Aerospace Materials: Chapter 2. Advanced Materials And Process Technologies For Aerosapace...

Chapter 2

Advanced materials and process

technologies for aerospace structures

Tsugio Imamura

Introduction

Considerable effort is being directed to reducing the weight of aerospacestructures and their associated manufacturing costs. Derivatives fromcurrent aluminium alloys are being developed to provide materials for lowfatigue crack propagation rate and damage tolerance design. For titaniumalloys the target is a better balance between strength and fracture toughnessand higher fatigue resistance. Process technology is being improved toproduce one-piece complex configuration parts, to reduce assembly costsand to reduce weight through fewer parts. For future structures, such as insupersonic transport, further developments in forming technology for light-weight heat resistant materials will be needed.

Materials

Aluminium alloys

Generally the fracture toughness of 2024 is improved by decreasing the volumefraction of constituent particles (Cu2FeAl17, Mg2Si etc.), and fatigue crackgrowth rate at high applied stress intensity�K is reduced with improved frac-ture toughness [1–3]. Investigation shows that: (a) fracture toughness isproportional to the square root of the spacing of the constituent particles;(b) fatigue crack growth rates at low and medium �K depend on the size ofdispersoids (Cu2Mn3Al20); and (c) coarse dispersoids reduce the fatiguecrack growth rate. Both fracture toughness and fatigue crack growth rateare thus improved by careful control of constituent and dispersoid particles.

Copyright © 2001 IOP Publishing Ltd

Fracture Toughness

Amount and Spacingof Constituent

Size of Dispersoid

Fracture Toughness

Fatigue CrackGrowth Rate

Amount of Fe, Si, Cu

Amount of MnHomogenizing Condition

Control ofMicrostructure

Alloy Compositionand Process

Properties

Properties

Fatigue CrackGrowth Rate

Amount of Fe+Si

Control of Microstructure

Amount ofConstituent

Alloy Composition

Current Approach

MHI/KSL

Approach

Figure 2.1. New approach to improving fracture toughness and fatigue crack growth rate.


This is shown in figure 2.1, giving details of an alloy/process combinationdeveloped by Mitsubishi Heavy Industries (MHI) and Kobe Steel.

The fracture toughness and fatigue crack growth rate in the newly designedalloy 2X24 have been measured as shown in figure 2.2 and figure 2.3. 2X24

Figure 2.2. Fracture toughness of 2X24 improved by 20% compared with 2024.

Figure 2.3. Fatigue crack growth rate of 2X24 improved by 50% compared with 2024.


shows approximately 20% higher fracture toughness than 2024 without lossof tensile yield strength, because of an increased spacing of constituentparticles in the microstructure. At the same time 2X24 shows approximately50% lower fatigue crack growth rate over the whole range of �K. Fromscanning electron microscope observations of the fracture surfaces of fatiguetest specimens, narrow and wavy striation patterns are observed. Coarsedispersoids are associated with the waviness, showing that they obstructthe crack propagation.

Figure 2.4. Crack growth simulation, showing the possibility of reducing the skin gauge by

21% compared with 2024.


A crack growth simulation and estimation of the margin of safety hasbeen performed on the application of 2X24 alloy for the centre fuselagecrown panel of a Global Express business jet, using a modified generalizedWillenborg model. The crack growth simulation shows that 2X24 hastwice the fatigue life of 2024 in the same thickness gauge, and a 21%thickness reduction can be achieved with the same fatigue life, as shown infigure 2.4.

Titanium alloys

The application of titanium alloys for aerospace is increasing because ofgood combinations of strength, toughness, corrosion resistance and compat-ibility with polymer composite materials. For the Boeing 777 aircraft rolledout in 1995, a variety of titanium alloys are used to make up 9% of the totalcraft weight. For example a large �þ � Ti-6Al-4V casting is used for theAPU duct panel, and a � Ti-10V-2Fe-3Al large forging is used for the landinggear truck beam. Figure 2.5 shows the material distribution of the newestfighter F-22 developed by the US Air Force [4], and the titanium alloyusage is over 30% of the structural weight. The major titanium alloys onthe F-22 are Ti-6Al-4V and Ti-6Al-2Sn-2Zr-2Mo-2Cr-Si.

Titanium alloys are also widely used for aerospace structures. The mostused alloy is �þ � Ti-6Al-4V, which has a wide variation of mechanicalproperties under different heat treatment conditions, as shown in figure2.6. The alloy has moderate tensile strength and moderate fracture toughnesswhen annealed in the �þ � region to produce a uniform equi-axed � phase

Composites35%

Others16%

Steel5%

Titanium33%

Aluminum11%

Figure 2.5. Material distribution on the F-22 fighter.


microstructure, and is used generally for many aerospace components. Athigher temperatures the �-annealed alloy shows very high fracture toughnessbut very low strength, and is used in high fracture toughness required partssuch as stop fitting. The � solution treated and over aged alloy has superiorstrength and fracture toughness, and is used for dynamic helicopter partssuch as rotors.

Recently an improved heat treatable alloy Ti-10V-2Fe-3Al has becomemore popular for aerospace applications. The alloy was developed byTimet as a high strength, high fracture toughness alloy. The alloy propertiesof strength and fracture toughness were re-designed by the airframe

Figure 2.6. Mechanical properties of �þ � Ti-6Al-4V heat treated under different

conditions.


manufacturer, MHI. At first, Fe segregation was found in the alloy, causing alow � transus temperature locally and resulting in � flecks, i.e. coarse � grainstructure areas with low elongation. It was necessary to remove the Fesegregation and ensure uniformity, selecting a higher solution temperatureto remove � flecks and ensure high fracture toughness. Figure 2.7 providesan example of a near-net-shape forging using a re-designed Ti-10V-2Fe-3Al alloy for the F-2 fighter.

The new � rich �þ � alloy SP700 (Ti-4.5Al-3V-2Fe-2Mo) wasdeveloped by NKK, and was designed to be more � stable than Ti-6Al-4V,by addition of molybdenum and iron. The SP700 alloy has a fine grainstructure and good superplastic formability at low temperature, as shownin figure 2.8 [5]. MHI is developing the application of superplastic formedparts by using SP700 for H-2A rocket components.

Process technology

General

Many unique manufacturing methods have been used to produce aerospacecomponents. Techniques have been developed to optimize difficult-to-workmaterials and complicated component configurations, resulting from apursuit of ultimate lightweight structures. On the other hand, manufacturingcost savings are a universal requirement even when there is a limited amountof parts production. Die-less forming has been used extensively to reducemanufacturing costs. Peen forming of complex curvature wing panels, rollforming of stringers and/or frames, and chip forming of the cylindrical

Figure 2.7. Application of Ti-10V-2Fe-3Al alloy for the F-2 fighter.


skins of rocket tanks and/or skin panels of airliners are typical examples ofrepresentative die-less forming used in the aerospace industries. Aerospaceapplications also require integrated components, large-size structures andpanel thickness control to achieve ultimate weight reduction. Superplasticforming and roll forming are typical examples of technologies combiningdie-less forming with integrated manufacture and thickness control toachieve substantial cost saving and weight reduction.

Superplastic forming

Superplastic forming was developed as a technology to form integrated singlepiece structures, which could replace assembled structures with intricate detailand fasteners, by making the most of the exceptional formability of super-plastic materials. Superplastic forming can reduce the numbers of parts andfasteners, which leads to considerable cost saving and weight reduction.Figure 2.9 [6] shows a superplastic formed inner skin made of 7475 aluminiumalloy, which is to be put together with an outer skin of the same material toform an access-door panel. The conventional door panel consists of 15 to 25detailed parts assembled with many fasteners. In this particular case, morethan 20% weight reduction and 40% cost saving was achieved by using super-plastic forming compared with the conventional manufacturing methods.Superplastic formed parts can have as close a tolerance as machined parts,allowing the manufacture of elaborate components like fuel tanks, whichrequire high accuracy and thin wall thickness. Figure 2.10 [7] shows a teardrop

Figure 2.8. Superplastic elongation and flow stress of SP700.


shaped fuel tank for a satellite fabricated by superplastic forming and electronbeam welding. The fuel tank is made of Ti-6Al-4V and has the optimum thick-ness distribution leading to exceptional weight reduction. The spherical areahas a constant thickness of 0.75mm and the cone area has a thickness distri-bution varying from 0.93 to 0.56mm corresponding to the curvature. Sincesuperplastic forming is necessarily accompanied by non-uniform thin out,

Figure 2.9. Superplastic formed inner skin of an access-door panel.

Figure 2.10. Teardrop shaped fuel tank for a satellite with the optimum thickness

distribution.


the stock sheets were machined to provide suitable thickness distribution afterforming.

Superplastic behaviour has been of great importance as a formingtechnology for difficult-to-work materials such as metal matrix compositesand intermetallic compounds. Superplastic forming has enabled advancedbut less workable materials to be plastically formed and therefore becomecost competitive, facilitating the practical use of high performance materialsand improving the performance of many aerospace components. Figure 2.11[6] shows a missile fin fabricated by superplastic forming of SiC whisker-reinforced 7075 composites, which replaced the conventional fin with aweight reduction exceeding 50%. The next generation of aerospacecomponents will require further weight reduction and cost saving, and thecombined process of superplastic forming and diffusion bonding is a promis-ing technology. The fabrication of various kinds of sandwich panel has beenunder development, focusing on the way to combine superplastic formingwith diffusion bonding and the edge structure needed to join panels toeach other. Figure 2.12 shows a hollow fan blade fabricated by 4-sheetsuperplastic forming/diffusion bonding, and a flat panel with the same corestructure.

Roll forming

Stringer and frames, major components of aircraft structure, are manu-factured by machining from extrusions or by roll forming from sheet.Figure 2.13 [8] shows a roll-formed stringer having an optimum thickness

Figure 2.11. Missile fin made of a SiC whisker-reinforced 7075 composite which replaced

the conventional fin with weight reduction of more than 50%.


Figure 2.12. Hollow fan blade fabricated by 4-sheet superplastic forming/diffusion bond-

ing and core structure.

Figure 2.13. Taper-rolled stringer with controlled thickness corresponding to stress distri-

bution.


distribution corresponding to variations of stress along the longitudinaldirection, which offers 20–30% weight reduction compared with a constantthickness stringer. The optimum thickness distribution is also achievedeconomically by taper rolling. The overall manufacturing process is taperrolling of annealed strip, solution heat treatment, ten steps of section rollforming and artificial ageing. The roll-formed stringers require a specialmicrostructure, fine enough to endure successive bending operations with abend radius as small as 1.3 times the thickness. On the other hand, thestringers have to be resistant to stress corrosion cracking, which requires arather coarse microstructure in directions perpendicular to the appliedstress. Thus the stringer strips need a grain structure with a large aspectratio, fine in the transverse direction to maintain formability, and coarse inthe transverse direction to provide good stress corrosion cracking resistance.Figure 2.14 [8] shows the dependence of threshold stress for stress corrosioncracking as a function of the aspect ratio of the grains, together with the flowstress during stringer manufacture. In order to maintain the large aspect ratiograin structure in strip rolled to various degrees at different positions, it wasnecessary to adopt special recovery heat treatments between the taper rollingand solution heat treatment steps.

Summary

Recently, new metallic materials technologies have been developed toenhance lightweight structure and reduce costs in aerospace applications.

Figure 2.14. Dependence of the threshold stress for stress corrosion cracking on the aspect

ratio of grains.


Strength, fracture toughness and corrosion resistance must be optimized atthe same time as reducing manufacturing costs, through the developmentof near-net-shape forming, and by using integrated components with reducedpart numbers, for example by superplastic forming. These developmentactivities show steady progress and achievement.

References

[1] Speidel M O 1975 Sixth International Light Metals Conference Lepven/Vienna p 67

[2] Rice J R and Johnson M A 1970 Inelastic Behavior of Solids p 641

[3] Staley J T et al 1970 Inelastic Behavior of Solids p 641

[4] JMIA 1992 Feb.

[5] Ogawa A, Fukai H, Minakawa and Ouchi C Beta Titanium Alloys in the 1990s p 513

[6] Tsuzuku T, Takahashi A and Sakamoto A 1991 Superplasticity in Advanced Materials

ed S Dori, M Tokizame and N Furushiro (The Japan Society for Research on Super-

plasticity) p 611

[7] Takahashi A , Shimizu S and Tsuzuku T 1999 J. Japan. Society Res. Superplasticity

31(356) 1128

[8] Hirota K, Ibaragi M et al 1996–5 Mitsubishi Heavy Industries Technical Review 33(3)


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