7075 Aluminium alloy in the extruded state (i.e. utilising
the RAP route to a thixoformable microstructure) can
be thixoformed and successfully fill the die. For one step
and two-step heating, and lower thixoforming temperatures,
defects occur including turbulence, liquid segregation, centreline
porosity and unrecrystallised grains which could be
obstructing flow. In addition, there may be some incorporated
oxide. Three-step heating and thixoforming temperatures
in the range 616.618 .C give improved results. An
intermediate ram velocity of 750 mm/s gives fewer defects
than 1000 mm/s. A graphite die and a heated tool steel
die both give successful die filling although the mechanical
properties of the thixoformed products are better with
the former. The average yield strength for material in the
T6 condition thixoformed at 616 .C by three-step heating (a
1 min hold at 500 and 575 .C) with a ram speed of 500 mm/s
in the graphite die is 477MPa with an elongation of 3.0%.
This compares with the target value of 505MPa and 11%
for the wrought material. Attention to the elimination of defects
through die design may help to improve the elongation
values. The highest result obtained was a yield strength of
478MPa with an elongation of 6.9%.