Thixoforming : Semi-solid Metal Processing

384j 10 Thixoforging and Rheoforging of Steel and Aluminium Alloys
and are also used in casting processes, verify the possibility of producing complex
components. Additionally, the process chain is shortened by reducing the forming
steps while comparable material properties are obtained, as exist for forging
components. The main advantage of the thixoforging process over thixo lateral
extrusion and the related thixocasting is the larger effective surface area during
compression after the complete die filling has been reached [40]. Subsequently,
shrinkage holes can be avoided due to the constant effective pressure during the
solidification. Depending on the pressure distribution during the forming operation,
for both processes segregations of the two phases can be observed. Especially for long
flow lengths, an increasing pressure during the forming operation results in
increasing segregation. Because of the better flexibility of the liquid phase, it is
pressed out and separated from the semi-solid bulk material. This leads to an
inhomogeneous phase distribution and local variations of the mechanical
properties [40].
Material Flow and Segregation
To clarify the influence of the geometry and forming parameters on the development
of segregation, a demonstrator part was investigated. As shown in Figures 10.11
and 10.12, the geometries of the flange and the upper cup of the component shown in
Figure 10.10b were varied.
Investigations regarding the design of the flange show different flow behaviour of
the semi-solid material in macroscopic view. Apart from different filling sequences of
the cups and the flange depending on the tool movement, the main difference
appears in the formation of the flow front. In the case of the flat flange, the flow front
breaks because of the growing surface of the flow front with increasing diameter
(Figure 10.11b). Semi-solid material leaks out of the bulk material and can lead to
inclusions in the flange due to its inhomogeneous filling. The angle of the modified
Figure 10.11 Filling behaviour of different shapes for thixoforging
of AlSi7Mg and X-ray analysis of resulting components [41].