Strength and (Extrinsic) Corrosion Resistance Improvements in New 7XXX-Series Alloys - Relative to 7075-T651 All Alloys Still Need Corrosion Protection Schemes 700 650 600 Corrosion Resistance Low Medium High Thin Products < 75 mm 7055-T7751 Yield 550 Strength (MPa) 500 450 400 7075-T651 7075-T7651 7075-T7351 7150-T651 7150-T7751 7050-T7451 Thick Products 75-200 mm 705-T7651 140 150 160 170 10 10 2000 Year First Used in Aircraft 2005 Filename.ppt 1
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
Composition (wt. %) of 7XXX Series Aluminum Alloys 7075 7175 7475 717 704 714 724 734 744 7050 7150 7040 7055 705 Zn 5.6 5.6 5.7 6. 7.7 7.7 7..1 6.2 6.4 6.2.0 7.5 Mg 2.5 2.5 2.2 2. 2.4 2.4 2.2 2.3 2.3 2.4 2.1 2.0 1.5 Cu 1.6 1.6 1.6 2.0 1.6 1.6 1.6 1.0 2.3 2.2 1. 2.3 1.6 Ti+Zr 0.25 max 0.12 0.12 0.10 0.16 0.12 Zr Cr 0.23 0.23 0.22 0.23 0.16 0.16 0.16 < 0.04 < 0.04 < 0.04 < 0.04 < 0.04 Si, max 0.40 0.15 0.10 0.40 0.25 0.15 0.10 0.12 0.12 0.12 0.10 0.10 0.06 Fe, max 0.50 0.20 0.12 0.50 0.35 0.20 0.12 0.15 0.15 0.15 0.13 0.15 0.0 Mn, max 0.30 0.10 0.06 0.30 0.20 0.20 0.10 0.20 0.10 0.10 0.04 0.05 0.04 Ti, max 0.20 0.10 0.06 0.20 0.10 0.10 0.06 Ti+Zr 0.25 max 0.06 0.06 0.06 0.06 0.06 Others, each, max 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Others, total, max 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 Al Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Filename.ppt 3
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
Properties of 1 in. and 3 in. thick aluminum plate alloys (* indicates S-basis values, (a) indicates values not yet approved) 1.000 in. 7050-T7651 7050-T7451 7150-6151 7150-T7751 7055-T7751 744-T7651 744-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 76 66 26 76 66 22 74* 64* 10 20 74 64 10 22 5 7 22 5 7 4 7 4 7 6 7 6 7 4 7 0(a) (a) LT LT LT T-L 76 66 24 76 66 74 64 1 74 64 4 77 20 4 7 4 77 4 77 5 5 4 77 Exfoliation EB EB EB EB EB Note - (*) indicates S -basis values and (a) indicates data calculated by supplier, but not approved 3.000 in. 3.001 in. 3.000 in. 2.500 in. 7050-T7651 7050-T7451 7040-T7451 7150-T7751 744-T7651 Properties S Min. A Min. A Min. A Min. A Min. L 76 76 73* 73 72 70 2 2 2 F tu (ksi) LT 76 76 73* 73 72* 71 2* 2 2 ST 70 70 6 6 6 6 77* 77 77 L 66 66 63* 63 62* 62 76 76 76 F ty (ksi) LT 66 66 63* 63 62* 62 75* 75 75 ST 60 60 5 5 5* 5 67* 67 67 L 7 7 7 e (%) - S basis LT 7 7 6 6 6 6 6 ST 1.5 1.5 3 3 3 3 1 1 3 L-T 24 27 31 21 22 K Ic (ksisqrt in.) T-L 23 24 26 1 20 S-L 20 21 24 Exfoliation EB EB EB EB EB SCC 25 35 35 25 25 Note - (*) indicates S -basis values Filename.ppt 5
6.000 in. 7050-T7451 7040-T7451 705-T7651 Properties A Min. A Min. S Min. L 70* 70 70* 70 74 F tu (ksi) LT 70 70 70* 70 73 ST 66 67 6 6 70 L 60 60 62* 62 6 6 F ty (ksi) LT 60 60 61* 61 67 67 ST 57 57 5* 5 63 63 L 7 e (%) - S basis LT 4 4 4 4 4 ST 3 3 3 3 3 L-T 24 2 2 K Ic (ksisqrt in.) T-L 22 23 22 S-L 21 24 22 Exfoliation EB EB EB SCC 35 35 26 Note - (*) indicates S -basis values.000 in. 7.000 in. 7050-T7451 7040-T7451 705-T7651 Properties A Min. A Min. S Min. L 6 6 6* 6 73 F tu (ksi) LT 6 6 6* 6 73 ST 65 65 66 66 6 L 5* 5 61 61 6 6 F ty (ksi) LT 5 5 60 60 65 65 ST 55* 55 57 57 63 63 L 6 6 6 6 6 e (%) - S basis LT 4 4 4 4 3 ST 3 3 3 3 3 L-T 23 26 27 K Ic (ksisqrt in.) T-L 21 22 20 S-L 21 23 21 Exfoliation EB EB EB SCC 35 35 26 Note - (*) indicates S -basis values and values for alloy 705 are tentative and not approved Properties of 6.000.000 in. thick aluminum plate alloys (* indicates S-basis values) Filename.ppt 6
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 (0.05-0.5 μm) Texture Second Phase Precipitates (0.01-0.1 μm) Dispersoids (0.05-0.5 μm) Constituents (0.1-10 μm) Strength / Ductility Anisotropy Durability / Damage Tolerance Environmental Resistance Corrosion Stress Corrosion Machining Forming Welding Chemical Processing Secondary Heat Treatments Hardware Filename.ppt 7
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
Typical Microstructure of a 7XXX alloy 100 nm Filename.ppt
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
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-T7651 0.40 0.50 0.30 32 7475-T7651 0.10 0.12 0.06 36 Filename.ppt 11
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
A Sample of the Microstructural Parameters Used in the Database Filename.ppt 13