Experimental study of friction in sheet metal forming

During deformation of the sheet metal over a tool, contact occurs only at the peak asperities of both surfaces. In the contact areas the processed material flows over the toolʹs surface, therefore all the models used to study forming processes must include a way to take into account the contact with friction phenomena. More widely used friction models are based in the Amontons-Coulomb theories. Unfortunately experience shows that for most applications the available models cannot accurately describe the friction phenomena. The determination of the friction coefficient in a sheet metal forming process is a complex procedure, because many variables influence the friction mechanisms. The aim of this research work is to apply an experimental approach in order to bridge simple benchmark friction experiments with real sheet forming applications. Two different techniques were used to assess friction, namely unidirectional crossed cylinders sliding with linear increase of the load and an equipment which allows measuring the friction coefficient under stretch-forming conditions in a sheet metal forming process. The tested materials are a cold-rolled advanced high-strength steel, DP600, and an aluminium 1100 alloy against heat-treated AISI D3 steel. The test protocols were established to allow the study of several effects: sliding speed, the surface roughness, the lubricant effect, the load and the running-in effect. The differences between the two techniques are widely discussed and laser profilometry and scanning electron microscopy are used to help understand the prevalent friction mechanisms. The present study allows concluding that: the friction results obtained by a load-scanning test are always higher than values assessed by a draw-bead test; roughness of the die material plays an important role on the friction coefficient; a significant reduction of friction was attained in multi-pass load-scanning tests due to running-in effect.