The objective of this work is to explore the feasibility of a novel severe surface plastic deformation process to simultaneously improve the corrosion resistance of the substrate while imparting desired surface chemistry and morphology. This limited scope project has two aims: (1) to quantify the ways in which a gradient microstructure achieved through this method can enhance the corrosion resistance of a treated aluminum alloy substrate and (2) extend and develop this surface treatment to engender a surface alumina layer in single-step method without hazardous chemical processes.
Surface Mechanical Attrition Treatment (SMAT) is a relatively unexplored severe surface deformation process that produces a nanocrystalline/ultra-fine-grain layer of varying depth and gradient. Reduction in grain size can lead to enhanced corrosion resistance and changes in compressive stress can improve pitting potentials and mitigate stress corrosion cracking. Additionally, the SMAT process offers the opportunity for both a large range of microstructural control and the opportunity for supplementary surface alloying. The severe plastic deformation imparted to the substrate surface from the SMAT process that engenders grain refinement can also reduce existing intermetallic/precipitate size– reducing localized corrosion susceptibility – and increase solid solution solubility of alloying elements, thus raising the potential of the matrix and possibly changing the composition of the passive film to an improved protective layer.
Application of the SMAT process alone has significant potential to improve the corrosion resistance of the substrate material and provide new avenues for exploring fundamental relationships between microstructure and corrosion response; however, it also has the capability to alter the surface chemistry of the substrate in addition to the surface morphology and microstructure through a direct mechanical (non-chemical) method. The desired surface coating results from incorporating material in powder form to be mechanical mixed with the substrate through the SMAT process. Mechanical alloying, as a solid state mixing process, has the energy necessary to create nanostructured surfaces on both metals and ceramics and to mix nanocomposites. As an alternative to chemical anodizing, the research team hypothesized that SMAT of alumina powder stock onto an aluminum alloy (AA) or other alloy substrate can provide a robust, adherent, corrosion protection coating that avoids both the toxicity of traditional anodizing baths (e.g. chromate-based, strong acids) and the large amount of generated hazardous waste. As both the incorporation of new material (alumina) as a surface layer and the desired redistribution of precipitate phases (aluminum substrate) are not well captured in the initial SMAT acronym, we devise a new process name: Surface Mechanical Alloying for Specialized Heterogeneity, or SMASH.
Grain refinement of the substrate was achieved using steel media in both the AA2024 and AA5083 substrates. Additionally, the distribution and size of precipitate phases was altered; in AA2024 the precipitates were reduced in size but that did not have an observable influence on the polarization behavior. However, the grain refinement in AA2024 led to an increase in exfoliation corrosion. The effect of the SMASH treatment on corrosion in AA5083 was far more positive – the chemical redistribution caused by the plastic deformation lowered the Mg content in the matrix such that the deleterious β phase precipitates are not expected to form. After sensitization heat treatments, no β phase was observed near the SMASH treated surface.
When using SMASH to produce an alumina surface layer from alumina powder, layers of 20-100 µm thick were formed on aluminum alloy substrates under a variety of processing routes – both alumina and steel impact media, and a variety of process times. Alumina media was more successful at producing a dense coating without cracking in the substrate.
This limited scope project has shown the feasibility of the SMASH treatment to improve the corrosion response of AA5083; the mechanism for this improvement – reduction of Mg content into the depth of the substrate as a result of plastic deformation – is not one that has been observed before in other methods of severe plastic deformation applied to AA5083 and opens up new methods for improving sensitization behavior of Al-Mg alloys.
The SMASH process was successful in producing an alumina layer on aluminum alloy substrates through a mechanical mixing approach that uses only powder as input and discard (e.g. no acid baths as hazardous waste). However, the coatings have a range of particle distribution and are morphologically very different from traditional anodized coatings; significant further testing is necessary to determine how this would affect adhesion and wear.