EVALUATION OF RHEOLOGICAL PROPERTIES OF MEDIUM FOR AFM PROCESS

Abrasive Flow Machining (AFM) is a new non-traditional machining process used to deburr, radius, polish, and remove recast layer of components used in a wide range of applications. Material removal in AFM takes place by flowing medium (i.e. carrier/or putty mixed with abrasive particles), across the surface to be machined. The medium is the key element in the process because of its ability to precisely abrade the selected areas along its flow path. From the literature review, it is found that there is a need to study how to evaluate rheological properties of the medium in general, and viscosity in particular. Viscosity of the medium has significant effects on the AFM process performance.

In the present work, effects of concentration and mesh size of abrasive particles, and temperature of medium on the medium viscosity have been studied. To determine the viscosity of the abrasive medium, a viscometer has been designed and fabricated based on the principle of capillary viscometry. Experiments have been conducted at different abrasive concentrations and mesh sizes, and medium temperatures. It is observed from the experiments that the viscosity of the medium increases with the abrasive concentration and decreases with the abrasive mesh size and medium temperature. Theoretical values obtained from mathematical model, and experimental results are compared. The results of viscosity are correlated with the process performance parameters, i.e. material removal and surface roughness. It is observed that there is an increase in material removal and decrease in surface roughness value as viscosity of the medium increases.     

纳米尺度下流体的流变及尺度效应

应用表面力仪研究了流体在纳米尺度下的流变及尺度效应。试验发现，体相状态时表现为典型牛顿流体的16 烷，在纳米膜厚状态下，出现了等效粘度随切应变速率的增大而减小的剪切稀化现象，并且等效粘度依赖于振动剪切的频率。随着膜厚从宏观量级进入到纳米量级，流体的特性具有了显著的膜厚相关性，其表现为等效粘度明显增大，流体出现了弹性特征，摩擦力表现出复杂的非牛顿特性。其他流体也表现出类似的特性，这些流变特性的改变，说明纳米尺度下流体具有与体相状态完全不同的特性。     

Abrasive flow machining (AFM) finishing of conformal cooling channels created by selective laser melting (SLM)

Conformal cooling channels are widely adopted in the mold industry because of rapid and uniform cooling during injection molding. These complicated cooling channel geometries become feasible via selective laser melting (SLM) technology. However, the SLM fabricated internal channel surface shows high surface roughness of about 10 μm Ra. This rough surface can cause stress concentration, reducing the fatigue life of the mold. Therefore, the objective of this study is to investigate the surface finish of the SLM fabricated conformal channels by abrasive flow machining (AFM), which is widely used in the surface finishing of internal channels. To fulfill this objective, a combination of single/multiple and straight/helical channels for conformal cooling channel geometries are employed. Seven different types of conformal cooling channels (ø3mm) inside the bar are fabricated using SLM. The bar is put in the AFM fixture, and the internal channels are polished by flowing AFM media (ULV50%-54) through the channel at the same extrusion pressure of 80 bars for ten cycles. Fourteen bars (seven before AFM and seven after AFM) are machined to have the internal channel surfaces exposed for surface roughness measurement. Surface topographies of the exposed surfaces of seven types of internal channels are obtained using focus variation microscopy. The areal roughness parameters, such as arithmetical mean height (Sa) on the internal channel surfaces before and after AFM. By comparing SLM as-built conformal channel surfaces with AFM finished ones, AFM is shown to be effective in improving all SLM conformal cooling channels’ arithmetical mean height, Sa. Areal roughness parameters, such as developed interfacial area ratio (Sdr), root mean square gradient (Sdq), reduced peak height (Spk), reduced valley height (Svk), and skewness (Ssk), on those internal surfaces, were found to be sensitive to surface finishing by AFM.     

IMPROVED FABRICATION METHOD FOR CARBON NANOTUBE PROBE OF ATOMIC FORCE MICROSCOPY（AFM）

An improved arc discharge method is developed to fabricate carbon nanotube probe of atomic force microscopy (AFM) here. First, silicon probe and carbon nanotube are manipulated under an optical microscope by two high precision microtranslators. When silicon probe and carbon nanotube are very close, several tens voltage is applied between them. And carbon nanotube is divided and attached to the end of silicon probe, which mainly due to the arc welding function. Comparing with the arc discharge method before, the new method here needs no coat silicon probe with metal film in advance, which can greatly reduce the fabrication's difficulty. The fabricated carbon nanotube probe shows good property of higher aspect ratio and can more accurately reflect the true topography of silicon grating than silicon probe. Under the same image drive force, carbon nanotube probe had less indentation depth on soft triblock copolymer sample than silicon probe. This showed that carbon nanotube probe has lower spring constant and less damage to the scan sample than silicon probe.     

Finishing effect of abrasive flow machining on micro slit fabricated by wire-EDM

This experimental research use the method of abrasive flow machining (AFM) to evaluate the characteristics of various levels of roughness and finishing of the complex shaped micro slits fabricated by wire electrical discharge machining (Wire-EDM). An investigative methodology based on the Taguchi experimental method for the micro slits of biomedicine was developed to determine the parameters of AFM, including abrasive particle size, concentration, extrusion pressure and machining time. The parameters that influenced the machining quality of the micro slits were also analyzed. Furthermore, in the shape precision of the micro slit fabricated by wire-EDM and subsequently fine-finished by AFM was also elucidated using a scanning electron microscope (SEM). The significant machining parameters and the optimal combinations of the machining parameters were identified by ANOVA (analysis of variation) and the S/N (-to-noise) ratio response graph. ANOVA was proposed to obtain the surface finishing and the shape precision in this study.     

EVALUATION AND COMPARISON OF LUBRICANT PROPERTIES IN MINIMUM QUANTITY LUBRICATION MACHINING

Minimum Quantity Lubrication (MQL) machining involves the application of a minute amount of an oil-based lubricant to the machining process in an attempt to replace the conventional flood coolant system. Understanding the correlations between fluid properties and MQL performance can help in selecting lubricants from a variety of choices without going through extensive machining tests. This study compared nine different MQL fluids in terms of their physical properties, wettability, tribological properties (lubricity and extreme pressure (EP) properties), mist characteristics and machinability to determine the correlation of measured properties and MQL drilling and reaming performance. Results show that low fluid viscosity, high mist concentration, large mist droplet diameter and high wettability were best correlated with good machinability. Although it is difficult to draw strong relationships, the optimal machining in a mild cutting condition was found with the low viscosity fluids, which may also have the highest mist concentration, largest drops and best wettability.     

磨料流加工技术现状及展望

磨料流加工技术是机械光整加工中新发展起来的一项新工艺、新技术。简要介绍了磨料流加工技术研究的现状和基本机理，并在此基础上对磨料流加工技术的研究进行了展望，首次提出了磨料流加工虚拟技术的概念。     

Effect on crystalline structure of AISI M2 steel using tungsten–thorium electrode through MRR,EWR, and surface finish

To investigate on the crystalline structure of AISI M2 steel by using tungsten–thorium electrode in electrical discharge machining (EDM) process was studied. Furthermore, the investigation were carried out for finding the value of material removal rate (MRR), electrode wear rate (EWR) and surface roughness (SR) of tool steel material depending upon three variable input process parameters. On the basis of weight loss, the value of MRR and EWR were calculated at optimized process parameter. Subsequently, surface topography of the processed material were examined through different characterization techniques like scanning electron microscopy (SEM), Optical surface profiler (OSP) and Atomic force microscopy (AFM), respectively. In XRD study, broadening of the peak was observed which confirmed the change in material properties due to the homogeneous dispersion of the particles inside the matrix. Lowest surface roughness and MRR of 0.001208 mg/min was obtained. Minimum surface roughness was obtained 1.12 μm and 2.18427 nm by OSP and AFM study, respectively. Also, minimum EWR was found as 0.013986 mg/min.     

Artesunate induced morphological and mechanical changes of Jurkat cell studied by AFM

In this study, we have used atomic force microscopy (AFM) to study the morphology and mechanical property changes of Jurkat cells exposed to different concentrations of Artesunate (ART) for 24 h at single cellular level. Cell viability and proliferation assays were performed by using the Cell Counting Kit‐8. The concentration of ART, which resulted in the inhibition rate >50% was selected. The AFM images revealed that the cell membrane changed and the ultrastructure also became complex. Mechanical properties of individual cell were tracked with AFM‐based force spectroscopy. The force curves revealed that when a cell was exposed to the ART, the mechanical properties changed obviously. Treated cells had a lower adhesion force of 416.8±37.9 pN, whereas control group had a higher adhesion force of 1064.2±97.0 pN. The Young's modulus decreased to nearly one‐third, from control group of 0.648±0.037 kPa to treated group of 0.254±0.035 kPa and the stiffness increased to nearly 1.5 times, from control group of 1.231±0.084 mN/m to treated group of 1.917±0.137 mN/m. These results suggest that ART can inhibit the proliferation of Jurkat and induce changes in the morphological structure and mechanical properties of Jurkat cells. The high resolution and high sensitivity of AFM can be used to detect morphological and mechanical properties of cells exposed to ART. The AFM may be developed to be a useful tool for detecting the cell death and evaluating the anti‐carcinogen efficacy against tumor cell. SCANNING 31: 83–89, 2009. © 2009 Wiley Periodicals, Inc.     

MULTI-OBJECTIVE OPTIMIZATION OF ABRASIVE FLOW MACHINING PROCESSES USING POLYNOMIAL NEURAL NETWORKS AND GENETIC ALGORITHMS

Abrasive flow machining (AFM) is an economic and effective non-traditional machining technique, which is capable of providing excellent surface finish on difficult to approach regions on a wide range of components. With this method, it has become possible to substitute various time-consuming deburring and polishing operations that had often lead to non-reproducible results. In this paper, group method of data handling (GMDH)-type neural networks and Genetic algorithms (GAs) are first used for modelling of the effects of number of cycles and abrasive concentration on both material removal and surface finish, using some experimentally obtained training and testing data for brass and aluminum. Using such polynomial neural network models obtained, multi-objective GAs (non-dominated sorting genetic algorithm, NSGA-II) with a new diversity preserving mechanism are then used for Pareto-based optimization of AFM considering two conflicting objectives such as material removal and surface finish. It is shown that some interesting and important relationships as useful optimal design principles involved in the performance of AFM can be discovered by the Pareto-based multi-objective optimization of the obtained polynomial models. Such important optimal principles would not have been obtained without the use of both GMDH-type neural network modelling and multi-objective Pareto optimization approach.     

MULTI-OBJECTIVE OPTIMIZATION OF ABRASIVE FLOW MACHINING PROCESSES USING POLYNOMIAL NEURAL NETWORKS AND GENETIC ALGORITHMS

Abrasive flow machining (AFM) is an economic and effective non-traditional machining technique, which is capable of providing excellent surface finish on difficult to approach regions on a wide range of components. With this method, it has become possible to substitute various time-consuming deburring and polishing operations that had often lead to non-reproducible results. In this paper, group method of data handling (GMDH)-type neural networks and Genetic algorithms (GAs) are first used for modelling of the effects of number of cycles and abrasive concentration on both material removal and surface finish, using some experimentally obtained training and testing data for brass and aluminum. Using such polynomial neural network models obtained, multi-objective GAs (non-dominated sorting genetic algorithm, NSGA-II) with a new diversity preserving mechanism are then used for Pareto-based optimization of AFM considering two conflicting objectives such as material removal and surface finish. It is shown that some interesting and important relationships as useful optimal design principles involved in the performance of AFM can be discovered by the Pareto-based multi-objective optimization of the obtained polynomial models. Such important optimal principles would not have been obtained without the use of both GMDH-type neural network modelling and multi-objective Pareto optimization approach.     

磨粒流加工的固液两相流模型及压力特性模拟

以FLUENT软件为计算平台,采用Spalart-Allmaras固液两相Mixture湍流模型对磨粒流加工过程中磨粒流的流动形态进行了数值模拟,结果表明：增大压力差可提高通道中流体的平均速度,增大边界层与壁面流速差可提高加工效率;通过改变进口压力得到非稳态流场,能够使近壁面处的磨粒数目增多,有利于加工效率的提高。同时,模拟结果还反映了黏度对磨粒流加工有重要影响。数值模拟结果为磨粒流加工过程中的参数选择提供了参考依据。     

Modelling of surface finish,electrode wear and material removal rate in electrical discharge machining of hard-to-machine alloys

Hard-to-machine alloys are commonly used for industrial applications in the aeronautical, nuclear and automotive sectors, where the materials must have excellent resistance to corrosion and oxidation, high temperature resistance and high mechanical strength. In this present study the influence of different parameters of the electrical discharge machining process on surface roughness, electrode wear and material removal rate have been studied. Regression techniques are employed to model arithmetic mean deviation Ra (μm), peak count Pc (1/cm), material removal rate MRR (mm³/min) and electrode wear EW (%). All these parameters have been studied in terms of current intensity supplied by the generator of the electrical discharge machine I (A), pulse time t_i (μs), duty cycle η and open-circuit voltage U (V). This modelling allows us to obtain mathematical data and models to predict that the most influential factor in MRR and Ra is the current intensity and in the case of EW and Pc is the pulse time.     

Experimental and numerical investigations of material removal process in electrochemical discharge machining of glass in discharge regime

Electrochemical discharge machining (ECDM) can be applied as a non-traditional processing technology for machining non-conductive materials such as glass and ceramics, based on the phenomena of evoked electrochemical discharges around the tool electrode. The material removal mechanism of ECDM is noticeably complex and difficult to experimentally characterize. In this paper, finite element models were proposed to predict the material removal in the ECDM discharge regime. First, the single-pulse discharge on a tapered electrode was modeled. It was found that about 30.5% of the discharge energy is transferred to the workpiece. The continuous discharge on a cylindrical electrode was thereafter modeled according to this phenomenon, in which the removal of a layer of the workpiece material starts from the projected contour of the edge of the electrode end and extends inward during the ECDM processing. The effective discharge ratio for material removal was calculated to be 10.1%. The drilling depths of holes at different applied voltages were predicted by the proposed finite element method. It was found that the predicted values were consistent with the experimental results.     

Feasibility study of micro-slit EDM machining using pure water

This study addresses micro-slit EDM machining feasibility using pure water as the dielectric fluid. Experimental results revealed that pure water could be used as a dielectric fluid and adopting negative polarity EDM machining could obtain high material removal rate (MRR), low electrode wear, small slit expansion, and little machined burr, compared to positive polarity machining. In comparing kerosene versus pure water, pure water was observed to cause low carbon adherence to the electrode surface. Also discharge energy does not decrease and the discharge processes are not interrupted. Therefore, MRR was higher, and related electrode wear ratio compared to kerosene use was lower. In a continual EDM with multi-slit machining, kerosene will cause carbon element adherence, creating an initially high MRR and electrode wear, with rapid decline. However, pure water will not cause carbon element adherence on the electrode surface, so MRR and electrode wear is always stable in this process.     

异形内孔曲面的磨料流均匀加工方法研究

针对磨料流加工异形曲面一致性较差的问题,提出通过设置相似模芯结构以改善剪切应力分布的状态,从而实现均匀化加工异形曲面的方法。根据磨料流的剪切率与黏度的关系建立了幂律模型,通过COMSOL Multiphysics软件的 CFD模块计算得到置入模芯时磨料介质在加工表面的速度、剪切率和剪切应力的分布。仿真与实验表明:置入相似的模芯后可形成等宽的流道,各曲面受到的剪切应力趋向一致。工件内壁表面粗糙度Ra最大差值由无模芯时的0.376μm降为置入模芯后的0.017μm。     

Simulation of abrasive flow machining process for 2D and 3D mixture models

Improvement of surface finish and material removal has been quite a challenge in a finishing operation such as abrasive flow machining (AFM). Factors that affect the surface finish and material removal are media viscosity, extrusion pressure, piston velocity, and particle size in abrasive flow machining process. Performing experiments for all the parameters and accurately obtaining an optimized parameter in a short time are difficult to accomplish because the operation requires a precise finish. Computational fluid dynamics (CFD) simulation was employed to accurately determine optimum parameters. In the current work, a 2D model was designed, and the flow analysis, force calculation, and material removal prediction were performed and compared with the available experimental data. Another 3D model for a swaging die finishing using AFM was simulated at different viscosities of the media to study the effects on the controlling parameters. A CFD simulation was performed by using commercially available ANSYS FLUENT. Two phases were considered for the flow analysis, and multiphase mixture model was taken into account. The fluid was considered to be a Newtonian fluid and the flow laminar with no wall slip.     

The morphological and biomechanical changes of keratocytes cultured on modified p (HEMA‐MMA) hydrogel studied by AFM

The poor integration with host cornea tissue and the low mechanical properties of pHEMA hydrogel for artificial cornea remains a difficult problem to solve. A modified pHEMA hydrogel, MMA copolymerized and type‐I collagen and bFGF immobilized, was previously prepared in an attempt to solve the problems. In this study, the cytotoxicity of Col/bFGF‐p (HEMA‐MMA) and p (HEMA‐MMA) was studied by cell adhesion assay and atomic force microscopy (AFM). The results of cell adhesion assay show that the attachment of keratocytes on the modified membrane is much higher than that of the unmodified membrane. This indicates that the material after modification have better cell–material interaction. The AFM images reveal that the morphology of keratocytes cultured on different substrate is obviously different. The cell cultured on modified membrane presented a completely elongated and spindle‐shape morphology. The force?distance indicates that the biomechanical of keratocytes changes significantly after culturing on different substrates. The adhesion force (2328±523 pN) and Young's modulus (0.51±0.125 kPa) of the cell cultured on modified membrane are much higher, and the stiffness (0.08±0.022 mN/m) is lower than those of the cell cultured on unmodified membrane. These results show that the cytotoxicity of Col/bFGF‐p (HEMA‐MMA) for keratocytes is much improved. SCANNING 31: 246–252, 2009. © 2010 Wiley Periodicals, Inc.     

A NEW METHOD FOR MACHINING OF ELECTRICALLY NONCONDUCTIVE WORKPIECES USING ELECTRIC DISCHARGE MACHINING TECHNIQUE

In this study, a new method for machining of nonconductive ceramic workpieces in electric discharge machining (EDM) was developed. Machining surfaces of nonconductive workpieces were coated with a conductive layer (CL) and graphite powder was added to dielectric fluid for machining. Al₂O₃, ZrO₂, SiC, B₄C and glass workpiece samples were machined by using the method. Different machining conditions were tested for each sample and optimum machining parameters were determined. Effect of electrical conductivity, thermal conductivity and melting point of workpieces on material removal rate (MRR) was investigated. Optical microscope and SEM (Scanning Electron Microscope) surface photographs of workpieces taken after machining are presented and discussed.     

Novel methods for high-speed observation of material removal and molten pool movement in EDM

This paper proposes two new methods to observe discharge phenomena without interference from the plasma in electrical discharge machining. The first method uses a bandpass filter with a bandwidth of 800–820 nm and laser illumination with a wavelength of 800–820 nm. The second method also uses a bandpass filter with a bandwidth of 800–820 nm; however, in this method, the tungsten material is used not only as the tool electrode but also as the illumination source. First, the discharge process was observed using traditional methods to investigate the influence of the plasma on the observation of the discharge process. Then, the process of removing molten material from both the tool electrode and workpiece, as well as molten pool movement, was observed using the first method proposed in this paper. The material removed from the tool electrode was scattered upward along the end profile of the tool electrode, while the material removed from the workpiece was distributed along the horizontal direction. To explain these phenomena, the flow distribution in the gap was qualitatively analyzed using a fluid simulation. Finally, the discharge process was also observed using the second method proposed in this paper. A tungsten tool electrode can emit light with a wavelength of 800–820 nm, which shines on the observed region during the discharge process. The observation results verified the phenomenon of multiple explosions of the molten pool during the discharge process. Moreover, it was found that the material removed by the explosion of the molten pool was scattered in different directions and hardly influenced by the flow distribution in the gap.