In laser transmission welding of thermoplastics the optical properties of the joining parts determine the quality of the welding result. Especially, the scattering of laser radiation in the transparent welding part has an impact on weld seam properties. This scattering is caused by additives. For polycarbonate (PC) with different additives the transmittance, the reflectance and the collimated transmittance are measured with a UV-VIS-NIR spectrometer. From this data, the optical properties, such as scattering coefficient, absorption coefficient, and anisotropy factor are calculated. The calculations are made with the aid of the four-flux model of radiation transport in the diffusive approximation. The results show that the additives have a significant influence on the scattering coefficient. For most additives under consideration the scattering is forward directed, which means that most of the radiation is transmitted into the absorbing welding part. However, the power density distribution of the transmitted radiation may differ significantly from PC without additives. So, the weld seam may also differ due to different additives

The laser-induced vaporization process of a metallic surface is incorporated into a simple model which describes the phase transformation and the expansion of the metal vapour against the ambient air as a function of the laser intensity and material properties. It is shown that there exists a material-dependent minimal laser intensity, where the vapour properties at the vaporization front become independent from further expansion against the ambient gas. This laser intensity is called critical intensity and also depends on the beam waist at the vaporization front. To check the model, a comparison with an experimental situation is performed, where a metal surface is irradiated by a Nd:YAG laser beam. The position of a shock front coming from the vaporizing metal surface were detected and compared with the calculated position.

The process of vaporisation of a light-absorbing metal under laser irradiation is investigated from a theoretical point of view. The tools are the Euler equations, the stationary heat-flow equation and the kinetic equations to calculate the gas dynamical properties of the edge of the Knudsen layer. Using the one-dimensional deflagration theory, it is shown that the local sound velocity is an upper bound for the metal vapour velocity at the phase boundary. Furthermore, it is shown that for an absorbed intensity Ia, which is greater than a certain threshold intensity Ic, the vaporisation process is independent of the ambient gas. This intensity Ic is calculated for aluminium and iron. On the assumption that a stationary limit of the vaporisation process is reached, the solution of the Euler equations is equivalent to the solution of a Riemann problem.

The ablation of Al2O3 by CO2 laser radiation is investigated both theoretically and experimentally. The model connects the laser-induced phase transition from condensed to vaporized state of the target and the dynamic of the emerging process plasma. The plasma is described in a two-fluid approximation by use of non-dissipative gas-dynamical equations incorporating absorption of laser radiation in the plasma and the dynamic of its ionization state. In the experimental part, the geometry of the luminous process plasma above the target at different instances is detected and the weight loss of the target as a function of the fluence is measured. At an Ar-base pressure below 1 mbar, both calculated and measured results reveal that there exist two zones in the process plasma: one which is directly attached to the target surface throughout the whole process, and another which is recognized as an outward moving shock front. Further, it is seen from both approaches that, due to absorption of laser radiation by the plasma, the weight loss has a local maximum.