The Microstructure And Properties Of Al-50%Si Alloy Formed Under Gradient Pressure

Al-50%Si alloy will be widely used in the field of electronic packaging materials for its properties, such as low density, high thermal conductivity and low coefficient of thermal expansion. It is cheap and easy to prepare Al-50%Si alloy by casting. But at the same time the alloy product of this method is coarse in microstructure and low density. This paper studies the microstructure refinement and densification of Al-50%Si alloy. Gradual squeeze casting method is introduced, combining with adding P and mechanical stirring alloy powder which is added in alloy melt, to refine the crystalline silicon and then densify the alloy. The microstructure of the alloy refined and densified is analyzed by the means of OM, TEM, BSE, XRD and then the mechanical and physical properties of the alloy is tested.In this paper, we used two different size formed punch to squeeze the semi-solid alloy. Adopt a gradual pressure, and ultimately the upper punch reach 190 MPa, the lower punch reaches 315 MPa so that we can achieve extrusion forming under gradient pressure. Analysis showed that the result of densification is very satisfactory. Tin cannot react with aluminum and silicon and the melting point is very low, so it can solidify in the hole of alloy which will further improve the density of alloy.We research the effect of addition of P in refining the primary crystal silicon. The results showed that: when adding 0.6%P, the refinement effect of primary silicon is the best. The primary silicon can be refined to about 100μm due to the addition of P.Isotype alloy powder has a good wettability with the alloy melt. By the method of mechanical stirring, the alloy powder uniformly mixed with the melt so that it can refine the primary silicon. By the composite technology of adding 0.6%P+3% alloy powder, the primary silicon can be refined to 90μm.We research the thermal diffusion treatment on eutectic silicon morphology, studies show that the fibrous eutectic silicon dissolve and turn into spherical particles when insulation 3h at the temperature 540℃. Tensile strength after heat treatment of alloy specimens is changed from 96.82 MPa to 102.96 MPa. The type of fracture is a typical brittle fracture before the heat treatment without obvious dimple and fracture dimple appear in the alloy after heat treatment.By measuring the thermal expansion coefficient of the alloy formed under different conditions show density having no impact on it. In the range of 25 ~ 225℃,the average of thermal expansion coefficient of formed under gradient pressure is 12.318×10-6/K.The thermal conductivity of the alloy decreases with increasing temperature and involve with interfacial thermal resistance and density. In the range of 25 ~ 230℃,the thermal conductivity of the alloy formed under gradient pressure is located in 116.42~140.35 W/(m·K), and the thermal conductivity of the alloy formed under a single punch is located in 105.97~127.23 W/(m·K). The airtightness of the alloy which is formed under gradient pressure is 4.7×10-9 Pa·m3/s.