Some tribological phenomena concerning the friction behavior of metallic or non-metallic materials at lower values of sliding speeds are essentially different from those corresponding to high speeds. Based on previous experimental and numerical studies developed in the GCGM laboratory of INSA Rennes concerning bulk and surface anisotropy of metallic thick plates together with their mechanical behavior evolution under important thermo-mechanical loading gradients, this scientific paper proposes to study the influence of a rolled thick sheet aluminum anisotropy on the stick-slip phenomenon concerning a range of low and very low sliding speeds (0.2-200 mm/min). It is known that the stick-slip phenomenon occurs if the static friction coefficient has a larger value as compared to the kinetic one, especially in the case of friction couplers with a dry or a limited friction regime (when the sliding speed is in the range of 0.01-3 mm/s or when the angular speed is somewhere in the range of 1-25 rad/s). For this study it was used an UMT Micro-Scratch Equipment of the Machine Elements and Tribology Department (OMTR)- University Politehnica of Bucharest (UPB). The equipment can provide rotational, translational or reciprocating motions with speeds starting from 0.1μm/s up to 10m/s. A constant or a progressive normal load between 0.05 N and 1000 N can be applied on the material sample.

Experimental friction tests were made on an anisotropic rolled aluminum alloy AA2024-T351 using an ultra-high-molecular-weight polyethylene (UHMWPE) cylindrical pion. The AA2024-T351 is one of the most popular high-strength aluminum alloys, having a low corrosion resistance and good mechanical properties at high temperatures: ultimate tensile strength > 420 N/mm2, yield strength at 0.2% > 260 N/mm2, HD harshness around 120 Kgf/mm2, LF 108 cycles of 125 MPa, Young modulus 71000-74000 MPa and Poisson coefficient of 0.33. The ultra-high-molecular-weight polyethylene is characterized by an excellent resistance to abrasion and its main mechanical properties are defined by a ultimate tensile strength around 231.1 MPa, a yield strength of 25.6±3.3 MPa, a Young modulus 915±42.3 MPa and a Poisson coefficient of 0.46. All the tests have been performed by moving the polyethylene pion along a specified trajectory under a constant normal force and at a uniform sliding speed. In order to analyze the influence of the aluminum alloy anisotropy, the tests were made along the three directions: a longitudinal one, corresponding to the rolling direction of the sample (0°), a transverse one, which represents the direction perpendicular to the rolling direction (90°), and a median direction (45°). Three different normal forces (3N, 5N, 7N) have been applied corresponding to three different sliding speeds (0.005 mm/s, 0.05 mm/s, 0.5 mm/s) along a specified trajectory of 3 mm length. It was observed that for a speed of 0.005 mm/s the friction coefficient decreases if the force increases for all the three principal anisotropic directions. When the sliding speed is increased up to 0.5 mm/s, the friction coefficient remains constant. It can be seen that the friction coefficient has different values along the three loading directions (longitudinal, transversal or median): the highest friction coefficient occurs for the direction perpendicular to the rolling direction and the lowest for the rolling direction. However concerning the median direction, the corresponding friction coefficient has little change with the sliding speeds or the normal forces. Specific theoretical and numerical analysis will be performed concerning the bulk and surface anisotropy of studied aluminum plate, especially regarding the influence on the tribologic properties and corresponding friction model formulations.