Mathematical Modelling Of Laser Material Surface Treatment

The mathematical modelling of laser material processes is important to understand the phenomena occurring in the region irradiated by the laser which is used as the heat source. A lot of theoretical and experimental work on the laser material processes has been accomplished and is continuing to be done. In this thesis, the finite element method is used to calculate the temperature field of workpiece and the velocity field in the laser pool for the laser material processes. The influences of laser power, laser distribution and translation velocity of workpiece on the temperature field are emphatically studied in detail. When a laser pool is formed, the velocity field of flow and its influence on the heat conduction are investigated in the two-dimension. For the one-step laser cladding, a model is developed to describe the interaction between the laser and the powder particles, in which the velocity distribution of powder particles is simulated assuming the stream of carrier as a steady jet.The obtained results have shown that the temperature field of workpiece is determined essentially by the laser power, laser distribution and translation velocity of workpiece for a given material. The finished numerical calculations have proved that the more strong the laser intensity(that is the density of energy flux), the higher the temperatures reached in the local region irradiated by the laser. The distribution of temperature field is dependent on the power distribution of laser and translation velocity of workpiece mainly. For the laser material processes in which the translation velocity is not great compared with the heat diffusing velocity, it is sure that the laser power distribution is the primary factor influencing on the temperature distribution and then the translation velocity of workpiece. The profile of local temperature field near the laser is similar to the distribution of laser. That is the different power distribution of laser will lead to a different temperature distribution. The differences of temperature fields and their gradients among the diverse conditions are revealed clearly in the figures plotted using the numerical results. The numerical calculations have also shown that the thermal diffusivity has great influences on the temperature field and further the final results. The hardened treatment experiments of workpiece have been finished to make a comparison with the theoretical calculations. It is shown that the theoretical results have reasonably good agreement with the experimental data. The velocity field of flow in laser pool for the laser remelting processes and its effects are also computed in the two-dimensional case. Due to the great temperature gradient existing on the surface of laser pool, the fluid in laser pool moves rapidly so that the heal convection is dominant in the region of laser pool and can influence the neighbourhood. The flow of fluid makes the temperature field in laser pool more uniform than without the effects of fluid. Moreover, the laser pool becomes flat and shallow owing to the heat convection of metal liquid. The finished calculations have shown that the flow field has a great influence on the laser material processing. However, it is difficult to calculate the velocity field of flow in general.An analytical model is accepted to simulate the velocity and temperature distribution of powder particles and the attenuation of laser by the cloud of powder particles in the onestep laser cladding processes. Our results have shown that the velocity and temperature distribution of powder particles depend upon the properties of the carrier gas and the process parameters such as the angle of the axis of gas-powder jet with respect to the surface of workpiece. The theoretical results have proved that the attenuation of laser is appreciable and leads to the profiles different from the initial distribution for the small laser-jet angles. When the laser-jet angle increases in excess of 20°, the attenuation of laser is small and its energy distribution looks like the incident one. It is convenient to use it as the input data in the further modelling of laser cladding. Finally, a review and some discussions on the theoretical modelling of laser cladding are given to terminate my thesis.