随着薄壁零件在航天航空、汽车制造等领域的广泛应用，薄壁零件的加工制造工艺成为当前研究热点。但传统加工方法工艺复杂、制造周期长、材料利用率较低，故以堆焊为基础的薄壁零件快速成形技术迅速兴起。但薄壁特征导致熔池“下淌”较为突出，严重影响其成形精度，为解决上述问题，论文结合快速成形原理，采用微束等离子焊接系统，提出利用外加电磁场辅助焊接过程以提高薄壁零件堆焊成形精度的控制策略。 论文以304不锈钢零件的堆焊为研究对象，应用COMSOL有限元分析软件建立数学模型，并对其熔池形态及流场特性进行研究。模型使用等效比热容法考虑相变潜热对温度场分布的影响，通过在动量方程中添加体积力体现熔池受力情况。仿真结果表明：薄壁特征导致熔池区域等温线为倒梯形，非熔池区域等温线为水平分布；表面张力是影响熔池流场特性的主要因素，在熔池中心温度高，表面张力较小，熔池边缘温度低，表面张力较大，故表面张力梯度驱使流体沿径向从中心向边缘流动，并在边缘处向下运动，将更多热量带入熔池底部，形成向两侧“下淌”的现象。 为探讨影响表面张力变化的关键因素，建立在氩气保护氛围下包含熔滴过渡、熔池自由表面变形及凝固在内的固、液、气三相统一模型。模型采用改进的水平集法捕捉气-液界面自由表面运动变化，通过对流体速度的散度求解，保证气液相体积的守恒。为处理气相区和液相区材料属性急剧变化引起的不收敛问题，引入Heaviside函数对密度、粘度及热导率等进行平滑处理。采用焓孔隙率法描述固-液界面熔化及凝固过程，将固化过程并入了体积力方程中，解决了在气液两相流模型中处理第三相的相转变问题。并对表面张力系数、表面张力温度系数以及接触角进行了详细的分析，确定了各参数对于熔池成形的影响规律。 为调控表面张力对熔池“下淌”产生的影响，提出利用非接触力-电涡流力提高堆焊成形精度的方法。该方法将高频正弦交流电通入励磁线圈产生感应磁场，同时此磁场在熔池表面感生电涡流，电涡流受到磁场的作用会产生指向熔池中心的电涡流力，达到抑制熔池向两侧“下淌”的目的。为了得到励磁线圈最佳结构参数和电参数，基于麦克斯韦方程给出高频交变电磁场中熔池表面的涡流密度、磁通密度及电场强度的解析方程，并通过建立高频励磁装置的数学模型，求解出工件表面上磁通密度、涡流密度和电涡流力的分布规律。随后建立了堆焊过程中温度场-流场-外加电磁场多场耦合数学模型，该模型揭示了外加电磁场对熔池流体流动特性的影响规律，通过抵消表面张力的作用，改变了熔池流体漩涡中心位置，使熔池内流体流速分布更加均匀，从而实现对熔池成形的精确控制。 搭建了微束等离子堆焊精确成形控制平台并进行了多层堆焊试验。通过对比施加电磁场前后堆焊层形貌的变化，对外加磁通密度与熔宽变化量、余高变化量进行回归分析并建立相应的回归方程，其熔宽变化量满足三次方程，余高变化量满足幂函数方程，从而为外加电涡流力控制成形技术提供理论依据。 关键词：薄壁零件；堆焊；熔池；表面张力；电涡流力

 With the wide application of thin-walled parts in aerospace, automobile manufacturing and other fields, the processing and manufacturing technology of thin-walled parts have become a current research focus. As the traditional processing methods are concerned, because of its complex process, long manufacturing cycle and low material utilization, the rapid prototyping technology of thin-walled parts based on surfacing is rapidly developing. Because of the thin-walled feature, the "downflow" of the welding pool is more prominent which seriously affects its forming accuracy. In order to solve the above problems, this paper proposes a control strategy which assist welding process to improve the forming accuracy of thin-walled parts by using external electromagnetic field, based on the rapid prototyping principle and micro-beam plasma welding system. In this paper, the surface of 304 stainless steel part is taken as the research object, and the mathematical model is established by COMSOL finite element analysis software. The shape of welding pool and flow field characteristics are studied. Equivalent specific heat capacity method is used to consider the effect of latent heat of phase change on temperature distribution, and the force of welding pool is reflected by adding volume force to momentum equation. The simulation results show that the thin-walled feature leads to the inverted trapezoidal isotherm in the welding pool area and the horizontal isotherm in the non-welding pool area. The surface tension is the most important factor affecting the flow field characteristics of the welding pool. The temperature in the center of the welding pool is high and the surface tension is small. The temperature at the edge of the welding pool is low and the surface tension is large. The surface tension gradient drives the fluid to flow radially from the center to the edge, and moves downward at the edge. The fluid brings more heat to the bottom of the welding pool and forms the "downflow" to both sides. In order to discuss the key factors which affect the change of surface tension, a solid-liquid-gas three-phase unified model was established. The model includes droplet transfer, free surface deformation and solidification of welding pool under argon atmosphere. The model captures the free surface motion of the gas-liquid interface by using an improved level set method to ensure the volume of the gas-liquid phase, which is conserved by solving the divergence of the fluid velocity.To deal with the problem of non-convergence caused by sharp changes with material properties in gas-liquid phases, Heaviside function is introduced to smooth the density, viscosity and thermal conductivity of materials. The enthalpy porosity method is used to describe the melting and solidification process at the solid-liquid interface. The solidification process is incorporated into the volume force equation to solve the phase transition problem of the third phase in the gas-liquid two-phase flow model. The surface tension coefficient, surface tension temperature coefficient and contact angle are analyzed in detail, and the influence laws of each parameter are determined on the welding pool forming . In order to regulate the influence of surface tension on the "downflow" of welding pool, a method of improving the forming accuracy of surfacing is proposed by using the eddy current force. In this method, high frequency sinusoidal alternating current is fed into the excitation coil to generate an inductive magnetic field, and the magnetic field induces eddy current on the surface of the welding pool at the same time. The eddy current generate an eddy current force directed to the center of the welding pool under the action of a magnetic field, so as to restrain the "downflow" of the welding pool to both sides. In order to obtain the optimal excitation coil parameters of structure and electric, analytical equations for eddy current density, magnetic flux density and electric field intensity on the surface of welding pool is given based on Maxwell's equation in high frequency alternating electromagnetic field. The mathematical model of high frequency excitation device is established, the distribution laws of magnetic flux density, eddy current density and eddy current force on the surface of workpiece are solved. A mathematical model of temperature field-flow field-external electromagnetic field is established, which reveals the influence of external electromagnetic field on the flow characteristics of welding pool. By counteracting the effect of surface tension, the center position of welding pool fluid vortices is changed, so that the distribution of fluid velocity in welding pool is more uniform and the precise control of welding pool formation is realized. The precise forming control platform of micro-beam plasma surfacing is built and the experiment of multi-layer surfacing has been carried out. By comparing the morphology changes of surfacing welding layer before applying electromagnetic field and that after, the regression analysis is made between the flux density and the variation of melting width, the variation of reinforcement. The corresponding regression equation is established. The variation of melting width satisfies the cubic equation and the variation of reinforcement satisfies the power function equation. Thus a theoretical basis is provided for the forming technology controlled by external eddy current force. Keywords: Thin-walled parts; surfacing; Welding pool; Surface tension; Eddy current force