Numerical simulation and experimental investigation of laser-based direct metal deposition

The laser-based direct metal deposition (LBDMD) has been developed as a valuable technique for rapid prototyping, rapid manufacturing and repair. This technology offers a reduction in the lead-time from design to part production, tailoring of material composition, and microstructure control.; For successful utilization of the LBDMD method, the operational parameters must be carefully controlled. There are complex relationships between the input variables such as the laser power intensity, deposition head traverse speed, and powder feed rate, and the process outputs such as the material deposition rate and track width. These relationships make the final results such as the surface finish, residual stress and material microstructure difficult to predict. One important factor contributing to the relationship is the variation in the powder stream concentration, because this affects the material delivery rate at the deposition point and the powder temperature as well as the degree of laser attenuation due to the material-radiation interaction. In this dissertation, a three-dimensional numerical model of gas-powder flow from four radially symmetric nozzles was developed to investigate powder distribution. However, attempts to understand the powder flow through simulation must involve concomitant well-designed experimental work to validate the model. The visualization and digital image processing techniques are used to analyze powder distribution. Both the experimental and numerical results revealed a zone of high powder concentration where the substrate should be located in order to achieve an accurate powder delivery to the molten pool as well as the required deposit geometry. An analytical model of laser power attenuation through a powder cloud was developed and verified experimentally. Finally, by knowing the material and energy input, it is possible to analyze the development of the deposited structure with the associated thermal behavior and development of residual stresses. To explore the deposition process of thin-walled structures, a three-dimensional thermo-structural finite element model was developed and calibrated using real processing parameters. The influence of different deposition strategies and geometries on residual stress development was investigated. The obtained results were confirmed in the real fabrication process of these structures.; This dissertation gives a good insight into the laser-based direct deposition phenomena that assists in the selection of initial processing parameters and their further optimization in order to produce deposits of highly accurate geometry and required material properties.