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Research and Development on Semisolid Slurry Processing Technology of Mg-and Al-based Alloy

Author: ZhangXiaoLi
Tutor: LiTingJu
School: Dalian University of Technology
Course: Materials Processing Engineering
Keywords: Semisolid metal processing Semisolid slurry Aluminum- and Magnesium-based alloy Non-dendritic structure Formation mechanism
CLC: TG249.9
Type: PhD thesis
Year: 2008
Downloads: 632
Quote: 2
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Microstructures are at the center of materials science and engineering. They are the strategic link between materials processing and materials behavior. Microstructure control is therefore essential for any processing activity. The most important processing route for metals and alloys is solidification. Solidification technologies of premium castings, including die-casting, investment casting, continuous casting & rolling, self-propagating high temperature synthesis and semisolid processing, etc, have been the dominant research and development. The specific microstructure, which meets the demand of performance, is conventionally obtained by casting and forging process. In comparison with those two methods, SSM(semisolid metal processing) is a new processing route, in which the processing feature depends on the thixotropic structure of the feedstock, which is so called non-dendritic microstructure. SSM is one of the most important processing technologies in 21st century due to its advantages, such as near net-shape forming, high quality and performance of products, and energy-saving, to name a few.The development of SSM, rheological behavior of the thixotropic structures and the formation mechanism of non-dendritic microstructure are summarized, and rheoforming route, which is the developing front of SSM, is introduced in the present paper. Research and development on semisolid slurry processing technology of Magnesium- and Aluminum-based alloys is fulfilled theoretically and experimentally, based on the abundant experience of EPM(Electromagnetic processing materials) in Foundry Engineering Center. Moreover, Damper cooling method (DCT) is developed to processing semisolid slurry for rheoforming Al-alloys, which is supported by National Natural Science Foundation(50374014) and cooperated with General Research Institute for Nonferrous Metals. The present doctoral dissertation is performed on basis of the above research works. Main research details and results are as follows:(1) AZ91 magnesium alloy is semisolid processed under experimental conditions designed to yield from 3% to 40% of the primary particles, and the semisolid slurry is produced by electromagnetic stirring after feedstock with small dimension has been isothermally heat treated at near-liquidus temperature. The thixotropic microstructures obtained are characterized in detail and linked to the corresponding tensile properties, and serial sectioning experiments are performed to construct three dimensional morphologies of clusters consisting of conglutinated primary grains. The experimental results indicate that incompact structure agglomerating a few primary solid particles is observed within structures of low solid fraction, while a reduction in the castings’ forming temperature and so increasing the fraction of primary solid phase bright out interaction among the primary particles within vigorously stirring melt, and thus causes slide and plastic deformation between or among the neighboring solid particles. In contrast with structure with low solid fraction, this slide and plastic deformation results in differently morphological 3D structures welded together with much more solid particles. The fractographic analysis reveals that fracture mechanism and corresponding morphology of the rapture surface of tensile bars depend on the solid fraction of the primary particles. Moreover, the increase in solid fraction mainly lies on the formation of new particles.(2) Microstructure evolution of AZ91 magnesium alloy, which is isothermally treated in solid-liquid region at a high volume fraction of solid from as-cast microstructure then transforms to semisolid state, is investigated. The results show that eutectic phase with net-work shape is first to melt, and second dendritic arms disappear or melt off and then re-precipitated on the dendritic trunk. Solid particles go through Ostwald ripening due to Gibbs-Thomson effect resulting from difference of curvatures, and transform into globular particles, and coalesce together with the time. Moreover, the entrapped liquid drops form intra-grain by two mechanisms owing to decreasing interfacial energy: one is that inclusions containing rich solute Al and Zn melt, diffuse and then coalesce together into larger liquid drops during hear treat process; and the other is that second dendrite arms, which melt off and re-precipitate on the trunk, enclose to form liquid drops by bridge-growth. In addition, a model is built and applied to calculate the spheroidizing time of dendritic grain during isothermal heat treatment, and of which calculating formula is proposed as(3) A novel approach is designed to obtain thixotropic structure of equiaxed or non-dendrtic grains for fine-grain and net-shape forming method, and demonstrated experimentally using A356 aluminum alloy. The circumstance in bulk liquid metal, which would burst into copious nucleation, and at the same time create homogeneous distribution of temperature and solute, is constructed by combined utilization of rotating magnetic fields and a cooling tube. Both microstructures solidified in metal mold and sand mold exhibit non-dendritic characteristic. Analyses of the rheocasting A356 microstructure indicate that high density of nuclei occurs by inserting a cooling tube into rotating slurry at liquidus temperature. In the case of slow cooling rate, mushy slurry obtained with high nuclei density keeps non-dendritic morphology of primary particles with holding time, accompanying with grain particles’ coarsening and spheroidizing.(4) Damper cooling tube method (DCT) processing semisolid slurry is designed, and demonstrated with A356 aluminum alloy in the case of different pouring temperatures. It is shown that there is a direct relationship between the pouring temperature and the grain size and shape factor after comparing and characterizing the castings microstructure: the lower the pouring temperature, the smaller the grain size and shape factor, due to more stray nucleus produced. The reason for this is that the alloy melt is stirred and cooled by DCT. In addition, in comparison with cooling slope process, the grains of castings produced by DCT exhibit finer and more round.(5) The formation mechanism of non-dendritic structure resulting from high density of nuclei is analyzed in foundation on the aforementioned investigation. The theory of crystal growth front nucleation is introduced with the effect of boundary layer resulting from viscous fluid, and "surrogacy-mechanism" is proposed to explain the copious nucleation of liquid alloy with coerced convection.

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