MIG/MAG Welding and Sectioning Joints
Process basics
MIG/MAG (GMAW) uses a continuous wire electrode with shielding gas. Two common collision-repair variants are steel wire with an argon/CO2 mix, and MIG brazing with copper-silicon wire where dilution of a coated or heat-sensitive base metal must be avoided. Some OEMs prohibit fusion welding on certain AHSS grades and require spot welding, rivet bonding, or adhesive instead.
Sectioning rules
- Section only at OEM-approved locations, typically away from crush zones, radii, and mounting points.
- Joint types include butt with backing plate, lap, offset lap, and joggled lap. The OEM procedure specifies which one is acceptable.
- Prepare by stripping coatings to bare steel, applying weld-through primer, holding a tight gap, and using backstep or staggered stitches to control heat input.
- Restore corrosion protection inside the joint before it is closed.
- Use the specified wire, gas, and settings. Do not bridge gaps with weld metal.
Worked example
A quarter panel must be sectioned. The OEM procedure specifies a butt joint with a backing plate at a set distance from a body feature. The reasoning: the joint must not sit at a stress concentration or inside a collapse zone, and the backing plate carries load across the joint. Placing the cut where it is easiest to grind would violate both conditions.
Traps that cost marks
Sectioning inside a crush zone; using MIG where the OEM requires spot welding; leaving galvanizing in the weld and creating porosity; omitting the backing plate; grinding welds flush until they are too thin to carry load.