The present thesis investigates the grain refinement in solidifying Al-7wt%Si hypoeutectic alloy driven by electric currents. The grain size reduction in alloys generated by electric currents during the solidification has been intensively investigated. However, since various effects of electric currents have the potential to generate the finer equiaxed grains, it is still argued which effect plays the key role in the grain refinement process. In addition, the knowledge about the grain refinement mechanism under the application of electric currents remains fragmentary and inconsistent. Hence, the research objectives of the present thesis focus on the role of electric current effects and the grain refinement mechanism under the application of electric currents.
Chapter 1 presents an introduction with respect to the subject of grain refinement in alloys driven by electric current during the solidification process in particular, including the research objectives; the research motivation; a brief review about the research history; a short introduction on the electric currents effects and a review relevant to the research status of grain refinement mechanism.
Chapter 2 gives a description of research methods. This chapter shows the employed experiment materials, experimental setup, experimental procedure, the analysis methods of solidified samples, and numerical method, respectively.
Chapter 3 focuses on the role of electric current effects in the grain refinement process. A series of solidification experiments are performed under various values of effective electric currents for both, electric current pulse and direct current. The corresponding temperature measurements and flow measurements are carried out with the increase of effective electric current intensity. Meanwhile, numerical simulations are conducted to present the details of the flow structure and the distribution of electric current density and electromagnetic force. Finally, the role of electric current effects is discussed to find the key effect in the grain refinement driven by electric currents.
Chapter 4 investigates the grain refinement mechanism driven by electric currents. This chapter mainly focuses on the origin of finer equiaxed grain for grain refinement under the application of electric current on account of the importance of the origin for understanding the grain refinement mechanism. A series of solidification experiments are carried out in Al-7wt%Si alloy and in high purity aluminum. The main origin of equiaxed grain for grain refinement is concluded based on the experiment results.
Chapter 5 presents three further investigations based on the achieved knowledge in chapter 3 and 4 about the role of electric current effects and the grain refinement mechanism. According to the insight into the key electric current effect for the grain refinement shown in chapter 3, this chapter presents a potential approach to promote the grain refinement. In addition, the solute distribution under the influence of electric current is examined based on the knowledge about the electric current effects. Moreover, the grain refinement mechanism under application of travelling magnetic field is investigated by performing a series of solidification experiments to compare with the experiments about the grain refinement mechanism driven by electric currents shown in chapter 4.
Chapter 6 summarizes the main conclusions from the presented work.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:105-qucosa-197915 |
| Date | 26 February 2016 |
| Creators | Zhang, Yunhu |
| Contributors | Technische Universität Bergakademie Freiberg, Werkstoffwissenschaft und Werkstofftechnologie, Prof. Dr.-Ing. Gotthard Wolf, Prof. Dr.-Ing. Gotthard Wolf, Prof. Dr.-Ing. Franz Josef Feikus |
| Publisher | Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola" |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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