Measurement of friction and material flow-stress by a plane-strain compression tribometer

Abstract

With the development of computer technology, many metal-forming processes have been investigated by numerical analysis [1]. A knowledge of friction and flow-stress material is extremely important for performing this analysis. The interfacial friction condition and the material flow stress are amongst the main concerns in metal-forming process design and analysis. Until now, there has not been a universal test method for friction and material flow-stress in bulk metal-forming processes [2]. The present paper introduces a recently developed, easy-to-handle, low cost, accurate tribometer which can measure the friction and material flow-stress in a simple plane-strain compression test. This tribometer can reproduce many characteristics of bulk-forming processes. The data acquisition is realized with a PCL 718 card. Menu controlled data processing software has been developed which can obtain the friction coefficients μ, m and q and the material flow curve σ f = f( ε i) by processing the same experimental data. Compared with the ring-compression test, this new method can obtain friction coefficients μ, m, q continuously without interrupting the experiment, which would change the interfacial conditions uncontrollably. Compared with the Rastegaev test, the material flow-stress can be obtained up to a fairly large strain with good accuracy. The newly developed tribometer can be used to evaluate the tribological properties of lubricants, tool materials, tool surface finishes and tool coatings. It can also be developed for use in elevated temperature measurements. Experimental results have been analyzed which show that the friction coefficients in the models commonly used (Coulomb, Von Mises, Ramaekers-Kals [3]) are not constant during the forming process, the R-K model forecasting the deformation load best amongst the three models.