A Review of Friction and Lubricant in Metal Forming

This chapter focuses on the effects of friction and lubrication in metal forming, the mechanisms of lubrication, and an account on lubricants used in metal forming leading toward sustainability of the lubrication in metal-working processes. In metal-forming processes, friction and lubrication are mutually related. Friction is an inherent characteristic in all forms of metal-forming processes and lubrication is often used to control friction in forming applications. Behavior of friction in metal forming should be carefully understood before taking process-related decisions. The Coulomb’s law of friction has been extensively used as an initial predictive model in the analysis of interface friction in many engineering applications. The Coulomb’s law of friction breaks down in applied metalworking processes especially when there is a lubricant present at the interface. The main function of lubricants used in metal forming is to control friction and tool wear. There are many supplementary requirements expected from forming lubricants: cooling material and tool are among them.

The ability to produce a variety of shapes from a block of metal at high rates of production has been one of the real technological advances of the current century. This transition from hand-forming operations to mass-production methods has been an important factor in the great improvement in the standard of living, which occurred during the period. With these forming processes, it is possible to mechanically deform metal into a final shape with minimal material removal. The use of metal forming processes is widely spread over many different industries. In metal forming processes, friction forces between metal and forming tools play an important role because of their influence on the process performance and on the final product properties. In many instances, this frictional behavior is often taken into account by using a constant coefficient of friction in the simulation of metal forming processes. Several different types of instruments are constructed to measure the coefficient of friction for different materials. In this chapter, the fundamental concept of forming processes and the influence of friction in metal forming are discussed. A case study on the influence of friction based on surface texture during metal forming is also presented.

Friction has an essential influence on metal forming processes and affects the mould filling strongly. Numerical simulation is widely used because they allow for a efficient product design without the time and cost intensive production of prototype moulds. The quality of the simulation results and thus their reliability is determined by the accuracy of the modelling. For this purpose the applied friction law is of great importance. Characteristic of sheet-bulk metal forming is the coexistence of moderate contact pressures like in sheet metal forming and high contact pressures like in bulk metal forming. The Coulomb friction law is suitable for the sheet metal forming process but it predicts too high friction forces for high contact pressures. On the other hand the Tresca friction law is suitable for bulk metal forming but overestimates the friction for low contact pressures. A smooth transition between the Coulomb and Tresca friction law is described by the Shaw friction law and the Wanheim-Bay friction law. An unresolved problem remains the influence of plastic surface smoothing of structured workpiece surfaces. The tribological properties of the surface are altered by the plastic deformation of the surface roughness. As a consequence the real area of contact and thus the friction are larger in unloading and reloading than in the first loading at the same surface pressure. This plays a role in forming processes with multiple stages, where the surface is smoothed by prior forming operations like for example the forming of tailored blanks. Therefore efforts have been made in the numerical modelling of elasto-plastic surface deformation with a halfspace model. This model allows for the efficient modelling of large rough surfaces because it uses only a surface mesh and not an numerically expensive volume mesh like a Finite-Element model. This halfspace model is calibrated and verified with experimental investigations. A friction law taking into account the plastic surface deformation has been developed based on the halfspace simulations. It distinguishes between first loading, where the current surface pressure is higher than all surface pressures which occurred previously, and unloading or reloading, where the friction is higher because the surface is smoothed plastically in a previous load step, where the surface pressure was higher than currently.

In the discrete element simulation of granular materials, the modelling of contacts is crucial for the prediction of the macroscopic material behaviour. From the tribological point of view, friction at contacts needs to be modelled carefully, as it depends on several factors, e.g. contact normal load or temperature to name only two. In discrete element method (DEM) simulations the usage of Coulomb’s law of friction is state of the art in modelling particle–particle contacts. Usually in Coulomb’s law, for all contacts only one constant coefficient of friction is used, which needs to reflect all tribological effects. Thus, whenever one of the influence factors of friction varies over a wide range, it can be expected that the usage of only one constant coefficient of friction in Coulomb’s law is an oversimplification of reality. For certain materials, e.g. steel, it is known that a dependency of the coefficient of friction on the contact normal load exists. A more tribological tangential contact law is implemented in DEM, where the interparticle friction coefficient depends on the averaged normal stress in the contact. Simulations of direct shear tests are conducted, using steel spheres of different size distributions. The strong influence of interparticle friction on the bulk friction is shown via a variation of the constant interparticle friction coefficient. Simulations with constant and stress-dependent interparticle friction are compared. For the stress-dependent interparticle friction, a normal stress dependency of the bulk friction is seen. In the literature, measurements of different granular materials and small normal loads also show a stress dependency of the bulk friction coefficient. With increasing applied normal stress, the bulk friction coefficient reduces both in the experiments and in the simulations.

The combined Coulomb constant shear friction law is widely used in commercial and research software for the finite-element analysis (FEA) of metalworking and is naturally more flexible and hence, more relevant to real-life manufacturing than the individual Coulomb and constant shear friction laws. In this work, a new mathematical model of coefficients in the Coulomb constant shear friction law for extruding a metal through narrow V-shaped channels with small convergence angles has been developed and evaluated and compared with laboratory measurements. The extrusion of the model material (lead) through narrow V-shaped channels with small convergence angles varying from 0 to 3.5 degrees has been studied. The Coulomb friction coefficient µ and the constant friction factor m appear to be independent of the dimension ratio and are influenced mostly by roughness and range from µ = 0.363 (with lubricant) to µ = 0.488 (without lubricant) and from m = 0.726 (with lubricant) to 0.99 (without lubricant). The relative length dominated by the Coulomb friction law is less than 1%, and the Coulomb’s coefficient of friction can be approximated as ½ the constant shear friction factor for all tested cases. The developed method and algorithm can be used in both FEA of manufacturing processes and efficiency tests for lubricants used in metalworking.

Publication schedule

Overview of friction modelling in metal forming processes

A simulator based on rigid-plastic finite element method is developed with two common friction law: Coulomb friction law and constant shear friction law are imposed. This project is to develop a simple method to identify tribological properties of various lubricants by metal forming method. In this project, the influence of different lubricants was studied by using ring compression test. The deformation of the ring compression test was measured to obtain an experimental friction calibration curves under different lubricants. To model the friction effect, theoretical friction calibration curves for Coulomb friction law and shear friction law are generated under various parameter of μ, coefficient of friction (Coulomb friction law) and m, shear factor (constant shear friction law). The experimental and theoretical friction calibration curves were compared and the result shows corresponding. The friction of the lubricants was further verified by using a common method: pulling a block on flat surface with load sensor yields the friction force, F in the basic equation F=μN where N is the normal force. The results match the calibration curves too.

A new friction law for sliding rigid blocks under cyclic loading

The present work discusses the use of model asperities as a method for studying asperity flattening in metal forming. Asperity flattening and the resulting real contact area largely influence friction in metal forming. In order to gain a fundamental understanding of asperity flattening, a method of choice often is to use model asperities. Here, macroscopic model surfaces are emulating real surface topographies. The work presents the method in a historic context, explains the method, lists advantages and disadvantages, and discusses the method in terms of whether the insights gained on model asperities are of qualitative or quantitative nature. In conclusion, the method is a valid tool to fundamentally study asperity flattening. It is, however, not always possible to transfer obtained conclusions quantitatively from model surfaces to real surfaces.

Review of friction modeling in metal forming processes

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 μ 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 μ 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 μ 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 μ simply decreased.

In recent years, a variety of surface treatments have been applied to press dies, jigs and tools, and parts to meet diversifying needs. DLC coating has features such as surface smoothness, low coefficient of friction, high hardness, anti-caking, wear resistance, optical properties, insulation, and corrosion resistance, and it is remarkably effective in areas where coatings such as TiN and CrN are not sufficiently effective. In this chapter, we first discuss the plasticity of the coatings. In this chapter, metal forming is first described, and then the role of coatings in metal forming is explained, including friction in metal forming, evaluation methods for friction, and the role of coatings in metal forming. Then, the role of DLC in metal forming is explained, including the required low friction and high galling resistance. Finally, examples of application to actual metal forming are described.

The adoption of advanced high-strength steels is growing in the automotive industry due to their good strength-to-weight ratio. However, the frictional contact conditions differ from the ones arising in mild steels due to the high values of contact pressure. The objective of this study is the detailed numerical analysis of the frictional contact conditions in the hole expansion test. The Coulomb friction law is adopted in the finite element model, using different values for the (constant) friction coefficient, as well as a pressure dependent friction coefficient. The increase of the friction coefficient leads to an increase of the punch force and a slight decrease of the hole expansion. The results show that increasing the friction coefficient postpones the onset of necking, but the localization does not change.

Effects of friction laws on metal forming processes