| This thesis has studied the effect of pulsed electric current onsolidification microstructures of eutectic Fe70Cr18Ni12 alloy andhypoeutectic ZGMn13Mo2 steel and the characteristics of solidificationmicrostructure of 0Cr18Ni7Mn5Mo2 steel under applied high-energy-densityintermittent direct current. The numerical simulation of solidification processof molten metal treated by pulsed electric current was carried out by theANSYS.A high-frequency and high-energy-density pulsed electric currentgenerator was designed and successfully fabricated. It has goodcharacteristics such as highest charge voltage at 50 kV, discharge peakcurrent at 80 kA, charge time less than 15 s, 4 discharge frequency selectionsof 1 × 10~5Hz, 5 × 10~4Hz, 2 × 10~4Hz and 1 × 10~4Hz. In addition, ahigh-temperature melt facility fitted together with the generator was designedand successfully fabricated, which has the characteristics of highest heatingtemperature at 1700 °C and heating rate higher than 30 °C/min, as well asinert gases protection. All of these were the basic experimental facilities forthis study.The effect of the pulsed current density, frequency and treatmenttemperature on solidification microstructures of eutectic Fe70Cr18Ni12 andhypoeutectic ZGMn13Mo2 steel were systemically studied by applying pulsedcurrent on the high-melt-point Fe-based alloys the pulsed electric currentgenerator. The result shows that the solidification microstructures of eutecticFe70Cr18Ni12 and hypoeutectic ZGMn13Mo2 steel can be remarkablyrefined by pulsed electric current. In addition, the refinement degree of thesolidification microstructure increases with increasing pulsed current densityand frequency. Under this experimental condition, the optimal refinementresults of the two alloys were that the austenitic grain sizes changed from 120μm to 0.6μm and from 240μm to 8μm respectively. The initial dischargetemperature is a very important parameter. If the temperature is overhigh,splash even explode can occur. If the temperature is underlow, it is difficultto obtain the homogeneous solidification microstructure. We found that theoptimal initial discharge temperature is be about the freezing point of treatedsteel.Nanograined solidification microstructure with face-center-cubicstructure was obtained in 0Cr16Ni22Mo2Ti steel processed by applyinghigh-energy-density and intermittent direct current together with pressure.Theoretical calculation indicates that the pulsed electric current couldcause the temperature increase of molten metal. However, a high-frequencypulsed current (1 × 105Hz) induced only several Celsius degrees intemperature increase for ZGMn13Mo2 steel, and less than 1 Celsius degreefor Fe70Cr18Ni12 alloy, which could be ignored. If pulsed current frequencywas very large, the increase of pulsed current density had no obvious effecton the temperature increase of molten metal. Therefore the negative effect oftemperature increase induced by the pulsed current on crystallizationnucleation of the molten metal could be ignored. Accordingly, it is furtherproved that the ultrafine-grained solidification microstructure can be obtainedby processing molten metal with high-frequency and high-energy-densitypulsed current.The hardness and the wear resistance of ZGMn13Mo2 steel subjected topulsed electric current could be markedly increased, and the wear resistanceincreased gradually with the increase of current density. In addition, pulsedelectric current processing can enhance strength and toughness of the eutecticFe70Cr18Ni12 alloy.The results of experiments and ANSYS theoretical calculation revealthat when high-energy-density pulsed current passed through the moltenmetal, strong electromagnetic oscillation and magnetostriction occurredinside it and the liquid metal was repeatedly compressed, and moved to andfro along the direction vertical to that of the current. Simultaneously,different electric field intensities and different magnetic field intensities indifferent positions of the samples caused pressure grads, which led to thedifference of local velocity of flow, and then resulted in a shear deformation.So dendrite broke into equiaxed grains, and solidification microstructures ofmetals were refined and crystal grains in solidification microstructurearranged regularly.
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