Thixoforming : Semi-solid Metal Processing

11.2 State of the Artj413
directly addressed in the available literature on semi-solid extrusion, the reported
data allow some conclusions to be drawn concerning process parameters and part
quality.
Semi-solid Bar Extrusion of Aluminium and Lead The first investigations on semisolid
extrusion were done by Kiuchi et al. [2] in 1979. On a horizontal forming device,
they studied the influence of the required extrusion force and the tool temperature
for Pb and Al alloys with a billet 40 mm in diameter and 25, 30 or 40 mm in height
under different test conditions. These conditions were the extrusion channel
diameter, varied between 2 and 10 mm, and the extrusion channel length, varied
from 4 to 100 mm. For the investigated alloys, it was found that the required initial
press force increases with decreasing liquid fraction. The extrusion ratio was varied
between 16 and 400. The press velocities were chosen between 37 and 47 mm s 1 .
The experiments were carried out using a simultaneous heating process , where the
tool and the billet were heated to the required temperature simultaneously, and a so
called no-preheating process , where the tool was not actively heated. A further
modification of the simultaneous heating process was an additional cooling of the
material with compressed air after the die. The authors pointed out that the optimum
liquid fraction for the semi-solid extrusion of the investigated alloys was in the range
5–10% and the required press force, which was only one-quarter to one-fifth of the
required press force in conventional extrusion, increased with decreasing liquid
fraction. When applying compressed air cooling directly after the forming die, the
temperature of the die decreased due to cooling by 20 C. Concerning the process
variations, the bars extruded with the no-preheating tool setup showed greater strain
hardening and less elongation than the bars in the simultaneous extrusion process.
Concerning tensile strength, both process variants showed the same results, but the
tensile strength decreased with increasing liquid fraction. The microstructure of the
extruded bars was generally fine for all process variants. When the liquid fraction was
higher than 20%, friction between the billet and container was negligible.
Much later, the same group [3] investigated, in addition to conventional aluminium
alloys, also the particle-reinforced aluminium alloy A-5056 þ 20 vol.% Al2O3 using
an extrusion channel diameter varied from 2 to 10 mm in 2 mm steps and a liquid
fraction of about 20–30%. To prevent cooling of the billet in the container, the tool was
preheated up to the billet temperature. The authors also mentioned that the
extrudable wall thickness decreased with increasing profile complexity. Generally,
they underlined the conclusion that the alloys can be extruded easily with low strain.
Heat treatment can improve the yield stress of the extruded products. Concerning the
particle-reinforced materials, the extruded products showed a smooth surface and no
extrusion failures. The dominant factor in semi-solid extrusion of this material is the
liquid fraction, which is responsible for the embedding of the particles.
M€oller [4] investigated billets of the aluminium alloy A356 þ 20 vol.% SiC with a
height of 100 mm and a diameter of 76 mm. With the chosen press velocity of
10 mm s 1 and a transverse extrusion tool setup, a maximum press force of 600 kN
was required irrespective of the tool temperature, which was varied between 200 and
400 C. The billets were extruded to a total height of about 600 mm using an extrusion