Keyhole/plasma sensing system for laser-welding control system

Abstract

A system has been developed for detecting the formation and failure of the plasma filled ‘keyhole’, produced during laser-welding. It allows the determination of the optimum position of focus, with respect to the workpiece, for processing, and forms part of a larger control system for a laser-welding cell.The major components of the system are acoustic and optical sensors connected to a microcomputer. The microcomputer processes the signals, deduces the presence of a keyhole, and produces a digital output.The acoustic signal, generated in the final mirror by the incident and back-reflected laser beam, changes significantly when a keyhole is formed. The optical signal is generated by the plasma above the keyhole.Experiments have been performed where the focus of the beam was taken through the surface of the workpiece, generating and then extinguishing the keyhole, whilst being monitored by the system. The experiments were repeated at various power levels illustrating the change in the acoustic signal. The signals, processed data, weld tracks and sectioned workpiece are shown. The cause of the changes in the signals are discussed and the significance of the phenomena observed.In conclusion it is shown that the system can be used to deduce the presence, or failure, of the keyhole, and that it offers the potential, after further development, for use in height-sensing and self-learning for optimal positioning of the beam-focus.A system has been developed for detecting the formation and failure of the plasma filled ‘keyhole’, produced during laser-welding. It allows the determination of the optimum position of focus, with respect to the workpiece, for processing, and forms part of a larger control system for a laser-welding cell.The major components of the system are acoustic and optical sensors connected to a microcomputer. The microcomputer processes the signals, deduces the presence of a keyhole, and produces a digital output.The acoustic signal, generated in the final mirror by the incident and back-reflected laser beam, changes significantly when a keyhole is formed. The optical signal is generated by the plasma above the keyhole.Experiments have been performed where the focus of the beam was taken through the surface of the workpiece, generating and then extinguishing the keyhole, whilst being monitored by the system. The experiments were repeated at various power levels illustrating the change in the acoustic signal. The ...