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金属表面电解质等离子抛光及其工艺的研究
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摘要
电解质等离子抛光是一种“绿色”高质高效的抛光金属工件的特种加工方法,通过在工件和抛光液之间形成的气层的放电去除作用实现对工件抛光。其抛光液为低浓度的盐溶液,并可通过补充抛光盐而循环使用,能够解决机械抛光难于加工形状复杂工件的问题,也可以解决化学和电解抛光难以避免的污染问题,应用前景广阔。
     本文研究该抛光方法能够实现微观整平的原因,揭示电解质等离子抛光作用机制。基于气体放电的相关理论,研究抛光液成分对抛光效果的影响,发现电解质等离子抛光是一个气体放电和化学反应同时进行的动态过程,放电去除速度大于反应生成速度是实现抛光的前提。通过实验和分析得出以不同的方式将工件潜入抛光液时的伏安特性曲线和电流、电压随时间变化的曲线,确定最合理的工件下潜方式。利用相关仪器研究了电解质等离子抛光前后不锈钢试件的外形尺寸、粗糙度、耐腐蚀性、微观形貌、表面化学成分和显微硬度等表面状态的变化。实验结果一方面证明了电解质等离子抛光可以达到理想的抛光效果,另一方面也在一定程度上验证了所揭示的抛光作用机制。
     分析电解质等离子抛光材料去除的热传导过程和工件表面获得能量的方式,发现工件表面的热流密度是影响去除速度的重要因素,工件表面所获得的能量主要来自电子冲击。根据材料去除机理,推导得出结论并实验证实:在稳定抛光状态下,材料去除速度与电流密度成正比。依据实验结果和分析,研究了电压、抛光液浓度和温度以及工件在抛光液中的下潜深度对材料去除速度的影响。建立表面粗糙度随抛光时间变化的数学模型,实验得出一定条件下经过不同的抛光时间后试件表面的实际粗糙度值,用这些实验数据和数学模型进行非线性拟合,并根据拟合结果对数学模型进行修正,修成后的数学模型与实验数据的拟合程度很好。在不同的抛光液温度下,又进行两组实验,验证了修正后的数模模型与实际抛光情况基本一致。
     基于抛光作用机制和表面粗糙度随抛光时间变化的数学模型,选取经过一定时间的抛光后的试件表面粗糙度值和抛光过程中的电流密度作为实验指标,进行四因素四水平的正交实验,对实验结果进行极差和方差分析,确定各因素影响粗糙度和电流密度的主次顺序和规律,获得只考虑抛光效果和综合考虑效果、成本、效率以及稳定性的最优的工艺参数组合。电解质等离子抛光过程中受抛光产生的金属微粒的干扰无法使用电导法对抛光液浓度进行检测,为了解决这一问题,基于实验提出两种确定抛光过程中硫酸铵抛光液浓度的方法。一种方法是利用硫酸铵抛光液的浓度低于2.5wt%以后抛光的电流密度会明显下降的现象,定时检测抛光过程中电流值和抛光液温度,以确定是否需要补充硫酸铵。另一种方法是通过实验得到一定抛光液温度下抛光量与硫酸铵消耗量的关系,再在抛光过程中记录抛光量计算硫酸铵抛光液浓度。前一种方法应用起来更为简便,可用于一般工业生产;后一种方法适用于对抛光效果要求更高的加工。经实验验证两种方法均具有可行性。针对形状复杂的工件,从气层厚度和电场强度两方面研究工件上的空间位置和形状对抛光的影响。对于一些形状特殊的工件的抛光方法进行研究。
     根据电解质等离子抛光及其工艺的研究成果,设计电解质等离子抛光设备,并使用该设备加工多种材质的工件,为该技术的工业化应用提供参考依据。
Electrolysis and plasma polishing is a “green” non-traditional machining that canefficiently polish metal workpieces and provide high-quality surface by removal effectof the gas layer discharge between the polishing solution and the workpiece. Itspolishing solution is salt solution with low concentration, which can be repeatedly usedby adding salt, so, with broad application prospect, it can solve the problems thatmechancal polishing is not fit for the workpieces with complex shapes and chemicalpolishing and electrolytic polishing are generally not environmentally friendly.
     The reason for microcosmic planarization and the polishing mechanism areexpounded on. Based on the gas discharge theory, influence of composition of thepolishing solution on the polishing effect is studied. It is found that electrolysis andplasma polishing is a dynamic process consisting of gas discharge and chemicalreactions and a precondition of the polishing is that the removal rate of the discharge isfaster than the production rate of the reactions. Curves of voltage and current with timeand volt-ampere characteristic curves dropping a workpiece into the polishing solutionin various ways are obtained by experiments and analyses to choose the best way todrop. Shape dimension, roughness, corrosion resistance, micromorphology, surfacechemical composition and microhardness of stainless steel before and after polishing arestudied by using relevant instruments. The experiment results show the ideal effects ofelectrolysis and plasma polishing and validate the mechanism to a certain extent.
     During the analyses on heat transfer process of material removal and energy gainsof the surface of metal workpieces in electrolysis and plasma polishing, it is discoveredthat the workpiece’s surface heat flux density has a great effect on the material removalrate and the gains come mainly from electron-bombardment. According to the materialremoval mechanism, it is deduced and testified by experiments that the materialremoval rate is direct proportion to the electric current density under steady state ofpolishing. Influences of voltage, temperature and concentration of the polishing solutionand the workpiece’s diving depth in polishing solution on the material removal rate areresearched basing on the experiments and analyses. The mathematic model of surfaceroughness with polishing time in electrolysis and plasma polishing is established. Theroughness of the specimen with different polishing time was measured. Nonlinear fittingof the experimental data and the mathematic model is carried out, by which themathematic model is improved. It is reasonable and feasible to use the improvedmathematic model to fit the experimental data. Two other group of experiments atdifferent temperature of the polishing solution were done to verify that the improved mathematic model is mainly in accordance with the actual polishing case.
     Orthogonal experiments consisting of four factors at four different levels wereperformed by using surface roughness of the specimens after some time of polishingand electric current density in the process of the polishing as experimental indexs basedon the polishing mechanism and the mathematic model of surface roughness withpolishing time. Primary and secondary order and regularity of the factors affecting theroughness and the electric current density are obtained by means of extreme differencemethod combined with variance analytic method. Optimal parameter combinationsfocusing on the polishing effects and considering fully the effects, cost, efficiency andstability are proposed. Conductometry is not available in detecting the polishingsolution concentration in electrolysis and plasma polishing because of the interferenceof the metallic particles produced in the polishing. To solve the problem, two methodsbased on experiment to detect concentration of ammonium sulfate in the polishingsolution are proposed. One method is to measure the current value and temperature ofthe polishing solution to determine whether ammonium sulfate needs to be added as theelectric current density drops significantly with the concentration less than2.5%. Theother method is to know correlations between the polishing quantity and theconsumption of ammonium sulfate in certain temperature of the polishing solution byexperiments and record the polishing quantity to work out the concentration ofammonium sulfate. The first method is simple to operate and suitable for generalindustrial production. The second method applies to the process requiring betterpolishing effect. The two methods are proved to be effective and feasible. Influence ofthe shape and spatial location on the polishing is studied for the workpiece withcomplex shape from two aspects: thickness of gas layer and electric field intensity. Thepolishing methods of specially shaped workpieces are investigated.
     According to the research results of the mechanism and process of electrolysis andplasma polishing, the equipment is designed and used to polish various materialworkpiece and supply a basis for the industrialized application.
引文
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