Effects of Sn/In additions on natural and artificial ageing of Al–Mg–Si alloys

Abstract

The effect of Sn or In addition on natural ageing and subsequent artificial ageing of Al–Mg–Si alloys was investigated. In addition not only retarded the natural ageing but also improved the T6 hardness. Although Sn addition retarded the natural ageing as well, precipitation kinetics was delayed. In addition, slightly refined the β′′ phase distribution. But β′′ phase was coarse with addition of Sn. During artificial ageing, the vacancies released from binding complexes promote the formation of β′′. The detrimental effects of Sn on artificial ageing could be attributed to the reduction of the amount of Sn solutes and of available Mg solutes forming β′′.

Effect of a Trace Addition of Sn on the Aging Behavior of Al–Mg–Si Alloy with a Different Mg/Si Ratio

“…It is well known that the formation of atom clusters is involved in the initial stage of the artificial aging of Al-Mg-Si alloys before large amount β″ precipitates are formed [19][20][21][22]. But the evolution of these clusters differs: (I) remain stable, dissolved and them nucleation of β″ precipitates and (II) nucleation or transformation of β″ precipitates from these clusters [8][9][10] according to the chemical composition and size of the clusters.…”

“…The decrease of the activation energy of the β" precipitates can be mainly attributed to Sn that affects the precipitation sequence of the alloy, and the schematic illustrations, as shown in Figure 8. Sn captures vacancies and prevents the formation of atomic clusters in the matrix during the initial stage of artificial aging [18][19][20]. This reason caused that the precipitation model of the β" precipitates have changed, it directly precipitates the β" precipitates by a supersaturated solid solution.…”

“…In recent years, several investigations [19][20][21][22][23][24][25] have revealed that the trace addition of Sn in this Al-Mg-Si alloy can reduce the negative effect of natural aging. At present, it is generally accepted that Sn can reduce the number density of atomic clusters formed in the alloys during the natural aging process [19][20][21]25], and thereby suppress the natural aging effect of the alloy. Tu et al found that the effect of Sn increased along with the amount added [21].…”

“…However, there are varying viewpoints on the effect of Sn on the artificial aging of the Al-Mg-Si alloy. Shishido et al and Cheng et al found that Sn can not only prevent the negative effect of natural aging, but also adversely affects subsequent artificial aging at 170 • C [19,20]. However, Liu et al and Xiang et al found that the precipitation sequence of alloy with high Mg content aged at 250 • C could be changed by adding Sn, and therefore the age-hardening ability of this alloy can be significantly improved [23,24].…”