Influence Of A Gradient High Magnetic Field On Solidification Of Al-Si Hypereutectic Alloys

A gradient magnetic field produces magnetizing force which can drive grains to move in the field. The solidified structures of alloys will be changed greatly by the force, such as grains' migration et al. Obvioursly, studying this effect of the gradient field on solidification may provide a new way for fabrications of in-situ composite and functional gradient materials. Furthermore, it may produce some new concepts and methods for removing inclusions in melt. So it is significant to investigate deeply the effects of a high magnetic field on the migration, refinement and distribution of the precipitated particles during solidification of alloys. In this thesis, commen-used aluminum silicon alloy was choosen as a studying object, by which the solidification behaviours of alloys have been investigated under a gradient magnetic field.In the first part of the thesis, the migration of the primary silicon grains during solidification of Al-18wt%Si alloy from semisolid zone has been investigated experimentally under a gradient high magnetic field, and a theoretical model responsible for the migration of a single grain in molten metal was proposed. The results showed that the migration of the primary silicon grains did not take place until the magnetic flux density was over a certain value when the flux density was kept at a certain value. The primary silicon grains tended to migrate and form a remarkable silicon-rich layer with the increment of the magnetic flux density. But the effect of the magnetic flux density on the migration of primary silicon decreased with further increases of the flux when it surpassed a certain value, indicating that the influence of the flux on the migration intended to get saturation. The migratory degree decreased with the increment of the flux density in the case of maintaining the same magnetization force, demonstrating that a resistance force was generated by the magnetic field when the primary silicon grains migrated. The amount of segregated primary silicon grains increased with increase of the magnetic gradient, but the grain size decreased when the magnetic flux density kept same. Theoretical analysis indicates that the effective viscosity of the melt increases linearly with the increment of the square of the magnetic flux density, its expression is asη' =η+ kB_z², the experimental results can be explained preferably via this model.In the second part, dynamics of precipitated grains migrating in molten metal under high gradient magnetic field has been studied theoretically. The equation above model was solved, and the theoretical expressions about the velocity and distance of migratory grains were derived, and they were expressed asthe migratory distance and the mobility ratio were closely related with the distribution of the magnetic flux density. The experimental results of liquid quenching indicated that the grains whose radii were larger than or equal to 40μm were mostly migrated in 120s, in good agreement with the theoretical calculation.In the third part, the effects of a gradient high magnetic field on the morphology and distribution of the primary silicon grains in semisolid Al-18wt%Si alloy have been investigated experimentally. It is shown that in the gradient high magnetic field the primary silicon phase grains, which are large plate-like or five-star-like in the case of solidification without magnetic field, are accumulated on the top of the specimen and refined remarkably with the morphology of equiaxed polygons when the alloy solidifies from the semisolid zone. In the segregated layer of the silicon, the distribution of the silicon grains is homogeneous. The size of the primary silicon grains decreases and the grain number density rises with the increase of the magnetic strength maintaining same magnetization force. It seems that the high magnetic field influences the diffusion of silicon atoms. A theoretical analysis has been proposed to explain the refinement and the distribution of the silicon grains, and expression on the relationships of the diffusion coefficient with the magnetic flux density wasobtained as D_M - D₀ exp . Moreover, it was discoveredthat the silicon particles' distribution in the segregated layer was homogeneous, and their distances were almost the same. A model about the distribution of silicon particles was established, and the relationship of the distance between arbitrary two primary silicon particles with the magnetic field was derived as, agreeing with the experiments. Thisdemonstrated that the orders of magnitude of repulsive and magnetization forces -acted on primary silicon grain were the same, and their interaction resulted in the uniform distribution of the primary silicon.In the fourth part, the effects of a high magnetic field on the primary silicon and the eutectic microstructures in hypereutectic Al-Si alloy solidified from the melt has been investigated. The results show that the primary silicon solidified from the melt migrated more or less under a gradient magnetic field, but did not form a segregated layer of primary silicon due to that the time of alloy's keeping semisolid state was short, this agreed with the theoretical calculation. The primary silicon precipitated on the walls of the sample without magnetic field, but under a stationary magnetic field, the primary silicon distributed on whole section of the sample, and the size of primary silicon became small when the field was set at 7 T. The theoretical analysis of the influence of magnetic field on the primary silicon's nucleation rate demonstrated that there was an maximum value of the rate with the increment of the magnetic flux density, this may qualitatively explain the experimental results. The metallographic analysis indicated that the lamellar spacing decreased with the increment of the magnetic flux density, the cause was the magnetic field retarding the convection and diffusion. The eutectic silicon morphologies of SEM were thick-short flake, whose distribution was sparse in the case of without magnetic field, but imposing a high magnetic field (5～12 T) the morphologies were thin-long flake, whose distribution was compact. This demonstrated that the eutectic silicon was refined by a high magnetic field. The cooling curves have been measured under different magnetic conditions, and the results showed that imposition of a magnetic field did not change the cold rate of the alloys. This demonstrated that the influence of the magnetic field on the heat transfer was ignored. Therefore, the refinements of the primary and eutectic silicon did not result from the cause of cooling speed, and this indicated further that the magnetic field retarding the diffusion, resulted in the refinements of solidified structures. From the measurement of the solidification curves, it can be seen that the eutectic temperatures were lifted more or less by imposition of a stationary or gradient magnetic field. This indicated that the field influenced the thermodynamicas of solidification of the the alloy and the -increasement of the eutectic composition, resulting in the increment of the amount of precipitated eutectic silicon. When the silicon particles congregated but did not touch each other under a gradient magnetic field, they consumed rapidly around solutes, so their growths were hindered. When the congregation of the silicon particles did not occur, the particle can grow sufficiently due to the sufficient solutes around them, and then the bulky silicon particles were formed.