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newSchoolWorks.com › VoTechWorks › Transportation › Auto Body & Collision › Welding for Auto Body & Collision Repair › Lesson 9
TRN-AUTO25-04 · Lesson 9 · 14 clock hours · theory

Weld Defect Identification and Correction

Objective. Students will identify six common MIG weld defects from physical specimens and photographs and prescribe the correct machine or technique adjustment to eliminate each defect.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Weld Quality Matters in Collision Repair

A vehicle's structural integrity depends on every weld meeting the original design strength. In auto body and collision repair, a weld that looks acceptable on the surface may contain hidden defects that cause a joint to fail in a future collision. I-CAR Weld Quality Standards and ASE B-series knowledge requirements both expect technicians to recognize defects visually, understand their causes, and make the correct adjustment before returning a vehicle to a customer. Learning to diagnose and correct defects saves rework time, protects the shop from liability, and—most importantly—protects the vehicle occupant.

Safety Before You Strike an Arc

Before examining or producing any weld:

The Six Common MIG Weld Defects

1. Porosity

Porosity appears as small pits or holes on or inside the weld bead. Cause: gas contamination during solidification. Typical sources include a loose or kinked gas hose, an empty shielding-gas cylinder, contaminated base metal (oil, paint, rust, moisture), or a draft blowing the shielding gas away from the puddle. Correction: Clean the base metal thoroughly; check all gas connections; verify flow-meter setting (typically 15–25 CFH for short-circuit MIG on sheet metal); eliminate drafts.

2. Undercut

Undercut is a groove melted into the base metal along the toe of the weld that is not filled by weld metal. It reduces the effective thickness of the base material and creates a stress riser. Cause: travel speed too fast, arc voltage too high, or incorrect gun angle. Correction: Slow travel speed slightly, reduce voltage, and adjust gun angle to keep the puddle flowing into the joint rather than undercutting the edge.

3. Overlap

Overlap (also called cold lap) occurs when weld metal rolls over the base metal surface at the toe without fusing to it. The bead appears wide and flat with a sharp, unfused edge. Cause: travel speed too slow, voltage too low, or wire feed speed too high relative to voltage. Correction: Increase voltage, increase travel speed, or decrease wire feed speed so the puddle wets out rather than piling up.

4. Burn-Through

Burn-through is a hole melted completely through the base metal. It is especially common on the thin sheet steel (18–22 ga) used in modern automotive panels. Cause: heat input too high—voltage too high, wire feed speed too high, travel speed too slow, or holding the gun stationary on thin metal. Correction: Reduce voltage and/or wire feed speed, increase travel speed, use a stitch or skip-weld technique on thin panels, or switch to a pulsed setting if available.

5. Incomplete Fusion

Incomplete fusion (lack of fusion) means the weld metal did not bond fully to the base metal or to the previous weld pass. The bead may look fine on the surface but has a cold, unbonded seam along one or both toes. Cause: insufficient heat, contaminated or painted surfaces, incorrect gun angle that directs the arc away from one side of the joint, or travel speed too fast. Correction: Increase voltage and/or wire feed speed slightly, clean and de-rust the joint surfaces, and direct the arc into the root of the joint.

6. Excessive Spatter

Spatter is molten metal droplets expelled from the weld puddle that land on the surrounding base metal. Some spatter is normal in short-circuit MIG; excessive spatter wastes wire, creates extra cleanup work, and can mask surface defects. Cause: voltage too low relative to wire feed speed (causes an erratic, popping arc), contaminated metal, or incorrect gas mixture. Correction: Tune voltage and wire feed speed to achieve a smooth, crackling arc sound; verify shielding gas type and flow rate; clean base metal.

Diagnosing Defects Systematically

Use a cause-and-effect approach:

1. Identify what you see: location, shape, and appearance of the anomaly.

2. Classify it using a reference such as I-CAR Weld Quality Standards.

3. Trace the cause to one of three categories: machine settings (voltage, wire feed speed, gas flow), technique (travel speed, gun angle, work distance), or material condition (cleanliness, fit-up, thickness).

4. Adjust one variable at a time and re-test on a scrap coupon before welding on the actual repair.

Visual Inspection Checklist

A passing MIG weld on auto body sheet metal should show:

I-CAR Weld Quality Standards provide specific visual acceptance criteria used by OEM repair procedures and insurance documentation. Refer to the current I-CAR reference for panel-specific requirements.

Key terms10

PorositySmall voids or pits in the weld caused by gas trapped during solidification, typically from contamination or shielding-gas loss.
UndercutA groove melted along the toe of the weld bead that weakens the base metal by reducing its cross-sectional thickness.
Overlap (cold lap)Weld metal that flows over the base metal surface but does not fuse to it, leaving a sharp, unbonded edge.
Burn-throughA hole melted completely through the base metal due to excessive heat input, common on thin automotive sheet steel.
Incomplete fusionFailure of the weld metal to bond fully with the base metal or adjacent weld pass, usually caused by insufficient heat or contamination.
Excessive spatterAn abnormally high number of molten metal droplets expelled from the puddle onto surrounding metal, indicating an unbalanced voltage-to-wire-feed-speed ratio or contamination.
Heat-affected zone (HAZ)The area of base metal adjacent to the weld that has been altered metallurgically by welding heat without being melted.
Wire feed speed (WFS)The rate at which filler wire is fed through the MIG gun, measured in inches per minute; increasing WFS raises amperage and heat input.
Shielding gas flow rateThe volume of shielding gas delivered to the weld puddle per unit of time, commonly measured in cubic feet per hour (CFH), protecting the molten metal from atmospheric contamination.
I-CAR Weld Quality StandardsPublished visual and procedural acceptance criteria issued by the Inter-Industry Conference on Auto Collision Repair, used to determine whether a collision-repair weld meets industry requirements.
In class: Instructor presents a defect specimen board with labeled examples of porosity, undercut, overlap, burn-through, incomplete fusion, and excessive spatter; students match each specimen to its cause using a cause-and-effect chart. · Students examine five unlabeled defect weld coupons, write the defect name and probable cause on a worksheet, then share findings with the class for instructor correction and discussion. · Students intentionally produce one defect weld (instructor assigns the defect type), photograph it, diagnose the setting error, correct the machine or technique, and re-weld a passing coupon; both coupons and the written diagnosis are submitted in the lab log.
Materials: Defect specimen board (pre-made by instructor), defect photograph reference sheet, MIG welders, 18-ga mild steel coupons, cause-and-effect chart, camera or smartphone, lab log forms, I-CAR Weld Quality Standards reference
Check: Worksheet scored at 80% or higher on defect identification and cause; corrected re-weld coupon must pass visual inspection using I-CAR quality checklist to complete the lab.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Weld defect identification and correction — I-CAR Weld Quality / ASE B-series
1. A MIG weld on an 18-gauge panel shows scattered round pits on the surface of the bead. The shop door is open to a stiff breeze. What is the most likely defect and its primary cause?
Weld defect identification — I-CAR Weld Quality / ASE B-series
2. A technician's weld bead has a sharp groove running along both toes of the bead where it meets the base metal. Metal thickness is reduced at that point. Which defect is present?
Weld defect correction — I-CAR Weld Quality / ASE B-series
3. A student examines a weld coupon and notices the bead edge rolled over the base metal surface but is not bonded to it; there is a distinct cold, unbonded line at the toe. Which adjustment is most likely to correct this defect?
Weld defect correction — I-CAR Weld Quality / ASE B-series
4. During a plug weld on a thin roof panel, a hole forms in the center of the joint before the weld fills it. What machine or technique adjustment should the technician make first?
Weld defect identification — I-CAR Weld Quality / ASE B-series
5. A weld bead appears visually complete but a bend test reveals a cold seam along one side where the weld metal did not bond to the base metal. Which defect does this describe?
Weld defect correction — I-CAR Weld Quality / ASE B-series
6. A technician notices the MIG arc sounds rough and irregular, like popping, and there are many metal droplets stuck to the base metal on both sides of the bead. Bead appearance is inconsistent. What is the most direct correction?
Weld quality inspection — I-CAR Weld Quality Standards
7. According to I-CAR Weld Quality Standards, which of the following is an acceptance criterion for a visual inspection of a MIG weld on automotive sheet metal?
Shop safety — PPE for collision repair operations
8. A student is about to grind a weld coupon to check its profile. Which PPE item is most critical to add beyond what was worn during welding?
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