Light Alloys- Aluminium Work Hardenable Alloys and Heat Treatment Mechanisms

Question 1

What are work hardenable alloys?

Answer 1

Those that cannot be hardened by precipitation mechanisms. Mechanisms of increasing strength limited to:
Refining grain size
Solid solution strengthening
Cold working (increase dislocation density)

Question 2

Which 3 Al alloy series are work hardenable?

Answer 2

1xxx (at least 99.00 wt% Al)
3xxx (Al-Mn)
5xxx (Al-Mg)

Question 3

How does the 1xxx series name system work?

Answer 3

Numbers indicate impurity level.
E.g 1145 is 99.45 wt% Al
And 1199 is 99.90 wt% Al

Question 4

1xxx series: properties, strengthening, applications, other info

Answer 4

Typical impurities are Fe and Si (beneficial). Yield strength can vary from 10MPa in annealed 1199 up to 165MPa in heavily worked 1350. Yield strength increases linearly with volume fraction Al-Fe-Si. Recrystallisation leads to drop in strength but increased ductility. Formation of intermetallics by DC process can affect surface quality. Applications: foil, architectural decorations, power transmission (pure grades).

Question 5

3xxx series: properties, 3004, recycling

Answer 5

Has 1 wt% Mn which forms dispersoids that refine the grain size (disrupt the structure so help recrystallisation). Annealed has 40MPa and 30%, cold worked has 185MPa ans 4%.
3004 (Al-1.2%Mn-1%Mg) has Mg as a solution strengthener. Is recyclable and used for cans (100billion in US, 30bill in EU). The higher energy content for Al than steel makes it more economical to recycle

Question 6

3xxx series: applications and strengthening

Answer 6

Used for higher strength than AA1100. Hardening by SS hardening from Mn and from dispersion strengthening via MnAl6 particles. Homogenisation replaces microsegregation of Mn with MnAl6 distribution. Replacing some Mn with Mg (AA3105) gives SS strengthening from Mg. suitable for can stock

Question 7

Why is there a yield plateau before work hardening on stress strain curve for 5xxx series alloys?

Answer 7

During this deformation is localised into bands surrounded bands of uniformed material (luders bands). E.g forming curved panel from flat sheet. Mg (in 5xxx series alloys) segregates to dislocations on GBs to stop them operating. Only locally when a source get activated will you get a burst of dislocation activity, very localised high strain level.

Question 8

Solution to yield point phenomena in 5xxx series

Answer 8

Lightly roll sheet (compressive deformation) to activate all dislocation sources throughout the material so don’t get luders bands when forming the panel. However must form just after rolling or Mg atoms will migrate back to dislocation sources during storage at room temperature

Question 9

5xxx series: what is it?, properties (YS and %), +ves, -ves

Answer 9

Up to 6 wt% Mg. Annealed: YS 40-160MPa, 25% elongation.
Cold work: YS 300MPa, 5% elongation.
Advantages (over steel): much more recyclable, lower density, does not rust.
Disadvantages: weldability (use adhesive bonding instead), repairability, yield point phenomena
Used in some car bodies

Question 10

5xxx series: strength, general properties, solubility

Answer 10

Up to 3% Mg gives precipitation hardening via Al3Mg2. Stronger than equivalent Al-Mn alloys. High strength, good formability, good corrosion resistance, bright surface finish, weldable. Main strengthening mechanism is SS strengthening by Mg dissolved in Al. Mg has high solid solubility in Al. 17.4% at 450C down to 1.7% at RT. More powerful SS strengthener on a weight basis than copper.

Question 11

5xxx series: effect of Mg

Answer 11

Strength increases in cold working (strain hardening) due to effect Mg has on lattice strain. Dislocation motion is restricted due to this. Mg atom has higher mobility in Al than a vacancy so moves to areas of higher dislocation density. This further reduces mobility and results in an increase in dislocation multiplication

Question 12

Which are the heat treatable alloys?

Answer 12

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

Question 13

What are the different phases considered for the heat treatable alloys?

Answer 13

2xxx: Al-Cu system is α+θ (CuAl2)
2xxx: Al-Cu-Mg system is α+S (Al2CuMg)
7xxx: Al-Zn-Mg system is α+η (MgZn2)
8xxx: Al-Li system is α+δ’ (LiAl3)

Question 14

First two steps of heat treatments after initial solidification of an Al-Cu alloy

Answer 14

Solution treatment: homogenisation. Alloy heated to temperature below the eutectic temperature and held for up to 24h to remove the eutectic and homogenise the solute content.
Quench to room temperature: cooling quickly prevents CuAl2 formation can Cu cannot diffuse at RT. gives super saturated solid solution.

Question 15

What happens if you slowly cool a Al-Cu alloy instead of quenching after solution treatment?

Answer 15

Al2Cu will form at the GBs. This is an embrittling intermetallic. Grains themselves are soft.

Question 16

Why does quenching alloy diffusion to occur at room temperature?

Answer 16

At higher temperatures (during homogenisation) there is a larger vacancy concentration. Quenching traps these vacancies so diffusion can occur at room temperature

Question 17

How does natural ageing work?

Answer 17

After quenching leave the Al-Cu alloy at room temperature. Al2Cu precipitates form inside the grains. Most effect is achieved within first 24h as supersaturation decreases and so does vacancy concentration. This is T4

Question 18

How does artificial ageing work?

Answer 18

After quenching heat the Al-Cu alloy to a temperature below the solubility limit. The solid solution become unstable. Cu can diffuse at these temperatures. Get formation of Al2Cu. Due to high undercooling get nucleation of Al2Cu inside Al grains. This is T6

Question 19

What does distribution of precipitates after ageing depend on?

Answer 19

Temperature, time and heating rate

Question 20

Effect of ageing temperature

Answer 20

Determines rate of nucleation and growth of precipitates. Increasing temperature decreases undercooling meaning less driving force for nucleation and get fewer nuclei, but increases diffusion of solute so precipitates grow faster.

Question 21

What does ageing time determine?

Answer 21

The size of the precipitates

Question 22

How doe strength and hardness vary with ageing time?

Answer 22

Start low in the supersaturated SS. Increase a bit during nucleation. Increase faster during growth (under aged, large number of very small underdeveloped precipitates). Reaches peak aged (optimum size and distribution of precipitates). Then decreases slowly during coarsening (over aged, a few large precipitates spaced far apart)

Question 23

Typical conditions when processing a commercial Al-Cu alloy

Answer 23

Typically 4wt% Cu. Solution treatment at 550C. Ageing at 150-200C. Precipitates of intermetallic CuAl2 (θ) are firmed and strengths similar to mild steel can be achieved.

Question 24

Al-Cu precipitation sequence

Answer 24

Stage 1: after solution treat and quench have supersaturated SS so substitutional SS strengthening.
Stage 2: GP (Guinier Preston) zones form, small clusters of atoms, disk shaped 0.5nm thick and 10nm diameter, coherent with Al matrix.
Stage 3 and 4: θ’’ forms, disk shaped 2nm thick 50nm diameter, tetragonal structure, coherent. θ’ plates, 150nm diameter, semi-coherent.
Stage 5: θ forms, stable phase BCT structure, completely incoherent with Al matrix, alternating Al and Cu atoms

Question 25

Al-Cu precipitation sequence summary

Answer 25

αss->α+GP->α+θ’’->α+θ’->α+θ

Question 26

Which parts of the Al-Cu precipitation sequence correspond to which parts of the strength vs ageing time graph?

Answer 26

This graph increases more at start, then levels off a bit, then increases steeper to a peak, then comes down fairly quickly. Time on a log scale.
Initial increase from α+GP. Second steeper increase from α+θ’’. After peak is α+θ’. Further down the decrease is α+θ.

Question 27

Dislocation motion by cutting

Answer 27

Where the slip plane goes through small precipitates, it cuts through them so one side of the precipitate is shifted by the Burgers vector. these are deformable particles. Strength proportional to square root of precipitate size.

Question 28

Dislocation motion by bowing

Answer 28

For large precipitates, the dislocation is pinned at them until it bows around them (like ox bow lake) and continues. These are non-deformable particles. Strength proportional to 1=distance between particles. Distance between particles decreases as they grow.

Question 29

Balance between cutting and bowing

Answer 29

Stress to move dislocation past particle vs ageing time or particle size graph. Cutting is concave curve from origin up with decreasing gradient. Bowing (looping) is like 1/x curve. Where they intersect is the optimum particle size for strengthening.