Modulated clustering and precipitation in an Al–Mg–Si alloy AA 6014 via enhanced pre-aging

The effect of an additional short spike aging treatment (SA: 180 to 220 °C, in tens of seconds) between the solution heat treatment and conventional pre-aging (PA: 85 °C, in hours) on the mechanical properties of an AA 6014 alloy after paint baking was evaluated. Tensile tests show that such enhanced pre-aging including SA and PA can significantly improve the paint bake response compared to conventional pre-aging (PA only). To interpret this phenomenon, Positron Annihilation Lifetime Spectroscopy, electrical resistivity measurements, Differential Scanning Calorimetry, Transmission Electron Microscopy, Atom Probe Tomography and MatCalc calculations were applied to identify the microstructural changes in different aging stages. It is found that the excess vacancies available after quenching assist in rapid nucleation of clusters during SA, which grow during subsequent PA and transform easily to β" precipitates during paint baking owing to their similar chemistry. The densely distributed β" precipitates are considered responsible for the improved paint bake strength as they efficiently hinder dislocation movement.