Thixoforming : Semi-solid Metal Processing

386j 10 Thixoforging and Rheoforging of Steel and Aluminium Alloys
Figure 10.13 Produced components (high-speed steel HS6-5-3)
by thixo lateral extrusion with their different stages of die filling.
components by using steel. Based on early experience with aluminium alloys,
different geometries were chosen to study the applicability of steels for forming
techniques in the semi-solid state. In addition to simple geometries shaped like a disk
with an arm to clarify the fundamental applicability of different steel alloys, further
components were realized (Figure 10.13). In case of the component in (a), the
speciality of the geometry is the filling of an undercut and the realization of sharp
contours. Investigations with different steel alloys (HS6-5-3, X210CrW12, 100Cr6,
C60, 41Cr4) demonstrate sufficient form filling in so-called step-shot trials except for
41Cr4 [42]. Even when the diameter of the channel increases, the flow front is
homogeneous for the testing procedure up to the chosen maximum punch speed
(80 mm s 1 ). Against first estimations, a steady filling from the beginning of the
gravity up to the end enables defects that would be caused by a transient and a
turbulent material flow, respectively, to be avoided or reduced. However, the
properties of the different materials show different values of the microstructure
depending on the location in the component due to segregations or pores that appear
in the component. Especially for the materials HS6-5-3 and 100Cr6 in locations near
the surface the fast cooling leads to higher strength, which was proven by hardness
measurements.
Further investigations to widen the range of possible geometries for the thixoforming
process were carried out with the concept of the component shown in
Figure 10.13b. Whereas the earlier components made of steel conformed to a
simple geometry of a drop forged part, with this part the flowability of semi-solid
material could be shown in its full potential. Illustrated by core pullings, which
cannot be realized in traditional forging operations, the excellent form filling and
the welding in areas of two flow fronts is demonstrated. To avoid problems of
shrinking of the component on to the tools, relatively large draft angles of about 5
were used.