等离子弧焊接小孔和熔池热—力耦合输运过程的模拟及验证

发布时间：2018-06-03 13:50

本文选题：等离子弧焊接 + 等效热源模型　；参考：《北京科技大学》2014年博士论文

【摘要】：焊接是现代制造业中非常重要的基础工艺技术,高效、优质和低成本的焊接工艺是生产过程永恒的需求。等离子弧焊接因为其潜在的优势,成为21世纪最具发展前景的高效焊接技术之一。与常规电弧焊相比,等离子弧焊接具有能量集中、电弧挺度大、一次焊透深度大、热影响区窄、焊接变形小、效率高等优点；而与激光焊和电子束焊相比,等离子弧焊接在设备造价、维护、操作复杂程度及焊枪运动灵活性等方面又具有较大优势。然而,等离子弧焊接在实际应用中存在一些尚未解决的关键问题,主要是小孔对工艺条件的变化比较敏感；焊缝成形的稳定性较差；获得优质焊接接头的适用参数区间窄、裕度小。这些问题制约着这种高效焊接技术的应用。本文针对该问题开展了焊接熔池与小孔动态耦合传热特性的研究,以期为优质焊缝成形的可控性提供理论指导。基于文献调研工作,确立了理论与实验相结合、机理分析循序渐进的研究方案。首先搭建了焊接实验系统,根据焊接过程中的关键参数和相应的热物理现象提炼出数学模型,并且开展了不锈钢板的焊接实验,为模拟结果提供验证。理论研究从宏观的传热过程出发,通过建立合适的体积热源分布模式来反映小孔效应的影响,暂时不涉及真实小孔的形成,而是将小孔的热-力作用体现在恰当的体积热源分布模式上。本文称之为等效热源模型,着重考察在这种体积热源模式作用下的焊接温度场。然后开始探讨焊接热场和温度场随小孔增长的演变机制,建立了小孔-熔池动态耦合的传热模型,利用流体体积函数(VOF)追踪小孔界面,研究了定点焊接条件下熔池穿孔的动态演变过程,同时分析了小孔和熔池之间的耦合热-力作用机制及其所引起的熔池流体流动。在定点焊接的动态热源模型的基础上,又开展了连续焊接条件下熔池穿孔的动态演变过程及熔池内传热和流动特性的研究,该模型不仅考虑了小孔动态变化的影响,同时也增加了焊接速度的影响。目前国内外针对穿孔等离子弧焊接的理论研究普遍不考虑电弧部分,仅关注熔池部分的传热。然而电弧的热力特性是小孔型等离子弧焊接稳定进行的基础,只有开展等离子电弧和熔池耦合的全模型才能准确认识其中的真实热物理机理。因此本文建立了一个包含电弧、小孔和熔池的二维焊接全模型,模型中不再包含内热源,而是通过等离子电弧向工件传热。该模型更为真实的描述了等离子电弧热场、电场、磁场和流场的耦合输运作用,更加接近实际的焊接热物理过程。最后应用该模型开展了7因素3水平的正交试验,有效地预测了各工艺参数对焊缝成形的影响水平。
[Abstract]:Welding is a very important basic process technology in modern manufacturing industry. High efficiency, high quality and low cost welding process is the eternal demand of production process. Plasma arc welding has become one of the most promising welding technologies in the 21st century because of its potential advantages. Compared with conventional arc welding, plasma arc welding has the advantages of concentrated energy, large arc stiffness, large penetration depth, narrow heat affected zone, low welding deformation and high efficiency, while compared with laser welding and electron beam welding, plasma arc welding has many advantages. Plasma arc welding has great advantages in equipment cost, maintenance, operation complexity and welding torch movement flexibility. However, there are some unsolved key problems in the practical application of plasma arc welding. The main problems are that the keyhole is sensitive to the change of process conditions, the stability of weld formation is poor, and the suitable parameters for obtaining high quality welding joints are narrow. The margin is small. These problems restrict the application of this high-efficiency welding technology. In this paper, the dynamic coupling heat transfer characteristics of weld pool and hole are studied in order to provide theoretical guidance for the controllability of high quality weld forming. Based on the literature research, the research scheme of combining theory with experiment and mechanism analysis is established. Firstly, the welding experiment system was built, and the mathematical model was extracted according to the key parameters and the corresponding thermo-physical phenomena in the welding process, and the welding experiment of stainless steel plate was carried out, which provided the verification for the simulation results. Based on the macroscopic heat transfer process, the theoretical study reflects the effect of the pore effect by establishing a suitable volumetric heat source distribution model, and does not involve the formation of the real pore for the time being. The thermal-force action of the pore is reflected in the proper volumetric heat source distribution model. In this paper, the equivalent heat source model is called, and the welding temperature field under the action of this volumetric heat source mode is investigated. Then the evolution mechanism of welding heat field and temperature field with the growth of small hole is discussed. A dynamic coupling heat transfer model between small hole and molten pool is established, and the pore interface is traced by fluid volume function (VOF). The dynamic evolution of weld pool perforation under fixed-point welding was studied. The coupled thermal-force mechanism between the holes and the weld pool and the fluid flow in the molten pool were analyzed. On the basis of the dynamic heat source model of fixed-point welding, the dynamic evolution process of weld pool perforation and the heat transfer and flow characteristics in the molten pool under continuous welding conditions are studied. The model not only takes into account the influence of the dynamic change of the pore, but also the heat transfer and flow characteristics in the molten pool. At the same time, it also increases the influence of welding speed. At present, the theory of perforated plasma arc welding is generally concerned with the heat transfer in the molten pool. However, the thermal characteristics of the arc are the basis for the stability of the keyhole type plasma arc welding. Only by developing the whole model of plasma arc and pool coupling can the true thermo-physical mechanism be accurately understood. In this paper, a two-dimensional welding model with arc, orifice and molten pool is established. The model no longer contains internal heat source, but transfers heat to the workpiece by plasma arc. The model describes the coupling transport of plasma arc heat field, electric field, magnetic field and flow field more realistically, and is closer to the actual thermal physical process of welding. Finally, the orthogonal experiments of 7 factors and 3 levels were carried out by using the model, which effectively predicted the level of influence of each process parameter on weld formation.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TG456.2

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