Welding and Joining Processes
Welding and joining asks you to classify processes, choose a flame or electrode, name joint types and identify defects. It rewards exact terms: fusion versus pressure, consumable versus non-consumable electrode.
Classification
- Fusion welding: gas (oxy-acetylene), shielded metal arc (SMAW), TIG (GTAW), MIG (GMAW), submerged arc, thermit, atomic hydrogen.
- Pressure or solid-state: forge, resistance spot, seam and butt, friction, ultrasonic, explosive.
Gas flames
Neutral flame, oxygen and acetylene about 1:1, temperature about 3200 °C, for welding steel. Oxidising flame has excess oxygen and suits brass and bronze. Carburising flame has excess acetylene and suits hard surfacing and some alloy steels.
Arc welding terms
Electrode coating stabilises the arc, shields the molten pool and adds alloying elements. DC straight polarity (electrode negative) gives deeper penetration; DC reverse polarity gives shallower penetration and suits thin sheet. Positions are flat, horizontal, vertical and overhead; flat is easiest.
Joints and defects
Butt, lap, tee, corner and edge joints. Defects: porosity from trapped gas, slag inclusion, incomplete penetration, undercut at the toe, spatter, cracks and distortion. Distortion is controlled by tacking, balanced sequences and back-step welding.
Worked example
Join mild steel sheet 2 mm thick. Choose TIG, or a small-diameter electrode at low current with DC reverse polarity, or a neutral flame for gas welding, because thin sheet overheats and burns through; low current with fast travel avoids burn-through. For 10 mm plate the opposite applies: high current, larger electrode, more passes, and a V-groove preparation for full penetration.
Traps
Using an oxidising flame on steel. Saying TIG uses a consumable electrode - it uses tungsten, with separate filler. Storing acetylene cylinders horizontally or using copper fittings with acetylene. Applying oil to an oxygen regulator.