the hardness of the extrusion after ageing is shown in Fig. 6.3(a) for three Al—Mg—Si alloys [5]. The controlled high-temperature soak and suitable cooling after homogenization give a structure which offers optimum surface quality, extrusion speed and mechanical properties (Fig. 6.3(b)). Two important factors which affect extrudability are Mg and Si in solid solution and coarse Mg2Si phases in the structure. The extrudability is reduced by increasing amounts of Mg and Si in solid solution. either by increasing alloy con- tent or by increasing cooling rate after homogenization. Equally, when coarse