On Parameters Affecting Metal Flow and Friction in the Double Cup Extrusion Test

Friction and lubrication in metal-forming processes are usually evaluated by a process test with a friction-sensitive divided flow like the ring-compression test. Parameters affecting metal flow are not only friction, but also strain hardening, tool geometry etc. The current friction models applied in numerical simulations often involve strain hardening because friction is expressed as a function of the flow stress or the flow shear stress. The double cup extrusion test is considered to be one of the process tests for determination of friction. In this test, varying heights of the upper and lower cups will develop with different friction and strain hardening conditions. In the present paper, an investigation of this test has been carried out experimentally and theoretically. Both annealed and work-hardened AA6082 specimens were extruded with varying degree of reduction and different lubricants. A friction model with constant absolute value excluding the influence of strain hardening on friction was adopted in FEM simulations, in order to more clearly study the individual influence of strain hardening and friction on the metal flow (upper and lower cup height ratio) without interference between the two parameters. The predicted results show a good agreement with the experimental data. The influence of material strain hardening. friction and tool geometry on the metal flow in the test has been investigated. By comparisons of the cup height ratio as a function of the punch travel estimated by experiments and FEM simulations, the friction has been evaluated. The parameters affecting the friction have also been discussed.