General Controlling Parameters
• Most structural metals and their alloys form a cold cathode, its area is small but large quantities of energy are generated to release the electrons needed to support an arc.
• High m. p. materials like carbon, tungsten and molybdenum easily supply electrons to sustain the arc due to their temperature. These metals are called thermionic.
• Change from cold cathode to thermionic emission is accompanied by a lowering of the heating energy and, therefore reduction in melting rate.
• Also any improvement to arc stability in a. c. or metal transfer mode in dc en is associated with a reduction in melting rate.
• Electrical resistance heating of the electrode by welding current affects the electrodes melting rate.
• Electrode melting rate can be expressed as :
M. R. = al + bLP...(3.11)
where a = anode or cathode constant of proportionality for heating. It depends upon polarity, composition and with dc en, the emissivity of the cathode. b = constant of proportionality for electrical resistance heating and includes the electrode resistivity.
L = electrode extension or stick out.
I = welding current.
Table 3.3. Relative magnitude of heating coefficients in the melting rate of 1.6 mm diameter wire electrode
a = Kg/hour. Amp. b = Kg/hour Amp.2 mm.
3.8.5 Melting Rates with GMAW
• Melting rate is controlled by:
(a) electrode diameter
(b) electrode extension
(c) cathode or anode heating (current polarity)
(d) current mangnitude
(e) Factors like shielding gas, arc length (arc voltage).
• Equation (3.11) for melting rate can be used to calculate melting rates for electrode positive. Problems develop with dc en, because the cathode heating value becomes quite sensitive to the presence of oxides alkali and alkaline earth compounds.
• The first term of the equation is more significant at low currents and with short electrode extension. The influence of second term becomes pregressively greater as the electrode diameter is reduced and its extension (resistivity) is increased and the current is raised. The relative magnitude of the heating coefficients with 1.6 mm diameter is shown in Table 3.3.
The values of the terms of the equation (3.11) depend upon the material (or alloy) being welded. First term is important for aluminium since its resistivity is low. It gains greater importance when the electrode is negative since the use of any additive that affects cathode emissivity will reduce the value of ‘a’ and thus reduce melting rate. Fig. (Fig. 2.20) shows that the electrode can be made so much thermionic as to reduce the heating effect represented by the term ‘a’ for electrode negative below that of electrode positive. Direct current electrode negative arcs have greater significance as they give very high melting rates (Fig. 2.20), but (unfortunately) the transfer is globular and spattery. When a. c. is used the values of ‘a’ are an average between the values obtained for dc ep and dc en.
When argon shields are used the upper limit of melting rates is determined by the formation of ‘jet-rotation’ which needs higher currents and consequently higher diameter electrodes to sustain higher currents. The extent of these ranges is shown in Fig. for steel. This is not true for aluminium. The upper current for aluminium is limited by the formation of a very rough weld surface.
With active gas welding, metal transfer is always globular for all current levels. At lower level of current there is random short circuiting, absence of wetting and power weld quantity. At upper limits of current, there is spatter, poor bead appearance and porosity. When very low melting rates are necessary, the short circuit technique is frequently used.
Melting Rates with SAW
In general the above discussion for GMAW applies to SAW also. The melting rate increases with current. Cathode or anode voltage changes due to change of flux.
3.8.6 Melting Rates with SMAW
• The SMAW is least efficient in converting electrical energy to useful weld heat.
• Current controls the melting rate to some extent, but as the current increases the electrode diameter must be increased proportionately.
• Lower limit of current is defined by incomplete fusion, high viscosity of flux. Upper limit causes excessive resistance heating of the electrode that damages the electrode flux covering and the flux constituents breakdown before reaching to the arc where products of combustion arc needed for shielding.
• Cellulose coating on E6010 electrode of 6 mm diameter is useful in the range between 200-300 A while for the same diameter, the rutile-base E6012 that does not rely on gas formers has a useful range between 200 and 400 A.