Thixoforming : Semi-solid Metal Processing

358j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Table 9.9 Process window of A356 for thixocasting and rheocasting [6].
Parameter Symbol Interval
Processing temperature
of the billet ( C)
Optimal
value Reason
TC 580–585 580 585 C:
decreasing fraction solid results in
increasing shrinkage porosity
Solid fraction fS 0.40–0.44 0.44
Mould temperature ( C) T F 250 For an optimal heat balance
Piston velocity (m s 1 ) vP 0.05–3 0.5 Depends on metal velocity
Metal velocity (m s 1 ) vM 7–30 Depends on mould-cavity
cross-section; warranty of laminar
form filling with closed flow front
Closing pressure (bar) PC 900 To reduce porosity
a critical two-channel flow at both ends of the component, which result in cold runs or
oxide layers.
9.4.3.1 Thixocasting: A356
With the modified mould, a small thixocasting batch was produced using the optimal
process parameters shown in Table 9.9. The cast components were torsion tested. For
that purpose, the tie rods were twisted until the first crack appeared (Figure 9.43). All
tested tie rods could be twisted about 525 , which relates to a tested elongation of
22%. Furthermore, the tie rods were investigated metallographically. In spite of thin
and long flow paths, no segregation, oxide layers or porosities could be determined.
This shows the high quality of the cast components. The industrially required
properties could even be outperformed.
9.4.3.2 Aluminium–Lithium Alloy
Wrought alloys in general are hardly liquidly castable. This is due to the high
solidification ranges, which lead to a high volume shrinkage. Furthermore, insufficient
feeding in hot tearing results. Oxygen-sensitive elements such as magnesium
and lithium impede conventional casting processes, and casting under particular
conditions is necessary to prevent combustion. To demonstrate the usability of the
RCP, the highly reactive wrought alloy AA1420 (AlLi2.1Mg5.5 þ Sr þ Zr) was
chosen. Porosity, volume shrinkage and hot tearing susceptibility should be significantly
improved due to the lower liquid phase content of 40–60% during the
thixoforming process.
9.4.3.3 RCP: AlLi2.1Mg5.5
Using the tie rod mould, the suitability of the RCP [6, 50] for the processing of highly
reactive Al–Li alloys was investigated [53].
The molten alloy (modified AA142, cf. Chapter 4) was poured at 630 C into the
container and processed at a temperature of 600 C and a solid fraction of 0.5