K-PAW准稳态过程小孔与熔池动态行为的数值模拟

NUMERICAL SIMULATION OF DYNAMIC BEHAVIOR OF KEYHOLE AND MOLTEN POOL AT K-PAW QUASI STEADY PROCESS

基于流体动力学原理，同时考虑电弧压力、表面张力、电磁收缩力、浮力和重力等因素影响，建立了随小孔深度增加热力作用二次变化的三维瞬态计算模型．利用上部双椭球体下部锥体的组合式体积热源描述等离子电弧对焊接工件的热作用，提出了可以维持小孔稳定的“孔内固体搅动式”计算方法．为了提高计算效率，建立了相对焊缝纵截面对称的计算区域：计算过程利用流体体积函数（VOF）法追踪小孔边界，基于FLUENT软件对穿孔型等离子弧准稳态焊接过程进行了数值模拟，得到了准稳态焊接过程中小孔、熔池及流场的动态变化行为，分析了穿孔型等离子弧焊接（K—PAW）准稳态过程的稳定性，探讨了影响小孔稳定的工艺因素，最后进行了计算模型的验证实验．结果表明，在设定的焊接工艺参数下，3．0s之后焊接过程达到准稳态，准稳态焊接过程中小孔前壁熔池较薄，平均厚度为0．6mm，且小孔前壁有一定倾斜现象，使得背面小孔中心相对焊接中心向后偏移，焊接不同时刻偏移量在0．46～0．97mm之间波动．在准稳态焊接过程中熔池内存在稳定的逆时针涡流，计算所得的背面小孔宽度与实验结果吻合良好．

The keyhole plasma arc welding （K-PAW） is widely applied in engineering project now as a high energy beam welding with its advantages of low-cost and easy operation. However, the arc instability may arise and welding defects will be produced in K-PAW due to the high current and strong plasma penetrating force when medium thickness plates are welded, finally weakening the efficiency of K-PAW. Furthermore, it is found that the flow field of liquid metal in the molten pool and the stability of keyhole have a critical influence on welding quali- ty. Therefore, modeling and simulating molten pool, keyhole and flow field in the K-PAW quasi steady process ex- cept for arc starting and ending phases are helpful to understand the welding process theory completely and pro- mote its application further. But to date, there is little study on the coupled analysis of molten pool and keyhole in the quasi steady welding process due to the difficulty to make keyhole stable. In this work, based on the principles of fluid dynamics with considering arc pressure, surface tension, electromagnetic force, buoyancy and gravity, a three dimensional transient model is established to reveal the secondary changing of heat and force effect regularly as the keyhole depth increases. To describe the welding heat process, a combined type volumetric heat source model of ＇double ellipsoid＋conical body＇ is employed. A keyhole inside solid agitated （KISA） calculated method is proposed to maintain the keyhole stability in the quasi steady welding process. To improve the computational efficiency, the calculated region is limited within the action region of a cone-symmetrical weld heat source. With volume of fluid （VOF） method to track the keyhole boundary, the dynamic behavior process of molten pool, keyhole and flow field are calculated using FLUENT software. The stability of K-PAW is analyzed and the factors affecting keyhole production are discussed. The calculated results show that under the welding current 140 A and plasma gas flow 3.5 L/ min, it needs 3.0 s to reach the quasi-steady state in which the average thickness of molten pool in keyhole front wall is 0.6 mm. The offset range of the keyhole center between top side and bottom side is 0.46-0.97 mm. There is the anticlockwise heat vortex appearing in molten pool of back side. The calculated width of keyholes on the bottom side is in good agreement with experimental results.