Thixoforming : Semi-solid Metal Processing

Figure 4.6 Grain structure of the billet centre, A356 after reheating.
4.2 Chemical Grain Refinement of Commercial Thixoalloysj115
form factor are comparable (Figure 4.6). During reheating, however, it was observed
that for the chemically grain-refined material less melt ran out of the billet than with
conventional raw material. This presents the possibility that when using grainrefined
raw material the dripping losses that are normally incurred during vertical
reheating may be reduced.
The reheated raw material is processed by die casting into the principle component
(Figure 4.7). A step form is used with which the form filling of a component
with decreasing wall thickness can be examined. In contrast to commercial raw
material, using chemically grain-refined raw material, with the same form filling rate
(vpiston ¼ 0.05 m s 1 ), even the overflow of the casting chamber could be filled. This
implies that an improved form filling capacity can be achieved.
Samples were taken from the component from steps of 15 and 25 mm height and
then metallographic tests were performed (Figure 4.8).
For both of the raw materials no significant variation in grain structure with the
step height was determined. The chemically grain-refined material had a somewhat
finer grain structure than the commercial raw material. The advantages that were
already apparent in respect of the aluminium phase and eutectic after reheating of the
Figure 4.7 Principle step-part made from thixo-adapted A356 (after chemical grain refinement).