The Study Of Hypereutectic Al-Si Alloys By Controlled Diffusion Solidification And Squeeze Forming

With the develop of lightweight vehicles, as a crucial part, piston is set higher demands upon the property. There are two important factors of effecting the property of piston: materials and forming method. Hypereutectic Al-Si alloy is optimal material of piston because of high hardness, lower density, lower linear expansibility. However, during the solidification process of hyper-eutectic Al-Si alloys, because of their large solidification range, high silicon fraction, high latent, primary silicon easily grow to large size. The large size of primary silicon heavily separate Al substrate, so it increases the alloy brittleness, decreases the mechanical property and cutting property of the parts, as well as, there are many defects with sand casting and metal mould casting, which limit its widely used. So it is a important issue to decrease the average size of primary Si and improve the solidification microstructure, reduce the quantity of defects.Using the Controlled Diffusion Solidification (CDS) technology, primary Si of hypereutectic Al-Si was studied, and optimum technology parameters of CDS was obtained. On this basis, it is investigated to the effect of squeeze technology parameters on microstructure and property of casts by squeeze. The results show that:(1) Controlled Diffusion Solidification can refine the size of primary silicon and improve the morphology and distribution of primary silicon for hypereutectic Al-Si alloy, and enhance mechanical property.(2) Adopting an appropriate precursor alloys mixing condition, which is liquid Al-25%Si alloy with800℃and pure Al with580℃, then pouring at640℃, the distribution of primary Si phase is uniformly and the average size is11.68μm in the solidification microstructure.(3) By means of temperature record at CDS process, the CDS process can be divide into three stages:mixing stage, temperature balance stage, uniform cooling stage.(4) The shape of casts is more complete by CDS squeeze forming than by conventional forming method, and the microstructure of casts is uniform, the property of casts is more better.(5) It can be obtained a perfect piston by CDS squeeze forming when the pressurized start time is60seconds, and extrusion pressure is81MPa, pressure holding time is12seconds.