Internal stress analysis for single and multilayered coating systems using the boundary element method

Various thin-coating films are designed and utilized for industrial applications to improve machining performance due to better temperature and wear resistant properties than their substrate counterparts. However, the widespread experimental research on thin coatings underlies a general lack of modeling efforts, which can accurately and efficiently predict the coating and thin film performance. In this paper, the boundary element method (BEM) for 2D elastostatic problems is studied for the analysis of single and multilayered coating systems. The nearly singular integrals, which is the primary obstacle associated with the BEM formulations, are dealt efficiently by using a general nonlinear transformation technique. For the test problems studied, very promising results are obtained when the thickness of coated layers is in the orders of 1.0E?6 to 1.0E?9, which is sufficient for modeling most coated systems in the micro- or nano-scales.     

The measurement of heat distribution in stereolithography electrodes during electro-discharge machining

Early work by the authors explored the potential for manufacture of electrodischarge machining (EDM) electrodes, using rapid prototyping (RP) technologies. A number of possible routes were identified to provide an integrated system from computer aided design (CAD) data through to production tooling. One such route is the processing of stereolithography (SL) RP models by electrodepositing copper onto the metallized surface. This strategy provides a process route from model to tool with minimum manual intervention. Initial studies using thin-coated SL models for EDM have identified a limitation to application due to heat build up in the electrode. Currently, the electrodes can be used at low material removal rates (MRR), commonly known as semi-roughing or finishing cuts. Higher MRR generates greater heat at the cutting face which causes failure of the RP electrodes. This failure is believed to occur through a combination of delamination, thinning and distortion of the electrode. The heat distribution and associated failure modes for these electrodes are being investigated to determine how the machining performance can be improved.     

Simultaneous determination of optimal machining conditions and tool replacement policies in constrained machining economics problems by geometric programming

This paper presents an analytical model for the simultaneous determination of the optimal machining conditions (cutting speed and feed) and the optimal tool replacement policy in constrained machining economics problems by geometric programming. The optimal preventive tool replacement policy is initially determined as a tool life fractile (independent of the underlying tool life distribution) and then it is expressed as actual tool life by utilizing the underlying tool life distribution applicable to the combination of tool material, workpiece properties, and machining conditions. Constraints on the optimal values of cutting speeds, feeds, and/or optimal tool replacement policy based on maximum allowable values and/or surface finish requirements are handled through the optimization of the dual objective function. It is shown that the optimal cost distribution does not depend on the cost coefficients in the objective function. Finally, the model is applied to two-stage systems where the necessary conditions are derived for increasing the synchronization between the two stages.     

Boundary element analysis of 2D thin walled structures with high-order geometry elements using transformation

Thin structures have been widely designed and utilized in many industries. However, the analysis of the mechanical behavior of such structures represents a very challenging and attractive task to scientists and engineers because of their special geometrical shapes. The major difficulty in applying the boundary element method (BEM) to thin structures is the coinstantaneous existence of the singular and nearly singular integrals in conventional boundary integral equation (BIE). In this paper, a non-linear transformation over curved surface elements is introduced and applied to the indirect regularized boundary element method for 2-D thin structural problems. The developed transformation can remove or damp out the nearly singular properties of the integral kernels, based on the idea of diminishing the difference of the orders of magnitude or the scale of change of operational factors. For the test problems studied, very promising results are obtained when the thickness to length ratio is in the orders of 1E?01 to 1E?06, which is sufficient for modeling most thin structures in industrial applications.     

Investigations into the effect of cathode material on temperature distribution during electrochemical machining

Electrochemical machining (ECM) is one of the most widely used unconventional manufacturing processes. Its capabilities have not been fully exploited mainly because of some inherent problems associated with tool design. Most of the models available for tool design in ECM ignore the effect of the heat transferred through the cathode and anode and its effects on the variation of different process parameters during ECM.

In the present paper, a simple thermal-resistance model of the ECM process is proposed. Using this model, the effects of the heat transferred through electrodes on temperature distribution, electrolyte conductivity, anode profile, metal removal rate (MRR), etc. have been studied. The experimental data have then been used to test the accuracy of the model.

The tests show that, for accurate determination of the anode profile, any model which takes no account of the heat transferred through the electrodes will yield erroneous results.     

空气介质微细电火花沉积加工微结构机理

对微细电火花沉积加工中沉积所得不同微细结构的成形机理进行了研究．在电火花成形机床上，通过合理选择工艺参数，用黄铜电极在高速钢工件表面稳定沉积出外径约0．20mm、线径约0．09mm的微螺旋结构和直径约0．20mm微圆柱体．通过有限元法对工具电极放电点的瞬态温度场进行了模拟，分析结果表明，不同的放电能量密度影响材料的蚀除形式，继而影响蚀除电极材料在放电通道中的运动，最终影响微细结构的成形过程．对沉积材料微观组织结构分析表明，沉积材料与基体结合层为冶金结合方式，结合紧密，并由于凝固过程极大的冷却速率，使沉积材料在凝固过程中发生了晶粒细化现象．     

Prediction of machining quality due to the initial residual stress redistribution of aerospace structural parts made of low-density aluminium alloy rolled plates

During the machining of thick, large and complex aluminium parts, the redistribution of initial residual stresses is the main reason for machining errors such as dimensional variations and the post-machining distortions. These errors can lead to the rejection of the parts or to additional conforming operations increasing production costs. It is therefore a requirement to predict potential geometrical and dimensional errors resulting from a given machining process plan and in taking into consideration the redistribution of the residual stresses. A specific finite element tool which allows to predict the behaviour of the workpiece during machining due to its changing geometry and to fixture-workpiece contacts has been developed. This numerical tool uses a material removal approach which enables to simulate the machining of parts with complex geometries. In order to deal with industrial problems this numerical tool has been developed for parallel computing, allowing the study of parts with large dimensions. In this paper, the approach developed to predict the machining quality is presented. First, the layer removal method used to determine the initial residual stress profiles of an AIRWARE^? 2050-T84 alloy rolled plate is introduced. Experimental results obtained are analysed and the same layer removal method is simulated to validate the residual stress profiles and to test the accuracy of the developed numerical tool. The machining of a part taken from this rolled plate is then performed (experimentally and numerically). The machining quality obtained is compared, showing a good agreement, thus validating the numerical tool and the developed approach. This study also demonstrates the importance of taking into account the mechanical behaviour of the workpiece due to the redistribution of the initial residual stresses during machining when defining a machining process plan.     

Performance of electrochemical micromachining of copper through infrared heated electrolyte

The application of Infrared (IR) light in electrochemical micromachining (EMM) is to focus the machining to confined areas so that material removal and productivity is improved. Predominantly, the localization mechanism in electrochemical machining (ECM) for material removal is determined by the temperature. In this study, a novel method of heating the electrolyte using IR has been attempted in EMM for the first time. IR light is considered for indirect heating of sodium nitrate (NaNO₃) electrolyte. Machining parameters such as machining voltage, duty cycle, and electrolyte concentration are varied by keeping the current and electrolyte temperature constant at 1 A and 37?±?0.5°C, respectively. The effect of these parameters on machining rate and overcut is studied. The result showed that the machining rate for heated electrolyte is 4.2 times better than the electrolyte at room temperature for 25% duty cycle, 35?g/L electrolyte concentration, and 9?V machining voltage. The change in electrolyte conductivity of heated electrolyte solution shows considerable effect on overcut. Field emission scanning electron microscope (FESEM) images show much difference in surface structure for electrolyte at room temperature and heated electrolyte. The findings provide valuable understandings on the use of IR light to heat the electrolyte in order to improve the performance of EMM systems.     

An inverse heat transfer problem for optimization of the thermal process in machining

It is evident that machining process causes development of large quantities of thermal energy within a relatively narrow area of the cutting zone. The generated thermal energy and the problems of its evacuation from the cutting zone account for high temperatures in machining. These increased temperatures exert a pronounced negative effect on the tool and workpiece. This paper takes a different approach towards identification of the thermal process in machining, using inverse heat transfer problem. Inverse heat transfer method allows the closest possible experimental and analytical approximation of thermal state for a machining process. Based on a temperature measured at any point within a workpiece, inverse method allows determination of a complete temperature field in the cutting zone as well as the heat flux distribution on the tool/workpiece interface. By knowing the heat flux function, one defines criterium and method of optimization, the inverse heat transfer problem transforms into extreme case. Now, the task of optimization is to determine most favourable ratio between heat flux parameters in order to preserve exploitation properties of the tool and workpiece.     

Influence of cryogenic coolant on turning performance characteristics: A comparison with wet machining

Machining of 17-4 Precipitation Hardenable Stainless Steel (PH SS) is one of the difficult tasks because of its high cutting temperatures. Conventional cutting fluids are used to overcome the high cutting temperatures, but these are not acceptable from the health and environmental sustainable points of view. Cryogenic cooling is one of the potential techniques to overcome such problems. In the current work, comparison is made of cryogenic turning results, such as tool flank wear, cutting forces (feed force, main cutting force), cutting temperature, chip morphology and surface integrity characteristics with wet machining during machining of heat-treated 17-4 PH SS. The result showed that in cryogenic machining, a maximum of 53%, 78%, 35% and 16% reductions was observed in tool flank wear, cutting temperature, surface roughness and cutting force, respectively, when compared with wet machining. It was also evident from the experimental results that cryogenic machining significantly improved the machining performance and product quality even at high feed rates.     

Broaching Performance of Superalloy GH4169 Based on FEM

The nickel-based superalloy GH4169 is an important material for high temperature applications in the aerospace industry. However, due to its poor machinability, GH4169 is hard to be cut and generates saw-tooth chips during high speed machining, which could significantly affect the dynamic cutting force, cutting temperature fluctuation, tool life, and the surface integrity of the parts. In this paper, the saw-tooth chip formation mechanism of superalloy GH4169 was investigated by the elasto-viscoplastic finite element method (FEM). Using the finite element software of ABAQUS/Explicit, the deformation of the part during high speed machining was simulated. The effective plastic strain, the temperature field, the stress distribution, and the cutting force were analyzed to determine the influence of the cutting parameters on the saw-tooth chip formation. The study on broaching performance has great effect on selecting suitable machining parameters and improving tool life.     

Comparative study of the effect of surface texturing on cutting tool in dry cutting

Recent researches in the field of dry machining have indicated that surface texture has the potential to influence tribological conditions. Researchers have studied the application of controlled surface microtextures on cutting tool surfaces to improve machining performance by changing the tribological conditions at the interfaces of tool–chip and tool–work piece. An experiment to study the performance of the microtextured high-speed steel cutting implement within the machining of steel and aluminum samples was performed. Surface textures were introduced using Rockwell hardness tester, Vickers hardness tester, and by scratching with diamond dresser on the face of single point cutting tool. Machining in dry conditions was applied on mild steel (EN3B) and aluminum (AA 6351) samples using lathe machine with microtextured and traditional cutting tool for the constant range of feed, depth of cut, and for varying range of cutting speeds. Measurement of cutting force, cutting temperature, and surface roughness of the work surfaces after machining were made. The results showed reduction in cutting forces and cutting temperature with textured tools in comparison with those of the untextured tool. Chips collected from different samples were studied under a microscope and the results showed that textures created on the tool surface by various methods exhibited variations in chip formation. Cutting tools without texture and with texture were comparatively studied and the outcomes of the experimental study are presented in this paper.     

采用疏水材料侧壁绝缘电极的微细电解加工实验

微细电解加工中工具电极和工件之间的加工间隙是影响加工效果的核心因素.工具电极侧壁施加绝缘膜的工艺方法,可显著改变侧面加工间隙内的电场分布,提高加工精度,同时保持原有的加工效率,有着良好的应用前景.本文基于仿真方法,分析了绝缘膜对端面加工间隙和侧面加工间隙的影响规律,研究了绝缘膜疏水性质对电解加工中产生的氢气泡运动的吸附作用对微细电解加工的影响.在直径为数十到数百微米的钨丝侧壁制备出可用于微细电解加工的几微米厚的硅胶疏水材料侧壁绝缘薄膜.在硝酸钠电解液中,实现了不锈钢材料上将微气泡附着于电极前端与绝缘膜结合处、保护绝缘膜和约束杂散腐蚀的微细电解端面分层扫描加工效果.     

Machinability studies on Al 7075/BN/Al2O3 squeeze cast hybrid nanocomposite under different machining environments

Enforcement of stricter environmental policies demands alternative methods that could reduce the usage of cutting fluid during machining. Thus, dry machining and minimum quantity lubrication (MQL) machining are gaining practical importance. Due to enhanced mechanical and thermal properties, aluminium based nanocomposites have wide application in aerospace and automobile industries. In this work, asqueeze cast hybrid nanocomposite is developed with the reinforcement of 0.5?wt% hexagonal boron nitride and 1?wt% alumina particles in the base matrix of Al 7075 that is subjected to a squeezing pressure of 150?MPa. During the turning of this hybrid nanocomposite, thefeed rate is varied (0.1, 0.2, and 0.3?mm/rev)and its influence on the generated forces, tool wear and surface roughness under dry and MQL environments is performed. The results are compared with squeeze cast unreinforced aluminium alloy and presented.     

Study of deformation zone effects of low,conventional, high and very high speed machining

The paper deals with cutting speed in range 3 m?min^‐1 up to 2200 m?min^‐1 and its complex impact mainly on chip macroscopic shape, chip microstructure, chip compression, tool wear, tool life and machined surface quality and interprets and compares the effects regarding low, conventional, high and very high speed machining based on the dry turning of carbon steel by sintered carbide coated by titanium nitride and ceramic cutting inserts. The deformation zone response for different cutting speeds at the tool‐chip‐workpiece interfaces and their effect on tool wear were studied. The extensive (so called complete) experiments within wide range of values and large number of measurements were carried out. The formation of secondary chip occurring in high speed turning is reported. Moreover, the paper analyses the total machining time involving tool replacement time in terms of high speed machining regarding the obtained experimental results.     

New method to characterize a machining system: application in turning

Many studies simulates the machining process by using a single degree of freedom spring-mass system to model the tool stiffness, or the workpiece stiffness, or the unit tool-workpiece stiffness in modelings 2D. Others impose the tool action, or use more or less complex modelings of the efforts applied by the tool taking account the tool geometry. Thus, all these models remain two-dimensional or sometimes partially three-dimensional. This paper aims at developing an experimental method allowing to determine accurately the real three-dimensional behaviour of a machining system (machine tool, cutting tool, tool-holder and associated system of force metrology six-component dynamometer). In the work-space model of machining, a new experimental procedure is implemented to determine the machining system elastic behaviour. An experimental study of machining system is presented. We propose a machining system static characterization. A decomposition in two distinct blocks of the system “Workpiece-Tool-Machine” is realized. The block Tool and the block Workpiece are studied and characterized separately by matrix stiffness and displacement (three translations and three rotations). The Castigliano’s theory allows us to calculate the total stiffness matrix and the total displacement matrix. A stiffness center point and a plan of tool tip static displacement are presented in agreement with the turning machining dynamic model and especially during the self induced vibration. These results are necessary to have a good three-dimensional machining system dynamic characterization (presented in a next paper).     

Experimental Study of the Tool Wear During the Electrochemical Discharge Machining

Electrochemical discharge machining is a nonconventional machining method which can be used to machine nonconductive materials such as glass and ceramics. However, machining of the refractory materials such as ceramics requires high voltages to produce the required thermal energy. In this condition, the tool wear would be increased significantly. This paper reports the study of the wear of the different tool materials. The selected tool materials have different melting/boiling temperatures responding to the high voltages in different ways. The possibility of using different tool materials in high voltages along with estimation of tool surface temperature is discussed in this paper.     

Simplified MQL system for drilling AISI 304 SS using cryogenically treated drills

In machining operations, cutting fluids have been comprehensively used to improve the cutting tool life, but the issues related to manufacturing cost, environment and health call for reducing their use by possible methods. Minimum quantity lubrication (MQL) is a technique that overcomes these problems by spraying a small amount of cutting fluid (<100?ml/hr) as mist using compressed air. In this work, the basic MQL technique is used to achieve flow rates slightly higher (～880?ml/hr) than MQL using simple techniques like paint sprayer and compressor, which is more generally called reduced quantity lubrication (RQL). Another method to increase the tool life is by cryogenic treatment, which increases the hardness of the tool. Tungsten carbide drill bits were subjected to cryogenic treatment (?185 °C). Drilling studies were carried out on AISI 304 stainless steel (SS) using untreated and cryo-treated WC drill bits under RQL and conventional wet lubrication conditions. The tool wear on the treated WC drill bits with RQL was comparatively less than on the untreated ones with RQL and wet lubrication. These improvements were established through microhardness, SEM images, XRD, wear studies and surface roughness measurements comparisons.     

Experimental investigations on cryogenic cooling by liquid nitrogen in the end milling of hardened steel

Milling of hardened steel generates excessive heat during the chip formation process, which increases the temperature of cutting tool and accelerates tool wear. Application of conventional cutting fluid in milling process may not effectively control the heat generation also it has inherent health and environmental problems. To minimize health hazard and environmental problems caused by using conventional cutting fluid, a cryogenic cooling set up is developed to cool tool–chip interface using liquid nitrogen (LN₂). This paper presents results on the effect of LN₂ as a coolant on machinability of hardened AISI H13 tool steel for varying cutting speed in the range of 75–125 m/min during end milling with PVD TiAlN coated carbide inserts at a constant feed rate. The results show that machining with LN₂ lowers cutting temperature, tool flank wear, surface roughness and cutting forces as compared with dry and wet machining. With LN₂ cooling, it has been found that the cutting temperature was reduced by 57–60% and 37–42%; the tool flank wear was reduced by 29–34% and 10–12%; the surface roughness was decreased by 33–40% and 25–29% compared to dry and wet machining. The cutting forces also decreased moderately compared to dry and wet machining. This can be attributed to the fact that LN₂ machining provides better cooling and lubrication through substantial reduction in the cutting zone temperature.     

MAGNETIC BEARING SPINDLES FOR ENHANCING TOOL PATH ACCURACY

Thin rib machining of electronic components or airframe structures can benefit from high speed machining for burr free cutting, improved surface quality and increased metal removal rate. It is suggested that the use of a magnetic bearing spindle can not only successfully provide the benefits of high speed machining but, more importantly, minimize tool path errors. In this paper the various sources of tool path error are discussed as functions of machine tool positioning errors and cutting force errors which are characterized as static, dynamic and stochastic. The operation of high speed magnetic bearing spindles is described and a control scheme whereby the spindle may be translated and tilted for minimizing tool path errors is discussed. This overall research activity is a cooperative effort between the University of Maryland, Cincinnati Milacron, Magnetic Bearings, Inc., The Uestinghouse Corporation, and The National Bureau of Standards.