Thixoforming : Semi-solid Metal Processing

The description of the yield stress is therefore of central relevance since it defines
when plastic deformation happens. Furthermore, it represents an important parameter
for the calculation of metal forming properties.
At high temperatures, the yield stress is especially dependent on the strain rate.
The viscoplastic constitutive equation describes the velocity dependence of plasticity.
The theory of the viscoplasticity dates back as early as 1922 and was developed by
Bingham [20]. This theory could only be applied to metal forming analyses when the
finite element-method (FEM) was introduced as a modification of the rigid-ideal
plastic model of Levy and von Mises [21].
6.1.4
Experimental Investigations to Determine Model Parameters
Experimental investigations of the rheological behaviour of metallic suspensions
have been conducted in different types of rheometers, which are mainly capillary or
rotational types, and using compression tests. Their range of application for semisolid
alloys together with advantages and disadvantages are given in Table 6.1.
6.1.5
Experimental Setups
6.1.5.1 Rotational Rheometer
The experiments were performed with two self-developed high-temperature Couette
rheometers – one for aluminium and the other for steel alloys. The steel rheometer
setup is shown schematically in Figure 6.5. The principle setup of the aluminium
rheometer is comparable and given elsewhere [22].
When developing the steel rheometer, the main challenge was device-related,
because a combination of steel and ceramic elements had to be employed in the
rheometer assembly. The measuring device consists of a rotating grooved rod (to
Table 6.1 Advantages and disadvantages of common rheometer
for analysing the rheological behaviour of semi-solid alloys.
6.1 Empirical Analysis of the Flow Behaviourj175
Capillary rheometer High shear rates up to 10 000 s 1
Conditions as during thixocasting (fS < 60%)
Dp measurements difficult
Material used is reheated into semi-solid state
Rotational rheometer Large variation of experimental procedure possible
(e.g. shear rate jumps, shear stress ramps, creep, oscillation)
Medium shear rates 60%)
No exact viscosimetric flow can be operated with wall slip as
elongation flow or without wall slip as shear flow
Simulation necessary to evaluate experimental data