Aluminium alloys and magnesium alloys
Al alloys:
Mg alloys:
Wrought Al alloys
Mg-Al-base alloys
Cast Al alloys
Zr-containing alloys
Precipitation hardening
Cast Mg alloys
Die casting
Aluminium is the most abundant metallic
element in the earth
Al:
atomic number
13
Atomic mass
26.982
Crystal structure fcc, a = 0.4041 nm
Melting poing
660ºC
Boiling point
2520 ºC
Density (r)
2.70 g/cm3
Elastic modulus E = 70GPa
Specific modulus E/r = 26
O
45.2%
Fe
5.8%
Si
27.2%
Ca
5.06%
Al
8%
Mg
2.77%
Applications:
Building and construction
Containers and packaging
Transportations
Electrical conductors
Machinery and equipment
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Alloy designation - International alloy designation system (IADS)
Wrought Al alloys:
Cast Al alloys
major alloying element(s)
1xx.x
pure Al
2xx.x
Cu
3xx.x
Si (Cu and/or Mg)
4xx.x
Si
5xx.x
Mg
7xx.x
Zn
8xx.x
Sn
9xx.x
other element
High purity Al: very low yield strength ~ 10 MPa, need to be alloyed
Al alloys
1. Wrought alloys (85%)
Non-heat-treatable Al alloys
High-purity Al alloys (1xxx series)
Al-Mn and Al-Mn-Mg alloys (3xxx series)
Al-Mg alloys (5xxxx series)
Heat-treatable alloys (respond to strengthening by heat treatment)
Al-Cu alloys (2xxxx series)
Al-Cu-Mg alloys (2xxxx series)
Al-Mg-Si alloys (6xxxx series)
Al-Zn-Mg and Al-Zn-Mg-Cu alloys (7xxxx series)
2. Cast alloys
Al-Si alloys
Al-Cu alloys
Al-Mg alloys
Al-Zn-Mg alloys
2
Wrought alloy productions:
Rolled plate (>6 mm in thickness)
Sheet
(0.15 – 6 mm)
Foil
(< 0.15 mm)
Extrusions
Tube
Rod bar and wire
Precipitation Strengthening
(p402 11.9)
• Internal wing structure on Boeing 767
Adapted from chapteropening photograph,
Chapter 11, Callister 5e.
(courtesy of G.H.
Narayanan and A.G.
Miller, Boeing Commercial
Airplane Company.)
• Aluminum is strengthened with precipitates formed
by alloying.
Adapted from Fig.
11.26, Callister 7e.
(Fig. 11.26 is
courtesy of G.H.
Narayanan and A.G.
Miller, Boeing
Commercial Airplane
Company.)
•alloy 7150-T651 (6.2 Zn,
2.3Cu, 2.3Mg, 0.12Zr, the
balance Al)
•transition phase h’ and
equilibrium phase h
1.5mm
3
Al-Cu alloys
1. The maximum solubility of Cu in Al: 5.65 wt% at 548ºC
2. Eutectic reaction: L fi a (Al) + q (CuAl2)
3. Alloy of interest: Al-4.5wt% Cu
Slow cooling from 550ºC to RT
a+q
Coarse precipitates form at grain boundaries in an Al-Cu(4.5%) alloy when
slowly cooled from the single phase a region to the two-phase (a+CuAl2)
region. The isolated precipitates do little to affect alloy hardness.
4
Precipitation hardening (age-hardening)
a+q
By quenching and then reheating an Al-4.5Cu alloy, a fine dispersion of
precipitates forms within the a grain. These precipitates are effective in
hindering dislocation motion and, consequently, increasing alloy hardness
(and strength). This process is known as precipitation hardening, or age
hardening
Peak hardness
overaging
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(a) A supersaturated a solid solution, (Cu: substitutional atoms) (b) A transition
q’’ precipitate phase, (c) the equilibrium q phase within the a-matrix phase.
1. GP (Guinier-Preston) zone formed at low
temperature, 130ºC
2. transition phase q’’, 130ºC for long time, or <
180ºC
GP zone
3. equilibrium phase q (CuAl2), formed at T>
190 ºC
Coherent interface
The precipitation hardening
characteristics of a 2014 Al alloy
(0.9% Si, 4.4% Cu, 0.8% Mn,
0.5%Mg)
(a) Yield strength,
(b) ductility (%EL)
f11_27_pg406
6
q’’ precipitates formed in a cast Al alloy. TEM. (a) bright field;
(b) dark field I; (c) Dark field II
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How can age-hardening happen?
1. There is a decrease in solid solubility of the alloying
element with decreasing temperature (see phase diagram).
2. The fine dispersed microstructure can be created during
ageing
How to perform an age-hardening treatment
1. Solution treatment
Heated to a single-phase region, e.g. the a (Al) region
2. Quenching
rapid cooled to room temperature to form a supersaturated solid
solution (SSSS*)
3. Aging
Decomposition of the SSSS - to form the fine precipitates
SSSS * - an unstable condition and easy to form metastable phases to
lower the energy of the system
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Age-hardening mechanisms
• dislocation by- passing
Needle-shaped
precipitates in a Mg alloy
Interaction of (001) glide dislocation with b1’
precipitates. Mg-8Zn-1.5RE. TEM. [0001]Mg
beam direction.
• dislocation cutting or
shearing of precipitates
Sheared g’ particles in Ni-19Cr-6Al
aged 540h at 750ºC and deformed 2%
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7075-T73: Al-5.6Zn-2.5Mg-1.6Cu,
Die-forged
Y.S = 430 MPa, T.S = 500MPa,
Elongation = 13
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Al casting alloys
Cast processes
Sand casting
Permanent mould casting (gravity die casting)
Die casting
Why cast
Low melting temperature, 660º-450ºC (Mg-Al alloys)
Negligible solubility for all gases (except H2)
Good surface finish
Good castability
Good fluidity
Good feeding ability
The major problem
The relatively high shrinkage (3.5-8.5%)
•Al-Si-based cast alloys
3xxx.x
•Maximum solubility of Si in Al: 1.65 wt% at 577ºC
•Eutectic type, eutectic composition: 12.7% Si
Hypoeutectic alloy, Si<12.7%
hypereutectic alloy, Si content >12.7%
• a(Al) - Si phases
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Si phase
•A390, Al-17Si-4Cu-0.55Mg
•coarse eutectic fi low ductility
•Modification: to refine the eutectic structure
•By adding sodium salts (0.005-0.015%), or phosphorus, or strontium (Sr) (0.03-0.05%)
Thin walled cast Al-Si alloy automotive transmission casing.
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Aluminum beverage can in various stages of production including:
drawing, dome forming, trimming, cleaning, decorating, and neck and
flange forming.
Mg
atomic number
Atomic mass
Crystal structure
Melting poing
Boiling point
Density (r)
Elastic modulus
Specific modulus
12
24.305
hcp, a = 0.32094 nm, c = 0.52107 nm
650ºC
1090 ºC
1.736 g/cm3
E = 45GPa
E/r = 26
1m3 of sea water contains 1.3 kg magnesium
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The advantages of magnesium alloys
• Lowest density (~1.8 g/cm3) of all metallic constructional materials
• High specific strength
• Good castability, suitable for high pressure die-casting
• Good machinability
• High thermal conductivity
• High dimensional stability
• Good electromagnetic shielding property
• High damping characteristics
• 100% recyclability
Applications:
•Transport industry
• Portable electronics
• Telecommunication
Designation of various Mg alloys
A
Al
M
Mn
W
Y
E
rare earth (Ce, La, Nd, etc.)
Q
Ag
S
Si
K
Zr
Z
Zn
Y
Sb (antimony)
L
Li
H
Th (thorium)
e.g.
AZ91: Mg-9Al-1Zn
AM60: Mg-6Al-0.3Mn
AE42: Mg-4Al-2RE
AS21: Mg-2Al-1Si
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Automotive Applications
Cylinder head cover
Hand break leverer-Porsche
Intake manifold -Daimler Chrysler
Intake manifold-Daimler Chrysler
Oil pan housing
Key lock housing
Door-Lupo
Oil pan-Honda
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Steering wheel-Ford
Steering wheel-Lupo
Transmission housing
Cock pit component.General Motors
Instrument panel-General Motors
Radiator support 1
Radiator support 2
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Non Automotive Applications
Handycam-Sony
Chainsaw-Stihl
Phone frame-Ericsson
laptop
stirrups
Speaker parts
Video camera-Sony
Prunning shears
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Suitcase frame
High pressure die-casting
Cold chamber process
Hot chamber process
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The disadvantages of magnesium alloys:
• Low elastic modulus
• Limited cold workability and toughness (hcp structure)
• Limited creep resistance at elevated temperatures (Tm = 650°C)
• High degree of shrinkage on solidification (high thermal expansion)
• High chemical reactivity (free 3s2 valence electron structure)
• In some applications limited corrosion resistance (electrode potential v = -2.31 V)
Focus:
• To improve high temperature performance
• To improve corrosion resistance
Cast Magnesium Alloys and their applicable temperatures
Mg-Al-based alloys
Y-containing alloys
• AZ91 (125°C)
• EZ alloys (Mg-RE-Zn-Zr) up to 200°C
• AM alloys - AM60, AM50, AM20
• QE alloys (Mg-Ag-RE-Zr) (200° -250°C)
• AS21 (150°C)
• WE alloys (Mg-Y-RE-Zr) (250° -300°C)
• AE42 (150° -175°C)
• Mg-Sc-Mn-X (300°C)
• Mg-Al-Ca (up to 200°C)
HZ22 (Mg-Th-Zn-Zr), applicable up to 350°C, but radioactivity problem
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Die-casting of the Mg-Al-based alloys
AZ91
AM50
Die cast AZ91 and AM alloys
5 mm
5 mm
AM50, as cast. SEM/SEI.
AZ91, as cast.
3 mm
Cooling rate during solidification: 100-1000ºC/Sec.
3 mm
Non-equilibrium solidification causes a coring
effect in the Mg-Al solid
solution
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