Good Welds vs. Bad Welds: Identification and Defects

A good weld is not simply smooth or attractive. It must have the required size, fusion, penetration, profile, and freedom from unacceptable defects for the drawing, welding procedure, and inspection standard. A bad weld shows visible or hidden discontinuities that exceed those acceptance limits. Visual inspection is the first filter, but it cannot prove internal soundness.

Start by checking bead consistency, weld toes, craters, surface pores, cracks, undercut, overlap, distortion, and evidence of shielding failure. Then compare what you see with the approved procedure and acceptance criteria. A neat stack-of-dimes pattern can still hide incomplete fusion, while a less decorative bead may be acceptable if its geometry and internal quality meet the specification.

Good Weld vs. Bad Weld: Quick Comparison

Inspection Point	Good Weld	Bad Weld or Warning Sign
Bead width	Consistent for the joint and position	Sudden widening, narrowing, wandering, or stop-start buildup
Weld profile	Specified fillet size or groove fill; smooth transition	Excessive convexity, concavity, overlap, underfill, or burn-through
Weld toes	Fused smoothly into the base metal	Undercut groove, cold lap, sharp notch, or visible lack of fusion
Surface	Clean and continuous within acceptance limits	Cracks, open pores, slag, pits, arc strikes, or heavy contamination
Root and penetration	Meets the drawing and procedure	Incomplete penetration, root concavity, excessive reinforcement, or collapse
Color and oxidation	Consistent with the process and material	Heavy oxidation, soot, sugaring, or heat tint beyond the allowed condition
Distortion	Within dimensional tolerance	Twist, bow, angular distortion, or joint movement outside tolerance

What Does a Good Weld Look Like?

A good weld has a stable profile from start to finish and blends into both sides of the joint without sharp notches. The required profile changes with the joint: a fillet weld is judged by leg size, throat, contour, and toe condition, while a groove weld is judged by fill, reinforcement, root condition, and fusion. Appearance must therefore be compared with the specified joint, not with a generic photograph.

The bead should be continuous and appropriately sized. Uniform ripples may indicate steady travel, but ripple pattern is not a strength test. On processes that produce slag, the surface can only be evaluated after the slag is removed. On stainless steel and aluminum, color and oxide condition can reveal shielding or heat-control problems, although the acceptable appearance still depends on the material and service requirement.

A reliable weld also begins before the arc or laser starts. Clean mating surfaces, consistent joint gaps, adequate clamping, correct filler material, stable shielding gas, and a qualified parameter window make good results repeatable. If fit-up changes from part to part, an operator may produce one attractive sample and still lose consistency during a batch.

Common Welding Defects: Signs, Causes, and Actions

Weld Cracks

Cracks may appear in the weld metal, crater, root, or heat-affected zone. They are high-risk indications because they can propagate under load or vibration. Likely contributors include excessive restraint, unsuitable filler, hydrogen, rapid cooling, poor crater filling, or an alloy-specific solidification problem. Stop production, preserve the evidence, and follow the approved repair and inspection procedure rather than grinding the crack away before evaluation.

Porosity

Porosity appears as pinholes or rounded cavities when gas becomes trapped during solidification. Surface contamination, moisture, gas leakage, drafts, unstable shielding, or an unsuitable parameter combination are common causes. Clean the joint and filler, verify gas type and delivery, inspect hoses and connections, and confirm that the torch or welding head angle is not pulling air into the pool.

Lack of Fusion and Incomplete Penetration

Lack of fusion means weld metal has not bonded adequately to a sidewall, root face, or previous pass. Incomplete penetration means the weld has not reached the required root depth. Both can remain hidden beneath a tidy surface. Check joint preparation, beam or arc position, power or current, travel speed, focus, torch angle, and fit-up. For critical joints, surface appearance alone cannot clear these defects.

Undercut, Overlap, and Underfill

Undercut is a groove melted beside the weld toe and left unfilled. It reduces the local section and creates a stress concentration. Overlap, sometimes called cold lap, occurs when weld metal rolls onto the base material without proper fusion. Underfill leaves the weld face below the required surface. Travel speed, heat input, filler delivery, working angle, scan width, and edge alignment should be reviewed together; adjusting one setting in isolation often shifts the problem elsewhere.

Burn-Through and Excessive Penetration

Burn-through creates a hole or collapsed root, most often on thin material or an unsupported gap. Excessive energy per unit length, slow travel, poor fit-up, or weak backing can be responsible. Reduce heat input within the qualified window, improve clamping, correct the gap, or use a backing method permitted by the procedure.

Slag Inclusions and Tungsten Contamination

Slag can become trapped between passes when the previous bead is not cleaned or when the joint geometry prevents slag from escaping. TIG welding can suffer tungsten inclusion if the electrode contacts the pool or filler. Remove contamination using the approved method, restore the joint preparation, and correct technique before depositing the next pass.

Spatter, Soot, and Discoloration

Spatter is not automatically a structural failure, but a sudden increase often points to unstable settings, polarity, wire delivery, surface contamination, or poor shielding. Soot and dark oxide can reveal contamination or gas coverage problems. On stainless steel, heavy heat tint may trigger cleaning or passivation requirements. Judge these conditions against the process and downstream finish rather than treating every mark as the same defect.

Welding Distortion

Distortion occurs when heating and cooling shrink the joint unevenly. It can move holes, twist assemblies, and create rework even when the bead itself is sound. Control it through joint design, clamping, tack sequence, balanced welding, travel planning, and heat input. If a part repeatedly pulls out of tolerance, correcting the sequence is usually more effective than forcing it straight after every weld.

How to Inspect a Weld Before, During, and After Welding

Before Welding

• Confirm the requirement: Identify the drawing, weld symbol, joint type, weld size, material, process, procedure, and acceptance criteria.
• Inspect fit-up: Measure the root opening, alignment, bevel, tack welds, and fixture condition across representative parts.
• Check cleanliness: Remove oil, moisture, paint, oxide, and debris to the level required by the process.
• Verify consumables and gas: Confirm filler type, condition, polarity where applicable, shielding gas, flow, and leak-free delivery.

During Welding

Watch arc or beam position, pool stability, wire feeding, shielding coverage, travel speed, and joint movement. A process that begins cleanly can drift when the gap changes, the nozzle accumulates spatter, a gas line leaks, or heat builds in the fixture. Recording the accepted parameter recipe makes troubleshooting much faster than relying on operator memory.

After Welding

Allow the joint to cool as required, clean it without hiding evidence, and inspect under adequate lighting. View the face, both toes, start and stop points, crater, root where accessible, and the surrounding heat-affected zone. Measure weld size and profile with suitable gauges, record defect location and length, and compare the result with the applicable acceptance criteria.

Good and Bad Welds by Welding Process

MIG welds are commonly checked for bead consistency, toe fusion, spatter, porosity, and cold lap. TIG welds should show controlled width and clean tie-in without tungsten contamination; stainless and aluminum also demand close attention to shielding and oxidation. Stick and flux-cored welds must be inspected after slag removal, especially between passes and at restarts.

Laser welding usually produces a narrow weld with concentrated heat input, but a smooth face does not rule out porosity, lack of fusion, unstable penetration, or root defects. Quality depends on material grade, joint design, fit-up, surface condition, shielding gas, laser power, travel speed, focus, scan width, and filler delivery. The relevant settings must be qualified as a group.

Laser Welding Defect Troubleshooting

Observed Problem	Likely Process Checks	Practical Response
Porosity or pinholes	Surface cleanliness, shielding gas, leaks, speed, keyhole stability	Clean and dry the joint; verify gas delivery; return to the qualified parameter window
Lack of fusion	Beam position, focus, power, speed, gap, scan width	Correct alignment and fit-up before increasing power
Undercut or underfill	Speed, energy, scan path, filler rate, joint gap	Balance travel and filler delivery; verify the required profile
Burn-through	Excess heat, slow travel, thin sheet, unsupported gap	Reduce energy per unit length and improve support or fit-up
Heavy oxidation	Gas type, purity, flow, coverage, drafts	Restore shielding and confirm the required surface condition
Variable bead width	Hand motion, robot path, scan setting, gap variation	Stabilize the path and measure actual production gaps

Joint gap deserves special attention in handheld laser welding. A wider or inconsistent gap changes how much parent metal is available and may require filler wire. Treat the feeder as a process tool, not a cure for poor cutting, bending, or clamping. Measure gaps over several batches before selecting wire diameter, single- or dual-wire feeding, scan width, and power.

When Visual Weld Inspection Is Not Enough

Visual inspection can find surface cracks, undercut, overlap, visible porosity, poor profile, arc strikes, misalignment, and distortion. It cannot reliably confirm internal fusion, penetration, embedded slag, subsurface porosity, or internal cracking. The inspection plan may therefore require penetrant testing, magnetic particle testing, ultrasonic testing, radiography, leak testing, macro sections, bend tests, or tensile tests depending on the material, joint, service risk, and governing specification.

Do not label every discontinuity a defect before checking the acceptance limit. A discontinuity becomes rejectable when its type, size, location, or quantity exceeds the applicable criteria. For load-bearing, pressure-containing, fatigue-sensitive, or regulated parts, acceptance belongs to qualified personnel working from the specified code and procedure.

Conclusion

The fastest way to separate good welds from bad welds is to follow a repeatable sequence: identify the joint requirement, inspect fit-up and cleanliness, examine the bead and toes, measure the profile, record any discontinuities, and escalate to the required test method when the surface cannot answer the question. Appearance is evidence of process stability, not proof of internal strength.

For laser welding, the most useful troubleshooting inputs are the actual material, thickness, joint design, gap range, drawings, shielding gas, filler wire, and photos of both the face and root. SENFENG can use those details to plan trials on its laser welding machines. For the current HWM Series handheld laser welding machine configuration, product literature lists autogenous fit-up up to 15% of plate thickness and no more than 0.5 mm, single-wire welding up to 1.2 mm, and dual-wire welding up to 2.0 mm. These are configuration limits, not automatic acceptance results; the finished joint still needs sample qualification and inspection.

FAQ

Can a good-looking weld still be defective?

Yes. A smooth surface can hide lack of fusion, incomplete penetration, internal porosity, slag, or cracking. Use the inspection method required by the joint and service condition.

What is the clearest visual sign of a bad weld?

Visible cracks are the clearest high-risk warning. Undercut, overlap, open porosity, burn-through, and major profile inconsistency also require evaluation against the acceptance criteria.

Does a stack-of-dimes pattern mean a weld is strong?

No. Consistent ripples may show steady travel, but they do not prove penetration, fusion, weld size, or internal soundness.

What causes porosity in a weld?

Common causes include contamination, moisture, gas leaks, drafts, poor shielding coverage, and an unstable parameter combination. Clean the joint and verify the complete gas path before changing equipment.

How do you confirm weld penetration?

Inspect the root when accessible and use the specified destructive or nondestructive test when it is not. A face-side visual check alone cannot confirm internal penetration.