Elevated temperature adhesion and junction growth mechanisms in Al–Mg alloys sliding in argon and air

Abstract

During hot forming of Al sheets, sliding contact against the forming die causes material transfer and adhesion to die surfaces. This work uses in-situ microscopy methods during sliding experiments conducted inside an environmentally controlled high temperature pin-on-disk tribometer to investigate both formation of junctions during initial contact and growth of adhesively transferred material. For this purpose, AA5083 (Al-0.1 wt%Cu-4.8 wt%Mg) pins placed in contact with a flat quartz glass counterface mounted on to the motorized stage of an optical microscope were tested at 573 K and 673 K in argon and ambient air atmospheres. The instantaneous changes in the contact area size and morphology were recorded along with the tangential force. Evidence for plastic deformation during junction formation and growth was provided by the formation of slip lines and grain boundary sliding. Junction growth occurred continuously in argon without separation of the adhesive junctions moving into the steady state friction stage, whereas in air, initial junction growth was interrupted due to oxidation, giving rise to a stick-and-slip behaviour, and the tangential force at the onset of junction break was determined as 1.75 ± 0.15 N at 573 K and 1.00 ± 0.08 N at 673 K. The junction growth rate in the absence of oxide formation followed an Arrhenius-type relationship in the temperature range investigated with an activation energy in the range of 144 ± 2 kJ mol-1, implying that the plastic deformation (creep) during junction growth was controlled by diffusional flow.