| Summary: | 博士 === 國立中正大學 === 機械工程學系暨研究所 === 102 === Abstract
This study proposed a tailored laser heat source model for the finite element analysis of the laser cladding process. The beam characteristics, including wavelength, beam radius, TEM mode and focusing conditions, were comprehensively considered in the heat source model. The model was integrated in a SYSWELD package to predict the temperature distribution and clad bead profile during laser cladding of preplaced powder layer or coaxial powder feeding on a steel substrate. Cladding process parameters were evaluated by varying the TEM mode, focusing conditions, wavelength and scanning speed. Single mode and tailored multi-mode TEMmixed laser beams were established for simulation. Validation of the heat source model was explored with laser cladding experiments at different beam powers and scanning speeds for both Nd:YAG and CO2 laser having a TEMmixed beam mode. As a comparison, the evolution of the melt pool isotherms and the clad bead profiles during laser cladding were simulated under identical laser parameter range as well as the materials and boundary conditions in the experiments. The threshold power for the clad bead formation and the feasible process parameter range for a successful laser cladding were verified. This work also presented an theoretical energy transfer model for laser cladding of preplaced powder layer. The energy transfer during laser cladding process was analyzed in the following consecutive steps: energy absorption by the preplaced layer, heat transfer from preplaced layer into base metal, melting of powder and base metal. To accomplish a successful cladding, two energy threshold values were proposed. The first threshold is necessary for melting of preplaced powder; the second threshold is required for melting of base metal and mixing of powder into a weld pool. The powder layer may be blown away without clad layer formation if the energy does not exceed threshold value. Feasible thickness range of the preplaced powder layer can be predicted with this model. The numerical results were verified by laser cladding experiment. The theoretical energy model shows good agreement with the clad layer thickness obtained from experiment.
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