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.     

Surface stresses and flow modes on contact surface in a combined double cup extrusion process

The main goal of this study is to examine the influence of geometrical parameters in double cup extrusion process (DCEP) on the surface stress profiles such as surface expansion, contact pressure, sliding velocity and distance. The flow modes, sliding over or sticking onto the punch surface, were also investigated to see the possibility of the metal-to-metal contact or the lubricant film breakdown along the punch and workpiece interface. Two major design factors chosen for analysis include the reduction in area in backward direction (RAB) and the wall thickness ratio (TR). A sequential operation has been also simulated to compare the surface stress profiles and flow modes with those in a combined operation.     

New wear tests of tool materials for metal forming

Tribological phenomena, i.e. friction, tool wear and material pick-up to the tool, in metal forming processes are influenced by various factors. Therefore, in friction tests in the laboratory, these factors should be varied widely and independently and they should be measured easily. A new test method which has these features is described; the test parameters are the tool pressure, sliding speed, enlargement of the workpiece surface, length of contact between the tool and workpiece, sliding distance, contact angle and viscosity of the lubricant. Some results of the tests on tool wear are presented and discussed.     

On the equivalent friction factor in hydrofilm extrusion

Hydrofilm extrusion is a kind of hydrostatic extrusion which uses a minimum amount of oil as lubricant and pressure-transmitting medium. In hydrofilm extrusion energy dissipation in the fluid lubricant between the die and working material corresponds to sliding friction in ordinary lubricated extrusion using solid lubricants. Utilizing the upper-bound theorm, an “equivalent” friction factor is defined so that the overall frictional effect between the die and working material can be conveniently investigated in terms of geometrical parameters and press velocity. On the basis of this definition, the effects of various process parameters on the frictional characteristics in hydrofilm extrusion are discussed. It is consequently found that the dominant contribution to frictional energy dissipation is made by reduction of area and press velocity. Die length is found to have very little influence on the equivalent friction factor in so far as it is longer than billet diameter.     

Analytical determination of friction resistance as a function of normal load and geometry of surface irregularities

A mathematical modeling and simulation of friction during steady state sliding of metals, based on the upper-bound approach, is demonstrated. The existence of wedge-shaped protrusions on the tool surface is assumed. Pressing these protrusions onto the workpiece and sliding the tool along the workpiece produces asperities on the surface of the workpiece. These asperities move in a wave-like motion along the surface layer and cause plastic deformation through a specified depth under the surface. This plastic deformation combines with local friction between the tool and the workpiece along the asperity interface to produce resistance to sliding. The relation between the normal pressure and the sliding resistance is established for the entire range of pressure levels from zero to infinity. The apparent Coulomb coefficient of friction for lower levels of normal pressure and the constant friction factor for excessive load levels are determined. The transition region from Coulomb coefficient of friction to constant friction factor also becomes clear. A mathematical determination is obtained by means of a force equilibrium considering the concept of a contact surface friction ratio. The force of resistance to sliding is related both to the geometry of the asperity of the surface of the tool and to the constant friction factor, which is used for measuring a local frictional force along the interface of each asperity.     

Effects of entraining velocity of lubricant and sliding velocity on friction behavior in stainless steel sheet rolling

A series of experiments was carried out using a rolling-type tribometer to investigate the effects on friction behavior of the entraining velocity of the lubricant at the inlet to the contact zone (V) and sliding velocity during deformation (ΔV). Experiments with stainless steel sheets of two different surface roughnesses showed that the variations in the friction coefficient with entraining velocity V and sliding velocity ΔV are largely dependent on the initial surface texture of the workpiece. For a smooth workpiece, the friction coefficient decreases with increasing sliding velocity ΔV but keeps almost constant with increasing entraining velocity V. However, for a rough workpiece, the friction coefficient initially decreases slowly and increases largely with increasing sliding velocity ΔV or decreasing entraining velocity V. Observation of the rolled surface for a smooth workpiece shows that, with increasing entraining velocity V, the slip band becomes more marked, and with increasing sliding velocity ΔV, the rubbed portions become more conspicuous. For a rough workpiece, galling occurs at high sliding velocity ΔV. The critical condition for galling outbreak is shown on the V-ΔV graph. The galling outbreak process is observed by interrupting the rolling process.     

Sliding wear response of a cast iron under varying test environments and traversal speed and pressure conditions

This study pertains to the examination of sliding wear behaviour of a gray cast iron over a range of sliding speeds and applied pressures in dry and (oil and oil plus graphite) lubricated conditions. Wear properties characterized were wear rate and frictional heating. The cast iron revealed various forms and sizes of graphite particles in a matrix of pearlite and limited quantity of free ferrite. Different solidification patterns, as controlled by the chemical composition and/or carbon equivalent of the alloy and rate of cooling, were thought to be responsible for the varying morphology of the graphite phase formed in the material matrix. Occasional decohesion of graphite at ferrite/graphite interfacial regions was also observed.The wear rate of the cast iron increased with the speed and pressure of sliding due to increasing severity of wear condition. The specimens tended to lose proper contact with the disc at larger pressures when slid dry. This was attributed to severe cracking tendency of the material. On the contrary, specimen seizure was noticed in the oil and oil plus graphite lubricated conditions; the seizure resistance (pressure) decreased with sliding speed in presence of the lubricants. The wear rate versus pressure plots attained different slopes, i.e. the rate of increase in wear rate with pressure, depending on the test environment. One slope and inappreciable effect of pressure on wear rate were noticed due to substantial cracking tendency of the cast iron when tested in dry condition. In the oil lubricated condition also, virtually one slope was observed but it was higher than that in dry condition indicating greater sensitivity of wear rate towards the applied pressure. Also, the samples attained lower wear rate in oil than in dry condition in view of suppressed cracking tendency causing more stable lubricating film formation in presence of the oil lubricant. Addition of graphite particles to the oil lubricant caused a further reduction in wear rate because of the enhanced possibility of a more stable lubricant film formation due to smearing of the graphite particles. In this case, the slope of the wear rate versus pressure plots was the least in the intermediate range of pressures irrespective of the sliding speed owing to more stable lubricating film formation.A higher rate of temperature increase with test duration (intermediate sliding distance) in the beginning was attributed to the abrasive action of the hard debris generated through the fragmentation of the initially contacting asperities. A subsequently observed lower rate of increase at longer durations could be owing to the occurrence of mild wear condition in view of less stressing of the contacting asperities and increased stability of the lubricant film formed. Increase in the rate of frictional heating at still longer durations resulted from destabilization of the lubricating film.Frictional heating increased with applied pressure and sliding speed in view of increasing severity of wear condition. The rate of increase in frictional heating was low initially up to a specific pressure followed by a higher rate of increase at still larger pressures when the tests were conducted in oil plus graphite at both the sliding speeds and in the oil lubricant at the lower speed. A constant (high) rate of increase in frictional heating with pressure was noticed in the dry condition at both the sliding speeds and in the oil lubricant at the higher speed. Low rate of frictional heating with pressure was attributed to the occurrence of mild wear condition while a higher rate of frictional heating with pressure resulted from the occurrence of severe wear condition. As far as the influence of test environment on frictional heating is concerned, least frictional heat was generated in the oil plus graphite lubricant mixture while the maximum was noticed in dry condition, intermediate response of the samples being observed in oil. Formation of more stable lubricating film was thought to be responsible for lower frictional heating in the lubricated conditions; the presence of graphite in the oil lubricant increased the extent of lubricating film formation and stability of the film so formed.The wear response of the samples has been explained in terms of cracking tendency and lubricating effects of graphite, predominance of the counteracting effects of the two parameters over each other, and lubricating film formation by the external oil (plus graphite) lubricant on the sliding surfaces in specific test conditions. Characterization of wear surfaces, subsurface regions and debris particles of the material enabled to further substantiate the observed wear performance of the samples.     

锻造加工中磨擦特性评价方法的研究

介绍了冷锻造中摩擦特性评价的一种新方法。将圆柱工件挤压成有2个突出锥形的平板边缘形状，通过测量工件的变形来计算硬模和工件之间的摩擦阻力。通过FEM分析，其摩擦阻力可以用摩擦系数定量表示。在实验中将8种润滑油应用于铝合金工件，结果表明，这种方法对降低摩擦力的评价和对润滑油防止表面磨损的评价都是有效的。     

Stress profiles at contact surface in ring compression test

A perfectly plastic material has been employed as a model material in simulation to analyze numerically the ring compression process, especially to examine the deformation patterns along the die/workpiece interface, which is strongly related to the frictional condition at the contact boundary. The main objective is to provide the deformation characteristics in detail in ring compression, especially at the tool/workpiece interface. The surface flow patterns at the contact boundary in ring compression are summarized and analyzed in terms of surface expansion, surface expansion velocity, pressure distributions exerted on the die surface, relative sliding velocity between die and workpiece, and sliding distance along the die surface. Movement of neutral positions and folding phenomenon are also investigated to see the effect on the deformation patterns at the interface, that is, geometrical change, which is important to measure the frictional condition at the interface using calibration curves. Finite element (FE) simulation using rigid-plastic finite element code has been performed for analysis. The results of this study reveal that surface expansion as well as other surface flow patterns, such as sliding velocity and so on, shows different and distinctive characteristics between low and high frictional conditions at the interface. This is directly related to the movement of neutral positions and folding, which affects the sensitivity of dimensional changes to tribological conditions at the interface.     

微动接触应力的有限元分析

以方足微动桥,试样接触几何条件为研究对象,应用ANSYS有限元分析软件对其接触面上的应力分布进行弹性有限元分析,验证用ANSYS所建计算模型的正确性,分别计算不同名义接触压力和不同摩擦因数条件下接触状态（粘着区、滑动区、张开区）和接触面应力分布,选取不同水平的循环载荷进行计算,研究接触状态和应力分布随循环载荷的变化情况。结果表明,微动疲劳过程中接触表面拉应力与剪应力在接触面的粘,滑交界区存在突变,微动疲劳裂纹正是在这一区域内萌生并扩展,计算结果与实验结果吻合很好。     

Rolling-Sliding Analysis in Ball Bearing Considering Thermal Effect

An analytical method is applied in the calculations of ball contact angles varying with the position angle of a ball bearing with a centrifugal force. This is further extended to investigate the thermal slipping behavior occurring at the contact surface of the inner raceway. The sliding velocities at the contact area are analyzed in the present study by the ball bearing contact mechanisms. The shear stress created at this contact surface is evaluated by considering the temperature-pressure dependence of the lubricant viscosity. The contact temperature of a ball and the inner raceway is obtained if the frictional heat created by the shear stress is available. Under the condition of controlling the outer raceway temperature, the thermal deformations of all components of a ball bearing are taken into account in the evaluation of bearing performance. Comparisons are made for several parameters, including the normal load, linear slip velocity, and linear traction force, between the results of considering the thermal effect or ignoring it. The friction torque evaluated by taking the thermal effect into account is higher than the torque evaluated when ignoring the thermal effect.     

In vitro frictional behavior and wear patterns between contemporary and aesthetic composite orthodontic brackets and archwires

This study evaluates the effects of surface roughness, saliva and bracket materials on in vitro frictional resistance of contemporary and aesthetic orthodontic brackets during the sliding of archwires against elastomeric ligature. Eight various brackets were investigated together with stainless steel (SS) archwires of several shapes. The sliding force involved in a ligated bracket-archwire combination was measured in an Instron by a self-designed jig. The interfaces were also examined using scanning electron microscope (SEM) to simulate sliding mechanics and abrasive/adhesive wear of the bottom surfaces of the bracket slots, which were observed under dry condition. Round SS archwires (SRO-OR) demonstrated the lower frictional force compared to coated round SS archwire (SRO-AW) and the flat SS archwire (rectangular and square) surfaces for most of the brackets. Braided composite bracket without a metal slot had the least resistance when sliding with SRO-OR archwire whilst the composite bracket (Spirit MB) with a metal slot demonstrated the lower resistance. Ceramic bracket (Inspire) without a metal slot, which also reflected severe wear patterns produced a significantly larger frictional force for SRO-AW. The presence of saliva had a consistent effect in increasing the friction, which counters the inconsistency reported in the literature. The soft-coated surface of SRO-AW archwire confirms sliding difficulty in sliding on the hard surface of bracket slot. A smoother surface correlates with a lower frictional force according to the surface roughness values.     

Deformation characteristics at contact interface in ring compression

The main objective of this study is to provide the deformation characteristics in detail in ring compression, especially at the tool/workpiece interface. The surface flow patterns at the contact boundary are analyzed in terms of surface expansion, surface expansion velocity, pressure distributions exerted on the die surface, relative sliding velocity between die and workpiece, and the sliding distance along the die surface. Movement of neutral positions and folding phenomenon is also been investigated to see its effect on the flow pattern, that is geometrical change, which is important to measure the frictional condition at the interface using calibration curves.     

Surface damage during machining of annealed 18% nickel maraging steel Part 2 — Lubricated conditions

The effect of cutting speed and tool wear land length on the surface damage produced during the machining of annealed 18% nickel maraging steel under lubricated conditions was determined. Machined test pieces were examined with a scanning electron microscope and an optical microscope. Surface roughness was determined with a profilometer.The results of the investigation show that during machining fine scale surface damage in a variety of forms is produced over the range of cutting conditions used. It is suggested that the fine scale surface damage is associated with the phenomena of continuous chip formation and interaction between the tool nose region and freshly machined workpiece surface.Comparisons are made between the characteristics of surfaces machined under lubricated and unlubricated conditions. The differences are interpreted in terms of the changes produced in the frictional conditions and stress distributions at the contact surfaces between the chip, tool and workpiece by application of a lubricant.It is shown that scanning electron microscopy is more indicative of the true condition of the surface than surface roughness measurements.     

Finite element simulation of high-pressure water-jet assisted metal cutting

Experimental studies have shown that improved metal cutting efficiency can be obtained when a high-pressure water/coolant jet is injected at the tool–chip interface. The pressure exerted on the chip face by the jet is expected to reduce, for example, friction along the tool–chip interface, temperature rise in the chip and the workpiece, the cutting force, and residual stress in the finished workpiece, leading to a longer tool life and a better surface integrity for the finished workpiece. This paper presents the results of finite element simulations of high-pressure water-jet assisted orthogonal metal cutting, in which the water jet is injected directly into the tool–chip interface through a small hole on the rake face of the tool. The mechanical effect of the high-pressure water jet is approximated as a pressure loading at the tool–chip interface. The frictional interaction along the tool–chip interface is modeled by using a modified Coulomb friction law. Chip separation is modeled by a nodal release technique and is based on a critical stress criterion. The effect of temperature, strain rate and large strain is considered. Cooling effect of the high-pressure jet on the temperature distribution is modeled with a convective heat-transfer coefficient. The effect of water jet hole position and pressure is studied. Contour plots showing the distributions of steady-state temperature and stress and the residual stress are presented. The simulation results show a reduction in temperature, the cutting force and residual stresses for water-jet assisted cutting conditions. The mechanical effect of the water jet is found to reduce the contact pressure and shear stress along the tool–chip interface and also the contact zone length for certain water jet hole locations.     

冷挤压塑性流体动力润滑分析

应用流体动力润滑理论和塑性成形原理，分析了冷挤压润滑过程。在挤压的起始和终止阶段为非稳态流体动力润滑，而中间阶段可近似为稳态流体动力润滑。考虑冷挤压在高压及大剪切应变率工况下润滑剂的非牛顿特性，运用Ostwald非牛顿体模型，分别建立了冷挤压非稳态和稳态的塑性流体动力润滑（PHD）模型。采用Monte Carlo法得到了冷挤压润滑过程的油膜厚度、油膜压力以及摩擦力的分布规律。     

Friction characteristics of a ferritic-austenitic stainless steel during lubricated reciprocating motion

This report describes friction measurements of stainless steel against stainless steel during lubricated, small-amplitude reciprocating motion. The experimental investigation was divided into two parts. First, four different lubricants were evaluated using a response surface design, during which the average contact pressure and the sliding velocity were varied. Secondly, a 2⁴ factorial design with three replicate runs was performed. Here, the coefficient of friction in the initial stage and the duration of that stage were studied. The independent variables were the average contact pressure, sliding velocity, surface roughness and type of lubricant. In the early state (stage I), the value of the frictional force is controlled by plowing of the surfaces by asperities. In many lubricated contacts, this is the practically useful stage. The experimental results from the response surface design show that the duration of stage I depends on the type of lubricant. Adhesive wear can take place before 100 cycles. The factorial design indicates that the coefficient of friction in the initial stage is affected by the type of lubricant, surface roughness and the simultaneous change of the surface roughness and type of lubricant. The duration of the initial stage is affected by a change in the surface roughness, average contact pressure and a simultaneous change in average contact pressure and surface roughness. A two-parameter Weibull analysis was performed on the data from the factorial design. For the tests where lubricant no. 3 was used, a mixed distribution was indicated for the duration of stage I. This mixed distribution indicates that a weakest-link process as well as a healing process were involved.     

柴油机滑动轴承热流体动力润滑仿真研究

根据径向滑动轴承热流体动力润滑理论,基于JFO理论提出的质量守恒边界条件,建立同时包含油膜完整区和空 穴压力变化的单缸柴油机滑动轴承热流体动力润滑模型,采用有限差分法求解模型方程,仿真分析滑动轴承的油膜厚度、油膜压力、润滑油流量和温度等参数对润滑性能的影响,分析内燃机滑动轴承润滑特性,为轴承润滑可靠性设计提供一定的理论依据.     

Evaluation of interfacial friction condition by boss and rib test based on backward extrusion

Proper consideration of tribological problems at the contact interface between the tool and workpiece is crucial in metal forming, since interfacial friction condition plays an important role in metal forming by influencing the metal flow, forming load, die wear, etc. In order to quantitatively estimate such friction condition, a new friction testing method “Boss and Rib Test” based on the backward extrusion process is proposed in this work. In boss and rib test, a key design is to use a tube-shaped punch so that the boss and rib at the deforming workpiece along the inner and outer surfaces of the punch are formed during backward extrusion. It was experimentally and numerically revealed that the heights of the boss and rib vary according to the friction condition applied. It was also found that the height of the boss is higher than that of the rib when the friction condition at the contact interface is severe. From this finding, the shear friction factor can be evaluated according to lubricant characteristics assigned. In addition, simulation results revealed that calibration curve demonstrating deformation pattern of the workpiece is affected by strain-hardening exponent of the workpiece.     

Nanoscopic surface texture formed by indentation and sliding of a smooth wedge tool

To achieve reliable replication of a flat tool surface through a metalforming process with nanometer-scale surface roughness, a wedge tool was indented and slid under a constant normal load. In a workpiece finished with abrasive paper, some of the initial valleys remain on the tested surface, though at the boundary contact area the surface roughness is as small as that of the tool. The final surface roughness of the workpiece was much reduced by starting with a smoother surface of the workpiece and by unlubricated short sliding. A nanoscopically smooth surface of 0.8 nm sRa was obtained with a gold-coated copper workpiece after unlubricated indentation and sliding. This suggests the beneficial effect of a soft surface film coated over a hard substrate, causing complete filling of the interfacial vacancies.