Thixoforming : Semi-solid Metal Processing

4
Design of Al and Al–Li Alloys for Thixoforming
Bernd Friedrich, Alexander Arnold, Roger Sauermann, and Tony Noll
4.1
Production of Raw Material for Thixoforming Processes
A summary of the most important process alternatives for the forming of partially
liquid metals is shown in Figure 4.1. In the left-hand column, conventional
thixoforming – which is currently most widely used for industrial applications –
is schematically represented and in the right-hand column, the so-called slug on
demand process.
The stress induced and melt activated (SIMA) process requires conventional
continuous-cast or extruded billets as feed material. Cold deformation of the billets
induces high residual plastic strain. On reheating, recrystallization leads to a fine
globulitic granular structure. The SIMA material exhibits very good properties,
although it is only economical for special applications due to its high cost [2].
Mechanical agitation is one of the principle processes for the production of raw
material [3], but was never really successful. It was not possible to counter the
corrosion and erosion of the agitating mechanism within the system [1], and on the
process side the continuous casting procedure could not be synchronized with
the agitation process.
The magnetohydrodynamic (MHD) process is to date the most widely used
industrial process for the production of raw material [1]. Grain refinement is achieved
by motion of the bath induced by a magnetic field. This involves installing a solenoid
coil on the permanent mould, fed with an alternating current. This measure can also
be used to keep the permanent mould temperature homogeneous at a desired value.
In the semi-solid processing (SSP) process, just as in the MHD process, magnetic
agitation is used to achieve grain refinement. The difference is that each billet is
individually produced in its own chamber [4]. This offers the advantage of flexible
alloy adjustment. Furthermore, the hot billets can be reprocessed straight from the
chamber, so that advantages can be achieved from the viewpoint of energy considerations.
On the other hand, there are economic advantages in the alternative of
semi-finished production.
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