Chapter 5. Aluminum Alloys. /MS371/ Structure and Properties of Engineering Alloys

Transcription

1 Chapter 5 Aluminum Alloys

2 Main characteristics light / non- properties high next to Ag, Cu, Au, Cr Al is similar with ceramics in the periodic table cast Al: for structure such as vehicle s transmission wrought Al: 단련, worked or processed

3 Production of Al Bauxite - Al 2 O 3 xh 2 O + SiO 2, FeO 3 - over 50~60% purity is valuable - by Bayer process pure Al Bayer process - Al 2 O 3 + 2NaOH 2NaAlO 2 + H 2 O ( C) - NaAlO 2 + 2H 2 O Al(OH) 3 + NaOH - Al(OH) 3 is then thickened, washed, calcined to produce Al 2 O 3 Hall process - Al 2 O 3 is dissolved in a bath of molten cryolite (Na 3 AlF 6 ) (F-very toxic) - electrolyzed with carbon electrodes ~99.9% Al is produced

4 Wrought Al Table 5-1 Wrought Aluminum Alloy Groups high strength good formability! corrosion resistance wide use good strength part: 2xxx good formability: 3xxx 2-piece Al can

5 Heat treatment of Al Properties of Al alloys are largely changed by its

6 Heat treatment of Al

7 Al-Mg alloys Basis for 5xxx series alloys High resistance, good and characteristics 451 o C solubility ~15% Al-rich end of Al-Mg phase diagram (after van Horn (ed.), Aluminum, vol.1, ASM, 1967, p.375) greatly decreasing solid with decreasing temp solubility at 451 C (~15%) non-precipitation hardening below 7wt% Mg, but solid solution hardening, Why? Because, e.g., Al-5.5Mg alloy on moderately quick cooling from 450C, it misses the nose of TTT. Thus, 5.5Mg is supersaturated giving rise to solid soln hardening

8 Al-Mg alloys High corrosion resistance Increase in strength and ductility with Mg However, ~5% (stress corrosion cracking)

9 Al-Cu alloys High strength by hardening structure materials due to high strength

10 Al-Cu alloys Precipitation hardening compression tension Al-rich end of Al-Cu phase diagram (after van Horn (ed.), Aluminum, vol.1, ASM, 1967, p.372) low solubility precipitation hardening Precipitation heat treatment - heat treatment (a solid solution phase field about 515 C) - rapidly to room temperature or below - artificial in the 130 to 190 C range

11 Al-Cu alloys Strength change with microstructure change GP zone, Guinier-Preston CuAl 2 compression tension Coherent precipitation Incoherent precipitation

12 Stages in the precipitation of CuAl 2 - Disc-shaped GP zones (b) nucleate homogeneously from supersaturated solid solution (a). - The disc faces are perfectly coherent with the matrix. - The disc edges are also coherent, but with a large coherency strain.

13 Stages in the precipitation of CuAl 2 - Some of the GP zones grow to form precipitates called θ. (The remaining GP zones dissolve and transfer Cu to the growing θ by diffusion through the matrix.) - Disc faces are perfectly coherent. - Disc edges are coherent, but the mismatch of lattice parameters between the θ and the Al matrix generates coherency strain.

14 Stages in the precipitation of CuAl 2 - Precipitates called θ nucleate at matrix dislocations. - The θ precipitates all dissolve and transfer Cu to the growing θ. - Disc faces are still perfectly coherent with the matrix. - But disc edges are now incoherent. Neither faces nor edges show coherency strain, but for different reasons.

15 Stages in the precipitation of CuAl 2 - Equilibrium CuAl 2 (θ) nucleates at grain boundaries and at θ matrix interfaces. - The θ precipitates all dissolve and transfer Cu to the growing θ. - The CuAl 2 is completely incoherent with the matrix. Because of this it grows as rounded rather than disc-shaped particles.

16 Al-Cu alloys Structural change during aging treatment role of vacancies significant effect on the formation of GP zones concentration of vacancies to increase with temp nucleation GP zones: homogeneous nucleation, critical vacancy concentration is required, lattice defects also to affect the nucleation of a new phase (grain boundaries, subgrain boundaries, dislocations, interphase boundaries nucleation of transient or equilibrium precipitates) precipitation on grain boundary: small effect on strength, but has a harmful effect on the corrosion resistance and promote intergranular failure dislocations: most effective for 2xxx and 8xxx alloys precipitation formed at high temperature or at grain boundary to result in phase precipitation free zone (PFZ)

17 Al-Cu alloys Microstructural change during aging treatment

18 Al-Cu alloys Microstructural change during aging treatment (a) disc shape (Al-Cu alloy) (b) q (c) q

19 Al-Cu alloys

20 Al-Cu alloys Precipitate free zone ( ) (a) Solute atom depleted (b) Vacancy depleted

21 Al-Cu-Mg alloys The precipitation-hardenable alloys to be discovered first by Alfred Wilm Structure supersaturated solid solution GP Zones S (Al 2 CuMg) S (Al 2 CuMg) 2024 T6 heat treatment 190 C 12h GP zones and S 2024 T81 cold stretched 1.5% & 190 C 12h aged GP zones and S 2024 T86 cold rolled 6% 190 C 12h aged GP zones and S

22 Al-Cu-Mg alloys duralumin super-duralumin high strength

23 Al-Mg-Si alloys Good formability and excellent corrosion resistance for construction materials 6013: Galaxy S6 엣지 (1.5 강도 of 6063) 6063: ease of extrudability for shapes, slightly lowerstrength alloy, Galaxy S6 6463: Fe level is kept low so that the brightness of the aluminum will be improved after anodizing

24 Al-Zn-Mg and Al-Zn-Mg-Cu alloys Primary material for applications, Extra Super Duralumin (weldable, high strength Al alloys) (super high strength Al alloys, but not weldable)

25 Al-Zn-Mg and Al-Zn-Mg-Cu alloys supersaturated solid solution GP zones h (MgZn 2 ) monoclinic h (MgZn 2 ) hexagonal strengthened by GP zones and/or semicoherent h (MgZn 2 ) h can be nucleated from pre-existing GP zones & deformation prior to aging has no effect on precipitation behavior highly susceptible to SCC to a certain extent through control of both composition and heat-treatment procedures (overaging: significant increase in resistance to SCC) T 73: improve the resistance to SCC with a small sacrifice in tensile properties (duplex aging treatment: low temperature aging (e.g. 120 C) + overaging ( C)

26 Al-Zn-Mg and Al-Zn-Mg-Cu alloys Microstructure fully hardened GP zones < 75A overaged h (100~300A) and h (400~800A) Grain boundary PFZ aged 16h at 90C and 24h at 150C, UTS = 49ksi 24h at 150C, UTS = 40ksi

27 Al-Ag alloys

28 Al-Ag-Zn alloys

29 Al alloys (summary) Aluminum Alloys Symbol # Primary properties Secondary properties Al-Zn-Mg-Cu 7xxx Tensile strength Al-Cu 2xxx Tensile strength Al-Mg 5xxx Corrosion resistance Formability Al-Mg-Si 6xxx Formability Corrosion resistance Ship is made by 5xxx aluminum alloys due to its excellent resistance