DUCTILE-REGIME MACHINING OF PARTICLE-REINFORCED METAL MATRIX COMPOSITES

ABSTRACT

This paper presents research results on ultraprecision machining of metal matrix composite (MMC) composed of aluminum matrix and either SiC or A1₂ 0₃ particles. Ductile-regime machining of both SiC and aluminum was evaluated to improve the surface integrity of the composite. Both polycrystal-line diamond (PCD) and single crystalline diamond (SCD) tools were used to ultraprecision machine the composites at a depth of cut ranging from 0 to 1μm using a taper cut. The feedrate was normalized to the tool nose radius. A model is proposed to calculate the critical depth of cut for MMCs reinforced with either A1₂0₃ or SiC. The critical depths of cut were found to be 1 p.m and 0.2 u.m for MMCs reinforced with A1₂ 0 or SiC₃, respectively. Both depth of cut and crystallographic direction of the ceramic particles are the sufficient conditions for ductile-regime machining. Although both tools produce similar surface finish, a SCD tool removed the MMC as chips while a PCD tool simply smeared the surface. A diffusion-abrasion mechanism was suspected to cause the surprising wear of the SCD tools when machining the aluminum/SiC composite.     

INVESTIGATION OF NANO-STRUCTURED PVD COATINGS FOR DRY HIGH-SPEED MACHINING

The objective of this paper is to investigate the performance of different categories of hard PVD coatings in terms of friction and tool wear under dry high-speed machining (HSM) conditions. In this study five different categories of commercially available coatings (nano-composite AlTiN/Si₃N₄, nano-crystalline Al₆₇Ti₃₃N and mono-layered Ti₁₀Al₇₀Cr₂₀N) and experimental nano-multilayered coatings (Ti₂₅Al₆₅Cr₁₀N/BCN and Ti₂₅Al₆₅Cr₁₀N/WN) were studied by machining hardened steel AISI H13 (HRC 50). The coefficients of friction against steel versus temperature were measured. Tool wear and cutting forces were measured in-situ under dry high speed machining conditions. The morphology of the worn tools and the chips collected during cutting were studied using an SEM (Scanning Electron Microscopy) and the EDX (Energy Dispersive X-ray analysis). The cutting temperatures were estimated based on the color of the chips generated during cutting. The comparison among these categories of coatings was conducted based on tool wear, coefficient of friction, cutting forces and chip formation. From this study, it was revealed that the solid self-lubricating layers, automatically formed in the cutting zone under elevated temperatures, play a key role in leading to a significant improvement of tool performance under dry high-speed machining.     

Electrochemical machining analysis on grid cathode composed of square cells

During the electrochemical machining (ECM), the cathodes designed by the existing methods are mainly unitary cathodes, which can be only used to produce the workpieces with the same shapes. However, there are few researches on designing cathodes for machining the different workpieces with the different surfaces. This paper presents the grid cathode composed of the square cells to produce the workpieces with different shapes. Three types of the square cells, 2.5 mm′2.5 mm, 3 mm′3 mm, and 4 mm′4 mm, are utilized to construct the plane, the slant, and the blade cathode. The material of the cathode and the anode is CrNi 18 Ti 9 , and the ingredient of electrolyte is 15% NaCl and 15% NaNO 3 . The machining equilibrium machining current and time are acquired and analyzed, the machining process and the workpiece quality are compared between using the grid cathode and the unitary cathode. Moreover, the machining errors on the workpiece surface are measured and analyzed, and the error reasons are traced and discussed to obtain the better surface quality of the workpiece. The experiment and analysis results show that the grid cathode can be used to manufacture the workpieces with complex shapes in certain range of the error. The workpiece quality improves with the size of the square cell being reduced, and if the square element is small enough, the workpiece quality is almost equal to the one machined by the unitary cathode. The proposed research realizes a single cathode machining the different workpieces with the different surfaces.     

An experimental work on using conductive powder-filled polymer composite cast material as tool electrode in EDM

This paper introduces the composite tool electrodes made of electrical conductive powder-filled polyester resin matrix material, providing promise for the electrical discharge machining (EDM) process. The dendrite-shaped copper powder, graphite powder, and their mixture were used as conductive fillers. Six different types of composite electrodes, namely, plain copper-polyester, pressed copper-polyester, furnaced copper-polyester, plain copper-graphite-polyester, pressed copper-graphite-polyester, and furnaced copper-graphite-polyester were prepared. It is found experimentally that increasing v _f improved workpiece material removal rate, tool wear rate, relative wear, and electrical conductivity of electrodes. The pressed copper-polyester electrodes were found to be promising in the ED finishing of workpieces at low machining current settings. The practical applicability of the proposed composite electrodes in the industry was also illustrated.     

MACHINING CHARACTERISTICS OF COMPLEX SURFACES USING FAST TOOL SERVO SYSTEM

Optical components with complex surfaces are more and more widely applied, but it is very difficult to manufacture these components by using traditional mechanical fabricating methods. Fast tool servo system can manufacture these complex surfaces or microstructures efficiently and accurately. The relative position between the tool and workpiece surface will vary continuously in the fast tool servo machining process, owing to the height change of workpiece profile in the same circle, and this will worsen the cutting conditions and debase the machining accuracy. In this paper, the cutting characteristics are studied in the fast tool servo machining process of complex workpiece, including the varying rule of cutting angle, and its influences on the rake angle and back angle, and the choice of machining parameters. Furthermore, the conditions for identifying tool interference are given. On the basis of the above work, two kinds of typical complex workpieces are manufactured by using fast tool servo system, including radial sinusoidal workpieces and lens array. The measuring results indicate that surface accuracy can reach 0.14 μm (peak-to-valley value) and the roughness is less than 10 nm (mean value).     

Experimental study on ECM with a grid cathode composed of circular cells

In order to resolve the problem of single machining object existing in traditional electrochemical machining (ECM) with unitary cathode, a grid cathode composed of circular cells is used to produce the workpieces with different shapes. Three types of circular cells, ??1.5 mm, ??2.0 mm, and ??2.5 mm, are utilized to construct the plane, slant, and blade cathode. The material of the cathode and the anode is CrNi₁₈Ti₉, and the ingredient of electrolyte is 15% NaCl and 15% NaNO₃. The machining balance current and the balance time are acquired and analyzed, the machining process and the workpiece quality are compared between using the grid cathode and the unitary cathode. Moreover, the machining errors and the error reasons of workpiece surface are analyzed. Research shows that the grid cathode can be used to manufacture workpieces with a complex shape, and the workpiece quality is better if the circular cell is smaller. If the circular cell is small enough the workpiece quality is almost equal to it machined by the unitary cathode.     

Mechanism of atomic removal in elastic emission machining

Elastic emission machining (EEM) can be thought of as a machining method utilizing the chemical activity of a particle surface, rather than a liquid echant as in chemical etching. Selecting the combinations of silicon as workpiece material and SiO₂, Al₂O₃ and ZrO₂ as the powder particles, the material removal rate during machining was examined. Different combinations of workpiece materials and powders affected removal rates strongly, so that the machining process was considered from the standpoint of the atomic interactions at the interface between the workpiece and the powder particles. The surface voltage of the workpiece with adsorbed powder particles was measured as one of the parameters representing the interfacial interactions, and correlations with removal rates were obtained.     

MACHINING OF FIBER-REINFORCED COMPOSITES

Abstract

Since the introduction of glass fiber-reinforced polymer composites in the early 1940s, composite materials development was driven by the needs of space, defense, and aircraft industries where performance rather than cost was the prime consideration. At the beginning, conventional machining techniques were adopted to machine glass fiber-reinforced composites for convenience as well as to keep the capital costs down. This was followed by significant advancements in tool materials and tooling design. With the development of new and more challenging metal-matrix and ceramic-matrix composites, conventional manufacturing processes proved to be inadequate or even inappropriate to process them. Need and opportunity, therefore, exists for alternate nontraditional machining operations, such as laser machining, water jet (WJ) and abrasive water jet (AWJ) cutting, electrical discharge machining, ultrasonic-assisted machining, and electrochemical spark machining. When composites become more popular and are used in large volume in the civilian sector, such as auto and other consumer industries, material and processing costs will be the driving factors. A high degree of automation for the mass manufacturing of composite parts will be required to bring the costs down and compete with other materials. Advancements in the nontraditional machining processes offer an opportunity to process these materials economically, thus realizing the full potential of the composite materials. This paper gives a broad overview on the various issues involved in machining (conventional and nonconventional) of fiber-reinforced composites. The field of composites, in general, and machining of composites, in particular, are so broad that it would not be possible to do justice by discussing each aspect of composite material machining without ending up with a voluminous document. This review, therefore has to be limited to a few aspects of composite materials and their machining techniques. It may also be pointed out that in this review certain areas are dealt more in-depth than others. Personal preferences and availability of material in the open literature are some of the reasons for this nonuniformity in coverage. Also, some areas are more actively pursued than others. An attempt is made to highlight some of the issues and opportunities in the area of machining of composites.     

A REPORT FROM THE CIRP WORKING GROUP ON MODELING OF MACHINING OPERATIONS

Abstract

This paper describes the goals and activities of the CIRP Working Group on Modeling of Machining Operations. The basic aim is to serve the metal-cutting industry through the development of better scientific models that will make it possible to reliably predict machining performance. Many different types of models for metal-cutting operations are classified and evaluated. The working group decided to concentrate on the development of models that could predict the precision of workpieces for the most frequently used operations such as turning, milling, and hole making. A large group of active international researchers are currently participating in this cooperative effort. Further contributions from other researchers all over the world are invited.     

The drilling of Al2O3 using a pulsed DC supply with a rotary abrasive electrode by the electrochemical discharge process

The electrochemical discharge machining (ECDM) process has the potential to machine electrically non-conductive high-strength, high-temperature-resistant (HSHTR) ceramics, such as aluminum oxide (Al₂O₃). However, the conventional tool configurations and machining parameters show that the volume of material removed decreases with increasing machining depth and, finally, restricts the machining after a certain depth. To overcome this problem and to increase the volume of material removed during drilling operations on Al₂O₃, two different types of tool configurations, i.e., a spring-fed cylindrical hollow brass tool as a stationary electrode and a spring-fed cylindrical abrasive tool as a rotary electrode, were considered. The volume of material removed by each electrode was assessed under the influence of three parameters, namely, pulsed DC supply voltage, duty factor, and electrolyte conductivity, each at five different levels. The results revealed that the machining ability of the abrasive rotary electrode was better than the hollow stationary electrode, as it would enhance the cutting ability due to the presence of abrasive grains during machining.     

薄壁弧形件装夹布局有限元优化

关于航空结构件加工变形控制的研究是高效数控加工研究的一部分。薄壁弧形零件加工中的弹性变形对加工质量影响很大,而装夹布局影响切削变形的大小和分布。以减少加工中工件最大弹性变形为目标,建立了弧形件铣削加工装夹布局的优化模型,采用商业有限元软件的设计优化模块进行计算。在对计算结果进一步分析的基础上,提出了最终的装夹布局方案,采用该方案可以得到整个加工过程中更低的变形量,变形分布更均匀,为采取相应数控补偿措施提供条件。优化方案和实际加工方案结果基本一致。所提出方法可推广至其他类型工件夹具布局优化设计。     

Complex phenomena study in dielectric fluid from gap during the W-EDM processing in ultrasonic field

Titanium alloys have been widely used in the aerospace industry due to its outstanding properties. However, the poor thermal conductivity and high chemical reactivity impair titanium alloys machinability in conventional machining, which make it to be one of the typical difficult-to-machine materials. Electrical discharge machining (EDM) becomes the best alternative to machine titanium alloys. This paper focuses on investigating surface integrity of low-speed wire electrical discharge machining (LS-WEDM) in machining Ti-6Al-4V (TC4) by multiple cuts namely one main cut (MC) followed by trim cuts (TC). The machining parameter levels of multiple cuts and offsets were modified aimed at TC4 material, and the surface roughness of 0.67 μm was obtained after one MC and three TC. In addition, scanning electron microscopy (SEM) was used to observe the surface microstructure, which is characterized by an uneven fusing structure, spherical droplets, irregular droplets, craters, cracks, and micro-void; Moreover, it can be observed that cracks usually began with the edge of micro-voids, and the debris on the machined surface were deformed fragments due to the low thermal conductivity makes TC4 material be ejected or solidified before completely melting. Furthermore, the foreign elements of Cu and Zn were detected in the white layer by energy dispersive spectrograph (EDS) integrated in SEM; it also can be found that the crater edge has more Cu and Zn elements than crater center. The white layers were predominantly nonuniform and discontinuous due to poor penetration hardening of TC4 material. After the third TC, the white layer has become more continuous and the thickness was reduced to 2.7 μm, which was nearly invisible. The hardness of the white layer was almost the same as the base material. Finally, the blueviolet film was observed on the TC4 workpiece surface due to the electrolysis making the surface oxidation. By using X-ray diffraction (XRD), it is confirmed that TiO₂, Ti₂O₃, and TiO existed in the oxidation film. The technique and knowledge that this study proposed could provide a significant contribution to electrical discharge machining surface improvement.     

ELECTRIC DISCHARGE MILLING AND MECHANICAL GRINDING COMPOUND MACHINING OF SILICON CARBIDE CERAMIC

Silicon carbide (SiC) ceramics have been widely used in modern industry. However, the manufacture of SiC ceramics is not an efficient process. This paper proposes a new technology of machining SiC ceramics with electrical discharge milling and mechanical grinding compound method. The compound process employs the pulse generator used in electrical discharge machining, and uses a water-based emulsion as the machining fluid. It is able to effectively machine a large surface area on SiC ceramics with a good surface quality. In this paper, the effects of pulse duration, pulse interval, peak voltage, peak current and feed rate of the workpiece on the process performance parameters, such as material removal rate, relative electrode wear ratio and surface roughness, have been investigated. A L₂₅ orthogonal array based on Taguchi method is adopted, and the experimental data are statistically evaluated by analysis of variance and stepwise regression. The significant machining parameters, the optimal combination levels of machining parameters, and the mathematical models associated with the process performance are obtained. In addition, the workpiece surface microstructure is examined with a scanning electron microscope and an energy dispersive spectrometer.     

INFLUENCE OF MACHINING ON FRICTION AND WEAR OF LUBRICATED CERAMICS IN SLIP-ROLLING CONTACTS

Abstract

The slip-rolling friction and wear tests were performed in a twin disc tribometer of the Amsler-type at a Hertzian pressure of 3 GPa over 2 million revolutions. Paraffinic oil with no additives was used as lubricant. The ceramics were machined with different processes, resulting in different surface roughness (i.e., rough and fine honed, rough and fine ground, rough and fine lapped, and rough and fine polished). Ceramic materials like HIP-Si₃N₄, S-SiC, HIP-ZrO₂, and GPS-Si₃N₄-TiN were investigated as self-mated couples. This paper summarizes the results. Si₃N₄, Si₃N₄-TiN, and ZrO₂ generally exhibit a small wear coefficient in the range of 10^?9 mm³/Nm in paraffinic oil and their wear coefficients correlate with the initial surface roughness and the material removal rate. The lowest wear coefficients were exhibited by ZrO₂- With a reduction of the Hertzian pressure to 1.5 GPa, S-SiC exhibits the same tribological behavior as the other ceramics.     

Parametric optimization of CNC turning process for hybrid metal matrix composite

Machining of hybrid metal matrix composite is difficult as the particulates are abrasive in nature and they behave like a cutting edge during machining resulting in quick tool wear and induces vibration. An attempt was made in this experimental study to evaluate the machining characteristics of hybrid metal matrix composite, and a mathematical model was developed to predict the responses, namely surface finish, intensity of vibration and work-tool interface temperature for known cutting condition while machining was performed in computer numerical control lathe. Design of experiments approach was used to conduct the trials; response surface methodology was employed to formulate a mathematical model. The experimental study inferred that the vibration in V _x, V _y, and V _z were 41.59, 45.17, and 26.45 m/s², respectively, and surface finish R _a, R _q, and R _z were 1.76, 3.01, and 11.94 μm, respectively, with work-tool interface temperature ‘T’ of 51.74 °C for optimal machining parameters, say, cutting speed at 175 m/min, depth of cut at 0.25 mm and feed rate at 0.1 mm/rev during machining. Experimental results were in close conformity with response surface method overlay plot for responses.     

Modeling of wire electro-discharge machining of TiC/Fe in situ metal matrix composite using normalized RBFN with enhanced k-means clustering technique

In the present work, wire electro-discharge machinability of 5 vol% TiC/Fe in situ metal matrix composite (MMC) has been studied. Four input process parameters such as pulse on-time, pulse off-time, wire feed-rate, and average gap voltage have been considered, while cutting speed and kerf width have been considered as the measure of performance of the process. The presence of nonconductive TiC particles and formation of Fe₂O₃ during machining make the process very much unstable and stochastic. Thus, modeling the process either by an analytical or numerical method becomes extremely difficult. In the present study, modeling of wire electro-discharge machining process by normalized radial basis function network (NRBFN) with enhanced k-means clustering technique has been done. In order to measure the effectiveness of this approach, the process has also been modeled by NRBFN with traditional k-means technique, and a comparison has been made between the two models. It is seen that both the models can predict the cutting speed and kerf width successfully, but NRBFN with enhanced k-means clustering technique yields better results than NRBFN with traditional k-means technique. Both the models have been used to carry out the parametric study and, finally, have been compared with the experimental results.     

LUBRICATION MECHANISMS OF LN2 IN ECOLOGICAL CRYOGENIC MACHINING

ABSTRACT

Cryogenic machining is considered an environmentally safe alternative to conventional machining where cutting fluid is used. In cryogenic machining, liquid nitrogen (LN2) is well recognized as an effective coolant due to its low temperature, however, its lubrication effect is less well known. Our previous studies of the change in cutting forces, tool wear, chip microstructure, and friction coefficient indicate a possible lubrication effect of LN2. This paper proposes two mechanisms on how LN2 can provide lubrication in the cutting process. To verify these proposed LN2 mechanisms and distinguish them, idealized disk-flat contact tests were performed. A low temperature can alter the material properties and change the friction coefficient between the specimens. However, from the test results, this lubrication mechanism was dependent on the material pairs. An uncoated carbide insert with a low carbon steel or titanium alloy disk test showed reduction of friction under LN2 cooling, but a coated insert increased the friction force. LN2 injection to form a physical barrier or hydrodynamic effect between two bodies is always effective in reducing the friction force.     

ASSESSMENT OF MACHINING MODELS: PROGRESS REPORT

Abstract

Progress in developing and assessing predictive modeling of machining processes has been hindered by the extremely localized nonlinear physical phenomena that occur in machining and the many different types of models ranging from theoretical to empirical. The difficulty in assessing models has been cited by industry as the major barrier to use of modern machining models. Current practice in industry is to machine and change tools conservatively, or to conduct costly empirical studies for a limited selection of tools and coolants. The Assessment of Machining Models project will assess the ability of modern machining models to predict the outputs of machining processes based upon a consistent, well measured calibration data set. The data set is nearly complete and is to be used in benchmarking the predictive capability of machining models in blind tests. This paper presents the project motivation, goals, and representative calibration data set results. The next steps in the effort include release of the calibration data, solicitation and collection of predictions, and evaluation and reporting of results.     

PREDICTION OF CUTTING FORCES IN MACHINING WITH RESTRICTED CONTACT TOOLS

Abstract

A semi-empirical method is described for predicting cutting forces in orthogonal machining with restricted contact tools. The method uses a well-established machining theory to predict cutting forces and tool-chip contact length for the corresponding plane face tool, i.e., a tool having the same cutting-edge geometry but no restricted contact. These predicted parameters and a set of empirical relations are then used to calculate the cutting forces for the restricted contact tool. A comparison between predicted and experimental results for two plain carbon steel work materials and a range of tool geometries and cutting conditions shows good agreement.