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AIRCRAFT MATERIALS

AIRCRAFT MATERIALS. Review of the Course AIRCRAFT MATERIALS. AIRCRAFT MATERIALS. Basic requirements High strength and stiffness Low density => high specific properties e.g. strength/density, yield strength/density, E/density High corrossion resistance

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AIRCRAFT MATERIALS

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  1. AIRCRAFT MATERIALS Review of the Course AIRCRAFT MATERIALS

  2. AIRCRAFT MATERIALS • Basic requirements • 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

  3. Development ofaircraft materialsfor airframe structures other materials Relative share of structural materials composites Ti alloys Mg alloys other Al alloys pure AlZnMgCu alloys AlCuMg alloys wood pure AlCuMg alloys new Al alloys steel Year

  4. Structural materials on small transport aeroplane

  5. Development of compositeaerospace applications over the last 40 years

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

  7. Structural materials on Eurocopter

  8. Comparison of mechanical performance of composite materials and light metals

  9. AluminiumAlloys

  10. 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 solutionswithalloying 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 • alloying atom > aluminium atom • pure aluminium • alloying atom < aluminium atom

  11. 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 (Al2O3) 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, Al-steel Characteristics of aluminium alloys

  12. Wrought alloys AL-PXXXX(A) designation basic alloying element 1XXX – pure aluminium 2XXX - copper (Cu) 3XXX - manganese (Mn) 4XXX - silicon (Si) 5XXX - magnesium (Mg) 6XXX - Mg + Si 7XXX - zinc (Zn) 8XXX - other (eg. Li) Casting alloys AL-CXXXXX designation basic alloying element 1XXXX - > 99.0 % Al 2XXXX - Cu 3XXXX - Si-Mg - Si-Cu - Si-Cu-Mg 4XXXX - Si 5XXXX - Mg 7XXXX - Zn 8XXXX - Sn Designation of aluminium alloys according to EN

  13. Important wrought aluminium alloys for aircraft structures • 2XXX (Al-Cu, Al-Cu-Mg) - high strength, lower corrosion resistance 2014 (0.8Si, 4.4Cu, 0.8Mn, 0.5Mg) 2017 (0.5Si, 4Cu, 0.7Mn, 0.6Mg) 2024 (4.4Cu, 0.6Mn, 1.5Mg) 2024Alclad (with the surface layer of Al) 2027 (4.4Cu, 0.9Mn, 1.3Mg, 0.2Zn, 0.05Cr, 0.25Ti) 2124 (4.4Cu, 0.6Mn, 1.5Mg, 0.1V) 2219 (6.3Cu, 0.3Mn, 0.1V) • 6XXX (Al-Mg-Si) -comparing to 2XXX - lower strength, better ductility and corrosion resistance 6013 (0.8Si, 0.8Cu, 0.50Mn, 1.0Mg) 6061 (0.6Si, 0.28Mn, 1.0Mg, 0.2Cr) 6061Alclad 6056 (1.0Si, 0.9Mg, 0.8Cu, 0.7Mn, 0.25Cr, 0.2Ti+Zr) • 7XXX (Al-Zn-Mg-Cu) – the highest strength, lower ductility, notch sensitivity 7050 (2.3Cu, 2.2Mg, 0.12Zr, 6.2Zn) 7075 (1.6Cu, 2.5Mg, 0.13Cr, 5.6Zn), 7075Alclad 7175 (1.6Cu, 2.5Mg, 0.23Cr, 5.6Zn) 7475 (1.6Cu, 2.2Mg, 0.22Cr, 5.7Zn)

  14. Most important tempers aluminium alloys • O - anealing • W - solution treating + quenching (non stabil state) • H - strain-hardening (strength is increased only due to cold working) • T3 - solution treating + quenching + cold working + room temperature aging • T351 - solution treating + quenching + stress relief due to controlled stretching + room temperature aging • T4 - solution treating + quenching + room temperature aging • T6 - solution treating + quenching + artificial aging • T651- solution treating + quenching + stress relief due to controlled streching + artificial aging • T7 - solution treating + quenching + artificial overaging • T73 - solution treating + quenching + artificial overaging for the best stress corrosion resistance • T8 - solution treating + quenching + cold working + artificial aging

  15. Reference aluminium alloys in airframe structure

  16. Typicalmechanical properties of alloy 20144.4Cu-0.8Si-0.8Mn-0.5Mg, E = 72.4 GPa , ρ = 2 .77 g/ccm

  17. Typicalmechanical properties of alloy 20244.4Cu-1.5Mg-0.6Mn, E = 72.4 GPa , ρ = 2 .77 g/ccm

  18. Typicalmechanical properties of alloy 21244.4Cu-1.5Mg-0.6Mn, E = 72.4 GPa , ρ = 2 .77 g/ccm

  19. Typicalmechanical properties of alloy 60611.0Mg-0.6Si-0.3Cu-0.2Cr ; E = 68.9 GPa; ρ = 2 .70 g/ccm

  20. Minimalmechanical properties of alloy 60561.0Si-0.9Mg-0.8Cu-0.7Mn-0.25Cr-0.2Ti+Zr; ρ = 2 .72 g/ccm

  21. Mechanical properties of alloy 70506.22Zn-2.3Mg-2.3Cu-0.12Zr; E = 70.3 GPa; ρ = 2 .83 g/ccm

  22. Typical mechanical properties of alloy 70755.6Zn-2.5Mg-1.6Cu-0.23Cr; E = 71.0 GPa; ρ = 2 .80 g/ccm

  23. Use of aluminum-lithium alloys in commercial aircraft

  24. Typical mechanical properties of aluminium- lithium alloys

  25. 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 1XX  99,0 % Al 2XX Al - Cu 3XX Al - Si - Mg Al - Si - Cu Al - Si - Cu - Mg 4XX Al - Si 5XX Al – Mg 7XX Al - Zn 8XX 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 - A201, 356 - A356, 357 - A357) Additional digit.0means shape casting, digit.1or.2 ingots

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

  27. 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

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

  29. Dendritic microstructure of hypoeutectic alloy AlSi10Mg – 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

  30. 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, A 201, AL 7 • 242, A242 • B295 • Application: • Smaller , simple, high-strength castings for service at higher • temperatures (cylinder heads, pistons, pumps, aerospace housings, aircraft • fittings)

  31. 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 and 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 isMg2Si Alloys Al-Si with alloying elements Mg andCuhave 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 orSrinto liquid metal before casting Results – increased tensile strength (40 %), impact strength (up to 400 %), ductility (2x) Mechanical properties can be improved also due to grain refinement buy rapid cooling in permanent metal molds

  32. Representative aluminum alloys – sand casting

  33. Magnesium Alloys

  34. General characteristicsof Mg alloys • Pure magnesium • Hexagonal crystal lattice • ρ=1,74 g/cm³ , Rm=190 MPa, Rp0,2=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 ~47GPa) • High specific bending stiffness (higher to 50 % comparing steel, to 20 % comparing Al) → highresistance 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.

  35. 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, oxidesMg, powder sulfur, gases SO2, CO2). • Lower corrosion resistance, generally difficult anti-corrosion protection • Corrosion environment (air, sea water), impurities Fe, Cu, Ni forming intermetallic compounds • Electrochemicalcorrosion – in contact with the most of metals (Al alloys, Cu alloys, Ni alloys, steel) • Low formabilityat 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)

  36. 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: SeriesAZ (alloying elements Al, Zn) SeriesAM (Al, Mn) SeriesQE (Ag, RE - rare earth) SeriesZK (Zn, Zr) SeriesAE (Al, RE) SeriesWE (Y, RE) SeriesHM, HZ, HK (Th, Mn, Zn, Zr) – high temperature alloys Two first digits– percentage of alloying elements Designation of Mg alloys

  37. 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 → ductilityimprovement (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 – AZ31B (applicable to 100 °C)

  38. Mg-Zn-Zralloys (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-Mnalloys (M) Good corrosion resistance, hot formability, weldability Not hardenable→lower strength

  39. Mg-Th-Zr (HK) High temperature alloys Example: alloy HK31A - 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

  40. Cast magnesiumalloys 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 and permanent mold castings - used mostly in heat treated state

  41. Typical properties of several cast magnesium alloys

  42. Titanium Alloys

  43. 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 solutionsαandβ • 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

  44. Shortages of titanium • High manufacturing costs =>high prices (~8x higher comparing Al) • Chemical reactivity above 500 °C – intensive reactions with O2, H2, N2, 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 highfor 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 3M, tubing… • Increasing usage (Boeing 727 – 295 kg, Boeing 747 – 3400 kg)

  45. Classification of titanium alloys • Alloying elements • α – stabilizers (Al, O, N, C) – stabilize solid solutionαand enlarge zone of its existence • β – stabilizers – stabilize solid solutionβ, decrease temperatureα-β transformation • βstabilizers forming eutectoid phase (Si,Cr, Mn, Fe, Co, Ni, Cu) • βstabilizersisomorphic (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)

  46. Classification of alloys according to microstructure after annealing • αalloys – microstructure consists of homogeneous solid solution α • pseudoαalloys (solid solution α + 5% solid solution β at most) • α+βalloys – microstructure consists of mixture solid solutions αandβ • βalloys – microstructure consists of homogeneous solid solution β • pseudoβalloys (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-P64005 (Ti-6Al-4V), TI-P99XXX (pure titanium) • Designation according to basic chemical composition (e.g. Ti-6Al-4V)

  47. Properties of important wrought titanium alloys

  48. 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

  49. Composite Materials

  50. 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 (SiC). 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 whiskerssuch as those made from silicon carbide and boron nitride (BN).

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