Study On Microstructure And Properties Of Hypereutectic Al-Si Alloy By CDS Casting

Hypereutectic Al-Si alloy is a good performance of casting alloy. It’s an ideal material of piston because of its small density, good thermal stability, low coefficient of linear expansion, good wear resistance and other excellent properties. However, primary silicon easily grows to large size during the forming process because of its large solidification range, high silicon fraction, and high latent. The large size of primary silicon heavily separates substrate, increases its brittleness, and decreases the mechanical property and cutting property of the parts, which limits its wide application. Many scholars both domestic and overseas have done large investigation on how to decrease the size of primary silicon, improve the morphology and distribution of primary silicon. Up to now, there are mainly two ways to refine the primary silicon:Chemical refining method and dynamic processing method. As to modification, it mainly refers to add phosphorus and rare-earth elements to the melt of hypereutectic Al-Si alloy. The way of dynamic processing method includes semi-solid stirring, rapid solidification, and melt temperature treatment. Meanwhile, the studies on properties of hypereutectic Al-Si alloy focused on mechanical properties and wear resistance. Preparation of hyper-eutectic Al-18%Si and Al-20%Si alloy is studied by the Controlled Diffusion Solidification (CDS) in this thesis. ZL102, pure Al and Al-25%Si were used as raw materials. Effects of mixing ways and mixing temperature of Al-25%Si on the microstructure (size, morphology, distribution) of primary silicon and mechanical properties, such as wear resistance, thermal expansion were studied. The refining mechanism of CDS was discussed. And the relationship between the properties and the primary Si phase was also analyzed. The results show that:(1) Controlled Diffusion Solidification can refine the size of primary Si phase, improve the morphology, distribution and raise the performance of the target Al-Si alloy. (2) Different mixing ways result to the different microstructure and performance of two target alloys. Samples, which are from liquid-liquid mixing method, especially liquid pure aluminum mixed with high silicon alloy, has the most even primary phase distribution, rounded morphology and comprehensive performances.(3) In the same mixing way, different mixing temperature of high silicon alloy contributes to the different results of refinement and performance of target alloy. The mixing temperature of high silicon alloy is controlled strictly when another master alloy is fixed. A casting with finer primary Si phase and its uniform distribution can be got when the mixing temperature is suitable. As a result, hypereutectic Ai-Si alloy with excellent performance can be also got.(4) Mixing temperature of high silicon alloy is a key parameter to control the morphology of primary silicon phase during CDS process.(5) There is a certain relationship between the size, shape, distribution of primary silicon phase and the performance of the target alloys.