Metals shed weight to compete

Metals shed weight to compete by George Marsh

If 'lighter, stronger, stiffer' is an applica- tion wish list that has driven the development of polymer-based composite materials, it has also prompted signifi-

cant deve lopment in performance metals of reduced density. So much so that metals, in new guises and forms, have come back into content ion for a range of transport applications where reinforced plastics once seemed to be the future.

Take for instance tomorrow's automo- biles which, to satisfy environmental imperatives, will have to burn less fuel

and emit greenhouse gases less copi- ously. For this they will have, for a start, to be lighter. Here composites and metals come head-to-head. While a team led by Volkswagen seeks, unde r Europe's Tecabs research project to design a carbonfiber automobile able to travel 100 km on a litre of fuel, automakers have been looking again at metals, notably aluminum.They accept

that soon it will no longer be justifiable to move perhaps a ton of steel around with every driver so aluminum - with a density less than haft that of steel - is seen as a possible alternative. This metal can be rolled and formed using processes familiar to steel body makers, can be welded (al though

continuously in seams rather than in spots) and can be produced with a Class A finish. A stiffness disadvantage with a luminum can be overcome by adopting a spaceframe-based construc- tion rather than a unibody as with steel. The lighter metal lends itself to this as, unlike steel, it is easily extruded into tube. Ford and Audi have produced, experimentally, certain of their models in aluminum, though without following through on consequential benefits such as lighter engines, drives and suspensions.

Unfortunately, a luminum is more cost- ly. Analyses by the Materials System Laboratory at the Massachusetts Institute of Technology (MIT) in the USA have suggested that, assuming a product ion rtm of 80 000 or so, a vehicle with an a luminum body would cost $500 to $1000 more than its equiva- lent in steel. (However, the differential is about half that for a carbonfiber

body for which the comparative eco- nomics typically become more unfavourable as product ion volume rises.) Low-weight steels have not been ruled out since steel companies have suggested that new alloys two or three times as strong as conventional carbon steel could yield bodies

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weighing up to 20% less than conventional equivalents, though they would cost an estimated 15% more. Some or all of the purchase price difference can be recovered through the fuel savings offered over the vehicle's service life, though environmental acceptability is presumably worth some premium.

Aluminum has received a further boost from alternatives to the space frame approach. Sandwich structures can, for instance, be used to confer stiffness. Nomex (aramid) and a luminum honey-

comb are used in aerospace, bu t automakers might prefer an emerging and highly compatible sandwich 'fill- ing' be tween alloy faces - foamed aluminum. It has proved possible to foam this, along with several other non-fer- rous metals, quite satisfactorily. At a recent conference, speakers from Fraunhofer USA (Delaware Center) and Frantisek Simancik ()4on-Ferrum of America LP) reviewed preparation of parts from moulded aluminum foam and aluminum foam sandwich (AFS) as well as foam-filled tubular products.

Advantages claimed include high strength and stiffness-to-weight ratios, sound absorption, fire resistance (espe- cially compared with plastics) and recyclability. Moreover, foamed metals - also called cellular metallic materials - can, upon impact, transform kinetic energy into deformation energy. This gives them significant advantage w h e n vehicles crash.

Now becoming established, metal foams are making it into design guides, such as one published recently by the UK's National Physical Laboratory. They are also progressing into 'live' applicat ions. Karmann GmbH, in another collaboration with Fratmhofer over AFS panels, achieved an eight-fold rise in stiffness and halved the weight w h e n it replaced a convent iona l stamped steel firewall in a vehicle body. Aluminum foam parts are also unde r deve lopment for other

Materials Today 25

Part of the load is 'bridged'

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Crack bridging in Fiber Metal Lamainates.

automotive frame components , though as yet there is no high-volume produc-

tion.

An alternative to foaming is syntactic metals, p roduced by incorpora t ing separately produced microspheres. A project subsidized by the German Research Society, for example, has yielded a syntactic magnesium which combines strength with a smooth pore structure and high energy absorption capability. A cellular magnesium produced by casting thin-walled hollow ceramic spheres into the metal proved

on lower fuselage r I

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Airbus A380 major potential technologies.

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Affordabil#y of metal composites - most cost effective fabrication route.

to have mechanical properties superi- or to those of aluminum foam. Work is

con t inu ing at the Universi ty of Nurnberg-Erlangen in Germany to opti- mize this material. Magnesium is wide- ly used in aerospace for its low density even though, because it is highly reac- tive, it requires an unbroken anti-cor-

rosion coating.

Sea and air At sea and in the air where sheet aluminum is a well-established weight saver, the material is facing the plastic composites challenge with a range of improved alloys. Marine grade alloys remain a popular choice for hulls and superstructures of fast commercial and

naval vessels where extra cost is justi- fied to secure low weight. One advantage is that when the surface oxidizes, a protective aluminum-oxide layer is formed to the extent that further protect ion is hardly necessary, though paint is usually added for cosmetic rea-

sons. Other alloys, with copper, zinc and magnesium being the usual alloying elements, are baseline structural materials for today's aircraft. Researchers, who have doubled the tensile yield strength of aerospace aluminum since the Junkers F13 first applied the material 80 years ago, con- t inue to develop new and improved

alloys.

In the airborne sector particularly, con- stant quest for weight reduction has spawned significant innova t ion in materials, both composite and metallic. Since UK researchers discovered that alloying aluminum with the even lower-density lithium would produce a usable low-weight material, aluminum- lithium has been developed for use in military aircraft, on the later Airbus

range and, as now seems likely, on Europe's new ultra-large passenger jet,

the A380. For every 1% of lithium added to a luminum there is a 3% increase in stiffness-to-density ratio. Current AI:Li alloys contain up to some 2.5% by weight of lithium, plus smaller percentages of copper and magnesium, resulting in densities up to 10% less than those of conventional 7000 series alloys. The downside of this material - apart from its cost - is that

26 Materials Today

fabricators have to handle it carefully

since any contact of the molten material with moisture can be explosive, and it is prone to surface oxidation during heat treatment.

Like convent ional aerospace alloys, Al:Li alloys attract the attention of researchers who would like to enhance them. For instance, GKN- Westland, noting shortcomings in the AA8090 alloy that has now been avail- able for some 12 years, has been working - with academic assistance - to develop a new alloy offering a similar density along with improved strength, fracture toughness, stress corrosion resistance and freedom from need for post-solution heat treatment.

In some applications, metals and fiber reinforced plastics have become part- ners.The Netherlands has been partic-

ularly influential in developing fiber metal laminates (FMLs), a notable collaboration be tween Fokker and the Technical University of Delft having delivered ARALL - an aramid-aluminum

hybrid in which alternate plies of sheet a luminum alloy and Kevlar composite are bonded together. A succes- sor, called GLARE - in which the rein- forcing fiber is glass rather than aramid - looks set to make a breakthrough for FMLs into major primary aerostruc- tures, via the upper fuselage of A380. Airbus Industrie is known to favor GLARE and, following complet ion of a

Dutch-led European programme to qualify the material, is likely to select it in a quest to make A380 up to 20% more efficient than its likely competition.

The material is 15% less dense than a luminum and offers fatigue and crack growth properties orders better than those of the monolithic metal. Fibers in

the plastic tend to bridge any incipient cracks in the alloy, arresting their development , while the outer aluminum layers protect the embedded organic composite from impact dam- age and moisture ingress.Tensile properties of the material are, again, greatly super ior to those of a luminum. Moreover, in many ways FMLs can be treated like aluminum sheet so that a wholesale change of manufacturing culture is not required.

High-load performance A dangerous enemy of conventional light metals, particularly in high-load applications, is fatigue cracking, with

cracks tending to propagate along the edges of crystals wi th in the microstructure. Developing metallic forms which do not have a crystalline structure is therefore a preoccupation among metallurgists. One such form, currently under development by the US Defense Advanced Projects Research Agency (DARPA) along with the Air Force Research Laboratory (AFRL), and by the Defence Evaluation and Research Agency (DERA) in the UK is amorphous a luminum.This would have strength levels about three times those of the conventional material. Another approach, adopted by aero-engine makers in realizing high- integrity turbine blades in nickel-based superalloys, is to produce single-crystal blade structures. A third method, currently engaging many researchers, is the powder metallurgy (PM) route. Compact ing high-quality fine metal powders through processes such as hot isostatic pressing (HIP was first developed by Battelle Laboratories in the USA), rapid solidification and spray forming can yield metals that exhibit superior performance under load.

Thus aircraft maker Boeing, working with casting specialist Howmet Corp,

found that HIP'd ti tanium castings are suitable for use in aircraft primary

structure. Castings save weight and enhance integrity by reducing part count, and are easier to incorporate into final assemblies than multiple sep- arate parts fastened together. An aft engine bulkhead for the Boeing 777 airliner is now fabricated this way. Howmet also produces directionally

solidified and single-crystal castings in ti tanium and superalloys for gas tur- b ine engines. Aluminum, too, is amenable to the HIP/PM treatment which, because the result can be cast to near-net shape, provides a potential- ly more economical route than forging or machining.

A further technique that has now become established is powder injection molding, for example of titaniurrr.

Some of the research groundwork for TiPIM was laid by Penn State

University and the University of Idaho. Injection molded ti tanium parts show outstanding corrosion resistance

Powder routes also facilitate alloying. In casting by traditional ingot pour ing or cont inuous casting techniques, a t e n d e n c y for alloy e lements to migrate inwards during cooling caus- es a degree of alloy separation.A non- uniform microstructure compromises propert ies and performance in fin- ished products. PM overcomes this p rob lem since each powder grain becomes , in effect, a microscopic ingot which, thanks to its tiny mass, cools so rapidly that segregat ion cannot occur.

PM offers a way of alloying light metals and other elements in proport ions which have not hitherto been possible. For instance, Sumitomo Electric Industries combined 40% by mass of silicon powder with 60% aluminum in a ne w material used in vacuum pump rotors for electric vehicles.The silicon confers hard-wearing properties to the otherwise light but easily abraided alu- minmn. In work which earned a Japan Powder Metallurgy Association Award, Sumitomo used r a p i d solidification powder and forging to provide the f'mal product.

In another example, Crucible Research of Pittsburgh, PA, USA, has alloyed aluminum with titanium to such effect that intermetallic titanium aluminide alloys may now challenge conventional titanium alloys and nickel superalloys in some aircraft and land vehicle engine applications where high thermal tolerance is required. Crucible's preferred process is gas atomization, followed by mixing and HIP'ing of the atomized powder. Titanium aluminide sheet is now being considered for applications such as hot skins on aerospaceplanes (it will be part of the thermal protection

shield on the X-33 single-stage-to-orbit vehicle), control surfaces and exhaust ducts. Other intermetallics, such as nickel alumim'des can be produced by similar methods or, in some cases, by mechanical alloying.

Current research may also yield further alloys offering high thermal tolerance. Imperial College of Science and

Materials Today 27

Technology, London, UK, is, for example, working to deliver by the inert gas atomization route aluminum alloys for potential use in temperatures

of 150-300°C. In Europe, participants in the Delta project, inc luding Aerospatiale and Matra, have collabo- rated over high-temperature alloys produced by rapid solidification and mechanical alloying.

Non-conventional alloys able to be produced by PM/rapid solidification, vapor

deposition or spray deposition, include aluminum-lithiums having lithium pro- portions of greater than about 2.7%; and alloys containing chrome, cobalt, nickel and other elements not normally soluble in aluminum. Metals produced by the powder route are amenable to conventional processes such as roll forming, or

to advanced processes such as laser

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Crucible Research has applied powder metallurgy to the aerospace performance metal titanium. This schematic diagram of Crucible's gas atomization system shows the equipment by which the charge is induction skull melted in a watercaoled copper crucible, tapped through the bo#om of the furance and atomized with argon gas. The powder solidifies as it falls through the cooling tower and is then collected in a cyclone separator.

forming. In the latter, powder introduced into a laser beam is melted, deposited and rapidly solidified. Penn

State University has been involved in a development in which parts can be built up under computer control directly from the powder, without any need for hard tooling. Substantial time and ~ energy are thereby

saved.

Parts currently being produced by PM in light metals include

camshaft bearing caps, mirror brack- ets, shock absorber components , con- nect ing rods and parts for generators and pumps. Parts can be cold (as well as hot) isostatically pressed. A predict- ed increased penet ra t ion of diesel engines with a luminum blocks might present opportunit ies for cylinder lin-

ers and valve seat inserts to be produced by this process.

A no t he r light meta l a m e n a b l e to powder, HIP and ext rus ion processes is beryl l ium. Costing dozens of t imes more than a luminum to produce , beryl l ium's applicat ions are l imited to space - satellites and launchers - where the slightest weight saving can justify a large p r e m i u m . The powder route overcomes poor out- of-plane loading characterist ics that const i tu te a disadvantage for berylli- um sheet.

Another way to enhance the performance of fight metals is to reinforce them with fibers.Aluminum particularly constitutes a light, ductile matrix which can be reinforced by carbon, silicon carbide or a lumina multifila- ments, or more expensive monofila- merits like boron and sili$on carbide in t roduced by vapor deposi t ion. Propert ies that c o m m e n d metallic matrix composites (MMCs) rather than organic reinforced composites include higher off-axis and transverse modulus and strength, good resistance to severe environments, strong joints achieved by diffusion bonding or brazing, good impact resistance and high temperature capability. A boron-reinforced aluminum alloy, for instance, will show lit- tle deterioration in properties in tem-

peratures up to 500°C.

Ceramic reinforcements can confer even higher thermal tolerance and stiffness, though at the expense of increased density. This may be com- pensated, though, by a reduction in volume of the much stiffer material required in a given applicat ion. However, when aluminum matrices are combined with particulate reinforcements such as silicon carbide, some mass reduction can be achieved as well. Particulate reinforced MMCs are also attractive in having isotropic properties, making them amenable to nor-

28 Materials Today

mal metal working techniques such as extrusion, forging and rolling.

MMCs have moved past the experi- mental stage and are in use. To quote one example, Eurocopter has achieved a 70 kg reduction in rotating mass on the rotor for the NH 90 helicopter by using MMCs instead of stainless steel. Weight has also been saved on the rotors of its smaller helicopters by sub- stituting for titanium. Aluminum can also be substituted usefully thanks to the greater fatigue strength of MMCs.

Researchers are focusing on MMC cost and processing. For instance, current work by a consort ium of companies from six European countries to develop a viable process for casting fiber reinforced aluminums was featured at

Materialica 2000. This effort, one of several EU Fourth and Fifth Framework programmes aimed at materials and sustainable transport, is being coordi- na ted by CORDIS, the European Community's facilitator for innovative

research. Infospace GmbH seeks to cast a luminum particulate MMCs into parts capable of performing under high loads. In another example, GKN Sinter Metals is one of several companies investigating new low-weight aluminum-based alloys, including MMCs, suitable for hot forging. Technische Universitat Hamburg has investigated

product ion of aluminum MMCs by the pre-preg route familiar in organic composites. UK's Defence and Research Agency (DERA) has been investigating how to produce silicon carbide-reinforced titanium.

A n u m b e r of companies supplying MMCs include a luminum alloy and spray forming specialist Osprey Metals

in the UK, which recently added particulate SiC-reinforced aluminum to its product line.

Clearly, with better aluminums, alloying of even lighter metals like lithium, development of powder routes able to overcome metals' Achilles' heel of crystalline microstructure and the emer- gence of metal matrix composites, the battle for the ground that seemed at one time about to be taken by polymer- based composites has been well and truly joined. Probably the future will be

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a par tnership in which, as with fiber/metal laminates, both classes of material have their place.

Europe's new large passenger jet, the AirbusA380, is likely to herald a breakthrough for fiber metal laminate hybrid materials, since GLARE is likely to be adopted for the upper fuselage. GLARE upper fuselage panels will be

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among a number of advanced mated- als helping A380 meet its weight and performance targets.

Crucible Research has applied powder metallurgy to the aerospace per- fo rmance metal t i t an ium. This schematic diagram of Crucible 's gas atomization system shows the equip- men t by which the charge is induc- t ion skull mel ted in a watercooled copper crucible, tapped through the bo t tom of the furnace and atomised wi th argon gas. The powder solidifies as it falls through the cooling tower and is then collected in a cyclone separator.

Materials Today 29

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Metals shed weight to compete