SEM and OM are used to analyze microstructure accurately. Results
indicate that the microstructure topography is not changed much
with age treatment and varied magnesium contents. Typical OM
image (Fig. 5a) indicates that microstructure of those alloys consists
of a-Al matrix (light field) and Mg2Si (dark field). However,
the size of a-Al dendrite grains is varied: the coarse and fine grains
which were formed in the shot sleeve and die cavity subsequently.
Grains nucleated once liquid alloy touched the wall of shot sleeve
and caused great undercooling, and then, initial grains were
pushed into die cavity by plunger and grew up. Finally, coarse
grains connected with finer grains formed in die cavity and the
solidification process was completed.