Abstract
In this work, we analyze the natural aging sensitivity of the Al–Mg–Si(–Cu )(6000) alloy family. We revisit old literature and show that the different aging behavior commonly observed in lean and dense alloys can be explained by the well-known theories of heterogeneous and homogeneous nucleation, here referred to as seeding and self-nucleation. We show that all alloys of the 6000 series can get their strength giving precipitates during two-stage aging from seeds which act as potent nucleation sites during the final annealing. However, in dense alloys, due to the increased supersaturation, the precipitates can self-nucleate freely within the matrix during one-stage heat treatments if no seeds are present. The transition from self-nucleation to seeding causes the drop in precipitate number density and thereby strength in the dense alloys and is commonly referred to “the negative effect of natural aging.” However, this drop is only transient. By creating enough seeds prior to the final aging stage, the negative effect can be overcome. We demonstrate that this can be done by keeping the alloy at a temperature > 40◦C prior to the actual artificial aging stage.