A systematic comparison of static and dynamic ageing of two Al-Mg-Si alloys

Aluminium alloys containing Mg and Si as the major solutes are strengthened by precipitation of the metastable precursors (beta") of the equilibrium beta(Mg2Si) phase. In this study, both static and dynamic ageing of two Al-Mg-Si alloys-the 6061 (Al-1.34% Mg2Si) and 6069 (Al-2.25% Mg2Si) alloys-were aged at 170degreesC and compared to evaluate the potential for improving the mechanical properties. Dynamic ageing was conducted through the procedure of equal channel angular extrusion (ECAE). It is shown that the ageing time scale is reduced from similar to1000 min for conventional static peak-ageing to similar to10 min by using ECAE-aided dynamic ageing. Compared to the significant strengthening effect in static peak-ageing treatment, a notable further increase in ultimate tensile strength (UTS) is achieved by dynamic ageing: over 40 MPa for the 6061 alloy and 100 MPa for the 6069 alloy. Ductility of dynamically aged alloys is found to be comparable to that of the statically peak-aged samples. Microstructures of both statically and dynamically aged alloys were characterised using transmission electron microscopy; dislocation-assisted precipitation was observed to be the primary precipitate nucleation and growth mechanism during dynamic ageing process. It is concluded that dynamic ageing using ECAE is efficient in executing ageing treatment that results in superior mechanical properties of the Al-Mg-Si alloys. (C) 2004 Elsevier B.V. All rights reserved.