Thin Products < 75 mm 7055-T7751. Strength (MPa) 500. Thick Products mm Year First Used in Aircraft

Transcription

1 Strength and (Extrinsic) Corrosion Resistance Improvements in New 7XXX-Series Alloys - Relative to 7075-T651 All Alloys Still Need Corrosion Protection Schemes Corrosion Resistance Low Medium High Thin Products < 75 mm 7055-T7751 Yield 550 Strength (MPa) T T T T T T7451 Thick Products mm 705-T Year First Used in Aircraft 2005 Filename.ppt 1

2 Engineering Properties of Interest Strength Fracture toughness Fatigue properties Crack initiation Crack growth Corrosion susceptibility Pitting corrosion Intergranular / Exfoliation corrosion Stress corrosion cracking Low residual stresses in engineered products Amenability to manufacturing processes (high speed machining, friction stir welding, etc.) Filename.ppt 2

3 Composition (wt. %) of 7XXX Series Aluminum Alloys Zn Mg Cu Ti+Zr 0.25 max Zr Cr < 0.04 < 0.04 < 0.04 < 0.04 < 0.04 Si, max Fe, max Mn, max Ti, max Ti+Zr 0.25 max Others, each, max Others, total, max Al Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Filename.ppt 3

4 Design Allowable Values A-basis: 5% confidence that at least % of all future material strength will be equal to or higher than this value. B-basis: 5% confidence that at least 0% of all future material strength will be equal to or higher than this value. Typical: The average strength. Half of all future material strength will be higher, half won't. S-basis: Value derived with little test data (hence no statistical assurance), but which is related to a minimum strength requirement in the material specification. Acceptance test results for future material lots must be higher than the specification requirements. Filename.ppt 4

5 Properties of 1 in. and 3 in. thick aluminum plate alloys (* indicates S-basis values, (a) indicates values not yet approved) in T T T T T T751 Properties S Min. A Min. A Min. S Min. A Min. A Min. A Min. F tu (ksi) F ty (ksi) e (%) - S basis K Ic (ksisqrt in.) L L L L-T * 64* (a) (a) LT LT LT T-L Exfoliation EB EB EB EB EB Note - (*) indicates S -basis values and (a) indicates data calculated by supplier, but not approved in in in in T T T T T7651 Properties S Min. A Min. A Min. A Min. A Min. L * F tu (ksi) LT * 73 72* 71 2* 2 2 ST * L * 63 62* F ty (ksi) LT * 63 62* 62 75* ST * 5 67* L e (%) - S basis LT ST L-T K Ic (ksisqrt in.) T-L S-L Exfoliation EB EB EB EB EB SCC Note - (*) indicates S -basis values Filename.ppt 5

6 6.000 in T T T7651 Properties A Min. A Min. S Min. L 70* 70 70* F tu (ksi) LT * ST L * F ty (ksi) LT * ST * L 7 e (%) - S basis LT ST L-T K Ic (ksisqrt in.) T-L S-L Exfoliation EB EB EB SCC Note - (*) indicates S -basis values.000 in in T T T7651 Properties A Min. A Min. S Min. L 6 6 6* 6 73 F tu (ksi) LT 6 6 6* 6 73 ST L 5* F ty (ksi) LT ST 55* L e (%) - S basis LT ST L-T K Ic (ksisqrt in.) T-L S-L Exfoliation EB EB EB SCC Note - (*) indicates S -basis values and values for alloy 705 are tentative and not approved Properties of in. thick aluminum plate alloys (* indicates S-basis values) Filename.ppt 6

7 Processing-Structure-Property Relationships for Aluminum Alloys Processing Microstructure Properties Melting Casting/ Solidification Homogenization Thermomechanical Processing Solution Heat Treatment Quenching Aging Mill Products Extrusion, Forging, Plate, Sheet Aluminum Matrix Grain Size (few μm - several mm) Degree of Recrystallization Subgrain Structure (0.5-5 μm) Grain Boundary Precipitate Free Zone ( μm) Texture Second Phase Precipitates ( μm) Dispersoids ( μm) Constituents ( μm) Strength / Ductility Anisotropy Durability / Damage Tolerance Environmental Resistance Corrosion Stress Corrosion Machining Forming Welding Chemical Processing Secondary Heat Treatments Hardware Filename.ppt 7

8 Schematic Age Hardening Curve for Aluminum Alloys and the Commercial Heat Treatment Tempers Hardness T6 T7 Aging Time T77 T76 T74 T73 T6 Solution treated and aged to peak strength T7X - Solution treated and artificially overaged to improve corrosion resistance T73 - Overaged to achieve the best stress corrosion cracking resistance T76 - Overaged to achieve good exfoliation corrosion resistance T74 - Overaged between T73 and T76 T7 - Very limited overaging T77 - Retrogression and reaging Peak strength (T6 temper) Point of transition from predominantly shearing to predominantly bypass of precipitate particles by dislocations Corrosion-resistant, overaged tempers (T7X) Particle bypass Filename.ppt

9 Typical Microstructure of a 7XXX alloy 100 nm Filename.ppt

10 Strength Superposition of precipitation and matrix strengthening Matrix (grain boundaries, dislocations, solid solution hardening) Precipitation hardening (shearable and non-shearable particles) Interactions (texture, precipitate orientations) Predominantly from precipitation hardening Typically modeled considering shearable and non-shearable separately For shearable precipitates, YS is proportional to (f v *R) 1/2 For non-shearable prcipitates, YS is proportional to (f v ) 1/2 /R Example of a model 1/2 σ p = M 3f v F 3/2 mean (2Γ) 1/2 b 2π R mean Filename.ppt 10

11 Toughness and Fatigue Properties Influenced by Coarse, insoluble intermetallic constituents Flow stress and work hardening Grain boundary PFZ and precipitate distribution Refinement of microstructure retards crack initiation (improves S-N behavior similar to toughness) Most commercial alloys show similar crack growth behavior For a constant yield stress K Ic {2σ y E(π/6) 1/2 D} 1/2 *f v -1/6 σy and E are yield strength and Young modulus, respectively D and fv are size and volume fraction of the particles K Ic Alloy (Plate) Max wt.% (MPa.m 1/2 ) Si Fe Mn L-T 7075-T T Filename.ppt 11

12 Stress Corrosion Cracking No models have been developed to correlate microstructure with SCC resistance Correlation based mainly on experience Microstruture As solution treated Underaged Peak aged Overaged Susceptibility Immune Maximum Susceptible Decreases with aging Filename.ppt 12

13 A Sample of the Microstructural Parameters Used in the Database Filename.ppt 13