Aluminium Flashcards
Describe light element
those with density less than 5g/cm^3
Describe properties of Al
- low density
- resistance to most corrosion
- great electrical conductor
- non-magnetic and non combustible
- non-toxic and impervious
- strong and ductile
Define extraction of Alumina from bauxite
- using bayer process
- bauxite (50% alumina) is dissolved in sodium hydroxide
Define Hall-Heroult
Alumina is dissolved in molten salt mixture of cryolite (Na3AlF6) and electrolytically reduced to molten aluminium. This is then tapped off.
Cathode tank and anode rods both Graphite (CO2 forms on these)
Al2O3 + 3/2C –> 2Al + 3/2CO2 (reality)
Describe effect of direct chill casting on microstructure
After direct chill casting there is macrosegregation due do different rates of cooling
What alloy is usually added to Al and what precipitates does it form
Cu, gets α phase and Al2Cu phase
Describe equilibrium solidification sequence
enough time is given for the material to cool and successfully end with only α phase grain structures
Describe non-equilibrium solidification sequence
- not a lot of time given to cooling process and therefore solidus line is depressed and grains are not uniform in their percentage of elements
- for example if 4wt%Cu then at grain centre %Cu «_space;4 and at grain boundary %Cu»_space; 4
- also eutectic point reached before all liquid has turned to α phase, so Al2Cu is formed
- heat treatment carried out to diffuse Cu into α phase
Why is non-equilibrium solidification done over equilibrium solidification
more economical with time and energy
List Aluminium alloy series and there alloys
1XXX - 99% min Al 2XXX - Cu 3XXX - Manganese 4XXX - Silicon 5XXX - Magnesium 6XXX - Magnesium & Silicon 7XXX - Zinc 8XXX - Other
What do, F, O, H, T stand for when naming Al alloys
F - as fabricated
O - annealed wrought products
H - Cold worked
T - Heat treated
List all 9 heat treatments
- Partial solution + natural ageing
- Annealed cast products
- Solution + cold work
- Solution + natural ageing
- Artificial ageing
- Solution + artificial ageing
- Solution + stabilising
- Solution + cold work + artificial ageing
- Solution + artificial ageing + cold work
List work hardenable alloy classification. List how they can be hardened
1XXX, 3XXX, 5XXX
- Solid Solution Strengthening
- Grain refinement
- Cold working (increase dislocation density)
List properties of 1XXX series
- work hardenable alloy
- indicates impurity level i.e. 1145 = 99.45wt%Al
- Typical impurities are Fe & Si (Can be beneficial by refining grain size)
- YS: 10-165MPa
- Recovery recrystallization reduces strength but increases ductility
- Formation of intermetallics by direct casting can affect surface quality
Give typical applications of 1XXX series
- Aluminium foil
- architectural decoration
- power transmission
List properties of 3XXX series
- about 1wt%Mn
- Mn forms dispersoids that refine grain
- 40-185MPa
- Recyclable
Uses of 3XXX series
Aluminium cans (additional 1wt%Mg) (recyclable)
List properties of 5XXX series
- up to 6wt%Mg which increases strength
- solid solution strengthening
- 40 - 300MPa
- Recyclable
- lower density
- does not rust
Uses of 5XXX series
- cars (95% recycled)
List disadvantages of 5XXX series compared with steel for use in cars
- weldability
- repairability
- yield point phenomena (yield plato before work hardening regime)
- deformation is localised into bands within the material during this period of strain
- Mg segregates to grain boundaries to prevent dislocation (solved by cold working, put needs to happen straight away)
List heat treatable alloys
2XXX, 6XXX, 7XXX
What does ageing temperature determine
determines the rate of nucleation and growth of precipitation
What does ageing time determine
determines size of precipitates
Describe the precipitation sequence of Al-Cu system
- After solution treatment and quench there is a supersaturated solid solution
- After quenching, there are a high number of vacancies
- The Cu will start to cluster and form GP (Gunier Preston) zones
- Then there is ordering of Cu and Al
- Then equilibrium phase occurs (Al2Cu) which is incoherent with Al matrix, therefore dislocations can’t pass through
What structure does GP zones have
similar structure to Aluminium
For deformable particles what is strength of material proportional to
strength ∝ (precipitate size)^1/2
For non-deformable particles what is strength of material proportional to
strength ∝ 1 / Distance between particles
When is aluminium at its greatest strength with relation to its phases
When phase is still coherent with Al (meta-stable phase)
Order heat treatable alloys in terms of strength
6 –> 2 –> 7
Alloying aluminium increases ______ but decreases __________
Increases strength
decreases extrudability, weldability and corrosion resistance
List properties of 2XXX series
- cladding required as stress corrosion cracking and corrosion resistance are poor
- Addition of Si or Mg help ageing process
- Excess Cu, so forms θ phase (Al2Cu) and S phase (Al2CuMg)
- High strength comes from cold working
- precipitation sequence:
SSSS –> Lath GP zones –> S’ laths –> S lath - High Cu ratio = θ phase
- Low Cu ratio = S phase
Uses of 2XXX series
- Aircraft structures: skin and lower wing
- useful for lower wing as can absorb damage from flying debris on the run way
If critical fracture stress is higher than yield strength, what does this imply about the material’s failure mechanism
the material will fail without warning which could be highly dangerous in many applications
List properties of 6XXX series
- good corrosion resistance
- good weldability
- easily extruded at high speeds
- precipitation sequence:
SSSS –> rod GP zones –> β’ needles –> equilibrium Mg2Si plates - split into 2 categories; Dilute and concentrate (Mg & Si > 1wt%)
Describe difference between dilute and concentrate 6XXX series alloys
Dilute:
- natural ageing creates metastable GP zones
- GP zone acts as nucleation site during artificial ageing
- homogeneous distribution of Mg2Si
Concentrate:
- natural ageing creates metastable GP zones
- artificial ageing will reduce strength as GP dissolve on heating meaning less nucleation sites for Mg2Si precipitates
- Therefore need to age immediately after quenching
- Adding Cu helps, but reduces corrosion resistance and welability
uses of 6XXX series
Window frames
List properties of 7XXX series
- highest strength Al alloy
- heaviest alloy due to Zinc
- age at much lower temperature which means problems occur when operating at high temperatures (Concorde)
- precipitation sequence:
SSSS –> GP zones –> η’ + η
Uses of 7XXX series
upper wing skin where compressive stresses dominate
Describe how stress corrosion cracking occurs
occurs under the joint action of a corrosive environment, a susceptible material and a mechanical stress
Would produce failure when either of these alone would not (stress can be residual)
What happens to stress corrosion resistance when material aged via ageing
- SCC resistance decreases as the precipitates that from along grain boundaries are more susceptible to corrosion and therefore act as a pathway for chemical corrosion
How to solve the fact stress corrosion resistance reduces with precipitate forming
solution is to use multistage age hardening
low temp first for precipitates in grains, then high temp to coarsen precipitates at grain boundaries
