【作者】 胡勇；

【导师】 闫洪；

【作者基本信息】 南昌大学 ， 材料加工工程， 2009， 博士

【摘要】 与其他金属结构材料相比,镁合金及其复合材料具有比强度、比刚度高,减振性、电磁屏蔽和抗辐射能力强,易切削加工等一系列优点,在汽车、航空航天等领域具有极其重要的应用价值和广阔的前景。复合材料的制备可分为外加复合法和原位生成法,与外加复合法相比,原位自生镁基复合材料的增强相在基体内反应生成,具有尺寸小、界面洁净无污染、热稳定性好、与基体相容性好及制备成本低等优点,已成为金属基复合材料中的一个重要发展方向。金属基复合材料在提高强度、硬度的同时,却降低了塑性,不利于复合材料的二次塑性加工,而传统铸造又易产生氧化夹渣、缩孔缩松等缺陷,半固态成形技术和理论不断成熟与发展,为金属基复合材料的成形提供了新的途径。本文采用原位自生法制备了Mg₂Si/AM60镁基复合材料,并对其进行了半固态流变成形研究。主要研究研究内容和获得的成果如下:从热力学和动力学两方面,探讨了原位生成Mg₂Si的机制,建立了体系原位生成Mg₂Si的动力学模型,推导了反应的动力学方程,得出影响反应速度的主要因素为:体系温度、结晶Si颗粒的大小以及扩散中间层的厚度。研究了不同结晶Si含量对原位自生Mg₂Si/AM60镁基复合材料组织性能的影响,确定了结晶Si的最佳加入量,深入分析了该复合材料的强化机理,发现增强相Mg₂Si以中国汉字状、板块状及树枝状的形态存在于基体中。研究了合金元素对Mg₂Si增强相的形态和分布及其对复合材料性能的影响,发现加入合金元素后中国汉字状的增强相转变为颗粒状,并均匀地分布于合金当中,且合金元素的加入量有一个最佳值。采用机械搅拌的方法,制备了Mg₂Si/AM60镁基复合材料半固态组织坯料,研究了不同工艺参数（如:剪切温度,剪切时间和剪切速率）对该复合材料半固态组织的影响规律,从理论和工艺上分析探讨了该复合材料晶粒球化细化的机制,发现机械搅拌可以抑制枝晶的生长,减小游离晶的长度,促进型壁晶的游离,并根据M-S理论,分析发现机械搅拌利于晶粒的球化。测定了半固态Mg₂Si/AM60镁基复合材料的稳态表观粘度,揭示了不同工艺参数（如:Mg₂Si的体积分数、剪切温度、剪切时间和剪切速率）对半固态Mg₂Si/AM60镁基复合材料的稳态表观粘度的影响规律,并建立了半固态Mg₂Si/AM60镁基复合材料的流变模型。在计算机模拟软件二次开发的平台上,将上述建立的稳态表观粘度模型嵌入模拟软件,成功地模拟了Mg₂Si/AM60镁基复合材料圆盘件的半固态流变成形的充型凝固过程,并对可能出现的铸造缺陷进行了预测,为流变压铸工艺提供了理论指导。进行了该复合材料的流变压铸试验,分析了铸件的微观组织特征。与液态压铸相比,半固态流变压铸可改善铸件的微观组织。更多还原

【Abstract】 Magnesium alloy and its composites have a great potential to be applied in automobile and aerospace industries because of their light weight, high specific strength, excellent anti-shock and good machine ability compared to other structural metals.The fabrication of magnesium composites includes ex situ method and in situ method. Compared to the conventional composites fabricated by ex situ method, the in situ synthesized composites exhibit the following advantages, such as finer grains, cleaner reinforcements and matrix interface, better thermodynamics stability, better wet ability and lower cost and so on, which makes it become an improtant development direction of fabrication magnesium composites. Metallic matrix composites reduce the plasticity, thus making against the second plasticty processing, despite the increasing of the hardness and strength. And conventional casting is prone to have the defects of oxide inclusion and shrinkage cavity and porosity. Present, with the increasingly develop of the semi-solid forming technique and theory, it offers a new way to the shaping of the metallic matrix composites. The Mg₂Si/AM60 magnesium matrix composites fabricated by in-situ synthesis and its semi-solid rheoforming were investigated in the dissertation. The main work and results are as follows:The mechanism of in situ synthesized Mg₂Si was discussed from thermodynamics and kinetics, the kinetic model of in situ synthesized Mg₂Si was established, the kinetics equation was deduced. And it was found that the major factors which influence reaction rate were: the system temperature, the size of Si particle and the thickness of diffusion middle layer.The effects of different content of crystal Si on the microstructures and mechanical properties of in-situ synthesized Mg₂Si/AM60 magnesium matrix composites were studied, the best content of crystal Si was established and the strengthening mechanism of the composites was analyzed thoroughly. It is found that the reinforced phase Mg₂Si with the morphology of the Chinese script type, polygonal type and dendritic type exit in the matrix microstructures. The effects of alloy elements on the morphology and distribution of reinforced phase Mg₂Si and the mechanical properties of in-situ synthesized Mg₂Si/AM60 magnesium matrix composites were investigated. The Chinese script type Mg₂Si were changed to granular type and distributed uniformly into the matrix microstructures by adding in alloy elements. And there is a best adding contents for elements.Semi-solid specimens of Mg₂Si/AM60 magnesium matrix composites were fabricated by mechanical stirring. The effects of different technological parameters such as shearing temperature, shearing time and shearing rate on the semi-solid microstructures of composites were investigated. The grain spheroidizing and fining mechanism of the composites was analyzed and discussed theoretically and technologically. The research indicates that mechanical stirring can restrict dendritic growth, reduce the length of free crystal and promote wall crystal to be free. According to M-S theory, it shows that it is favorabal to make the grains global by mechanical stirring.The steady apparent viscosity of semi-solid Mg₂Si/AM60 magnesium matrix composites was tested. The effects of different technological parameters such as the volume fraction of Mg₂Si, shearing temperature, shearing time and shearing rate on the steady apparent viscosity of semi-solid Mg₂Si/AM60 magnesium matrix composites were investigated. And the rheological model of semi-solid Mg₂Si/AM60 magnesium matrix composites was established.Based on the second developing platform, the steady apparent viscosity model was embedded in simulation software and the simulation of semi-solid rheoforming of Mg₂Si/AM60 magnesium matrix composites disc part was sucessfully done. The filling and solidification process were analyzed, and the casting defects were predicted, which can provide theoretical guidance for rheological casting. The rheological casting test was carried out by using the composites and the microstructures of the cast were analyzed. Compared to the conventional casting, the semi-solid rheological casting can improve the microstructure of the cast.更多还原