In the present investigation, tip test based on backward extrusion was utilized to characterize the effect of surface roughness of the billet and forming tools and type of lubricants on friction. For the test, cylindrical specimens made of aluminum alloys of 6061-O and 2024-O with four lubricants such as VG32, VG100, corn oil, and grease were used. Single punch and two die sets with different surface topologies were manufactured in order to investigate the effect of surface conditions on friction and flow behavior. The load levels and tip distances were measured for both materials and compared with each other to determine shear friction factors at the punch and counter punch interfaces separately depending on the variation of surface topologies using the finite element simulations. As a result, a linear relationship among the dimensionless load, tip distance, and shear friction factors at punch and counter punch interfaces was derived for the experimental conditions investigated. The slope change of this linear relationship from negative to positive clearly depends on the variation of the surface conditions at the billet/punch and billet/counter punch interfaces. Also, it was clearly demonstrated that the dimensionless tip distance for the frictionless case can be extrapolated from the experimental data based on the simulation results. The value for the frictionless case can be used for characterizing the relative effect due to surface topologies at punch and counter punch and lubrication qualities of lubricants under various processing conditions.

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