Welding Processes: Arc Physics, Transfer Modes and Heat Input
What to master
Candidates must connect process selection to arc stability, shielding, transfer mode, deposition rate, penetration and heat input. For SMAW, know electrode coating functions, polarity and the effect of moisture. For GMAW, distinguish short-circuiting, globular and spray transfer, and how gas composition, current density and wire diameter shift the mode. For FCAW, distinguish self-shielded and gas-shielded, rutile versus basic slag. For SAW, know flux handling, high current density and the risk of excessive heat input; for GTAW, know arc cleaning, polarity and the need for separate filler. Heat input is a comparative tool, not a measure of actual weld cooling rate unless efficiency and preheat are considered.
Traps
Questions often confuse deposition rate with travel speed, or assume that higher current always means more penetration - polarity, arc length and joint geometry can reverse that. Duty cycle is specified at a particular current and ambient temperature, so it cannot be extrapolated directly. Process efficiency differs: SAW and GMAW are more efficient than SMAW, so equal calculated heat input does not mean equal actual energy into the plate. Do not confuse alternating current with variable polarity, or 'reverse polarity' with 'electrode negative'.
Worked example
Heat input (kJ/mm) = voltage × current × 60 / (travel speed in mm/min × 1000). At 25 V, 250 A and 300 mm/min: 25 × 250 × 60 / 300,000 = 1.25 kJ/mm. If travel speed doubles, heat input halves. Record voltage, current and speed at the same point in the weld; a nominal machine setting is not evidence.