Study Of The Solidification Structure Evolution Of Aluminum Alloys Inoculated By Al-Ti-B(C)

Aluminum alloys have been widely used in the field of aerospace,automobiles,construction industry and daily life since the abundant reserves,low density,high strength-to-weight ratio,good corrosion resistance,excellent machinability and so on.The quality of ingots has an important effect on the final structures and performance of the components,which is dependent on the solidification microstructure evolution of the ingots,A fine,equiaxed microstructure can ensure the uniformity of composition,good mechanical processing performance and also greatly reduce the tendency of hot cracking of castings,shrinkage,porosity and other casting defects.At present,chemical inoculation by Al-Ti-B(C)has been widely used in the industrial practices to refine the grains of aluminum alloys.It is of great scientific and practical significance to study the mechanism and the control method of solidification structure of aluminum alloy inoculated by Al-Ti-B(C).The solidification microstructure evolution of alloys is very complex,so the mathematical modeling and the numerical simulation have been carried out for the necessity.Among them,the Cellular Automaton(CA)method is a powerful tool to simulate the solidification process of aluminum alloys due to its high computational efficiency and clear physical meanings.However,until now,the nucleation calculations in the CA method not only rely on the experimental data,but also neglect the dynamic behavior of TiB2/TiC particles in Al melt.Therefore,it cannot describe a real solidification process of aluminum alloys inoculated by Al-Ti-B(C).In this paper,the influence factors and the control methods of the microstructure evolution of aluminum alloys inoculated by Al-Ti-B(C)will be investigated experimentally and numerically.The detailed contents are as follows:A model has been developed by coupling the Population Dynamics approach and the CA method to simulate the solidification microstructure evolution of aluminum alloys inoculated by Al-Ti-B(C)refiner.The model takes into account the nucleation,diffusional growth/dissolving and Ostwald ripening of TiB2/TiC particles in Al melt;the heterogeneous nucleation and spherical growth/dissolving behaviors of α-Al at the early stage of solidification as well as the dendritic growth of α-Al.The dynamic behavior of TiB2/TiC particles and α-Al are calculated by using the Population Dynamics Approach.The dendritic growth process of α-Al is calculated by the CA method.The comparisons between the simulated results and the experimental ones verify the rationality of the proposed model.Experiments and simulations have been carried out for the Al-Cu alloys inoculated by Al-Ti-C master alloys.The effects of the additive amount of refiner,the time of holding temperature and the cooling rate of the melt on the solidification microstructure evolution of inoculated Al-Cu alloys have been investigated.The results demonstrate that the kinetic behaviors of TiC particles in the melt can change the number density and size distribution of TiC particles at the moment that the nucleation of α-Al starts.The heterogeneous nucleation of α-Al can be divided into two stages,Ⅰ):In the early stage of nucleation,there are enough effective inoculant particles in the melt.The heterogeneous nucleation process is dominated by the cooling rate of the melt;II):After a short time,the nucleation of α-Al is dominated by the number density of the effective inoculant particles in the melt until the nucleation process stops.The empirical formula has been put forward for predicting the grains size of aluminum alloys inoculated by Al-Ti-C refiners.Continuous solidification experiments and simulations have been carried out with the Al-Zn-Mg-Cu alloys inoculated by Al-Ti-B master alloys.The effects of the additive amount of refiner,the cooling rate of the melt and the size distribution of inoculant particles on the microstructure evolution of inoculated Al-Zn-Mg-Cu alloys have been investigated.It demonstrates that some nulcei will dissolve during the continuous solidification.The proportion of dissolved nuclei to the total nuclei increases with the increase of the additive amount of refiner and the cooling rate of the melt.When the additive amount of refiner and the cooling rate of melt reach a certain critical value,the proportion of dissolved nuclei tends to keep a constant(～1/3).The grains size of the alloys decreases with the increase of the additive amount of refiner and the cooling rate of melt.After the additive amount of refiner and the cooling rate of the melt reach a certain critical value,the grains size changes slightly.The efficiency of grains refinement depends on the additive amount of refiner,the cooling rate of the melt and the size distribution of TiB2 particles in Al-5Ti-1B refiner.An empirical formula has been proposed for predicting the grains size of aluminum alloys inoculated by Al-Ti-B refiner.