Influence of Natural Aging Time and Mg/Si Ratio (Mg + Si = 1.3 mass%) on the Two-Step Aging Behavior in Al–Mg–Si Alloys

Two-step aging with the different natural aging times and chemical compositions was carried out in Al–Mg–Si alloys to investigate the clustering and two-step aging behavior. The activation energies of cluster formation through the Kissinger method were analyzed as 69.4, 67.3 and 73.8 kJ/mol for the 9M4S, 6M6S and 3M10S, respectively. A similar hardness was identified for short-term natural aging for 3.6 ks regardless of the Mg/Si ratio. On the other hand, a lower hardness was identified in the 3M10S (Mg/Si = 0.3) than in 9M4S (Mg/Si = 2.5) and 6M6S (Mg/Si = 1) for long-term natural aging for 4838.4 ks. The hardness increase during the two-step aging was due to the formation of a β″ phase during artificial aging based on structure analysis of high resolution transmission electron microscope images. There was a hardness decrease at the initial stage of the two-step aging at 170 °C in the case of long-term naturally-aged specimens. The hardness decrease was caused by the partial dissolution of the Mg–Si co-cluster, which was influenced by the Mg/Si ratio. Meanwhile, the hardness increase during two-step aging was promoted for 6M6S and the higher number density of the β″ phase was confirmed.