Modeling of surface roughness effect on dry contact friction in metal forming

Interfacial conditions such as friction and roughness substantially affect the process characteristics of metal forming. This study developed a dry friction model that accounted for the adhesion and interference effects of surface roughness. A sliding friction coefficient was suggested to provide fundamental information about the interfacial conditions of the contact surface. The proposed model was easily verified by published experiments and predicted values agreed with experimental results. Accordingly, friction coefficient ?? clearly increased as relative roughness R _m (=?roughness of tool $ R_a^T $ /roughness of workpiece $ R_a^M $ , measured as interference effect) increased. Simulations confirmed that the friction coefficient ?? decreased as dimensionless stress S _m (=?contact pressure p _m /tensile strength $ \sigma_u^0 $ ) increased at small strain hardening exponent n-values. Under the conditions of large n and small R _m values, the friction coefficient ?? initially decreased and then increased. It then slightly decreased as dimensionless stress S _m increased. However, this trend became less apparent as relative roughness R _m increased since friction coefficient ?? simply decreased.     

Multi-scale friction modeling for sheet metal forming: The boundary lubrication regime

A physical based friction model is presented to describe friction in full-scale forming simulations. The advanced friction model accounts for the change in surface topography and the evolution of friction in the boundary lubrication regime. The implementation of the friction model in FE software codes is discussed. Results show that friction coefficients vary in space and time, and depend on local process conditions such as the nominal contact pressure and the plastic strain in the sheet material. The advanced friction model is validated by two small-scale forming processes, proving the enhanced predictive capabilities of FE simulations. The moderate increase in FE computation time, compared to using a Coulomb based friction model, demonstrates the efficiency of the proposed friction model.     

Real area of contact and friction stress — The role of trapped lubricant

A theory is developed which shows the influence of the normal pressure, asperity slope, friction factor and pressure-sensitive lubricant bulk modulus on the real area of contact and the friction stress between a rough specimen and a smooth tool surface. Theory as well as experiments show that the normal pressure and bulk modulus have great influence on the real area of contact, whereas the asperity slope and friction factor are of minor importance.     

Comparisons of friction models in bulk metal forming

A friction model is one of the key input boundary conditions in finite element simulations. It is said that the friction model plays an important role in controlling the accuracy of necessary output results predicted. Among the various friction models, which one is of higher accuracy is still unknown and controversial. In this paper, finite element analyses applying five different friction models to experiments of upsetting of AA 6082 lubricated with four lubricants are presented. Frictional parameter values are determined by fitness of data of friction area ratio from finite element analyses to experimental results. It is found that calibration curves of the friction area ratio for all of the five chosen friction models used in the finite element simulations do fit the experimental results. Usually, calibration curves of the friction area ratio are more sensitive to friction at the tool/workpiece interface than those of the normal pressure.     

An upper-bound analysis of friction in metal forming

In the analysis of metal forming processes, a knowledge of friction is important, especially when the microstructure evolution and criteria for limiting phenomena are predicted by numerical simulation. The friction wave model has been studied by several researchers. Their analyses are mainly based on the assumption that there is no plastic deformation of the bulk material. However, it is necessary to clarify the influence of bulk material deformation on the surface asperity deformation. This paper deals with the development of a friction wave model by considering the influence of bulk material on the surface asperity deformation. The situation of rough tool—smooth workpiece (RT—SW) contact during forming process has been investigated. Based on this condition, an admissible velocity field is constructed for the upper bound analysis. The relationship between the normal pressure and the sliding resistance is established over a large range of pressure. The role of surface roughness, bulk displacement and bulk strain on metal forming friction is analysed.     

Shear characteristics of a boundary film for a paper-based wet friction material: friction and real contact area measurement

A paper-based wet friction material lubricated with automatic transmission fluids (ATFs) produces plateau and positive slope regions in the friction–sliding velocity diagram. This feature observed at less than 1 mm/s of sliding velocity is discussed in this paper. The relationship between the friction and contact pressures of less than 5 MPa was determined by a reciprocating friction test rig lubricated with mineral oil including phosphoric acid ester, one of the additives for ATFs. The real contact area for dry condition was also determined by optical interferometry. The consistency of the real contact pressure dependency of the shear strength of the boundary film was indicated at pressures of less than 1 GPa obtained by steel-on-steel boundary friction (oiliness) test. This suggests that the generation of the positive slope region is due to thin film lubrication. To the plateau region, possibility of application of the phase transition model is considered.     

Effects of friction laws on metal forming processes

In this paper, the Coulomb friction law and the constant shear friction law were compared and investigated in detail using a rigid-plastic finite element method with emphasis on their application in bulk metal forming. The ring compression test for two different materials was used to evaluate the two friction laws, and then their effects on metal flow lines and forming loads for various friction-sensitive metal forming processes including strip rolling, ring gear forging, multistep extrusion, and pipe shrinkage and expansion were investigated. It was shown that considerable differences exist between the two friction laws, especially in friction-sensitive metal forming processes.     

Modelling of static friction in rubber–metal contact

A static friction model for contact between rough rubber and metal surfaces is developed. This model is based on the contact of a viscoelastic–rigid asperity couple. Single asperity contact is modelled in such a way that the asperities stick together in a central region and slip over an annulus at the edge of the contact. The slip area increases with increasing tangential load. Consequently, the static friction force is the force when the slip area is equal to the contact area. Using the model, the traction distributions, contact area, tangential and normal displacement of two contacting asperities are calculated. The single asperity model is then extended to multi-asperity contact, suitable for rough surfaces. This model allows calculation of the above-mentioned parameters for two rough surfaces (a rubber and a metal one) subjected to normal and tangential loads. A parametric study will be presented. The results are qualitatively in good agreement with those found in literature.     

Numerical modeling of the thermal contact in metal forming processes

Heat flow across the interface of solid bodies in contact is an important aspect in several engineering applications. This work presents a finite element model for the analysis of thermal contact, which takes into account the effect of contact pressure and gap dimension in the heat flow across the interface between two bodies. Additionally, the frictional heat generation is also addressed, which is dictated by the contact forces predicted by the mechanical problem. The frictional contact problem and thermal problem are formulated in the frame of the finite element method. A new law is proposed to define the interfacial heat transfer coefficient (IHTC) as a function of the contact pressure and gap distance, enabling a smooth transition between two contact status (gap and contact). The staggered scheme used as coupling strategy to solve the thermomechanical problem is briefly presented. Four numerical examples are presented to validate the finite element model and highlight the importance of the proposed law on the predicted temperature.     

Dissipative function of coulomb friction on elliptical contact area

The problem of calculation of the moduli of the resultant vector and the basic moment of the forces of the dry rolling friction of a solid body against a rough plane surface in the presence of spinning around a vertical axis is solved in quadratures. The elementary forces of sliding friction are directed opposite to the points of the elliptical contact area fulfilling plane motion. The friction coefficient is accepted to be independent of velocity. The proposed method of computing the resultant vector and the basic moment of friction forces on the elliptical contact area makes it possible to find with a preset accuracy the forces and the friction torque in the contact problems by using numerical procedures. In particular, it is used to determine the tangential forces arising during the motion of a rail vehicle. With increasing vertical load on the wheels, the contact spot area in which the friction forces of sliding play the main role becomes larger. The proposed method can also be used with some modifications, e.g., by introducing a corrective factor, to calculate the contact spots having a configuration different from elliptical but restricted by a closed curve.     

The actual contact area and friction properties of elastomers under frictional contact with solid surfaces

Using a specially designed apparatus simultaneous measurements were carried out of the area of contact and the friction between elastomers and solid surfaces. For a wide range of load, the friction force is proportional to the actual contact area and the friction constant depends only on the nature of the rubbing solids. In the range of low sliding velocities the actual contact area remains almost constant. At high sliding velocities increase of the friction force is associated with the process of the mechanical glass-transition of the surface layer of the cross-linked rubber which causes the actual contact area to decrease.     

Actual contact area growth caused by interaction of junctions in dry friction

Experimental data characterizing dry friction processes between strain-hardenable metallic bodies under various normal loads are presented. It is shown that changes both in tangential force and friction coefficient observed in experiments are accompanied by a corresponding growth in the actual contact area (i.e. total area of metallic junctions) between contacting surfaces. Variations of actual contact area are explained as a result of interaction of metallic junctions, caused by their coalescence, which depends on the distances between actual contact area spots. The results obtained reveal some peculiarities of the process seizure.     

Observation of proportionality between friction and contact area at the nanometer scale

The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Müller–Toporov continuum mechanics model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Evaluation of the real contact area in three-body dry friction by micro-thermal analysis

Many tribological properties and wear mechanisms occurring on the micro-and nanoscale are strongly controlled by the so-called real contact area (A_r) which is a small fraction of the nominal or apparent contact area (A_a). The determination of A_r is often based on either (i) a geometrical approach describing the real geometry of contacting surfaces or (ii) a mechanical approach involving contact mechanics and physical-mechanical properties. In addition some experimental methods have also been attempted but they generally do not take into account the presence of third body at the interface—i.e. the wear debris trapped within the contact. In this paper we propose an experimental approach to estimate the dynamic real contact area from the operating parameters (F_n, v, T) and the tribological responses (μ, F_t) in presence of third body. A scanning thermal microscope (SThM) is used for determining both the thermal conductivity of the third body and the relationship between the contact temperature and the thermal power really dissipated at the micro-asperity level. These results are combined with a thermal model of the macro-tribocontact for computing the real contact area and the real contact pressure. Validation of these results is carried out using a classical Greenwood Williamson model and finite element models built from the real AFM maps.     

Evaluation of friction coefficient using indentation model of Brinell hardness test for sheet metal forming

This work presents an indentation model of the Brinell hardness test, which is a rigid ball-deformable plane contact model (RB-DP model), to elucidate the sliding friction mechanism of sheet metal forming. In the proposed model, the friction force can be defined as a combination of shear (shearing effect) and plough (ploughing effect) forces. The real contact area ratio α is determined from the RBDP model under sliding condition. Moreover, the lateral contact area ratio A _c /A _r can be specified as a function of the real contact area ratio α. Based on Meyer’s law and Hertz contact problem, the maximum contact area ratio α _u , a limiting condition of the real contact area ratio α, can be described as a function of the strain hardening exponent n. Additionally, a limiting condition applies: the strain hardening exponent n must be less than 0.64 in the present model. The present friction model reveals that the friction coefficient μ _d is a function of strain hardening exponent n, the real contact area ratio α and the maximum contact area ratio α _u . The calculated friction coefficient μ _d agrees with the published experimental results.     

Evaluation of electrical contact area between metal and semiconductor using photo-induced current

Photo-induced current (PIC) is produced from excited electrons and holes in a semiconductor by irradiation of laser light. Since the intensity of the PIC depends on the traveling distance of carriers, measuring current between a metal and a semiconductor can be utilized for analyzing electrical contact. The spatial resolution of the PIC depends on the attenuated length and the thickness of the semiconductor, and its intensity is proportional to the contact pressure. By scanning laser light, we obtained two-dimensional distribution maps of electrical contacts between solids.     

Modelling of contact and friction in aluminium extrusion

A physical model capable of predicting the friction and sticking/slipping lengths in the bearing channel during unlubricated aluminium extrusion processes is presented. The model takes into account the pressure build up in the extrusion direction in relation with the bearing–extrudate friction and how it will be influenced by die geometry such as die angle and bearing length. The results show that a high friction coefficient in the bearing will increase the sticking length and shorten the slipping length. The results are in good agreement with experimental findings documented by previous researches.     

基于一种广义摩擦公式的金属非稳态成型过程弹塑性行为的分析

通过理论分析推导,归纳总结了一个广义摩擦公式。同时应用该公式,对典型金属成型过程非稳态轧制过程进行了三维大变形弹塑性有限元分析。对轧辊与工件之间的接触(摩擦)问题,用这种新的摩擦公式进行了较精确计算。对数值计算结果(包括轧制力、轧制力矩以及界面间的正应力和剪应力)进行了分析讨论。证明广义摩擦公式是切实可行的。