Fundamental Study On Solidification Of Al Alloys Under Electromagnetic Fields

This work is a part of the items, solidification of Al alloys under outer fields, in the National Key Fundamental Research Program (973). The variation rule of liquidus and solidus temperature of Al alloy solidified under electromagnetic fields was examined. The effect of electromagnetic fields on the solid solubility and macro-segregation of alloying elements, the microstructures and solidification structures of Al alloy were all studied. And the mechanism of solidification of Al alloy under magnetic fields was discussed. The purpose of this work is, to provide new fundamental knowledge for EPM theories and basis for EPM technology. The following results were obtained: For the first time, the liquidus temperature, solidus temperature and solidification interval of Al, Al-3.01Cu, Al-5.69Cu, Al-11.4Cu, Al-13.3Cu and Al-17.4Cu alloys were determined by testing the change of the resistance of these alloys solidified under a AC or DC magnetic fields. The liquidus temperature and solidus temperature of Al-Cu alloys increased, the solidification interval decreased, the slope of liquidus and solidus decreased and the equilibrium solute distribution coefficient k0 increased by application of an AC magnetic field. Furthermore, these variation were more evident when the magnetic fields became higher. When these alloys solidified under a DC magnetic field, their liquidus and solidus temperature decreased, and the solidification interval, the slope of liquidus and solidus and k0 all became bigger. Besides, the effect of DC or AC magnetic fields on the liquidus and solidus temperature of 7075 Al alloy was studied by the same way. Under a DC magnetic field, the variation tendency of liquidus and solidus temperature of 7075 Al alloy was same as Al-Cu binary alloys. Under an AC magnetic field with different frequencies and densities, it was found that the higher the frequency and the stronger the density, the larger the increment for liquidus and solidus temperature in 7075 Al-alloy. It was put forward that the electromagnetic fields could be regarded as work done by environment, AC magnetic field do positive work while DC magnetic field do negative work to the melt alloys. So the barrier potential was changed, which required by the atom cluster jumping from liquid phase to solid phase at the front of liquid-solid interface. Therefore the work of nucleation was decreased or increased, and finally, the melting point of alloy was raised or reduced. The influence of magnetic field on solidification process in Al-alloy was analyzed by dynamics and thermodynamics point of view. It was found through experiments that solid solubility of solute elements of Al alloys obviously increased during solidification under electromagnetic field. When Al-5.3Zn, Al-2.8Mg or Al-1.2Cu binary alloys solidified under a DC magnetic field, AC magnetic field or pulsed magnetic field, LY12 alloy solidified under a pulsed magnetic field or pulsed current, also 7075 alloy solidified under magnetic fields with different frequencies or densities, the solid solubility of solute elements in α-Al was increased notably. When the frequency or magnetic density was increased, the variety law of solid solubility of the alloying elements in 7075 alloy including Zn, Mg and Cu, was firstly increased and then decreased. It was put forward from microscopic point of view, for the first time, that the actions of electromagnetic fields were the action to the particle with charge in the molten alloy. The relative movement between solute particle and Al matrices was produced by the electromagnetic field, and the diffusion of solute particle in Al was enhanced by this movement, which results an increase in the solute in α-Al. In the present thesis, the effect of electromagnetic field on the orientation of crystal and the lattice constant of Al-alloy was studied first time. It was found that the orientation of planes (111) of grains in Al-5.3Zn, Al-2.8Mg or Al-1.2Cu binary alloys were strengthened with application of DC magnetic field, AC magnetic field or pulsed magnetic field, the orientation of planes (200) of grains of these alloys were strengthened with application of DC current, AC current or pulsed current, and the diffraction peaks (111) and (200), in the diffraction patterns, were both strengthened in LY12 alloy applied with pulsed magnetic field or pulsed current. This phenomenon was got by the directional movement of particle under electromagnetic field. It was also found that lattice constant of α-Al in Al alloys obviously changed with application of electromagnetic fields. The lattice constant of α-Al in Al-5.3Zn and Al-1.2Cudecreased and the one of Al-2.8Mg increased. Besides, the solute elements in LY12 alloy could be made homogeneous under pulsed magnetic field or pulsed current, the negative segregation phenomena of Cu was deleted and the segregation of Mg was decreased. The macro-segregation of the elements Zn, Mg and Cu in 7075 Al alloy was decreased with the application of magnetic field with different frequencies or densities during solidification. When the magnetic density was set at 0.06T, the best frequency was 15Hz or 30Hz, under which the distributing of the alloying elements was most homogeneous and the macro-segregation rate was smallest, on the other hand, when the frequency was set at 0.06T, the best magnetic density was 0.03T or 0.06T. The grain of LY12 alloy was remarkably refined under either pulsed magnetic field or pulsed current with different densities. However, the grain size was not diminished singly with increase in the electromagnetic density. The refinement was most remarkable when the capacitor was 80μF and the discharge voltage was 5kV. The refinement mechanism of solidification structures under pulsed electromagnetic field was considered, i.e. the severe force convection under electromagnetic field promoted the grain dissociate from the mold wall, thus the nucleation number was greatly increased, thereby the grain was refined. The dissociative grains were saved and the priority growth at one direction was restrained because of the relative uniform temperature field, so that the dendritic structure was restrained and nondendritic structure was formed. The solidification structures of LY12 Al-alloy were remarkably changed under pulsed electromagnetic field, the eutectic structure, which accumulated at crystal boundary and dendrite net, was reduced obviously, changed from continuous dendrite net to thin discontinuous structure. The influence of different magnetic fields during solidification on macrostructures in 7075 Al-alloy, Al-6.0Zn, Al-2.2Mg and Al-1.9Cu alloy was studied. It was found that, the structures of alloy couldn't be refined under any magnetic fields, the proper magnetic field conditions could largely reduce the grain size, and however, the improper magnetic field conditions would make the grain bigger than that without magnetic field. With application of magnetic field, on one hand, the convection becomes large and the grains were refined; on the other hand, the undercooling was decreased and the grain was coarsened. The solidification structure was the result of the restriction by these twoactions. At the same time, the macrostructures was also determined by the other two actions, which were nucleation and growth. It was also found that the grain size was tended to enlargen when the frequency or the magnetic density was too big. It is put forward the first time that there is a best density at a fixed frequency, under which the grain size was smallest. It is also pointed out that the higher the frequency , the smaller the magnetic density, under which the grain size was smallest.