Martin Aggerbeck from DTU mechanical Engineering defends his PhD, "Tailored Aluminium based Coatings for Optical Appearance and Corrosion Resistance". The defence takes place Friday the 21th March at 13:00, in Auditorium S09, Building 101, DTU.
The current project investigated the possibility of designing aluminium based coatings focusing on the effect of composition and surface finish on the optical appearance and on the alkaline corrosion properties using titanium as the main alloying element. The main results and discussions of this work are presented in manuscript form as four appended papers communicated for journal publication.
Aluminium is widely used in applications such as transportation, building, heat exchangers, packaging, and design products. Optical appearance after anodisation is a key aspect for many of these applications, but the use of recycled aluminium compromises this due to the presence of increased levels of impurity and alloying elements. Knowledge on how different alloying elements affect the optical appearance might therefore increase the applicability of recycled aluminium.
It was investigated how the optical appearance is affected by the alloy composition, surface morphology, and the microstructure. Four commercial aluminium alloys were studied before and after polishing, etching, anodisation, and hot water sealing, giving an overview on how the alloy composition affects the appearance. It was found that the roughness after etching increases with higher amounts of alloying elements (especially iron and silicon). Proper polishing requires some alloy hardness, while alloy purity is required for a glossy appearance after anodisation.
Magnetron sputtered aluminium based coatings containing up to 18 wt. % titanium were deposited, heat-treated, and anodised. The microstructure of the as-deposited coatings was layered, and Al3Ti phases formed during the heat treatment. During anodisation, the heat-treated specimens containing Al3Ti phases turned dark, and the specimens were investigated as a model system on the optical effect of partially oxidised intermetallics and the subsurface morphology after anodisation. It was suggested, that the darkening of the anodised specimens happened due to roughness of the oxide-substrate interface causing light trapping and optical scattering and absorption by the partially anodised intermetallics in the anodised layer.
The transport industry has a big share of the CO2 emission in the world, which can be decreased by reducing the weight of the vehicles itself. More than half of the weight of a car comes from steel, which can almost be reduced to half by replacing steel with aluminium. Unfortunately, aluminium corrodes heavily in the alkaline environments known e.g. from a brush less car wash (> pH 12). Today nickel salt sealing is used to protect e.g. aluminium wheel rims, but an alternative is needed due to environmental and health reasons.
Investigations using the previously described magnetron sputtered Al-Ti coatings showed that 13 wt. % titanium and more improved the corrosion resistance at pH 13.5 and this was further improved by heat treatment, especially at 400 °C and more. The improved corrosion properties were ascribed to structural relaxation, decreased galvanic potential differences in the microstructure, and protection from the network of the Al3Ti phases precipitated during the heat treatment.
Laser surface cladding of aluminium containing up to 20 wt. % Ti6Al4V were studied focusing on the microstructure and the alkaline corrosion properties. Due to precipitation of large Al3Ti phases during the cladding process, the microstructure consisted of an almost pure aluminium matrix, which corroded preferentially when exposed to pH 13.5. Additional heat treatment did not break down the solidified microstructure and the corrosion properties were not improved.