Thixoforming : Semi-solid Metal Processing

6
Modelling the Flow Behaviour of Semi-solid Metal Alloys
Michael Modigell, Lars Pape, Ksenija Vasilic, Markus Hufschmidt, Gerhard Hirt,
Hideki Shimahara, Rene Baadjou, Andreas B€uhrig-Polaczek, Carsten Afrath, Reiner Kopp,
Mahmoud Ahmadein, and Matthias B€unck
6.1
Empirical Analysis of the Flow Behaviour
In recent years, the demand for an industry-related and target-oriented enhancement
of the semi-solid processing of metallic alloys and composites has grown. Until then,
die design and process parameters such as holding time, pressure and piston velocity
had been a matter of trial and error, because design rules from classical casting or
forging cannot be transferred to thixoforming [1, 2].
As mentioned in previous chapters, metallic alloys in the mushy state consist of
solid particles suspended in a liquid matrix. Hence the flow behaviour is comparable
to the one of classical suspensions and in principle the methods to evaluate and
model the flow properties can be taken from classical suspension rheology. The main
differences for metallic suspensions are related to changes in particle diameter due to
Ostwald ripening (see Section 6.1.1) and non-isothermal effects such as solid fraction
distribution during die filling (see Section 6.1.5).
The flow properties of suspensions are shear-thinning and thixotropic. They depend
on solid content, particle shape and particle diameter. Figure 6.1 shows the viscosity as
a function of solid content for ideal particles, titanium oxide and soot. The shearthinning
behaviour is displayed in Figure 6.2 for latex particles suspended in water.
The shear-thinning and time-dependent flow properties of semi-solid metal alloys
were first discovered by Flemings and co-workers [3] in the early 1970s for a lowmelting
tin–lead alloy. Since then, numerous groups (e.g. [2, 4–9]) have determined
the rheological characteristics experimentally and set up empirical models which are
used for numerical simulation.
The latest investigations concentrated on the flow properties of industrially
relevant aluminium and steel alloys whose metallurgical characteristics are described
in Chapter 2. In the following, we focus on the flow behaviour of the steel alloy
A List of Symbols can be found at the end of
this chapter.
j169