Thixoforming : Semi-solid Metal Processing

332j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.21 Plate sample mould with use of tool materials and schematic division.
in 16 installation spaces according to the order shown in Figure 9.21. Using this
order, the different materials were tested at the same time, with four particularly
promising candidates having the metal stream over one edge. As parent metals the
steels 1.2343 and 1.2999 and the molybdenum-based alloy TZM were used. Steel
HSS 6-3-2, the temperature-resistant copper-based alloy CuCoNiBe (2.1285) and
silicon nitride ceramic, soldered to a metal socket and as a bulk insert were
investigated in detail. With the steel X210CrW12 at 1290 C, clear conclusions show
the suitability already after 10 shots; TZM pieces were broken off by brittle failure and
blocked the ejectors. The soldered armour plating made of silicon nitride was broken
repeatedly and the uncoated steel was damaged by abrasion in the area of the
overflows and redirections.
For the optimization of the coatings and comparison with the model tests
performed at the collaborating institutes, the quantification of the load spectrum
is of great importance. For its description, the piston pressure during form filling and
pressurization and also the surface temperature of the tools can be measured directly
after casting the components. Therefore, the measurement of the surface temperatures
is of great importance.
The suitability of different materials for the thixocasting process in terms of
mechanical strength was estimated by comparing the individual mechanical
strengths at the real temperatures appearing on the tool surface. The piston force
delivers the hydrostatic pressure to the semi-solid material.
A high resolution, a short-wave thermal image camera was used in the temperature
measurement, since the measurements cannot be carried out with thermocouples on
the surfaces. For metals, this method leads to strong deviations of the measured
temperature because of the very different emissivity in the infrared area. The
regulation for any single tool material or coating by calibration measurement
increased the precision considerably.
Table 9.2 gives information about the emissivity coefficients found and the
temperatures corrected for the tool surfaces.