Materials Science and Engineering, A 135 ( 1991 ) 295-297 295

Unlocking the potential of metal matrix composites for civil aircraft

David Charles British Aerospace (('ornmercial Aircraft) Ltd., Bristol (U, K.)

Abstract

Metal matrix composites offer considerable potential for widespread application in most aerospace fields. The potential for the use of these materials in civil aircraft is largely dependent on costs and currently these are too high. However, by the use of appropriate design and manufacturing technology this intrinsic high cost can be overcome to unlock their potential. Development activities at the Airbus Division of British Aerospace aimed at unlocking this potential will be reviewed.

1. Introduction potential of MMCs by applying these techniques to the design, manufacture and test of structural

There is considerable potential for the applica- demonstrators. tion of aluminium-based metal matrix composites Two basic types of MMCs have been under (MMCs) to commercial aircraft for providing evaluation consisting of a continuous fibre- lightweight components exhibiting high strength, high stiffness, good wear resistance and improved reinforced aluminium alloy and discontinuous elevated temperature properties in comparison particulate-reinforced aluminium alloys. Details

primarily of the work carried out with the latter with the matrix alloy. Consequently, they are materials are presented in this paper. under consideration for the manufacture of a wide range of components. However, achieving The development work carried out at British this potential is restricted owing to high material Aerospace with the discontinuous reinforced cost and the limited understanding of the design aluminium alloys was carried out in parallel with

that on continuous fibre-reinforced aluminium and manufacturing techniques required for unlocking the potential of this class of material, but commenced slightly later such that an initial

Because of intense development activities, in programme had been successfully completed the U.S.A., Japan and Europe, major price reduc- with fibre-reinforced aluminium alloys. The tions are envisaged in the near future. This has objective of the first Government-supported resulted recently in material suppliers in the programme was to design, manufacture and test

idealized aircraft structures. To achieve this two U.S.A. delivering the first production compo- nents for airframeuse, large compression panels representative of a

fuselage and wing sections were manufactured. These panels consisted of particulate MMC skins and unidirectional continuous fibre-reinforced

2. Metal matrix composite development "Z'" section stringers. These panels were subse- at British Aerospace quently compression tested and either met or

The Airbus Division of British Aerospace exceeded the predicted design failure loads Commercial Aircraft has been involved with the (Fig. 1). evaluation of MMCs for several years. Since Following the success of the initial programme, 1984 the project has been partially grant sup- design studies were carried out to identify poten- ported by the British Government. The overall tial structural applications for MMCs. These objective of the project is to develop low cost design studies indicated that two components manufacturing techniques and to demonstrate the from the British Aerospace 146 centre fuselage

Elsevier Sequoia/Prinled in The Netherlands

296

w , , ~ TABLE 1 • llKl~wOl~lgO~ r 1 i ~ F a l l l n g Load KN Comparisonofdatafortensiletesting2124-25vol.%SiCp

P r ~ a ~ Material F~l F~2 F~ E Elonga- - - - - - - - - -+--- - (MPa) (MPa) (Mpa) (GPa) tion

(%/

~ 4 Extruded plate(L) 449 483 605 129 4.0 Strut extrusion (L) 351 376 525 108 3.5 Rolled sheet (L) 324 355 535 113 5.3 Rolled sheet (T) 322 350 492 118 3.3 Suppliers' data -- 414 565 114 5.6

Fig. 1. MMC skin stringer compression panel.

extrusions and then machined to final size. For production of the MMC demonstrator com-

could offer significant weight savings if re- ponents it was proposed to adopt a similar fabri- designed and manufactured from MMCs. cation route as this offered the best material

To achieve component design, manufacture utilization. and test within the time scales a broad-based Two basic extrusions were purchased con- programme was constructed which was divided sisting of an "I" section for floor beam strut into several key topic areas: production process manufacture and a tubular section for keel long- development; mechanical property assessment; eron tube manufacture. Although some machin- demonstrator studies, ing difficulties were encountered the extrusions

2.1. Production process development were successfully machined to final design by the use of polycrystaUine diamond tools. Process Typical commercial aircraft structural com-

ponents are complex, involving varying thick- developments have also been carried out to nesses, angled or joggled sections and with evaluate the weldability of the material and to numerous cut-outs or access holes. In addition, establish appropriate chemical pretreatment virtually all components require joining to other procedures. Forming trials have successfully parts of the structure as well as finishing treat- demonstrated the ability to form sheet material at ments, such as painting. The production cost of a room temperature to provide bend radii similar finished structural part comprises the initial to thoseofconventionalalloys. material cost and the added value of component manufacture. The cost of MMC material will 2.2. Mechanical property assessment always be greater than the unreinforced matrix The material selected for the demonstrator alloy and consequently it is essential to develop components designs was a 2124 matrix alumin- cost-effective manufacturing techniques achiev- ium reinforced with 25% silicon carbide particu- ing as close to near-net-shape production as late by volume. This material was selected on the possible, basis that the proposed particle loading would

The British Aerospace programmes have been offer considerable improved tensile and compres- specifically aimed at establishing low cost manu- sive properties, in addition to increased stiffness facturing methods for both continuous fibre- and over conventional aluminium, and still offer a particulate-reinforced aluminium. These activ- fracture toughness in excess of 20 MPa m ~/2. ities have included both part manufacture "and A comparison of the results obtained from the secondary processes such as machining, adhesive evaluation with manufacturers' data is presented bonding and forming. Some of this work with in Table 1. From these results it can be observed discontinuous reinforced aluminium alloys which that there are some differences between the has been carried out during the design, manu- British Aerospace data and the manufacturers' facture and test of various structural demonstra- data. Discussions with the material supplier have tors is described in more detail in the following indicated that the material chemistry has been sections, slightly modified to improve the material's work-

The two selected components, the British ability andtoughness. Aerospace 146 floor beam strut and keel long- This modification was carried out after pro- eron bay tube, are currently manufactured as duction of the initial extruded plate material.

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Fig. 2. Particulate-reinforced floor beam strut. Fig. 3. Particulate-reinforced keel longeron tube.

Consequently, the sheet material and component 3. Future commercial aircraft applications extrusions were expected to exhibit a small drop in elastic modulus and strength but with the The next phase of the British Aerospace pro- correct degree of processing have improved gramme commenced in mid-1989 and is aimed at ductility and toughness. Toughness testing indi- assessing whether the components identified by cated the initial extruded plate material to have a the design studies can be manufactured to give plane strain fracture toughness of 17 MPa m 1/2. cost-effective weight savings. Central to this This testing was carried out using compact ten- phase is a cost-benefit analysis. This analysis has sion specimens tested according to A S T M E399. now been almost completed for the discontin- It proved very difficult to obtain results that con- uous reinforced aluminium alloys. From this anal- formed to the full requirements of the ASTM test, ysis it has been concluded that these materials can with many specimens exhibiting excessive crack be used to produce viable components through front curvature. This has resulted in considerable the utilization of near-net-shape extrusions. This collaboration between various U.K. organizations production route maximizes material utilization to examine the exact requirements for establish- and minimizes secondary processing operations ing the true fracture toughness of this class of such as machining. If this is coupled with corn- material. In the light of this it is believed that the ponent redesign it may be possible to produce

some large extrusions that not only save weight figure quoted is probably lower than the intrinsic material toughness, which is expected to be in but may be cheaper than existing components. At excess of 25 MPa m 1/2. full production it has been estimated that up to

10% of the structural weight of a modern sub- 2.3. Demonstrator studies sonic civil airliner could be made from discontin-

uous reinforced aluminium alloys and exhibit The design studies enabled a range of MMC cost-effective weight savings. The main areas of

components to be successfully designed, manu- pbtential application are at present considered to factured and tested. These components included floor beam struts made from both continuous be the upper wing and the internal structure of

the fuselage. fibre- and particulate-reinforced aluminium (Fig. 2) and a keel longeron tube from particulate- reinforced aluminium (Fig. 3). Studies into a

Acknowledgment future civil aircraft structural demonstrator are still ongoing with areas of the wing and fuselage In thanking British Aerospace for permission showing considerable promise for subsonic air- to publish this paper the author emphasizes that, liners. Potential applications on a next-generation although it is based on his professional work on supersonic aircraft could be widespread but true the Company's behalf, any views expressed are potential cannot be assessed until long-term his own and do not necessarily represent those of elevated temperature test data are produced, the Company.