Influence of Cu Content on the Negative Effect of Natural Aging in SiC/Al-Mg-Si-Cu Composites

Most Al-Mg-Si-Cu alloys and their composites are affected by natural aging processes. When natural aging precedes artificial aging, it impairs the hardening during artificial aging. This study investigates the influence of Cu content on the negative effects of natural aging in silicon carbide (SiC) (17% volume fraction) reinforced Al-1.2Mg-0.6Si-xCu (x = 0, 0.2, 0.6, 1.0, and 1.2, mass fraction, %) composites. The samples were investigated by hardness analysis, DSC, and TEM. For comparison, Al-1.2Mg-0.6Si-xCu alloys were examined by the same methods. The difference in hardness (Delta H) between samples in the direct artificial aging state and those that were naturally aged for 14 d before artificial aging were compared with Cu-containing and Cu-free samples. The values of Delta H were lower in the Cucontaining samples, indicating that Cu mitigated the negative effects of natural aging. However, the values of Delta H fluctuated as the Cu content increased. DSC and TEM results revealed the addition of Cu promoted the precipitation of beta '' phases (the primary strengthening phases in Al-Mg-Si alloys) and the formation of stable L phases during artificial aging. This morphological behavior explained why Cu inhibited the negative effects of natural aging. On the downside, Cu aggravated the formation of clusters during natural aging, which resisted precipitation and negatively affected the hardening during artificial aging. The contrasting beneficial and adverse influence on the effects of natural aging caused the fluctuations in Delta H. The mitigating effect of Cu differed between the 17%SiC/Al-1.2Mg-0.6Si-xCu composites and Al-1.2Mg-0.6Si-xCu alloys. A small amount of Cu (0.2%, mass fraction) significantly reduced the Delta H of the composite, but the Al alloy with 0.2%Cu failed to elicit this effect. This result can be explained by two observations. First, the DSC results showed that in the Al-1.2Mg-0.6Si-0.2Cu alloys, Cu significantly aggravated the clustering of solute atoms during natural aging, whereas in the 17%SiC/Al-1.2Mg-0.6Si-0.2Cu composites, formation of clusters was low because the vacancies were annihilated by interfaces and dislocations. Second, the TEM results revealed the presence of L phases in the 17%SiC/Al-1.2Mg-0.6Si-0.2Cu composites, which were absent in the Al-1.2Mg-0.6Si-0.2Cu alloys.