Research On Forming And Filling Flow Law In Microscale For Micro Precision Casting Process Based On Investment

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.