Experimental Study On Fabrication Of Semisolid Billet Of AlSi30 Alloy Based On New SIMA Method

AlSi30 alloy belongs to typical hypereutectic Aluminum-Silicon alloys, which is well-known for its lightness, outstanding wear resistance, low thermal expansion and good casting characteristics. It has received broad applications, such as electronic packaging materials, wear resistant automobile parts and etc. However, due to the disadvantages of conventional ingot metallurgy (IM) and powder metallurgy (PM) forming, these refer to relatively coarse and brittle primary Si particles with shape angle in conventional casting of AlSi30 alloys and poor formability of AlSi30 alloy powder in the solid state in conventional PM forming, and not achieving net-shape forming, these greatly confine its further application.The conventional strain induced melt activating (SIMA) method includes several typical technical processing as follows: hot extrusion processing, cold deformation and subsequent isothermal heat treatment processing, while new SIMA is to fabricate semisolid billet by using equal channel angular pressing (ECAP) method as a substitute for thermal and cold plastic deformation of the conventional SIMA method. The main object of this dissertation is to fabricate semisolid billet of AlSi30 and conduct some experimental investigations based on new SIMA method.In this work, firstly, influences of ECAP temperature and ECAP route on microstructures and properties of AlSi30 powder preforms were investigated. The results show that primary Si particles are broken, rounding and the hardness of preforms rise greatly with some deformation energy accumulation, which create a favorable strain inducement condition for subsequent isothermal treatment in semisolid state. The densification and deformation energy accumulation mechanism -s were studied during AlSi30 alloy powder in tubes-ECAP processing. The results show that the preform densification and its deformation accumulation during ECAP mainly have something to do with the interaction between inner composited phases of the individual powder particles and between powder particles.Secondly, influences of ECAP temperature, ECAP route, heat holding temperature and heat holding time on semisolid microstructure evolution were also analyzed through orthogonal test. The results indicate that the main factor for grain refinement and spheroidization of primary silicon is the holding temperature; the secondary factor for grain refinement is holding time while the temperature of ECAP for grain spheroidization; the best technical parameters for experiment are ECAP temperature 500℃, ECAP route A, heat holding temperature 605℃and heat holding time 55min. The semisolid microstructure evolution mechanism of hypereutectic AlSi30 alloy was investigated. The results indicate that semisolid microstructure evolution of hypereutectic AlSi30 alloy is related to two courses. One is precipitation, amalgamation, growing and coarsening of Si grain in low melting point matrix. The other is Si, which is around the primary Si particles belonging to low melting point matrix, segregation to the boundary of primary Si particles resulting in primary Si coarsening. In one word, the combination of the two courses realizes the semisolid microstructure evolution of hypereutectic AlSi30 alloy.In conclusion, the semisolid billet of AlSi30 alloy was obtained from its powder through new SIMA method. On one hand, this method can resolve the problem of relatively coarse and brittle primary Si particles with shape angle in conventional casting of AlSi30 alloys. Meanwhile, it can also overcome the flaw of poor formability of AlSi30 alloy powder in the solid state in conventional PM forming. On the other hand, it makes net-shape forming of AlSi30 alloy become feasible, with high theoretical research value and application prospect.