Welding, Cutting, and Joining of Structural Materials
Process selection
GMAW (MIG) is the workhorse for steel structural repair: solid wire, 75/25 argon/CO2 shielding, steady travel speed. Squeeze-type resistance spot welding (STRSW) reproduces factory welds and is preferred where the OEM specifies it. TIG suits aluminum and thin specialty work. MIG brazing with silicon bronze joins HSS with far less heat input and is often the OEM method on coated or high-strength panels. Oxy-acetylene welding is essentially obsolete for structure.
Joints and weld types
Butt, lap, plug/rosette, stitch, and seam welds each have a place; the OEM says which. A plug-weld hole of about 8 mm is typical for 0.8 mm sheet, spaced like the original spot welds. Butt joints in thin sheet need a backing insert.
Defects the exam loves
Porosity (wind, moisture, dirty or galvanized metal), incomplete fusion (too little current, wrong travel speed), undercut (too much current or wrong angle), burn-through (too hot, too slow, thin metal), and distortion (too much heat in one place). Back-step welds and let panels cool.
Cutting and preparation
Plasma, reciprocating saw, air chisel, and spot-weld cutter are standard. Grind galvanized coating back from the weld zone, weld with ventilation, and recoat with zinc-rich primer. Aluminum needs a dedicated stainless brush, separate wheels, and a clean, steel-free area.
Rule of thumb: match the number, size, and spacing of the factory welds - never fewer. Traps: torch-cutting structural steel, welding over paint or sealer, and arc-welding galvanized metal without respiratory protection.