A critical variable in Gas Metal Arc Welding (GMAW)
The mode of metal transfer required for a specific application can vary based on many factors, including base metal thickness and position. The mode of metal transfer is also affected by the shielding gas and the welding parameters used in the welding procedure. Filler metal can be transferred from the electrode to the work in two ways: when the electrode contacts the molten weld pool, thereby establishing a short circuit, which is known as short circuiting transfer (short circuiting arc welding); and when discrete drops are moved across the arc gap under the influence of gravity or electromagnetic forces. Drop transfer can be either globular or spray type.
Shape, size, direction of drops (axial or non-axial), and type of transfer are determined by a number of factors. The factors having the most influence are: • Magnitude and type of welding current. • Current density. • Electrode composition. • Electrode extension. • Shielding gas. • Power supply characteristics.
Axially directed transfer refers to the movement of drops along a line that is a continuation of the longitudinal axis of the electrode. Non-axially directed transfer refers to movement in any other direction. The stability of the welding arc and the metallurgical changes in the electrode wire are dependent on the mode of transfer. Welding procedures are categorized according to the mode of transfer.
There are four modes of metal transfer: 1. Short Circuit Transfer 2. Globular Transfer 3. Spray Transfer 4. Pulsed-Spray Transfer
Variations on the above modes of transfer are: 1. Electronically Controlled Short Arc (Miller RMD, Lincoln STT) 2. Double Pulsed Spray (Kemppi Double-pulsed MIG-MAG)