Preparation of Materials and Equipment
Domain 3 - Preparation of materials and equipment. This domain tests cleaning, fit-up, jig and fixture use, and machine setup: the steps that decide whether the weld can be sound before the arc even starts.
Base metal cleaning
Remove oil, grease, paint, rust, mill scale, and moisture at least 25 mm (1 in) beyond the joint faces. Degrease before grinding so you do not drive oil into the surface, then grind or wire-brush. Stainless needs a dedicated stainless brush. Dry the joint - trapped moisture produces porosity.
Fit-up
Check bevel angle, root face, root gap, alignment, and joint geometry against the drawing. Tacks must be the correct size, made with the same consumable, and either consumed by the weld or removed; a small or cracked tack becomes a defect. Control distortion with balanced sequences, backstep, presetting, and restraint.
Jigs and fixtures
A fixture should locate the part repeatably, clamp without crushing the root gap, allow torch access and visibility, and let the weld shrink without cracking. Clamp the work lead to clean bare metal on the workpiece, not to the fixture body.
Machine setup
- Constant-voltage power source; DCEP for solid wire.
- Drive roll tension just enough to feed without flattening the wire; correct groove size.
- Contact tip bore matched to the wire; nozzle clean and undamaged.
- Gas mix per the WPS (for example 75% Ar/25% CO2 or 100% CO2), flow about 12–20 L/min (25–40 cfh) on a flowmeter, lines purged and leak-checked.
- Set wire feed speed (which controls amperage) and voltage per the WPS; confirm stickout.
Worked example: 6 mm mild steel with 1.2 mm ER70S-6 in short-circuit transfer. Clean 25 mm each side, cut a 60° included bevel with a 1.5–2 mm root gap and 1.5 mm root face, tack both ends, set DCEP, 75/25 gas at 15 L/min, and start from the WPS parameters rather than guessing.
Traps: welding over mill scale because it looks clean; using the fixture as the ground path; setting gas flow by ear instead of a flowmeter.