Theoretical research on rheological behavior of semisolid slurry of magnesium alloy AZ91D
Graphical abstract
The curves are the calculated results of the steady-state apparent viscosity by using the model developed in this paper and the scatters are the experimental results.
Introduction
Magnesium alloys have been accepted as the lightest structural material and gained increasing attention owing to their excellent properties, such as light weight, high specific strength and stiffness as well as recyclability [1]. However, magnesium has a hexagonal close-packed crystal structure that results in its poor plasticity at room temperature and it is also easy to oxidize and burn. All of these cause low production efficiency and high price of magnesium alloys. How to process magnesium alloys more effectively has become an important subject for the researchers all over the world [1], [2]. As an advanced near-net shape processing technique, semisolid metal processing has many advantages such as short forming process, easy formation and good product property; and it has been regarded as an ideal way to process magnesium alloys [1], [2], [3], [4].
In recent years, lots of experimental researches have been done on the semisolid behavior of magnesium alloy AZ91D [5], [6], [7]; but it is very difficult in practice to obtain the reliable data of semisolid metal slurry because of the experimental difficulties [8]. And thus modeling of semisolid behavior has become a necessary aid to experimental researches. In this paper, a microstructure model describing the rheological behavior of semisolid slurry of magnesium alloy AZ91D will be built up so as to analyze the rheological behavior of this kind of alloy. Semisolid metal slurries used in semisolid metal processing can be roughly divided into two broad categories, liquid-like slurry and solid-like slurry. The former contains dispersed solid particles and behaves like a liquid under external forces, while the latter contains an interconnected solid phase and behaves like a solid. This work confines itself to liquid-like slurries with a solid fraction less than 0.6 and a globular solid morphology, which usually exhibit two unique rheological properties, thixotropy and pseudoplasticity.
Section snippets
Rheological model for system with a fixed solid volume fraction and a constant shear rate
In this work, semisolid metal slurry is considered as a suspension with the interacting spherical solid particles dispersed in a liquid matrix [9]. In the simple shear field, the agglomeration and deagglomeration synchronize; and the degree of agglomeration depends on the nature of system such as particle size, volume fraction, and external flow conditions [9].
In order to model the rheological behavior of semisolid metal slurry, the following assumptions are proposed:
- (i)
the solid particles are
Steady-state rheological behavior of semisolid slurry of AZ91D – pseudoplasticity
Fig. 1 shows the agreement between the theoretical prediction and the experimental data of the steady-state apparent viscosity changing with shear rate. As shown in Fig. 1, the semisolid slurry of AZ91D behaves as non-Newtonian fluid, that is, the apparent viscosity of AZ91D depends on shear rate and exhibits shear-thinning, which is generally known as pseudoplasticity [3], [4].
According to the model developed in this paper, the average size of agglomerate influences the rheological behavior of
Conclusions
In this paper, a microstructure model is developed to investigate the rheological behavior of semisolid metal slurry including steady-state behavior and instantaneous behavior, and it successfully predicts the rheological behavior of semisolid metal slurry of magnesium alloy AZ91D. The calculated results indicate that there is a very close coupling between the rheological behavior and the microstructure of semisolid slurry, and the external conditions influence the rheological behavior of
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