Page 90 - eBook_Proceedings of the International Conference on Digital Manufacturing V2

Proceedings of the International Conference on Digital Manufacturing –
Volume 2

i. Shear-Thinning
When a semi-solid metal experiences shear stress, its
structure breaks down, leading to a decrease in viscosity,
allowing it to flow more easily. This happens due to particle
interactions within the material break down, resulting in a
more fluid-like state. As shear stress increases, the material
flows more easily until the shear-thinning effect stabilises
(Keung, Lee, Shan & Luo, 2008). High shear rates
significantly decrease apparent viscosity, facilitating better
mould filling during processing (Czerwinski, 2006).
ii. Time-Dependent Viscosity
The viscosity of thixotropic materials changes over time
under constant shear conditions. Initially, as shear is applied,
viscosity decreases rapidly. However, if the material is
allowed to rest after shearing, its viscosity increases again
due to the reformation of particle networks. This reversible
behaviour is critical for applications requiring controlled
flow (Alexandrou, Burgos & Entov, 1999).
iii. Thixotropic Strength
Thixotropic strength is the minimum shear stress required to
initiate flow in a semi-solid metal. This initial flow resistance
shows the material's ability to resist flow until a certain shear
stress is applied. Thixotropic strength is influenced by factors
such as temperature, solid fraction, microstructure (size and
shape of solid particles), and shear rate (Alexandrou, Burgos
& Entov, 1999; Keung, Lee, Shan & Luo, 2008). Researchers
have found that the microstructure of semi-solid metals
significantly influences thixotropic strength, which affects
their mechanical properties. The way metal flows in semi-
solid form are largely affected by the size of the particles, the
shape and how evenly they are spread out. A well-distributed
globular microstructure enhances flowability compared to
dendritic structures (Modigell, Pola & Tocci, 2018; Hirt &
Kopp, 2009; Martin-Raya, Menargues, Martin, Baile &
Picas, 2023; Li, Wang, Li, Liu & Zhao, 2023; Al-Sahlani,
Broxtermann, Lell & Fiedler, 2018; Kim et al., 2017; Jiang
et al., 2023; Bhattacharya, Bhattacharyya & Haldar, 2020).

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