Study On The Mechanism Of Grain Refinement Under Low Frequency Pulsed Magnetic Field During Solidification

Low frequency pulsed magnetic field (LF-PMF) can effectively refine solidified grain, but there is in dispute with the mechanism of grain refinement due to the experimental verification of high temperature process is difficult. In order to achieve industrialization of LF-PMF, the further research must be developed on grain refinement mechanism of LF-PMF.In this paper, a common pulsed electromagnetic force field calculation model, a transient multi-cycle pulsed magnetic field turbulent flow model in which a Fourier series method is used to implement the boundary condition of magnetic flux density under multiple period of PMF, a model of metal solidification which piecewise function is converted to a continuous function in order to improve the convergence of the nonlinear temperature equation under PMF and small wetting angle solidification nucleus model are developed by mathematical analysis, numerical simulation and experimental method. Using the model established, the electromagnetic force, melt flow and solidification are obtained from comparing the difference between time-harmonic magnetic field(HMF) and LF-PMF. On this basis, the grain refinement model of dendrite broken under LF-PMF is proposed. Research results are summarized as follows:1. The feature of low frequency pulsed electromagnetic force(1) Compared with HMF, the electromagnetic force has two main features of a large transient extremes and intermittent under PMF. Compared with PMF, the application of combined static magnetic field with PMF increases magnitude of electromagnetic force and promotes effect depth of force.(2) It is proved that the velocity of melt does not oscillate under PMF by numerical results. Unlike electromagnetic oscillating proposed by Vives C, the average impulse of pulsed magnetic pressure is 99 times as much as that of pulsed magnetic pull force in each cycle.2. The feature of melt flow under LF-PMF(1) It is found that toroidal vortex pair is the dominating structure within the melt flow, and the velocity field of molten melt is be quasi steady state after multiple period. The mean velocity increases with the magnetic flux density, frequency and pulse applying time under PMF. When t=0.1 s under PMF, the electromagnetic force does not exist intermittent period, and the steady-state velocity magnitude of melt is about 1.4m·s-1, which is the same as under HMF.(2) Compared with HMF, the fluctuation of melt velocity is the main feature under PMF. The average velocity which is 0.47m·s-1 under PMF is much less than that which is about 1.4m-s-1 under HMF, but the relative amplitude of velocity which is about 20% under PMF is much greater than that which is 0.67% under HMF. The feature can cause higher secondary dendrite growth, and increase the chance of the dendrite broken under PMF.3. The feature of metal solidification under LF-PMF(1) The error of our model is less than 10% by verified, compared with pouring hot temperature measurement experiments. The temperature gradient is 0.4K·mm-1 under PMF, which is more than 0.1 K·mm-1 under HMF during initial solidification. However, the average temperature gradient is 0.9K·mm-1 under PMF, which is less than 1.1 K·mm-1 under HMF during whole solidification. Effect time of PMF which is 80s is twice as much as that of HMF which is 40s during solidification.(2) The ratio of Joule heat flux and melt flux, solidification latent flux is respectively 4.63×10-7,2.22×10-3 by quantitative analysis, so effect of Joule heat from PMF on the melt temperature and the solidification process is negligible. With increasing magnetic flux density, the solidification cooling rate G×V value first increases and then decreases, and G×V value is maximum when B=0.1T; with increasing the pulse frequency, the solidification cooling rate G×V value increases and then decreases after the first, and GxV value is maximum when f= 5Hz. The minimum grain size can be got when B=0.1T and f= 5Hz.4. The mechanism model of refining grain under LF-PMF and its industrial design(1)The critical nucleus radius and nucleus under-cooling can effectively reduced by strong PMF. but there is almost no effect on the critical nucleus radius and under-cooling under weak PMF.(2) Without considering the Joule heat condition. the refining grain mechanistic models of weak PMF is established because of added equiaxed grain during dendrite growth process. Grain refinement mechanism of PMF is attributed to transient extremes characteristic of pulsed electromagnetic force which causes increasing the probability of dendrite broken. At the same time the survival of dendritic fragments in melt zone is increased, because the temperature gradient is more uniform and grow rate is increased by the melt motion under PMF. The number of equiaxed grain obtained per unit of time under PMF is(3) Industrial applications of LF-PMF is offered based on dendrite broken mechanism during continuous casting. The required critical magnetic flux density is respectively 0.0845T, 0.122T and 0.182T when the PMF is applied to foot roll segment, secondary cooling section and final solidification of the high-temperature alloy in continuous casting.