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Modes of Transfer

Globular

Globular 1

Globular metal transfer is a mode of metal transfer where a wire electrode is continuously deposited in a combination of short circuits with gravity assisted large drops.  These larger drops are irregularly shaped.

During Globular transfer is a point where short circuit transfer ends and the globular transfer begins.  The irregular shaped molten droplets that are produced when globular takes over from the short circuit transfer can fall out of the path of the weld which make globular transfer welding more difficult to control and spatter a little more severe.

During the 1960’s and 70’s globular transfer was the popular mode of transfer for high production metal fabrication – using mainly 100% CO2 as the shielding gas.  Mixtures of Argon and CO2 started to used form the 1980’s and on. 

Advantages

  1. Uses inexpensive CO2 gas
  2. Capable of high travel speeds
  3. Uses inexpensive solid wires

Limitations

  1. High spatter levels result in costly clean up
  2. Weld bead shape is convex and welds exhibit poor wetting at the toes
  3. High spatter reduce electrode efficiency to 87-93% range

Axial Spray Transfer

Axial Spray Transfer photo

Axial spray transfer is where a continuous wire electrode is deposited at a higher energy level resulting in a stream of small molten droplets.  These  droplets, unlike Globular Transfer are propelled axially across the arc. 

To achieve axial spray transfer, binary blends of gas containing 95% Argon and 5% Oxygen, or 82-90% Argon and 18 – 10% CO2 are used.  Axial spray transfer may be used with all common alloys including Aluminum, Carbon tell, stainless steel, Nickel alloys and Copper alloys.

For most wire diameters the change to axial spray transfer takes place at the globular to spray transition current.  A stream of fine metal droplets that travel axially from the end of the electrode characterizes the axial spray mode of metal transfer.  The high puddle fluidity restricts its use to the horizontal and flat welding positions.

For Carbon steel, axial spray transfer is applied to heavier section thickness material for fillets and for use in groove type weld joints.  The use of Argon shielding gas compositions of 95%, with a balance of Oxygen, creates a deep finger-like penetration profile, while shielding gas mixes that contain more than 10% CO2 reduce finger-like penetration profile and provide a more rounded type of penetration. 

The selection of axial spray metal transfer is dependent upon the thickness of base material and the ability to position the weld joint into the horizontal or flat welding positions.  Finished weld bead appearance is excellent, and operator appeal is high. 

Advantages

  1. High deposition rates
  2. High electrode efficiency of 98% or more
  3. Wide range of filler metal types and diameters
  4. Excellent weld bead appearance
  5. Ease of use
  6. Low spatter
  7. Excellent weld fusion

Limitations

  1. Restricted to flat horizontal
  2. High weld fume
  3. Very bright arc requires extra protection
  4. High shielding gas cost

Pulsed Spray Transfer

Pulsed Spraty Transfer

Pulsed spray metal transfer is a highly controlled variant of axial spray where the welding current is cycled between a high peak current level to a low background current level.  Metal transfer occurs during the high energy peak level in the form of a single molten droplet.
Pulsed spray transfer was developed for two reasons: control of weld spatter and the elimination of incomplete fusion defect that are common in globular and short circuit transferring.  This mode employees wire diameters of .030 – 1/16” and metal cored wires from .045 to 5/64” and used for many material types.  Typically Argon based shielding gas blends with a maximum of 18% COS are used in this spray transfer.

The welding current alternates between a peak current and a lower background current and this controlled dynamic of these currents results in a lower average current than is found in axial transfer.  The time, which includes the peak current and the background current, is a period and this period is known as a cycle (Hz).  The high current excursion exceeds the globular to spray transition current, and the low current is reduced to a value lower than is seen in short circuiting.

Ideally, during the peak current- the high point of the period – a single droplet of molten is detached and transferred across the arc.  The descent to the lower current is responsible for the overall heat input into the weld.  The frequency is the number of times the period occurs per second.  The frequency of the period increases in proportion to the wire feed speed.  Taken together they produce an average current, which leverages its use in a wide material thickness range.

Advantages

  1. Low Spatter levels
  2. Excellent weld bead appearance
  3. High operator appeal
  4. Controls weld fume generation
  5. Low level of heat distortion
  6. Ability to weld out of position
  7. High electrode efficiency 98%
  8. Lends itself for robotic applications
  9. Capable of high travel speeds – 50 inches per minute

Limitations

  1. Equipment to support the process is expensive
  2. Shielding gases are more expensive
  3. Higher arc energy requires additional safety protection

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