A number of factors influence the final strength of the finished extrusion. The strengthening due to both Mg and Si has been well documented. Figure 6.2 [2] gives the response of extrusion produced with condition help constant at extrusion temperature = 540 degree C ; exit velocity = 30m min-1 ; quench rate 4 DC/s ageing = 8 h at 175 C. It is very important to know that strength variations within a given composition are due to two causes; some result from compositional differences and others result from MG2Si transformations conditions. Of these two causes, approximately 25% of the mechanical property variation observed in final extrusion form is due to compositional variations existing within the alloy. The remainder is due to all the other variations in processing conditions.