The effect of homogenization practice on the microstructure of AA6063 alloys

Homogenization is an essential step in the preparation of aluminum billets for extrusion and is performed to produce a homogeneous solid solution and to transform the beta-AlFeSi particles to the finer and more equiaxed and thus more acceptable a variety. Homogenized billets require lower extrusion pressures and give extrusions with better surface finish and higher strength than as-cast billets. The extrudability of the billet is maximized once the solutionizing of the Mg2Si particles is optimized and the beta-->alpha transformation is complete. The latter takes higher temperatures and longer times and often requires help through revision of the alloy chemistry, i.e. addition of some Mn. The cooling practice in a homogenization treatment is just as significant as soaking and a variety of microstructures ranging from a fully solutionized homogeneous matrix to a heterogeneous one with a coarse dispersion of Mg2Si particles can be produced by simply adjusting the cooling rate. The optimum rate is that which gives Mg2Si precipitation readily redissolvable during subsequent processing and is dictated by the preheating, practice. The present work was undertaken to identify the optimum homogenization soaking and cooling practice for a semicontinous DC-cast AA6063 billet which, due to its composition (very low Mn), relies solely on the soaking practice for the beta-->alpha transformation.