Determination of friction models for metallic die-workpiece interfaces

A two-parameter friction model is used which combines the Coulomb friction model and the friction factor yield stress model. The drawback of this two-parameter model is the complex nature of its calibration. In this paper a new technique is proposed to calibrate the model, which utilizes two testpiece geometries, namely the solid cylindrical compression testpiece and the ring compression testpiece. In addition, a mathematical model is required of the true stress-true strain behaviour of the material, so that finite deformation/finite element techniques can be used to accurately predict the compression behaviour of both testpieces. By a combination of careful experimentation, carried out on aluminium alloy and copper testpieces, and of finite element analyses of the testpieces made using the two-parameter friction model, it has been shown that it is possible to derive the true stress-true strain curve for the workpiece materials; and, to calibrate the friction model. The geometrical changes of all testpleces, carefully measured throughout the tests, for a range of four different friction conditions, dry friction, lubricant, lead metal and nylon, have been predicted with good accuracy using the true stress-true strain constitutive models, the two-parameter friction model, and the finite-element analysis procedures. In this way, the proposed approach has been validated. (C) 2005 Elsevier Ltd. All rights reserved.