Friction in bulk metal forming: a general friction model vs. the law of constant friction

Commercially available finite-element programs for the simulation of bulk metal-forming processes usually model the frictional restraint acting at the interface between the workpiece and the tools according to the law of constant friction. Such description is often inadequate and does not represent the state-of-the-art in tribology. In the present paper it is shown that the application of the general friction model, developed by Wanheim and Bay, involves a major improvement in the ability to simulate processes where low tool-workpiece interface stresses may prevail. This is confirmed by experimental and numerical investigations into the upsetting of a semi-tapered specimen between parallel dies. Additionally, it has led to the proposal of a new ring-compression test geometry intended to complement the conventional ring test for the calibration of friction models under conditions where the normal stresses over considerable parts of the tool-workpiece interface may be lower than the yield stress of the material.