Meaning
Structural discontinuities in the laser or ultrasonic joint of a cell cap represent a failure mode for energy storage devices. Presence of a cylindrical header weld microcrack can compromise the hermetic seal of the canister, leading to electrolyte leakage or moisture ingress. These flaws often appear at the start or stop point of the weld path where heat dissipation is inconsistent.
Formation Cause
Rapid cooling of the molten metal creates internal stresses that exceed the material’s tensile strength. During high speed production, a cylindrical header weld microcrack might develop if the power profile of the laser is not finely tuned to the thickness of the cap assembly. Variations in the surface finish of the aluminum or steel components also influence the absorption of energy and the subsequent solidification rate.
Electrical Risk
Contact resistance increases when the conductive path between the jelly roll and the terminal is partially severed. If a cylindrical header weld microcrack exists, the increased impedance generates localized heat during high current discharge. This temperature rise can degrade the nearby gasket or safety vent, potentially leading to a thermal event if the cell is pushed to its limits.
Inspection Method
Non-destructive evaluation using high resolution X-ray or acoustic microscopy identifies sub-surface fissures. Because a cylindrical header weld microcrack is often too small for visual detection, automated optical systems must be supplemented by electrical testing.
