Thixoforming : Semi-solid Metal Processing

The cooling to the process temperature of 585 C lasts 6.5 min. Manually unfastening
the oxide-wiper ring leads to an overall cycle time of about 15 min. This cycle time
could be easily reduced by using more containers simultaneously, assuming that the
batch time for high-pressure die-casting is fast enough.
The casting velocity during form filling was always 0.4 m s 1 . A scavenging gas
treatment to reduce hydrogen was applied before casting. The hydrogen index was
2.02 at the beginning of the experiments and 2.63 at the end. To ensure a steady state,
the first five cast parts were discarded. During subsequent experiments, 20 tie rods
were produced and partitioned into three sets for subsequent heat treating and tensile
testing. One-third (six samples: 1st, 4th, ...) were tested as-cast, the remaining
samples were T6 heat treated with the object of high strengths [heat treatment (HT)
A: seven samples: 2nd, 5th, ...] and high elongations (HT B: seven samples: 3rd,
6th, ...). The solution annealing was applied at 535 C for 17 h and the ageing
temperature in all experiments was 160 C. The difference between HT A and HT B
was the ageing time. To achieve high strengths, the ageing time was 2.5 h for high
elongations in contrast to 7 h. The increase in the ductility is caused by the rounding
of the silicon precipitations during ageing. The success of flushing and heat
treatment and the suitability of rheocasting becomes apparent on examining the
surface of the cast parts, where almost no blistering is visible.
Similarly to the sampling in thixocasting of microalloyed AA1420, flat bar tensile
specimens were taken from all cast parts (40 samples in total) and tested. The results
are illustrated in Figure 9.48. As the result of heat treatment, the mechanical
Figure 9.48 Mechanical properties of rheo-cast tie rods (A356);
HT A, aged 2.5 h; HT B, aged 7 h (HT, heat treatment).
9.4 Rheoroutej363