Fig. 5 shows low magnification images of sections through
fingers thixoformed using one-step heating schedules with
varying thixoforming temperatures and constant ram velocity
of 500 mm/s. Typical defects are labelled. Oxide skin may
be deflected by the sharp corner at the entrance of the die
and incorporated into the essentially laminar flow. The sharp
corner can also cause some turbulence, which is undesirable
in thixoforming [18]. Shrinkage porosity tends to occur on
the centreline near the ends of the fingers and at the lower
thixoforming temperatures, suggesting that the cooling rate
is too fast for a material with a relatively wide temperature
range over which the final solidification is occurring (see
Fig. 4). The oxide incorporation could be avoided with oxide
stripping devices, which are used by some of the commercial
manufacturers to skim the oxide off the surface of the billet
before it enters the die. Fig. 6 shows defects in more detail
with unrecrystallised grains potentially obstructing the flow
of slurry in Fig. 6a, centre-line shrinkage porosity in Fig. 6b
and liquid/solid segregation with turbulent flow taken from
the central edge of the finger in Fig. 6c. The excessive liquid
generated at above 620 ◦C may cause the turbulence by
the sharp corner on the die.