Light Alloys- Aluminium Processing and Alloying

Question 1

Different directions relative to rolling direction

Answer 1

Longitudinal is parallel to rolling so along length of the grains.
Long transverse is perpendicular to the rolling direction but in the plane of the sheet.
Short transverse is perpendicular to both longitudinal and long transverse so is out of plane of the sheet

Question 2

What does rolling cause?

Answer 2

Anisotropy if the grain shape
Grain boundary phases
Crystallographic texture

Question 3

Which direction favours crack propagation in rolled sheet?

Answer 3

Loading in short transverse. Like opening up the grain boundaries. Easier route for crack to propagate through brittle impure phases along the GBs

Question 4

Anisotropy of strength and elongation for rolled sheets

Answer 4

Longitudinal has highest proof stress and tensile strength and % elongation. Long transverse direction just behind in all of these. Short transverse similar amount lower than this for strength but more of a decrease for elongation. Designer assumes loading in the ST direction

Question 5

ST/L ratios for strength and elongation

Answer 5

Ratios of values for short transverse loading to longitudinal loading. For strength can be between 0.8 and 0.95. For elongation is much lower anywhere below 0.5

Question 6

Downsides of alloying Al

Answer 6

Deformability is impaired
Corrosion resistance best for pure Al
Surface finish best for pure Al
Density often increased by alloying
Conductivity best for pure Al

Question 7

Figures that show how alloying can greatly increase the strength of Al

Answer 7

99% pure Al has 90MPa UTS and YS of 30MPa. Work hardening can increase UTS to 160MPa. But strongest age hardenable alloys have UTS of 700MPa

Question 8

How to improve fabrication properties of Al alloys

Answer 8

Additions of Si or Sn improve castability

Question 9

Constitutional effects of alloying and types of strengthening

Answer 9

No allotropic changes with temperature
Useful solid solubility is limited to a few additions
Compound formation is common
Solid solution strengthening
Dispersion strengthening
Precipitation hardening

Question 10

Effect of different alloying additions on density and YM

Answer 10

Pb increases density most. Cu next and increases YM by least. Mn next highest increase to density but second highest YM increase. Zn just below for density. Then Ti for density. Si decreases the density a little and increases YM little more than Cu. Mg decreases density quite a bit but decreases YM. Li decreases density a lot and increases strength the most

Question 11

Age hardening alloys

Answer 11

Al-Cu
Al-Cu-Mg
Al-Mg-Si
Al-Zn-Mg
Al-Zn-Mg-Cu

Question 12

Casting alloys

Answer 12

Al-Si
Al-Si-Cu

Question 13

Work hardening alloys

Answer 13

Al-Mg
Al-Mn
Al-Li-Mg-Cu

Question 14

Al alloy designation system. 4 digits and the main alloying elements they correspond to

Answer 14

1xxx - 99.00% min
2xxx - Cu
3xxx - Mn
4xxx - Si
5xxx - Mg
6xxx - Mg and Si
7xxx - Zn
8xxx - others

Question 15

Letters for Al designation system

Answer 15

F - as-fabricated
O - annealed wrought products
H - cold worked (strain hardened)
T - heat treated (stable), has numbers for type of heat treatment

Question 16

Numbers for each type of heat treatment

Answer 16

1- partial solution and natural ageing
2- annealed cast products
3- solution and cold work
4- solution and natural ageing
5- artificial ageing only
6- solution and artificial ageing
7- solution and stabilising
8- solution, cold work and artificial ageing
9- solution, artificial ageing and cold work

Question 17

What does AA6061.T6 mean?

Answer 17

Is an Al-Mg-Si alloy which has been solution heat treated followed by artificial ageing

Question 18

More detail on the three most common temper designations

Answer 18

T3- solution treated, cold worked and naturally aged to a substantially stable condition. Allows for cold working to increase strength and also to be involved in flattening or straightening.
T4- solution treated and naturally aged to a substantially stable condition.
T6- solution treated and artificially aged
There are many more tempering designations not covered here or before

Question 19

What is solution treatment?

Answer 19

A heat treatment where you heat a material to temperatures sufficient for the dissolution of its soluble phases so a solid solution forms. Generally into a single phase region. Then rapidly cool to obtain supersaturated solid solution.

Question 20

What are the major alloying elements for Al?

Answer 20

Zn, Mg, Cu, Mn, Si

Question 21

What is Hume-Rothery limit?

Answer 21

The difference in atomic radio between the solute and solvent atoms should not be greater than 15%

Question 22

Effect of size of alloying atoms

Answer 22

Larger atoms expand the Al lattice and smaller ones contract it.
Hardening is largest for largest difference in atomic radii as long as under 15%

Question 23

What is required for effective solid solution strengthening?

Answer 23

The solid solution must be stable at service temperatures

Question 24

What is required for precipitation hardening?

Answer 24

A steep drop in the equilibrium concentration of the alloy addition with temperature

Question 25

What is hard to avoid in Al alloys?

Answer 25

Mg

Question 26

What does diffusion control?

Answer 26

Rate of precipitation
Rate of coarsening of secondary phases: clustering of solute at RT, stability of properties at elevated temperatures

Question 27

Lattice self diffusion formula

Answer 27

D=D0exp(-Q/RT)
Values depend on alloying element
Assume it’s all to do with diffusion coefficients

Question 28

Why is resistance to coarsening important?

Answer 28

Particle coarsening leads to drop off in elevated temperature properties. Resistance increases with decreasing D of the slowest diffuser involved. Dispersoids containing V show resistance to coarsening at 425C similar to that of Cu bearing precipitates at 200C.

Question 29

Coarsening parameter formula

Answer 29

K=(r^3-r0^3)/t
r is precipitate radius
t is time