• This technique is an alternative of dip transfer for welding in positions and when thin plates are to be welded. This type of transfer is shown in Fig. 3.21 (a) and (b).

Low-current arc keeps weld pool molten.

Arc returns to low background current.

Time for complete 1

sequence = — th second.

Fig. 3.21 (b) Pulsed transfer in MAGS welding

• Current pulses back and forth between the globular and spray transfer are superim­posed on the normal background current.

• Time duration between consecutive pulses must be less than that required for globular transfer.

• Droplets are ejected from the electrode tip at regular intervals corresponding to the frequency of current pulses.

• Currents and deposition rates can be decreased so that welding speed can be reduced to cope more easily with thicknesses down to 1.0 mm or even thinner.

3.8.4 Covered Electrode Transfer

• In general the metal transfer is globular on one extreme and spray type on the other.

• Showery spray transfer is desirable. In some cases, however, spray transfer is not used because of spatter associated with it.

• Most of the electrodes contain cellulose or metal carbonates that burn in the arc forming a gas shield to protect the weld from atmospheric contamination. This shield contains mainly active gases like carbon dioxide, carbon monoxide, hydrogen and oxygen. These gases do not develop a highly conductive arc plasma, the current distribution is such that the liquid metal is forced out of the arc and weld pool as massive drops and spatter.

• These reactions are more intense when electrode is negative, Reverse polarity is, therefore, used with electrodes that do not contain cathode stabilizers (cellulosic elec­trodes).

• Coverings can be made thermionic by adding rutile, lime and iron-oxide in combina­tion. Such electrodes produce more stable arc, less spatter and form smaller drops with direct current electrode negative.

• With AC, current reduces to zero when polarity changes. The binders for such elec­trodes is changed from sodium silicate to potassium silicate. Potassium has lower ionisation potential, it also increases cathode emissivity to permit an easy reignition.

• Electrodes containing rutile or lime in sufficient quantities are also thermionic and do not require substitution of potassium binders to make them suitable for AC weld­ing.