Thixoforming : Semi-solid Metal Processing

414j 11 Thixoextrusion
channel with a rectangular cross-section of 10 10 mm. The edges were filled out
well. In the microstructure analysis, the author detected the phenomenon of shell
formation. The extruding material which entered the forming die first cooled on
coming in contact with the extrusion channel wall. This cold material solidified and
stuck to the extrusion channel wall. The following still semi-solid material flowed
through the reduced extrusion channel. The phenomenon of shell formation was not
solved by increasing the tool temperature. A plug to increase the pressure in the
extrusion channel did not increase the heat transfer and did not ensure rapid
solidification of the material before leaving the extrusion channel.
Semi-solid Impact Extrusion of Steel Some authors have described experiments in
which they extruded billets into a closed die of limited length. These processes,
which are similar to impact extrusion, were mainly used to study the influence of the
process parameters on solid–liquid segregation and part properties.
Miwa and Kawamura [5] carried out experiments similar to semi-solid impact
extrusion, denoting them thixoextrusion into a mould. They extruded billets (diameter
30 mm, height 20 mm) of the stainless-steel alloy UNS: s30400 (AISI 304), held in
a ceramic container, into a metallic mould to investigate the influence of the press
velocity on the tendency for segregation. Depending on the applied forming velocities,
they observed effortless form filling, but phase separation of the liquid and solid
fractions at 100–1800 mm s 1 press velocities, whereas at a low press velocity of
10 mm s 1 a homogeneous phase distribution was observed, but poor surface quality.
An unintended increase in the required forming pressure occurred. The authors
attributed the latter effects to undesired rapid solidification of the semi-solid slurry in
the comparatively cold extrusion dies. Unfortunately, they did not mention either any
dimensions of the forming die diameter or extrusion channel length or give an
illustration of the tool setup.
Rouff et al. [6], in impact extrusion experiments, investigated the influence of
different shear rates on the steel grade C80 (AISI 1080). The extruded billets had a
diameter of 30 mm and a height of 45 mm. The desired extrusion temperature was
1430 C, which corresponds to a liquid fraction of about 40%. In order to reach a large
range of strain rates, the authors investigated two possibilities. First, the press
velocity was kept constant and the extrusion diameter was changed from 8 to 15 mm.
Second, the extrusion tool geometry was kept constant and the press velocity was
changed. For the first possibility, where the calculated shear rate was 243 or 215 s 1 ,
the extruded bars showed a homogeneous material flow and no kind of segregation.
In the second case, when the extrusion channel diameter was kept constant at 12 mm,
inhomogeneous material flow could be detected for shear rates between 10 and
95 s 1 , whereas a homogeneous material flow was observed for higher press
velocities, which resulted in shear rates between 95 and 475 s 1 .Aninfluence of
different shear rates could not be detected if the shear rate was lower than 10 s 1 . The
results of the extrusion experiments were compared with simulated values. With
inverse modelling, using a viscoplastic material law, the authors concluded that the
form filling behaviour of this steel grade is dependent on the shear rate. No data were
provided on the applied tool materials and die temperatures.