Study On Grain Refinement Process Of High Temperature Ni-based Alloy Welds And Numerical Simulation

Nickel based superalloys are widely used in the key structural materials of nuclear steam generator,pressure vessel and heat dissipation evaporator because of their excellent high temperature strength,corrosion resistance,thermal stability and low temperature properties.However,in the process of welding,it is found that Ni-based superalloy and its matched metal are very easy to form a kind of intergranular crack under solidstate,called ductility-dip cracking(DDC),especially in high constraint narrow gap pipe weld manufacturing is more prone to appear.The existence of DDC directly affects the life and operation safety of nuclear power plants.The studing results show that the minimum critical strain and DDC resistance could be significantly improved of high temperature nickel base alloy by grain refinement.Based on the above,in this paper,the Inconel690 alloy with FM-52 M as filling material are welded by pulsed tungsten inert gas welding(P-TIG).After the adjustment and optimization of the key welding parameters,such as peak current,basic current,pulse frequency and duty ratio,P-TIG is more easier to produce equiaxed grains with less orientation than TIG in the surfacing layer,which reduces the appearance of coarse columnar crystals.So,the P-TIG welding method is used to weld high temperature nickel base alloy.The peak current,pulse frequency and duty ratio in P-TIG welding parameters can increase the plasma flow force of the arc to the molten pool.The larger pulse arc pressure is produced at the peak time,the increase of the dendrite nucleation rate and the grain refinement of the weld,and the increase of the grain size of the weld.Large current value can increase the heat input of the weld pool,and makes the weld grain has a coarsening tendency.In addition,the technical welding test was carried out with the auxiliary longitudinal magnetic field assisted TIG welding method,and the weld pool was stirred by the strong electromagnetic oscillation produced by the applied magnetic field to refine the grain of the weld.Secondly,based on the magnetohydrodynamic(MHD)theory,a two-dimensional axisymmetric TIG arc model is established.The arc behavior in the process of TIG welding is simulated using the fluid computing software FLUENT,and the mechanism of refining weld grain by the P-TIG welding method is explained.The TIG arc under constant current steady combustion presents the shape of bell,and the heating radius and arc temperature increase with the increasing of current density.The arc pressure distribution is characterized by small arc column area and large cathode and anode.On the basis of the two-dimensional axisymmetric TIG arc model,the arc model was extended to three dimensions.Therefore,a mathematical physical model of the interaction of three dimensional arc,molten pool and tungsten pole is established for TIG welding.The coupling effect of arc behavior on the molten pool is studied systematically,and the temperature field and flow field are analyzed.The results show that the arc(cathode)of the fixed-point TIG welding is a typical bell cover shape,the temperature gradient is very large,the highest temperature appears at the bottom of the tungsten pole,which is in accordance with the two-dimensional axisymmetric simulation results.While,the bath(anode)isotherm is slow and the temperature gradient is small,and the molten pool shows a shallow and wide shape.The simulation results agree well with the experimental results.In addition,a three-dimensional coupling arc and weld pool model is applied to TIG welding assisted longitudinal magnetic field.The effects of the external longitudinal magnetic field on the behavior of the TIG arc and the flow state of the molten pool are studied by simulation.The results of numerical analysis show that the arc shape of TIG welding with the added longitudinal magnetic field is changed from TIG's bell shape to hollow bell shape,and the highest arc temperature is slightly increased.And arc plasma near cathode is compressed,which explains the arc temperature is slightly higher here,and the plasma body divergence near anode shows the temperature drop of the arc here.Low temperature cavity cause the heat flux density of the anode surface to change from the Gauss distribution without magnetic field to the bimodal distribution.Compared with TIG welding,the peak pressure of TIG welding with the auxiliary longitudinal magnetic field drops,and the maximum arc pressure is no longer in the center of the arc,but the central axis of the arc.The negative pressure zone appears near the central anode of the center axis of the arc.The effect of external longitudinal magnetic field on the pool of TIG welding is increased by increasing the flow velocity and decreasing the penetration depth.Compared with TIG welding,the flow velocity of the weld pool in the circumferential direction is increased by the effect of the longitudinal magnetic field,and the energy is homogenized rapidly on the circumference,which shows the weld pool morphology with a slight increase in the melting width and a decrease in the depth of the melt.As a result,the reheat zone of the next weld pool will be reduced and the tendency of hot cracking will be reduced during the multi-pass welding process of high temperature Ni-based alloy.It is the increasing of the flow velocity and the decreasing of the peak temperature that results in the rapid homogenization of the heat of the arc afferent pool during the TIG welding of the nickel based alloy,which makes the weld overheating tendency decrease,and the strong stirring of the molten pool can also play the effect of refining the grain of the weld with the broken dendrite.Therefore,the application of longitudinal magnetic field assisted TIG welding to high temperature nickel based alloy welding could effectively improve the sensitivity of welding cracks.