中文摘要：

本文建立了描述等离子弧焊接熔池相变传热与流动的三维数理模型，考虑表面张力、电磁力和浮升力的作用，并针对等离子弧焊接特点，改进组合式体积热源模型，上部采用双椭球热源，下部采用圆锥体热源。重点分析了焊接熔池形状和温度场的演变过程，熔合线的模拟形状与实验焊缝吻合，较好地呈现了焊缝的凸起和熔池宽度，验证了数学模型和热源模型的正确性。本文还进一步开展了焊接功率和焊接速度的影响分析。研究结果表明，流动对焊缝形状的影响不容忽略，而表面张力在三个流动驱动力中占主导地位；焊接功率越大，焊接速度越小，越有利于焊件焊透，数值模拟得到优化的焊接功率和速度有益于实际焊接生产质量及效率。

英文摘要：

A three dimensional numerical model has been developed to simulate the plasma arc welding （PAW） process, featuring a modified compound volumetric heat source together with enhanced heat transfer, phase change and fluid flows in the melten weld pool driven mainly by surface tension. The new heat source, which can reasonably characterize the keyhole PAW phenomenon, is composed of a double-ellipsoidal volumetric heat source at the upper part and a cone-shaped volumetric heat source at the lower part. Great emphasis is put on the melten weld pool evolution and temperature spread, and the simulation results, which accurately demonstrate size of the molten pool and apophysis in the fusion line, show quite good agreement with experimental image on the profile of the weld bead. Fhrther study continues to analyze the effect of welding power and welding velocity. Research results validate the practical application of the modified compound volumetric heat source to the PAW simulation, and display the considerable effect of fluid flows in the pool as well. Large welding power or lower welding velocity is both conducive to full penetration, but optical power and velocity are the best choice to ensure both the weld quality and penetration.