Thixoforming : Semi-solid Metal Processing

grain fragment density [mm -1 ]
200
160
120
80
40
0
0,0 0,1
(a) cooling rate [K/s]
(b)
9.4.1.4 Oxidation
To assure good mechanical properties of manufactured components, prevention of
deleterious oxide layers embedded in the cast parts is essential. In that case, the high
affinity of aluminium to oxygen is also advantageous, because instantaneously
during pouring over the channel a tube-like oxide layer, where the melt flows
through, forms and shelters from the atmosphere. Hence no additional encapsulation
or similar is necessary.
9.4.1.5 Summary
The cooling channel process is an easy to use and stable process to provide excellent
semi-solid precursor material for subsequent HPDC processing. The process
window was definitely determined. The cooling channel enhances nucleation and
hence benefits the globular microstructure. An additional air cooling can minimize
the cooling time to less than 3 min, with enhancing effects on the grain sizes and
maybe also the mechanical properties, which has still to be proved.
In addition, in forming experiments very good mechanical properties were
achieved. Investigations have shown that a simple and cheap implementation to
industrial manufacturing is possible and particularly with regard to the expensive
thixoroute, the cooling channel process is an advantageous option.
9.4.2
Rheo Container Process (RCP)
0,2 0,3 0,4 0,5
Figure 9.40 Grain fragment density (a) and shape factor (b) versus cooling rate.
The RCP, shown in Figure 9.41, was developed on the basis of the Constant
Temperature Process and the cooling channel process. The special feature is the
encapsulation and consequently the suitability for processing of highly reactive alloys
such as magnesium or aluminium–lithium alloys.
To diminish the contact with the atmosphere of a high oxygen-sensitive alloy, the
slightly superheated molten metal is poured directly into a 1 mm wall thickness
shape factor
1,0
0,9
0,8
0,7
0,6
0,5
9.4 Rheoroutej355
measurements simulation
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
cooling rate [K/s]