Thixoforming : Semi-solid Metal Processing

11.2 State of the Artj415
In similar experiments, Abdelfatah et al. [7] reported a strong increase in the
required extrusion pressure with increasing extrusion ratio. The billets used had a
diameter of 30 mm and a height of 45 mm. The chosen liquid fraction of the
investigated steel grade C80 was between 20 and 30%. The container diameter was
40 mm. In two test series, the diameter of the extrusion channel was reduced from 12
to 8 mm, which correspond to extrusion ratios of 11 and 25. On increasing the
extrusion ratio, the required press force had to be increased from 15 to 20 tons. The
ultimate tensile strength and yield strength of the extruded bars were comparable to
values from conventional forged parts. Other mechanical properties such as percentage
reduction of area after fracture were lower than for conventional forged parts
due to the appearance of microporosity in the thixoforged parts.
Semi-solid Bar Extrusion of Steel Sugiyama et al. [8] investigated the semi-solid bar
extrusion process of steel. In their experiments, the steel alloy C22 (AISI 1020)
(billet of diameter 18 mm and height 35 mm) was extruded through a forming die
channel 7 mm in diameter, corresponding to an extrusion ratio of 10. The billet was
inserted in a graphite case and the gap between the billet and case was filled with
alumina powder. Then the graphite case was inserted in a graphite block which was
located directly in the inductive heater, where the billet was heated to the desired
temperature. With a press velocity of 8.8 mm s 1 , the billets were extruded at hot
forming and semi-solid temperature. After the forming die, active cooling with
compressed air and water dust was applied. A comparison of the resulting press
force showed that the press force for semi-solid extrusion is half of the force
required for conventional extrusion. Furthermore, the authors observed shell
formation in the extrusion channel when the semi-solid material touched the
extrusion channel wall. Due to segregation, poor quality at the top of the extruded
bar was observed. The other parts of the bar showed a good microstructure –
independent of the process parameters chosen. The hardness in the surface area
was similar to that for conventional extruded products but the centre was about two
times harder.
M€oller [4] extruded billets (diameter 34 mm, height 40 mm) of the steel grade
X210CrW12 with variation of the press velocity from 20 to 100 mm s 1 . After the
extrusion channel exit, the tool was equipped with an active water dust cooling device.
With variation of press velocity, the author observed defects such as porosity in the
extruded bars. The phenomenon of shell formation occurred during the extrusion
process. Due to the billet size and the tool temperature of 300 C, the billet solidified
quickly. In the experiments, only a small bar could be extruded. In contrast to the
conventional extrusion process, no stationary press force was realizable.
Owing to the aforementioned difficulties, the number of publications dealing with
steel semi-solid extrusion is low, compared with thixocasting and thixoforging of
steels. The above-mentioned studies [2–8] have demonstrated the feasibility of semisolid
extrusion. Experiments were performed on the laboratory scale and the
experimental results such as extrusion load development and microstructure analysis
of the extruded parts were presented. Mechanical properties such as yield stress and
hardness were also presented. However, in all these studies, no tool and process