Optical monitoring of laser generated plasma during laser welding

Process monitoring is a vital part of industrial laser applications that enables intelligent control of processes by observing acoustic, optical, thermal and other emissions. By monitoring these emissions during laser processing, it is possible to ascertain characteristics that help diagnose features of the laser processed material and hence to optimise die technique. An experimental set up of observing plasmas during laser spot welding is described here. A pulsed Nd:YAG laser was used to spot-weld a variety of materials of different thickness, the plasmas generated during welding were monitored by a number of techniques, and the data obtained was used to characterise the welds. In the study photodiodes were set at different angles and observed the intensity and generation of the plasmas during the laser spot-welding process thereby giving a weld "signature". A portable spectrometer was used off-asis to obtain spectra of the emissions from the plasmas. Post process analysis was performed on the materials by mechanical polishing and chemical etching and observations of weld penetration depth and weld quality were correlated with the data collected on the plasmas. Different cover gases were also used during laser welding and the results of the effects of the various gases on the plasma are shown. The results indicate the relationship between laser weld generated plasma characteristics and and features such as penetration depth. A direct correlation between the intensities of the photodiode and portable spectrometer signals was observed with weld penetration depth.