Microstructure Evolution of Friction Boundary Layer during Extrusion of AA 6060

The tribological behavior between the aluminium alloy AA 6060 and the hot working steel 1.2344 has been recently studied using a new axial friction test for extrusion processes. With this new test, the friction forces as well as the specimens’ plastic deformations generated during sliding tests can be investigated. Selected specimens tested at high temperature (300-500 °C), normalized normal stress (σn/kfo=1.5) and high relative speed (0.1, 50 mm/s) have been sectioned and the microstructure on their mid-planes has been investigated. Light optical microscope analysis of the friction boundary layer revealed highly stretched grains and a thickness variation of the shear layer from 750 to 1600 μm depending on testing conditions. Moreover, EBSD analysis showed a grain refinement in the high shear zone especially at 300 °C with a grain size about 4 μm. Dynamic recrystallization was observed at 400 °C and an abnormal grain growth at 500 °C. Hardness measurements revealed a light hardening effect of 8% at 300 °C as well as a softening effect of 6% at 400 °C and 500 °C.