Efficient alloys

Materials World magazine
,
1 Nov 2008

Professor George Thompson, Principal Investigator for the LATEST portfolio at the School of Materials, The University of Manchester, UK, discusses the progress made in lightweighting alloys for transport applications and reducing carbon dioxide emissions.

The University of Manchester, UK, received a £5.93m research grant from the UK Engineering and Physical Sciences Research Council to address important issues limiting the wider application of light alloys in transport. The LATEST (Light Alloys Towards Environmentally Sustainable Transport) Portfolio Partnership is a flexible and adaptive research programme, responding to the changing needs of industry and academia. The initiative takes a multidisciplinary approach to developing solutions for environmental sustainability, integrating the core areas of microstructure/process control, joining/forming, and surface modification.

Increased light alloy usage is one way to cut fuel consumption and greenhouse gas emissions, with a one kilogramme drop in automotive vehicle mass approximating to a 20kg reduction in carbon dioxide output over a typical lifetime. Cost remains a barrier to using more light alloys in car bodies, with steel costing about one-third of the price of aluminium. Widespread use of full aluminium bodies is unrealistic, so developments in hybrid or multi-material solutions remain the future of lightweight design.

In the grain

Magnesium alloys, with a density of approximately two-thirds of aluminium and one-fifth of steel, and with competitive specific strength, are attractive for weight critical applications. Advances in alloy and coating technologies have greatly reduced corrosion susceptibility. However, several challenges limit use in wrought applications, including extrusions and sheets for the automotive body-in-white.

Magnesium has a hexagonal close packed structure and, at ambient temperature, deforms by slip on the basal (close packed) plane, suggesting it is too brittle for an engineering material. However, heating magnesium alloys to moderate temperatures can give excellent ductility, enabling forming by rolling, extrusion and forging. Large elongations (over 1,000%) are possible in certain alloys deformed under superplastic conditions. These properties result when additional deformation modes are active. Empirically, fine grain size, the addition of certain solute elements and a favourable distribution of grain orientations (texture) can enhance ductility. These give scope for alloy design and process optimisation. But improved understanding of alloy deformation is required.

Research at Manchester uses advanced analytical techniques, such as electron backscattered diffraction (EBSD) and in situ neutron diffraction, to enhance understanding of deformation. In situ neutron diffraction reveals the response of individual grain families during deformation, giving insight into how deformation is shared between differently-oriented grains. The EBSD provides fundamental knowledge of grain structure and texture development in magnesium alloys.

The research is providing scientific underpinning for the design of novel wrought-specific magnesium alloys with improved mechanical properties. Their increased use may save weight and improve efficiency.

Joined up thinking

Joining is crucial to increased application of light alloys in transport, with cost savings in manufacture and weight reduction. Complex body structure subassemblies can be produced in single-piece, high pressure magnesium die castings with large savings in part count and improved vibration damping dimensional accuracy and weight over traditional steel structures. These also allow additional joining operations to be eliminated.

The substituted light alloy structures need to be joined to themselves, other alloys and product forms, as well as other materials such as composites, laminates and plastics, to produce weight-efficient hybrid structures. This is often performed with fasteners requiring electrical insulation to prevent galvanic corrosion.

Welding of light alloys is difficult because of their sensitivity to solidification cracking, thermal damage and distortion. Solid state friction-based welding techniques provide solutions and dramatically reduce energy usage. Friction welding requires two metal surfaces coming into contact to form atomic bonds, which is achieved by pressure, heating to soften the material sufficiently to flow, and by destroying oxide films on the adherents. The joints form at relatively low temperatures with reduced thermal damage and distortion. Large local plastic strains at the join line give highly refined microstructures, providing superior properties to a fusion zone.

For automotive bodies, alternative technologies for electrical resistance spot welding are being explored, as this is difficult for aluminium panels due to high conductivity, low strength at high temperatures, and reactivity with electrode materials. Self-piercing rivets provide an effective joining method, with high consumable costs. Friction spot welding, a cheaper solution, is under development. Friction stir spot welding uses a high-speed rotating tool inserted into the seam between two lapped sheets. Acceptable shear strengths are obtained, with energy requirements one-tenth of resistance spot welding. This minimises surface preparation. Liquid glue layers that are squeezed out in the process cater for effective hybrid adhesive bonded-welded joints.

Ultrasonic friction welding is a further low energy process for joining with low thermal damage, and is being developed by Ford for automotive applications.

Modelling behaviour

Friction welding techniques are compatible with other forming operations since the welds have good ductility - microstructural refinement by friction stir welding improves cold formability and induces localised superplastic behaviour in aluminium sheet. Recently it has been used to successfully produce tailor blanks with superior formability to laser welded alternatives.

Friction welding allows the joining of dissimilar material combinations, with few problems. It is feasible to produce dissimilar metal joints between zinc-coated automotive steel grades and aluminium, and between magnesium and aluminium.

Research at Manchester is focused on understanding material interactions in friction welding, with a view to developing microstructure and property models so the processes can be optimised for specific applications, as well as predicting and controlling residual stresses. Additional studies are directed at the problems associated with joining dissimilar materials and friction stir processing, where advantage can be taken of the local microstructure refinement that occurs under ultra-high plastic strains.

Surface solutions

Advances in surface treatments of light alloys are producing alternatives to chromate-based formulations, which are restricted due to the toxicity of hexavalent chromium and waste disposal costs. Various methods are being researched, including anodising, conversion treatments, sol-gel processes and combinations. Several functionalities may be tailored for specific applications including adhesion, wear resistance, flexibility, hardness, wettability, and thermal barrier and corrosion-resistance properties.

An essential target is active corrosion protection, following damage that exposes the underlying alloy to the environment. Self-repair is a feature of chromate coatings, and is a main reason for their use. The body of the coating comprises trivalent chromium oxide, providing an excellent base for organic top-coats. Active corrosion protection is achieved by the reservoir of residual chromate ions which inhibit corrosion in failed regions.

Integrating such performance characteristics into new coatings requires an understanding of the relationships between alloy microstructure resulting from fabrication and joining, pre-treatments, and coating formation and performance. To this end, sol-gel coatings are being designed with balanced organic /inorganic constituents for compatibility with substrates and top-coats, and with nanoscale modulation for active corrosion protection. The coatings are produced by simple dip processes, employing environmentally-compliant baths, followed by curing.

Wide ranging processing routes enable coatings of diverse compositions, for instance those based on silica, alumina or zirconia, with thicknesses up to tens of microns. These usually only offer barrier type protection. However, inhibiting species can be incorporated to increase corrosion resistance.

Coatings are being formulated that contain nanoparticles, generated in situ or ex situ, and controlled additions of organic functional groups. These can have high resistance to cracking and are being assessed for active corrosion protection.

Further information: The School Materials, The University of Manchester