An approach to calculating the energy-power parameters of metal pressing process by the conform method

The kinematics of the plastic flow of the metal processed by pressure along the contact surface of the tool is largely determined by the direction of the friction forces acting in the boundary layer. It is possible to forcibly create conditions under which these forces will not hinder but promote the improvement of the metal forming process. Since the main friction process occurs in the near-contact layer “deformable metal-tool,” the maximum possible value of the friction stress ( τ ) cannot exceed the intensity of shear stresses acting on this boundary. In calculations related to the flow of metal along the contact surface of the tool, the friction coefficient is usually assumed to be no more than 0.58. According to a number of researchers, when using the Siebel law, this value can reach more than 1.0 and is called the “friction index.” This is due to the fact that the maximum value of resistance to deformation in the near-contact layer can significantly exceed the value averaged over the deformation zone. However, the determination of the true resistance to deformation in a thin near-contact metal layer is very difficult, so this value in the calculations can be corrected by the friction index ( μ ) obtained by experiment. As a result of this work, by solving the equation of the balance of forces required for a stable process of metal extrusion into a die hole, based on the actual length of the contact arc between the workpiece and the container, the calculated value of the friction index was obtained ( μ = 1.56). This confirms the position that the maximum value of friction stresses can significantly exceed the yield strength of the workpiece material averaged over the volume of the deformation zone under pure shear in the process of continuous pressing by the conform method.