The microstructure of the 5182 strip cast by the unequal
diameter twin roll caster is shown in Figure 6. The microstructure
of as-cast strip was not uniform and symmetrical at the thickness
direction as shown Figure 6a. The reason of unsymmetrical
microstructure was that the solidification length of the upper and
lower roll was not same. The solidification length of the upper roll
was shorter than that of the lower roll, and the thickness of
solidified layer by the upper roll was thin. The microstructure at
interface between the upper and lower solidification layer was
globular as shown in (2) of Figure 6a. This is the typical
microstructure by the low temperature casting. In this area, Mg
segregated at the grain boundary. The microstructure became coarse
at the upper side of the solidification layer cast by the lower roll as
shown in (3) of Figure 6a. This reason is that the heat transfer
between the lower roll and solidification layer became small when
upper side solidified, and the cooling rate of upper side became low.
The strip shrinks at width direction as the temperature of the
solidification layer becomes low, as the result, the contact condition
between the roll and the solidification layer becomes worse. If the
solidification length is shorter or the roll speed is higher, this coarse
microstructure becomes fine. The microstructure near the lower side
was fine as same as that near the upper side.