The characterization of near‐surface defects evolved on aluminum–manganese alloys during hot rolling

Abstract

Hot rolling laboratory experiments were performed on an Al–Mn alloy, utilizing a rolling tribo‐simulator to characterize the evolution of near‐surface defects induced by tribological interactions between the work roll and alloy surfaces. A ten pass hot rolling schedule was conducted on Al–Mn samples using an AISI 52100 work roll polished to a surface roughness (Ra) of 0.01 μm. Micro‐cracks formed on the alloy surfaces at the first pass were observed to initiate at grain boundaries and propagate less than 1.5 μm deep into the subsurface region. The faces of the micro‐cracks in the subsurface region were observed to be covered with MgO. The surface of the samples were observed to possess a top 90‐nm‐thick oxide‐rich layer, which was mostly composed of MgO. MgAl2O4 and Al2O3 were also observed on the alloy surface at this stage of rolling. After ten passes, near‐surface features on the Al–Mn alloy samples included MgO‐rich islands and transverse cracks. Cracks extended more than 2 μm deep into the subsurface. Crack faces were covered with surface deformation induced crevices as well as MgO. The near‐surface was also composed of nano‐crack induced damaged areas, filled with porous nanocrystalline oxides. These damaged areas extended 0.5 μm into the subsurface region beneath a 60‐nm‐thick MgO film. The nanocrystalline oxide layers possessed embedded aluminum nano‐particles. The microstructure of the near‐surface layer suggests that magnesium diffusion to the free surfaces and crack formation play an important role in its evolution. Copyright © 2015 John Wiley & Sons, Ltd.