Lecture 10 Metals III

8/13/2019 Lecture 10 Metals III

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Non-ferrous metals

From last lecture

Division of metals into ferrous and non-ferrous categories: Iron and steel = general purpose metals

over 90% byweight of alloys in general industrial use (1995)

Non-ferrous metals

used in niche applications where

properties of iron and steel are inadequate

Main drawbacks of iron and steel:

Low corrosion resistance

High density

High temperature strength could be better

(Poor electrical conductivity)


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high temperaturestrength

corrosionresistancelow density

Mg

Al

Ti

Ni

Co

Cu

noble metals

(Au, Pt, Ag)

Ta

Pb

ZnSn


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high temperaturestrength

corrosionresistancelow density

Mg

Al

Ti

Ni

Co

Cu

noble metals

(Au, Pt, Ag)

Ta

Pb

ZnSn


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Light alloys: Mg, Ti, Al

Problem:

Does lower density mean lower mass of component?

1.72.7

4.5

7.98.9 8.9

10.5

19.3

21.5

0

5

10

15

20

25

Mg Al Ti Fe Cu Ni Ag Au Pt

Density (g/cm3)


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Solution:

Ashby diagram


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density (log scale)

Youngs modulus(log scale)


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Designing for minimum weight

e.g. Beam (thickness variable, other dimensions constant)

Wish to achieve a given stiffness for minimum weight

For given stiffness and mass,

CE

=

3/1


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0

0.2

0.4

0.6

0.8

1

1.2

Mg Al Ti Fe

Mass

(nominal units)

Mass required to produce beam of given stiffness


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Aluminium

Properties:

Low density (2.7 g/cm3

) Very good corrosion resistance in common environments (due to

protective oxide layer, can be improved by anodising)

Ductile (fcc crystal structure)

High electrical and thermal conductivity

BUT, low melting point: 660C (e.g. mp of iron is 1535C)

restricts use at high temperatures

Also, Youngs modulus not particularly high (E = 70 GPa)


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Applications: Aerospace & air travel: structural components of planes, fuel

tanks in spacecraft

Building and construction: panels, roofs, window frames

Packaging: beverage cans, foil

Transport: bikes, car engine parts, bus bodies

Electrical: e.g. overhead cables


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International Alloy Designation System

(Classification of wrought alloys)

Designation Main alloying elements Applications (e.g.) 1xxx >99 wt% Al Foil, cables

2xxx Cu Aircraft

3xxx Mn Cooking pans

4xxx Si5xxx Mg Beverage cans

6xxx Mg & Si Window frames

7xxx Zn Aircraft

8xxx Others, including Li

(No universally accepted designation system for cast alloys)


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Strengthening mechanisms in wrought alloys

Distinguish between heat-treatable(precipitation hardeningpossible) and non-heat-treatable

Non-heat-treatable alloys strengthened through work-hardening(1xxx, 3xxx and 5xxx series) and solid solution

strengthening (5xxx series)


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Maximise strength by optimising precipitate distribution

age-hardening

Precipitation hardening

T(C)

t

550

150

1. Solution treat

uniformdistribution of Cu atoms

2. Quench

freezesolid solution inplace

3. Age

allow precipitation to occur

fine

distribution of second phase


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Lecture 10 Metals III

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