Bake hardening of twin roll cast At-Mg-Si sheet

The bake hardening response of a twin roll cast Al-Mg-Si (aluminium alloy AA6016) sheet, submitted to pre-aging treatments at 180&DE;; C shortly after solutionising, has been investigated by means of tensile testing, microhardness and electrical conductivity measurement, differential scanning calorimetry and transmission electron microscopy. During pre-aging at 180&DE;; C, stable Guinier-Preston type 1 (GP1) zones form first and suppress any subsequent natural aging. Clusters and small GP1 zones are reverted, and stable β" nuclei are formed. These nuclei grow readily to become coherent β" particles during the paint bake cycle, giving a considerable increase in strength. A pre-aging treatment for 2-10 min at 180&DE;; C offers a sufficient bake hardening response for the twin roll cast 6016 alloy sheet.