Friction during forward and backward extrusion focusing on the lubrication regime change

An experimental and numerical work on tribological phenomena focusing on lubrication regime change (LRC) during forward and backward extrusion (F&BEx) of a carbon steel, S20C and an aluminum alloy, A6061 alloy is comprehensively conducted. The specific F&BEx process is purposely designed to extremize its sensitivity to the tribology. To decouple the friction from the flow behavior of the material, accurate flow curves of the materials are obtained using the combined flow characterization approach of the implicit elastoplastic finite element method (FEM) and tensile test. The friction coefficient function of the material's effective strain at the material-die interface is described by the piecewise linear functions defined by unknown friction coefficients at the junctions, which are optimally determined. It is revealed by comparing the experiments and finite element (FE) predictions of the backward and forward heights of the F&BEx process that any traditional constant friction coefficient and friction factor cannot supply the acceptable FE predictions. However, the optimized friction coefficient function, exhibiting the extreme change in friction coefficient with respect to the surface effective strain of the test material, yields the acceptable FE predictions for both S20C and A6061 alloy in terms of both forming load and the ratio of upper (backward extrusion) height to the lower (forward extrusion) height. It is also emphasized that effect of the friction on the macroscopic responses depends much on the material’s strain-hardening.