Transient Rheological Behavior of Semisolid SEED-Processed 7075 Aluminum Alloys in Rapid Compression
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RHEOLOGICAL properties of semisolid slurries are the key parameter that require a further understanding for successful development of industrial semisolid forming processes and technologies due to the complex rheological phenomena during the forming process.[1–3] The rheological properties (i.e., viscosity vs shear rate) of the semisolid slurries rely on the non-dendritic microstructure of the semisolid metallic alloys consisting of solid globular grains in a liquid matrix.[4] The viscosity of the semisolid slurry drops under shearing, and the slurry flows. The semisolid slurry can stand its own weight when allowed.[5] In the practice, the semisolid forming of aluminum alloys takes place in a fraction of a second.[6] Thus, the transient rheological behavior governs the flow behavior of the semisolid slurry within the die cavity during the semisolid forming process.[7] On the other hand, it is widely reported that the viscosity strongly depends on the solid content of the
AMIR BOLOURI is with the Department of Engineering Design and Mathematics, University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK. Contact e-mail: amir.bolouri@ uwe.ac.uk X.-GRANT CHEN is with the Department of Applied Science, University of Quebec at Chicoutimi, Saguenay, QC G7H 2B1, Canada. Manuscript submitted May 10, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
semisolid slurry and the process temperature,[7–9] in which 1 K variation in temperature can have a substantial influence on the viscosity.[8] This implies that the semisolid slurry will experience a significant variation in the viscosity during the die filling due to the rapid heat loss of the slurry in the contact with the die. The issue can be very critical for the semisolid forming of high-strength wrought aluminum alloys such as 7xxx series,[10] in which there are sharp changes in the solid content of the semisolid slurry as a function of the semisolid temperature.[11] This can cause on the one hand several manufacturing defects such as incomplete die filling that requires an extensive die modification and process optimization.[12] On the other hand, the precise modeling and simulation of the flow behavior of the slurry within the die will become very complex.[13,14] In order to minimize the complexity of the transient rheological behavior of the semisolid slurries, the transient rheological behavior of the semisolid slurries needs to become less sensitive to the solid content of the semisolid slurries. A significant improvement in the semisolid forming process can be achieved by using the semisolid slurry feedstock showing consistent transient rheological properties within a wide range of solid contents. A detailed study is required to understand the dependency of the transient rheological behavior of the semisolid slurries to the solid contents and the microstructural characteristics such as grain size and morphology of the slurries.[15]
There have been several publications on the use of different methods to study the rheological behavior of the semisolid slurri
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