| As known to all, due to the high specific strength, high thermal conductivity, high electrical conductivity, good corrosion resistance, good wear-resistance, and excellent performance in casting, machinability, workability, etc., aluminium and its alloys have been wildly used in advanced technology fields of machinery, automotive, aerospace, military, and so on, which also have become the second largest metals materials after the steel. However, in the casting process, aluminium alloys are generally easy to form coarse equiaxed grains, columnar grains and feathery grains. Particularly, the coarse columnar or feathery grains will greatly reduced the mechanical properties of the alloys, especial for the workability, yield strength and elongation, etc.. Meanwhile, it also easily causes many surface defects of the casting. In addition, as the most varieties and largest amount alloys in aluminium casting alloys, Al-Si series casting alloys will form a lot of coarse acicular/flaky eutectic Si in the casting process except having the common microstructure defects of aluminium alloys above. These coarse eutectic Si are easy to arouse stress centralization, cause micro-cracks in the deformation process, and also seriously deteriorate the mechanical properties and processability of the alloys by separating the a-Al matrix, especially the ductility. Recently, with the rapid development of study on aluminium alloys, more and more industrial practice have proved that the Al-Ti-B and Al-Sr master alloys are the most efficient refiner and modifier for a-Al grains and eutectic Si phases, respectively, which therefore expanding their scopes of application in aluminium industry. However, because the study on refiner and modifier are late in our country, the current quality of domestic Al-Ti-B and Al-Sr master alloys are still a gap compared with that of abroad, which can't meet the needs of high-quality aluminium alloy materials in aerospace and national defense technology fields. Therefore, the development of high efficiency, high purity, fast effect Al-Ti-B and Al-Sr master alloys will have great practical significance and economic benefits for aluminium processing industry.In the present work, based on the theoretical calculations of thermodynamics and kinetics, as well as the results of basic experimental research, we improved the conventional preparation processes of "the reaction of fluoride salts with aluminium" and "direct reaction method", respectively, and successfully developed novel methods to prepare high-quality Al-Ti-B and Al-Sr master alloy wire/rod. Meanwhile, on the basis of these new processes, we also developed a novel method to prepare high-quality Al-Ti-B-Sr composite master alloy rod. Besides, we had investigated the microstructures and performances of these master alloys from various aspects. Overall, our work had made a certain contribution to further improving the quality of Al-based master alloy refiner and modifier.This dissertation includes seven chapters.Chapter one is the introduction. We first introduced the research background, significance and importance of our work. Then, we summarized the development history and research status of the refiners and modifiers at home and abroad, emphasizing the research progresses of Al-Ti-B master alloys prepared by "the reaction of fluoride salts with aluminium" and Al-Sr master alloys prepared by "direct reaction method", respectively. What's more, an overall review had been made for the grain refinement mechanisms and eutectic Si modification mechanisms of Al-Ti-B refiners and Al-Sr modifiers. Finally, the origin, main contents and innovations in this work had been briefly described.In chapter two, it detailed illustrated the experimental materials, equipments and processes used in all parts of work in this dissertation, which mainly included three parts:the adoption of the materials, equipments and processes to prepare Al-Ti-B, Al-Sr and Al-Ti-B-Sr master alloys in the basic experimental research and industrial experimental research; the characterization and analysis methods of the chemical compositions, phase compositions and microstructures in the alloy samples; the testing methods of refinement performance, modification performance and mechanical properties in the alloy samples.In chapter three, we detailed explored the main factors to affect the growth of TiAl3and TiB2, the purity of Al-Ti-B, and the recovery of Ti and B elements in the reaction of fluoride salts with aluminium, based on the thermodynamic and kinetic analysis of the preparation processes of Al-Ti-B master alloys (Ti/B mass ratio>2.2). Meanwhile, the operation parameters, such as the feeding order of K2TiF6and KBF4, the granularity of fluoride salts, the melt temperature, the holding time etc., were also optimized in this work, which had provided a basis for preparation of high-quality Al-Ti-B master alloys.In chapter four, the high-quality Al-Ti-B master alloy wire was successfully prepared by using a new process of "two-stage-feedatock method", based on the improvement of the conventional preparation process of "the reaction of fluoride salts with aluminium" and the optimization of the correlative processing parameters. Meanwhile, the macroscopic morphology, chemical compositions, phase compositions, microstructure, fracture characteristics, mechanical properties as well as the refinement performance of the Al-5Ti-1B master alloy wire prepared by this new process were detailed studied in this work, which also did a comparison of the Al-5Ti-1B master alloy wire prepared by the world famous Al-Ti-B manufacturer of KBM company. In addition, the refinement mechanism of Al-Ti-B master alloys in pure aluminium grains was also discussed.In chapter five, the high effective and fast modification Al-Sr master alloy rod was successfully prepared by using a novel process of "direct reaction-hot extrusion" based on the improvement of the conventional preparation process of "direct reaction method" and the optimization of the correlative processing parameters. Meanwhile, the macroscopic morphology, chemical compositions, phase compositions, microstructure, fracture characteristics, mechanical properties as well as the modification performance of the Al-lOSr master alloy rod prepared by this new process were detailed studied in this work, which also did a comparison with the Al-1OSr alloy trapezoidal block prepared by conventional "direct reaction method". Besides, the Al-20Sr master alloy rod was also successfully prepared by using this novel process, which had overcome the weakness of producing high Sr content Al-Sr master alloy rod in conventional "direct reaction method". Finally, we also provided a way to prepare the coiled Al-1OSr master alloy wire by using a reasonable heat treatment to improve its mechanical properties.In chapter six, the high-quality Al-Ti-B-Sr composite master alloy rod was successfully prepared by using a novel process of "three-stage-feedatock method" and "hot extrusion process". Meanwhile, the macroscopic morphology, chemical compositions, phase compositions, microstructure, fracture characteristics, mechanical properties as well as the refinement and modification performance of Al-5Ti-1B-10Sr composite master alloy rod prepared by this new process were also systematic studied in this work.Chapter seven is the conclusions of.all the research works mentioned in this dissertation. Finally, a brief introduction of the published papers and patents, participated projects and academic conferences, obtained awards, curriculum vitae and acknowledgements during the doctorate study period were given. |
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