The initiation of roll coating buildup during thermomechanical processing of aluminum-magnesium alloys

The roll coating developed on an AISI 440C steel work roll during the laboratory hot rolling of an Al-Mg alloy was examined and its microstructure and composition were characterized. The AISI 440C steel work roll had a surface roughness (Ra) of 0.02 μm and the hot rolling schedule involved 20 passes under lubricated conditions. Initial examination of the roll coating generated on the work roll surface revealed it was patchy, discontinuous, streaked in the rolling direction and composed mainly of aluminum, magnesium, oxygen and carbon. Further analysis revealed that the roll coating possessed a complex layered microstructure. Under these rolling conditions, the roll coating microstructure comprised of an amorphous magnesium-rich oxide layer lying on an amorphous mixture of aluminum, magnesium, carbon and oxygen with amorphous iron and chromium-rich oxide particles embedded within it. Damage to the work roll surface and work roll debris observed within the roll coating highlighted that the work roll surface was involved in the roll coating formation. Analysis of the roll coating suggested that the initial roll coating composition and microstructure were influenced by the work roll and work piece material composition.