Experimental Investigation and Optimization of Machining Process Parameters in AISI D2 Steel Under Conventional EDM and Cryogenically Cooled EDM Process

Electrical discharge machining (EDM) is a well-established machining option for manufacturing geometrically complex or hard material parts that are extremely difficult-to-machine by conventional machining processes. In this research work, investigations were carried out on the machining of AISI D2 tool steel under conventional EDM and cryogenically cooled EDM process (CCEDM). The input process parameter such as current (I_p), pulse on time (T_on) and gap voltage (V) were investigated and the output responses like electrode wear ratio, metal removal rate and surface roughness were explored. Experiments were conducted using L₁₈ orthogonal array in EDM machine. Experimental results showed reductions in the electrode wear of 16% over CCEDM process. A grey relational analysis was used to solve the EDM process with multi performance characteristics to optimize EDM parameters.     

正交磁场对电火花单脉冲放电中电蚀凹坑的影响规律

电火花放电通道在正交磁场的作用下向洛伦兹力方向偏转、延伸,进而导致电蚀凹坑形貌发生变化,研究磁场辅助电火花加工(MF-EDM)过程中电蚀凹坑的形貌变化规律及特点,对进一步明晰电火花加工机理具有重要意义. 基于MF-EDM气中单脉冲放电试验,使用表面轮廓仪观测电蚀凹坑延伸长度、深度、宽度及放电起始点偏移量,并得出磁场及放电参数对电蚀凹坑的影响规律. 结果表明:电蚀凹坑长度随着磁感应强度、开路电压的增大而增大;电极外伸长度的影响结果相反;电蚀凹坑深度随着磁感应强度、开路电压、电极外伸长度的增加没有明显的变化规律;电容与磁感应强度存在最优参数组合以使凹坑长度最大;随着磁感应强度及放电能量的增加放电起始点的偏移量增加.      

压痕对车轴钢疲劳极限的影响

利用硬度计在光滑沙漏状车轴钢疲劳试样上制造压痕,同时利用电火花在试样上加工缺陷,通过疲劳试验研究两种缺陷尺寸与试样疲劳极限之间的关系.将两类试样的测试结果和基于材料硬度、缺陷投影面积的Murakami模型计算结果进行对比.利用扫描电镜观察试样疲劳断口.结果表明,与计算结果相比较,压痕局部塑性变形导致的加工硬化和残余应力对试样的疲劳强度没有影响,裂纹依然从应力集中最大的压痕底部起裂.电火花缺陷表面粗糙度较大引起二次缺口效应,表面硬脆的重铸白层上还有微孔和微裂纹存在,导致此类试样疲劳强度低于模型计算结果,裂纹从电火花缺口底部多处萌生.      

Effect of Method of Sample Preparation on Strength of Zirconium Diboride (ZrB<Subscript>2</Subscript>)

Different surface finish of ZrB₂ was generated by machining the hot pressed monolithic ZrB₂ by wire-electrical discharge machining (EDM) as well as conventional diamond grinding. The effect of surface finish on strength was studied. Our results showed that wire-EDM produced very high roughness (Ra = 1.2) as compared with diamond grinding (Ra = 0.2 and 0.33) which resulted into lower strength of ZrB₂ for EDM machined sample as compared to diamond ground samples.     

How Suspended Particles Affect Surface Morphology in Powder Mixed Electrical Discharge Machining (PMEDM)

The effect of SiC powder mixing into the dielectric liquid on surface topology and structure of interstitial free (IF) steel has been studied. Four process parameters, namely pulse duration, pulse current, concentration of powder, and type of dielectric liquid material, have been selected as varying parameters. Surface modifications due to suspended SiC particles were identified by using optical, scanning electron microscopy, and X-ray diffraction analysis. It was observed that suspended particles around discharge column accelerated and gained sufficient velocity to penetrate to the molten pool before solidification by means of electrophoresis and negative pressure induced after cessation of a discharge, which leads a surface embedded with added fine particles under that prevent formation of penetrating cracks during machining. However, surface cracks that formed due to high transformational stresses developed during solidification were found to be unaffected by means of mechanical action of the particles.     

不同电极材料对电火花成型加工特性的实验研究

电火花成型加工过程中,不同的电极材料对成型工件的表面质量及加工时间均有影响,本文在相同的电参数与非电参数下分别以石墨、钨铜、紫铜3种工具电极材料来加工不锈钢工件并对实验的结果进行分析.实验结果发现:钨铜电极成型的工件表面质量最好,可以达到0.841um的高精度水平,满足航空航天精密零件的加工要求,同时加工时间为12.6...     

电火花加工硬质合金模具的研究

对电火花加工硬质合金模具的电参数选择进行了实验研究,对工具电极材料和尺寸计算进行了探讨,分析了影响电火花加工的因素和电火花加工对硬质合金模具寿命的影响,简述了葬送３复杂的工具电极加工应用ＣＡＤ／ＣＡＭ来完成和复杂表腔的模具可采用数控电火花加工。     

Influence of Dielectric Type on Porosity Formation on Electrical Discharge Machined Surfaces

This work presents a comparative study on the performance of three types of dielectrics at different electrical discharge machining (EDM) parameter combinations favorable for generating porosity. The characteristics of the porosity formation in the recast layer formed in water/oil emulsions (W/O) were investigated by comparing them with those present in the recast layer formed in kerosene and deionized water dielectric. The results showed that the porosity characters were influenced significantly by the dielectric type. The formation mechanism of the porosity in different dielectrics is presented in this article.     

Preparation of Metallic Aluminum Compound Particles by Submerged Arc Discharge Method in Aqueous Media

Fine metal particles are produced by chemical methods, which add surfactants to control particle size and concentration. This study used the submerged arc discharge method (SADM) to prepare metal fluid containing nanoparticles and submicron particles in pure dielectric fluid (deionized water or alcohol). The process is fast and simple, and it does not require the addition of chemical agents. The SADM uses electrical discharge machining (EDM) equipment, and the key parameters of the production process include discharge voltage, current, and pulse discharge on-off duration. This study added a capacitive component between the electrodes and the electrode Z-axis regulation in the control parameters to render the aluminum fluid process smooth, which is the main difference of this article from the literature. The experimental results showed that SADM can produce aluminum particles from nanometer to submicron grade, and it can obtain different compounds from different dielectric fluids. The dielectric fluids used in this study were deionized water and ethanol, and aluminum hydroxide Al(OH)₃ particles with suspending power and precipitated aluminum particles were obtained, respectively. The preparations of metal colloid and particles by the SADM process have the characteristics of low cost, high efficiency, high speed, and mass production. Thus, the process has high research value and developmental opportunities.     

A Comparative study on the Performance of Different Sintered Electrodes in Electrical Discharge Machining

In this research two different materials have been machined by sintered copper, brass, and bronze electrodes in electrical discharge machine. The input parameters are varied and output responses have been studied by adopting design of experiments. Response surface methodology has been adopted for analysis of the parameters. Mathematical model has been formulated for all sintered electrodes and to optimise the parameters to obtain maximum material removal rate , minimum tool wear rate and minimum surface roughness. The effects of interaction of parameters has been studied among the three sintered electrodes. Copper electrodes removes high rate of material compared with sintered brass and sintered bronze electrode. Tool wear rate is high for brass electrode.     

Surface Integrity Assessment Upon Electric Discharge Machining of Die Steel Using Non-Destructive Magnetic Barkhausen Noise Technique

Surface integrity characterization of manufactured component is very important as it significantly affects the in-service performance of the component. Till now, surface integrity was evaluated using conventional measurement technique like microhardness tester, X-ray diffraction, optical microscopy and surface roughness tester. But, this technique being laboratory based cannot be used for in-service monitoring of the surface integrity. The present study focuses on the characterization of surface integrity upon electric discharge machined sample using non-destructive magnetic Barkhausen noise technique. Electric discharge machining was performed in die-sinking mode on die steel using copper–tungsten electrode (negative polarity). Experiment was performed by selecting different levels of peak current, gap voltage and pulse on time. Surface integrity characteristics like microhardness change, residual stress, microstructural alteration and surface roughness were analysed using microhardness tester, X-ray diffraction, optical microscopy and surface roughness tester, respectively, and were then correlated with magnetic parameter like root mean square value and peak value obtained from Barkhausen noise signal. The results show a good correlation between magnetic parameter (RMS and Peak value) of Barkhausen noise with the microhardness and surface roughness of the machined sample.

     

Machining Characteristics of Ce-ZrO2/CePO4 Ceramics

Two-phase mixtures of Ce-ZrO2 and monazite-type CePO4 were fabricated. Drilling and grinding experiments were carried out to investigate the machining characteristics of Ce-ZrO2/CePO4 ceramics. The machined surfaces of ceramics and wear surfaces of drill bits were observed with scanning electron microscope. Material removals and grinding forces were measured. The transgranular fracture of CePO4 grains, intergranular fracture between ZrO2 and CePO4 grains, and ductile deformation of ceramics were observed on Ce-ZrO2/CePO4 machined surfaces. With the increase of CePO4 proportion to composites, drilling material removal rates increases and specific normal grinding forces decreases.There existed rapid wear of conventional metal cutting tool is caused by abrasive wear. The experimental results indicate that the weak interfaces and properties of Ce-ZrO2/CePO4 ceramics have influences on material removal and machinability.     

Microstructural Investigations of the White and Deformed Layers Close to the Turned Surface of Ti-6Al-4V

In the aircraft industry, along with geometrical and dimensional integrity, the surface integrity of manufactured parts is a necessity. In fact, severe anomalies generated during machining may have a substantial impact on the lifetime of the parts. Nevertheless, these anomalies are not well known in terms of microstructures such as the white layer in titanium alloys. Based on this observation, the present paper deals with microstructural investigations performed on Ti-6Al-4V white and deformed layers generated during turning with a round uncoated carbide insert. The aim of this study is to characterize these anomalies in terms of microstructure and phases. In particular, this study provides a better understanding of metallurgical transformations in the sublayer of machined surfaces through qualitative models.

     

Mechanics of Intermittent Plasticity Punctuated by Fracture During Shear Deformation of Mg Alloys at Near-Ambient Temperatures

Microstructure evolution of basal-textured Mg alloy AZ31B (Mg: Al: Zn; 96: 3: 1 wt pct) during simple shear deformation at near-ambient temperatures was studied by plane-strain machining. Using Schmid factor calculations in conjunction with quantitative electron microscopy, it was found that plastic deformation in AZ31B in the primary deformation zone of machining commences by extension twinning followed by basal slip. Characteristics of twinning in individual grains were described by correlating the direction of twinning with the principal stress state. The implications of these deformation mechanics for the microstructure inherited by the freshly generated surfaces in shear-based material removal processes are examined. These include the identification of extensive surface texture reorientation at machined surfaces via extension twins, limits on surface integrities wrought by fracture events that punctuate plastic deformation, and their relationship to the cutting tool geometry.     

The effects of surface hardening on fracture toughness of carburized steel

This research program was carried out to evaluate the effects of surface hardening on the fracture toughness of carburized steel. The materials AISI 8620 steel was machined into compact-tension (CT) specimens. The specimens were pack carburized at 930°C (1706°F) for different periods of time, cooled to ambient temperature and subsequently tempered at various temperatures for one hour. The fractured specimens were examined by hardness tests, metallography, X-ray diffraction analysis for retained austenite in the case, and scanning electron microscope fractographic analysis of the fracture surfaces. The experimental results revealed that theKIC values of the carburized, AISI 8620 steels were improved by the increase in case depth. Martensitic/tempered-martensitic structure in the case was the major constituent contributing to the improved toughness. The amount of retained austenite at the case increased as the thickness of the hardened layer increased. But retained austenite as well as large grain size were found to have adverse effects on fracture toughness of the carburized steel. The tempering temperature of 500°C (932°F) provided maximumKIC values. Higher tempering temperatures resulted in sharp decrease of fracture, toughness values. W{upeio}-Y{upoue} H{upo}, formerly a Graduate Student, in the Department of Materials Engineering Tatung Institute of Technology, is in compulsory 0 ROTC military service of Republic of China.     

The influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L

In this article, the influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L austenitic stainless steel bar is examined. Hot-forged bar and the quenched bars (at different quenching temperatures, 1050 °C, 1100 °C, 1150 °C, and 1200 °C) are machined at a high cutting speed. It was noted that the inhomogeneous distribution of grain size in the surface area, within a depth of 15 mm of the workpiece, resulted in tool edge breakage and lower tool life when machining the hot-forged bar compared with all of the quenched bars. In addition, a slight decrease in tool life was observed as the grain size increased in the quenched bars. The chip studies revealed that a higher segment height ratio of chip was gained when machining the hot-forged bar, compared to machining the quenched bars, due to the inhomogeneous distribution of grain size. Moreover, the thickness of the secondary shear zone was reduced as the grain size increased. Interestingly, it was noticed that the chip work hardened during the machining process due to strain-induced twinning and ɛ martensite transformation. The studies of tool wear and failure revealed that a crack was initiated on the flank face at the interface between the deposited workpiece and the tool substrate when machining the hot-forged bar. This crack was formed due to either the thermal and mechanical fatigue or plastic deformation of the tool substrate. The fatigue crack propagated into the tool substrate through the decohesion of interface between carbides. The criterion of tool life when machining all of the quenched bars was normal flank wear. Based on the studies of chip deformation and the mechanisms for tool wear and failure, the effects of austenite grain size and its distribution on tool life were explained.     

Simulation of Surface Integrity for Nanopowder-Mixed Dielectric in Micro Electrical Discharge Machining

The use of powder-mixed dielectric for electrical discharge machining (PMD-EDM) and micro-electrical discharge machining (PMD micro-EDM) has been shown to reduce machined surface roughness and surface defects, such as, recast layer thickness. However, these were mainly empirical studies which had limited capacity to elucidate the physics and characteristics of the PMD-EDM process. In the current study, a new numerical model has been developed to describe the PMD micro-EDM process. The model has taken into account the larger expansion of the plasma channel and reduction in heat flux distributed to the workpiece because of the presence of powder particles. It has been analyzed using finite element methods (FEMs) and based on the multiple discharge approach. The heat source expansion factor due to the presence of powder particles has been estimated to be 1.07 at 0.02 g/L powder concentration. The fraction of heat flux to the workpiece has been found to decrease from 0.24 at 0.02 g/L to 0.11 at 0.2 g/L powder concentration. For model validation, simulated R _max and recast layer thickness values were compared with their corresponding measured values. The simulated and measured results varied with the same general trend which suggested that the proposed process model for PMD micro-EDM was fundamentally sound.     

On Heat Treatment and Surface Characterization of Spark Eroded Nickel-Based Superalloy Developed by Additive Manufacturing

In this study, a nickel-based superalloy Inconel 718 material was developed through direct metal laser sintering process. To enhance the mechanical properties, printed material was heat-treated at two different conditions in the combination of solution and ageing. The transformations of microstructure from original anisotropic phase were studied using optical microscope. The changes in mechanical properties were evaluated. The hardness of the heat-treated samples obtained is 35HRC and 39HRC for the heat treatment plan at 1100 °C + 845 °C and 980 °C + 720 °C (solution + ageing), respectively. Further, micro-holes were machined on these two material samples using spark erosion micro-machining or micro-electric discharge machining (µEDM) process. Applied voltage, capacitance and material hardness were three major variable parameters. Experiments were performed with hollow tungsten carbide rod of 300 µm in diameter as a drill tool rotating at a speed of 1500 rpm for a constant machining time of 30 min. Optical and scanning electron micrographs were used to analyse the hole quality and therefore the performance of µEDM. Perfect concentric drill hole was produced with 35HRC sample than the other. Weld spatters and metal drops were found around the surface of 39HRC sample. The range of drill hole varied to a maximum of ϕ385 µm in low-hardness and ϕ400 µm in high-hardness samples. The depth of drill hole was maximum with high hard sample at 100 V and 100 nF process condition. Light spectroscope-based roughness characterization revealed that the minimum surface roughness measured was 6.946 µm in low hard and 9.829 µm in high hard material.     

基于微晶刚玉砂轮的20 CrMnTi齿轮成型磨削表面完整性

为研究微晶刚玉砂轮成型磨削20CrMnTi齿轮的表面完整性,开展了20CrMnTi齿轮成型磨削试验,分析了砂轮线速度、轴向进给速度及径向进给量对齿面粗糙度、表层/次表层显微硬度、微观组织和残余应力的影响规律,探讨了由磨削引起的磨削烧伤、微观裂纹等损伤缺陷的形成机理,结果表明:径向进给量对表面粗糙度的影响最显著,砂轮线速度次之,轴向进给速度最不显著;磨削温度过高会导致磨削烧伤,淬火烧伤使得表面硬度提高5%~20%,回火烧伤则导致表面硬度不同程度地下降;表层组织从外至内分别为白层、暗层和基体组织,白层主要由致密的马氏体+碳化物+残余奥氏体组成;砂轮线速度和径向进给量的增大使得由磨削引起的残余拉应力增大,表面残余压应力下降并逐渐向拉应力转变,当表面最终残余拉应力大于材料的断裂强度时,表面产生微观裂纹.