【作者】 伏云昌；

【导师】 李俊昌； A.B. Vannes；

【作者基本信息】 昆明理工大学 ， 材料学， 2004， 硕士

【摘要】 在激光材料过程中，激光辐照工件时，相当于一个热源，辐照区会发生很多现象，数值模拟对理解这些现象非常重要。对激光材料过程，已完成了大量的理论和实验研究工作，而且这方面的研究还在继续。在本论文中，采用有限元法计算激光材料过程工件的温度场，模拟激光熔池中的流体速度场，详细地重点研究了激光功率、激光能量分布和工件移动速度对温度场的影响；如果有激光熔池的重熔处理，就二维情况，模拟了流体速度场，研究了流体流动对热传导的效应；建立了一个模型模拟一步法激光涂覆过程中粉末粒子与激光的相互作用，将载流气体假定成一个稳定的射流，数值模拟了粉末粒子的速度分布。模拟结果表明对给定的材料，工件的温度场主要由激光的功率、激光的能量分布和工件的移动速度决定。数值计算证明激光的光强越大，激光辐照区域达到的温度越高；温度场分布主要取决于激光能量分布和工件的移动速度；与热扩散速度相比，工件移动速度不大的激光材料过程，可以肯定激光能量分布是影响温度场分布的第一因素，其次才是工件的移动速度；激光照射附近区域温度场的轮廓与激光功率分布相似。根据计算数据，得到的图清楚地显示出不同条件下温度场及其梯度的差别。数值模拟还表明材料的热扩散系数对温度场和处理结果有很大影响。完成了工件的激光硬化处理实验，与理论计算做了比较，结果表明理论结果与实验数据吻合。对二维情况，还计算了激光重熔处理过程激光熔池流体速度场及其效应。由于激光熔池表面存在很大的温度场梯度，熔池中流体快速运动，所以熔池区热量主要通过流体对流传递，并进一步影响熔池临近区域；流体流动使熔池区温度场比不计入流体效应的均匀。还有，由于金属流体的对流传热，相比不考虑流体影响，激光熔池变扁变浅。计算结果表明流体场对激光材料过程有很大影响，但一般来讲，数值计算流体的速度场非常困难。提出了一个解析模型模拟一步法激光涂覆过程中粉末粒子的速度分布和温度分布以及粉末粒子云对激光的衰减效应。我们的结果表明粉末粒子速度分布和温度分布依赖于载流气体的性质和过程参数，如气体-粉末喷枪轴与工件表面之间的夹角。理论模拟结果证明对小的激光-喷枪夹角，激光的衰减显著，激光的能量分布不同于原始分布；随激光-喷枪夹角增大，超过20度，激光的衰减较小，分布与入射光相似。这个解析模型的数据可以很方便地作为进一步模拟激光图覆的输入数据。最后，作为论文的结束，对激光涂覆理论模拟进行了一些讨论和综述。更多还原

【Abstract】 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.更多还原