Thixoforming : Semi-solid Metal Processing

362j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.47 Mechanical properties of thixo-cast
AlLi2.1Mg5.5ScZr; all heat-treated samples were solution
annealed at 460 C for 24 hours.
Due to the heat treatments, a large increase in the mechanical properties could be
achieved. As a result of the longer ageing time, concomitantly with the reduced
ageing temperatures, the elongation to fracture decreased; notwithstanding the
strength remained at a similar level.
The mechanical properties were improved during the heat treatment, the tensile
strength to almost 500 MPa and the yield strength to a maximum of 437 MPa. The
elongation to fracture could be doubled during ageing at 180 C from an average of
1.57 to 2.98%, but these results fall short of the expectations (cf. the results of the RCP
experiments).
During the examination of the fracture surfaces, in almost at all samples dark
microstructure areas were observed. Because these are probably the reason for the
substandard elongations, the fracture surfaces were investigated by means of
scanning electron microscopy, which showed porosity to be the cause of early
fracture. This porosity is the result of the particularly high volume shrinkage of
this special alloy, which likewise shows the importance of an accurate mould layout.
9.4.3.5 Cooling Channel Process: A356
To demonstrate the serviceability of the cooling channel process for industrial
application, a small batch of a real component was produced. For this purpose, the
tie rod mould was also used to ensure comparability to the other investigations. In
this case, batch production means a continuous steady-state casting of components
without any longer break. No additional pressurized air cooling was implemented.