Directional solidification studies, simulating billet casting, have been made off alloys with balanced and not balanced Mg and Si additions, and with different alloying levels ranging from 6063 to 6082. The effects of growth rate, grain refinement, tempera-ture gradient and composition on struc-ture formation have been investigated. Increased alloy content strongly influence the solidification structure, but during cooling the coarsening process decrease the differences, and relatively similar structures are obtained at room temperature. Segregation to the surface zone is proposed to have an effect on surface defect formation.