Technique And Theory Of Semi-continuous Casting Of Aluminum Alloys Under Low-frequency Electromagnetic Field

This research is a part work of solidification of Al alloy under outer fields, which comes from the project of Improvement of the Quality of Aluminum Products supported by the Major State Fundamental Research Projects of China (973). Through studying on the such factors as sump shape,refinement and formation of non-dendritic structure of ingot,solubility of alloying elements and macroscopic distribution of solute elements,formation and elimination mechanism of hot cracking in the process of semi-continuous casting of 7075 aluminum alloy under low frequency electromagnetic fields or simultaneous application of DC and AC electromagnetic fields. The aim is to provide some new fundamental knowledge of theories and experiments for improving the micro-structure and the mechanical property of aluminum alloy ingot produced by semi-continuous casting. The dependence of meniscus surface shape on the electromagnetic field intensity and frequency have been investigated. The results show that the curvature of the meniscus decreases with the enhancement of intensity and frequency of the applied coil current. The changes of solidified shell height and surface quality of 7075 aluminum alloy ingots produced by CREM process are also measured. Crystal nuclei-dissociating theory is used to explain the formation mechanism of primary solidified shell under the effect of electromagnetic field. It is pointed out that under the effects of electromagnetic field, the increase in crystal nuclei dissociated from the crystallizer wall results in the difficulty of formation of solidified shell, which is the key reason that delays the time and decreases the position of the primary solidified shell. On the basis of mentioned above, The microstructures of 7075 aluminum alloy ingots DC cast under electromagnetic field with low frequency was studied. The formation mechanism of non-dendritic structure can be explained that crystal nuclei are easily dissociated from the crystallizer wall under the effects of electromagnetic field, which increases amount of the grain nuclei in the melt. Furthermore, the movement of free grains together with the melt convection and self-rotation of grains restrain them from growing into dendrite, which results in the formation of fine and uniform spheroid-shaped and rosette-shaped grains after solidification. Based on the analysis of previous works on hot cracking of alloys, a new formation mechanism of hot cracking was proposed. An explanation was made on the mechanism of eliminating hot cracking by application of electromagnetic field with low frequency in the process of continuous casting aluminum alloy. It can be explained that electromagnetic field, on the one hand, reduces the external factors for initiating hot cracking through improving the casting condition to decrease the size of unfeeding zone, reduce the thermal stress and strain and make the brittleness zones in high temperature in the compression state. On the other hand, the electromagnetic field increases the ability of alloy to endure the stress and strain by reducing the size of brittleness zone in high temperature, increasing the extensibility of solid/liquid phase, decreasing the quantities of eutectic of low melting point and the linear contraction of effective solidifying zone. The two aspects mentioned above contribute to decrease the possibility of initiating cracking. Low frequency electromagnetic field significantly increases the solubility of alloying elements within grains of 7075 aluminum alloy ingot. The reasons can be explained as follows: (1) under the effects of electromagnetic field, The difference of increase in liquidus and solidus temperature of 7075 aluminum alloy results in an increase in the non-equilibrium solute distribution coefficient, which decreases the level of micro-segregation and promotes the solution of alloying elements in matrix; (2) Electromagnetic field reduces the non-equilibrium solidification interval of 7075 alloy, which prevents alloying elements from forming coarse second-phase and reduces the quantity of inter-dendritic compounds of low melting point; (3) Electromagnetic field increases the amount of grain nuclei in the melt and uniforms temperature field and concentration field of alloying elements. Grain nuclei grow up in manner of equiaxed grains, which diffuses the distribution of the alloying elements and reduces the degree of micro-segregation; (4) The formation of non-dendritic structure avoids the macro-segregation caused by "dendrite bridge". Low frequency electromagnetic field is effective on decreasing the degree of macro-segregation of alloying elements in 7075 aluminum alloy ingot. An explanation is given as follows: The prevention of grain nuclei from growing into dendrite avoids the macro-segregation caused by formatting "dendrite bridge". Meanwhile, electromagnetic field reduces the forming position of primary solidified shell, makes the sump height become shallow, increases the non-equilibrium solute distribution coefficient, decreases the solidification interval and uniforms the temperature field and concentration field, all of...