Rear Rail Plug Welds: Peel Test Failures, Primer, and a Short Weld Count
After sectioning a 1.2 mm HSLA rear rail, a technician makes 10 plug welds in 8 mm holes with 0.9 mm ER70S-6 at 19 V and 3.5 m/min. The OEM procedure calls for 14 plug welds per flange, a minimum nugget diameter of 1.5 times the thickness of the thinner sheet, and no weld-through primer between faying surfaces. The technician applied weld-through primer at roughly 25 µm to both faying surfaces. A peel test on a sacrificial coupon and on two production welds shows 6 of 10 failures at the faying surface with smooth, unfused interfaces; the other 4 tear out of the base metal. Two production welds show porosity and one has a 1 mm crater. The vehicle is scheduled for delivery in four hours.
1. Interpret the peel test results: identify the failure modes, what each indicates, and the most likely root cause.
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Read the failure mode before the numbers. A smooth, unfused faying surface means the weld never penetrated the lower sheet - lack of fusion, not merely a small nugget. The four base-metal tears are the only acceptable mode and show the process is capable when the faying surface is clean. The 25 µm primer is the leading cause: weld-through primer is designed for thin films, and 25 µm between faying surfaces adds a resistive layer that can stop fusion and create porosity. The 19 V and 3.5 m/min with 0.9 mm wire in an 8 mm hole is also a cold plug weld for 1.2 mm sheet; current is spent melting the wire rather than the faying surface. The crater and porosity are secondary defects, but the crater reduces throat thickness and is a crack starter. The joint fails on fusion mode, not simply on weld count.
2. Determine whether the repair complies with the OEM procedure and whether the vehicle can be delivered on schedule. Justify the decision.
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No. The repair is noncompliant on at least three points: 10 welds versus 14, primer between faying surfaces contrary to the procedure, and 6 of 10 peel failures showing lack of fusion. The four good welds do not offset the six bad ones; weld quality is not averaged. A vehicle with a structural rail repair that fails the OEM weld schedule should not be delivered. The four-hour deadline is irrelevant to the compliance decision. The technician should stop, quarantine the vehicle and notify the supervisor or engineer. If the OEM procedure allows a documented repair - for example, removing the primer, re-welding and adding the specified welds - that can be evaluated; otherwise the section must be redone or the rail replaced. Delivery without correction would leave a known structural nonconformance.
3. Write the corrective action and verification plan, including parameter changes, primer removal, weld count, and documentation.
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Remove the primer from the faying surfaces with an approved method and solvent, clean to bare metal where the procedure requires, and re-establish coating only where specified after welding. Reset the machine on a coupon of the same 1.2 mm HSLA: 0.9 mm ER70S-6 is acceptable, but increase heat input by raising wire-feed speed or voltage, or use a synergic plug-weld program, until the peel test gives a button through the lower sheet; do not simply weld hotter on the vehicle without a coupon. Make 14 welds at the OEM spacing, fill craters, and inspect each weld. Re-test with a coupon and, where the procedure allows, a production weld; record diameters, machine settings, primer removal and QC sign-off. If the rework cannot meet the schedule, replace the rail section and document the reason.