What's inside
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A MIG weld can look smooth and still be weak. The most useful inspection is not simply checking whether the bead is shiny; it is looking for metal missing from the joint, trapped gas, and poor bonding at the edges or root. You can find many problems with a bright light, a wire brush, and a small inspection gauge before they become cracks or broken parts.
Prepare the Weld Before Inspecting
Let the weld cool to a safe handling temperature, then remove slag-like residue, spatter, smoke film, and loose soot. MIG welding does not normally create a thick slag coating, but spatter and surface contamination can hide a defect. Use a wire brush suited to the base metal and wipe the area with a clean rag.
Inspect under strong, angled light. A flashlight held nearly parallel to the plate makes shallow grooves and undercut easier to see. A magnifying glass can help with small work, but it will not prove that a weld is sound beneath the surface.
Useful basic tools include a fillet-weld gauge, steel rule, flashlight, small mirror, and a pointed pick for locating holes or crater cracks. For repeated fabrication work, a welding inspection gauge set is more useful than relying on visual judgment alone.
Start With a Visual Inspection
Check the entire weld, including the start and stop points. Look for consistent width, a reasonably even ripple pattern, and a smooth transition into both pieces of base metal. A bead that is extremely tall is not automatically strong; excess reinforcement can indicate slow travel, low voltage, or poor technique.
Also check the joint itself. Uneven gaps, misalignment, mill scale, paint, oil, and rust can cause defects that are incorrectly blamed on the welder. Measure the finished weld where size matters. For a fillet weld, the leg length is the distance from the root to the toe along each joined surface. Do not assume a larger bead compensates for poor fusion.
Acceptance limits depend on the drawing, welding procedure, material, and applicable code. A small decorative bracket may have different requirements from a trailer frame or pressure component. If the part is safety-critical, use the specified welding standard and have an inspector or qualified welding professional make the final decision.
How to Find Undercut
Undercut is a groove melted into the base metal alongside the weld toe or, less commonly, at the root. It leaves a sharp notch where stress can concentrate. Run a fingernail or the edge of a weld gauge across the toe; a distinct groove is a warning sign even when the bead itself looks attractive.
Common causes include excessive voltage, excessive amperage, travel speed that is too fast, an electrode aimed too much toward one side, and excessive weaving. Long arc length can also spread the arc and wash metal away from the toe. Undercut is especially concerning on cyclically loaded parts because the notch can become a starting point for fatigue cracks.
Do not confuse a normal toe transition with undercut. A proper weld blends into the base metal without a visible groove. If the defect is shallow and the part specification permits repair, clean the area and fill it with a controlled pass. Do not simply bury deep undercut under another bead without removing sharp edges and confirming that the repair procedure allows it.
How to Find Porosity
Porosity is gas trapped in the solidifying weld metal. Surface pores may appear as round pinholes, small craters, or clusters of dark openings. Some pores are only exposed after grinding or cutting. Internal porosity cannot be reliably identified by looking at the bead, so visual inspection has limits.
Shielding gas problems are a common cause. Check for an empty or nearly empty cylinder, a loose connection, a damaged hose, leaks, blocked diffuser holes, and an incorrect flow setting. For many indoor MIG jobs, roughly 20 to 30 cubic feet per hour is a practical starting range, but the machine manual, nozzle size, joint, and surrounding drafts may require a different setting. Too little gas allows air in; too much can create turbulence and draw air into the shield.
Wind, fans, dirty steel, moisture, excessive stickout, and an obstructed nozzle can all produce porosity. A clean nozzle and replacement contact tip are inexpensive fixes, making a MIG nozzle cleaning kit worthwhile for regular users.
Small isolated surface pores may be acceptable under some specifications, while clustered or elongated porosity is more serious. If you find repeated pinholes, stop and correct the process before welding farther. Grinding out only the visible holes will not fix a gas-flow problem.
How to Identify Lack of Fusion
Lack of fusion occurs when the weld metal does not properly bond to the base metal, the previous pass, or the joint root. It may appear as a dark line along a toe, an unfused corner, or a bead sitting on top of the joint rather than flowing into it. The surface can look smooth while the joint remains weak.
Typical causes include low heat input, travel speed that is too fast, incorrect gun angle, an oversized wire for the settings, poor cleaning, and insufficient root opening. On a fillet weld, holding the gun roughly 45 degrees into the corner and using a slight work angle can help distribute heat, but the correct angle depends on access and joint geometry.
| Defect | What you may see | Likely causes | First corrective step |
|---|---|---|---|
| Undercut | Groove along the toe or root | High voltage, fast travel, long arc, excessive weaving | Reduce travel speed or voltage as appropriate and improve angle |
| Porosity | Pinholes, pits, or crater-like openings | Drafts, leaks, dirty steel, poor gas flow, long stickout | Clean the joint and verify gas delivery before rewelding |
| Lack of fusion | Dark toe line or bead sitting on the surface | Low heat, fast travel, poor angle, inadequate joint preparation | Expose clean metal and increase effective heat or improve technique |
When Visual Inspection Is Not Enough
Visual inspection cannot reliably find internal lack of fusion, buried porosity, or cracks. For important work, dye penetrant testing can reveal surface-breaking cracks and pores on suitable nonporous metals. Ultrasonic testing and radiography can examine deeper defects, but they require appropriate equipment and trained interpretation.
A practical home-shop test is to cut a sample weld through the joint, polish or grind the cross-section, and inspect the fusion profile. This is destructive, but it shows whether the weld actually reached the root and sidewalls. A bend test can also expose poor fusion, although it should be performed on a test coupon rather than a finished load-bearing part.
Before repairing a questionable weld, mark the defective area and remove it completely with grinding or gouging. Clean to bright metal, correct the process problem, and reweld using settings tested on scrap of the same thickness. A mild-steel MIG wire suited to the base metal is inexpensive, but the correct wire cannot compensate for contaminated surfaces or inadequate shielding.
Inspect Safely
Wear a properly rated welding helmet, safety glasses under the helmet, gloves, long sleeves, and footwear that covers the ankle. Grinding out a defect creates sparks and fine metal dust, so use a face shield over safety glasses and suitable hearing and respiratory protection. Keep ventilation adequate, especially when cleaning painted, galvanized, or coated steel.
If a weld carries people, vehicles, lifting loads, pressure, or structural loads, treat visual inspection as a screening step—not proof of safety. Follow the drawing and welding procedure, document repairs, and get qualified inspection when the consequences of failure are serious.