Influence of surface roughness on friction during metal forming processes
Introduction
Real metal surfaces are not smooth; asperities of different kind and different distributions exist, representing the roughness of the surface. Workpiece as well as tool are characterised by surface roughness. This has an influence on the friction properties at those surfaces, especially at the beginning of metal forming processes, till all asperities are flattened. During the forming process asperities plough into each other, and thus a small sliding always exists. Since, at the beginning of the process, the tool is in contact just with the peaks of the asperities, the friction properties depend on the distribution of asperities, on their height and their deformation during the process, e.g. on the roughness of the contact surface. With the flattening of asperities the contact with the tool gets larger and this leads to varying friction properties. A non-linear non-local friction law will be applied here in order to obtain the influence of the surface roughness on the normal compliance law. A comparison with the solution obtained by means of the classic friction law will be made and the influence of the surface roughness on the friction coefficient will be shown.
Section snippets
Mechanical model
Oden and Pires [1], [2] proposed a non-linear, non-local friction law for describing the influence of surface roughness on friction. According to that law, the friction stress at a given point of the contact surface depends not only on the normal pressure at that point (the local pressure) but also on the normal pressure in a neighbourhood, surrounding the point under consideration (non-local pressure):where μ0 is the prescribed friction coefficient and ut the relative
Numerical simulation
The commercial FE package MARC is used for the calculations, and user subroutines are created in order to take into consideration the isotropic and anisotropic non-local friction laws. The friction coefficient at each nodal point is calculated by means of the values of the normal nodal forces from the previous time increment in a neighbourhood determined by a circle or ellipse. Saving of those normal forces, as well as the numbers and coordinates of the contact nodes at each time increment, is
Conclusions
The illustrated examples showed that the profile of the roughness of the contact surface has an influence on the normal compliance law. The non-homogeneity, as well as the density of asperities influence the normal compliance law. The influence of tangential tool velocity is rather negligible. Anisotropy of friction properties changes the form of the plastic zone under the asperity and influences the normal compliance law.
The profile of the surface roughness of the metal workpiece can be
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