METAL TRANSFER AND MELTING RATES
3.8.1 Metal Transfer
• Shielded metal arc welding processes are used extensively since filler metal is deposited more efficiently and at higher rates than is possible with other processes.
• For better efficiency, the spatter losses should be reduced to minimum and uncontrolled short circuits between the electrode and work should be avoided.
• Metal transfer can be studied with motion pictures and by the analysis of the short circuit oscillograms.
• Metal transfer may be classified as:
(a) globular (massive drops, short circuiting occurs)
(b) spray (shower of a large number of small drops).
Generally the metal transfer occurs in some combination of both.
• In GMAW process with argon shielding, when the current is above the transition level, the transfer mechanism can be described as axial spray. With active gases, however, the transfer is globular and some short circuiting is unavoidable.
• Study of metal transfer in arc welding is difficult because the arcs are too small and their temperatures too high and the metal transfers at high rates.
• A combination of the following forces functions to detach the droplet against the force of gravity.
(a) Pressure generated by the evolution of gas at the electrode tip.
(b) The electrostatic attraction between the electrodes.
(d) The ‘pinch effect’ caused by a momentary necking of the liquid drop that is, conducting current.
(e) Explosive evaporation of the necked filament between the drop and electrode due to the very high density of the conducting current.
(f) Electromagnetic action produced by a divergence of current in the plasma around the drop.
(g) Friction effect of the plasma jet.
3.8.2 Polarity and Metal Transfer
• At low welding currents the size of the droplet in argon develops to a diameter more than the diameter of the electrode.
• The droplet size is roughly inversely proportional to the current and only a few droplets are released per second.
• With long arc length, the droplets are transferred without short circuit, no spatter, and arc is stable.
• Above a critical current level, the characteristics of this transfer change from globular to spray transfer mode.
• In spray transfer, the tip of the electrode becomes pointed and, from it, minute drops are transferred at a rate of about a hundred per second. The current at which this occurs is called transition current. The change is usually abrupt. See Fig. 3.19.
• Axial spray transfer is stable. There is no spatter, the drops are transferred in line with electrode and not through the minimum path. The metal can therefore be directed where needed for making fillet vertical or overhead welds.
• The key to the spray transfer is the ‘pinch effect’ which automatically squeezes the drops off the electrode; this occurs as a result of the electromagnetic effects of the current.
• The transition current depends upon :
(a) electrode diameter, Fig. 3.20 shows the effect.
(b) electrode extension (distance between the point of current pick-up and the arc). As extension increases current for spray transfer decreases (extended wire gets heated).
(c) electrode composition.
(d) metal being welded (less for aluminium and more for steel).
• Spray transfer can be achieved at average current levels below the transition current by using pulsed current. Drops are transferred at the frequency of the current pulses. This technique increases the useful operating range of a given electrode size.
• When useful upper range of the welding current is exceeded a spatter-forming rotation of the arc is initiated on the electrode tip. This is called “Jet rotation”.
• GMAW arc becomes unstable and spattery when electrode negative is used. The drop size is big and due to arc forces the drops are propelled away from the workpiece as spatter.
• Spray transfer is observed in argon shielded consumable electrode arc only. It appears that argon provides the unique plasma properties with the self-magnetic force to develop axial spray transfer through the arc.
A. C. Arcs
• Arc is extinguished during each half cycle and is reignited as the voltage rises again, current increases and the electrodes get heated again, arc path gets ionised.
• As arc length increases, the arc gas gets less heated and a higher reignition potential is required.