Thixoforming : Semi-solid Metal Processing

370j 10 Thixoforging and Rheoforging of Steel and Aluminium Alloys
Figure 10.1 Schematic view of semi-solid forging processes.
thixocasting. Oxides on the surface of the material can be introduced into the bulk
material, the geometry complexity is limited and lower liquid-phase fractions are
used.
Compared with drop forging, the resulting mechanical properties are similar to
those of forging parts whereas geometries adapted to requirements of lightweight
components can be achieved more easily because of existing component potentials.
Additionally to the increase in the possible component geometries and material
savings, aggregates with lower capacities can be used owing to the reduced
capacities that are needed for the forming operation. Concerning the mechanical
loads of the tools, the low forces that are required for the forming operation reduce
the mechanical wear and permit a longer tool life in comparison with the drop
forging process. However, there are also disadvantages regarding higher cycle
times, higher thermal load of the tools and limited suitability of alloys for the
forming operation.
One goal of thixoforging is to replace forged components. In comparison with
industrial forging of complex geometries, advantages of thixoforging could be the
realization of more complex geometries with slight spline and long flow lengths.
Even the shortening of several processing stages (roll forging, upsetting, forging,
deburring, piercing and calibration) that are not required for semi-solid forming
operations (Figure 10.2) are of interest. The process is shortened right at the
beginning because pre-forming of the precursor material is not necessary. There
are significant differences in the forging operation itself. Whereas conventional
forging requires few forming steps until the end geometry is reached, semi-solid
forming is carried out in just a single step. Draft angles can be designed very flat and
core pullers enable near-net-shaped geometries to be produced without piercing the
component after forming. Also, deburring of the component can be minimized or
even avoided. In particular, this leads to advantages regarding process costs, mainly
based on material savings [5, 6].
Open challenges are the development of economic working tools (as shown in
Chapter 8), even though first investigations with promising results were started in the