Investigation of rheological and microstructural properties of semi-solid aluminium copper alloy during isothermal shear

  • Untersuchung der rheologischen und mikrostrukturellen Eigenschaften von teilerstarrten Aluminium-Kupfer Legierungen während isothermischen Scherens

Harboe, Siri Johanna; Modigell, Michael (Thesis advisor); Ratke, Lorenz (Thesis advisor); Singheiser, Lorenz (Thesis advisor)

1. Auflage. - Aachen : Shaker (2017, 2018)
Book, Dissertation / PhD Thesis

In: Berichte aus der Verfahrenstechnik
Page(s)/Article-Nr.: 1 Online-Ressource (xxi, 190 Seiten) : Illustrationen

Dissertation, RWTH Aachen University, 2017

Abstract

Semi-solid metal alloys (SSM) are suspensions of solid alloy particles segregated in the liquid alloy due to solubility differences. The rheological and microstructural behaviours of these materials are intertwined. Throughout literature, there are strong hypotheses stating that agglomeration and de-agglomeration behaviours are linked to the SSM characteristic thixotropic flow behaviour. However, limited published experimental proof of this was found at the onset of this research. In this work, theoretical and experimental investigations of the rheological and microstructural behaviour of semi-solid AlCu10 %wt of volume solid fraction 18-40 % during isothermal shear 1-1000 1/s were performed. This was used to evaluate the connections between the microstructural and rheological behaviours. From the microstructural analysis of the investigated system, it was observed that at low shear rates, the primary solid forms a porous, percolated network. At high shear rates the solid forms separate, densely packed cellular clusters. Agglomeration was observed at all levels of shear, but in different manners depending on the rate of shear. A mechanistic model which gives the dependency of the steady-state cluster radius on the shear rate was developed. The agglomeration process was found to be partly reversible by de-agglomeration at increasing shear, but it also contributed to irreversible coarsening by sintering and coalescence of the structure. It was discovered that the crystallographic misalignment in the agglomeration process decreases with increasing shear rate, so that at high shear rates the clusters become monocrystalline. The coarsening rate was found to generally increase with increasing shear rate. For the microstructural time dependency, published analytical Ostwald ripening models were compared to the coarsening observed in the experiments herein. The investigated SSM macroscopic flow behaviour was as expected found to be thixotropic (shear thinning in the steady-state). This was semi-quantitatively linked to de-agglomeration and agglomeration processes. As expected, a significant irreversible decrease in steady-state viscosity over time was observed, especially at higher rates of shear, meaning that a fully thixotropic recovery cannot be achieved after a certain duration of shear. This was, as in previous literature works, linked to the microstructural coarsening phenomenon. With the input of the experimentally determined microstructural properties, a semi-empirical model for the steady-state viscoplastic rheological behaviour was set up. In conclusion, the presented results give strong empirical indication of the relation between agglomeration and de-agglomeration of the microstructure and the macroscopically observed thixotropy of SSM.

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