【作者】 肖泽辉；

【导师】 罗吉荣； 吴树森；

【作者基本信息】 华中科技大学 ， 材料科学与工程， 2005， 博士

【摘要】 镁合金具有小的密度、高的比强度、比刚度,好的导热性,良好的阻尼减震能力等优良性能,符合对材料的轻量化和绿色化的要求,其表现出强劲的发展势头。半固态技术综合了液态铸造成形、固态压力加工的优点,被认为是二十一世纪最具有发展前途的近净成形方法之一,得到了越来越广泛的关注。目前国内外针对镁合金半固态流变成形方面的研究相对较少,本文采用自我开发的双螺杆机械搅拌制浆机制浆,与冷室压铸机配合,进行镁合金半固态流变压铸成形研究。该技术为镁合金产品的开发和半固态技术的发展提供了新的有效途径,具有重要的学术意义和较大的实际应用价值。针对镁合金高温熔体易氧化、燃烧的特点,研究开发了容积定量、气压输送的镁合金高温熔体输送装置,填补了国内空白。该装置能有效防止输送过程中镁液产生氧化、夹杂等现象,输送量误差能控制在±5%范围内。分析了镁合金半固态浆料的特点,设计了组合式、多壁厚型腔的压铸成形模具。设计了一套简易、可行的压铸压射速度测量装置,该测量装置能测量的最短距离为3cm,时间精度为1ms。通过压射压力曲线分析及理论计算,确立了镁合金半固态浆料在压射室内存在理想流变形态时的流变压铸工艺参数:压铸模温度、压射室温度、慢压射速度和一快压射速度的影响; 并通过传热学分析,建立了浆料在压射室内停留,其温度的降低与压射室温度、固相率、压射室涂料及停留时间的本构关系方程。试验研究了镁合金液态和半固态浆料在不同内浇口尺寸和不同内浇口速度条件下的流变压铸充填过程。在试验条件下,液相表现为强烈的紊流充型,而固相率为15 %半固态镁合金则表现为弱的紊流充型; 固相率为40 %、50 %呈现明显的层流充填,并同时表现出一定的胀流型特征。研究了镁合金半固态流变压铸成形宏观组织与流变流程的关系。结果表明,流变流程对组织的影响与固相率密切相关,其横断面组织呈现不均匀性,表面存在固更多还原

【Abstract】 Magnesium alloy, which fulfills the requirements of lightweight and friendly to environment for materials, has very vast development foreground as a new engineering materials because of its low density, high strength-to-weight ratio, high modulus, good heat conductivity, superior damping characteristic, good machinability, and so on. Semi-solid metal processing technology, synthesized the merits of liquid-casting technology and solid-press forming technology, is catching many researchers’attention in various fields, and regarded as one of the near net-shape forming technologies with wide application in 21 century. However, research on semi-solid rheomoulding for magnesium alloys is seldom uptodate worldwide. In this paper, the rheo-diecasting process of semi-solid magnesium alloy has been studied with a self-made twin-screen slurry mixer, cooperated with cold-chamber die casting machine. Therefore, this advanced technique opens a new route for application of magnesium alloy products and development of semi-solid metal processing, and it is of great importance for academic research and industrial application. In order to solve the problem that magnesium alloy is easy to be oxidizd and combust at elevated temperature, a delivery pump has been developed with capacity dosing and gas-pressure transferring methods, and it is a originated technology in China. Delivery error can be controlled within ±5%. The characteristic of magnesium semi-solid slurry has been analyzed. A die casting mold was designed with assembly multi wall thickness cavities. A measure device was set up for shot speed, which was simple in structure and reliable in accuracy. The least length can be measured was 3cm, and the measure time error was in 1ms. By analysis of injection pressure curve and theoretical calculation, the influence of the die temperature, shot chamber temperature, slow shot speed, first fast shot speed on the rheological flow of semi-solid slurry of magnesium alloy in shot chamber were investigated. The constitutive equation of the slurry temperature reduction was established in relation with shot chamber temperature, solid fraction, shot chamber coating and stay time after thermodynamic analysis. The effects of the ingate velocity and ingate dimension upon mould filling process of liquid magnesium and semi-solid slurry were systematically studied. The results showed that: liquid flows in strong turbulent flow mode; and the slurry at solid fraction of 15 % flow in weak turbulent flow mode; and the slurry at solid fraction of 40 % or 50 % flows in laminar current obviously, but also has feature of expansion fluid. The relation between the macrograph structure of the semi-solid magnesium alloy formed by rheo-diecasting and mould filling route was comparatively surveyed. The results showed that: the correlation between the structure and solid is great, the texture of the cross-section presents non-homogeneous feature, the surface appears chilling layer, and the center appears aggregation of the solid grains. At the meantime, the thicker the wall and the bigger the solid fraction, the more non-homogeneous structure, and the more the aggregation in the center area. When the thickness reaches 5 mm , the chilling layer appears in multi-layers, and fluid transition region arises. The surface of the primary crystal grains was studied. In the process of making slurry by the twin-screen mixer, a turbulent boundary layer emerges on the surface of the grain with constitutional undercooling, the surface of the grains has cellular structure, which promotes the primary crystal as granule. The groove of the surface increases surface energy, and accelerates aggregation and agglomeration of the grains. The grooves enlarge the meshing system, increase the transient shear stress. The mechanical properties of the rheo-diecasting were tested. The hardness and density increase with increasing of the solid fraction, and when the solid fraction was 40 % approximately, the tensile strength and the elongation were best, and the solid fraction was up to 58 % or so, the tensile strength and the elongation were decreased to low level. After heat treatment, the mechanical properties can be increased, overcoming the nonheat-treatable shortage of normal die casting.The microscopic fractographs of the rheo-diecasting specimen were carefully analyzed. The fractographs of the liquid diecasting sample were typical quasi-cleavage fracture, and as the solid fraction increased, the equiaxed dimple increased in the fractograph of the rheo-diecasting on the SEM. The liquid quantity around the primary crystal grains affected the mechanical properties greatly. When the solid fraction is low, the liquid quantity around the primary crystal grains is large, the mechanical properties are decided by the liquid mainly; as the solid fraction increases, the liquid quantity decreases, the primary crystal grains impact, the properties improve. As the solid fraction increases to some degree, the liquid film of the grains can not be integrate, and create holes, the properties decrease. The aggregation of the grains aggravate the tendency of the decreasing. When the solid fraction is 50 % or so, there are many holes among the grains, and the properties decrease abruptly.更多还原