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Aluminium alloys and magnesium alloys

Al alloys:

Wrought Al alloys

Cast Al alloys

Precipitation hardening

Mg alloys:

Mg-Al-base alloys

Zr-containing alloys

Cast Mg alloys

Die casting

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

Applications:

Building and construction

Containers and packaging

Transportations

Electrical conductors

Machinery and equipment

Aluminium is the most abundant metallic element in the earth

O 45.2% Fe 5.8%

Si 27.2% Ca 5.06%

Al 8% Mg 2.77%

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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

Alloy designation - International alloy designation system (IADS)

Wrought Al alloys: Cast Al alloys

High purity Al: very low yield strength ~ 10 MPa, need to be alloyed

Al alloys1. 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

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Wrought alloy productions:

Rolled plate (>6 mm in thickness)Sheet (0.15 – 6 mm)Foil (< 0.15 mm)ExtrusionsTubeRod bar and wire

• Internal wing structure on Boeing 767

• Aluminum is strengthened with precipitates formedby 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.)

1.5mm

Precipitation Strengthening

Adapted from chapter-opening photograph, Chapter 11, Callister 5e. (courtesy of G.H. Narayanan and A.G. Miller, Boeing Commercial Airplane Company.)

(p402 11.9)

•alloy 7150-T651 (6.2 Zn, 2.3Cu, 2.3Mg, 0.12Zr, the balance Al)

•transition phase h’ and equilibrium phase h

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Al-Cu alloys

1. The maximum solubility of Cu in Al: 5.65 wt% at 548ºC2. Eutectic reaction: L fi a (Al) + q (CuAl2)3. Alloy of interest: Al-4.5wt% Cu

Slow cooling from 550ºC to RT

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.

a+q

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Precipitation hardening (age-hardening)

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

a+q

overaging

Peak hardness

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GP zone

Coherent interface

1. GP (Guinier-Preston) zone formed at low temperature, 130ºC

2. transition phase q’’, 130ºC for long time, or < 180ºC

3. equilibrium phase q (CuAl2), formed at T> 190 ºC

(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.

f11_27_pg406

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)

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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

Interaction of (001) glide dislocation with b1’precipitates. Mg-8Zn-1.5RE. TEM. [0001]Mgbeam direction.Needle-shaped

precipitates in a Mg alloy

• dislocation by- passing

Sheared g’ particles in Ni-19Cr-6Al aged 540h at 750ºC and deformed 2%

• dislocation cutting or shearing of precipitates

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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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•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%)

Si phase

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 12Atomic mass 24.305Crystal structure hcp, a = 0.32094 nm, c = 0.52107 nmMelting poing 650ºCBoiling point 1090 ºCDensity (r) 1.736 g/cm3Elastic modulus E = 45GPaSpecific modulus E/r = 26

1m3 of sea water contains 1.3 kg magnesium

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• 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

The advantages of magnesium alloys

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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Cylinder head coverHand break leverer-Porsche

Intake manifold-Daimler Chrysler

Intake manifold -Daimler Chrysler

Automotive Applications

Key lock housing

Door-Lupo

Oil pan housing

Oil pan-Honda

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Cock pit component.General Motors

Steering wheel-Ford

Steering wheel-Lupo

Transmission housing

Instrument panel-General MotorsRadiator support 1

Radiator support 2

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Chainsaw-Stihl

Handycam-Sony

laptop

Phone frame-Ericsson

Non Automotive Applications

Prunning shears

Speaker parts

stirrups

Video camera-Sony

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Suitcase frame

Cold chamber process Hot chamber process

High pressure die-casting

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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

Mg-Al-based alloys

• AZ91 (125°C)

• AM alloys - AM60, AM50, AM20

• AS21 (150°C)

• AE42 (150° -175°C)

• Mg-Al-Ca (up to 200°C)

Cast Magnesium Alloys and their applicable temperatures

Y-containing alloys

• EZ alloys (Mg-RE-Zn-Zr) up to 200°C

• QE alloys (Mg-Ag-RE-Zr) (200° -250°C)

• WE alloys (Mg-Y-RE-Zr) (250° -300°C)

• Mg-Sc-Mn-X (300°C)

HZ22 (Mg-Th-Zn-Zr), applicable up to 350°C, but radioactivity problem

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AM50

AZ91

Die-casting of the Mg-Al-based alloys

Die cast AZ91 and AM alloys

5 mm

3 mm

3 mm

AM50, as cast. SEM/SEI. AZ91, as cast.

5 mm

Cooling rate during solidification: 100-1000ºC/Sec.

Non-equilibrium solidification causes a coring effect in the Mg-Al solid solution