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Advanced Aluminium Alloys for Aircraft and Aerospace Applications

RC Dorward and TR Pritchett Center for Technology, Kaiser Aluminium & Chemical Corporation, Pleasanton, CA 94566, USA

Abstract Aluminium has been the dominant material in the aircraft industry for a half century due to its attractive combination of light weighL strength, ductility, corrosion resistance, ease of assembly and low eosL This dominance is being challenged by new materials offering potential weight savings and fuel economies. The aluminum industry has countered this challenge by developing a number of advanced materials of its owrL. rapidly solidified alloys, metal matrix composites and aluminum-lithium alloys. Performance and cost considerations favour the last in most situations, and a considerable effort is under way to commercialize Al-Li alloy products. State.of-the.art aluminum lithium alloys show promising property levels, particularly strength and elastic modulus. Steady improvements in ductility and fracture toughness are resulting from industry.wide development work, particularly with the Al. Cu-Li and AI-Li-Cu-Mg alloy systems.

Introduction The structural integrity and performance of aircraft structures are a function of the materials from which they are made. The most effective materials are those that meet the engineering requirements of the various components at the lowest possible cost. In the early days of aviation, airplanes were built of wood and fabric. As aircraft speeds and technology advanced, it was inevitable that metals would be used to substitute for these fragile materials, albeit with occasional relapses such as Howard Hughes' amphibious C5. However, by this time, aluminum had become the favourite material of construction in the industry due to its attractive combination of properties: light weight, high strength, good corrosion resistance and fabricability. The dominance of aluminium in the aircraft market was threatened in the 1950s by the promotion of substitute materials touting higher strength, lighter weight or improved strength-to-weight ratios: titanium alloys, new high-strength steels and organic resin composites. However, their penetration was slowed when the aluminum industry introduced a

number of new alloys and tempers providing dramatically improved performance characteristics in the areas of fracture toughness and stress corrosion resistance. More recently, rising fuel costs and higher performance requirements have resulted in renewed interest in alternative materials distinguished by greater strength-to-weight ratios. The early thermosetting resin systems consisting typically of fiberglass or carbon in an epoxy or polyester matrix suffered from poor toughness, low ductility and instability at temperatures above 135°C. Bismaleimide resins are superior to epoxy type resins in resisting moisture and elevated temperatures to 190°C. However, they have an even lower elongation to failure and are prone to microcracking. The newer thermoplastic resins, such as polyetheretherketone (PEEK), aramids, polyimides and polyphenylene sulfides have overcome most of these disadvantages except the high cost. For example, polyetheretherketone resins have heat deflection temperatures of 320°C or more, are tougher than brittle crosslinked thermosetting resins, can withstand impacts without cracking or delam-

MATERIALS & DESIGN Vol. 9 No. 2 MARCH/APRIL 1988

inating, do not absorb moisture and may be easier and cheaper to process. Even so, thermoplastic pre-impregnated shapes cost about $35-45/kg and require elevated temperatures of 300-425°C to process. Predictions as to the timing and extent of penetration of these new materials into the aircraft market vary greatly from one source to another. Some years ago, "advanced" composite materials were to have captured a large sector of the structural aircraft market by 1985; yet the actual use of composites today is largely restricted to secondary or lightly stressed components. Nevertheless, it is these materials with which aluminum must compete in the future. Factors that will impact on the timing and market penetration of both advanced aluminum alloys and plastic composites include material properties and costs, aircraft performance criteria, conversion economics and the price of jet fuel.

Aluminum Technology Developments New materials under development by the aluminum industry include alloys produced by rapid solidification

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processing (atomized, melt spun, strip or roller quenched), light weight AI-Li alloys, and composites (metal matrix, resin/polymer laminates). Continuing improvements in the performance of conventional alloys are also being made through composition optimization, impurity control, minor element additions and modified heat treatment & thermo-mechanicai processing practices. In addition, new manufacturing techniques such as superplastic forming of complex shapes, precision die forgings and one-piece non-critical castings are being used to reduce the weight and cost of some components. RSP Alloys Rapid solidification processing (RSP) allows the use of higher concentrations of alloying elements with simultaneous decreases in solute segregation and grain size. Alloys can be designed or modified to specific compositions and microstructures with higher strength, ductility and fracture toughness; improved corrosion and stress corrosion resistance; and elevated temperature strength. At present these materials are most effectively utilized for retrofitting parts requiring higher stress levels, improved stress corrosion resistance or better fracture toughness. While a number of techniques are used to produce alloys by rapid solidification, the powder metallurgy approach is most often used today. In this process, molten aluminum alloys (frequently exceeding solid solubility limits) are atomized in air or inert gases and rapidly solidified to powder. The powder is then sized, isostatically compacted and encapsulated in an aluminum container. Residual gases are evacuated from the container and the powder is then degassed/ cleaned by a number of techniques. Kaiser Aluminum, for example, has developed a depurative (wash) gas technique that results in optimum degasification of aluminum powder compacts. -

300

l 2

l 5

l i0

, 20

, 50

i00

AGING TIME (HR)

Fig 4

Isothermal aging curves at 177°C for AA 2090 alloy extrusions stretched 0.4%. Yield strengths are for long-transverse direction

that products containing non-homogeneous cold work will have internally varying strength levels and temper conditions. Cryogenic Properties: An Unexpected BenefiL Although the highest volume usage of AI-Li alloys is likely to occur in conventional aircraft structures, they will probably be most effectively utilized in certain space applications involving cryogenic environments. Preliminary results show that the yield strength, ultimate tensile strength, fracture toughness, and elongation of 2090-T8X plate all improve as the test temperature is reduced to 4K (27)- see Table V. Consequently, the strengthtoughness combination improves dramatically as temperature de-

creases, making the material for superior to any aluminum alloy currently used in cryogenic situations. Initial weldability studies also suggest that with appropriate processing, acceptable post-weld properties can be achieved. AI-H alloys can therefore be seriously considered for applications such as the fuel tanks in the space shuttle, or for the proposed hypersonic and trans-atmospheric vehicles. Commercial Status. General goals have been established to develop substitutes for damaged tolerant 2024-T3, medium strength 2014-T6, high strength 7075-T6, stress corrosion resistant 7075-T73, and a low-density medium-strength alloy. As shown in Table Vl, con-

MATERIALS & DESIGN Vol. 9 No. 2 MARCH/APRIL 1988

Density (g/cm 3)

Product Form

0 . 2 % Proof Strength (MPa)

2090-T8X 2090-T8E41 2090-T8E41

2.60 "

Sheet Sheet Plate

350 495 470

2091 -T3 2091 -T8

2.58 "

Sheet Sheet

8090-T3X 8090-T3X51 8090-T6 8090-T8 8090-T651

2.54

8091 8091 -T8 8091 -T651

2.55 "

Alloy/Temper

"

" " " "

"

Elastic Modulus (GPa)

K o or K_* (MPam ~=)

11 8 7

78.5 "

100 45 25

285 335

19 15

78 "

130

Sheet Plate Sheet Sheet Plate

325 345 365 390 410

9 6 7 7 5.5

79.5 " " " "

100 35 60 55 30

Sheet Sheet Plate

400 440 475

7 7 5

80 " "

% Elongation

"

50 45 18

* The toughness levels listed refer to 1986-era properties. Significant improvements in some alloy/tempers have been claimed more recently.

Table VI Typical Properties of Semi. Commercial AloLi Alloys (L T Direction) ~2s3°~ siderable success has been made in developing replacements for 2024T3, 2014-T6 and 7075-T6; however, a stress corrosion resistant substitute for 7075-T73 remains elusive. Commercial production of several AI-Li alloys has been initiated by the aluminum industry. Samples of extrusions, forgings, sheet and plate have been provided to aircraft companies for evaluation and qualification testing. These individual and joint development programs will accelerate alloy acceptance and subsequent use of these new products.

Even so, this represents a much smaller increase than the 10 to 20fold cost increase of plastic composites. Some of the newer manufacturing techniques such as superplastic forming and precision die forging, coupled with greater use of special extruded shapes and structural sheet may well lead to reductions in buy-to-fly ratios and hence to lower costs of the completed assembly.

Conclusions

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The aluminum industry is moving rapidly to meet the aircraft industry's need for lighter structural materials. Current AI-Li alloys provide the opportunity of achieving 8 to 10% weight savings and 15% greater stiffness while satisfactorily meeting other performance criteria. Conventional aircraft assembly methods can be employed, thus avoiding the high conversion costs inherent with the use of carbon resin composites. The cost of lithium metal, safety precautions in casting, the need for scrap segregation, and closer control of processing parameters all continue to increase products costs two to three times above those of conventional aluminum aircraft alloys.

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References 1

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SG Roberts, US Patent Nos 3,954,458 (1976) and 4,104,061 (1978) JW Bohlen, RJ Kar and GR Chanani, in Rapidly Solidified Powder Aluminum Alloys, ASTM STP 890, 1986, p 166 SL Langenbeck,WM Gdt~h, GJ Hildeman and J W Simon, ibid, p 410 DJ Skinner, K Okazaki and CM Adam, ibid, p 211 PJ Meschter, PS Rao, RJ Lederich and JE O'Neal, ibid, p 512 PJ Meschter, RJ Lederich and JE O'Neal, Fin a] Report, NASA CR-178145 GAJ Hack, Metals and Materials, Vol 3 (1987); p 457 WR Mohn, Research and Development, July, 1987, p 54 TR Pritchett, Aluminum Technology '86, London, March, 1986 J White, IR Hughes, TC Willis and RM Jordan, 4th lntemational AI-Li Conference, Paris, 1987 JC Bittence, Advanced Materials and Processes, July, 1987, p 45 EJ Stefanides, Design News, Sept 8, 1986, p 66

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LB Bogelesang and JW Gunnink, Materials & Design, Vol VII 1986 No 6, p 278 IM LeBaron, US Patent No 2,381,219 1945 HK Hardy and JM Silcock, J Inst Metals, Vol 84 1955-56 p 423 JM Silcock, ibid, Vo188,1959-60, p 357 RJ Payne and JD Eynon, British Patent Specification 787,665 1957 IN Fddlyander, VF Shamrai and NV Shiryaeva, lzv Akad Nauk USSR Metally, No 2, 1965, p 153 EA Starke, TH Sanders and IG Palmer, J Metals, 33 1981 No 8, 24 TH Sanders, EA Ludwiczak and RR Sawtell, Mater Sci Eng, Vol 43 1980 p 247 AK Vasudevan, EA Ludwiczak, SF Baumann, RD Doherty and MM Kersker, MaterSci Eng, Vol 72 1985 p 125 TH Sanders and PW Niskanen, Res Mech Lett Vol 1 1981 p 363 CB Cdner, US Patent No 2,915,391 1959 RE Lewis, "Advanced Aluminum Alloys From Rapidly Solidified Powders," R & D Status Report, Contract No F33615-78(:5203, September, 1980 IN Fridlyander, e t a l , British Patent Specification 1,172,736 1969 RF Ashton, DS Thompson, EA Starke and FS Lin, in AI.Li Alloys /I/, Inst of Metals, 1985 p 66 J Glazer,SLVerzasconi, EN Dalder, WYu, RA Emigh, RO Ritchie and JW Morris, Int Cryogenic Materials Conference, Cambridge,/v~ USA 1985 PE Bretz, Al-Li Symposium Proceedings, Los Angeles, March, 1986 Pechiney Aluminum Data Sheet, June, 1986 British Alcan LITAL Data Sheet, Sept 1986

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