【作者】 杨闯；

【导师】 傅恒志； 李邦盛；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 微熔模精铸工艺可以低成本、大批量地高效制备三维复杂的微金属铸件,是近几年来问世的最具竞争力的微构件微细加工工艺之一。本文自主研发了超声场下制备纳米晶石膏铸型的新工艺,并利用离心力场下微熔模石膏型精铸工艺制备成形了直径小至100μm的微尺度铸件。并对微熔模石膏型精铸过程中涉及的超声细化形核规律、微尺度液态金属润湿规律、微充型流动规律及其停止流动机理、微圆丝的显微组织与力学性能等问题进行了深入系统研究,揭示了微尺度条件下,润湿、充型流动、形核、显微组织和力学性能等方面的微尺度效应及微观机理。研究了微尺度金属液滴在原子级平整的云母平面上的微润湿规律,发现微观尺度时,润湿角则随着金属液滴尺寸的减小而降低,表现出了显著的微尺度效应,微尺度与宏观尺度的临界分界点为rc=0.33 mm,只有在金属半径为大于此值的宏观尺度时,润湿角才不随半径变化而保持常数。在经典的Young方程基础上,改进并推导出了微尺度液滴在理想平面上的铺展动力学方程表达式,并借助此公式进行了铺展动力学的计算机数值模拟计算。依据霍尔兹曼自由能保持不变的热力学判据,推导出了微小液滴在测量微润湿角时重量可否被忽略的判据。借助超声空化理论和形核热力学理论,推导出了超声力场下石膏晶粒形核率的表达式。依据形核率公式得出了形核率与温度和压力的相互影响规律,研究发现,超声力场作用可以极其显著地改变石膏晶粒的形态和细化石膏晶粒尺寸,施加超声力场后,石膏晶粒由重力场下长径比高达十几倍的细针状,转变为超声力场作用后的近球形;而石膏晶粒尺寸由重力场下的15~30μm,显著细化至超声力场的500 nm。通过研究不同烧结温度以及铸型成份对微铸型表面粗糙度的影响,确立了微熔模精密铸造用铸型的最佳烧结温度和最佳铸型成份,并制备具有低表面粗糙度的纳米晶石膏微铸型,可完全满足成形微米尺度铸件对于低表面粗糙度铸型的严苛要求。在重力场条件下宏观尺度流动的传统Navier-Stokes微分方程以及微尺度下流动的新型Navier-Stokes微分方程的基础上,本文改进并建立了离心力场下的微尺度流动的Navier-Stokes全新微分方程。借助相似物理模拟理论,以及离心力场下的微尺度流动的Navier-Stokes微分方程,推导出了离心力场下流体在微尺度下离心流动的相似准则。以特殊成份的水溶液作为模拟流体,对Zn-4%Al合金熔体在石膏铸型流道内的微尺度流动规律进行了相似物理模拟研究。结果表明:离心力场下流体在横浇道内的微充型流动过程中,优先通过横截面积最大的流道;在充型流动过程中,流体的总能量保持不变,即位能、压力能、平动动能和离心转动动能的总和不变;在流体流动过程中,会发生动能和势能之间的相互转化,并以逐层方式平稳的充填流道;在整个充型流动过程中,流体的自由液面始终以转轴为圆心的规则圆弧面。在重力场下伯努利流动方程的基础上,附加考虑离心力的作用,推导出了离心力场中微尺度条件下流体总流流动的伯努利方程。并在此伯努利方程的基础上,推导出了离心力场下微尺度充型流动时浇注系统可否充满流动的判据表达式,以及离心力场下微尺度充型流动时阻流断面的水力学计算公式。借助自主研制的纳米晶石膏基铸型以及离心力场微熔模精密铸造工艺,实际铸造成形了不同直径的Zn-4%Al合金微尺度铸件。实验结果表明:液态金属在微流道中充型流动时,其充型长度随着微流道半径的减小而降低,随着旋转速度和铸型预热温度的增加而提高,并呈现出明显的微尺度效应。采用离心力场微熔模精密铸造工艺,可制备出的微圆丝最小直径细至100μm,充型长度可高达20mm,长径比高达200倍。当微圆丝的直径大于400μm时,液态金属在微流道中充型长度增加非常明显,从而确定出400μm为充型流动的宏观尺度与微尺度的临界分界点。研究发现微熔模石膏精密铸造过程中,离心力场下微尺度管道内液态金属的停止流动机理为“整体堵塞”的停止流动机理,明显不同于液态金属宏观尺度流动时的“端部堵塞”停止流动机理,呈现出显著的微尺度效应。研究发现,微圆丝的显微组织和力学性能表现出了强烈的微尺度效应:预热温度显著影响微尺度条件下晶粒尺寸:随着铸型预热温度从130℃增加到270℃,微圆丝的晶粒尺寸增大约10倍,共晶组织平均片层间距也增加约5倍。当平均晶粒尺寸减小10倍时,微圆丝抗拉强度提高约1.7倍,拉伸强度与平均晶粒尺寸之间符合Hell-Petch公式;随着微圆丝直径从300μm到100μm,微圆丝的纳米硬度增大约1.1倍,与宏观尺度铸件的硬度0.72 Gpa相比,达到了宏观尺度铸件的4.9倍左右。更多还原

【Abstract】 Micro precision casting based on gypsum can effectively prepare complex three dimensional micro-metal components with low cost and mass. It is one of the most competitive micro-component processing crafts in the last few years.With the aid of self-developed nano scale gypsum as mold materials and using the centrifugal micro precision casting craft based on gypsum, we actually prepare micro scale casting in the diameter of 100μm. The questions in this work including the refining grain law of nucleation in ultrasonic field, the wetting law of liquid metal in micro scale, the filling law and mechanism of cessation in micro scale and the microstructure and mechanical properties of micro castings were researched. The micro scale effects and microscopic mechanism on wetting, filling flow, nucleation, microstructure and mechanical properties have been found in the work.The micro wetting law of micro scale metal droplet on mica with atomically smooth was studied. The significant micro scale effects were found in micro scale that the contact angle decreases with reducing the size of metal droplet. The critical demarcation point is rc=0.33 mm between the macroscopic and the microscopic scale. The contact angle is not changed with varing the radius of contact line when r>rc.Spreading kinetics equation of micro scale liquid droplet on ideal surface was improved and derivated by classic Young equation, and with which the calculation of numerical simulation was completed. The criterion that gravity is ignored when contact angles are tested was derivated through thermodynamic criterion on the constant of Holzman free energy. The nucleation rate eqution of gypsum grain in ultrasonic field was derivated bycavitation theory and thermodynamic theory of nucleation, and the relationship between nucleation and temperature or pressure was acquired by the factors of nucleation rate. It is found that the grain shape and the grain size of gypsum were significantly changed and refined by ultrasonic field. The needle shape of gypsum grain was varied to nearly spherical in the ultrasonic field, and the grain size with 15~30μm was refined to the 500 nm in the ultrasonic field. We studied the influence of different sintering temperature and the mold component to the surface roughness of micro mould. As a result, the micro mould of gypsum with nano grain in high surface roughness were prepared, and which meet fully the needs of micro scale castings.Based on the macroscopic Navier-Stokes equation in the gravity field and the micro scale Navier-Stokes equation, micro scale Navier-Stokes equation was established and improved under the centrifugal field. The microscale similar criterion in centrifugal force was derivated by the theory of similar criterion and microscale Navier-Stokes equation in centrifugal force. Taken water solution of special ingredients as simulation fluid, the flow rule of Zn-4%Al alloy in micro scale mould prepared by gypsum was simulated. It is found that the liquid pass firstly through the runner with biggest cross section area during filling of fluid in centrifugal force. The total energy of fluid including potential energy, pressure energy and kinetic energy is an constant during filling, and transforms between kinetic and potential energy. The liquid fills smoothly channel in the way to successive step, and the fluid free head is regular circular arc surface which takes the revolution axis as the center of circle.The total flow Bernoulli’s equation under the condition of micro scale in the centrifugal force was acquired by adding centrifugal force into Bernoulli’s equation in the gravity. According to it, hydraulics equation of section and gating system been fall of liquid with micro scale in centrifugal force were deduced.With the aid of self-developed nano grain of gypsum as casting mould and using the micro precise centrifugal casting, different diameter parts of Zn-4%Al alloy with micro scale were perpared. It is found in the experiments that the flow length of micro round wires shows obviously micro scale effects, or the flow length decreased with decreasing diameter of micro round wires and incresed with increasing centrifugal speed and preheating temperature. The smallest diameter of micro round is 100μm, its flow length achieves 20 mm and the aspect ratio reaches up to 200. The flow length increased significantly when the diameter of micro round wire is more than 300μm. And it is determined in the research that the critical demarcation point of diameter is 300μm between macroscopic scale and microscopic scale. At the same time, the mechanism of cessation in the micro channel of liquid metal in centrifugal field is mechanism of the overall blockage. It shows micro scale effects and is sigificantly different from the cessation of end blockage in macro scale.It is found in the research that microstructure and mechanical properties of micro round wires appear obviously micro scale effects.The grain size also affected by preheating temperature under the micro scale condition. With increasing preheating temperature from 130℃to 270℃, the grain size increased about 10 times, and the interlamellar spacing of eutectic structure increased also approximately 5 times.The tensile strength of micro round wires increased approximately 1.7 times when the average grain size decreased 10 times. The relationship between them is in accordance with hell-petch equation; The nano hardness of micro round wires increased approximately 1.1 times with deducing the diameter from 300μm to 100 μm, and is approximately 4.9 times to the castings in macrosopic scale whose hardness is 0.72 GPa.更多还原