The effect of homogenization practice on the microstructure of AA6063 billets

It is well established that homogenized billets extrude easier and faster and give better surface finish and higher tensile properties than as-cast billets. Hence, the production of AlMgSi extrusions from DC-cast billets almost always starts with a homogenization cycle which typically consists of a soaking treatment followed by cooling at a predetermined rate. While soaking is performed to produce a homogeneous solid solution and to transform the β-AlFeSi particles to the more acceptable α variety, the motivation behind controlled cooling is heterogenization. The cooling cycle has to be designed to reprecipitate as much Mg2Si as possible, in a form and size easily redissolvable during subsequent processing for high extrudability and surface quality. The effect of various soaking and cooling cycles on the formation and transformation of insoluble and soluble constituents was investigated in order to identify the optimum homogenization practice for a semicontinuous DC-cast AA6063 billet. A homogenization practice with a 6 h soak at 580 °C followed by step-cooling at 250–300 °C was found to be optimum. Step-cooling gave a more complete depletion of the aluminum solid solution, i.e. lower flow stress, without forming coarse and stable Mg2Si particles which are very difficult to solutionize during reheating and thus survive the extrusion process impairing both surface quality and mechanical properties.