Study On Heat Transfer And Solidification Of Magnesium Alloy Slab By DC Casting Processing

The metallurgical quality and dimension of magnesium slab are the important factors affecting the rolling processing and the quality of plate and sheet in the slab hot-rolling cogging process. High quality and large-scale slab of magnesium alloy is the primary requirement for producing wide plate and wide sheet coil. However due to the low conductivity of heat, volume specific heat, latent heat of solidification, and high hydrogen, the structure inhomogeneity, macrosegregation, cracks and porosity occurred during the solidification and casting process. The situation become more serious in the slab preparation of width charaterstic feature plane-symmetry or in large volume solidification processing, so the fabrication of large-scale slab become the bottle neck factor in the development of magnesium plate wide sheet/coil. Above all, it is significant to develop the new low-cost high quality slab processing. In the present study, the temperature real-time measurement method was used to obtain the thermal evolution of melt during the semicontinuous casting process of AZ80-1Y magnesium slab with the cross-section of 130mm×300mm. The effect of technique parameters such ascasting velocity, first cooling, second cooling and current intensity on the heat transfer behavior of semicontinuous casting of magnesium slab were studied and the solidification behaviors were analyzed through the observation of solidification microstructure. The main conclusions were followed:(1) The microstructur are related with the area and the shape together with its temputer field of the mushy zone.The gradually increasing thickness in the casting direction from the centre to the edge shows the high cooling rate in the centre; The isothermal curves in the mushy region show that the cooling rate in the centre appeared a high value without change in different condition; the cooling rate decreases gradually in the initial solidification areas and was invariant in the end of the solidification. the cooling rate shown obvious increase during the solidification; Microstructures of the cross section are composed of four region, which are coarse equiaxed grain region in the centre, columnar grain region, fine equiaxed grain region and chill crystal on the surface. columnar grain region and fine equiaxed grain region always appear under most situations, but coarse equiaxed grain region and chill crystal on the surface are rarely, or even disappear. The columnar grain growth direction point at the centre with an upward angle.(2) The casting velocity shows the more pronounced effect on the heat transfer and solidification of magnesium DC casting process, especially in the thickness direction. Shallow liquid sump, increasing curvature radius was observed when the casting velocity increase from 80mm/min tol20mm/min, and more remarkable near the centre.The thinner mushy region and more uniform temperature field in it was appeared with the casting velocity increasing. High casting velocity can lead to internal stress decreasing, holistic cooling rate of melt improving. The difference of cooling rates between sections, solidification stroke, the columnar grains range and difference of microstructure all decrease with the increasing of casting velocity; The impact by increasing the secondary cooling water flow rate is similar to increasing the casting velocity and without significant difference between direction of thickness and width.(3) The addition of electromagnetic field significantly reduced the thickness in casting direction and improved the uniformity of temperature field of mushy region. The overall cooling rate and its homogeneity in melt were improved, with the shallow liquid sump. The difference of microstructure was reduced as well as the fine grain, but the effect on columnar grain zone was closely related to casting velocity; the influence of casting velocity did not vary by the addition of electromagnetic field, but, with the addition of electromagnetic field and high casting velocity, non-oriented columnar grain zone was appeared in the width direction; the decrease of secondary cooling water flow caused the thin thickness of mushy region and temperature homogeneity along the thickness direction under electromagnetic field; the electromagnetic intensity had no significant effect on the heat transfer, however, Magneto forced convection distinctly refined the microstructure of the slab.(4) The melt temperature rise during the LFEC process with the mold that was pasted by aluminosilicate fiber paper to reduce the first cooling and the effect of EM field enhance. Under the above condition, the mushy region become thin as well as the cooling rate at the beginning of solidification increase significantly. The columnar grain disappeared, fine grain and little difference was shown in the width direction. When the aluminosilicate fiber paper was only pasted on the narrow side, the cooling rate increase only near the narrow side but move down at the position away from the narrow side where the coarse columnar grains appeared, while it’s detrimental to the solidification structure.