Study On The Si Phase, Modification And Properties Of Al-25%Si Alloy

Hypereutectic aluminum-silicon alloy is an important casting alloys, and widely used in such industrial fields as aerospace engineering and automotive manufacture. However, the casting of hypereutectic Al-Si alloy always present relatively coarser and brittle primary silicon particles with shape angle which degrade significantly mechanical properties, and limit the further application. In order to expand the application range of hypereutectic Al-Si alloy, the fundamental research on Al-25%Si alloy was studied in this article.In order to lucubrate the microstructure characteristic of Al-25%Si alloy, different etching and extracting methods were used to study the morphology of the octahedral primary silicon, five pyramid prismatic-shaped primary silicon and five petal star-shaped primary silicon by means of the optical microscope, SEM, X-ray. Growth process of octahedral primary silicon, five pyramid prismatic-shaped primary silicon and five petal star-shaped primary silicon was revealed. The growth traces of octahedral primary Si and five pyramid prismatic-shaped primary Si existed layer by layer, it indicates that the growth step sources required for Si phase were supplied by sub-steps, which created on {111} growth surface of silicon crystal by stacking faults, not by the helix dislocation. The interface morphology of primary Si was five pyramids polyhedron pit-shaped, which likes the nucleus coagulated by octahedrons. Therefore the sub-step theory can explain the growth mechanism of five pyramid prismatic-shaped primary Si. Five pyramid prismatic-shaped primary Si is the initial shape of five petal star-shaped primary Si. Impurities effected the growth process of five petal star-shaped primary Si and flat-plate primary Si. The growth of flat-plate primary Si follows not only the twin plane reentrant edge(TPRE) mechanism, but also layer mechanism as well.The effect of P,RE and Sr modification is investigated on the microstructure of primary silicon and eutectic silicon, the modification mechanism and strengthening mechanism of Al-25%Si alloy are discussed. Depending on the single and dual modification of P, RE, Sr elements on primary and eutectic silicon, the investigator find an efficient P-RE-Sr triplex modifier. Modified by the triplex modifier, the coarse primary silicon of Al-25%Si alloy turns to be obtuse, and its average diameter decreases sharply from 150~200μm to 20~25μm. In order to research the different modification on Al-25%Si alloy, some modifications are prepared in the present study. The better modification parameters is: modification temperature(Sr-750℃,P-840℃,RE-840℃), adding order(Sr,P,RE), the modification adding quality(Sr-0.06%,P-0.12%,RE-0.8) and modification times(Sr-30min,P-10min,RE-10min).The article researches the connection among ingredients, structure and property of Al-25%Si alloy through normal experiment measure, maximal dispersion and curvilinear regression analysis. The results reveal that influents of Cu, Mg, Mn elements onσb,300℃,u andαhave markedly difference. According to the analysis, ingredients of Al-25%Si alloy are established: 24~26%Si, 2.2%~2.5%Cu, 0.3%~1.0%Mg, 0.2%~0.3%Mn, 0.35%Ti, 0.2%Zn.The effect of particle size and morphology of Si phase in Al-25%Si alloy with or without modified on high temperature tensile properties, wear resistance and thermal expansion are studied in the paper. The properties test shows that, the plasticity and strength of hypereutectic Al-25%Si alloys are improved obviously with the refinement and spheroidization of primary silicon, while thermal expansivity of materials has limited relationship with the morphology of primary silicon partieles.Wear resistance test shows that the mass loss of all materials tested inereases with the inerease of load while mass loss deereases with the inerease of load; the wear resistance can be improved by the refinement and spheroidization of Si partieles.The result shows that the diverdified properties of Al-25%Si alloy with compound modified and composition optimizated can achieve:σb-230.33MPa,σb,300℃-142.64MPa,δ300℃-1.073%, HBS-158.17, u-1.063,α-18.32×10-6/℃.