The Welded Bracket Failure
A fabricator produces steel mounting brackets from 6 mm mild steel plate. The process is: laser cut, bend on a press brake, then MIG weld two tabs to the base. Inspection of a recent batch of 200 finds 14 brackets with cracks at the toe of the weld and 9 with the tabs 2 mm out of position. Production records show the welder was changed mid-batch, the shielding gas cylinder was replaced, and the press brake tooling was swapped to a different V-die to speed up bending. The customer's drawing specifies a 1 mm positional tolerance on the tabs and a weld that must pass a visual inspection to a recognised standard. The fabricator must decide whether to rework or scrap the affected units and what to change before the next batch.
1. Distinguish the likely causes of the two defect types and explain why they should be investigated separately.
Read the worked answer
The two defects have different origins and should not be lumped together.
Cracks at the weld toe. These are weld-quality defects. Candidate causes: the gas cylinder change may have introduced a period of poor shielding (air in the line, wrong flow rate), producing porosity or brittle weld metal; a new welder may have used incorrect current, travel speed or technique, giving poor fusion or an unfavourable bead profile that concentrates stress at the toe. Cracking at the toe is a stress-concentration failure, so bead shape and heat input matter.
Tab position error (2 mm vs 1 mm tolerance). This is a dimensional/forming issue, not a welding issue. Swapping to a different V-die changes the bend allowance and springback, so the bent base geometry shifts and the tabs, welded to it, end up out of position. The welder change would not move tabs by 2 mm.
Separating them matters because the corrective actions are unrelated: weld parameters and gas supply versus bend tooling and setup. Treating all 23 units as one problem risks fixing neither.
2. Recommend disposition of the 23 affected brackets and the process changes for the next batch.
Read the worked answer
Disposition
- The 9 out-of-position tabs: if the error is 2 mm against a 1 mm tolerance, they do not meet drawing. Rework is only viable if the tabs can be cut free, repositioned in a fixture and re-welded without distorting the base - and then re-inspected. Otherwise scrap them. Do not ship out-of-tolerance parts.
- The 14 cracked brackets: cracks at a weld toe are a structural reject. Welding over a crack is not acceptable without a documented repair procedure and re-inspection; in most cases scrap is the correct call. Cracks indicate the joint may be weak even where no crack is visible, so consider whether the whole batch needs closer inspection.
Process changes
- Restore the correct V-die and re-qualify the bend setup; verify the first-off part against the drawing before running.
- Purge and check the gas line after any cylinder change; record flow rate.
- Require welder qualification on the joint and record parameters (current, voltage, travel speed).
- Add a first-off inspection and in-process check at intervals, with the 1 mm tab tolerance measured on a fixture.
- Keep a traceable record linking each unit to welder and setup.