Non-equilibrium models for the arc in arc welding processes

该项目旨在描述弧焊过程中电弧与电极的相互作用。特别是，弧根和鞘电压的建立以及由此产生的向电极的能量转移应该由钨极惰性气体(TIG)工艺和气体保护金属弧焊(GMAW)工艺的例子的模型来确定。除了钨阴极处的电弧附着外，还应详细分析TIG工艺中工件上的阳极附着和脉冲GMAW工艺中的Eh液滴库。重点是金属在电极上蒸发的影响和描述。与以往的方法不同，应该在自己的基础上建立并应用于Ar-Fe蒸气混合物的非平衡模型。这样的模型能够连续地描述从靠近局部热力学平衡(LTE)状态的中心到电极前面发生明显偏离热、化学和电离平衡的区域的弧等离子体。附加考虑假设每个电极前面的空间电荷区是无碰撞的，并且考虑穿过该区域的粒子通量，使得能够确定鞘电压。整个模型将基于对等离子体和电极的热和电流平衡的磁流体力学多流体模拟，包括电极表面的发射机制。在这种模型中考虑了鞘电压，最终实现了对弧根结构和电极蒸发过程的自洽描述。在第一种情况下，模拟应限于给定旋转对称电极几何形状的准稳态情况。伴随着TIG和GMAW工艺的实验，将为电极几何形状的选择和非平衡模型的验证提供支持，特别是通过光学发射光谱进行验证。模拟的结果将大大提高对TIG和GMAW工艺中的弧根结构、金属蒸发对弧根的影响以及电极鞘区域对电弧功率分配和向电极的能量转移的作用的理解。此外，还应分析电弧在下落液滴上的换向。从理论上推导出直流工艺和脉冲GMAW工艺中选择的状态下的鞘电压和输入电极的能量值，将提供改进的弧长、电流和总电压的关系，可用于进一步研究焊接过程控制等。此外，研究结果还为推导和采用简化的弧根描述方法提供了视角，这些方法也可以改进基于LTE假设的焊接过程的磁流体力学模拟。

The project is aimed at the description of the interaction of arcs and electrodes in arc welding processes. In particular, the establishment of the arc roots and the sheath voltages as well as the resulting energy transfer to the electrodes should be determined by a model for the examples of a tungsten inert gas (TIG) process and a gas metal arc welding (GMAW) process. Beside the arc attachment at the tungsten cathode the anode attachment at the workpiece in the TIG process and at eh droplet depot in a pulsed GMAW process should be analysed in detail. Main emphasis will be put on the impact and the proper description of the metal evaporation at the electrodes.In contrast to previous approaches, a non-equilibrium model for argon-iron vapour mixtures should be worked out on the base of own groundwork and applied. Such a model enables a continuous description of the arc plasma from the centre where it is near to the local thermodynamic equilibrium (LTE) state to the regions in front of the electrodes where distinct deviations occur from the thermal, chemical and ionisation equilibrium. The additional consideration of a space charge region in front of each electrode assumed as collision less and of the particle fluxes across this region enables the determination of the sheath voltage. The whole model will be based on a magneto-hydrodynamic multi-fluid simulation of the plasma and the heat and current balances of the electrodes including the emission mechanisms at the electrode surfaces. The consideration of the sheath voltages in such a model finally enables the self-consistent description of the arc root structure and the electrode evaporation. In the first instance, the simulations should be limited to quasi-steady-state situations with given rotationally symmetric electrode geometries. Accompanying experiments on TIG and GMAW processes will support the choice of the electrode geometries and the validation particularly of the non-equilibrium model by means of optical emission spectroscopy.The results of the simulations should provide a significantly improved understanding of the arc root structures in TIG and GMAW processes, of the impact of metal evaporation on the arc roots and of the role of the electrode sheath regions for the power budget of the arc and the energy transfer to the electrodes. Beside others, the commutation of the arc across a falling droplet should be analysed also. Theoretically deduced values of the sheath voltages and the energy input into the electrodes for dc processes and selected states in pulsed GMAW processes will provide improved relations of arc length, current and total voltage which can be used for further studies among others for the welding process control. In addition, the results deliver the perspective to deduce and adapt simplified approaches for the description of the arc roots which can also improve established magneto-hydrodynamic simulations of welding processes based on the LTE assumption.