The tribological behaviour of AA5083 alloy plastically deformed at warm forming temperatures

The plastic deformation behaviour and deformation induced microstructures of the tribolayers formed on the surface of aluminium AA5083 alloy sheets subjected to tensile strain were studied while measuring the variation of their coefficients of friction as a function of temperature. A strain rate of 4×10−2 s−1 was applied at temperatures ranging between 25 °C and 450 °C to simulate the deformation conditions encountered during warm and hot forming of this alloy. Surfaces of the AA5083 samples tested at 150 °C were covered with tribolayers formed during the preceding rolling process and showed no significant damage but increasingly significant damage occurred at higher temperatures due to the softening of the alloy and the increasing ductility of the tribolayers. The coefficient of friction was observed to increase with the temperature. The surface roughness showed two plateaus: one with low roughness at temperatures less than 300 °C and the other above 300 °C, attributed to the roughening due to grain boundary sliding of near surface grains of AA5083. Fibrous structures between the edges of cracks on fractured tribolayers were observed at temperatures of 250 °C and higher. These fibrous structures, composed of aluminium and magnesium oxides, were connected by rib-like membranes. At high temperatures, the fibres reached lengths as long as 3.13 μm and had diameters less than 200 nm, as such they revealed local superplastic behaviour. The appearance of these fibrous structures coincided with the temperatures where surface damage and aluminium adhesion to the counterface were observed.