Prozessmodelle in der Lasermaterialbearbeitung
Process models for laser treatment of materials
Four different models for the laser surface hardening, laser cladding, laser fusion welding and deep penetration welding are presented in this paper. They are able to predict the results of treatment and, partly, also to estimate those process parameter values which yield the desired results. From such process modells even equivalent heat sources may be derived which can be used for the calculation of thermal stresses and distortions by means of commercial finite element software packages. The laser hardening simulation tool GEOPT of the IWS consists of a data base to be evaluated by means of an included optimisation algorithm which is able to solve systems of non-linear equations and inequalities. This data base contains a lot of numerical results describing the relationship between process parameters, characteristical temperature field data and geometry parameters of hardening and annealing zones. Thus even inverse problems may be solved by GEOPT within a few seconds. The laser cladding process is modelled by the IWS software LAVA. First the interaction between laser beam and powder jet is simulated. From the resulting energy and mass flow densities acting on the substrate surface the bead geometry and the temperature field are calculated in a self-consistent manner. Recommendations are given how to correct the process parameter values in order to achieve prescribed results. Calculation times amount to a few minutes per simulation run on a standard PC. The weld seam shape in laser fusion welding is strongly influenced by the thermocapillary fluid flow. The latter depends very sensitively on the surface temperature gradients and the contents of surface active impurities such as oxygen or sulphur. This opens a possibility to enlarge the penetration depth of the melt and/or to reduce the required power input. Such effects can be simulated by means of the program MELTFLOW of the IOF. The software tool DC-LASIM of the DaimlerChrysler AG has been developped for the simulation of the deep penetration laser beam welding processes. Within the scope of a joint project of the German Federal Ministry of Education and Research this software was combined with the FEM software package ANSYS to form a 'welding simulation tool' with the aim to facilitate welding stress computations. Such tools, however, are also required for other technologies, e. g. for the laser cladding.