Welding Processes and Equipment
This module tests whether you can classify a process by heat source and transfer mode, then match the power-source characteristic, consumable and shielding medium to the job. Questions usually ask why a parameter is chosen, not just its value.
How the processes divide
- Arc: MMAW/SMAW (covered electrode), TIG/GTAW (non-consumable tungsten), MIG/MAG/GMAW, FCAW, SAW, plasma.
- Resistance: spot, seam and projection, where heat comes from I²Rt at the faying interface.
- Beam and solid state: laser, electron beam, friction stir, stud, explosive.
The static characteristic drives selection
- MMAW and TIG use a drooping (constant-current) characteristic: a change in arc length produces only a small current change, so the electrode does not stub out and the welder controls penetration by arc length.
- GMAW uses a flat (constant-voltage) characteristic; the welder sets wire-feed speed and voltage and the arc self-regulates current.
- SAW combines high current with a flat characteristic for deep penetration and high deposition rate, with granular flux providing shielding and alloying.
Worked example: duty cycle
A set is rated 300 A at 60% duty cycle, defined over a 10-minute period at 40 °C ambient. At 100% duty cycle the permissible current is 300 × √0.60 ≈ 232 A. A job requiring 280 A continuously cannot be run at 60%: the set is thermally undersized, and the correct answer is a larger machine or an interlocked work/rest cycle, not a lower voltage.
Traps that cost marks
- Treating duty cycle as arc time or operating factor; it is a purely thermal rating.
- Claiming MIG/MAG is constant current because the display shows amps.
- Forgetting that short-circuit transfer depends on inductance setting and gives low heat input, so it suits thin sheet and positional work only.