The Study Of Aluminium Silicon Alloy Solidification Process And Its Simulation Under Modification

Aluminum silicon alloy is one of the important commercial alloys, and its castings are widely used for different purposes, especially for castings with complicated shapes, ranging from thin wall components and high static loads parts used for aviation industry to cylinder blocks and cylinder head. In order to get better casting products, it has warranted a more thorough understanding of the solidification process of aluminum silicon alloys. Based on the statistical analysis of the experimental data, the undercooling prediction models and the micro structure quantitative models of the modified aluminum silicon alloys are developed. The solidification process and micro structure evolution are studied by numerical simulation so as to optimize processing technique, improve casting quality and shorten design cycles.The nucleation undercooling is one of the problems paid more attention by people in the solidification process. With the study of the solidification characteristics of aluminum silicon alloys, which are modified and solidified in permanent mold, the undercooling prediction models during different solidification stages have been developed and these models are related to cooling rate. To investigate this question, the aluminum silicon alloys with silicon content from7wt%to17wt%were selected for experiment. The different cooling rates are realized by a step casting mold whose steps are in different thickness. The temperature of the center of each step was measured and recorded by a data acquisition system. The relationship between nucleation undercooling and cooling rate during the melting solidification has been analyzed with the help of computer aided cooling curve analysis technique, then the prediction models have been achieved.Because of the fact that the alloy performance prediction was based on the microstructure, the rules of the secondary dendrite arm spacings of hypoeutectic and eutectic aluminum silicon alloys modified by Al-Sr master alloy were studied by means of cooling rate data and metallographic quantitative analysis techniques. Meanwhile, the nucleation numbers of primary silicon phase of hypereutectic aluminum silicon alloys modified by Al-P master alloy were measured and the area density and body density of primary silicon phase with different cooling rates have been determined.The deterministic models for the nucleation and growth of eutectic cells have been founded for the eutectic solidification stage of aluminum silicon alloys. For the purposes of both computation efficiency and microstructure evolution during solidification process, the macro method was used for the primary phase stages and the macro method coupled with the micro deterministic method was used for the eutectic phase stage in this study. The parameters for the nucleation models and the growth models were determined by the experimental data and validated by numerical simulation.Latent heat has an impact on the simulation accuracy of the thermal field. A latent heat computation model of aluminum silicon alloy has been created based on the silicon content in the alloys. Two different methods were used for studying the latent heat release rules of aluminum silicon alloys. One was Computer Aided-Cooling Curve Analysis method (CA-CCA). The results of the CA-CCA experiment showed that this method gave users a latent heat value with larger relative error, which could not be used for numerical simulation. However, this method provided information about fraction of solid and so on that can help us to better understand the solidification process. The other method is Differential Scanning Calorimeter method (DSC). The high purity aluminum silicon alloys were first used for DSC experiment. It is found that there is a linear relationship for the latent heat of hypoeutectic or hypereutectic alloy and a mathematical formula has been obtained. The experimental castings were also carried out the DSC experiment for each step center. The values of them are smaller than the ones of high purity alloys but show regular variations so that it could also be estimated by the latent heat formula mentioned above.In order to shorten the computation time of the macro-micro coupled method, the macro method for the primary phase and the macro-micro coupled method for the eutectic phase were used for computation based on the undercooling prediction models developed in this study. Then the Scheil equation was used to deal with the fraction of solid of the primary phase solidification and the micro models of nucleation and growth developed in this study were employed for the eutectic phase solidification. The program was developed on Windows XP and the programming language was C++. The simulation results for the eutectic alloys have been verified by good consistence of calculated results with experimental ones. For the alloys with primary phase, there is a difference on the cooling rate variations but the solidification time also shows a certain degree of accuracy. With the SDAS models the values of SDAS could also be good predicted. This method neglected the problems such as micro structure patterns and improved the computation efficiency. The good prediction results could also be applied for industry application. According to the comparison of the results, the disadvantage of the models was also be analyzed and some of the future research directions have been pointed out.