Troubleshooting Projection Welding on Galvanized Steel

Projection welding a steel weldnut to galvanized sheet fails most often when zinc remains trapped at the weld interface or the weld head cannot follow the collapsing projections. Start with fast mechanical follow-up, then use high current over a short weld time to clear zinc and form the steel-to-steel joint. Inspect the projection roots for zinc-related cracking before releasing production.

Reject the usual quick fixes

Do not respond to a weak weld by extending weld time first. A longer cycle adds heat after zinc has already melted, giving it more opportunity to remain at the interface, contaminate the growing weld, or attack the highly stressed projection root. It can also overheat the surrounding sheet without correcting poor projection collapse.

Quick fix	Why it fails	Correct direction
Increase weld time	Prolongs zinc exposure and spreads heat without necessarily clearing the interface	Move toward higher weld current and shorter weld time
Add more force without checking motion	A heavy or slow head may still lose contact as the projections collapse	Correct follow-up speed and head inertia
Judge the weld by appearance	A centered nut can still contain an incomplete weld or cracked projection root	Use destructive and mechanical verification
Accept heavy expulsion as proof of heat	Expulsion can remove useful molten steel and produce an erratic joint	Find the shortest repeatable schedule that clears zinc and forms the weld

Check before continuing: confirm that the troubleshooting plan separates mechanical follow-up from the electrical schedule. Changing both at once hides the actual cause.

Confirm the joint and coating condition

Inspect the sheet surface, weldnut projections, tooling alignment, and contact faces. Zinc melts before the steel weld develops. As the projections heat and collapse, the coating must move out of the active interface so clean steel can form the joint. If zinc remains trapped, resistance and heat distribution change from weld to weld, producing incomplete fusion, porosity, expulsion, or inconsistent projection collapse.

1. Verify that every projection contacts the sheet before current flows. Rocking or a visible gap concentrates load and current on one projection.
2. Check the nut and sheet for oil, loose debris, oxide, damaged coating, or burrs. Do not grind away production coating merely to make a trial weld pass; that creates a different joint.
3. Inspect the upper and lower contact faces for zinc pickup and uneven wear. Contaminated tooling changes current density and mechanical alignment.
4. Confirm that the sheet is supported beneath the complete weld pattern. Local deflection consumes head travel and prevents uniform projection collapse.
5. Compare coating and material lots when failures are intermittent. Treat a lot change as a schedule-validation event rather than compensating blindly at the machine.

Check before continuing: close the head without welding and confirm simultaneous, stable contact at all projections with no rocking, sheet deflection, or tooling interference.

Set fast weld-head follow-up

Projection collapse is a moving joint. Force must remain on the weldnut while the projections shorten. If the head cannot descend as fast as the collapse, contact force falls, the electrical path opens or becomes unstable, and arcing or expulsion can interrupt weld formation. Static force alone does not prove that the mechanism can follow the joint dynamically.

A heavy head has greater inertia and may lag during rapid collapse. Check guides, pivots, cylinders, bearings, springs, and tooling mass for drag or excess inertia. A low-inertia head with correctly sized springs is one established way to obtain fast follow-up. Whatever actuator is fitted, remove binding and lost motion before tuning current.

1. Isolate welding current and cycle the head through the production stroke.
2. Watch for hesitation as the tooling contacts the nut and continues through the expected collapse travel.
3. Check that the tooling remains square and that force does not shift to one side.
4. Confirm that the return mechanism, guides, hoses, or cables do not restrain the final movement.

Check before continuing: prove that the head maintains contact throughout simulated projection collapse. Stop here if motion sticks, rebounds, or visibly lags; an electrical adjustment will not repair a mechanical follow-up fault.

Build a short-time, high-current schedule

After correcting contact and motion, develop the weld around high current for a short time. The rapid heating helps drive zinc away from the projection interfaces so the steel surfaces can join. Extending time is the wrong direction when zinc is the barrier.

1. Record the current production settings and preserve the last known acceptable schedule.
2. Use the machine’s approved setup method to establish a short weld time. Do not guess beyond the machine, tooling, nut, or sheet ratings.
3. Increase weld current in controlled changes while holding the verified force and follow-up condition constant.
4. Make a small group of test welds at each setting. Reject a schedule that works only once.
5. Watch and listen for unstable arcing, excessive expulsion, incomplete collapse, sheet distortion, or nut displacement.
6. Select the shortest repeatable condition that clears the coated interface and produces acceptable weld strength without destructive expulsion.

Current and time interact with projection geometry, coating condition, contact resistance, force, and machine capability. Copying settings from an uncoated joint or a different weldnut does not establish a valid galvanized-steel schedule.

Check before continuing: obtain repeatable projection collapse and acceptable destructive-test results across several consecutive welds, with no worsening zinc buildup on the contact faces.

Inspect the projection roots for zinc cracking

Do not release the joint on pull strength alone. Zinc can contribute to cracking along the root of the weld, where thermal stress, mechanical strain, and the edge of the fused region meet. A weld may carry one test load yet have a crack that reduces fatigue life or separates during forming, painting, or service.

Finding	Likely condition	Action
Unfused projection area	Zinc remained at the interface, contact was uneven, or energy was insufficient	Recheck alignment and follow-up before changing the schedule
Crack along the weld root	Zinc exposure combined with high local strain or excessive thermal input	Quarantine the welds and review time, current, force, and collapse behavior
Heavy expelled metal	Energy application or force control is unstable	Back away from the unstable condition and inspect tooling contamination
One strong projection and weak remaining projections	Uneven initial contact or support	Correct nut, sheet, and tooling alignment

Use the plant-approved destructive method to expose every projection interface. Where the consequence of failure demands it, section representative welds and inspect the root region using the approved metallographic or crack-detection method.

Check before continuing: confirm that all projection locations meet the defined acceptance criteria and show no root cracking, not merely that the nut stayed attached.

Verify the complete production cycle

Run the final check with production parts, normal loading, actual tooling, and the normal interval between welds. A cold setup weld does not test heat buildup, zinc accumulation, electrode wear, or lot variation.

1. Clean or dress contact faces using the approved maintenance method, then record their condition.
2. Run enough consecutive welds to expose drift as the tooling warms and collects zinc.
3. Track current delivery, collapse consistency, expulsion, nut position, thread condition, and sheet distortion.
4. Destructively test samples from the beginning and end of the run. Examine every projection and the root region.
5. Record the accepted current, time, force condition, tooling state, material identification, test method, and inspection result in the controlled setup record.
6. Define maintenance triggers based on measured weld deterioration or contact-face condition rather than waiting for detached nuts.

Final check: release the process only when the complete run maintains weld strength, uniform collapse, acceptable appearance, intact threads, and crack-free projection roots.

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