【作者】 罗武生；

【导师】 王成勇；

【作者基本信息】 广东工业大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 微磨料浆体射流抛光技术是在磨料水射流加工技术基础上发展起来的集流体力学、表面技术于一体的一种新型精密加工技术。其具有切削力低、无热变形、无污染、材料利用率高、几乎能抛光所有的材料和几何形状等优点,特别是在微小复杂曲面零件的精密抛光等方面,具有其它传统技术无可比拟的灵活性。目前国内外对微磨料浆体射流抛光技术只进行了一些探索性实验研究,还没有形成系统的理论体系。本文研究的微磨料浆体射流抛光技术结合了射流抛光技术和非牛顿流体技术的特点,利用添加高聚物聚丙烯酰胺(PAM)对射流束产生集束稳定作用,提出利用非牛顿流体的低压微磨料浆体射流对复杂精密金属模具局部实现可控切除量的精密抛光加工基础理论问题和加工工艺。采用理论、实验与计算机仿真相结合的研究方法,以高速摄影、冲击力测量、扫描电镜为手段研究了在普通微磨料水射流中加入高分子长链聚合物包覆微磨料和用流态化理论来混合浆体与微磨料来改善浆体的射流特性；采用计算流体力学和理论推导研究了微磨料浆体射流形成过程中浆体和磨粒的动力学特性、喷嘴的效率及系统能量传输效率；采用有限元分析研究微磨粒冲蚀工件的材料去除机理和工件材料的应力、应变分布规律及工件材料的等效塑性应变的影响因素：实现用微磨料浆体射流抛光40CrMnMo7模具钢。本文主要研究内容如下：(1)流化混合时,流化床混合罐内微磨粒在流化床中心位置上升,而在流化床沿壁的环形区下降,这个上升和下降的反向零点速度发生在无因次半径的0.6～0.7的位置范围内。反向零点的无因次半径位置的值随着浆体粘度和磨料粒径的增加而增加。流化床内沿着高度径向方向的浆体速度和浓度断面呈现抛物线形状。(2)在射流压力分别为0.6MPa,1.2MPa和1.8MPa的压力下,用计算流体力学(CFD)仿真喷嘴出口的浆体最大速度分别为34.2m/s,48.4m/s和59.3m/s。CFD模型预测射流速度无论在趋势上还是数值上都能很好地符合实验数据和理论公式得出的数据。喷嘴出口到大气的一段时间内,动压、浆体速度继续增大,当在一定距离处达到最大的动压和速度后,动压和速度开始线性递减,然会非线性递减。磨料浆体射流的加工应用区域是速度常数和线性递减的区域。在微磨料浆体射流过程中,磨粒首先被加速直到接近浆体相的速度,当浆体和磨粒在集中管同时推进时,浆体保持一个比磨料高的加速度,从出口到大气,浆体相开始减速,由于磨粒和浆体存在相对速度,磨粒继续以较低的加速度加速,直到与浆体相速度达到一致。磨粒和浆体的滑移速度随着离出口越远而减少,直到滑移速度收敛到零。(3)纯水射流、磨料浆体射流和加入高聚物的磨料浆体射流对Cr40MnMo7工件材料的冲击力信号的密集程度、峰值大小依次为添高聚物磨料浆体、磨料浆体、纯水射流。微磨料浆体射流与纯水射流的冲击力约相差20%。随着射流压力增大,喷嘴效率会降低,射流冲击力与水力能量之比也会降低,可通过射流冲击力与水力能量之比来评估喷嘴效率和优化喷嘴的结构。(4)磨粒撞击工件的最大残余应力、最大应力与撞击速度呈线性关系,冲蚀率与撞击角度呈类似抛物线关系。在75°撞击角度出现一个冲蚀应力的峰值,在55°左右出现应变的峰值。经过3个粒子冲击后体积去除率保持稳定。带尖角的磨粒形状对冲击工件点冲击导致较大的应力和材料去除率。当冲击角小于15°时的小角度冲击工件时,浅的耕犁和粒子的卷边是主要的冲蚀方式,当冲击角大于15°小于750时,材料的冲蚀方式主要是微切削和深犁耕,当以冲击角大于75°小于90°时,则主要是以凹坑和隆起对材料去除起作用。入射角度在30°～60°之间时,材料去除量随着入射角度的增大而增大,当入射角在60°到75°范围内时,材料去除量达到最大,然后在750～90°范围内,材料去除量又随着入射角度的增大而逐渐减少。(5)微磨料浆体射流抛光40CrMnMo7模具钢表面较微磨料水射流抛光时,磨粒对工件的嵌入少,定点较大材料的去除抛光较喷嘴有一定移动速度的较少材料的去除抛光嵌入工件表面的磨粒相对较多。长链高聚物聚丙烯酰胺(PAM)的加入,磨料浆体射流抛光特性在过渡面陡峭切痕较深,在较小喷射距离的范围内,添加高聚物的磨料射流对材料的切除深度是普通磨料水射流的1.5-2倍,但在较大的喷射距离范围内(通常大于40mm),去除深度存在明显的差异；添加高聚物#2000SiC磨料颗粒对工件抛光45min,工件表面粗糙度能够达到Ra=0.049μm。微磨料浆体射流抛光,微磨粒适宜的使用周期是连续循环使用一次。更多还原

【Abstract】 With the development of abrasive water jet machining technology, micro-abrasive slurry jet polishing technology is proposed. It is a new type of precision processing technology which is combined the set of fluid mechanics with surface technology. Micro-abrasive slurry jet polishing technology is high flexibile compared with other traditional machining techniques due to its characterization of low cutting force, no heat distortion, non-polluting, high material utilization ratio and polishing workpieces with almost all of the materials and geometry shapes, especially the small complex surface parts. Currently, it is only carried out some exploratory experimental study on the micro-abrasive slurry jet polishing technology in our country and abroad, and there is no theoretical system. In this paper, it is proposed that complex precision metal molds is polisheded partially and cutting amount is controllable by polishing basic theory and machining technology of micro-abrasive slurry jet, which is combined jet polishing technology and low pressure characteristics of non-Newtonian fluid. At the same time, polyacrylamide (PAM) polymer added into slurry can stabilize the jet beam and have effect of cluster. Long-chain polymers are added into micro-abrasive water jet and mixed with water in the fluidized bed to improve the properties of the slurry jet It is also studied the dynamics of slurry and abrasive particles motion, nozzle efficiency and system energy transfer efficiency, material removal mechanism of micro-abrasive particles erosion into the workpiece, effect factors analysis of stress and strain and equivalent plastic strain during the process of polishing mold steel 40CrMnMo7 with micro-abrasive slurry jet. The paper uses the research methods combined theory with experiment and computer simulation. The results are characterized by high-speed photography, impact measurement, scanning electron microscopy and three-dimensional surface morphology. Main researches and conclusions are described as follows:When micro-abrasive particles are mixed with water and the slurry is fluided in the mixing tank as the fluidized bed, they are raised in center position of the fluidized bed and decreased in the annular region near the wall. The rise and fall positions of the reverse zero point velocity are in the range of 0.6-0.7 times of non-dimensional radius, which is increased with the increasing of slurry viscosity and abrasive particle size. The slurry velocity and cross-section concentration distribution is parabolic along the radial direction of the fluidized bed.Established a micro-abrasive slurry jet computational fluid dynamics (CFD) model, The maximum slurry velocity in the nozzle exit is simulated respectively as 34.2,48.4, 59.3m/s under the condition of jet pressure 0.6,1.2 and 1.8MPa based on FLUENT analysis software. Simulation and experimental results and theoretical calculation results are in good agreement. With the increasing of jet distance from nozzle exit to the atmosphere, the dynamic pressure and slurry velocity increases continuously, up to the maximum, then linearly decreases, then is non-linear decline. The machining application region of micro-abrasive slurry jet is the region where the velocity keeps constant and linearly decreases. In the process of micro-abrasive slurry jet, abrasive particles is firstly accelerated until its velocity is close to the slurry velocity, then the slurry maintains a higher acceleration than abrasive particles when they are advancd simultaneously in the concentration tube, finally the slurry begins to slow and abrasive particles continues lower acceleration until the both velocity reaches the same when they are advanced from nozzle exit to the atmosphere. The slip velocity of slurry and abrasive particles is decreasing with farther away from the exit, until they converge to zero.By measuring impact on the workpiece of Cr40MnMo7 materials and nozzle efficiency of different jet parameters, the maxium signal intensity and peak size of impact force is abrasive slurry jet added polymer, next abrasive slurry jet, then pure water jet, and the difference of impact force between micro-abrasive slurry jet and pure water jet is about 20%. Similarly, one can conclude that with jet pressure increasing, the nozzle efficiency will reduce, the ratio of jet impact force and hydraulic energy will reduce, too. The impact force calculated theoretically is 20 percent larger than that measured actually. Therefore, the nozzle efficiency is evaluated by the ratio of jet impact force and hydraulic energy, and the nozzle structure is also optimized by it.As for the research of materials erosion processes and erosion mechanisms, we researched the effect factors like impact velocity, impact angle, shape and number of abrasive paticles, etc. After the workpiece is impacted with abrasive particles, the relationship between maximum residual stress and maximum stress and impact velocity is linear, the erosion rate and impact angle is similar parabolic relationship, erosion stress peaks in a 75°impact angle, strain peaks at 55°, the volume removal rate remains stable with three particles, the stress and material removal rate is greater with abrasive particles with sharp corner shape. When the impact angle is less than 15°, erosion mechanism of the workpiece is shallow plowing and particle curling, it is mainly micro-cutting and deep plowing when the impact angle is greater than 15°and less than 75°, there are material pits and uplifts removal work when the impact angle is greater than 75°and less than 90°. The material removal amount is increasing when the incident angle increase from 30°to 60°, until reaches the maximum at 60°～75°, then is reducing with the increasing of the incident angle from 75°～90°.Compared micro-abrasive slurry jet polishing 40CrMnMo7 mold steel surface with micro-abrasive water jet polishing, the former is embedded less abrasive particles into the workpiece. It is embedded more abrasive particles into the workpiece with the larger material removal polishing with fixed point than that of relatively less materials with a slip velocity of nozzle. The abrasive slurry jet polishing added long-chain polymer PAM is different characteristic with different jet distance. The steep cutting depth is deeper in the transition region. The depth of material removal is 1.5-2 times than ordinary abrasive water jet in the range of smaller jet distance, but it is obvious different in the range of larger jet distance (typically greater than 40mm). The surface roughness Ra of the workpiece reaches 0.049μm after it has been polished 45 minuters with #200040CrMnMo7 abrasive jet added polymer. In micro-abrasive slurry jet polishing, micro-abrasive particles can be reused, the suitable recycling is a continuous loop cycle.更多还原