Semi-solid Extrusion Of Spray-deposited Al-27%Si Alloy

High silicon aluminium alloys are novel electronic packaging materials. Al-27%Si alloy was prepared by spray deposition and subsequent semi-solid extrusion in this study. The microstructure of spray deposition Al-27%Si alloy and their evolution at different re-heating temperatures have been studied by optical microscopy (OM) and scanning electron microscopy (SEM). The influence of liquid volume fraction during semi-solid extrude were studied. The results were as follows.1) Spray deposition is an excellent way for preparing Al-27%Si alloy for semi-solid forming. The alloy have fine, homogeneous primary Si particles which uniformly dispersed in Al matrix, it is fit for semi-solid extrusion.2) There are three stages during the re-heating treatment of spray deposition Al-27%Si alloy, Si phases coarsen at the grain boundaries, then the liquid volume fraction increases with the re-melting of a-Al phase and Si phase, and finally achieves the equilibrium of Si phase and liquid phase, the volume fraction of Si phase and liquid phase keeps constant, but the dimension of Si phase increases with the decrease of the number of Si particles. The shapes of Si phases convert from near-spherical to the lamellae during the re-heating treatment. With the increase of the temperature, the time of each transition stages decrease.3) Semi-solid extrusion can eliminate the hole in alloy and its microstructure and properties affected by liquid volume fraction. Extrusion after reheating at600℃for10-12min can eliminate the hole in the alloy and decrease the size of Si particles, extruded bar have good surface quality and homogeneous microstructure after extrusion. When semi-solid extrusion at higher liquid volume fraction there have macroscopic inhomogeneous and liquid separate from semi-solid Al-27%Si alloy, and crack appear in the surface when extrusion at lower liquid volume fraction.4) The best technique for re-heating treatment of spray deposition Al-27%Si alloy is holding12min at600℃, The alloy can achieve the highest properties after semi-solid extrusion at this re-heating treatment:relative density (99.5%), tensile strength (195MPa),elongation (6.7%), coefficient of thermal expansion (16.1×10-6K-1) and thermal conductivity (140.3W·m-1·K-1).