旁路耦合微束等离子三维焊接数值研究

发布时间：2019-01-26 15:32

【摘要】：三维焊接是增材制造中的一类,直接从零件的数字化模型(例如CAD文件)中制造金属部件。特别适用于生产形状复杂,高价值和定制化零件,以及修复报废零件。目前,三维焊接的主要挑战包括成形缺陷(如气孔),表面光洁度低,材料性质在空间上不均匀分布。这些挑战主要来自于对增材制造复杂的物理过程认识不足,尤其是熔池动态,比如熔池液态金属流动,热传导,合金元素的蒸发以及固化。由于熔池面积小,同时瞬态变化较大,采用实验的方法直接测量比较困难。而且,目前的光学和红外摄像机在观察熔池表面时也受到限制。所以,很难通过实验研究熔池中熔融金属的流动、熔池动态和宏观缺陷。另一方面,基于严格求解质量、动量和能量守恒方程的数值模拟可以提供发生在增材制造过程中复杂传输现象的重要定量认识。首先,通过分析电弧增材制造中遇到的种种问题,发现电弧增材制造精度不高,组织性能不好的部分原因是对工件的热输入太大,因此提出了旁路耦合微束等离子弧焊(Double Electrode Micro Plasma Arc We l d i n g,D E-M PAW)。搭建了D E-M PAW三维焊接平台,整个平台由三维运动系统、DE-MPAW焊接系统、信号采集系统和焊接控制系统组成。运用CCD摄像机实时跟踪熔滴过渡,采用热电偶测量对母材的热输入并且实时采集焊接过程中的电流电压。对DE-MPAW焊进行了一系列的基础研究。分析了旁路电流对焊接过程中熔滴过渡、母材热输入和焊缝形貌的影响并采用优化的工艺参数进行了单道多层堆积试验。其次,采用FLOW-3D软件,使用有限差分算法,开发了熔滴、热源和熔池受力子模型,建立了考虑自由表面的DE-MPAW焊三维瞬态熔池模型。计算了熔池温度分布和流动状况,深入理解熔池中熔融金属的传热过程和流动模式,为获得高性能的SUS304不锈钢DE-MPAW三维焊接成形件打下良好的基础。在该模型中,将熔滴处理为在熔池上方固定高度,恒定温度和初始速度的球形液滴。对模型中主路,旁路和熔滴之间的能量分配做出分析,假设热源和电流密度符合高斯分布模式,考虑了外界与工件之间的蒸发、辐射和对流热损失,以及马兰戈尼效应、电磁力、熔滴冲击力、电磁力、浮力和等离子流力等对熔池对流的影响。然后,从模拟的角度分析了DE-MPAW三维焊接中单一熔滴定点动态堆积过程。熔滴由重力,电磁力和电弧等离子流力驱动,携带质量、动量和能量周期性的冲击基板,形成一个液态熔池。计算了主要由熔滴冲击动量,马兰戈尼效应、电磁力等综合作用下的瞬态熔池形状和熔池中复杂的流动。并且分析了旁路电流和冷却时间对熔池温度分布、流动模式和形貌的影响。最后,从理论角度分析了熔池中的各种作用力对流场流动方向、最大流速及熔池形貌的影响。在较全面的考虑了众多因素后,研究D E-M PAW焊熔池的流场,并重点分析熔池中液态金属的流动模式及固化后的焊缝组织。
[Abstract]:Three-dimensional welding is a kind of material augmentation manufacturing, which manufactures metal parts directly from the digital model of parts (such as CAD files). It is especially suitable for producing complex, high-value and customized parts, as well as repairing end-of-life parts. At present, the main challenges of 3D welding include forming defects (such as porosity), low surface finish and uneven distribution of material properties in space. These challenges stem mainly from a lack of understanding of the complex physical processes of fabricating materials, especially pool dynamics, such as molten pool metal flow, heat conduction, evaporation of alloying elements, and solidification. Because of the small area of the molten pool and the large transient change, it is difficult to measure directly by the experimental method. Moreover, current optical and infrared cameras are limited in their observation of molten pool surfaces. Therefore, it is difficult to study the flow, dynamics and macroscopic defects of molten metal in molten pool. On the other hand, the numerical simulation based on the strict solution of mass, momentum and energy conservation equations can provide an important quantitative understanding of complex transmission phenomena in the process of material augmentation manufacturing. First of all, by analyzing the problems encountered in the arc material increasing manufacturing, it is found that the low precision and poor structure and properties of the arc material increasing manufacturing are partly due to the excessive heat input to the workpiece. Therefore, a by-pass coupled microbeam plasma arc welding (Double Electrode Micro Plasma Arc We l d i n) D E-M PAW). Is proposed. The 3D welding platform of D E-M PAW is built. The platform consists of three dimensional motion system, DE-MPAW welding system, signal acquisition system and welding control system. The CCD camera is used to track the droplet transfer in real time, the thermocouple is used to measure the heat input of the base metal and the current and voltage in the welding process are collected in real time. A series of basic research on DE-MPAW welding is carried out. The effects of by-pass current on droplet transfer, heat input of base metal and weld morphology were analyzed. Single pass multilayer stacking test was carried out with optimized process parameters. Secondly, by using FLOW-3D software and finite difference algorithm, the submodels of droplet, heat source and molten pool force are developed, and the 3D transient molten pool model of DE-MPAW welding considering free surface is established. The temperature distribution and flow state of molten pool were calculated, and the heat transfer process and flow mode of molten metal in molten pool were deeply understood, which laid a good foundation for obtaining high performance SUS304 stainless steel DE-MPAW 3D welding forming parts. In this model, the droplet is treated as a spherical droplet with fixed height, constant temperature and initial velocity over the molten pool. The energy distribution between main circuit, bypass and droplet in the model is analyzed. Assuming that the heat source and current density conform to Gao Si distribution mode, the evaporation, radiation and convection heat loss between the outside and the workpiece, as well as Ma Lan Goni effect, are considered. The influence of electromagnetic force, droplet impact force, electromagnetic force, buoyancy force and plasma flow force on the convection of molten pool. Then, the dynamic stacking process of a single fusion titration point in DE-MPAW three-dimensional welding is analyzed from the point of view of simulation. Droplets are driven by gravity, electromagnetic force and arc plasma flow, carrying mass, momentum and energy periodically to form a liquid melting pool. The shape of the transient molten pool and the complex flow in the molten pool under the combined action of droplet impact momentum Ma Lan Goni effect and electromagnetic force are calculated. The effects of bypass current and cooling time on the temperature distribution, flow mode and morphology of the molten pool were analyzed. Finally, the effects of flow direction, maximum velocity and shape of molten pool on the flow direction of various forces in the molten pool are analyzed theoretically. The flow field of the molten pool in D E-M PAW welding is studied, and the flow mode of liquid metal in the molten pool and the weld microstructure after solidification are analyzed.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG44

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