Thixoforming : Semi-solid Metal Processing

and are plausible on the left. Furthermore, it can be recognized that the coated tools
allow a simpler interpretation of the temperature because of the uniform emission
coefficient of more than 0.75 (see Table 9.2).
9.3.4
Demonstrators and Real Parts
9.3.4.1 Impeller
In consideration of the previously obtained good results of the fundamental experiments
on the flow and mould filling behaviour of semi-solid steel casting on a highpressure
die-casting machine, a more complex part with practical application was the
next step of the investigation.
Alongside the realization of a complex part in semi-solid steel casting, the
experiments delivered first results on the use of rapid prototyping steel tools. The
die tools were produced by direct metal laser sintering (DMLS) in cooperation with
the Fraunhofer Institut f€ur Produktionstechnik in Aachen and EOS GmbH [37, 38].
A schematic drawing of the part (diameter 51.5 mm, volume 7525 mm 3 and wall
thicknesses between 1.2 and 6 mm) is shown in Figure 9.24. The completed die tool
of the moving plate is built of the laser-sintered geometry layer and a welded steel back
plate to reduce the production time (Figure 9.24b). The mould, displayed in
Figure 9.24c, has a triple layout with a centred ingot and can be heated to 270 C
by oil tempering.
In the run up to the experiments, numerical simulation of the mould filling and
solidification and the temperature distribution inside the dies were performed with
the software MAGMAsoft. Figure 9.25 shows the development of the temperature of
the semi-solid steel X210Cr12 during the filling of the impeller cavity. The initial
temperature of the billet was 1290 C, which approximates a solid fraction of about
70%. Due to the huge temperature difference between the cast metal and the mould
material, rapid cooling occurs at the thin sections of the impeller vanes. Figure 9.25b
displays the metal velocity during the mould filling process at an initial piston
speed of 0.3 m s 1 . Due to the reduction in wall thickness at the ingate, the velocity
increases to nearly 45 m s 1 . The validation of the simulation was done by step-shot
Figure 9.24 (a) Schematic drawing of the impeller geometry [38];
(b) rapid prototyping tool insert [39]; (c) complete moving plate
with centred ingot and triple tool set.
9.3 Thixocasting of Steel Alloysj335