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Peer Reviewed, Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructura, Postprint (published version)

Friction plays an important role in metal-forming processes by pressure and largely controls material flow wear, adversely affecting the wear resistance of dies employed to form metals. Therefore, 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. Accurately determining coefficients in the Coulomb constant shear friction law is critically important for the correct assignment of the boundary conditions in the FEA of metal forming needed to develop proper dies. 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 com-pared with laboratory measurements. The model is based on an approximate solution of the equilibrium equations of elasticity, the kinematically admissible velocity field, and the distribu-tion of tangential stresses in the plastic deformation zone that meet the certain boundary condi-tions. For the zone, where a transition from the Coulomb friction law to the constant shear fric-tion law occurs, analytical formulas relating the corresponding friction coefficients have been derived. Based on the aforementioned mathematical model, an algorithm allowing us to derive the Coulomb constant shear friction law coefficients from laboratory measurements has been developed. 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 under laboratory con-ditions. The Coulomb friction coefficient µ and the constant friction factor m calculated for the model material appear to be independent of the dimension ratio and are influenced mostly by roughness and range from µ =, Peer Reviewed, Postprint (published version)

Burnishing, the plastic deformation of the workpiece surface due to sliding contact with a tool called burnisher, is a finishing operation widely used in various industries. In this work, impacts of the initial surface roughness Ra of the workpiece being burnished, the nature of friction in the contact zone, and the clamping force on the stability and energy efficiency of burnishing have been investigated. Experiments have been conducted with and without lubricant, represented by low-viscosity deep-hydrogenated fraction of sour oils, at initial surface roughness Ra of 0.8 and 1.25 µm and variable (100–200 N) clamping force. A key process indicator, which largely controls mechanics of burnishing, the temperature in the tool-workpiece contact zone has been measured using natural thermocouple method. Microhardness of the workpiece surface after burnishing has also been measured. It has been shown that changes in the temperature of the tool-workpiece contact zone are proportional to the changes in the squared tool clamping force. This dependence appeared to be universal and equally applicable to burnishing with and without lubrication. Based on the analysis of the experimental data, ¿ new criterion of the burnishing efficiency has been developed. The new criterion simplifies the choice of optimum operational parameters and helps in preventing adverse impacts of structural phase transformations in the workpiece surface layer that unavoidably lead to reduced product quality and operational reliability and in reducing tool wear, which is critically important in the case of dry burnishing. The obtained results show that the nature of friction accompanying the surface plastic deformation has a significant impact on the stability and energy efficiency of the burnishing process. While the clamping force is equally important for burnishing with and without lubrication, the initial roughness Ra has an impact on dry burnishing only. Application of minimum quantity lubrication (MQ, Peer Reviewed, Postprint (published version)