Aluminium-Lithium alloys

Question 1

What properties does the addition of Lithium have on Aluminium

Answer 1

• Can increase Youngs Modulus and decrease density
• Also can produce age hardening precipitation which is homogeneous and creates Al3Li particles
• Strength at high temperatures is better retained

Question 2

What are the disadvantages of adding Lithium

Answer 2

• lithium is expensive and highly reactive during DC casting
• scrap must be segregates as Li can be highly deleterious in conventional alloy
• fracture toughness is slightly lower than 2/7XXX series

Question 3

Why do Al-Li alloys need to be cold worked before ageing process

Answer 3

• to develop work structure high dislocation density and provide network of rapid diffusion for Cu, Mg
• this allows nucleation sites for S and T type precipitates
• if not cold worked, Li would prevent widespread nucleation of S and T phases which would lead to sparsely distributed colonies of coarse particles

Question 4

What are the issues encountered when cold rolling Al-Li alloys

Answer 4

• can only be worked via cold compression which limits configuration and section size of forging
• compression doesn’t have uniform effect on material and so precipitation network might be different at the centre of the material then that of the outside

Question 5

Why isn’t Al-Li alloys more widely used

Answer 5

• price of kerosene was kept low so demand for lighter aircraft reduced
• end of Cold War in late 80’s meant less demand for lighter more manoeuvrable military aircraft
• initial cost of Al-Li did not warrant the savings made when aircraft was in operation
• other Al alloys were too competitive

Question 6

Give uses for Al-Li alloys

Answer 6

• fuel tanks for rockets

- Merlin helicopter (uses AA8090)

Question 7

Give advantages of 3rd generation Al-Li alloys

Answer 7

• good at FSW (Friction Stir Welding)
• rescent coining evaluation
• no delamination issues when plate drilling
• better machinability