Materials transactions jim vol:40 issue:1 pages:34-41
Artificial ageing of several Al-Mg-Cu alloys for car-body applications has been studied. The Cu:Mg ratio was between 0.14 and 0.29 (in mass) and the ageing temperatures were in the range 60-180 degrees C. Strengthening curves during ageing at 140 to 180 degrees C show several typical stages, namely an initial jump compared to the as-annealed state, followed by a linear increase and finally a peak in strength which appears after considerable ageing time. A two-fold precipitation sequence has been confirmed. On one side S "- and further on S'-phase (precursors of the Al2MgCu phase) precipitate heterogeneously on dislocation loops and helices, on the other hand a homogeneous precipitation sequence in the matrix of the type: Cu/Mg clusters (GPB zones) --> S " --> S' has also been observed. The ageing at 60 degrees C follows the same scheme but at much lower speed, so up to three months only the first precipitation stages (i.e. clusters in the matrix and S " on dislocations) were found. An increased amount of Cu increases the bake hardening response, not only at the peak but over the whole ageing curve. Increasing the amount of Mg (or lowering the Cu:Mg ratio), retards the precipitation sequence which increases the stability against natural ageing. It does not severely decrease the precipitation hardening response since for this practical application the hardening is given by the initial jump. This last appears promptly in the first minutes of ageing at 140-180 degrees C, irrespective of the Mg content. Decreasing the baking temperature from 180 to 140 degrees C is also not affecting the first two hardening stages which means that traditional baking temperatures (160-180 degrees C) can be decreased to 140 degrees C and may be less.