A multi-scale model for friction in cold rolling of aluminium alloy

A simple and robust friction model is proposed for cold metal rolling in the mixed lubrication regime, based on physical phenomena across two length scales. At the primary roughness scale, the evolution of asperity contact area is associated with the asperity flattening process and hydrodynamic entrainment between the roll and strip surfaces. The friction coefficient on the asperity contacts is related to a theoretical oil film thickness and secondary-scale roll surface roughness. The boundary friction coefficient at the “true” asperity contacts is associated with tribo-chemical reactions between fresh metal, metal oxide, boundary additives, the tool and any transfer layer on the tool. The asperity friction model is verified by strip drawing simulations under thin film lubrication conditions with a polished tool, taking the fitting parameter of the boundary lubrication friction factor on the true contact areas equal to 0.1. Predicted values of average friction coefficient, using a boundary friction factor in the range 0.07–0.1, are in good agreement with measurements from laboratory and industrial rolling mill trials.     

Mechanics of roll edge contact in cold rolling of thin strip

Simulation of cold rolling of thin strip due to roll edge contact with oil lubrication was performed successfully using a developed influence function method. Roll edge contact and related surface roughness was discussed in this paper. The calculated rolling force, intermediate force and work roll edge contact force increase significantly when the reduction increases. The strip profile becomes poor with a higher reduction, and the calculated rolling forces are consistent with the measured values. A modified edge shape of work roll determined from the roll edge contact length and roll edge flattening value is helpful to reduce the work roll edge wear and to extend the work roll life. Surface roughness and asperity of the rolled strip are characterized by surface profilometer and atomic force microscope. The research shows that the surface roughness reduces with a higher reduction or rolling speed. The effect of the strip width on surface roughness is not significant.     

Bridging the Gap Between the Atomic-Scale and Macroscopic Modeling of Friction

A short survey of a modern view on the problem of friction from the physical viewpoint is presented. An atomically thin lubricant film confined between two substrates in moving contact has been studied with the help of molecular dynamics (MD) based on Langevin equations with coordinate- and velocity-dependent damping coefficient. Depending on model parameters, the system may exhibit either the liquid sliding regime, when the lubricant film melts during sliding (the “melting-freezing” mechanism of stick-slip motion), the “layer-over-layer” sliding regime, when the film keeps a layered structure at sliding, or the solid sliding regime, which may provide an extremely low friction (“superlubricity”). Atomic-scale MD simulations of friction, however, lead to a “viscosity” of the thin film, as well as to the critical velocity of the transition from stick-slip to smooth sliding, which differ by many orders of magnitude from the values observed in macroscopic experiments. This contradiction can be resolved with the help of the earthquakelike (EQ) model with a continuous distribution of static thresholds. The evolution of the EQ model is reduced to a master equation which can be solved analytically. This approach describes stick-slip and smooth sliding regimes of tribological systems within a framework which separates the calculation of the friction force from the atomic-scale studies of contact properties.     

Finite Volume Modeling of Gas Flow in Microbearings with Rough Surface Topography

The random surface roughness effects on the performances of gas-lubricated slider microbearings are investigated using finite volume analysis. The rough surface morphologies with the atomic force microscope (AFM) data and the numerical procedures used to generate self-affine surfaces by the midpoint displacement method are proposed and compared. Three-dimensional finite volume modeling of the gas microbearing and its flow field mesh are described considering the velocity-slip boundary condition. The results indicate that surface roughness causes a deviation in the pressure distribution, load-carrying capacity, velocity profile, and local Reynolds number from conventional theory with various values depending on the degree of surface roughness used. The pressure decreases irregularly due to the fractal surface and produces a larger change. The larger the roughness exponent is, the larger the gas slip velocity at the bottom of the wall. In addition, the velocity-slip boundary condition can cause a decrease in the gas flow velocity. The surface roughness effect also leads to random variations in the local Reynolds number. This demonstrates that the random surface roughness potentially causes very complicated flow behavior in the ultra-thin-film bearing lubrication in microelectromechanical systems (MEMS) and should be adequately characterized in terms of the fractal nature of the bearing surfaces.     

Experimental study and evaluation methodology on hard surface integrity

The Taguchi method is adopted experimentally to investigate the surface integrity (surface roughness, residual stress, and thermal damage layer) of hardened bearing steel in hard dry turning, and the validation experiments are consequently performed. It was revealed that the value and effect sequence of optimal hard turning parameter varies with different objectives of surface integrity. However, it is quite difficult to select or determine the optimal combination of hard turning parameters. A hard-turned component performance, which reflects an integrated impact of surface integrity, should be fully recognized to resolve the inherent conflict in the selection process. Based on it, an evaluation methodology composed of four steps is proposed that surface integrity should be evaluated by the service/fatigue life of hard-turned components and therefore turning parameters. It bears significance for super-finish hard turning further application in respect that it provides an integrated approach for hard turning parameter optimization to achieve a superior surface integrity. Funded by the Ministry of Education of China- “985” of international cooperation project “Clean Manufacturing Technology”.     

Coupled effects of surface roughness and flow rheology on elastohydrodynamic lubrication

The coupled effects of surface roughness and flow rheology on elastohydrodynamic lubrication (EHL) circular contact problems are analyzed and discussed. The averaged type Reynolds equation utilizing the average flow model on the interactions between couple stress fluids and surface roughness, the elastic deformation equation, the viscosity–pressure and density–pressure relations equations, and the force balance equation are solved numerically by the multilevel multi-integration (MLMI) algorithm to calculate the pressure distributions and film thickness shapes. The results show that the transverse type roughness and standard deviation of composite roughness enhance the pressure and film thickness in the central contact region. Moreover, the longer the characteristic length of the couple stress fluids is, the smaller the pressure distribution is in the central contact region and the greater the film thickness is in all regions.     

磨料流微去除力学分析与可控因素影响

研究了磨料流抛光中磨粒微去除力学建模方法以及可控因素影响抛光效果的问题。以力为纽带,提出磨粒去除工件表面微凸材料的动力来源于三个方面--介质作用力、磨粒挤压载荷和磨粒冲击载荷。利用建立的力学模型,分析了磨料流加工的内在因素,其中可控因素包括：加工温度、加工压力、活塞的移动速度、磨料黏度、磨粒物理性质（如尺寸、硬度）等;研究了各可控影响因素与工件表面抛光质量及效率的关系;量化了可控因素的大小对磨粒作用在工件表面的力的影响程度;将磨粒作用在工件表面的力合成并分解为与活塞运动方向相同的轴向力和垂直于工件壁面的切向力,指出微去除效果随轴向力与径向力的比值改变而发生变化,设计出简易的测量轴向力和径向力的方案。用试验验证了所建模型和可控因素对抛光效果影响,以及工件表面的加工纹理方向直接影响工件表面粗糙度的减小率和材料去除率的正确性。     

Adhesion-Induced Instability in Asperities

Adhesive forces between two approaching asperities will deform the asperities, and under certain conditions this will result in a sudden runaway deformations leading to a jump-to-contact instability. We present finite element-based numerical studies on adhesion-induced deformation and instability in asperities. We consider the adhesive force acting on an asperity, when it is brought near a rigid half-space, due to van der Waals interaction between the asperity and the half-space. The adhesive force is considered to be distributed over the volume of the asperity (body force), thus resulting in more realistic simulations for the length scales considered. Iteration scheme based on a “residual stress update” algorithm is used to capture the effect of deformation on the adhesion force, and thereby the equilibrium configuration and the corresponding force. The numerical results are compared with the previous approximate analytical solutions for adhesion force, deformation of the asperity and adhesion-induced mechanical instability (jump-to-contact). It is observed that the instability can occur at separations much higher, and could possibly explain the higher value of instability separation observed in experiments. The stresses in asperities, particularly in case of small ones, are found to be high enough to cause yielding before jump-to-contact. The effect of roughness is considered by modeling a spherical protrusion on the hemispherical asperity. This small-scale roughness at the tip of the asperities is found to control the deformation behavior at small separations, and hence are important in determining the friction and wear due to the jump-to-contact instability.     

Design and static performance of high speed machining centre with “direct drive”

“Direct drive” is an ideal method for speeding machine tools. In the structure of a CNC machining centre with “direct drive”, the linear motor’s primary and secondary parts are assembled into the worktable and machine bed respectively to directly drive the worktable. The built-in rotary motor is assembled in the spindle to realize the main transmission system of the machine tool “direct drive”. All mechanical transmission elements in machine tools are eliminated. High speed, efficiency, and accuracy are easily obtained. However, in this type of “direct drive” machining centre, the magnetic attraction force between the primary and the secondary linear motors and the dynamic impact at acceleration and deceleration are directly imposed on the machine tool, and influence the performance of the machining centre. This paper analyzes the special demands of “direct drive” on the machine centre, and introduces a new structure of the machining tool. The worktable and machine bed are optimized to meet the special demands of “direct drive”. Static performance simulation on the machining centre is done to reveal an ideal result.     

Optimization of cutting conditions for minimum residual stress,cutting force and surface roughness in end milling of S50C medium carbon steel

Residual stresses are usually imposed on a machined component due to thermal and mechanical loading. Tensile residual stresses are detrimental as it could shorten the fatigue life of the component; meanwhile, compressive residual stresses are beneficial as it could prolong the fatigue life. Thermal and mechanical loading significantly affect the behavior of residual stress. Therefore, this research focused on the effects of lubricant and milling mode during end milling of S50C medium carbon steel. Numerical factors, namely, spindle speed, feed rate and depth of cut and categorical factors, namely, lubrication and milling mode is optimized using D-optimal experimentation. Mathematical model is developed for the prediction of residual stress, cutting force and surface roughness based on response surface methodology (RSM). Results show that minimum residual stress and cutting force can be achieved during up milling, by adopting the MQL-SiO₂ nanolubrication system. Meanwhile, during down milling minimum residual stress and cutting force can be achieved with flood cutting. Moreover, minimum surface roughness can be attained during flood cutting in both up and down milling. The response surface plots indicate that the effect of spindle speed and feed rate is less significant at low depth of cut but this effect significantly increases the residual stress, cutting force and surface roughness as the depth of cut increases.     

Modeling of Head/Disk Interface—An Average Flow Model

The average flow model is utilized to derive an average Reynolds type equation, which includes the effects of surface roughness, gas rarefaction, and accommodation coefficients (ACs). The related flow factors (pressure flow factors, shear flow factors) are derived in closed form and are expressed as functions of roughness parameters (standard deviation of the composite roughness, Peklenik number and film thickness ratio), Knudsen number, and ACs. Finally, the effects of surface roughness, gas rarefaction, and ACs on the bearing performance of magnetic head sliders with fixed configurations are considered.     

Effect of extrusion pressure and number of finishing cycles on surface roughness in magnetorheological abrasive flow finishing (MRAFF) process

Magnetorheological abrasive flow finishing (MRAFF) was developed as a new precision finishing process for complicated geometries using smart magnetorheological polishing fluid. This process introduces determinism and in-process controllability of rheological behaviour of abrasive laden medium used for finishing intricate shapes. Magnetorheological polishing (MRP) fluid is comprised of carbonyl iron powder and silicon carbide abrasives dispersed in a viscoplastic base of grease and mineral oil and exhibits change in rheological behaviour in presence of external magnetic field. This smart behaviour of MRP fluid is utilized to precisely control finishing forces. The process performance in terms of surface roughness reduction depends on process variables like hydraulic extrusion pressure, magnetic flux density in the finishing zone, number of finishing cycles, and composition of MRP fluid. In the present work, experiments were conducted on a hydraulically powered MRAFF experimental setup to study the effect of extrusion pressure and number of finishing cycles on the change in surface roughness of stainless steel grounded workpieces. A new observation of “illusive polishing” action with the initial increase in number of finishing cycles is reported. The actual finishing action is possible only after removal of initial loosely held material remaining after grinding.     

Atomic-scale finite-element model of tension in nanoscale thin film

In this paper, a 2D atomic-scale finite-element model of tension in nanoscale thin film is developed in which Morse’s potential energy function is used to model the interactive forces between atoms. The model is fed into the finite-element package LS-DYNA and both a single integration point and an explicit solution method are used for solving the tension process rapidly to investigate the size effect of different film thicknesses and the effect of different atomic vacancy ratios on nanoscale thin film under tension. The results show that since the applied displacement is exerted at both ends for different thickness of a perfect crystal, a neutral line is formed at the middle of the material. The material slides along the easiest slip direction to cause a “necking” feature on both sides. The stress initially increases with the gradual increase of strain and thicker film shows a larger tensile stress. After the film experiences the peak stress, the stress then decreases with the gradual increase of strain. While the applied displacement is applied at both ends for different vacancies, a neutral line is formed at the middle of material, but this is not apparent due to the random scattered vacancies. The material slides along the easiest slip direction from left to right, and the stress concentration areas near the constrained ends form “necking” features. Stresses are not zero at zero strain. Tension tests for different vacancy ratios show different maximum stresses. Film with a larger vacancy ratio shows a lower stress at the same strain. As the vacancy ratio of the film under tension increases, the strength and elastic modulus reduces.     

Adhesion Force Measurements of Polymer Particles by Detachment Field Method

The adhesion force distributions of polymer particles to aluminum substrates were measured by the detachment field method. Polymer particles with conducting surface treatment were used for the measurements.Further the conventional detachment field method was modified to be applicable to the adhesion force measurements of a single particle. The adhesion force of the polymer particles increased with an increase in relative humidity. The surface roughness of the substrate influenced the adhesion forces of particles significantly. The influence of the CF4 plasma treatment of the polymer particles and thin layer coating of the substrate surface on the adhesion forces of the polymer particles was also studied, and factors affecting adhesion forces of polymer particles are discussed.     

Grain-Flow Lubrication of Finite-Width Slider Bearings with Rough Surfaces

In this paper, the average lubrication equation for grain flow is solved using a control volume method to analyze the performance of rough slider bearings. The effects of surface characteristics and film particle ratios under various slenderness ratios were investigated. The mean shear stress is derived in terms of shear stress factors. The effects of particle size are discussed and show that roughness effects are pronounced at smaller film particle ratios.     

Parameter optimization of non-vertical laser cutting

In some cases, in order to avoid interference during 3D laser cutting of thin metal a laser head could not be kept vertical to the surface of a work piece. In such situations, the cutting quality depends not only on “typical” cutting parameters but also on the slant angle of the laser head. Traditionally, many tests had to be done in order to obtain the best cutting results. In this paper, an experimental design is employed to reduce the number of tests and an artificial neural network (ANN) is set up to describe quantitatively the relationship between cutting quality and cutting parameters in the non-vertical laser cutting situation. A quality point system is used to evaluate the cutting result of the thin sheet quantitatively. Testing of this novel method shows that the calculated “quality point” using ANN is quite closely in accord with the actual cutting result. The ANN is very successful for optimizing parameters, predicting cutting results and deducing new cutting information.     

Measurement of developing turbulent flow in a U-bend of circular cross-section

Hot-wire measurements of the full mapping of the velocity and Reynolds stress components are reported for developing turbulent flow in a strongly curved 180 deg pipe and its tangents. A slanted wire is rotated into 6 orientations and the voltage outputs from wires are combined to obtain the mean velocity and Reynolds stress components. The strength of secondary flow reaches up to the 28% of bulk mean velocity. The strong counter-rotating vortex pair induced by the transverse pressure gradient and centrifugal force imbalance grows up to Θ = 67.5° into the bend. But the vortex pair breaks down into two cell pattern after Θ=90° Core vortex formation and reversal of secondary flow direction along the bend symmetry plane is cleanly found in the secondary vector plot. At Θ=67.5° and Θ = 90° into bend a large “trough” develops in the longitudinal velocity toward the inside of the bend due to the breakdown of secondary flow. In the bend, the mean longitudinal velocity component changes little after Θ=90°, but secondary flow never achieves fully-developed state. Similar behaviors are observed in the radial and circumferential stresses.     

An Average Flow Model for Couple Stress Fluids

The interactions of surface roughness and flow rheology of couple stress fluids on thin film lubrication problems are modeled. The generalized average Reynolds equation as well as the flow factors is derived. The effects of couple stress parameters (l), the standard derivation of surface roughness ( _i ), the Peklenik number ( _i), and the roughness orientation angle ( _i) on the flow factors ( ^p _ij , ^s _ij) are discussed. In results, the related Reynolds-type equations and flow factors for Newtonian fluids, power-law non-Newtonian fluids, mixtures of Newtonian and power-law non-Newtonian fluids, and couple stress fluids are tabulated.     

微通道内壁面粗糙度对电渗流影响有限元模拟

Lab-on-a-Chip(芯片实验室)系统迅速发展对微流体的研究提出更高的要求,微通道壁面粗糙度对微尺度下的流体存在显著影响。采用有限元模拟研究了壁面机糙度对圆形截面微通道内电渗流的影响机理。壁面粗糙度几何模型采用三角形波形并考察了微通道两端存在阻碍压力作用下的情况。结果表明,壁面粗糙度宽度和间隔增加时微通道中截面流速先减小后增加。相对粗糙度值在0.01～6%之间时截面流速随着相对粗糙度的增加非线性减小,但减小趋势变缓。相对粗糙度增加时压力与截面流速线的斜率减小,截面流速不易受压力变化的影响。     

Studies on friction and transfer layer: role of surface texture

Friction influences the nature of transfer layer formed at the interface between tool and metal during sliding. In the present investigation, experiments were conducted using “Inclined Scratch Tester” to understand the effect of surface texture of hard surfaces on coefficient of friction and transfer layer formation. EN8 steel flats were ground to attain surfaces of different textures with different roughness. Then super purity aluminium pins were scratched against the prepared steel flats. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the morphology of transfer layer. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the texture of hard surfaces, but independent of surface roughness of hard surfaces. It was observed that on surfaces that promote plane strain conditions near the surface, the transfer of material takes place due to the plowing action of the asperities. But, on a surface that promotes plane stress conditions the transfer layer was more due to the adhesion component of friction. It was observed that the adhesion component increases for surfaces that have random texture but was constant for the other surfaces.