Light Alloys- Aluminium Heat Treatable Alloys

Question 1

Which Al alloys are heat treatable?

Answer 1

2xxx: Al-Cu(-Mg)
6xxx: Al-Mg-Si
7xxx: Al-Zn-Mg(-Cu)
8xxx: Al(-Li)

Question 2

Trends for strength between different series

Answer 2

Strength increases 6xxx to 2xxx to 7xxx. Increased volume fraction of precipitates possible. Strength increases with increased alloying

Question 3

What else does increased alloying do?

Answer 3

Decreases extrudability, weldability and corrosion resistance.

Question 4

Trends for thermal stability between series

Answer 4

Thermal stability increases 7xxx to 6xxx to 2xxx. Reflects higher solubility and diffusivity of Zn then Si and Cu

Question 5

What temperature are high temperature applications of Al at?

Answer 5

Around 200°C

Question 6

2xxx series: applications, corrosion, additions, precipitation sequence

Answer 6

Aircraft structures and heavy engineering. First family of age hardenable alloys. Stress corrosion cracking and corrosion resistance poor so cladding sometimes used. Modern alloys have added Si to increase artificial ageing and added Mg to increase natural ageing. To get highest strengths cold work between quenching and artificial ageing. Precipitation sequence:
SSSS->lath GP zones->S’ laths->S laths

Question 7

2xxx series ternary phase diagram

Answer 7

wt% Mg vs wt% Cu with dashed lines (like arcs) for different temperatures. High Cu:Mg ratio results in θ phase (Al2Cu) above certain temperature. Low Cu:Mg ratio results in S phase (Al2CuMg) above certain temperature. Very low Cu:Mg ratio give T phase but doesn’t tend to have commercial applications

Question 8

Toughness vs yield strength for 2xxx and effect of tempers

Answer 8

Linear decrease in fracture toughness with increasing yield strength. T4 (natural ageing) given lower yield strength and higher fracture toughness than T6 (artificial ageing). However for T6 critical fracture stress is lower and so fast fracture will occur before yielding. T 4 has higher critical fracture stress than yield stress so the warning of plastic deformation occurs before fast fracture.

Question 9

Formula for fracture toughness

Answer 9

Kc=Yσrt(πacrit)
Where Y is some constant
σ is yield stress?
a sub crit is critical crack length

Question 10

Where is 2xxx series Al used on aircraft and why?

Answer 10

Airframe and skin structure. T4 is damage tolerant and used for lower wing skin. This is in tension so cracks tend to open and is prone to damage from material on runway

Question 11

6xxx series: general uses and advantages over 2 and 7xxx

Answer 11

Accounts for 70% of extruded sections. Used in structural and architectural applications. General purpose medium strength heat treatable alloys. Advantages are: easily extruded at high speed, good anodisable surface finish, good corrosion resistance and weldability better than 2 or 7xxx, less dense than 2xxx

Question 12

6xxx: disadvantages compared to 2 and 7xxx, other additions, precipitation sequence

Answer 12

Not as strong as 7xxx or as thermally stable as 2xxx. Add Cu to increase strength and Cr to restore corrosion resistance. T6 achieves 310MPa. Precipitation sequence:
SSSS->rod GP zones->β’ needles->equilibrium Mg2Si plates

Question 13

What defines 6xxx as dilute or concentrate?

Answer 13

When Mg+Si < 1wt% is dilute
When Mg+Si > 1wt% is concentrate

Question 14

Dilute 6xxx

Answer 14

Age well at room temperature (metastable GP zones). GP zones act as nucleation sites during artificial ageing (T6). Homogeneous distribution of Mg2Si precipitates.

Question 15

Concentrate 6xxx

Answer 15

Initially higher strength were achieved by increasing Mg and Si content above 1wt%. Age well at room temperature (metastable GP zones). If carry out T6 artificial age strength is reduced. GP zones redissolve on heating so decrease in nucleation sites for Mg2Si precipitates. Need to age immediately after quenching (for artificial ageing) to get highest strength.

Question 16

Effect of adding Cu to 6xxx

Answer 16

Possible solution to artificial ageing problem of concentrate. counteracts deleterious natural ageing. Reduces natural ageing response. But weldability decreases and this is the main advantage of 6xxx over 2 and 7xxx. Other effects Ar unclear. Either reduction in corrosion resistance or no effect on it. Increases strength by refining Mg2Si and small amount of SS hardening. No effect on toughness. Improve alloy brightness after bright dipping and anodising

Question 17

Where is 6xxx used in aircraft?

Answer 17

Lower fuselage skin

Question 18

Other precipitates in 6xxx to Mg2Si

Answer 18

Si combined with Fe in preference to Mg to form AlFeSi or AlCrSi. AlFeSi phase is very detrimental to biller properties in its unspheroidised form (β). Homogenisation must change this phase to the α form. Extrudability linked to the levels of Si found in the alloy

Question 19

Effect of Mg in 6xxx

Answer 19

Effect on SS strengthening negligible. If in excess causes dramatic decrease in extrudability. Changes the mechanism by which tears are initiated. Also decreases ductility and toughness. Has beneficial effect on corrosion resistance.

Question 20

Effects and details of Mg2Si in 6xxx series

Answer 20

Mg and Si combine to form Mg2Si which is the principal strengthening phase in 6xxx series. Equilibrium Mg2Si has Mg;Si ratio of 1.73:1. Increasing levels of Mg2Si: improves tensile properties, decreases extrudability due to cracking problems, reduces required storage time between extrusion and artificial ageing

Question 21

Why is there a storage period between extrusion and artificial ageing of 6xxx?

Answer 21

Due to the growth of fine precipitates after ageing. If the GP zones are allowed to reach a certain size reversion of these zones will not occur during artificial ageing.

Question 22

Effect of Mg2Si on storage period and controlling precipitates

Answer 22

Effect of storage period between extrusion and artificial ageing more pronounced when ageing T is low and Mg2Si content high. To maximise properties Mg2Si precipitates must dissolve into solution, usually during extrusion process. To ensure this precipitates must be in correct form. Homogenisation process must be carefully controlled. Mg2Si precipitates must be fine and uniformly dispersed. Reheating to extrusion T also vital. For best combination of pressure, ram speed, surface finish and aged strength precipitate size of 0.5μm at 450C desirable.

Question 23

Effect of iron in 6xxx

Answer 23

Increase in Fe content can reduce strength. However Fe can refine the Mg2Si precipitates and so theoretically increase the achievable strength. Fe can reduce corrosion resistance but increase toughness. Also possible that Fe has a GB pinning effect. Alloys containing small amounts of Fe exhibit normal grain growth and recrystallisation as GBs are not pinned

Question 24

Effect of Mn and Cr in 6xxx

Answer 24

Present either as impurities or deliberate additions. Increase toughness by forming incoherent stable dispersoids during homogenisation. Both increase quench sensitivity by providing sites for Mg2Si nucleation. Mn preferred as is less harmful to extrusion speed and finish. Mn also improves surface finish

Question 25

Other additions to 6xxx

Answer 25

V, Pb, Zn, Ti, B.
V increases strength and toughness by refining grain size during casting. Pb (and B) improve machinability. Zn can provide some SS strengthening. Ti and B increase solidification rate and so can influence final grain size of the alloy

Question 26

7xxx series: what is it?, precipitation sequence, solution and ageing treatment temperatures, uses

Answer 26

Based on Al-Zn-Mg(-Cu). Highest strength Al alloy on market (7075).
αss->GP zones->η’+η (MgZn2).
Solution treat st 360C (500 for 2 and 6xxx)
Ageing treat at 120C (160 for 2 and 6xxx)
Used in upper wing skin, tanks, Saturn 1 rocket, 7475 has best damage tolerance

Question 27

Where is 7075 used in aircraft?

Answer 27

Highest strength Al alloy. Used in upper wingskin as compressive stresses dominate here and sustained tensile stresses need to be avoided

Question 28

What factors combine to give stress corrosion cracking (SCC)?

Answer 28

Susceptible material, tensile stress, environmental conditions.

Question 29

Effect of T6 on SCC resistance and strength

Answer 29

Plot each as d’onction of ageing time. SCC forms a u-shape whereas strength forms an n-shape. Two points of intersection

Question 30

Ways of reducing SCC in 7xxx

Answer 30

Add Cu?
Modify heat treatment (different ageing times)

Question 31

How do precipitate free zones (PFZs) form in 7xxx series and what is the problem with them?

Answer 31

Heterogeneous nucleation leads to precipitation of equilibrium phase on GB (MgZn2?). Homogeneous nucleation (in grains) provides a uniform distribution of second phase and can lead to non-equilibrium precipitates like GP zones depending on temperature and surface energy. Leads to solute and vacancy depletion in outer parts of grains close to GBs forming PFZs. These are a near continuous active path for corrosion (anodic).

Question 32

Solving the problem of PFZs in 7xxx

Answer 32

Can do a 2 step heat treatment. First at lower temperature and second at higher temperature. Diffusion is faster in GBs than in grains. Means second step at higher temperature coarsens the GB η particles (MgZn2) without fully coarsening the particles in the grains. Reduces the size of PFZs as coarser and fewer η particles are present to fill the space.

Question 33

Requirements of upper and lower wing skin

Answer 33

Upper is in compression. Needs good compressive yield strength, modulus, fatigue resistance, fracture toughness, mainly uses 7xxx.
Lower is in tension. Needs good tensile strength, fatigue crack growth, fracture toughness, mainly uses 2xxx.
All need good corrosion resistance