Research On Precise Forming Control Method Of Plasma Surfacing Welding Of Thin-walled Parts

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.