MODEL OF A LASER-HEATED PLASMA INTERACTING WITH WALLS ARISING IN LASER KEYHOLE WELDING

In laser welding with laser intensities of approximately 16(11) W/m2, a hole, called a keyhole, is formed in the material. In this keyhole a plasma is detected, which is characterized by high pressure as well a s being influenced by the boundary of the keyhole. Experimental data o n plasma parameters are rare and difficult to obtain [W. Sokolowski, G . Herziger, and E. Beyer, in High Power Lasers and Laser Machining Tec hnology, edited by A. Quenzer, SPIE Proc. Vol. 1132 (SPIE, Bellingham, WA, 1989), pp. 288-295]. In a previous paper [C. Tix and G. Simon, J. Phys. D 26, 2066 (1993)) we considered just a simple plasma model wit hout excited states and with constant ion-neutral-atom temperature. Th erefore we neglected radiative transport of excitations and underestim ated the ion-neutral-atom temperature and the ionization rate. Here we extend our previous model for a continuous CO2 laser and iron and tak e into account radiative transfer of excitations and a variable ion-ne utral-atom temperature. We consider singly charged ions, electrons, an d three excitation states of neutral atoms. The plasma is divided in p lasma bulk, presheath, and sheath. The transport equations are solved with boundary conditions mainly determined through the appearance of w alls. Some effort is made to clarify the energy transport mechanism fr om the laser beam into the material. Dependent on the incident laser p ower, the mean electron temperature and density are obtained to be 1.0 -1.3 eV and 2.5 X 10(23)-3 X 10(23) m-3. Radiative transport of excita tions does not contribute significantly to the energy transport.