Microstructure And Defect Of ZL205A Alloy Solidified Under Traveling Magnetic Field

ZL205A is one cast alloy with the higher strength, comprehensive properties and good processability, which is one important structural material in aerospace and automobile industry. However, the poor castability and shrinkage and segregation defects which form easily in large castings are the dominant factors of affecting the large casting quality. So it is important for ZL205 A alloy to develop new casting process or to improve the existing casting process. In this paper, the traveling magnetic field(TMF) was used to eliminate macro segregation and shrinkage porosity since the electromagnetic force induced by TMF can has a significant effect on the solute distribution and shrinkage.The effect of the intensity and frequency of excitation current and the relative position and thickness of castings on the intensity at different sites and the average intensity of electromagnetic force induced by TMF were studied by numerical simulation. The intensity at different sites and the average intensity of electromagnetic force in the alloy melt have a linear relation with the square of the excitation current. The intensity of electromagnetic force increases with increasing intensity of the excitation current. The intensity of electromagnetic forces increases firstly and then decreases with increasing the frequency of the excitation current. The intensity of electromagnetic forces decreases with increasing the relative distance and the thickness of casting.The attenuation coefficient was used to characterize the attenuation rate of electromagnetic force. The higher value indicates the higher decay rate of electromagnetic force. The model on the pressure gradient and rotational flow of alloy melt under TMF was established using the distributed function which is expressed by the attenuation coefficient. The effect of the average electromagnetic force intensity and attenuation coefficient on the distribution of flow rate and the position of zero velocity in the alloy melt for casting with different thickness was analyzed. The flow rate increases linearly with increasing the average intensity of electromagnetic forces in the alloy melt. The flow rate increases first and then decreases with increasing the attenuation coefficient. The position of zero velocity shifts toward to the TMF generator with increasing the casting thickness and the attenuation coefficient.ZL205A alloy is a multicomponent and multiphase alloy. The relative density difference of constitute phases will induce phase aggregation and even segregation defect. Segregation could be occurred in the solidification process under the electromagnetic field due to the conductivity difference of the phases. Segregation is characterized with the non-uniform distribution of solutes and phases. In this paper, the distribution of the solute Cu and high density phase in ZL205 A alloy were studied by the solidification experiments under TMF. The distribution of Cu and high density phase become more uniform with increasing the excitation current intensity and the grain size of casting is also more uniform since high density phase has a refining effect on ZL205 A alloy, which are attributed to the melt flow induced by TMF. With increasing the flow rate, the interphase viscous force increases, and the difference of the phase in gravity and electromagnetic force is counteracted.The effect of TMF on casting density, microstructure, mechanical properties and fracture morphology were analyzed by the solidification experiments of different thickness castings under TMF. It is found that the microporosity in casting decreases with increasing the excitation current intensity. The feeing effect is affected by the direction of TMF and casting thickness. TMF can promote feeding when the direction of TMF is consistent with that of feeding. There is an optimal excitation current intensity for the feeding, and the optimal value decreases with increasing the casting thickness. The rotation flow induced by TMF forms more easily with increasing the casting thickness, which reduce the resistance of feeding by hindering the join of grains or dendrites. However, melt flow rate increases with increasing the excitation current intensity, which results in the decrease of the temperature difference in the casting and the increase of the length of mushy zone. Feeding is hindered due to the length of mushy zone. So there is an optimal feeding effect for castings solidified under TMF.