AIRCRAFT MATERIALS Review of the Course AIRCRAFT MATERIALS

AIRCRAFT MATERIALS Review of the Course AIRCRAFT MATERIALS

AIRCRAFT MATERIALS 1. Basic requirements • • • 2. High strength and stiffness Low density => high specific properties e. g. strength/density, yield strength/density, E/density High corrossion resistance Fatigue resistance and damage tolerance Good technology properties (formability, machinability, weldability) Special aerospace standards and specifications Basic aircraft materials for airframe structures • • Aluminium alloys Magnesium alloys Titanium alloys Composite materials

Development of aircraft materials for airframe structures Relative share of structural materials other materials composites Ti alloys Mg alloys other Al alloys wood Al. Cu. Mg alloys pure Al. Zn. Mg. Cu alloys pure Al. Cu. Mg alloys new Al alloys steel Year

Structural materials on small transport aeroplane

Development of composite aerospace applications over the last 40 years

Composite share in military aircraft structures in USA and Europe Structural materials on Eurofighter

Structural materials on Eurocopter

Comparison of mechanical performance of composite materials and light metals

Aluminium Alloys

Aluminium – Al • • plane centered cubic lattice melting point 660 °C density 2. 7 g/cm³ very good electrical and heat conductivity very good corrosion resistance low mechanical properties solid solutions with alloying elements maximum solubility is temperature dependent – – Cu: 6 % at 548 °C; 0. 1 % at RT Mg: 17 % at 449 °C; 1. 9 % at RT Zn: 37 % at 300 °C; 2 % at RT Si: 1. 95 % at 577 °C; 0 % at RT Substitution solid solution a) alloying atom > aluminium atom b) pure aluminium c) alloying atom < aluminium atom

Characteristics of aluminium alloys Advantages • Low density 2. 47 - 2. 89 g/cm³ • Good specific properties – Rm/ρ, E/ ρ • Generally very good corrosion resistance (exception alloys with Cu) • Mostly good weldability – mainly using pressure methods • Good machinability • Good formability • Great range of semifinished products (sheet, rods, tubes etc. ) • Long-lasting experience • Acceptable price Shortcomings • Low hardness, susceptibility to surface damage • High strength alloys (containing Cu) need additional anti-corrosion protection: – Cladding – surface protection using a thin layer of pure aluminium or alloy with the good corrosion resistance – Anodizing – forming of surface oxide layer (Al 2 O 3) • It is difficult to weld high strength alloys by fusion welding • Danger of electrochemical corrosion due to contact with metals: – Al-Cu, Al-Ni alloys, Al-Mg alloys, Alsteel

Designation of aluminium alloys according to EN Wrought alloys Casting alloys AL-PXXXX(A) designation basic alloying element • • 1 XXX 2 XXX 3 XXX 4 XXX 5 XXX 6 XXX 7 XXX 8 XXX – pure aluminium - copper (Cu) - manganese (Mn) - silicon (Si) - magnesium (Mg) - Mg + Si - zinc (Zn) - other (eg. Li) AL-CXXXXX designation basic alloying element • 1 XXXX • 2 XXXX • 3 XXXX • • 4 XXXX 5 XXXX 7 XXXX 8 XXXX - > 99. 0 % Al - Cu - Si-Mg - Si-Cu-Mg - Si - Mg - Zn - Sn

Important wrought aluminium alloys for aircraft structures • 2 XXX (Al-Cu, Al-Cu-Mg) - high strength, lower corrosion resistance 2014 (0. 8 Si, 4. 4 Cu, 0. 8 Mn, 0. 5 Mg) 2017 (0. 5 Si, 4 Cu, 0. 7 Mn, 0. 6 Mg) 2024 (4. 4 Cu, 0. 6 Mn, 1. 5 Mg) 2024 Alclad (with the surface layer of Al) 2027 (4. 4 Cu, 0. 9 Mn, 1. 3 Mg, 0. 2 Zn, 0. 05 Cr, 0. 25 Ti) 2124 (4. 4 Cu, 0. 6 Mn, 1. 5 Mg, 0. 1 V) 2219 (6. 3 Cu, 0. 3 Mn, 0. 1 V) • 6 XXX (Al-Mg-Si) -comparing to 2 XXX - lower strength, better ductility and corrosion resistance 6013 (0. 8 Si, 0. 8 Cu, 0. 50 Mn, 1. 0 Mg) 6061 (0. 6 Si, 0. 28 Mn, 1. 0 Mg, 0. 2 Cr) 6061 Alclad 6056 (1. 0 Si, 0. 9 Mg, 0. 8 Cu, 0. 7 Mn, 0. 25 Cr, 0. 2 Ti+Zr) • 7 XXX (Al-Zn-Mg-Cu) – the highest strength, lower ductility, notch sensitivity 7050 (2. 3 Cu, 2. 2 Mg, 0. 12 Zr, 6. 2 Zn) 7075 (1. 6 Cu, 2. 5 Mg, 0. 13 Cr, 5. 6 Zn), 7075 Alclad 7175 (1. 6 Cu, 2. 5 Mg, 0. 23 Cr, 5. 6 Zn) 7475 (1. 6 Cu, 2. 2 Mg, 0. 22 Cr, 5. 7 Zn)

Most important tempers aluminium alloys • • • O W H T 3 - anealing - solution treating + quenching (non stabil state) - strain-hardening (strength is increased only due to cold working) - solution treating + quenching + cold working + room temperature aging T 351 - solution treating + quenching + stress relief due to controlled stretching + room temperature aging T 4 - solution treating + quenching + room temperature aging T 6 - solution treating + quenching + artificial aging T 651 - solution treating + quenching + stress relief due to controlled streching + artificial aging T 7 - solution treating + quenching + artificial overaging T 73 - solution treating + quenching + artificial overaging for the best stress corrosion resistance T 8 - solution treating + quenching + cold working + artificial aging

Reference aluminium alloys in airframe structure Structure Part Control parametr Reference alloys Wing Upper panels Upper stringers Lower panels Lower stringers Beams, ribs compression damage tolerance (DT) tension + DT static properties 7150 -T 6/T 77 7050 -T 74 2024 -T 3, 2324 -T 39 2024 -T 3 7050 -T 74, 7010 -T 76 Fuselage Upper panels Lower panels Stiffeners Main frame compression, DT, formability tension + DT tension/compression complex 2024 clad-T 3 7175 -T 73 7010+7050 -T 74 All types 7010/7050/7075 Other parts

Typical mechanical properties of alloy 2014 4. 4 Cu-0. 8 Si-0. 8 Mn-0. 5 Mg , E = 72. 4 GPa , ρ = 2. 77 g/ccm Temper Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 500 mil. cycles Bare sheet 2014 0 186 97 18 90 T 4 427 290 20 140 T 6 483 414 13 125 Alclad sheet 2014 0 172 69 21 - T 3 434 273 20 -

Typical mechanical properties of alloy 2024 4. 4 Cu-1. 5 Mg-0. 6 Mn, E = 72. 4 GPa , ρ = 2. 77 g/ccm Temper Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 500 mil. cycles Bare 2024 0 185 75 20 90 T 3 485 345 18 140 T 4, T 351 470 325 20 140 Alclad 2024 0 180 75 20 - T 3 450 310 18 - T 4, T 351 440 290 19 -

Typical mechanical properties of alloy 2124 4. 4 Cu-1. 5 Mg-0. 6 Mn, E = 72. 4 GPa , ρ = 2. 77 g/ccm Temper Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 500 mil. cycles 9 - 5 - Plate 2124 - L T 851 490 440 Plate 2124 - LT T 851 490 435 Plate 2124 - ST T 851 470 420 Better transvers properties, good strengths and creep resistance at higher temperatures - for application between 120 – 175 °C.

Typical mechanical properties of alloy 6061 1. 0 Mg-0. 6 Si-0. 3 Cu-0. 2 Cr ; E = 68. 9 GPa; ρ = 2. 70 g/ccm Temper Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 500 mil cycles Bare 6061 0 124 55 25 62 T 4 241 145 22 97 T 6 310 276 12 97 Alclad 6061 0 117 48 25 - T 4 228 131 22 -

Minimal mechanical properties of alloy 6056 1. 0 Si-0. 9 Mg-0. 8 Cu-0. 7 Mn-0. 25 Cr-0. 2 Ti+Zr; ρ = 2. 72 g/ccm Temper Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 500 mil cycles Thin extrusions - L T 4511 355 245 16 - T 6511 380 360 10 - T 78511 360 335 10 - Bare sheet - LT T 4 265 135 18 - T 78 340 315 8 -

Mechanical properties of alloy 7050 6. 22 Zn-2. 3 Mg-2. 3 Cu-0. 12 Zr; E = 70. 3 GPa; ρ = 2. 83 g/ccm Direction Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 10 mil. cycles Minimum properties - Die forgings T-736 (T-74), thickness up to 50 mm L 496 427 7 - L-T 469 386 5 - Minimum properties – Hand forgings T 73652, thickness up to 50 mm L 496 434 - - L-T 490 421 - - 11 170 -300 Typical properties – Plate T 73651 L 510 455 Typical properties – Forgings T 73652 L 524 455 15 210 -275

Typical mechanical properties of alloy 7075 5. 6 Zn-2. 5 Mg-1. 6 Cu-0. 23 Cr; E = 71. 0 GPa; ρ = 2. 80 g/ccm Temper Tensile strength MPa Yield strength MPa Elongation % Fatigue strength MPa At 500 mil. cycles Bare 7075 0 228 103 17 - T 6, T 651 572 503 11 159 T 73 503 434 - 159 Alclad 7075 0 221 97 17 - T 6, T 651 524 462 11 -

Use of aluminum-lithium alloys in commercial aircraft

Typical mechanical properties of aluminium- lithium alloys Temper Direction Tensile strength MPa Yield strength MPa Elongation % Alloy 2090: 2. 7 Cu-2. 2 Li-0. 12 Zr; E = 76 GPa, ρ = 2. 59 g/ccm L 527 3 LT 505 503 6 45° T 83 (near peak aged) 530 440 Alloy 8090: 2. 45 Li-1. 3 Cu-0. 95 Mg-0. 12 Zr; E = 77 GPa; ρ = 2. 55 g/ccm L T 8 X (near peak aged) 480 400 4. 5 LT 465 395 5. 5 45° 400 325 7. 5

Casting aluminum alloys • Designation (in addition to EN) – Often used system (Aluminum Association - USA): three digit designation - the first digit indicates a main alloying element • 1 XX • 2 XX • 3 XX • • 4 XX 5 XX 7 XX 8 XX 99, 0 % Al Al - Cu Al - Si - Mg Al - Si - Cu - Mg Al - Si Al – Mg Al - Zn Al – Sn A letter ahead of designation marks alloys with the same content of main alloying elements but with different content of impurities or micro alloying elements. (e. g. 201 - A 201, 356 - A 356, 357 - A 357) Additional digit. 0 means shape casting, digit. 1 or. 2 ingots

• Typical castings in aircraft structures Al – front body of engine 32 kg - D=700 mm Al – casing - 1, 3 kg 470 x 190 x 170 mm Al- steering part - 1, 1 kg 390 x 180 x 100 mm Al – pedal - 0, 4 kg 180 x 150 x 100 mm

• General characteristics – Micro and macro structures of metal are influenced by conditions of metal solidification – quantity of nuclei, temperature interval of solidification, cooling rate … A fine, equiaxed grain structure is normally desired in aluminum casting (Al-Ti or Al-Ti-B alloys are most widely used grain refiners) – Mechanical properties are influenced by existence of casting defects – porosity, inclusions (mainly oxides), shrinkage voids …. – Alloys – heat treatable , non heat treatable – Mechanical properties are mostly lower comparing wrought alloys of the similar chemical composition – High quality aircraft casting need careful metallurgical processing of liquid metal • • • Degassing – hydrogen elimination (hydrogen causes porosity) Grain refinement and modification for better mechanical properties Filtration for inclusions removing

Solubility of hydrogen in aluminum Alloy Al-7 Si – the effect of grain refinement During solidification - dissolved hydrogen can precipitate and form voids.

Dendritic microstructure of hypoeutectic alloy Al. Si 10 Mg – sand casting wall thickness 2 mm wall thickness 10 mm There is direct relation between mechanical properties and dendrite arm spacing (DAS) → different properties in different portions of casting

• Alloys of Al–Cu system - Composition 4 – 6 % Cu - Copper substantially improves strength and hardness in the as-cast and heat- treated conditions - Copper generally reduces corrosion resistance and, in specific compositions stress corrosion susceptibility - Copper also reduces hot tear resistance and decreases castability - Main advantage: high strength up to 300 °C - Basic alloys • ČSN 424351, 201, AL 7 • 242, A 242 • B 295 - Application: Smaller , simple, high-strength castings for service at higher temperatures (cylinder heads, pistons, pumps, aerospace housings, aircraft fittings)

• Alloys of Al–Si + (Mg, Cu, Ni) system – – The most important alloys for aircraft castings Silicon improves casting characteristics (fluidity, hot tear resistance, feeding), Si content depends on casting methods • • • – – Sand plaster molds, investment casting 5 -7% Si Permanent molds 7 -9% Si Die casting 8 -12% Si Alloys containing Mg are heat treatable, hardening phase is Mg 2 Si Alloys Al-Si with alloying elements Mg and Cu have after heat treatment high mechanical properties but lower plasticity and corrosion resistance Ni is alloying element in hypereutectic alloys for service at higher temperatures (e. g. engine pistons) Strength and ductility can be improved using modification for refinement of eutectic phases • Principal – addition small quantities of Na or Sr into liquid metal before casting • Results – increased tensile strength (40 %), impact strength (up to 400 %), ductility (2 x) – Mechanical properties can be improved also due to grain refinement buy rapid cooling in permanent metal molds

Representative aluminum alloys – sand casting Mechanical properties Alloy Temper Rm HB MPa Rp 0, 2 MPa A % A 201. 0 Al. Cu 4, 5 Ag 0, 7 Mg 0, 25 Mn 0, 3 T 7 496 448 - 6 A 356. 0 Al. Si 7 Mg 0, 35 F T 61* 159 278 283 83 207 75 90 6 6 10 A 357. 0 Al. Si 7 Mg 0, 55 Zn. Be 0, 05 T 6* 317 359 248 290 85 100 3 5 * permanent mold casting F as cast. 0 shape casting

Magnesium Alloys

General characteristics of Mg alloys • Pure magnesium – Hexagonal crystal lattice – ρ=1, 74 g/cm³ , Rm=190 MPa, Rp 0, 2=95 MPa – Used in metallurgy (alloying element in Al alloys, titanium metallurgy, ductile iron metallurgy). – Not used for structural purposes – magnesium alloys have better utility values • Advantages of Mg alloys – – – Low density (ρ = 1, 76– 1, 99 g/cm³ ) → high specific strength (Rm/ ρ) Comparing Al alloys, lower rate of strength decrease in relation with temperature Lower notch sensitivity and higher specific strength at vibrating loads High damping capacity (influence of low modulus of elasticity ~47 GPa) High specific bending stiffness (higher to 50 % comparing steel, to 20 % comparing Al) → high resistance against buckling – High specific heat → minor temperature increasing at short time heating – Very good machinability – Applicability – most alloys up to 150 °C, some of them up to 350 °C.

• Shortcomings of Mg alloys – High reactivity at increased temperatures • Above 450 °C rapid oxidation, above 620 °C ignition (fine chips, powder) • Melting and casting – protection against oxidation (chlorides, fluorides, oxides Mg, powder sulfur, gases SO 2, CO 2). – Lower corrosion resistance , generally difficult anti-corrosion protection • Corrosion environment (air, sea water), impurities Fe, Cu, Ni forming intermetallic compounds • Electrochemical corrosion – in contact with the most of metals (Al alloys, Cu alloys, Ni alloys, steel) – Low formability at room temperature - most alloys cannot be formed without heating – After forming – high strength anisotropy along and crosswise deformation –→ differences 20 to 30 %. – Low shear strength and notch impact strength – Low wear resistance – Low diffusion rate during heat treatment → longtime processes , artificial aging is necessary at precipitation hardening – Relatively difficult joining – possible electrochemical corrosion, limited weldability (hot cracking, weld porosity, possible welding techniques - inert gas welding, spot welding)

Designation of Mg alloys • Designation according to EN 2032 -1 – Wrought alloys MG-PXXXXX – Casting alloys MG-CXXXXX – In numerical designation, one or two digits represent one or two main alloying elements according to their weight percentage. The third digit is zero, the last two digits represent serial number. (1 - Al, 2 – Si, 3 – Zr, 4 – Ag, 5 – Th, 6 – rare earth, 7 – Y, 8 – Zn, 9 - other) • More common designation - according to ASM: – – – – Series AZ (alloying elements Al, Zn) Series AM (Al, Mn) Series QE (Ag, RE - rare earth ) Series ZK (Zn, Zr) Series AE (Al, RE) Series WE (Y, RE) Series HM, HZ, HK (Th, Mn, Zr) – high temperature alloys Two first digits – percentage of alloying elements

Basic wrought Mg alloys • Mg-Al-Zn (AZ)alloys – The most common alloys in aircraft industry, applicable up to 150 °C – Composition – 3 to 9 % Al, 0. 2 to 1. 5 % Zn, 0. 15 to 0. 5 % Mn – Increasing Al content → strength improvement , but growth of susceptibility to stress corrosion – Zn → ductility improvement – (Cd + Ag) as Zn replacement → high strength up to 430 MPa – Precipitation hardening → strength improvement + decrease of ductility – The most common alloy for sheet and plates – AZ 31 B (applicable to 100 °C) Alloy Composition Semi-product Rm, MPa Rp 0. 2, MPa Ductility, % AZ 31 B-F 3. 0 Al-1. 0 Zn bars, shapes 260 200 15 AZ 61 A-F 6. 5 Al-1. 0 Zn bars, shapes 310 230 16 AZ 80 A-T 5 8. 5 Al-0. 5 Zn bars, shapes 380 240 7 AZ 82 A-T 5 8. 5 Al-0. 5 Zn bars, shapes 380 275 7 AZ 31 B-H 24 3. 0 Al-1. 0 Zn sheet, plates 290 220 15

• Mg-Zn-Zr alloys (ZK) – – – Zn → strength improvement Zr → fine grain → improvement of strength, formability and corrosion resistance Better plasticity after heat treatment Alloying with RE a Cd → tensile strength up to 390 MPa Application up to 150 °C • Mg-Mn alloys (M) – Good corrosion resistance, hot formability, weldability – Not hardenable → lower strength Alloy Composition Semi-product Rm, MPa Rp 0. 2, MPa Ductility, % ZK 60 A-T 5 5. 5 Zn-0. 45 Zr bars, shapes 365 305 11 M 1 A-F 1. 2 Mn bars, shapes 255 180 12

• Mg-Th-Zr (HK) – High temperature alloys – Example: alloy HK 31 A - service temperature 315 to 345 °C • Mg-Th-Mn (HM) – Medium strength – Creep resistance → service temperature up to 400 °C • Mg-Y-RE (WE) – Hardenability, formability, good weldability – Y → strength after hardening, Nd → heat resistance, Zr → grain refinement – Application to 250 °C alloy composition semi-product Rm, MPa Rp 0. 2, MPa ductility, % HM 21 A-T 8 2. 0 Th-0. 6 Mn sheet, plates 235 130 11 HK 31 A-H 24 3. 0 Th-0. 6 Zr sheet, plates 255 160 9 Mg-RE (WE) 8. 4 Y-0. 5 Mn 0. 1 Ce-0. 35 Cd bars, shapes 410 360 4

Cast magnesium alloys • Basic systems – – Mg-Al-Mn with or without Zn (AM, AZ) Mg-Ag-RE (QE) Mg-Y-RE (WE) Mg-Zn-Zr with or without rare earth (ZK, ZE, EZ) • Pressure die castings - alloys AZ → excellent castability, good corrosion resistance in sea water - aloys AM → good castability, corrosion resistance, better ductility and lower strength - castings are not heat treated • Sand permanent mold castings - used mostly in heat treated state

• Typical properties of several cast magnesium alloys alloy composition product Rm MPa Rp 0. 2 MPa ductility % AM 60 A-F 6. 0 Al-0. 13 Mn pressure die casting 205 115 6 AZ 91 A-F 9. 0 Al-0. 13 Mn 0. 7 Zn pressure die casting 230 150 3 AZ 63 A-T 6 6. 0 Al-3. 0 Zn 0. 15 Mn sand casting 275 130 5 AZ 91 C-T 6 8. 7 Al-0. 13 Mn 07 Zn sand casting 275 145 6 AZ 92 A-T 6 9 Al-2 Zn-0. 1 Mn sand casting 275 150 3 AM 100 AT 61 10. 0 Al-0. 1 Mn sand casting 275 150 1 QE 22 A-T 6 2. 5 Ag-2. 1 RE-0. 7 Zr sand casting 260 195 3 WE 43 A-T 6 4. 0 Y-3. 4 RE-0. 7 Zr sand casting 250 165 2 ZK 61 A-T 6 6. 0 Zn-0. 7 Zr sand casting 310 195 10 EZ 33 A-T 5 3. 3 RE-2. 7 Zn-0. 6 Zr sand casting 160 110 2

Titanium Alloys

Characteristics of titanium and titanium alloys • Pure titanium - 2 modifications – αTi – to 882 °C, hexagonal lattice – βTi – 882 to 1668°C, cubic body centered lattice – With alloying elements, titanium forms substitution solid solutions α and β • Commercially pure titanium can be used as structural material in many applications, but Ti alloys have better performance. • Basic advantages of Ti – Lower density comparing steel ( ρ = 4. 55 g/cm³) – High specific strength at temperatures 250 – 500 °C, when alloys Al, Mg already cannot be used – High strength also at temperatures deep below freezing point – Good fatigue resistance (if the surface is smooth, without grooves or notches) – Excellent corrosion resistance due to stabile layer of Ti oxide – Good cold formability, some alloys show superplasticity – Low thermal expansion => low thermal stresses

• Shortages of titanium – High manufacturing costs => high prices (~8 x higher comparing Al) – Chemical reactivity above 500 °C – intensive reactions with O 2, H 2, N 2, with refractory materials of furnaces and foundry molds => brittle layers, which are removed with difficulties – Lower modulus of elasticity comparing steel ( E = 115 GPa against 210 GPa) – Poor friction properties, tendency for seizing – Poor machinability (low thermal conductivity → local overheating, adhering on tool, above 1200 °C danger of chips and powder ignition. – Welding problems (reactivity with atmospheric gases => welding in inert gas, diffusion welding, laser beam welding, electron beam welding) – Special manufacturing methods (vacuum melting and heat treating, manufacture of castings in special molds – graphite molds and/or ceramic molds with a layer of carbon, hot isostatic pressing - HIP) • Preferred use of titanium alloys – – If strength and temperature requirements are too high for Al or Mg alloys At conditions, when high corrosion resistance is required At conditions, when high yield strength and lower density comparing steel are required Compressor discs, vanes and blades, beams, flanges, webs, landing gears, pressure vessels, skin up to 3 M, tubing… – Increasing usage (Boeing 727 – 295 kg, Boeing 747 – 3400 kg)

Classification of titanium alloys • Alloying elements – α – stabilizers (Al, O, N, C) – stabilize solid solution α and enlarge zone of its existence – β – stabilizers – stabilize solid solution β, decrease temperature α-β transformation • β stabilizers forming eutectoid phase (Si, Cr, Mn, Fe, Co, Ni, Cu) • β stabilizers isomorphic (V, Mo, Nb, Ta) – Neutral elements (Sn, Zr) – only small influence on the α-β transformation Phase diagrams of Ti with different stabilizers (solid state)

• Classification of alloys according to microstructure after annealing – – – α alloys – microstructure consists of homogeneous solid solution α pseudo α alloys (solid solution α + 5% solid solution β at most) α+β alloys – microstructure consists of mixture solid solutions α and β β alloys – microstructure consists of homogeneous solid solution β pseudo β alloys (solid solution β + small amount solid solution α) Alloys consisting of intermetallic compouds • Classification according to usage – Wrought alloys – Cast alloys • Designation of titanium alloys according to EN 2032 -1 • • Wrought material TI-PXXXXX Cast material TI-CXXXXX Product of powder metallurgy TI-RXXXXX First two digits represent main alloying elements (1 -Cu, 2 -Sn, 3 -Mo, 4 -V, 5 -Zr, 6 -Al, 7 -Ni, 8 -Cr, 9 -others), TI-P 64005 (Ti-6 Al-4 V), TI-P 99 XXX (pure titanium) • Designation according to basic chemical composition (e. g. Ti-6 Al-4 V)

Properties of important wrought titanium alloys Alloy Temper Rm, MPa Rp 0. 2, MPa Elongation, % E, GPa α and pseudo α Ti-5 Al-2, 5 Sn annealed 790 - 860 760 - 807 16 110 Ti-5, 6 Al annealed 875 750 8 - Ti-11 Sn-1 Mo-2, 2 Al 5 Zr-0, 2 Si annealed 1000 - 1100 900 - 990 15 114 α+β Ti-3 Al-2, 5 V annealed 620 - 690 520 - 585 20 107 Ti-6 Al-4 V hardened annealed 1170 900 - 990 1100 830 - 920 10 14 114 Ti-6 Al-2 Sn-2 Zr-2 Cr 2 Mo-0, 25 Si hardened 1275 1140 11 122 pseudo β and β Ti-10 V-2 Fe-3 Al hardened 1170 - 1275 1100 - 1200 10 112 Ti-15 V-3 Cr-3 Al-3 Sn hardened 1095 - 1335 985 - 1245 6 - 12 -

Cast titanium alloys • Comparison with wrought alloys – – Similar chemical composition Higher content of impurities, specific casting structure and defects (e. g. porosity) Lower ductility and fatigue life Often better fracture toughness • Manufacture of shape castings – Good casting properties (fluidity, mold filling) – Hydrogen absorption, porosity – Vacuum melting, special molds, hot izostatic pressing of castings (HIP) • HIP – heating close to solidus + pressure of inert gas (elimination and welding of voids due to plastic deformation) – conditions 910 to 965 °C/100 MPa/2 h. Examples of cast alloys Alloy Heat Treatment Rm, MPa Rp 0. 2, MPa A 5 , % Ti-6 Al-4 V stress relief annealing 880 815 5 Ti-6 Al-2 Sn-4 Zr-2 Mo 970°C/2 h + 590°C/8 h 860 760 4 Ti-15 V-3 Cr-3 Al-Sn 955°C/1 h + 525°C/12 h 1120 1050 6

Composite Materials

Most composites consist of a bulk material (the ‘matrix’), and a reinforcement, added primarily to increase the strength and stiffness of the matrix. This reinforcement is usually in fibre form. Today, the most common man-made composites can be divided into three main groups: Polymer Matrix Composites (PMC’s) – These are the most common and will be discussed here. Also known as FRP - Fibre Reinforced Polymers (or Plastics) – these materials use a polymer-based resin as the matrix, and a variety of fibres such as glass, carbon and aramid as the reinforcement. Metal Matrix Composites (MMC’s) - Increasingly found in the automotive industry, these materials use a metal such as aluminium as the matrix, and reinforce it with fibres such as silicon carbide (Si. C). Ceramic Matrix Composites (CMC’s) - Used in very high temperature environments, these materials use a ceramic as the matrix and reinforce it with short fibres, or whiskers such as those made from silicon carbide and boron nitride (BN).

Polymer fibre reinforced composites Common fiber reinforced composites are composed of fibers and a matrix. Fibers are the reinforcement and the main source of strength while the matrix 'glues' all the fibres together in shape and transfers stresses between the reinforcing fibres. Sometimes, fillers or modifiers might be added to smooth manufacturing process, impart special properties, and/or reduce product cost.

Polymer matrix composites • The properties of the composite are determined by: - The properties of the fibre - The properties of the resin - The ratio of fibre to resin in the composite (Fibre Volume Fraction) - The geometry and orientation of the fibres in the composite Properties of unidirectional composite material

Main resin systems • Epoxy Resins The large family of epoxy resins represent some of the highest performance resins of those available at this time. Epoxies generally out-perform most other resin types in terms of mechanical properties and resistance to environmental degradation, which leads to their almost exclusive use in aircraft components • Phenolics Primarily used where high fire-resistance is required, phenolics also retain their properties well at elevated temperatures. • Bismaleimides (BMI) Primarily used in aircraft composites where operation at higher temperatures (230 °C wet/250 °C dry) is required. e. g. engine inlets, high speed aircraft flight surfaces. • Polyimides Used where operation at higher temperatures than bismaleimides can stand is required (use up to 250 °C wet/300 °C dry). Typical applications include missile and aero-engine components. Extremely expensive resin.

Fabric types and constructions • Unidirectional fabrics – The majority of fibres run in one direction only, a small amount of fibre may run in other directions to hold the primary fibres in position – Prepreg unidirectional tape- only the resin system holds the fibres in place – The best mechanical properties in the direction of fibres • Basic woven fabrics – Plain -Each warp fibre passes alternately under and over each weft fibre. The fabric is symmetrical, with good stability. However, it is the most difficult of the weaves to drape. – Twill - One or more warp fibres alternately weave over and under two or more weft fibres in a regular repeated manner. Superior wet out and drape, smoother surface and slightly higher mechanical properties

Fabric types and constructions – cont. – Basket -Basket weave is fundamentally the same as plain weave except that two or more warp fibres alternately interlace with two or more weft fibres. An arrangement of two warps crossing two wefts is designated 2 x 2 basket. It is possible to have 8 x 2, 5 x 4, etc. Basket weave is flatter, and, through less crimp, stronger than a plain weave, but less stable. • Hybrid fabric – A hybrid fabric will allow the two fibres to be presented in just one layer of fabric. – Carbon / Aramid - The high impact resistance and tensile strength of the aramid fibre combines with high the compressive and tensile strength of carbon. – Aramid / Glass - The low density, high impact resistance and tensile strength of aramid fibre combines with the good compressive and tensile strength of glass, coupled with its lower cost. – Carbon / Glass - Carbon fibre contributes high tensile compressive strength and stiffness and reduces the density, while glass reduces the cost.

Properties of composites • UD laminate Tensile strength, MPa Properties directionally dependent Angle between fibers and stress, ° • Quasi-isotropic laminate Properties nearly equal in all directions

Properties of epoxy UD prepreg laminates Fibre fracture volume typical for aircraft structures Prepreg Fabrics and fibres are pre-impregnated by the materials manufacturer with a pre-catalysed resin. The catalyst is largely latent at ambient temperatures giving the materials several weeks, or sometimes months, of useful life. To prolong storage life the materials are stored frozen (e. g. -20°C). High fibre contents can be achieved, resulting in high mechanical properties.

Fiber metal laminates • Consist of alternating thin metal layers and uniaxial or biaxial glass, aramid or carbon fiber prepregs

Fibre metal laminates • Developed types - - ARALL - Aramid Reinforced ALuminium Laminates (TU-DELFT) GLARE - GLAss REinforced (TU-DELFT) CARE - CArbon REinforced (TU-DELFT) Titanium CARE (TU-DELFT) HTCL - Hybrid Titanium Composite Laminates (NASA) CAREST – CArbon REinforced Steel (BUT - IAE) T i. Gr – Titanium Graphite Hybrid Laminate (The Boeing Company) • Advantages Fibre metal laminates produce remarkable improvements in fatigue resistance and damage tolerance characteristics due to bridging influence of fibres. They also offer weight and cost reduction and improved safety, e. g. flame resistance. They can be formed to limited grade.

Standard FML configurations Type Configuration Metal alloy Prepreg constituents Prepreg orientation ARALL 2 2/1 – 6/5 2024 -T 3 Aramid-epoxy unidirectional ARALL 3 2/1 – 6/5 7475 -T 76 Aramid-epoxy unidirectional GLARE 1 2/1 – 6/5 7475 -T 76 Glass-epoxy unidirectional GLARE 2 2/1 – 6/5 2024 -T 3 Glass-epoxy unidirectional GLARE 3 2/1 – 6/5 2024 -T 3 Glass-epoxy Cross-ply GLARE 4 2/1 – 6/5 2024 -T 3 Glass-epoxy Cross-ply /unidirectional

Mechanical properties of FML Laminate Metal thickness mm Prepreg thickness mm Tensile strength MPa Yield strength MPa E Density GPa g/ccm ARALL 1 0. 3 0. 22 897 535 67. 5 2. 16 ARALL 2 0. 3 0. 22 849 411 68. 3 2. 16 GLARE 1 0. 3 0. 25 1494 530 62. 2 2. 42 GLARE 2 0. 25 1670 416 60. 9 2. 34 0. 3 0. 25 1449 406 63. 0 2. 42 0. 4 0. 25 1295 399 64. 5 2. 47 0. 3 0. 25 849 382 51. 3 2. 42 GLARE 3

Fatigue resistance of FML comparing to 2024 alloy

GLARE fire resistance comparing to 2024 alloy

Fiber metal laminates - application AIRBUS A 380 Panels of fuselage upper part – 470 m² , GLARE 4 Maximum panel dimensions 10. 5 x 3. 5 m Weight saving - 620 kg Adhesive bonded stringers from 7349 alloy

Sandwich materials • Structure – consists of a lightweight core material covered by face sheets on both sides. Although these structures have a low weight, they have high flexural stiffness and high strength. • Skin (face sheet) – Metal (aluminium alloy) – Composite material • Core – Honeycomb – metal or composite (Nomex) – Foam – polyurethan, phenolic, cyanate resins, PVC • Applications – aircraft flooring, interiors, naccelles, winglets etc. Sidewall panel for Airbus A 320

Effectivness of sandwich materials

List of problems (light alloys) What are the main advantages of aluminium alloys for applications in aircraft structures? What numerical designation system is used for identification of wrought aluminium alloy? What is meaning of the first digit? What groups of wrought aluminium alloys are usually used in aircraft structures? Explain the designation of the following alloys: - 2024 T 4 - 7075 T 6 - Alclad 2219 Why is sheet from 2 xxx alloys often clad with pure aluminium? What group of wrought aluminium alloys exhibits the best mechanical properties? Compare alloys 6056 and 7050! What are the main advantages and limitations of Al-Li alloys comparing to other Al alloys? What is a common value of aluminium alloys elastic modulus in tension? Recommend the alloys for aircraft skin! Why are Mg alloys valuable for aerospace application? What is damping capacity of magnesium alloys? What are the main reasons for using of titanium alloys in airframe and engine structures? What titanium alloy is the most widely used? Compare the specific tensile strength and specific tensile modulus of 2090 and Ti-6 Al-4 V alloys! - (Specific value = value/density)

List of problems (composite and sandwich materials) What is composite material? What are advantages of composites comparing to metals? What is prepreg? What are common types of fibres? What fibres have the highest specific tensile strength and specific tensile modulus? ( the specific property is the ratio value/density ) What is main role of matrix? What are main advantages of epoxy, phenolic and bismaleimide (polyimide) matrices? What are main advantages of using prepregs? How the fibre orientation influences resulting mechanical properties of a composite? What are typical tensile properties of epoxy prepregs UD laminates along and across fibres? What is structure of sandwich material? What are main advantages of sandwich panels compared to solid panels? What materials are usually used for sandwich skins and core?