Fundamental Research On Refinement Of Solidification Structure Of Metals Under Pulse Magneto Oscillation

In this paper,a new developed technique,pulse magneto oscillation(PMO),was introduced.The effects of PMO on the solidification structures and behavior of metals were systematically studied by employing thermal analysis and directional solidification techniques. The experimental devices were combined with the self-developed facilities including high voltage pulse power source,solidification tester and EMV directional solidification device etc.As a result, the regularity of structure evolution under the influence of PMO was obtained,and the mechanism of structure refinement was discussed.Based on the analysis of solidification process of pure metals by classical electrodynamics and acoustics,the model about the molten metals and mechanism of solidification structure refinement under PMO was presented.Generally,Lorentz force f=J×B can be generated due to the combined effects of pulsed magnetic field and the induced current.This force orients from the surface to the inside of the specimen,causing an intense oscillation on the surface of melt on account of the "skin effect" of current.Meanwhile,a pulsed force from surface to inside was produced on the nucleus owing to the extremely high difference between conductance of the solid and liquid of the metal and thereby an increase of the induced current appeared.As a result,the free nucleus fell from the surface were forced to move inside.When the original small nucleus moved deeper enough out of the area of the "skin effect" area,they moved slowly and got together to form the nucleus clusters with many holes.As the size of these nucleus clusters exceeds exceeded some critical size which is was bigger enough to reflect the oscillation wave,nucleus clusters could be forced to move into the melt by the press of the wave.When the nucleus clusters reaches reached inside of the melt,it might be melt into some smaller nucleus because of around higher temperature of the melt around it and therefore the number of the nucleus in the melt was increased and equiaxed crystals were formed.The PMO was applied at three different stages during the cooling process of aluminum melt, i.e.incubation period,solidification stage and whole course from melt to total solid.The results showed that optimum refinement was achieved by second and third processes,with a change from coarse columnar crystals to fine equiaxed ones.In the experiment with stainless steel net in the melt,the solidification structure refinement appeared between the stainless steel net and the surface,but inside the stainless steel net the solidification structure was coarse columnar crystals. It was deemed that the reason of solidification structure refinement was the increasing of nucleus inside the melt which fell off from the mould wall into the surface near the place treated by PMO, i.e.crystal rain effect was produced by the oscillation.Based on another systemic experimental result,the quantificational relation between the average grain size and the PMO parameters was established.It was found that the press produced by oscillation wave resulted from the surface oscillation of the PMO promoted the nucleus inside.In different shield condition experiments treated by the PMO,the results showed that the shield effect was less with the smaller conductance of mould and greater with the bigger conductance of model than the melted metal, but the solidification structure could be refined if the intensity of the PMO was increased.It was shown that the solidification structure was obviously refined and the macro-segregation was improved by PMO treatment.The study about the effect of PMO on theβphase of the solidification structure of Sn-10.4%Sb indicated that the size of theβphase in the cast ingot diminished.Clearly,the more intensity of the PMO,the more obviously diminished effect was achieved.It is reasonable to come to the conclusion that the PMO can break the dendrite and multiply the nucleus,and therefore largely refine solidification structure.In normal solidification condition,PMO can obviously refine the macrostructure of ferrite stainless steel.When the treatment frequency and intensity of PMO was increased the size of equiaxed grains was correspondingly decreased and the equiaxed grain area was increased.In an optimal intensity range,the proportion of the equiaxed grains can reach as high as 100%.The macrostructure became coarse again when the intensity of pulse magneto oscillation was reasonably increased because the Joule heart produced by pulse magneto oscillation was too much to solidify.Besides,the directional solidification under the effect of PMO was carried out.As the increase of magnetic intensity,the amount of grains was increased and the space between the first dendrite arm spacing of columnar grain was decreased at a given temperature gradient and crystal growing rate.The results showed that the PMO could accelerate the transformation of the liquid-solid interface in the mode of plane interface(?)cell interface(?)cell/dendrite interface.It was deemed that this transformation was resulted from the effect of cut force of PMO on the columnar grains.