Rheological behavior, microstructure and hardness of A356 aluminum alloy in semisolid state using backward extrusion process

The influence of temperature on the flow behavior and rheological characteristics of an A356 alloy in the semi-solid state was investigated using backward extrusion process. Experiments were performed at 5 temperatures and 4 different wall thicknesses. Viscosities were determined using the force-displacement graphs obtained form back extrusion tests. As observed experimentally, at a constant temperature, the increase of shear rate results in the decrease of alloy viscosity exponentially. Raising the temperature increases the liquid fraction hence reduces the semi-solid alloy viscosity. Metallographic and image analyses show that, because of low forming speed, liquid has time to escape from solid phase forward the sample wall. This condition is the main reason for the segregation phenomenon seen in the base and walls. Vickers hardness test on samples reveals that the hardness increases with the decrease of temperature and wall thickness.