Effects of natural aging on precipitation behavior and hardening ability of peak artificially aged SiCp/Al-Mg-Si composites

Abstract

Silicon carbide particles reinforced 6xxxAl (SiCp/6xxxAl) composites have high strength but low elongation after artificial aging (AA), leading to poor formability. To obtain components with precise dimensions, complex shapes and great load-bearing abilities, it is demanded for SiCp/6xxxAl composites to be cold plastically formed during natural aging (NA) and served after AA. However, the process is limited due to the unclear effects of NA on AA hardening. In this study, the influence of NA on precipitation behaviors and hardening abilities during AA of a SiCp/Al-Mg-Si composite and unreinforced Al-Mg-Si alloy was investigated using hardness, tension, differential scanning calorimetry, electron probe and transmission electron microscopy tests. It is shown that in both the composite and Al-Mg-Si alloy, clusters formed during NA inhibit the formation of β'' phases and promote the formation of coarse β′ phases, so NA exhibits negative effects on AA hardening. However, what different from the Al-Mg-Si alloy is that the quenching dislocations in the composite promote the formation of precipitates and thus alleviate the inhibition of NA on precipitation. In addition, the composite has a lower proportion of coarse β′ phases due to the depletion of Mg by chemical reactions at or near SiCp-Al interfaces. Consequently, the negative NA effect in the composite is much weaker than that in the Al-Mg-Si alloy. This study clarifies how SiCp affect the negative NA effect on AA hardening and is expected to deepen the understanding of aging hardening behaviors of SiCp/Al composites.