Applying optical coherence tomography for weld depth monitoring in remote laser welding of automotive battery tab connectors

Abstract

This paper addresses in-process monitoring of weld penetration depth (WPD) during remote laser welding of battery tab connectors using optical coherence tomography (OCT). The research aims at studying the impact of welding process parameters on the accuracy of WPD measurements. In general, the highest measurement accuracy is achievable by positioning the OCT measuring beam toward the bottom of the keyhole. However, finding and maintaining the alignment between the OCT measuring beam and the bottom of the keyhole is a challenging task because of the dynamic changes in the size and shape of the keyhole itself. The paper addresses the above challenge by (1) developing welding process parameters for the Al-Cu thin foil lap joint (Al 1050 foil 450 μm and Ni-plated Cu foil 300 μm) using a novel adjustable ring mode (ARM) laser and (2) integrating OCT technology with two beams: one targeting the bottom of the keyhole and another as a reference to the part surface (TwinTec technology). The methodology is underpinned by the “Keyhole Mapping” approach, which helps one to identify the optimal placement of the OCT measuring beam with considerations to both measurement accuracy and stability of the keyhole. Findings indicated that welding with the ARM laser results in a more stable process, reduces fluctuations of the keyhole opening, and, therefore, helps one to improve the measurement accuracy by a factor of 50% (from the average error of 0.22 mm to 0.11 mm). Results further identified that the feasible operating window of the OCT measuring beam, corresponding to the highest measurement accuracy, is below 20 μm in length.