【作者】 李鹏宇；

【导师】 王旭东；

【作者基本信息】 大连理工大学 ， 材料与化工（专业学位）， 2023， 硕士

【摘要】 脉冲微孔喷射法制备微米级球形粒子是典型的基于液滴喷射成形、无容器传热和凝固过程,制备出的球形粒子具有粒径均一、圆整度高、热履历一致等特点,在增材制造、微电子等领域具有不可替代的作用。鉴于粒子在成形、尺寸、传热特征及制备过程等方面的特殊性,建模和数值计算成为探讨液滴成形、粒子传热与凝固行为的首要途径。本文基于有限差分法,在三维球坐标系下建立了描述金属粒子传热、凝固过程的数值计算模型,对粒子冷却过程中的温度变化、传热特点和凝固进程进行计算和分析,把握影响粒子换热的主导机制和主要因素,在此基础上,针对POEM法喷射特征建立了微球粒子喷射模型,还原液滴成形过程,计算并讨论不同物性参数对液滴成形行为的影响,本文主要工作内容如下:不同直径金属粒子在初速为2m/s、1个标准大气压Ar气环境中冷却时,150～400μm粒子在0.23s以内完全凝固,400μm直径粒子的凝固时间是150μm粒子凝固时间的3倍以上,凝固过程中径向温度梯度峰值在14527～23197°C/m范围变化,冷却速率随直径的增加大幅下降,晶粒尺寸随冷却速率呈负指数幂形式变化,150μm粒子的冷却速率与凝固速度峰值分别达到了7720°C/s和31250μm/s;在粒子冷却的不同阶段,对流换热始终是影响总体热流的主导因素,凝固过程中对流换热比例不断增大,且对流换热的比例随粒子直径的减小显著增大,是粒子凝固过程传热的主要机制;粒子在He气环境中换热效率远大于Ar气,200μm粒子在He气环境中的平均凝固速度约为Ar气中的4.2倍;随下落初速和压强增大,粒子凝固时间缩短,200μm粒子在Ar气环境中冷却时,2、3atm压强下的凝固时间分别为1个标准大气压的84.8%,76.1%;液滴成形过程可分为液面下沉、液滴颈缩、液面回弹三个阶段;液滴体积与微孔直径d_N、传动杆运动位移?z、传动杆向下运动时间t、坩埚内外差压P_b及坩埚内金属熔体液面高度h等因素呈正相关;喷射过程中,表面张力越大,克服表面张力束缚所需能量越多;黏度越大,金属熔体克服阻力流动消耗能量越多;金属熔体与坩埚材料之间应互不润湿,润湿角越大,金属熔体填满微孔所需能量越多,上述因素都会导致形成新表面能量减少,从而引起液滴体积减小。此外,表面张力增大会使液滴出现时间变晚,黏度增大会延迟液滴颈缩时间。传动杆位移与传动杆位置是影响液滴体积的主要因素,传动杆位移增大,液滴体积增大,初始速度越大,相同时刻距微孔底部距离越远,位移小于15μm时金属熔体无法喷出微孔完成颈缩;传动杆运动相同位移时,临界范围内,液滴体积随传动杆距微孔距离增大表现出先增大,后减小的趋势;临界范围外,金属熔体无法喷出微孔。液面差压是影响液滴体积的次要因素,液面差压增大,液滴体积小幅增大。上述计算结果为POEM法微米级球形粒子制备及凝固过程调控提供支持。更多还原

【Abstract】 The preparation of micron-sized spherical particles by the Pulsed Orifice Ejection Method is a typical process based on droplet spraying,unconstrained heat transfer,and solidification.The prepared spherical particles have the characteristics of uniform particle size,high roundness,and consistent thermal history.Material manufacturing,microelectronics and other fields play an irreplaceable role.In view of the particularity of particles in terms of forming,size,heat transfer characteristics,and preparation process,modeling and numerical calculations have become the primary way to investigate droplet formation,particle heat transfer,and solidification behavior.In this paper,a numerical calculation model is established utilizing the finite difference method to describe the heat transfer and solidification of metal particles within a three-dimensional spherical coordinate system.The model is employed to analyze and calculate the temperature variation,heat transfer characteristics,and solidification process of particles during cooling,with the aim of comprehending the dominant mechanism and principal factors influencing particle heat transfer.Subsequently,a microsphere particle injection model is formulated based on the injection features of POEM,and the droplet forming process is reconstructed.Furthermore,the influence of distinct physical parameters on the droplet forming behavior is evaluated and discussed.The primary objective of this study is to present the following content:The solidification behavior of metal particles with varying diameters in an Ar gas environment was investigated in this study.At an initial velocity of 2 m/s and 1 standard atmospheric pressure,the 150-400μm particles were completely solidified within 0.23 s,with the solidification time of the 400μm diameter particles being more than three times longer than that of the 150μm particles.The peak value of the radial temperature gradient changed in the range of 14527～23197°C/m,and the cooling rate decreased sharply with increasing particle diameter.The grain size changed with the cooling rate in the form of a negative exponential power.The peak cooling rate and solidification rate of 150μm particles were found to be7720°C/s and 31250μm/s,respectively.Convective heat transfer was identified as the dominant factor affecting the overall heat flow during different stages of particle cooling.The proportion of convective heat transfer increased continuously during solidification,with a significant increase observed with decreasing particle diameter,which was identified as the main mechanism of heat transfer during particle solidification.Notably,the heat transfer efficiency of particles in a He gas environment was much higher than that in an Ar gas environment,and the average solidification speed of 200μm particles in He gas was approximately 4.2 times higher than that in Ar gas.Additionally,the solidification time of the particles decreased with increasing falling initial velocity and pressure.When the 200μm particles were cooled in an Ar gas environment,the solidification times under pressures of 2and 3 atm were 84.8%and 76.1%of the standard atmospheric pressure,respectively.In the droplet forming process,three stages were identified,namely liquid surface sinking,droplet necking,and liquid surface rebound.The droplet volume and micropore diameter d _N,transmission rod movement displacementΔz,transmission rod downward movement time t,differential pressure P_b inside and outside the crucible,and the height h of the metal melt in the crucible were found to be positively correlated.A greater surface tension was found to require more energy to overcome surface tension constraints,while higher viscosity consumed more energy to overcome resistance flow.Wetting between the metal melt and crucible material was deemed unfavorable.The larger the wetting angle,the greater the energy required for the metal melt to fill the microholes.These factors caused a reduction in the energy required to form new surfaces,leading to a reduction in droplet volume.Furthermore,an increase in surface tension delayed droplet emergence time,and an increase in viscosity delayed droplet necking time.The displacement and position of the drive rod were identified as the primary factors influencing the volume of the droplet.An increase in displacement resulted in an increase in droplet volume and initial velocity,accompanied by an increase in the distance from the bottom of the microhole.When the displacement was below 15μm,the metal melt failed to be ejected from the microhole to achieve necking.When the drive rod moved with the same displacement,the droplet volume initially increased and then decreased within the critical range of the distance between the drive rod and the microhole.Outside this critical range,the metal melt was unable to be ejected from the micropores.The liquid level differential pressure was determined to be a secondary factor influencing the droplet volume,with a slight increase observed as the liquid level differential pressure increased.These findings support the preparation of micron-sized spherical particles and the regulation of solidification processes by POEM.更多还原