Lithium cures the woes of aluminium producers

It has been clear that polymer composites are attracting most of the limelight in innovation for lightweight structural solutions; however as Dr Neil Calder explains, the field of lightweight metallics is by no means static.  Because the value chains for creating and processing components constructed from these two material types is so different, both in their participants and what they are doing, and possibly because it is easy to pull out figures from an airframe's design such as mass % by material type (Al, carbon, Ti), this provides a naturally adversarial position amongst those with vested interests in the disparate camps. In reality, aircraft parts are constructed from whichever material is most suitable, given the sometimes competing parameters of specific stiffness, specific strength, damage tolerance, initial and lifetime cost, corrosion potential etc. Airbus certainly makes a big play of their material agnosticism. The biggest thing to hit the world of aluminium in recent times has been the advent of a new group of lithium bearing wrought aluminium alloys that are commercially and technically viable. Since the introduction of this current breed of aluminium lithium alloys, commonly referred to as its ‘third generation', these have become the preferred choice for any new airframe designs, both military and civil. The exploitation of Al-Li alloys could reasonably be said to have started with the use of alloys like the Soviet Union's 01420 in the MiG21 in the 1970s. The next wave of development produced the ill-fated AA8090 which was scheduled for wide scale Eurofighter production until insurmountable problems with short transverse properties emerged. These early alloys typically contain between 2-3% lithium, whilst the current generation manages to achieve major property improvements with around half that. As the lightest solid metal element, third in the periodic table, lithium displaces aluminium atoms in solid solution up to a solubility limit of 4%. Every 1% addition of Li as an alloying constituent means a 3% strength increase and 6% stiffness increase coupled with a decrease in density of 3%. In reality, with the composition of AA2050, AA2198 etc. we see performance benefits compared with the incumbent AA7050-T74 or AA2024-T351 materials, in plate and sheet respectively, of strength increases up to 44%, fatigue resistance up to 25% and corrosion resistance up to 46%. These property advantages all add up to give a solution for many structural applications that can significantly outperform the conventional 2000 and 7000 series alloys. Airframers who have been averse to taking the technical and business risk to move to entirely carbon fibre structures have found this an absolute boon. More from the ore As a naturally abundant material, and relatively easy to extract from its ores, the raw material costs for aluminium alloys have been significantly more stable than for many other engineering material commodities. There is the potential of a future resource issue with lithium supply, however, through competition for the raw material with auto battery producers. Raw material producers predict that battery use of lithium will swamp alloying needs by 2020 with four times as much required by then for batteries as for alloying. There is current global overcapacity in lithium production caused by the recent economic downturn but the auto market for lithium batteries is also widely predicted to explode over the next decade. The last year has seen some major announcements on strategic supply chain relationships with programmes and airframers. Both Constellium and Alcoa have signed multi-year deals with Airbus, totalling $2 billion and $1.4 billion respectively. This material is scattered throughout Airbus' significant current programmes, with the Al-Li alloys from both producers used extensively on the A350 and A380 airframes. Long-term relationships like this are conducive to the joined up thinking evident in the Constellium Airware total material recycling programme. Some distinct pairings between aluminium supplier and airframer have emerged, notably Embraer with Alcoa, although on more global programmes such as the A350 and CSeries the preference seems to be for multiple material suppliers. The extensive use of aluminium lithium alloys is particularly evident in the upcoming regional aircraft programmes. These alloys have been cited for fuselage and wing application on the ARJ21, MJ21, MRJ and CSeries. Predictions of 20-25% weight reduction on comparative structures from more conventional 2000 and 7000 series aluminium alloys have been made once all the mechanical performance parameters of strength, stiffness, damage tolerance are summed in a structural design. Alcoa engineers believe that the initial performance advantages of CFRP are eroded significantly in real applications. CFRP starts off with a 45% density advantage for a comparable volume of raw material. Twenty five per cent of that advantage is lost because CFRP needs to be applied in multiple layers to balance the structural load equally. Ten per cent of the remaining advantage is lost due to the addition of other materials and components to provide functionality like lightning strike protection, fasteners and electrical bus bars needed in aircraft. The last 10% is lost to the addition of other materials for strength e.g. Ti fittings and to the inability to taper CFRP around cut-out features to the same extent as aluminium.  The triangular relationship between material, process and product is never more evident than in the rhetoric from the key material producers. The evidence is that these basic building block players within the supply chain have been listening to their customers and coming up with technical and business solutions that bring aluminium very much back into the game. The best bar none? The Constellium Airware concept is finding real traction now within the aerospace manufacturing sector. The Airware product range from Constellium has been configured around three product groupings: I-Gauge plate, typically for wing ribs and nose landing gear, and has been selected by Airbus for the A350; I-Form sheet aimed at fuselage, nose and tail components and  selected by Bombardier for CSeries fuselage barrel components to be manufactured in Shenyang; and I-Core extrusions which meet the requirements of hybrid structural environments and with good damage tolerance,  ideal for components like floor beams. The Airware customer focus goes further though, in closing the loop with recycling and reuse of material throughout the manufacturing supply chain and at end of life. This leads to more of a leasing of material than an outright purchase as the material supplier is involved at all the stages where scrap or recovered material can be added back into the primary production process.  The most significant observation which can be made of this sector is that through materials engineering and a re-energised customer focus the use of light alloys in airframes is holding its ground and there may even be a future reversal of the trend towards predominantly CFRP aerostructures. neil@engineeredcapabilities.co.uk