Continuous cooling precipitation diagrams of Al–Mg–Si alloys

The temperature- and time-dependent precipitation behaviour of Al-based 6060, 6063, 6005A and 6082 alloys at different cooling rates after solution annealing has been investigated. The continuous cooling precipitation diagrams of these alloys were recorded by differential scanning calorimetry. The cooling rate was varied over five orders of magnitude (0.05–20,000 K/min). Cooling-rate-dependent precipitate formation was analysed by light microscopy, scanning and transmission electron microscopy. Cooling-rate-dependent hardness was tested after artificial ageing. Over an appropriate range of cooling rates all alloys show similar precipitation behaviour. At least two precipitation reactions were observed in different temperature ranges. The high-temperature reactions correspond to the precipitation of the equilibrium phase Mg2Si, and the low-temperature reactions correspond to the precipitation of precursor phases such as β′ and B′. The precipitation kinetics depends on the alloy composition. Maximum hardness values are to find as long as the materials were cooled faster than alloy specific critical cooling rate, which increases with increasing alloy content.