Noncontact inspection of weld condition in cylindrical lithium-ion battery cap utilizing laser ultrasonic

Abstract

A laser weld placed on a center of a cylindrical lithium-ion battery cap is a component of a safety device called a current interrupt device (CID) that cuts off the current from the damaged battery cell. In this study, a laser ultrasonic inspection system is developed for noncontact and nondestructive inspection of laser welding conditions of the battery cap. An Nd: YAG pulse laser is used for Lamb wave generation, and the Lamb waves are measured from the front surface of the battery cap using a Laser Doppler Vibrometer. As the welding condition deteriorates, it induces the bending stiffness reduction, consequently increasing the Lamb wave amplitude. Based on this observation, an automated defect detection algorithm for welding condition inspection is developed. The proposed inspection system is potentially more attractive than X-ray methods for in-line inspection of battery caps because it can inspect them in a noncontact and nondestructive manner with high speed. Furthermore, the proposed inspection system can detect invisible weld defects on the back surface of the battery cap by inspecting its front surface. Finally, the effectiveness of the proposed inspection system is theoretically and experimentally examined using actual cylindrical battery caps, and its performance is cross-validated through conventional destructive testing.