Aluminium fights back!

Dr Neil Calder discovers that while titanium and carbon composites still vie for dominance in the manufacture of sustainable aerostructures, aluminium remains well positioned to produce components like wing ribs with realistic weight/cost trade-offs.
There have always been tensions between the various material camps in aerostructure design since the supply and expertise in production and processing of material types has generally come from completely different places, and has been the product of different organisation and individuals.
Long ago, towards the end of the 1920s, spruce was displaced in favour of duralumin. After the Second World War, titanium displaced some uses of aluminium and medium strength steels and more recently carbon composites have disrupted the balance yet again. Apart from the possibility of resurgence in the use of natural wood as the ultimate in sustainable manufacturing of aerostructures, this leaves the three major material categories of aluminium, titanium and carbon still trying to achieve an optimum balance.
In responding to the worldwide prevalence of carbon fibre composites, the aluminium industry has not been standing still. As a global supplier of aluminium material and technology and Europe's major player in this field, Alcan has been visibly active through its research centre in Voreppe, France.
Recently designed civil aerostructures for the B787 and A350 have highlighted the interactions between the various material types and the differing ways in which the eternal engineering compromises are resolved. Carbon fibre composite components in fuselage and wing structures of these aircraft have been designed to interface with titanium parts where possible. Alcan's development personnel cite a shortfall in development time for the technology portfolios of these aircraft as the reason for the extensive use of titanium and carbon fibre in inner structures as there are less compatibility issues with this material pairing, particularly regarding corrosion and thermal expansion. Aluminium, however, still remains well positioned to produce affordable aerostructures with its realistic weight/cost trade-off and wing ribs have persistently been designed in this material. Lithium lightens the way
A high potential approach for alloy performance improvements has been in the optimisation of Al-Cu-Li-(Mg-Ag-Zn) alloys where every weight percent of lithium in the alloy results in a 4% reduction in density of the metal. This third generation of lithium bearing alloys has been developed as damage tolerant variants of military and space metals in order to meet the demands of future commercial airframes. AA2198 and AA2050 are typical of these damage tolerance alloys. Material performance improvements are only part of the potential developments of metallic solutions for airframes however, with gains of similar magnitude in component weight and cost achievable by applying new technologies and new design solutions to metallic structures.
In a classic three-way relationship, this eternal triangle of advanced engineering, between material, process and product design has provided the inspiration for Alcan's actions. It is impossible to take action in one of these factors without the others being affected, or to optimise one in isolation. Nowhere is this more evident than at the interface between aluminium and carbon in hybrid composite/metallic structures.
The electrochemistry of the aluminium/carbon pairing counts against it in hybrid structures: the difference in electronegativity between these two elements is sufficient to create a battery or, in the presence of an electrolyte, accelerated galvanic corrosion. Perhaps counter-intuitively, the addition of lithium as an alloying element, itself well known as an electrochemically active material, provides lower corrosion potential through the formation of nanoscale dendritic microstructures. Because of these specific nanoparticulates, corrosion is mainly due to pitting and is intergranular in nature. When considering al-li alloys, Alcan is creating dendrites within the microstructure that are actually nanomaterials, and by looking at these alloys in a wider context than just corrosion it has been possible to alter performance at two levels, at macro and nano scales.
In tests with these new alloys, some 30% improvement in corrosion potential is seen compared with standard 7000 series alloys. This eases an already tight situation in joint design but does not eliminate the corrosion problem for designers and aircraft operators. Three pillars of development
In addition to alloy development, Alcan has identified three other pillars for the development of its ability to satisfy customer demands: integrated solutions, sustainability and smart materials.
In addressing integrated solutions, it has simultaneously tackled alloy and design and joining development with the aim of making aluminium live in a hybrid environment. Alcan has been investing a lot of effort in this field, characterised by the establishment last year of a research chair in multi-materials and interfaces at the Ecole Polytechnique Fédérale de Lausanne. This is significant, as there are now two professors working on this issue of the interface between materials.
Regarding sustainability, Alcan's model is to offer a closed loop solution to the customer by designing metals and alloys that can be recycled. Here is where aluminium has a specific advantage over carbon fibre in its inherent recyclability. This means actions alongside the value chain as well as disposal at end of life, and is happening partly as a result of the EU funded PAMELA project.
Last but not least, Alcan is active in the field of smart materials investigating technologies to enable enhanced and specific properties beyond the ‘lightweighting' that is already achievable. They have developed cost-effective, functionally graded materials so that close to the wing box you can have a totally different behaviour than on top of the wing.
Alcan has patented a process to insert a sensor fibre as an active part of a plate or extruded part, providing the possibility of detection alongside the structural properties of the material and is working with universities to see where this innovation can be taken. Structural health monitoring is an expanding field in aerostructures, but for aluminium components it is not as necessary as the material is well understood and it is well known how cracks propagate. There is, however, the possibility to extract other information from structures such as local stresses or temperatures and this creates the possibility of reacting to this information to manage effects like aeroelasticity.
With all this activity going on in developing technical and business solutions, it is unlikely that any future aircraft will fly without a significant fraction of aluminium on board. www.alcanaerospace.com