【作者】 吴晓；

【导师】 李建军；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2011， 博士

【摘要】 微型零器件在微机电系统(MEMS).通讯、电子、国防、医学等领域具有广阔的应用前景,这些行业的发展对微成形技术(Micro-forming Technology)提出了更高的要求。非晶合金一系列优良性能使其成为最理想的微成形材料之一,因此,非晶微成形技术引起了人们的广泛关注。但人们对非晶材料在微成形过程中的变形和流动特征、尺寸效应的产生机理以及影响因素仍不清晰；非晶合金微成形工艺参数选择、微成形模具的设计与制造理论还很不完善；另外,超塑性微成形是否影响非晶材料组织和性能也不得而知。有鉴于此,本文以锆基非晶合金为对象,系统地开展了非晶合金超塑性微成形技术的基础研究和工艺研究。首先,系统地研究了块体非晶合金Zr55Cu30Al10Ni5在其过冷液态区内不同温度下的单轴压缩变形行为以及不同载荷模式下的微成形能力。结果表明,其流变特性强烈依赖于温度和应变速率,在690 K的温度下压缩时,该合金过冷液表现出明显的非牛顿流特征,而在690 K以上的过冷液态区内,随着应变速率的增加,Zr55CU30Al10Ni5非晶过冷液的流动特性逐渐由牛顿流向非牛顿流体转变,而且发生这种转变的临界应变速率值随温度的升高而增大,725K时,临界应变速率降到约为10-2s-1。另外,振动载荷能降低非晶合金的流动粘度,提高其微成形能力。增加载荷的振幅或者在低频阶段增加频率都有利于提高非晶合金的微成形能力。选用DEFROM软件,对块体非晶合金Zr55Cu30Al10Ni5在过冷液态区内的超塑性微反挤压过程进行了有限元模拟分析。结果表明,随着挤压速度的升高,非晶合金由牛顿流转向非牛顿流体,指出块体非晶合金Zr55Cu30Al10Ni5的最佳微反挤压工艺参数为：温度725 K,挤压速度高于4μm.s-1。在数值模拟研究的指导下,分别选用块体非晶ZrssCu30Al10Ni5和Zr65Cu15Ni10Al10作为实验材料,对不同壁厚的简单杯形零件和变截面三维复杂零件的微反挤压工艺进行了实验研究,分析了温度、挤压速度等工艺参数对非晶合金的微成形能力以及成形质量的影响规律。成功挤压出外径为2.2 mm,壁厚只有50μm的简单杯形非晶零件以及变截面三维复杂微型非晶零件,零件的形状和尺寸符合设计要求,表观质量良好。选用非晶合金Zr65Ni10Al10Cu15为实验材料,分别设计了外径为1.2 mm和0.6mm两个系列杯形零件的微反挤压方案,通过理论分析和实验研究,系统地探讨了非晶合金在微反挤压过程中的尺寸效应现象。结果表明,随着坯料尺寸或者零件壁厚的减小,表观粘度单调增加；当零件内外径之比大于0.83后,非晶合金微反挤压成形表现出明显的尺寸效应。进一步分析发现,零件尺寸和壁厚减小引起的比表面积增加,导致微反挤压过程中摩擦、表面能等因素的影响急剧上升,这是产生尺寸效应的根本原因。在泊肃叶方程的基础上,结合尺寸效应分析,推导出计算非晶合金微反挤压成形载荷的理论公式。利用显微维氏硬度计(Vickers Indenter).纳米压痕仪(Nano Indenter)系统地分析了不同尺寸非晶零件在微反挤压成形过程中的变形特征和性能变化。研究发现,对于大壁厚零件,其硬度与铸态非晶合金的硬度接近；而对于薄壁零件,其纳米压痕硬度低于铸态非晶合金。另外,零件纵截面上的显微硬度明显不均匀,杯壁内侧拐角处的硬度最大,而杯口处的硬度最低,相对差值到达14%。分析认为,成形零件的性能变化以及分布不均匀主要是由于非晶坯料在复杂应力状态和局部强烈剪切变形引起的自由体积变化所致。本论文的研究结果为开发非晶合金微成形工艺提供了可靠的理论依据,同时,也为扩大非晶合金的工业化应用范围奠定坚实的理论基础。更多还原

【Abstract】 Micro-parts and micro-devices have broad application prospect in many areas, such as micro-electro-mechanical-system (MEMS), communication, electronics, national defense, medical science, etc. The development of these industries put forward a higher requirement on micro-forming technology. Bulk metallic glass (BMGs) have lots of superior propertiesis and they are one of the most ideal micro forming materials, micro forming technology of BMGs have attracted wide attention in recent years. But the deformation and flow characteristics of BMGs during the micro forming, size effect and the influencing factors are still not clear; the selection of micro forming parameters and the design and manufacture theory of micro die are not perfect; In addition, whether the micro-structure and properties of BMGs will be influenced by superplastic forming is also unknown. In view of those, systematically basic research and technology research of superplastic micro forming technology of BMGs are carried out in this paper.First, the uniaxial compression deformation behaviors of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) in the supercooled liquid region and the micro-formability under different load mode are studied systematically, the results show that the deformation behavior of the BMG is strongly dependent on the temperature and strain rate, and there exists obvious stress overshoot when deformed at a low temperature or high strain rate. It exhibited non-newtonian behavior at a temperature of 690 K, but when deformed above 690 K, it will chang from a Newtonian flow at low strain rates to a non-Newtonian flow at high strain rates, the critical strain rate increased with the increasing in temperature and is about 10-2 s-1 at the temperature of 725 K. In addition, the study shows that vibration load can improve the micro-formability of MBGs by reducing the viscosity, higher amplitude or higher frequency is helpful to improve the micro-formability.The superplastic micro back-extrusion process of Zr55Cu30Al10Ni5 MBG supercooled liquid is investigated by using DEFROM software.The results shows that with the increasing of extrusion speed, the MBG supercooled liquid change form a Newtonian flow to a non-Newtonian flow, and points out that the optimum extrusion process parameters of Zr55Cu30Al10Ni5 MBG are:at a temperature of 725 K and extrusion speed above 4μm.s-1.Choosing cup-shaped parts with simple cross-section and three-dimensional complex variable cross-section as research objects, the micro back-extrusion process of Zr55Cu30Al10Ni5 and Zr65Cu15Ni10Al10 BMGs in their supercooled liquid region are studied under the guidance of numerical simulation, the influence law of the main process parameters, such as temperature and extrusion speed, on the forming quality and micro formability of the BMGs are analysed. A micro-cup-shaped object with an outer diameter of 2.2 mm and a single side wall thickness of only 0.05 mm, as well as a micro part with a three-dimensional complex variable cross-section are successfully extruded. Scanning electron microscopy shows that the parts, with good apparent quality, maintain amorphous structures, the shapes and sizes of the parts meet the design requirements.Two series of cup-shaped micro parts with the out diameter of 1.2 mm and 0.6 mm but different wall thickness were set as the objects, the size effect during the micro back-extrusion process of the Zr65Cu15Ni10Al10 BMG are systematically investigated. The results show that the apparent viscosity monotonously increases with the decreases of overall dimension or the wall thickness, and there exits obviously size effect when the ratio of inner diameter to outer diameter is greater than 0.83. Further analysis found that the rapid increase in the specific surface area resulted by the decreases in parts size and wall thickness causes the rapid increase in the micro friction and surface energy, and those contributes to size effect. Based on the Hagen-Poiseuille-equation, a equation for calculating the required extrusion load is deduced.The deformation characteristics and property changes of the BMGs during in the micro back-extrusion process were systematically analyzed by means of micro-hardness measurement and nanoindentation, the study found that the hardness of thick-walled parts is similar to this of as-cast amorphous alloy, but the nano-indentation hardness of thin-walled parts is smaller than that of as-cast amorphous alloys. In addition, the micro-hardness distribution of the longitudinal section is far from uniform, the largest hardness was found at the inside corner and the lowest hardness at the edge place, the relative difference value is about 14%. The free volume change caused by the complex stress state and local strong shear deformation is the main cause for the property changes and the uneven distribution of microhardness.These research results supply a credible theoretics bases for the development of BMGs micro forming technology, and also provide a stable academic foundation for expanding the industrialized application range of BMGs.更多还原