【作者】 殷凤良；

【导师】 胡绳荪；

【作者基本信息】 天津大学 ， 材料加工工程， 2007， 博士

【摘要】 等离子弧焊具有电弧能量密度高、阳极电弧力作用强的特点,可以实现单道焊双面成形,当阳极电弧力足够大时还可以形成小孔效应,所以具有较高的焊接生产效率。但影响等离子弧焊接过程的因素较多,特别是穿孔等离子弧焊中的小孔状态不易保持,限制了其在工业生产中的广泛应用。如何提高等离子弧焊接过程的稳定性一直是人们致力要解决的问题。随着计算机技术的发展,数值模拟技术在焊接研究中得到了广泛应用。本文对等离子弧焊接过程进行数值模拟研究,为确定等离子弧焊接工艺、提高焊接过程稳定性提供理论依据。首先,根据流体动力学的质量守恒、动量守恒和能量守恒以及麦克斯韦方程组,建立了二维等离子弧焊电弧的数值分析模型。因为所建模型包括了一部分约束喷嘴和钨极,所以该模型可以反映焊枪结构和钨极形状等对等离子电弧的影响。在确定合适的边界条件后,利用基于有限体积法的流体分析软件,选用SIMPLEC算法求解所建电弧模型,可以得到等离子弧焊电弧温度场、流场、电磁场等。模拟研究了焊接电流、离子气流量、弧长、钨极内缩量等工艺参数对电弧特性的影响,以及钨极形状和喷枪结构等对电弧的影响。另外,本文建立了湍流状态下的等离子电弧模型,并将湍流等离子电弧模型与层流等离子电弧模型计算结果进行了比较。根据湍流模型得到的平均有效粘性系数与动力粘性系数之比和层流模型得到的马赫数,认为等离子电弧一般处于层流不可压缩状态。其次,在分析等离子弧焊接工艺特点的基础上,分别建立了三维熔透型静止电弧和运动电弧作用下的熔池数值分析模型。在模型中,等离子电弧作用于工件上表面的热流密度和电流密度分布为上述等离子电弧模型计算结果。通过粘性系数的设置来解决固液同一化问题,即当温度在熔点以下时给定一个较大的粘性系数。并采用大型有限元分析软件ANSYS求解得到了熔池温度场与流场分布情况。最后,在考虑等离子弧焊接小孔存在的情况下,建立三维非轴对称等离子电弧模型,以研究电弧反翘现象。研究结果表明,电弧反翘与小孔尺寸之间存在一定的关系,随小孔尺寸的增大电弧反翘变弱;电弧轴线与小孔轴线之间存在一定的偏差是形成电弧反翘的必要条件,在适当的焊接速度下,可以形成电弧轴线与小孔轴线之间的偏差。对单探针有源电弧反翘检测电路也进行了模拟研究,结果表明,附加电阻上的电压信号与小孔尺寸间存在规律性关系,其电压随小孔尺寸的增加呈减小趋势。根据模拟研究结果,设计了一种两探针的有源电弧反翘检测电路,可以检测出小孔的尺寸状态及变化情况。更多还原

【Abstract】 The plasma arc welding (PAW) possesses a higher productivity for its high energy density arc and a powerful arc force at the anode. This kind of welding technology can finish the welding process only with single pass, and even keyhole effect appears when the arc force is strong enough. But too many factors affect the stability of the PAW process, especially the status of the keyhole in keyhole PAW, which restricts its widely application in industry. How to keep the stability of PAW process has been concerned always. While, with the development of computer technology, numerical simulation is widely used in the welding technology. So simulation study on the PAW process is put forward here, to provide theoretical instruct for the PAW process.Based on the set of conservation equations for mass, momentum, energy and the Maxwell equations, a two-dimensional mathematical model has been developed. The model includes a part of nozzle and tungsten electrode to study the influence of nozzle configuration and electrode shape on the plasma arc characters. Setting proper boundary condition, some kind of CFD software based on finite volume method is used to solve the model with SIMPLEC algorithm. The temperature field, flow field and electromagnetic field of the plasma arc are gotten and analyzed from the model. And the influence of current, ionized gas flow rate, arc length etc on plasma arc character are also studied numerically, as well as the nozzle configuration and electrode shape. To verify the supposition that the plasma arc is in laminar flow state, another plasma arc model is built in which arc flow state is supposed to be turbulent. The results of the two kinds of arc model are compared, and the plasma arc is considered to be laminar judged by the ratio of effective viscosity and dynamic viscosity calculated in turbulent arc model. Also, the plasma arc is considered to be incompressible by the Mach number gotten form the laminar arc model.Analyzing the PAW process, three-dimensional weld pool models acted by static arc and moving arc respectively have been developed to predict the temperature and flow of it. The heat flux and current density distributions at the anode are taken from the results of plasma arc model. The solid part of workpiece is taken as liquid by setting a great viscosity to the workpiece when the temperature is above the melting point. Finally, the weld pool models are solved by ANSYS software. A three-dimensional non-axisymmetrical plasma arc model has been developed to study the plasma arc reflection phenomenon in the keyhole PAW, considering the existing of keyhole. The simulating results show that the arc reflection gets weaker with the increase of keyhole dimension, and a higher welding velocity to induce the offset between the arc axis and keyhole axis is necessary to the appearance of arc reflection. The theory of arc reflection detecting circuit is also simulated studied. The result shows that there is some certain relation between the keyhole dimension and voltage signal on the additional resistance, that is, the voltage signal on the additional resistance decreases with the increase of keyhole dimension. According to the simulated results, a new kind of arc reflection detecting circuit with two probes is designed which can detect the keyhole dimension and its changing.更多还原