Study On Process And Formation Mechanism Of Microstructure Of Al-20%Si Alloy Prepared By CDS Casting

Hypereutectic Al-Si alloy is an ideal material of piston because of its several advantages, such as its low density, good thermal stability, low coefficient of linear expansion, good wear resistance and other excellent properties. However, during the forming process of hypereutectic Al-Si alloys, primary silicon particles easily grows to large size because of their large solidification range, and high latent. The large size of primary silicon would limit hypereutectic Al-Si alloy widely used because of its heavily separate the matrix, thus decrease the mechanical property and cutting property of the parts. Many scholars at home and abroad have done large researches on how to decrease the size, improve the distribution and morphology of primary silicon. Up to now, there are mainly two ways to refine the primary silicon:Chemical modification and dynamic processing method. As to modification, it mainly refers to add P and RE elements to hypereutectic Al-Si alloy melt. The way of dynamic processing method includes rapid solidification, melt temperature treatment and semi-solid casting. In the present study, preparations of Al-20%Si alloy have been studied by Controlled Diffusion Solidification (CDS) associated with other processes. Pure Al and Al-30%Si alloy were used as raw materials. Combined with cooling channel, effects of mixing speeds of Al-30%Si alloy and angles of cooling channel on the microstructure (size, distribution and morphology) of primary silicon have been studied. The results show that:(1) Prepared Al-20%Si alloy by Controlled Diffusion Solidification (liquid-liquid mixing) associated with other processes can refine the size, improve the distribution and morphology of primary silicon, and improve the mechanical properties;(2) Different mixing speeds have different performances on microstructure and mechanical properties of target alloy. In this study, the lower mixing speed can better refine the primary silicon than the higher mixing speed, meanwhile, the morphology and distribution of primary silicon also be better improved.(3) During the mixing process, the nucleation of primary Si occurred in the pockets that formed from the breakdown of Al-30%Si alloy. The smaller mass mixing rate was, the greater tendency of breakdown of Al-30%Si alloy was, and which can better refine the primary Si and improve its morphology and distribution in the matrix.(4) The crystal nucleus mainly comes from three ways:high melting-point particle, cooling channel chilling nucleation and the free crystal promoted by cooling channel during the application of cooling channel after CDS. The application of cooling channel after CDS can cause the forced convection of melt, make the temperature and solute distribution field more even and enhance the solid-liquid interface stability.(5) Different pouring temperatures result in different microstructures during the process of application of cooling channel after CDS. And with the decrease of the pouring temperature, the average size of primary Si is less; however, a small number of primary Si occurred.