Inline Process Monitoring of Hairpin Welding Using Optical and Acoustic Quality Metrics

Due to the electrification of the drive train, hairpin technology for stators has established itself in the automotive industry. Hairpins are rectangular copper wires connected to full stator coils using laser beam welding. The welding results depend on the preliminary processes and tolerances of the welding system. The quality of the connection is defined by its cross-sectional area that is influenced by porosity. Another quality characteristic are spatters. The quantification of the cross-sectional area and porosity of the welding seams is usually carried out post-process using X-Ray computed tomography (CT). This paper investigates spatters using high-speed camera-based image processing and airborne sound using an optical microphone. By varying influencing factors of the welding result (position deviations, maximum laser power, focal position shift, stripping quality), different quality levels of the welding joints are generated. With the developed method for in-process evaluation of high-speed camera images, a spatter detection and quantification of their number and size is enabled. The method shows an accuracy of 84%. In addition, the characteristics of the recorded airborne sound during the process allows conclusions to be drawn about the welding results. Deviations in maximum laser power, height offset, focal position shift and stripping quality can be detected. Both methods provide a reliable inline monitoring of the welding process as well as its quantitative quality assessment.