Origins and Evolution of Near-Surface Microstructures and their Influence on the Optical Property of AA3104 Aluminium Alloy

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Abstract

The microstructures of the near-surface layers on AA3104 aluminium alloys and their evolution through rolling and deep drawing processes have been investigated. The effect of the near-surface layers on the optical property of AA3104 aluminium alloy has also been assessed. It was revealed that two types of near-surface deformed layers, both with different microstructures different from the underlying bulk alloy, were generated on the surface of AA3104 aluminium alloy during rolling. Both of them are characterized by ultrafine, equiaxed grains, with diameters <100 nm for type A near-surface deformed layer and <200 nm for type B near-surface deformed layer. A high population density of nano-sized, oxygen-rich particles is present along grain boundaries within type A deformed layer. But type B deformed layer is free of oxygen-rich particles. Type A deformed layer was generated through two mechanisms, i.e. geometric dynamic recrystallization and mechanical alloying. Rolling introduced plastic strain in the surface/near-surface region of aluminium sheet was of sufficient magnitude to cause geometric dynamic recrystallization and thus microstructure refinement. In addition, the incorporation of oxides into the near-surface region was also involved in the formation of type A deformed layer. However, the formation of type B deformed layer was only attributed to severe strain induced geometric dynamic recrystallization. Type A deformed layer was mainly formed at the early stages of hot rolling. The subsequent rolling and deep drawing reduced the thickness of type A deformed layer by distributing it over a larger surface area. During cold rolling, type A deformed layer broke into patches with the extension of alloy surface. Type B deformed layer may be generated on the nascent surface if the strain is sufficiently severe to cause geometric dynamic recrystallization. For the hot rolled alloy sheets, the surface/near-surface region is mainly covered by type A deformed layer. However, for the alloys after cold rolling, only limited area is covered with type A deformed layer. The thicknesses of the near-surface deformed layers are not uniform across the surface of AA3104 aluminium alloy. The maximum thickness of type A deformed layer on transfer slab is approximately 4 µm, while that on re-roll gauge sheet is ~1 µm, and ~0.8 µm on the final gauge sheet, ~400 nm on formed cup and ~100 nm on formed can. Type A deformed layer is randomly distributed as patches on the cold rolled aluminium sheet. The reflectivity of oxygen-rich particles is lower compared with the reflectivity of aluminium. As a result, the type A deformed layer patches absorb more incident light than the area without type A deformed layer. Further, there are plenty of micro-scale mini-cracks present on type A deformed layer, their opening sizes are in the equivalent scale of the wavelength of visible light. The incident light may not able to be reflected out if they go into these mini-cracks. It is more prone to happen for short wavelength light since it is easier for them to go into the mini-cracks than long wavelength light. As a result, less short wavelength visible light is reflected from the type A deformed layer patches. Thus, such patches exhibit a yellow appearance while the surrounding area appears the original silver white aluminium appearance. The aluminium sheet with a high coverage of type A deformed layer exhibits a low total reflectance. Further, its total reflectance is with a significant “red shift”. Neither the macro-scale surface roughness nor the ultrafine grain size affects the total reflectance of aluminium alloys. The total reflectance of aluminium alloys is primarily dependent on the presence of type A deformed layer.

Keyword(s)

Aluminium alloy; Appearance; Deformed layer; Optical property; Reflectance; Rolling; Ultrafine grain

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