When a MIG weld bead comes out wrong, the natural reaction is to grab the nearest dial and start turning. That approach usually makes things worse, because two different defects can look almost identical at a glance but stem from entirely different causes. Porosity and arc instability both leave a rough surface. Lack of fusion and cold lap both look like the bead never bonded. Without a systematic way to match what you see to what is really happening, you end up chasing the wrong setting for an hour or more.
This article provides a visual identification chart that matches bead appearance to likely root causes before you change any machine settings. It is designed for DIY welders, hobbyists, and entry-level fabricators who need a clearer diagnosis path without guesswork. The chart uses general welding knowledge and points you back to your machine manual whenever settings vary. It does not claim universal numbers that cannot apply across every welder, and it treats safety as the first priority for any joint that could affect structural or load-bearing work.
Quick answer
The fastest way to diagnose a MIG welding defect is to look at the bead surface first, match the visual symptom to one of the defect categories in the chart below, and then follow the likely-cause checklist for that category. Do not adjust voltage, wire speed, or gas flow until you have identified which defect you are dealing with. Most common MIG defects fall into four visual groups: surface porosity and gas shield issues, fusion and penetration problems, bead shape and profile defects, and feed system or arc symptoms. Each group has a distinct appearance and a different set of root causes. The chart that follows lets you compare your weld to these groups and move directly to the most probable fix.
Main visual identification chart
The chart is organized by what you see on the finished bead. Find the row that matches your weld appearance, check the likely defect, read the common cause, and then follow the next step. Settings listed are starting points only. Your machine manual is the final authority for voltage, wire speed, inductance, and gas flow specifications.
Surface porosity and gas shield issues
| Visual sign (what you see) | Likely defect | Common cause | Next step |
|---|---|---|---|
| Pinholes scattered across the bead surface | Porosity | Gas shield disruption, draft, contaminated base metal, or incorrect gas flow rate | Check gas flow setting against your manual. Verify the nozzle is clean and the gas hose is not kinked. Eliminate drafts in the work area. Clean base metal to bright metal before welding. |
| Large, irregular cavities or a spongy bead face | Severe porosity | Empty gas cylinder, blocked gas diffuser, or excessive nozzle spatter blocking gas flow | Check cylinder pressure. Replace or clean the gas diffuser. Remove spatter from the nozzle bore. See the porosity guide for a full step-by-step sequence. |
| Fine, wormhole-like porosity running along the bead center | Hydrogen porosity | Moisture in the gas line, damp filler wire, or humid conditions in the work area | Store filler wire in a dry, heated environment. Purge the gas line before welding. Use a shorter stick-out to keep the arc zone protected. |
| Bead surface has a sooty, dark appearance with tiny pits | Gas coverage loss at the trail edge | Travel speed too fast for the gas flow rate, or torch angle pushing gas away from the trailing edge of the puddle | Reduce travel speed. Adjust torch angle so the nozzle stays ahead of the puddle. Increase gas flow within your manual’s recommended range. |
Fusion and penetration defects
| Visual sign (what you see) | Likely defect | Common cause | Next step |
|---|---|---|---|
| Bead sits on top of the base metal with no visible blending at the toe | Lack of fusion | Travel speed too fast, voltage too low, or torch angle not directing heat into the joint root | Slow down travel speed. Verify voltage is within the range recommended for your wire diameter. Experiment with a slight push angle (10 to 15 degrees) to direct heat forward. See the lack of fusion guide for a full diagnostic workflow. |
| Bead lip overlaps the base metal without fusing at the edge | Overlap (cold lap) | Wire feed speed too high for the voltage setting, or travel speed too slow, causing the puddle to roll over uncoupled base metal | Reduce wire feed speed in small steps. Increase voltage to improve wetting at the toes. Keep the arc on the leading edge of the puddle. |
| Insufficient root penetration; the weld does not reach the far side of the joint | Incomplete penetration | Joint preparation too narrow, root opening too tight, or travel speed too fast for the material thickness | Verify joint geometry matches the welding procedure. Widen the root opening or add a bevel on thicker material. Slow travel speed to allow the arc to dig into the root. |
| Groove or depression along the toe of the bead, often with a sharp edge | Undercut | Voltage too high, travel speed too fast, or torch angle too steep, causing the arc to wash away base metal at the bead edge | Reduce voltage slightly. Slow travel speed to let filler metal fill the edge. Use a weave technique on wider passes. Check torch angle against your manual’s recommendation. |
Bead shape and profile defects
| Visual sign (what you see) | Likely defect | Common cause | Next step |
|---|---|---|---|
| Bead is excessively tall and narrow with a rounded crown | Excessive convexity | Wire feed speed too high for the voltage setting, or travel speed too slow, piling filler on top of the joint | Reduce wire feed speed or increase voltage to flatten the bead profile. Increase travel speed slightly to spread the deposit more evenly. |
| Bead is too wide and flat with poor wetting at the toes | Excessive concavity (on fillet welds) | Voltage too high for the wire feed speed, or travel speed too slow, causing the puddle to spread too thin | Reduce voltage in small steps. Increase wire feed speed to add more filler. Check the inductance setting on your machine; a higher inductance setting can improve puddle control on thin material. |
| Bead width varies significantly along the joint with inconsistent ripple spacing | Erratic bead profile | Unsteady travel speed, inconsistent torch angle, or arc instability caused by feed or electrical problems | Practice a steady, controlled travel speed. Check for arc instability causes such as a worn contact tip or fluctuating voltage. See the arc instability section below. |
| Crack visible at the crater (end of the weld bead) | Crater crack | Arc extinguished too quickly, leaving an unfilled crater that shrinks and cracks on cooling | Use the crater fill or burnback function on your machine if available. Pause at the end of the weld and let the puddle fill the crater before releasing the trigger. On critical joints, grind out crater cracks before welding over them. |
Feed system and arc symptoms
| Visual sign (what you see) | Likely defect | Common cause | Next step |
|---|---|---|---|
| Wire fuses to the contact tip; the arc stops or stutters | Burnback | Wire feed speed too slow for the voltage, contact tip worn or oversize, or tip-to-work distance too short | Replace the contact tip. Increase wire feed speed or reduce voltage. Check tip-to-work distance against your manual’s range. See the burnback guide for a complete troubleshooting sequence. |
| Excessive spatter around the joint and on the nozzle | Spatter | Voltage too low for the wire feed speed, arc length too short, or gas flow too low to stabilize the arc | Increase voltage in small steps to smooth the arc. Adjust wire feed speed if needed to maintain a steady arc sound. Clean the nozzle and apply anti-spatter compound or gel to reduce buildup. |
| Arc sounds harsh or irregular; bead surface looks rough and uneven | Arc instability | Loose ground connection, dirty base metal, worn contact tip, or voltage/wire feed mismatch | Clean the ground connection area to bare metal and tighten the clamp. Replace the contact tip. Verify voltage and wire feed speed are within the ranges in your machine manual. A smooth, steady buzzing sound indicates a stable arc. |
| Bead has a small, tight profile and the weld pool seems sluggish | Cold weld (low heat input) | Voltage too low for the material thickness, or travel speed too fast for the heat input to develop proper fusion | Increase voltage in steps of 0.5 to 1.0 volts. Slow travel speed to allow more heat to build in the joint. Verify that your wire diameter matches the material thickness and the recommended settings in your manual. |
How to use the chart as a diagnosis flow
A visual identification chart works best when you treat it as a sequence rather than a menu. Start with the bead surface. Ask yourself whether the problem is on the surface (pinholes, soot, spatter), at the fusion line (lack of bond, undercut, overlap), in the bead shape (too tall, too flat, uneven), or in the arc behavior (stuttering, harsh sound, burnback). Each of these four categories points to a distinct family of causes.
Once you have identified the category, move through the checklist for that table in order. The first cause listed in each row is the most common, so check that one before you adjust anything else. If the first likely cause does not match your situation, move to the second, then the third. This approach prevents the common mistake of changing three variables at once and not knowing which one helped.
If the defect is intermittent, it usually points to a variable that changes during the weld, such as travel speed, torch angle, or a loose connection that vibrates. If the defect is consistent along the entire bead, it usually points to a fixed setting such as voltage, wire feed speed, or gas flow rate. Use this distinction to narrow your diagnosis further.
When the manual matters most
The chart above provides starting points for common causes, but every MIG welder is different. Voltage ranges, wire feed speed calibrations, gas flow recommendations, and inductance curves vary significantly between machines, even within the same brand and model family. That is why the machine manual is the final authority for any setting that affects arc performance, weld profile, or safety.
If your manual gives a specific voltage range for a given wire diameter and material thickness, start at the lower end of that range and increase only if the arc needs more heat. If your manual specifies a gas flow rate, do not exceed it on the assumption that more gas means better coverage. Excessive flow creates turbulence that pulls air into the shield, causing the porosity you were trying to fix in the first place. The manual exists because the manufacturer has tested the machine with specific wire, gas, and material combinations. General advice, including the chart in this article, is a starting point that must be cross-checked against that tested data.
This is especially important for any weld that could affect structural integrity, load-bearing performance, or pressure retention. If your work will carry a load, contain pressure, or support a structure, a visual identification chart is not enough. Stop and consult a certified welder or a welding engineer. The chart helps you diagnose a bead appearance, but it cannot substitute for a qualified procedure, a weld test, or an inspection method such as bend testing or radiographic examination. When safety is on the line, the manual is the floor, not the ceiling, and professional review is the correct next step.
Related reading
The chart in this article helps you identify the defect. Once you know which defect you are dealing with, the dedicated guides in this cluster provide the full step-by-step correction for each problem. These articles go deeper into the causes, the adjustments, and the verification steps that a single chart cannot cover:
- Burnback guide – complete troubleshooting for wire fusing to the contact tip
- Porosity guide – detailed sequence for eliminating pinholes and gas shield problems
- Lack of fusion guide – diagnostic workflow for beads that will not bond to the base metal
Each guide builds on the visual identification step you have already completed here. Use the chart to find your defect, then follow the linked article for the fix.
If the defect pattern seems to point upstream to machine setup rather than one isolated symptom, the wire feed problems guide and the systematic setup checklist are the best next stops.
Final note
A visual identification chart is only as useful as the diagnosis it leads to. The goal is never to memorize every possible defect and cause. The goal is to have a repeatable sequence: look at the bead, match the visual symptom to a category, check the most likely cause in that category, and then change exactly one setting at a time. That sequence keeps you out of the guesswork loop and puts you back in control of the weld.
Keep the chart accessible in your workshop. Use it when a bead looks wrong. Cross-check any setting change against your machine manual. And for any weld that matters structurally, get a qualified second opinion before treating the joint as sound. That approach will make you a more consistent welder and a safer one.
