Metal near the weld expands and then contracts non-uniformly. Thin sheet has low bending rigidity, so shrinkage readily becomes angular distortion, waviness and flatness error. The core controls are lower effective heat input, balanced shrinkage and controlled degrees of freedom—not aggressive straightening after welding.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
QUANTIFY HEAT INPUT
A common estimate is H=ηVI/(1000v) in kJ/mm, where V is volts, I amperes, v travel speed in mm/s and η process efficiency. At 18 V, 100 A, 6 mm/s and η=0.8, H≈0.24 kJ/mm. Use it for relative comparison within one process; efficiency, pulsing, joint heat sinking and transfer mode change the real thermal cycle.
CONTROL SEQUENCE
1. Reduce weld volume first: use intermittent or plug welds when load analysis permits, rather than defaulting to continuous welds. 2. Control fit-up; variable gaps force extra filler and heat. 3. Tack symmetrically and use skip/back-step sequences from the center outward, allowing local cooling. 4. Fixture datums without over-restraint that springs back after release; record clamp points and release temperature. 5. Inspect the first part in final condition using a surface plate/feeler gauges or CMM. Any thermal straightening requires an approved method and temperature.
RESEARCH AND STANDARDS
Rosenthal’s 1946 moving heat-source theory and the Goldak–Chakravarti–Bibby 1984 double-ellipsoidal model are classic foundations for welding thermal analysis. ISO 13920:2023 addresses general dimensional tolerances; ISO 5817:2023 provides imperfection quality levels, but a workmanship level does not replace structural fitness calculations.
PROJECT CHECKLIST
Thickness; fit-up gap; weld size/length; heat-input window; sequence; fixture; interpass cooling; final flatness and datums.