Underlying Bulk Alloy Research Articles

Forming Process during Electron Beam Surfi-Sculpt<sup>TM</sup> on Ti-6Al-4V Alloy

Electron beam Surfi-SculptTM is a novel surface treatment technique applied to produce high level performance Composite-Metal-Weld (ComeldTM) joints. Investigation on forming process during electron beam Surfi-SculptTM on Ti-6Al-4V alloy showed protrusions were formed via a layer-by-layer mode like additive manufacturing process. The near-surface region of electron beam Surfi-Sculpted Ti-6Al-4V alloy was occupied by fusion zone, heat-affected zone and base metal from the outermost surface to the underlying bulk alloy. The microstructure of fusion zone was characterized by a high density of fine acicular martensite phase, leading to a higher micro-hardness. A heat-affected zone was sandwiched between fusion zone and the underlying base metal, with different microstructural features compared to both fusion zone and the base metal.

Electron beam processed surface textures on titanium alloys for fluid-drag reduction

Ti-6Al-4V and Ti-4Al-1.5Mn alloys were employed for electron beam processing with the aim to produce surface textures efficient for fluid-drag reduction. The as-resulted surface textures were examined by using advanced optical microscopy for surface morphology analysis, optical microscopy and scanning electron microscopy for near-surface microstructure observation. The titanium model with electron beam processed surface textures was tested in wind tunnel to evaluate their fluid-drag reduction efficiency. It showed that the as-resulted non-smooth surface was characterized by parallel ridges and grooves, with height and spacing able to be customized by adjusting processing parametres. The ridges displayed continuous scales while the valley of grooves presented V-shaped ripples. Their dimensions were also related to and could be controlled by processing parametres. Further, the near-surface region was occupied by fusion zone, heat-affected zone and base metal from the outermost surface to the underlying bulk alloy. The microstructure of fusion zone was characterized by martensite phase. A heat-affected zone was sandwiched between fusion zone and the underlying base metal, with different microstructural features compared to both fusion zone and the base metal. With respect to fluid-drag reduction efficiency, titanium model with electron beam processed surface textures exhibits a reducing efficiency over 15% at attack angles of 0° and ±1°, with an air flow velocity at 24 m/s.

Near-Surface Microstructure on Twin-Roll Cast 8906 Aluminum Alloy

The microstructure of the near-surface region of twin-roll cast foil stock 8906 AlFeSi alloy has been investigated. It was revealed that a near-surface layer with microstructure differing from that of the underlying bulk alloy was generated due to severe surface shear strain during twin-roll casting. Scanning and transmission electron microscopy showed that the near-surface layer consists of nano-sized grains with diameters varying in the range of 50 to 600 nm. Energy dispersive X-ray spectroscopy revealed that grain boundaries within the near-surface layer were decorated by aluminum oxide particles and carbon-containing lubricant inclusions. Preferential precipitation of a high population density of cubic α-AlFeSi dispersoids, of 30 to 150 nm diameter, was observed within the near-surface layer. The near-surface layer was associated with shingles on the twin-roll cast alloy surface and the maximum layer thickness of approximately 6 µm was observed at the shingles. Other areas of the surface were either free of the near-surface layer or had a thinner near-surface layer.

Delamination of near-surface layer on cold rolled AlFeSi alloy during sheet forming

A near-surface layer existed on a cold rolled and annealed foil stock AlFeSi alloy due to severe shear strain during twin-roll casting and subsequent cold rolling. The near-surface layer, with a thickness up to ~700nm and consisting of ultrafine grains with diameters up to 100nm and high population of voids/microcracks, was present as randomly distributed patches on the alloy surface. The grain boundaries within the near-surface layer were decorated by oxide particles and inclusions of lubricant residue, which reduce the ductility of the near-surface layer. During sheet forming, further shear deformation occurred in the near-surface region due to the friction caused by the sliding contact between the alloy sheet and the forming tool. The differences in the microstructure and the ductility between the near-surface layer and the underlying bulk alloy resulted in cracking within the near-surface layer and/or at the near-surface layer/bulk alloy interface, finally leading to the delamination of patches of the near-surface layer. The delamination generated flake-like alloy debris may significantly modify the property of forming lubricant and influence the tendency to galling during sheet forming.

Evolution of Near-Surface Deformed Layers on AA3104 Aluminium Alloy

The present work studied the microstructure of near-surface deformed layers and their evolution from the transfer slab to the cold rolled final gauge sheet of an AA3104 aluminium alloy. Electron microscopy of ultramicrotomed cross-sections revealed two types of near-surface deformed layers, i.e. type A and type B, both with different microstructures to the underlying bulk alloy. A typical feature of the deformed layers is the nano-sized ultrafine grains, with diameters ≤ 200 nm for the type A and ≤ 500 nm for the type B deformed layer. Electron energy loss spectroscopy (EELS) indicated that oxide particles are present along grain boundaries within the type A deformed layer, while the type B deformed layer is free of oxide particles. The type A deformed layer is mainly generated at elevated temperatures during the early stages of hot rolling. Its thickness is non-uniform across the surface, with a maximum of ~4 µm on the transfer slab, ~1 µm on the re-roll gauge sheet and ~0.8 µm on the final gauge sheet. While the surface of the hot rolled sheet is mainly covered by the type A deformed layer, the surface undergone cold rolling is alternately covered by the type A and the type B deformed layers.

Mechanism of Localized Breakdown of 7000 Series Aluminum Alloys

The localized breakdown behavior of AA7055 alloy, and an Al-Zn-Mg alloy having no Cu, with different tempers was studied in 3.5 wt% NaCl solution by an in situ observation system during polarization. Three potentials at which the current increased rapidly were found for AA7055-T6 (peak-aged) samples. The first breakdown potential corresponded to the transient dissolution of an active surface layer formed during surface preparation by abrading with grinding paper. The second potential at which the current increased rapidly was associated with crevice corrosion and was not a true breakdown potential. The second breakdown potential was associated with pitting of the underlying bulk alloy. However, no surface layer attack and a single breakdown potential were observed for AA7055-T73 (over-aged) samples and for the Cu-free Al-Zn-Mg samples in the peak-aged condition. All three samples contained surface layers consisting of nano-grains with the solute-rich bands formed by surface preparation. Both the Zn-rich bands in the surface layer and the Cu content in matrix solid solution of the underlying bulk alloy are responsible for surface layer attack. The change in pitting potential with over-aged tempers could be rationalized based on the Cu content in the matrix.

Electrocatalysis of the Oxygen Reduction Reaction at Pt and Pt-Alloys

We have clarified for Pt nanoparticles supported on carbon black that the oxygen reduction reaction (ORR) activities are not affected by differences in the particle size. Enhanced activities in the ORR have been demonstrated at Pt alloyed with non-precious metals such as Fe, Co, and Ni. By use of multiple analysis methods, we found that the dissolution of the surface alloy layer was followed by a rearrangement of the remaining Pt layer, resulting in a Pt skin layer, which can protect the underlying bulk alloy. It was shown that a modified electronic structure at the Pt skin layer induced enhanced ORR activity due to an increased coverage of oxygen atoms. These results contribute to establishing a clear strategy for the design of highly active electrocatalysts for fuel cells.

Near-Surface Deformed Layers on Rolled Aluminum Alloys

Near-surface deformed layers, which are characterized by nano-sized fine grains, are generated in aluminum alloys by hot and cold rolling. During the rolling processes, the alloy surface and near-surface regions experience a high level of shear deformation that results in significant microstructure refinement, leading to formation of near-surface deformed layers with microstructures different from that of the underlying bulk alloy. Two types of near-surface deformed layers are observed. Type A is characterized by fine grains with grain boundaries decorated by oxide particles; type B is characterized also by fine grains but with the grain boundaries free of oxide particles. The high levels of shear deformation result in dynamic recrystallization. Together with mechanical alloying, this is responsible for the formation of the near-surface deformed layer. Furthermore, the structure in the near-surface deformed layer can survive the typical annealing process particularly if the grain boundaries are pinned by oxide particles.

Corrosion Control of Friction Stir Welded Aluminium Airframe Alloy

In the present study, microstructure and its influence on corrosion behaviour of friction stir welded aluminium airframe alloy have been investigated. Further, the effect of laser surface treatment on the microstructure and the corrosion behaviour was also assessed. The welded AA2024 alloy showed the expected zones associated with friction stir welding. Corrosion testing showed that modified microstructure reduced its corrosion resistance to localized corrosion, with the regions immediately outside TMAZ being most susceptible to corrosion. Laser treatment resulted in a melted near-surface layer, up to 5 (m thick, where normal constituent particles are absent. Scrutiny of the melted near-surface layer revealed continuous segregation bands, approximately 10 nm thick, containing mainly copper. Corrosion testing showed that laser treatment increases the short term resistance to localized corrosion due to the removal constituent particles. However, prolonged testing revealed corrosion within the melted near-surface layer and delamination of the melted near-surface layer from underlying bulk alloy. The corrosion is associated with copper rich segregation bands that promote local microgalvanic action.

Precipitation in an AA6111 aluminium alloy and cosmetic corrosion

The near-surface deformed layer on AA6111 automotive closure sheet alloy, generated by mechanical grinding during rectification, has an ultrafine grain microstructure, of 50–150 nm diameter, and a sharp transition with the underlying bulk alloy microstructure. Grinding and heat treatment to simulate rectification and paint baking processes result in the nucleation and growth of ∼20 nm diameter precipitates at grain boundaries within the near-surface deformed layer. High-resolution transmission electron microscopy has shown Q phase precipitates in the deformed layer, giving dramatically increased corrosion susceptibility compared with the bulk microstructure, and this is responsible for the rapid-onset filiform corrosion. Transmission electron microscopy of the corrosion attack showed directly that the mode of corrosion was intergranular and that the Q phase precipitates were preserved after the passage of the corrosion front.

The influence of surface features on bacterial colonization and subsequent substratum chemical changes of 316L stainless steel

Biofilm-forming bacteria were found to selectively colonize specific surface features of unpolished 316L stainless steel exposed to flowing aqueous media. Depending on the types of bacteria present, selective colonization resulted in significant depletion of Cr and Fe relative to Ni in the surface film at these features. No such depletion was observed on uncolonized surfaces exposed to sterile flowing aqueous medium. The results demonstrate that non-random, initial colonization of 316L stainless steel surfaces by these bacteria leads to changes in alloy elemental composition in the surface film that are enhanced with time. These chemical changes may be a critical step that weakens the oxide film at specific locations, allowing halides such as Cl − ions greater access to the underlying bulk alloy, and thereby facilitates localized attack and pit formation and propagation.

Use of ion beam analysis in metal modification by means of ion implantation

Ion implantation is being investigated as a technique for the beneficial modification of surface sensitive, life-limiting properties of metals, including resistance to corrosion, wear, and fatigue. Ion implantation can produce a graded alloy from the surface to the unchanged underlying bulk alloy so that both the surface alloy and bulk alloy can be independently optimized, and the technique promises the ability to produce unique metastable surface alloys. With typical implantation energies in the range of 10–200 keV, implantation depths are less a few hundred nanometers. These implantation depths are well suited for study by means of ion beam analysis techniques. This paper reviews examples of mechanical and corrosion property changes induced by ion implantation and describes in each case how ion beam surface layer analysis provided information useful to the present understanding of the effects observed.