Aluminium and its alloys

112 Pages·2007·3.7 MB·English

Lecture 2
AAlluummiinniiuumm aanndd iittss aallllooyyss
Subjects of interest
• Why aluminium alloys?
• Production of aluminium
• Physical metallurgy of aluminium alloys
• Heat treatments of aluminium alloys
• Corrosion in aluminium alloys
• Mechanical properties of aluminium alloys
• Corrosion resistance of aluminium alloys
• Classification of aluminium alloys
• Wrought aluminium alloys
• Cast aluminium alloys
Suranaree University of Technology Tapany Udomphol May-Aug 2007
OObbjjeeccttiivveess
• This chapter provides fundamental knowledge of different
methods of productions / heat treatments of aluminium alloys
and the use of various types of cast and wrought aluminium
alloys.
• The influences of alloy composition, microstructure and
heat treatment on chemical and mechanical properties of
aluminium alloys will be discussed in relation to its
applications.
Suranaree University of Technology Tapany Udomphol May-Aug 2007
Introduction- Why aluminium alloys?
• Abundant element of 8% on earth crust and normally found in
oxide forms (Al O ), i.e., bauxite, kaolinite, nepheline and alunite.
2 3
• Found in United states, Italy, France.
• Have not been found in Thailand but in Malaysia, Vietnam.
Bauxite with penny Bauxite-pebbly Kaolinite
http://www.galleries.com
Suranaree University of Technology Tapany Udomphol May-Aug 2007
Introduction- Why aluminium alloys?
www.corusgroup-koblenz.com http://ngdir.ir/minemineral
Comparison of Al and steel
Attractive properties
• High corrosion resistance
• Excellent machining properties
• Light weight
• High thermal/electrical conductivity
• High ductility/easily deformable
http://ngdir.ir/minemineral
Suranaree University of Technology Tapany Udomphol May-Aug 2007
Applications for aluminium alloys
Construction
Containers
& Equipment
& Packaging
Automotives
Aerospace
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PPrroodduuccttiioonn ooff AAlluummiinniiuumm
France 1855: • H. Sainte-Claire Deville first reduced
aluminium chloride with sodium.
Austria 1888: • Karl Josef Bayer first patented the Bayer process
(digesting crushed bauxite in strong sodium
hydroxide solution at temperatures upto 240oC.
Germany : • Hall-Héroult introduced Hall-Héroult process by
dissolving the alumina in molten cryolite (Na AlF )
3 6
United states • Alcoa commenced a chloride-based
1976: smelting process using alumina combined
with chloride.
Suranaree University of Technology Tapany Udomphol May-Aug 2007
EExxttrraaccttiioonn ooff aalluummiinniiuumm
Aluminium can be extracted from bauxite, Kaolinite or nepheline.
30-50% Alumina (Al O )
2 3
3-13% Silica (SiO )
Bauxite: 2
10-18% Titanium oxide (TiO )
2
Balanced water (H O)
2
30-32% Alumina (Al O )
Kaolinite 2 3
Balanced Silica (SiO ) and water (H O)
2 2
30% Alumina (Al O )
2 3
Nepheline 40% Silica (SiO )
2
20% Na O + K O
2 2
Note: extraction processes of aluminium require high electricity
(cid:1)
consumption. suitable for countries where cheap electricity is supplied.
Suranaree University of Technology Tapany Udomphol May-Aug 2007
Bayer process
Alumina (Al O ) is extracted by digesting crushed bauxite in
2 3
strong sodium hydroxide (NaOH) solution at Temp upto 240oC.
First step
• Most of alumina is dissolved to give sodium aluminate (NaAlO )
2
which is soluble in water.
• Insoluble residues or ‘Red mud’, mainly composed of oxide of Fe
and Si, are removed by filtration.
Al O .xH O + 2NaOH → 2NaAlO + (x +1)H O
…Eq. 1
2 3 2 2 2
Note: Optimisation of the process with the aid of temperature and
pressure, depending on the nature of bauxite ores, (to obtain different
forms of alumina αααα, ββββor γγγγ).
Suranaree University of Technology Tapany Udomphol May-Aug 2007
Bayer process
Second step
• The next step is to decompose NaAlO to give alumina (Al O ), at
2 2 3
temp ~ 50oC.
• This reverse reaction is achieved by adding crystals of trihydrate
Al O .3H O (seeding agent) to promote fine precipitates of this
2 3 2
compound.
…Eq. 2
2NaAlO + 2H O → 2NaOH + Al O .3H O
2 2 2 3 2
• The water is then removed by calcining at high temperatures.
(cid:1)
At Temp ~ 400-600oC γγγγalumina (chemically active).
(cid:1)
At Temp ~ 1200oC ααααalumina (inert)
∆
…Eq. 3
Al O .3H O → Al O
2 3 2 2 3
Suranaree University of Technology Tapany Udomphol May-Aug 2007
Bayer process
• In the case of bauxite containing high silica, alumina loss
during the process is increased.
• Silica reacts with NaOH at the first step to give sodium silicate
Na SiO .
2 3
SiO + 2NaOH → Na SiO + H O
…Eq. 4
2 2 3 2
• Sodium silicate then reacts with sodium aluminate to give
nonsoluble sodium alumosilicate.
2NaAlO + 2Na SiO → Na Al Si O .2H O + 4NaOH
…Eq. 5
2 2 3 2 2 2 8 2
Solution: 1) Using bauxite with high alumina, low silica.
2) Adding lime or lime stone to react with bauxite to give
calcium silicate Ca SiO .
2 4
Suranaree University of Technology Tapany Udomphol May-Aug 2007