Influence Of Superposing Electricity And Magnetic Field On The Solidification Structure Of Metals

It is well known that the mechanical properties of a metal or metallic alloy, such as strength and toughness, are highly dependent on its grain size. Electromagnetic vibration has proven to be effective in controlling columnar dendritic structure, reducing the size of equiaxed grains, and under some conditions, producing globular non-dendritic grains. However, most of the studies are generally quantitative, limited to a small range of vibration conditions on the solidification, in addition, most of the magnetic fields used in vibration are AC magnetic fields or pulsed magnetic fields. With the development of superconductor technology, the application of strong static magnetic fields becomes more and more wide. It is found that the strong magnetic field has multiple effects on the solidification of metal, and it is became more complicated by simultaneously imposing an AC current and strong static magnetic field. However, the reports about the effects of complex fields, besides vibration on the material processing were seldom available. Therefore, it has great significance to research the influence of superposing electricity and magnetic field on the solidification structure of metals.In this paper, effects of complex field, including electromagnetic vibration, MHD and magnetization effects on the solidification structures of pure Al and alloys have been investigated experimentally. The results show that their macrostructures of pure aluminum are remarkably refined by simultaneously imposing an AC current and a high static magnetic field. At certain a magnetic field, the extent of grain refinement increased with the increasement of alternating currents. At the AC current of 10A, the refinement firstly increased with the increasement of magnetic fields and reached the optimum at a 6T magnetic field, however, as magnetic fields further increased, the refining effect diminished.With simultaneously imposing a high magnetic field and AC current during solidification of Al-6wt%Si alloy, the size of the grains was remarkably decreased and the microstructure evolved from dendrites to rosette-shaped grains. The size of grains was increased with the increase of current density when the intensity of magnetic fields was below 4T. Nevertheless, the size of grains was unchanged when the intensity of high magnetic field was above 4T. At certain a current density, the size of grains was decreased with the increase of the intensity of magnetic fields. Moreover, with the increase of electromagnetic vibration intensity, not only the morphology of primaryα-Al phase transformed from dendrite to equiaxed crystal, but also the eutectic Si phase was modified from a mainly lamellar morphology to a mainly fully divorced needle.For Al-12.6wt%Si eutectic alloy, it was found that the eutectic structure had been refined by only imposing a magnetic field while it coarsened under the electromagnetic vibration. Furthermore, divorced eutectic structure formed when the electromagnetic vibration was strong enough.For Al-18wt%Si alloy, the primary silicon particles were refined by solely imposing a magnetic field. However, when simultaneously imposing a magnetic field and AC current, many primary single silicon particles were congregated under lower vibration intensity. However, at higher vibration intensity, the primary silicon particles dispersed, and migrated at a critical size. Finally homogenous refining primary silicon particles formed at high vibration intensity. Furthermore, thermal analysis experiments were performed. The results show that the nucleation temperature have been elevated which indicated a smaller undercooling, and the steady growth time of primary silicon increases under electromagnetic vibration. Experimental results also show that the primary silicon refinement may be attributed to the change of growth pattern from continue growth to laminar growth under electromagnetic vibration.Based on the above experiments, a new process was proposed by simultaeous imposition of AC current and high magnetic field at the top of specimen. Effects of complex field on the distribution of refinement grains have been investigated. At certain a magnetic field, the size of Al grains decreased and the depth of refinement region increased with the increase of alternating currents. At certain an AC current, the refinement region firstly increased, reached an optimum and then decreased with the increase of magnetic fields. The analysis indicates that the nuclei dissociate from the top surface and migrate under gravity is the dominant factor for grains refinement.Owing to the difference in susceptibility between the liquid and the solid of pure aluminum ( 0 <χl <χs), an upward effective magnetic force is applied on the solid aluminum, when the specimen are laid in a positive gradient magnetic field, the magnetic force may restrain crystal shower and improve the distribution of the refining grains on the specimen. Dynamics of grains migrating in molten metal under high gradient magnetic field has been studied theoretically. The equation of above model was solved, and the theoretical migrating velocity is expressed asIn order to further study the effects of electromagnetic complex field on the characteristic of the structure, the EBSD analysis technology is adopted. The results show that the fraction of low-angle grain boundaries (LAGB) is increasing with the increase of vibration intensity, this modification is benefical to the corrosion resistance. Moreover, the Al-4.5%Cu alloy was chosen to investigate the influence of electromagnetic complex field on the crystal orientation. X-ray diffraction indicated that the refining grains were oriented with <111> towards the magnetic field direction. The orientation degree was increased with the decreasing of grain size. The relation between orientation and grain size was analyzed from crystal orientation and migration theory.Finally, A new method (AC+EMCC) was supposed by applying alternating current (50Hz) directly to the melt during electromagnetic continuous casting (EMCC) process. Results show that the application of AC current during EMCC is capable of improving the the surface quality and physical properties of copper billets significantly. Moreover, the application of electromagnetic complex field has also been explored and attempted to welding. A new Tig welding process with tailoring magnetic field was supposed according to magnetism len technique. The result indicates that the introducing of proper magnetic field not only can control the shape and rotational frequency of arc shape, but also can refine grains and improve mechanical properties of welding joint remarkably.