The modification effects of alloying element Sb and heat treatment on Al-15.5wt%Mg2Si alloy were investigated by Olympus microscopy （OM）, scanning electron microscopy and energy disperse spectroscopy （SEM-EDS）, and X-ray diffraction （XRD）. It is found that Sb plays a significant role in shaping primary Mg2Si phase and eutectic Mg2Si phase in Al-15.5wt%MgzSi alloy. The Sb addition of about 1.0wt% makes the resultant alloy show the finest primary Mg2Si phase and the eutectic Mg2Si phase with well distribution. But further increasing the Sb content decreases the amount of primary Mg2Si phase, and some segregated phases appear at regions between the grains. In addition, heat treatment can modify the microstructural feature of Sb-modified Al-15.5wt%Mg2Si alloy in terms of obviously shortening the nodulizing time of primary Mg2Si phase and eutectic Mg2Si phase.
Al-Mg-Si-Cu alloys with and without Zn addition were fabricated by conventional ingot metallurgy method. The microstructures and properties were investigated using optical microscopy （OM）, field emission scanning electron microscopy （FE-SEM）, transmission electron microscopy （TEM）, tensile test, hardness test, and electrical conductivity measurement. It is found that the as-cast A1-Mg-Si-Cu-Zn alloy is composed of coarse dendritic grains, long needle-like ~/5-A1FeSi white intermetallics, and Chinese script-like a-A1FeSi compounds. During high temperature homogenization treatment, only harmful needle-like ^-A1FeSi phase undergoes fragmentation and spheroidizing at its tips, and the destruc- tive needle-like 5-phase does not show any morphological and size changes. Phase transitions from ^-A1FeSi to ^-A1FeSi and from 6-A1FeSi to [3-A1FeSi are also not found. Zn addition improves the aging hardening response during the former aging stage and postpones the peak-aged hardness to a long aging time. In T4 condition, Zn addition does not obviously increase the yield strength and decrease the elongation, but it markedly improves paint-bake hardening response during paint-bake cycle. The addition of 0.5wt% Zn can lead to an increment of 99 MPa in yield strength compared with the value of 69 MPa for the alloy without Zn after paint-bake cycle.
