The kinetics of lubricant penetration action during machining

A new model of cutting fluid penetration in the interface zone during metal cutting is proposed. This model includes three stages relating to processes in a single capillary: 1. liquid phase entry, 2. ‘micro-droplet explosion’, and 3. filling the capillary by gas phase lubrication. Calculations of model parameters have been done for two different liquids: water and castor oil. The correlation between model data and known experimental results is also given.     

刀-屑接触区摩擦润滑的毛细管模型研究

切削时刀-屑接触区为边界摩擦状态,冷却润滑剂难以进入,其作用可用毛细管理论解释.在切削试验与理论分析的基础上,提出了圆锥形毛细管模型.动力学分析结果表明:冷却润滑剂的微粒尺寸越小,冷却润滑效果越好.刀-屑接触面的吸附作用能形成边界润滑膜,圆锥形毛细管能限制冷却润滑剂的渗透深度,负压力可为冷却润滑剂的渗透提供动力.     

在刀-屑界面持续润滑刀具切削45钢的性能及润滑机理

为提高刀具润滑性能,尽量减少切削液的使用,制备出在刀屑界面持续润滑的新型刀具,能够将切削液通过微通道直接输送到刀屑接触界面内部。采用该新型刀具与普通刀具在干切削和浇注切削液条件下分别进行切削45钢试验,测量了切削三向力,对刀具前刀面磨损面进行SEM微观形貌分析及元素检测,分析了刀具的摩擦磨损特性及润滑机理。试验结果表明,与普通刀具在干切削和浇注切削液条件下相比,刀屑界面持续润滑刀具能够有效减少切削过程中的摩擦磨损,而切削液用量只有传统浇注式切削的1/120。分析前刀面的元素可知,切削液能够更加深入到离主切削刃更近的区域,并能持续供给,这是该刀具具有更好的减摩抗磨效果的主要原因。尽管新型刀具的黏结情况大大缓解,但刀具的磨损机理仍然以黏结磨损和氧化磨损为主。     

A Theoretical Study of Vapor Lubrication for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising technique to overcome the superparamagnetic limit to further increase the areal recording density of hard disk drives. However, HAMR brings about serious problems to the slider-disk interface, such as lubricant depletion on disk surface caused by laser heating. It is proposed to overcome the lubricant depletion problem by using vapor lubrication. The lubricant film formation process on disk surface in vapor lubrication is studied theoretically based on fundamental adsorption and desorption theories. The controlling parameters of lubricant film thickness and film formation time are identified. It is found that the lubricant film thickness is controlled mainly by lubricant vapor pressure and molecular weight. The film formation time can be shortened by using low molecular weight lubricant and high temperature lubricant vapor.     

钛合金切削润滑研究现状与发展趋势*

钛合金在航空航天、生物医疗等领域具有广泛而重要的应用,但钛合金是典型的难加工金属,其切削润滑问题是制约钛合金加工效率与质量的关键所在,目前尚未得到较好的解决。从钛合金切削中的摩擦学问题、切削润滑问题、水基润滑问题三方面介绍钛合金切削润滑研究现状,以有望解决钛合金切削润滑问题的水基润滑为基础,从适合于钛合金切削润滑的微量润滑技术及纳米颗粒增效两方面探讨钛合金切削水基润滑研究的发展趋势,并总结以水基润滑剂为基础的高性能钛合金切削液体系设计是未来研发新型高效环保钛合金切削液的重要途径。     

分子动力学模拟巴氏合金界面磁性液体润滑行为

磁性液体因其顺磁可控的流变特性,可满足极端工况对滑动轴承润滑油膜稳定性不断提高的要求,在轴承润滑方面具有良好的应用前景。为探究磁性液体微观润滑机制,采用分子动力学模拟方法构建和优化巴氏合金界面磁性液体润滑的微观模型,并根据实际工况进行限制性剪切模拟,研究温度和剪切速度对PAO6基磁性液体在巴氏合金界面润滑行为的影响;通过分析滑动过程中相对浓度分布、温度分布、速度分布、均方位移和界面吸附能等参数的变化,从分子层面揭示磁性液体微观润滑的作用机制。结果表明：PAO6基磁性液体具有良好的扩散性和散热性,可以粘附在巴氏合金摩擦界面起到很好的承载和减磨作用;在高温和高剪切速度下,磁性液体润滑膜仍呈现出良好的稳定性,磁性颗粒具有良好的扩散能力。研究结果有助于完善纳米薄膜润滑理论,对磁性液体的工程应用具有现实指导意义。     

A Mixed Lubrication Model for Computer Simulation of Extrusion Processes

A mixed lubrication/friction model for extrusion process is developed in the present research. The model combines a rigid-plasticity finite element code to simulate the interface condition between the tooling and workpiece in the extrusion operation. The influence of surface roughness on lubricant flow is treated by using the average Reynolds equation. The active lubrication regime and appropriate friction factor were determined from the current local values of interface variables such as mean lubricant film thickness and workpiece and tooling roughness, in addition to the more traditional external variables such as interface pressure, node sliding velocity and strain rate of the workpiece. Numerical results using the coupled code include friction stress and normal pressure under different lubrication conditions are compared with experimental investigation. The discrepancy is very small and the proposed model proved to be very efficient in predicting interface friction condition in the extrusion processes.     

Modeling Sliding Contact of Rough Surfaces with Molecularly Thin Lubricants

The sliding contact between two rough surfaces in the presence of a molecularly thin lubricant layer is investigated. Under very high shear rates, the lubricant is treated as a semi-solid layer with normal and lateral shear-dependent stiffness components obtained from experimental data. The adhesive force in the presence of lubricant is also adapted from the Sub-boundary lubrication model and improved to account for variation in surface energy with penetration into the lubricant layer. A model is then proposed, based on the Improved sub-boundary lubrication model, which accounts for lubricant contact and adhesion and its validity is discussed. The model is in good agreement with published experimental measurements of friction in the presence of molecularly thin lubricant layers and suggests that a molecularly thin lubricant bearing could be successfully used to reduce solid substrate damage at the interface.     

Investigation of the strain distribution with lubrication during the deep drawing process

A lubrication/friction model can be implemented in FEM codes to predict the contact area ratio, friction coefficient and strain distribution in lubricated deep drawing process. In the lubrication analysis, the surface roughness effect on lubrication flow is included by using Wilson and Marsault's average Reynolds equation that is appropriated for mixed lubrication with severe asperity contact. With regard to the asperity contact theory, the well-known flattening effect is considered. Friction is expressed in terms of variables such as lubricant film thickness, sheet roughness, lubricant viscosity, interface pressure, sliding speed, and strain rate. The proposed lubrication/friction model combined with a finite element code of deep drawing process to predict the contact area ratio, friction coefficient and strain distribution. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

Mathematical model of friction and wear of polycrystalline solids

Thermodynamic analysis of the cyclic process of contact appearance and breaking in a closed system is used to develop a mathematical model of friction and wear of polycrystalline solids without lubrication and with boundary lubrication. When calculating friction in the case of boundary lubrication, it is assumed that the clearance in the contact is a microbearing with a mobile wall, while the adsorption is described by the Dubinin-Radushkevich equation. Because the physically sorbed molecules are not localized on the surface, the conditions of existence of the lubrication layer is the equality of its counterpressure to the normal pressure in each contact point. The counterpressure of the lubricating layer was found by integration of the Pointing equation describing the condition of equilibrium of two phases (the lubricating layer and the gas phase) coexisting under different pressures and at the same temperature. Comparison of the calculated and experimental results confirms the adequacy of the advanced model.     

An explicit finite element model to study the influence of rake angle and friction during orthogonal metal cutting

A fundamental understanding of the tribology aspects of machining processes is essential for increasing the dimensional accuracy and surface integrity of finished products. To this end, the present investigation simulates an orthogonal metal cutting using an explicit finite element code, LS-DYNA. In the simulations, a rigid cutting tool of variable rake angle was moved at different velocities against an aluminum workpiece. A damage material model was utilized for the workpiece to capture the chip separation behavior and the simultaneous breakage of the chip into multiple fragments. The friction factor at the cutting tool–workpiece interface was varied through a contact model to predict cutting forces and dynamic chip formation. Overall, the results showed that the explicit finite element is a powerful tool for simulating metal cutting and discontinuous chip formation. The separation of the chip from the workpiece was accurately predicted. Numerical results found that rake angle and friction factor have a significantly influence on the discontinuous chip formation process, chip morphology, chip size, and cutting forces when compared to the cutting velocity during metal cutting. The model was validated against the experimental and numerical results obtained in the literature, and a good agreement with the current numerical results was found.     

2D FEM estimate of tool wear in turning operation

Finite element method (FEM) is a powerful tool to predict cutting process variables, which are difficult to obtain with experimental methods. In this paper, modelling techniques on continuous chip formation by using the commercial FEM code ABAQUS are discussed. A combination of three chip formation analysis steps including initial chip formation, chip growth and steady-state chip formation, is used to simulate the continuous chip formation process. Steady chip shape, cutting force, and heat flux at tool/chip and tool/work interface are obtained. Further, after introducing a heat transfer analysis, temperature distribution in the cutting insert at steady state is obtained. In this way, cutting process variables e.g. contact pressure (normal stress) at tool/chip and tool/work interface, relative sliding velocity and cutting temperature distribution at steady state are predicted. Many researches show that tool wear rate is dependent on these cutting process variables and their relationship is described by some wear rate models. Through implementing a Python-based tool wear estimate program, which launches chip formation analysis, reads predicted cutting process variables, calculates tool wear based on wear rate model and then updates tool geometry, tool wear progress in turning operation is estimated. In addition, the predicted crater wear and flank wear are verified with experimental results.     

Theoretical and experimental study of the chatter vibration in wet and MQL machining conditions in turning process

Turning is one of the most commonly used cutting processes for manufacturing components in production engineering. The turning process, in some cases, is accompanied by intense relative movements between tool and workpiece, which is called chatter vibrations. Chatter has been identified as a detrimental problem that adversely impacts surface finish, tool life, process productivity, and dimensional accuracy of the machined part. Cooling/Lubrication in the turning process is normally done for some reasons, including friction and force reduction, temperature decrement, and surface finish improvement. Wet cooling is a traditional cooling/lubrication process that has been used in machining since the past. Besides, a variety of new cooling and lubricating approaches have been developed in recent years, such as the minimum quantity lubrication (MQL), cryogenic cooling, nanolubrication, etc., due to ecological issues. Despite the importance of cooling/lubrication in machining, there is a lack of research on chatter stability in the presence of cutting fluid in cutting processes. In this study, the chatter vibration in turning process for two cooling/lubrication conditions of conventional wet and MQL is investigated. An integrated theoretical model is used to predict both the metal cutting force and the chatter stability lobe diagram (SLD) in turning process. This model involves deriving a math equation for predicting metal cutting force for both wet and MQL conditions using experimental training force data and a Genetic Expression Programming (GEP)-based regression model. Also, the traditional single degree of freedom chatter model is used here for predicting the SLDs. The chatter model is discussed and verified with experimental tests. Then, the experimental results of the tool's acceleration signal, work surface texture, surface roughness, chip shape, and tool wear are presented and compared for wet and MQL conditions. The results of this study show that the cooling/lubrication systems such as wet or MQL have a considerable effect on the SLDs. Also, the predicted results of metal cutting force and SLD for both wet and MQL techniques are in good agreement with the experimental data. Therefore, it is recommended that for each lubrication condition including wet, or MQL, the SLD be determined to achieve higher machinability.     

Boundary lubrication by an adsorption layer

Some key directions of study of the friction and wear of solids under conditions of boundary lubrication by an adsorption layer are analyzed. The ideas and methods of the studies are considered in sequence starting from works of the founder of the boundary lubrication concept W. Hardy and proceeding to the results of the outstanding scientists of the following generations—F.P. Bowden, D. Tabor, B.V. Deryagin, A.S. Akhmatov, G.I. Fuks, R.M. Matveevskii, and others—and then to those of contemporary researchers. Tribochemical aspects of lubrication by an adsorption layer are discussed. Special attention is paid to attempts to develop physical and mathematical models of the boundary lubrication process.     

Cutting force reduction and surface quality improvement in machining of aerospace duralumin AL-2017-T4 using carbon onion nanolubrication system

In machining of very high precision Duralumin AL-2017-T4 for aerospace applications, the shape varieties of the product lead to many different complicated shapes to be developed. The computer numerical control (CNC) milling machine facilities provides a wide variety of parameter set-up, making the machining process on the Duralumin AL-2017-T4 excellent in manufacturing complicated special products compared with other machining processes. However, the demand for high quality and fully automated production focuses attention on the cutting process, which are partial determinant of the quality of surface and affects the appearance, function, and reliability of the products. The key solution is to increase the effectiveness of existing lubrication systems in the machining process in order to improve product quality as it could reduce the friction component at the tool–chip interface. For further improvement, introducing the nanolubrication system could reduce the cutting force and produce much better surface quality as the rolling action of billions units of nanoparticles at the tool–chip interface could reduce the coefficient of friction significantly. In this study, carbon onion has been used as nanoparticle mixed with ordinary mineral oil at different concentrations to investigate the cutting force reduction and the surface quality improvement of CNC end-milling machined Duralumin AL-2017-T4. From the results, with using of carbon onion nanolubricant, the cutting force and surface roughness values are reduced by 21.99 and 46.32 %, respectively, compared with the case of using ordinary lubrication systems. This can be attributed to the tribological properties of carbon onion, which reduces the coefficient of friction at the tool–chip interface during the machining process.     

Ejecting performance simulation of an innovative piezoelectric actuated lubrication generator for space mechanisms

A novel lubrication generator combing piezoelectric drop-on-demand (DOD) oil-jet technology is proposed for space mechanisms. A finite difference numerical model was established to analyze the design parameters of droplet ejection. The volume-of-fluid piecewise linear-interface construction (VOF-PLIC) interface-capturing method was adopted to represent the fluid domain and to track the evolution of its free boundaries whereas the continuous surface force (CSF) mode was chosen to model the interfacial physics. To explore the practicability of the proposed new lubrication generator, the flow behavior during the stages of fluid ejection and droplet formation are examined with Krytox 143AB lubricant as the baseline test fluid for a ejection cycle of 100 μs. To improve the droplet ejecting performance of lubricant, the characters of inject flow part have been investigated with the variation of the pulse voltage for different spatial temperature. Injection process in the extra-low pressure is also investigated. The developed model helps to understand the drop formation process and this approach can be applied in various inject head designs.     

Numerical modeling of the influence of process parameters and workpiece hardness on white layer formation in AISI 52100 steel

White layer formation is considered to be one of the most important aspects to take into account in hard machining. Therefore, a large number of experimental investigations have been carried out in recent times on the formation mechanisms and properties of the white layer. However, up to now, only very few studies have been reported on modeling of the white layer formation. This paper presents a finite element model which predicts the white layer formation during machining of hardened AISI 52100 steel. This numerical model was properly calibrated by means of an iterative procedure based on the comparison between experimental and numerical data. The empirical model was also validated for a range of cutting speeds, uncut chip thickness, and material hardness values. This study provides excellent results concerning cutting force, temperature, chip morphology, and white layer. From this study, it was also possible to properly analyze the influence of process variables on the white layer formation.     

Multi-factor coupling system characteristic of the dynamic roll gap in the high-speed rolling mill during the unsteady lubrication process

In the present work, a multi-factor coupling dynamic model of a rolling mill system for a dynamic roll gap during an unsteady lubrication process was developed on the basis of the rolling theory, lubrication and the friction theory, and the mechanical vibration theory. The multi-factor coupling model of interfacial film binding was coupled with the rolling force model, dynamic roll gap interface friction model and work roll movement model. The corresponding distributions of friction and pressure at varying surface roughness and times were systematically analyzed during the unsteady mixed lubrication process. The effects of the main processing parameters on the critical speed and amplitude for self-excited vertical vibration were investigated.     

A numerical analysis of the elastohydrodynamic line contact film formation at motion start-up

A mathematical model is proposed of the process of formation of the elastohydrodynamic (EHD) lubricant layer between resilient cylinders that begin to rotate in the lubricating material medium from the resting state. The model assumes division of the whole contact region into three zones: the zone within which dry motion is described by the equations of the elastohydrodynamic theory of lubrication, the transient zone, and the dry contact zone. The method of the numerical solution of this system of equations is presented. The calculations are performed for the lubricating material that was used in the published experimental study of the process of formation of the EHD lubricating layer between the resilient ball and the flat resilient base. It is shown that the calculation results well agree with the experimental data both qualitatively and quantitatively providing that the transient region dimensions are adequately selected. The function of the pressure distribution, the lubricating layer thickness, the lubricating material flow, the rate of approach of the surfaces over the contact region at different moments of time, the time dependencies of the lubricating layer thickness at different points of the contact region, and the coordinates of the boundary points of the dry contact region is also presented.     

Finite element simulation for burr formation near the exit of orthogonal cutting

A coupled thermo-mechanical model of plane-strain orthogonal metal cutting including burr formation is presented using the commercial finite element code. A simulation procedure based on Normalized Cockroft–Latham damage criterion is proposed for the purpose of better understanding the burr formation mechanism and obtaining a quantitative analysis of burrs near the exit of orthogonal cutting. The cutting process is simulated from the transient initial chip formation state to the steady state of cutting, and then to tool exit transient chip flow by incrementally advancing the cutting tool. The predicted burr profile is compared with experimental data and found to be in reasonable agreement. The effect of the tool conditions and cutting conditions on the burr formation process was also investigated.