PLASTICS

MAKING A GOODIMPRESSION

uperplastic aluminium alloys - thosewhich behave like plastic when heated and

metal when cold - have been commercially ex-ploited by the aerospace industry since the early1970s. Yet it is only during the last decade or so thatthey have begun to find favour with manufacturingindustry at large, leading to fresh applications infields as diverse as electronics, medical equipment,architectural components, defence, the auto indus-try and public transport (especially railways).

Technically, a superplastic aluminium alloy isone which exhibits high tensile durability at lowstrain rates, coupled with high uniform elongationsand low flow stress at elevated temperatures. Ofthese characteristics, the most important is theelongation factor. In most applications this will beslightly less than 200%, in order to maintain a viablematerial thickness, although it is technicallypossible to achieve a factor of more than 1000%. Itis of course this stretching capability which facili-tates the compressed air forming of complex shapesfrom a single sheet, a capability which embodiesobvious attractions for industrial designers.

Lital 8090-SPF, for example, is an aluminiumlithium alloy specially developed for aerospace ap-plications which offers a reduced density (of up to10% less than conventional aluminium alloys),together with a similar degree of increased elasticmodulus, or stiffness. For increased room tempera-ture ductility and corrosion resistance there arealloys such as 5083-SPF, a non-heat treatable alloywith a 4.5% magnesium content which offers a levelof post forming mechanical properties that make itideal for many semi- or non-stressed aerospacecomponents. All of these alloys are of the fine par-ticle stabilised type (true superplasticity necessi-tates a uniform grain size, usually of less than 10microns).

The use of compressed air forming techniquesbrings the further advantages of high finish quality,

When is a plastic not a plastic?When it's superplastic aluminium, of course!

Russell Stracey extols the benefits of hot, runny metal

since only one component surface comes into con-tact with the tool; together with low tooling costs -the majority being machined from aluminium alloysor ferrous metals - and shorter lead times. Theseare now four individual forming methods in com-mon use, the chosen technique depending on size,base alloy, the required level of structural integrityand the complexity of the design itself. These are,respectively, male and female forming, drape for-ming and diaphragm forming.

All take place within specially designed formingmachines which maintain the necessary combina-tion of temperature and air pressure throughout theforming cycle. Typically, temperatures will be be-tween 470°C and 520°C, with maximum air pres-sures of up to 10 bar. In addition to these techniques,a fifth, back pressure forming, is employed for theproduction of components for which the higheststructural integrity is called. Since this demands inturn a reduction of the microporosity which is a

MANUFACTURING ENGINEER NOVEMBER 1991

PLASTICS

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normal feature of the superplastic forming (SPF)process, a special chamber is employed in which airpressures of up to 40 bar are applied to both the frontand back of the component.

Regardless of the method used, however, it is theexceptional design freedom offered by the combina-tion of air pressure forming with a metal which iscapable of behaving like plastic which remains thesingle most important benefit to manufacturing in-dustry. It is this unique combination which makesthe SPF process particularly suited to the productionof components with double curvature, and thosewith steps, angles and styling features. The enduringappeal of the process for aircraft constructors is thusreadily understood, and yet it is only recently thatother valuable manufacturing advantages havebegun to be widely appreciated.

Four superplastic formingtechniques have beendeveloped to satisfy thedemands of complex shape,component integrity andoverall size. All four methodsoperate at elevatedtemperature (470°C-520°C)

Foremost among these is the ability to producecomplicated parts from a single sheet, minimisingthe need for additional fabrication and assembly. Aswell as reducing production lead times and costs,this facility frequently enables designs to be simpli-fied, component weights to be reduced and structu-ral integrity to be substantially enhanced. In excep-tional circumstances, some components have beenreduced from as many as 10 sheet metal detailsdown to a single forming. Since SPF aluminiumalloys possess excellent strength-to-weight ratios,this capability is now broadening the material'sappeal in areas where high strength is important,such as the automotive industry, public transportand, of course, architectural cladding.

Strength and performanceStrength is by no means the only inherent ad-

vantage offered by the alloys, however. Althoughplastics steadily continue to improve, their flamma-bility performance (aggravated by high heatemission and toxicity levels) remains relativelypoor. This factor is clearly a major cause of concernfor aerospace designers and civil aviation auth-orities, for example, and it is no surprise to learn thatSPF aluminium alloys are natural beneficiaries ofthe search for improved fire performance.

Other material benefits include excellent EMIscreening properties, now being exploited in a var-iety of telecommunications and electronics applica-tions within both the defence and medical equip-ment industries. Again, the importance of this factorcan only increase in the future as design criteria areinfluenced by fresh legislation, such as the introduc-tion of the EC Directive on Electromagnetic Com-patibility (EMC).

Finatly, in direct contrast to plastics, aluminiumis of course a recyclable material. While this fact isalready having an effect upon consumer productsand packaging - witness the recent opening ofBritain's first aluminium can recycling plant andcollection centre in Birmingham- it will clearly takemuch longer to influence the decisions of industrialbuyers and specifiers. Nevertheless, the public rela-tions opportunities arising from a more Green ap-proach to manufacturing are already making them-selves felt; and will doubtless continue to do so asindustry seeks new ways of reducing its impact uponthe planet.

Cost advantagesAttractive as they may be, all these benefits have

to be put into perspective by costs, and here againthe SPF process can claim considerable advantagesover rival manufacturing techniques.

Given the qualification of low to medium totalproduction volumes of anything between 50 to sev-eral thousand units, SPF aluminium will usuallyprove far more cost effective than die casting, thematched die pressing of metal components, or theinjection moulding of plastics. One of the primaryreasons for this is the modest cost of the singlesurface tooling, which can also be readily modifiedvia machining and/or welding during the productdevelopment phase, again increasing flexibility fordesigners.

The range of applications on which these con-siderable advantages are now being exploited is, asnoted earlier, quite diverse, and growing broadereach year. Refreshingly, the commercial success ofsuperplastic forming is also very much a Britishsuccess, with Worcester based Superform Metals -a British Alcan company - a pioneer in the early1970s and still claiming to be the largest singlesupplier of SPF aluminium components world-widetoday. Since its inception, the firm has establisheda second design and production facility in Californiaand is planning a third in France.

The growth in new markets for SPF is im-pressive, with turnover for Superform up 30% lastyear, attributable in part to increasing use by theaerospace industry, which is now for the first timegradually beginning to accept certain SPF alloys infully heat treated condition for use in leading edgepositions which are prone to bird strike.

Indeed, the growth in the range of SPF alloysnow available has also contributed to the diversityof applications. For example, in addition to the longestablished heat treatable alloys containing copperand zirconium as alloying elements, a number ofalloys are now being produced in a way whichendows them with a degree of superplasticity, whileretaining the higher strength of the base material.Still others offer increased room temperature duc-tility and corrosion resistance, or incorporate l i -thium for reduced density and an increased elasticmodulus.

To summarise then, there seems little doubt thatsuperplastic aluminium is set to carve out an everlarger slice of the manufacturing cake, at least forsmall to medium production volumes where someform of sheet metal or plastic would be the conven-tional alternative. A potent combination of increaseddesign freedom, high finish quality, good mechan-ical properties, short lead times and modest toolingcosts is proving highly attractive in an increasinglycompetitive manufacturing environment EQFor more information enter ME33

Russell Stracey is Product Development Managerwith Superform Metals of Worcester.

MANUFACTURING ENGINEER NOVEMBER 1991