I-CAR SWQT cheat sheet

GMAW open-circuit voltage typically runs 15-35 V DC; anything above 30 V with wet gloves or damp floors raises shock severity because body resistance drops from ~100,000 ohms dry to ~1,000 ohms wet.

Fume extraction must be at the arc, not the ceiling: keep the extraction hood within roughly 12 in (300 mm) of the arc to capture zinc oxide and manganese fume before it enters the breathing zone.

Galvanized and zinc-coated parts produce zinc oxide fume; welding requires supplied-air or PAPR respiratory protection plus local exhaust, and metal-fume fever symptoms appear 4-12 h after exposure.

Squeeze-type resistance spot welding (STRSW) uses line voltage at the transformer primary and low voltage/high current (typically 8,000-20,000 A) at the secondary; never open a transformer or touch secondary tips while the foot switch is live.

Fuel, brake, and A/C lines within 12 in (300 mm) of a weld must be removed, shielded with a heat-resistant barrier, or protected per OEM procedure; brake fluid is flammable and A/C refrigerant decomposes into toxic gas when heated.

Before any welding on a vehicle, disconnect the 12 V battery negative cable and both battery terminals on vehicles with start-stop or dual-battery systems, and wait the OEM-specified time (often 1-10 min) for capacitor discharge.

Fire watch: combustibles must be 35 ft (11 m) away or protected by welding blankets; the watch must continue 30 min after hot work ends per NFPA 51B practice.

Cylinder rules: acetylene cylinders are stored and used upright, never laid down; oxygen and fuel cylinders are separated by 20 ft or a 5 ft noncombustible barrier with a 30-min fire rating.

Personal protective equipment for GMAW: shade 10-13 filter lens depending on current, leather gauntlets, flame-resistant jacket, and safety glasses under the helmet for chipping and grinding.

AWS symbol anatomy: reference line is horizontal; arrow line points to the joint; the tail carries process or spec; the weld symbol sits above the reference line for the arrow side and below for the other side.

Fillet weld size is written to the left of the fillet triangle; length and pitch follow to the right (length-pitch format for intermittent welds, e.g., 2-4 means 2 in weld, 4 in pitch).

A groove weld symbol (V, bevel, U, J) is placed on the reference line with the groove opening facing the arrow side; a square groove symbol is two parallel lines.

A supplementary flush contour is a straight line above the symbol; convex contour is a curved line above; concave is a curved line below.

All-around weld is shown by a circle at the junction of the arrow line and reference line; field weld is a flag at that same junction.

Tail notes commonly specify process (GMAW, STRSW), filler classification (ER70S-6), and OEM weld procedure number rather than a code.

Joint types: butt (groove), lap, T-joint, corner, and edge. Automotive body panels are mostly lap and T-joints welded with fillet or plug welds, not full-penetration butt welds.

Plug weld symbol is a rectangle on the reference line with the hole size to the left and the number of holes above or below; OEMs use plug welds to replace factory spot welds in non-visible areas.

Unibody construction: rockers, frame rails, and strut towers are high-strength or UHSS; mild-steel outer panels and bolt-on parts are the usual candidates for GMAW replacement.

Short-circuit transfer uses 60-75% argon/CO2 (or 75/25) at 15-20 V and works on thin sheet; spray transfer needs 85%+ argon and higher voltage (~26-32 V) and is not used on thin auto sheet because of burn-through.

C25 (75% Ar/25% CO2) is the standard auto-body mix; 100% CO2 gives deeper penetration and more spatter and is used for thicker mild steel.

Wire feed speed sets current; voltage sets arc length. Increasing WFS without voltage increase causes stubbing; increasing voltage without WFS causes spatter and poor fusion.

ER70S-6 (0.023-0.030 in) is the common mild-steel filler for auto sheet; ER70S-2 is used where cleanliness is poor; ER80S-D2 or ER90S-class fillers match higher-strength base metals only when OEM specifies.

For 18-gauge (0.048 in) mild steel, typical settings are ~0.023 in wire, 40-70 A, 14-16 V, and 15-20 cfh shielding gas; 16-gauge runs ~60-90 A.

Contact tip-to-work distance (CTWD) of 3/8-1/2 in (10-13 mm) controls current; doubling stick-out reduces current and causes cold lap.

Shielding gas flow of 15-25 cfh (7-12 L/min) is typical indoors; drafts above ~5 mph (2 m/s) blow away coverage and cause porosity.

Polarity: GMAW uses DCEP (electrode positive) for steel; DCEN is used for some self-shielded flux-cored wires, not for standard MIG on auto sheet.

A liner, drive roll, and contact tip matched to wire diameter prevent bird-nesting; knurled drive rolls are for flux-cored wire, V-groove for solid wire.

STRSW produces a nugget by resistance heating at the faying surface; heat is proportional to I squared times R times t (H = I²Rt), so current and time dominate nugget size.

Typical automotive STRSW parameters: 8,000-14,000 A secondary current, 5-30 cycles (0.08-0.5 s) weld time, and 400-1,200 lbf (1.8-5.3 kN) electrode force.

Electrode force must be applied before current and held after current (squeeze, weld, hold, off) so the nugget solidifies under pressure; forge cycles are added on some high-strength steels.

Electrode tips: copper-chromium-zirconium or Class 2 alloy, domed or truncated-cone face; face diameter should be about 5 times the square root of sheet thickness in mm; tips are dressed and replaced when face diameter grows ~20%.

Tip force must match the welder: a 16-gauge (1.5 mm) to 18-gauge pair needs roughly 500-700 lbf; too little force causes expulsion and surface marking.

Shunt current: welds placed too close together share current through the previous nugget; minimum spacing is typically 4 times the nugget diameter or per OEM (often 3/4-1 in).

Weld-through adhesive reduces current flow and requires higher current or longer time; OEM procedures specify whether adhesive is allowed at a spot weld location.

Weld nugget diameter target for automotive sheet is typically 4 times the square root of the thinner sheet thickness in mm (4√t), with a minimum around 3.5 mm for 0.7 mm sheet.

STRSW cannot weld through sealers, primers thicker than OEM spec, or gaps greater than ~1 mm; poor fit-up causes cold welds and surface expulsion.

Steel strength classes: HSS is generally 280-700 MPa tensile; AHSS includes DP (dual phase), TRIP, CP (complex phase), MS (martensitic), and press-hardened (PHS) grades up to 1,500-2,000 MPa.

DP steels (e.g., DP600, DP780, DP980) contain ferrite plus martensite islands; heat input above ~4 kJ/cm can soften the martensite and reduce the HAZ strength by 10-30%.

GMAW on DP and TRIP steels requires OEM-approved filler and low heat input; ER70S-6 is acceptable on DP600-780 only where OEM procedures permit, and never on PHS without OEM direction.

Welding of AHSS is generally limited to the OEM-approved method; many OEMs prohibit GMAW on 1,000 MPa+ grades and require STRSW or rivet-bonding instead.

STRSW on AHSS needs higher electrode force and current, and martensitic or PHS grades may require tempering pulses or post-weld heat treatment to avoid brittle nuggets.

Bake-hardening and TRIP steels are sensitive to preheat and interpass temperature; OEM procedures usually cap interpass temperature and prohibit preheat.

Repair limits: OEMs commonly restrict sectioning to designated cut lines on AHSS parts; heat straightening of AHSS is typically prohibited because it destroys the temper.

Weld-through primer on AHSS must be OEM-approved; zinc-rich primers can cause porosity and reduce nugget size, so current is increased per OEM tables.

UHSS and press-hardened steel (PHS, 22MnB5, ~1,500 MPa) are boron-alloyed martensitic grades; welding them without OEM procedure creates a brittle, untempered martensite HAZ prone to cracking.

Most OEMs prohibit GMAW, torch heating, and sectioning on PHS parts such as B-pillars, roof rails, and door intrusion beams; replacement is by full-part substitution with OEM fasteners or approved STRSW.

HAZ width in GMAW is roughly 1-3 mm on each side of the fusion line; heat input (kJ/in or kJ/cm) = (voltage × current × 60) / (travel speed in in/min or mm/min).

Lower heat input reduces HAZ softening but too low heat input causes lack of fusion and cold lap; OEM weld procedure sheets specify both current range and travel speed.

Preheat is generally prohibited on AHSS/UHSS because it widens the HAZ and tempers the base metal; it is used only on thick mild steel or per OEM.

Cooling rate control: forced air or heat sinks can be used per OEM to avoid martensite cracking on thick sections; rapid quench in still air is normal for thin sheet.

Post-weld inspection on UHSS joints should include visual for cracking plus destructive testing on coupons; magnetic particle or dye penetrant can reveal surface cracks.

Heat straightening temperatures above roughly 1,200°F (650°C) will destroy the strength of AHSS/UHSS; OEMs set lower limits or prohibit heating entirely.

Porosity is caused by gas entrapment (drafts, moisture, paint, galvanized coating, low gas flow); it appears as rounded internal voids on a break test.

Lack of fusion appears as a smooth, unfused interface on a peel or chisel test; causes are low current, fast travel, wrong angle, or poor fit-up.

Undercut is a groove melted into the base metal at the weld toe and not refilled; it reduces throat thickness and is rejected when deeper than OEM limits (often 0.5 mm or 10% of thickness).

Spatter, burn-through, and overlap (cold lap) are surface defects; overlap is a rolled-over bead not fused to the base metal and is a rejectable discontinuity.

Destructive test methods for auto body: peel test (bend the coupon to failure), chisel test (drive a chisel between sheets at the weld), and twist test for spot welds.

Spot weld quality is judged by nugget diameter and failure mode: a button pull-out (nugget remains in the coupon) passes; an interfacial (plug) failure indicates a cold weld.

Weld macro-section: measure penetration, HAZ, and nugget; a proper fillet weld on sheet shows fusion to the root and a leg length at least equal to sheet thickness.

Visual inspection criteria: uniform bead, no cracks, no undercut beyond limit, no burn-through, and complete fusion at the start and stop; crater cracks are common at the end of a bead and are removed by backstepping.

Cracks in the HAZ of AHSS are typically longitudinal and appear within minutes to hours; inspection should occur after the part cools, not while hot.

OEM repair information (procedures, sectioning guides, weld symbols, and fastener part numbers) supersedes generic training; the OEM position statement on collision repair defines allowed and prohibited methods for each model.

Sectioning: cut only at OEM-designated locations, typically 3-6 in (75-150 mm) from a body line or feature, and stagger seams between inner and outer panels; never section at a crush zone or on a PHS part.

Weld replacement rule: replace one factory spot weld with one plug weld or one STRSW; do not substitute a continuous seam unless the OEM specifies it.

Squeeze-type resistance spot welders must be capable of the OEM-required nugget size; OEMs publish minimum welder output (kVA or secondary amps) for each steel grade, and a 240 V unit may not meet the spec for AHSS.

MIG brazing (CuSi3 filler) is specified by several OEMs for galvanized and high-strength outer panels because it joins at ~1,800-2,000°F (1,000-1,100°C), below the steel melting point, preserving coating and strength.

Structural adhesive plus spot welds (weld-bonding) is used on many OEMs; adhesive must be OEM-approved, applied per bead width and cure schedule, and welds placed before adhesive sets.

Corrosion protection after repair: apply OEM-specified zinc-rich primer to bare steel, cavity wax to enclosed sections, and seam sealer to all factory seams; skip this and the repair fails long-term.

Battery electric vehicles add high-voltage safety: disable the HV system per OEM procedure, wait the discharge time, verify zero voltage, and never weld near HV cables or the battery pack.

Documentation: record welder settings, filler, gas, and procedure number on the repair order; OEM warranty and insurer audits require traceability.