Spectroscopic measurements of plasma inside the keyhole in deep penetration laser welding

Deep penetration laser welding is associated with violent plasma generation characterized by high charge densities. The plasma resides both outside and inside the keyhole, and is called plasma plume and keyhole plasma, respectively. The plasma plume outside the keyhole has been studied extensively because it can be conveniently observed; however, very little work has concentrated on the analysis of the keyhole plasma. In this paper, a specially designed set-up was used to take firsthand measurements of the light emission of the keyhole plasma in deep penetration laser welding aluminium films, clamped in between two pieces of GG17 glass, which we called a 'sandwich' sample, thus triumphantly eliminating the effect of the plasma plume covering the keyhole on the observation of the keyhole plasma. Results of spectroscopic measurements of both the plasma plume and keyhole plasma under welding conditions were obtained with an orthogonal experimental design. It was shown that the keyhole plasma had a considerable effect on the energy transfer efficiency of the incident laser beam to the material, exhibiting various melting widths and depths; a deeper welding depth as well as a lower temperature of the keyhole plasma was obtained when the density of the keyhole plasma was decreased by reducing the thickness of the aluminium films.