| Hypereutectic Al-Si alloys such as A390 (Al17Si4CuMg) exhibit several specific and interesting properties, such as high wear resistance, high strength and hardness, and a low thermal expansion coefficient. As a result, they are used in heavy wear applications. However, their use has always been restricted by several difficulties such as long solidification time, die wear, segregation of large primary silicon particles, and unfavourable shrinkage behaviour. Semi-solid processing allows hypereutectic Al-Si alloys to be shaped into complex near net shape components, with a microstructure of fine and compact Si particles.;The objective of the present work is the determination the rheological characteristics of A390 alloy in the semi-solid state using a Couette type (Haake RV12) viscometer. The rheological behaviour was investigated for continuous cooling and during isothermal conditions at 565°C and for a range of solid fractions, followed by step change tests. In the continuous cooling test, the alloy is cooled at a rate of -2°C/min and sheared at constant rates of 15, 59, 123, 250 and 507 s-1 in separate experiments. The effect of shear rate and the solid fraction (temperature) on morphological evolution of the silicon crystals are investigated. A numerical calculation of the phase diagram of A390 alloy is also presented using the FACTSAGE software developed by the CRCT at Ecole Polytechnique de Montreal in order to interpret the solidification path and the compounds and phases that are formed during the solidification period. |
