It is useful in all welding positions and for a wide range of material thickness.

Pulsing the welding current extends the range of spray transfer operation well below the natural transition from dip to spray transfer. This allows smooth, spatter-free spray transfer to be obtained at mean currents below the transition level, e.g. 50-150A and at lower heat inputs. Pulsing was introduced originally for control of metal transfer by imposing artificial cyclic operation on the arc system by applying alternately high and low currents.

Globular Transfer mode

A typical pulse waveform and the main pulse welding variables are shown in Figure below. A low background current (typically 20-80A) is supplied to maintain the arc, keep the wire tip molten, give stable anode and cathode roots and maintain average current during the cycle. Droplet detachment occurs during a high current pulse at current levels above the transition current level. The pulse of current generates very high electromagnetic forces, which cause a strong pinch effect on the metal filament supporting the droplet; the droplet is detached and is projected across the arc gap. Pulse current and current density must be sufficiently high to ensure that spray transfer (not globular) always occurs so that positional welding can be used. Pulse transfer uses pulses of current to fire a single globule of metal across the arc gap at a frequency of 50-300 pulses.

Characteristics (Advantages & disadvantages)

  1. Low heat input metal transfer mode.
  2. Very low spatters.
  3. Reduced risk of lack of fusion compared with dip transfer due to controlled heat input.
  4. Control of weld bead profile for dynamically loaded parts.
  5. Process control/flexibility to control welding parameters.
  6. Enables use of larger diameter, less expensive wires with thinner plates, more easily fed (a particular advantage for aluminum welding).

What is Synergic MIG

In normal MIG/MAG welding power source, welder required to set the wire feed speed (Current) and select an appropriate voltage. The two variables are dependent on the wire diameter and gas used. This requires the welder/operator to have knowledge on the relationship between current and voltage for having good welding results.

In a synergic (non-pulse) MIG/ MAG welding set, there is a one knob dial that defines the wire feed speed. The microprocessor within the equipment will select the optimum voltage from a look up table (a synergic curve) to match the given current. The synergic curve has been developed to give the best possible settings for a particular current/wire feed speed.

So, now the welder is not responsible to select the right voltage. A trim button can be used, which allows the user to decrease or increase the voltage by a small percentage. The trim action allows the welder to make small corrections in voltage to suit the variables at the workpiece.

Synergic MIG

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