混粉准干式放电加工机理及工艺研究
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摘要
气中放电加工以气体介质取代传统液体介质进行电火花加工,具有工具电极损耗低、工件表面受到的破坏力小、以氧气为介质时材料去除率高、对环境污染小等优点,备受人们关注。但是,气中放电加工的放电间隙小、气体的比热低,存在加工区域热量传散困难、熔融材料冷却缓慢、加工屑难于从放电间隙排出等固有缺陷,造成短路率高、脉冲利用率低、加工过程不稳定。采取恰当措施,抑制气中放电加工缺点,充分发挥其优势,是推进气体介质中放电加工技术实用化的关键。混粉准干式放电加工技术在气体连续介质中加入微量的固、液相成分,在有效提高加工效率的同时保留气中放电加工技术工具电极损耗率低的优势,是一种极具潜力的高效、绿色电火花加工技术。本文重点研究混粉准干式放电加工机理,确定基础工艺参数,研究加工工艺规律,并进行加工参数优化。
从微量固、液成分的作用机制入手,开展混粉准干式放电加工机理研究。借助Wagner模型对固液混合相的介电系数进行计算,分析固相、液相和固液混合相的介电性能,证实气体连续相的介电系数小于其内分散相的介电系数。基于非单一介质中电场强度变化对介质放电特性的影响,开展三相流介质的击穿机理研究。选用球面坐标系中高斯定律微分形式的拉普拉斯方程对三相流介质中的电位分布进行分析,结果表明,分散相的加入引发了极间均匀电场畸变；在一定的假设条件下,建立三相流介质中的电场强度表达式,得到放电间隙计算公式,并对三相流引起的放电间隙变化进行分析,分析结果证实,三相流介质能够增大放电间隙,有利于介质击穿和电蚀产物的排出；通过对比试验验证三相流介质的放电间隙增大效果,通过放电电压波形对比分析,证实了介质击穿频率的提高。理论计算得出分散相所在位置场强畸变严重,易于引发“工具电极-分散相-工件电极”串联放电,据此建立了串联放电理论模型。研究工具电极表面的材料附着现象,借助扫描电镜观察工具电极表面微观形貌,利用能谱分析仪分析薄膜状附着区、片状附着区、含球状附着物区和孔隙状附着区的成分组成,证实工具电极表面存在来自工件的附着物成分,提出了混粉准干式放电加工的工具电极损耗机理。研究结果表明,混粉准干式放电加工技术的工具电极损耗是材料去除过程和材料附着过程综合作用的结果。
将混粉准干式放电加工的工艺参数划分为基础工艺参数和加工工艺参数两大类,并进行了相关研究。
从放电加工常用粉末材料入手,综合考虑粉末材料的加入对放电加工效果、工作介质稳定性和工件被加工表面综合机械性能的影响,分析粉末材料和粒径对沉降稳定性的影响,并从充分发挥粉末性质对固相成分放电改善作用的角度出发,选用粒径2.5μm的硅粉颗粒作为混粉准干式放电加工用固态相。从放电加工常用电极材料入手,选用紫铜作为混粉准干式放电加工试验研究用工具电极材料。开展加工极性对比试验,结果表明,工具电极接脉冲电源负极时(正极性加工时),能够获得更高的工件材料去除率和更低的工具电极损耗率,因此,混粉准干式放电加工适于采用正极性加工。在传统极性效应理论的基础上开展混粉准干式放电加工技术极性效应理论分析；计算得到等离子放电通道中的电子群和正离子群撞击正负极时的动能差异,建立适于混粉准干式放电加工技术的极性效应理论；实验结果与极性效应理论分析结果一致。
基于单因素试验设计,开展混粉准干式放电加工工艺研究,获取峰值电流、脉冲宽度、脉冲间隔、固液混合物流量、粉末颗粒的浓度和工具电极转速对工件材料去除率和工件被加工表面粗糙度的影响规律,揭示其规律成因。研究表明,峰值电流主要通过改变工件被加工表面获取的脉冲放电能量密度影响放电加工效果。随着峰值电流的增大,蚀坑变得大而深,工件材料去除率和工件被加工表面粗糙度值(Ra值)均增大。脉宽主要通过改变脉冲能量供给时间影响放电加工效果。随着脉宽的增大,脉冲能量供给时间延长,且等离子通道沿径向扩展,蚀坑变得大而深,工件材料去除率增大,工件被加工表面Ra值增大。脉间对放电加工效果的影响主要表现在两个方面：一是影响极间介质的消电离效果,二是影响脉冲放电占空比。消电离效果间接影响单脉冲放电过程和单脉冲放电能量,进而影响工件材料去除率和工件被加工表面粗糙度。随着脉间增大,工件材料去除率降低,工件被加工表面Ra值降低。流量的变化引起进入放电间隙的固相体积和液相体积的变化,液相和固相体积的变化影响介质击穿过程和消电离效果；随着流量的增长,工件材料去除率增大,工件被加工表面Ra值降低。颗粒浓度的变化能够影响介质击穿过程和极间介质消电离效果；随着浓度的增长,工件材料去除率先增大后减小,工件被加工表面Ra值先减小后增大。工具电极旋转运动的存在对于混粉准干式放电加工效果的改善意义显著,但转速的过分增大使得放电加工稳定性变差,导致工件材料去除率和工件被加工表面Ra值均随转速增大而减小。从稳定放电过程进而改善加工效果的角度考虑,应选取较小的工具电极转速。
借助正交设计方法进行试验设计,系统研究工件材料去除率、工件被加工表面粗糙度和工具电极损耗率的变化规律。根据望大特性信噪比计算公式将工件材料去除率的正交试验结果转换成相应的信噪比值,根据望小特性信噪比计算公式分别将工具电极损耗率和工件被加工表面粗糙度值的正交试验结果转换成相应的信噪比值；借助信噪比分析方法和理论,分别得到取得最大工件材料去除率、最小工具电极损耗率和最小Ra值的最优加工工艺参数组合。验证实验证实单目标优化结果的可靠性。
对正交试验结果开展方差分析,找到混粉准干式放电加工各个加工效果评价指标的信噪比的显著影响因子。工件材料去除率信噪比的显著影响因子是峰值电流、脉宽和流量；工具电极损耗率信噪比的显著影响因子是峰值电流；工件被加工表面粗糙度Ra值信噪比的显著影响因子是脉宽和峰值电流。以非线性回归分析的相关理论和方法为指导,借助统计分析软件Minitab16进行非线性回归计算,分别得到工件材料去除率、工具电极损耗率和工件被加工表面粗糙度Ra值的信噪比数学表达式；借助显著性检验证实回归结果的显著性,借助验证实验和回归方程预测结果误差分析证实回归分析数学表达式的预测效果。综合考虑工件材料去除率、工具电极损耗率和工件被加工表面粗糙度,借助粒子群算法开展混粉准干式放电加工多目标优化研究,得到工件材料去除率和工件被加工表面粗糙度Ra值、工件材料去除率和工具电极损耗率、工件被加工表面粗糙度Ra值和工具电极损耗率的帕累托前沿。当以较大的加工效率进行加工时,并不会造成工具电极的过快损耗及其带来的电极频繁更换等一系列问题；当以获取较低的被加工表面粗糙度Ra值为目的进行加工的同时,也能够获得较小的工具电极损耗。
In the traditional electrical discharge machining (EDM) process, the discharge is induced in the liquid medium. However, in the dry EDM process, the liquid medium is replaced by the gas medium. Dry EDM has many advantages. For example, the tool wear rate (TWR) of it is low, its acting force on the machined surface of the workpiece is low, its material removal rate (MRR) with oxygen gas as its dielectric medium is high, and environmental pollution is reduced. So the dry EDM is paid close attention to. Nonetheless, dry EDM has disadvantages, such as small discharge gap and low specific heat of the gas. Its residual heat transfer is difficult, the cooling of the molten materials produced during its discharging is slow, and it is hard for the corrosion particles to be expelled out of the discharging areas. Therefore, its short circuit rate is high, its utilization efficiency of pulse energy is low, and the machining stability of it is poor. Only by taking appropriate measures to overcome shortcomings of the dry EDM process while taking advantages of it, can we promote the practical application of this process. In the powder mixed near dry electrical discharge machining (PMND-EDM) process, trace amounts of powder particles and liquid phase are added into the gas continuous phase. Its MRR is higher while the TWR is lower. So it is thought to be a potential green processing technology with high machining efficiency. In this paper, the mechanism of PMND-EDM process is researched, its basic processing parameters are determined, its technological characteristics are studied, and its processing parameters are optimized.
Start from the function of the solid phase and the liquid phase, research is done on the mechanism of the PMND-EDM process. Based on the Wagner model, the dielectric coefficient of the solid-liquid mixed phase is calculated. The dielectric properties of the solid phase, the liquid phase and the solid-liquid mixed phase are analyzed. And it is verified that the dielectric coefficient of the continuous gas phase is lower than its inner dispersed phases. Research is done on the medium breakdown mechanism of the three phase dielectric medium based on the influence of variation of electric field strength on the discharge characteristic of dielectric medium in non-monolithic medium. Analysis is done on the electric potential distribution in the three phase dielectric medium based on the Laplace's equation of differential form of the Gauss's law in the spherical coordinates. Results show that the distortion of the uniform electric field can be induced due to the addition of dispersed phases. The expression for electric field strength is established under a certain assumed conditions. The calculation formula for the discharge gap is obtained. And the change of the discharge gap, which is caused by the three phase dielectric medium, is analyzed. The theoretical analysis results indicate that the discharge gap can be enlarged by the three phase flow, the breakdown of the medium can be improved, and the expelling of the debris formed during discharging can be facilitated. The enlargement of discharge gap for the three phase flow is verified by the comparison test. The discharge voltage is recorded during the comparison test and the comparative analysis is done. It is confirmed that the medium breakdown frequency is increased. The theoretical calculation reveals that serious distortion of the electrical field strength is easier to be found at locations with dispersed phased, and the series electrical discharge among the tool electrode, the dispersed phases and the workpiece electrode can be induced easily in these places. Based on the above, the theory model of series electrical discharge is establised. Research is done on the phenomena of material attachments on the surface of tool electrode. The surface micro-morphology of the used tool electrode is observed by the scanning electron microscope (SEM). The composition of the thin-film attachment region, the lamellate attachment region, the region with spherical attachments and the porosity attachment region are analyzed by the energy dispersive spectrometer (EDS), respectively. It is verified that there are attachments, which are elementary compositions of the workpiece, on the surface of the tool electrode. And the mechanism of tool electrode wear for the PMND-EDM process is proposed. The research results show that the tool electrode wear in the PMND-EDM process is caused by the material removal from the tool electrode and the material attachments on the surface of tool electrode.
The technical parameters of the PMND-EDM process are divided into two categories:basic processing parameters and machining parameter. And relevant researches are done.
Starting from the general powders in EDM process, comprehensively consider the addition of powder particles on the EDM machining, the dielectric medium stability, and the synthesized mechanical properties of the machined workpiece surface, and do analysis on the influence of powder ingredient and powder size on the dispersion stability. From the perspective of giving full play to the effect of powder properties on the improvement of electrical discharging, silicon powders with grain size of2.5μm are selected as the solid phase of the PMND-EDM process. Starting from the general tool electrode materials in EDM process, copper is selected as the tool electrode material for the PMND-EDM process. Comparative test is done on the influence of polarity of electrodes on the machining. And it is found that the higher MRR and the lower TWR can be obtained when the tool electrode is connected to the cathode of the pulse electrical source, the machining polarity for which is normal. Therefore, normal polarity is more appropriate for the PMND-EDM process. Theoretical analysis is done on the polarity effect of the PMND-EDM process based on the conventional theory of polarity effect. The difference in the kinetic energy for the positive ion swarm and the electron swarm, when they impact the negative electrode and the positive electrode respectively, is got by calculating. The theory of polarity effect for the PMND-EDM process is established. And the experimental results coincide with the theory analysis results based on the theory of polarity effect for the PMND-EDM process.
Based on the single factor experimental design, research is done on the technological characteristics of the PMND-EDM process. The influences of peak current, pulse on time, pulse off time, flow rate of solid-liquid mixture, concentration of powder particles and rotational speed of tool electrode on the MRR and the surface roughness (SR) are achieved, and the reasons for these influences are revealed. Research results reveal that the effect of peak current on the EDM machining mainly based on its effect on the obtained energy density of the machined workpiece surface. With the increase of peak current, the crater becomes larger and deeper, both the MRR and the SR for the machined workpiece increase. The effect of pulse on time on the EDM machining mainly based on its effect on the supply time of the pulse discharge energy. With the increase of pulse on time, the supply time of pulse discharge energy increases and the plasma channel expands in its radius direction; as a result, the crater becomes larger and deeper, both the MRR and the SR of the machined workpiece increase. The influences of pulse off time on EDM machining are mainly based on two main aspects. One is its influence on the deionization effect of the dielectric medium in the discharge gap. The other is its effect on the duty circle. The deionization effect indirectly influences the discharge course of single pulse and the discharge energy of single pulse, and then influences the MRR and the SR of the machined workpiece. With the increase of pulse off time, both the MRR and the SR decrease. Flow rate of solid-liquid mixture determines the volume of solid phase and liquid phase that are delivered into the discharge gap. The volume of the liquid phase and the solid phase influence the breakdown of the dielectric medium and the deionization effect of the dielectric medium. With the increase of the flow rate, the MRR of the workpiece increases, however, the SR of the machined workpiece decreases. The concentration of powder particles influences the breakdown of the dielectric medium and the deionization effect of the dielectric medium. With the increase of the concentration, the MRR of the machined workpiece increases first and then decreases, however, the SR decreases first and then increased. The existence of rotation for the tool electrode is helpful to improve the machining process of the PMND-EDM technology. However, the excessive increase of the rotational speed for the tool electrode will deteriorate the machining stability, which results in the decrease of the MRR and the SR for the machined workpiece along with the increase of the tool rotational speed. A low tool rotational speed should be selected from the perspective of stabilizing the machining stability and then improving the machining.
Based on the theory of orthogonal experimental design, systematical research is done on the MRR, the SR and the TWR. The MRR obtained in the orthogonal design experiment is converted to the corresponding value of signal to noise ratio based on the expression for the signal to noise ratio of the higher the better characteristics. The TWR and the SR obtained in the orthogonal design experiment are converted to the corresponding value of signal to noise ratio based on the expression for the signal to noise ratio of the smaller the better characteristics. The optimal combinations of machining parameters for the highest MRR, the lowest TWR and the lowest SR are obtained based on the theory and the method of the analysis on the signal to noise ratio. The results of single objective optimization are verified by the verification experiments.
The significant impact factors for the signal to noise ratio of each machining evaluation index of the PMND-EDM process is found out by doing analysis of variance (ANOVA) on results of the orthogonal design experiment. The significant impact factors are peak current, pulse on time and flow rate for the signal to noise ratio of MRR, peak current for the signal to noise ratio of TWR, and pulse on time and peak current for the signal to noise ratio of SR. The mathematical expressions for the signal to noise ratio of the MRR, the TWR and the SR are established by the statistical analysis software Minitab16based on the relative theory and method of nonlinear regression analysis. The significance of the regression model is verified by test of statistical significance. The predictive effects of the established models are verified through verification experiment and the error analysis for the predictive results of the regression models. Taking into account the MRR, the TWR and the SR, multi-objective optimization is done on the PMND-EDM process based on the particle swarm optimization (PSO) algorithm. The pareto fronts of the MRR and the SR, the MRR and the TWR, and the SR and the TWR for the PMND-EDM process are obtained respectively. The process could decrease the TWR and overcome the problems accompanied with the large TWR, such as the frequent replacement of the worn tool electrode, when machined with large MRR; meanwhile, lower TWR can be obtained along with lower SR.
从微量固、液成分的作用机制入手,开展混粉准干式放电加工机理研究。借助Wagner模型对固液混合相的介电系数进行计算,分析固相、液相和固液混合相的介电性能,证实气体连续相的介电系数小于其内分散相的介电系数。基于非单一介质中电场强度变化对介质放电特性的影响,开展三相流介质的击穿机理研究。选用球面坐标系中高斯定律微分形式的拉普拉斯方程对三相流介质中的电位分布进行分析,结果表明,分散相的加入引发了极间均匀电场畸变；在一定的假设条件下,建立三相流介质中的电场强度表达式,得到放电间隙计算公式,并对三相流引起的放电间隙变化进行分析,分析结果证实,三相流介质能够增大放电间隙,有利于介质击穿和电蚀产物的排出；通过对比试验验证三相流介质的放电间隙增大效果,通过放电电压波形对比分析,证实了介质击穿频率的提高。理论计算得出分散相所在位置场强畸变严重,易于引发“工具电极-分散相-工件电极”串联放电,据此建立了串联放电理论模型。研究工具电极表面的材料附着现象,借助扫描电镜观察工具电极表面微观形貌,利用能谱分析仪分析薄膜状附着区、片状附着区、含球状附着物区和孔隙状附着区的成分组成,证实工具电极表面存在来自工件的附着物成分,提出了混粉准干式放电加工的工具电极损耗机理。研究结果表明,混粉准干式放电加工技术的工具电极损耗是材料去除过程和材料附着过程综合作用的结果。
将混粉准干式放电加工的工艺参数划分为基础工艺参数和加工工艺参数两大类,并进行了相关研究。
从放电加工常用粉末材料入手,综合考虑粉末材料的加入对放电加工效果、工作介质稳定性和工件被加工表面综合机械性能的影响,分析粉末材料和粒径对沉降稳定性的影响,并从充分发挥粉末性质对固相成分放电改善作用的角度出发,选用粒径2.5μm的硅粉颗粒作为混粉准干式放电加工用固态相。从放电加工常用电极材料入手,选用紫铜作为混粉准干式放电加工试验研究用工具电极材料。开展加工极性对比试验,结果表明,工具电极接脉冲电源负极时(正极性加工时),能够获得更高的工件材料去除率和更低的工具电极损耗率,因此,混粉准干式放电加工适于采用正极性加工。在传统极性效应理论的基础上开展混粉准干式放电加工技术极性效应理论分析；计算得到等离子放电通道中的电子群和正离子群撞击正负极时的动能差异,建立适于混粉准干式放电加工技术的极性效应理论；实验结果与极性效应理论分析结果一致。
基于单因素试验设计,开展混粉准干式放电加工工艺研究,获取峰值电流、脉冲宽度、脉冲间隔、固液混合物流量、粉末颗粒的浓度和工具电极转速对工件材料去除率和工件被加工表面粗糙度的影响规律,揭示其规律成因。研究表明,峰值电流主要通过改变工件被加工表面获取的脉冲放电能量密度影响放电加工效果。随着峰值电流的增大,蚀坑变得大而深,工件材料去除率和工件被加工表面粗糙度值(Ra值)均增大。脉宽主要通过改变脉冲能量供给时间影响放电加工效果。随着脉宽的增大,脉冲能量供给时间延长,且等离子通道沿径向扩展,蚀坑变得大而深,工件材料去除率增大,工件被加工表面Ra值增大。脉间对放电加工效果的影响主要表现在两个方面：一是影响极间介质的消电离效果,二是影响脉冲放电占空比。消电离效果间接影响单脉冲放电过程和单脉冲放电能量,进而影响工件材料去除率和工件被加工表面粗糙度。随着脉间增大,工件材料去除率降低,工件被加工表面Ra值降低。流量的变化引起进入放电间隙的固相体积和液相体积的变化,液相和固相体积的变化影响介质击穿过程和消电离效果；随着流量的增长,工件材料去除率增大,工件被加工表面Ra值降低。颗粒浓度的变化能够影响介质击穿过程和极间介质消电离效果；随着浓度的增长,工件材料去除率先增大后减小,工件被加工表面Ra值先减小后增大。工具电极旋转运动的存在对于混粉准干式放电加工效果的改善意义显著,但转速的过分增大使得放电加工稳定性变差,导致工件材料去除率和工件被加工表面Ra值均随转速增大而减小。从稳定放电过程进而改善加工效果的角度考虑,应选取较小的工具电极转速。
借助正交设计方法进行试验设计,系统研究工件材料去除率、工件被加工表面粗糙度和工具电极损耗率的变化规律。根据望大特性信噪比计算公式将工件材料去除率的正交试验结果转换成相应的信噪比值,根据望小特性信噪比计算公式分别将工具电极损耗率和工件被加工表面粗糙度值的正交试验结果转换成相应的信噪比值；借助信噪比分析方法和理论,分别得到取得最大工件材料去除率、最小工具电极损耗率和最小Ra值的最优加工工艺参数组合。验证实验证实单目标优化结果的可靠性。
对正交试验结果开展方差分析,找到混粉准干式放电加工各个加工效果评价指标的信噪比的显著影响因子。工件材料去除率信噪比的显著影响因子是峰值电流、脉宽和流量；工具电极损耗率信噪比的显著影响因子是峰值电流；工件被加工表面粗糙度Ra值信噪比的显著影响因子是脉宽和峰值电流。以非线性回归分析的相关理论和方法为指导,借助统计分析软件Minitab16进行非线性回归计算,分别得到工件材料去除率、工具电极损耗率和工件被加工表面粗糙度Ra值的信噪比数学表达式；借助显著性检验证实回归结果的显著性,借助验证实验和回归方程预测结果误差分析证实回归分析数学表达式的预测效果。综合考虑工件材料去除率、工具电极损耗率和工件被加工表面粗糙度,借助粒子群算法开展混粉准干式放电加工多目标优化研究,得到工件材料去除率和工件被加工表面粗糙度Ra值、工件材料去除率和工具电极损耗率、工件被加工表面粗糙度Ra值和工具电极损耗率的帕累托前沿。当以较大的加工效率进行加工时,并不会造成工具电极的过快损耗及其带来的电极频繁更换等一系列问题；当以获取较低的被加工表面粗糙度Ra值为目的进行加工的同时,也能够获得较小的工具电极损耗。
In the traditional electrical discharge machining (EDM) process, the discharge is induced in the liquid medium. However, in the dry EDM process, the liquid medium is replaced by the gas medium. Dry EDM has many advantages. For example, the tool wear rate (TWR) of it is low, its acting force on the machined surface of the workpiece is low, its material removal rate (MRR) with oxygen gas as its dielectric medium is high, and environmental pollution is reduced. So the dry EDM is paid close attention to. Nonetheless, dry EDM has disadvantages, such as small discharge gap and low specific heat of the gas. Its residual heat transfer is difficult, the cooling of the molten materials produced during its discharging is slow, and it is hard for the corrosion particles to be expelled out of the discharging areas. Therefore, its short circuit rate is high, its utilization efficiency of pulse energy is low, and the machining stability of it is poor. Only by taking appropriate measures to overcome shortcomings of the dry EDM process while taking advantages of it, can we promote the practical application of this process. In the powder mixed near dry electrical discharge machining (PMND-EDM) process, trace amounts of powder particles and liquid phase are added into the gas continuous phase. Its MRR is higher while the TWR is lower. So it is thought to be a potential green processing technology with high machining efficiency. In this paper, the mechanism of PMND-EDM process is researched, its basic processing parameters are determined, its technological characteristics are studied, and its processing parameters are optimized.
Start from the function of the solid phase and the liquid phase, research is done on the mechanism of the PMND-EDM process. Based on the Wagner model, the dielectric coefficient of the solid-liquid mixed phase is calculated. The dielectric properties of the solid phase, the liquid phase and the solid-liquid mixed phase are analyzed. And it is verified that the dielectric coefficient of the continuous gas phase is lower than its inner dispersed phases. Research is done on the medium breakdown mechanism of the three phase dielectric medium based on the influence of variation of electric field strength on the discharge characteristic of dielectric medium in non-monolithic medium. Analysis is done on the electric potential distribution in the three phase dielectric medium based on the Laplace's equation of differential form of the Gauss's law in the spherical coordinates. Results show that the distortion of the uniform electric field can be induced due to the addition of dispersed phases. The expression for electric field strength is established under a certain assumed conditions. The calculation formula for the discharge gap is obtained. And the change of the discharge gap, which is caused by the three phase dielectric medium, is analyzed. The theoretical analysis results indicate that the discharge gap can be enlarged by the three phase flow, the breakdown of the medium can be improved, and the expelling of the debris formed during discharging can be facilitated. The enlargement of discharge gap for the three phase flow is verified by the comparison test. The discharge voltage is recorded during the comparison test and the comparative analysis is done. It is confirmed that the medium breakdown frequency is increased. The theoretical calculation reveals that serious distortion of the electrical field strength is easier to be found at locations with dispersed phased, and the series electrical discharge among the tool electrode, the dispersed phases and the workpiece electrode can be induced easily in these places. Based on the above, the theory model of series electrical discharge is establised. Research is done on the phenomena of material attachments on the surface of tool electrode. The surface micro-morphology of the used tool electrode is observed by the scanning electron microscope (SEM). The composition of the thin-film attachment region, the lamellate attachment region, the region with spherical attachments and the porosity attachment region are analyzed by the energy dispersive spectrometer (EDS), respectively. It is verified that there are attachments, which are elementary compositions of the workpiece, on the surface of the tool electrode. And the mechanism of tool electrode wear for the PMND-EDM process is proposed. The research results show that the tool electrode wear in the PMND-EDM process is caused by the material removal from the tool electrode and the material attachments on the surface of tool electrode.
The technical parameters of the PMND-EDM process are divided into two categories:basic processing parameters and machining parameter. And relevant researches are done.
Starting from the general powders in EDM process, comprehensively consider the addition of powder particles on the EDM machining, the dielectric medium stability, and the synthesized mechanical properties of the machined workpiece surface, and do analysis on the influence of powder ingredient and powder size on the dispersion stability. From the perspective of giving full play to the effect of powder properties on the improvement of electrical discharging, silicon powders with grain size of2.5μm are selected as the solid phase of the PMND-EDM process. Starting from the general tool electrode materials in EDM process, copper is selected as the tool electrode material for the PMND-EDM process. Comparative test is done on the influence of polarity of electrodes on the machining. And it is found that the higher MRR and the lower TWR can be obtained when the tool electrode is connected to the cathode of the pulse electrical source, the machining polarity for which is normal. Therefore, normal polarity is more appropriate for the PMND-EDM process. Theoretical analysis is done on the polarity effect of the PMND-EDM process based on the conventional theory of polarity effect. The difference in the kinetic energy for the positive ion swarm and the electron swarm, when they impact the negative electrode and the positive electrode respectively, is got by calculating. The theory of polarity effect for the PMND-EDM process is established. And the experimental results coincide with the theory analysis results based on the theory of polarity effect for the PMND-EDM process.
Based on the single factor experimental design, research is done on the technological characteristics of the PMND-EDM process. The influences of peak current, pulse on time, pulse off time, flow rate of solid-liquid mixture, concentration of powder particles and rotational speed of tool electrode on the MRR and the surface roughness (SR) are achieved, and the reasons for these influences are revealed. Research results reveal that the effect of peak current on the EDM machining mainly based on its effect on the obtained energy density of the machined workpiece surface. With the increase of peak current, the crater becomes larger and deeper, both the MRR and the SR for the machined workpiece increase. The effect of pulse on time on the EDM machining mainly based on its effect on the supply time of the pulse discharge energy. With the increase of pulse on time, the supply time of pulse discharge energy increases and the plasma channel expands in its radius direction; as a result, the crater becomes larger and deeper, both the MRR and the SR of the machined workpiece increase. The influences of pulse off time on EDM machining are mainly based on two main aspects. One is its influence on the deionization effect of the dielectric medium in the discharge gap. The other is its effect on the duty circle. The deionization effect indirectly influences the discharge course of single pulse and the discharge energy of single pulse, and then influences the MRR and the SR of the machined workpiece. With the increase of pulse off time, both the MRR and the SR decrease. Flow rate of solid-liquid mixture determines the volume of solid phase and liquid phase that are delivered into the discharge gap. The volume of the liquid phase and the solid phase influence the breakdown of the dielectric medium and the deionization effect of the dielectric medium. With the increase of the flow rate, the MRR of the workpiece increases, however, the SR of the machined workpiece decreases. The concentration of powder particles influences the breakdown of the dielectric medium and the deionization effect of the dielectric medium. With the increase of the concentration, the MRR of the machined workpiece increases first and then decreases, however, the SR decreases first and then increased. The existence of rotation for the tool electrode is helpful to improve the machining process of the PMND-EDM technology. However, the excessive increase of the rotational speed for the tool electrode will deteriorate the machining stability, which results in the decrease of the MRR and the SR for the machined workpiece along with the increase of the tool rotational speed. A low tool rotational speed should be selected from the perspective of stabilizing the machining stability and then improving the machining.
Based on the theory of orthogonal experimental design, systematical research is done on the MRR, the SR and the TWR. The MRR obtained in the orthogonal design experiment is converted to the corresponding value of signal to noise ratio based on the expression for the signal to noise ratio of the higher the better characteristics. The TWR and the SR obtained in the orthogonal design experiment are converted to the corresponding value of signal to noise ratio based on the expression for the signal to noise ratio of the smaller the better characteristics. The optimal combinations of machining parameters for the highest MRR, the lowest TWR and the lowest SR are obtained based on the theory and the method of the analysis on the signal to noise ratio. The results of single objective optimization are verified by the verification experiments.
The significant impact factors for the signal to noise ratio of each machining evaluation index of the PMND-EDM process is found out by doing analysis of variance (ANOVA) on results of the orthogonal design experiment. The significant impact factors are peak current, pulse on time and flow rate for the signal to noise ratio of MRR, peak current for the signal to noise ratio of TWR, and pulse on time and peak current for the signal to noise ratio of SR. The mathematical expressions for the signal to noise ratio of the MRR, the TWR and the SR are established by the statistical analysis software Minitab16based on the relative theory and method of nonlinear regression analysis. The significance of the regression model is verified by test of statistical significance. The predictive effects of the established models are verified through verification experiment and the error analysis for the predictive results of the regression models. Taking into account the MRR, the TWR and the SR, multi-objective optimization is done on the PMND-EDM process based on the particle swarm optimization (PSO) algorithm. The pareto fronts of the MRR and the SR, the MRR and the TWR, and the SR and the TWR for the PMND-EDM process are obtained respectively. The process could decrease the TWR and overcome the problems accompanied with the large TWR, such as the frequent replacement of the worn tool electrode, when machined with large MRR; meanwhile, lower TWR can be obtained along with lower SR.
引文
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