Thixoforming : Semi-solid Metal Processing

Semi-solid viscosity (Pa s)
into clusters able to form a stiff network. Mechanical loading is transmitted through
interglobular bonds and the system behaves like a solid, leading to a high viscosity.
In the calculations, the solid fraction in the active zone f s
A is found to be higher than
0.4. As the shear rate increases, more and more bonds between solid particles are
broken, leading to a progressive decrease in f s
A down to 0.4. A more dispersed
suspension having a fluid-like behaviour is thus obtained and, in the calculation, a
strong decrease in the active zone viscosity is observed. These results prove that the
shear-thinning behaviour of semi-solid alloys is due to an evolution of the liquid–
solid spatial distribution within the material. This evolution is attributed to local
deformation mechanisms, namely the fact that deformation is mainly carried by the
solid bonds and the non-entrapped liquid.
The prediction capability of the model is tested by calculating the semi-solid steadystate
viscosity for a solid fraction of 0.45, keeping all the parameters constant. Very
good agreement between the calculated and experimental results (Figures 7.5 and 7.6)
Table 7.1 Set of identified parameters for the Sn–15wt%Pb semisolid
alloy simulations of steady-state viscosities for f s ¼ 0.5.
K s
70
60
50
40
30
20
10
0
0
50
100
m K dg/K ag n f A
1 MPa 0.43 0.8 0.2 0.007
150
Shear rate (s -1 )
Figure 7.6 Evolution of the semi-solid viscosity as a function of the
shear rate at high shear rates for the Sn–15wt%Pb alloy.
Experimental data were obtained using concentric cylinder
viscometer tests.
200
7.4 Results and Discussionj231
Joly & Mehrabian (1976) ; f s = 0.5
Micro-macro modelling ; f s = 0.5
Joly & Mehrabian (1976) ; f s = 0.45
Mictro-macro modelling : f s = 0.45
250
300