Modeling of friction in cold forging considering a wide range of tribological conditions

In cold forging processes, the workpiece is deformed under high pressure resulting in material flow at the interface between the workpiece and the die, causing large friction forces in opposing direction to the relative movement between the workpiece and the die. Friction is a very complex physical phenomenon, which affects the material flow, the forming load, the surface quality of the workpiece, and the service life of the dies. A general friction model was furtherly confirmed by experimental and numerical investigations into the upsetting of a semi-tapered specimen. The Coulomb friction law and the constant shear friction law were compared using a rigid-plastic finite element method, and considerable differences existing in simulation results can be observed between the two friction laws. Different friction models generate different friction stress distributions, and it can be found that calibration curves of the friction area ratio are more sensitive to friction. Based on compression-twist testing results, a mathematical model was established for friction as a function of normal pressure and tool/workpiece interface temperature. Another friction model considering the sliding velocity between tools and the workpiece was developed. A critical normal pressure was defined between the Coulomb friction law at low normal pressures and the constant friction model at high normal pressures, and a new law of friction involved with the effect of the ratio of real contact area under oil-lubricated condition was proposed. As reviewed by Nielsen and Bay, during the last 80 years, most important contributions on theoretical models of friction in metal forming are based on the analysis of the real contact area and different understandings of asperity flattening in tool-workpiece interfaces. In tribological systems of metal forming, the tribological loading conditions can mainly be described with four quantitive parameters including contact normal stresses, surface expansion ratio, relative sliding velocity between tool and workpiece and initial temperature. There are relatively large differences between the empirically determined friction coefficients from different tribometers, but it can be well explained when the respective tribological loads are considered. Against this background a friction model for metal forming can be created, considering the relationship between tribological loads and friction force. In this work, four tribometers were used to measure friction forces for a wide range of tribological conditions, typically occurring in cold forging. The collected data is used to create different friction models, using mathematical fitting and machine learning algorithms, to linking the tribological loads and the coefficient of friction.