Thixoforming : Semi-solid Metal Processing

Figure 9.18 (a) Meander tool for flow length investigations of
SSM alloys; (b, c) X210CrW12 cold work steel for two different
piston velocities at a given fraction solid of about 75%
(Tf 1290 C); (d) flow lines of cast metal at beginning of
meander, interface healed with eutectic melt; (e) at high piston
velocities no healing at the interface, here at the end of the
meander.
9.3 Thixocasting of Steel Alloysj329
castable part geometries [31]. As illustrated by two meander parts in Figure 9.18, the
different piston velocities have a significant influence on the flow length. While at a
piston velocity of 0.05 m s 1 (metal velocity 3.4 m s 1 ) a total flow length of about
320 mm can be achieved, the flow length increases to more than 1.3 m at a piston
velocity of 0.5 m s 1 . The flow tip shows typical laminar characteristics. Detailed
investigations on the evolution of the microstructure depending on the flow length
have shown a very similar development of the solid phase to that already observed in
the described step specimen. The formation of a distinctive boundary layer caused by
a thin fluid film at the tool surface due to the high cooling rate is observed in the
meander experiments where the skin forms a flow channel. While the boundary layer
is healed with eutectic phase in most of the experiments, at very high piston velocities
at the flow tip of the meander healing can no longer be achieved.