Non-equilibrium models for the arc in arc welding processes

该项目旨在描述电弧焊接过程中电弧和电极的相互作用。特别地，电弧根部和鞘电压的建立以及由此产生的到电极的能量传递应当通过用于钨极惰性气体（TIG）工艺和气体保护金属极电弧焊（GMAW）工艺的示例的模型来确定。除了钨极电弧附着外，还应详细分析TIG焊过程中阳极附着在工件上和脉冲GMAW过程中熔滴库处的情况。重点讨论了电极处金属蒸发的影响和正确描述，并在自己的基础上建立了氩-铁混合蒸气的非平衡模型，并将其应用于实际。这样的模型使得能够连续地描述电弧等离子体从其接近局部热力学平衡（LTE）状态的中心到电极前方的区域，在该区域中发生与热平衡、化学平衡和电离平衡不同的偏差。假设碰撞少的每个电极前面的空间电荷区域和穿过该区域的粒子通量的额外考虑使得能够确定鞘层电压。整个模型将基于等离子体的磁流体动力学多流体模拟以及电极的热和电流平衡，包括电极表面的发射机制。在这样的模型中考虑鞘层电压，最终使电弧根结构和电极蒸发的自洽描述。在第一种情况下，模拟应限于准稳态的情况下，给定的旋转对称电极的几何形状。对TIG和GMAW工艺的实验将支持电极几何形状的选择和通过光学发射光谱特别是非平衡模型的验证。模拟结果将提供对TIG和GMAW工艺中电弧根结构的显著改进的理解。金属蒸发对电弧根部的影响以及电极鞘层区域对电弧的功率预算和向电极的能量传递的作用。除此之外，还应分析电弧穿过下落液滴的换向。从理论上推导出的鞘层电压和能量输入到电极的直流过程和选择状态的脉冲GMAW工艺将提供改进的弧长，电流和总电压，可用于进一步研究的焊接过程控制等的关系。此外，结果提供的角度来推导和适应简化的方法来描述的电弧根，这也可以提高建立磁流体动力学模拟的基础上的LTE假设的焊接过程。

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.