Light Alloys- Introduction

Question 1

What are light elements?

Answer 1

Defined as those with density less than 5g/cm3

Question 2

Narrowing down the light elements to use

Answer 2

31 light elements
14 are metals
8 are alkali or alkaline earth metals which are not suitable for engineering metals
Of remaining 6 Sc and Y are too rare and costly to use as alloy bases
Leaves Be, Mg, Al, Ti

Question 3

Physical properties that are service limiting

Answer 3

Density ρ, Mg lowest
Melting point Tm, Ti highest
Modulus E, Be highest

Question 4

Deformability and strength of Al

Answer 4

Only one with FCC
Easily drawn, extruded, rolled at low temperatures
Can be strengthened without loss in ductility

Question 5

Deformability and strength of Ti, Mg, Be

Answer 5

When all HCP difficult to deform due to restricted slip modes
Pure Ti BCC above 882°C but needs alloying to stabilise this at room temperature (increases density)
Limitation of Be is brittleness and toxicity

Question 6

Dimensional changes with temperature of Al and Mg

Answer 6

Both have high solidification contractions (so high shrinkage) compared to Fe. Can rectify by adding Si to Al castings to reduce αT and reduce solidification contraction to about zero at eutectique (13% Si) composition.

Question 7

Conductivity of Al

Answer 7

Pure Al has 2/3 electrical conductivity and 1/2 density of Cu. Used for heavy applications in overhead wires and busbars. Cu mainly restricted to internal wiring

Question 8

Fabrication of light metals

Answer 8

Mg has exceptional machinability (2x Al and 10x steel) because of high conductivity and low ductility. Combined with high castability makes Mg attractive. Ti shows poor machinability, formability, castability and conductivity.

Question 9

Surface characterisation of light metals

Answer 9

All 4 form stable thin oxide films
High reflectivity and corrosion resistance of Al
Film can be thickened by anodising

Question 10

Beryllium characteristics and uses

Answer 10

High specific modulus and melting point
Limited formability and high cost
Toxic
Low x-ray and neutron absorption
Limited to P/M and special applications
X-ray windows, satellite antennae, heat sinks for brakes, Cu-Be alloys, nuclear

Question 11

Magnesium characteristics

Answer 11

Main competitor is Al
Processed mainly into castings
Good machinability
Lower latent heat per volume than Al
Weight advantage over Al
Worse formability and corrosion resistance
Production is only 2% of Al, 1/3 of it goes into Al alloys

Question 12

Aluminium characteristics and uses

Answer 12

Good corrosion resistance and conductivity
Low weight
1/3 stiffness of steel, lower atom and higher cost
Poor fatigue resistance
Processed as wrought, cast or powder products
Building and domestic, transport and containers
Ti better in some high T applications
Compares favourably to plastics and is recyclable

Question 13

Titanium characteristics and uses

Answer 13

Corrosion resistance food
Strength to weight ratio good
High Tm and good fatigue resistance
Biocompatible
High cost
Aerospace and aircraft engines/airframe
Non-aero uses depend mainly on corrosion resistance
Hip replacements, competes with stainless steel
Special melting procedures and equipment

Question 14

Properties to consider when selecting light alloy

Answer 14

Cost, strength, density, stiffness,
Ductility, fracture toughness, fatigue resistance,
Creep resistance, corrosion, stress corrosion cracking

Question 15

What is materials cost made up of?

Answer 15

Cost of purchase
Cost of ownership

Question 16

How to minimise cost of ownership

Answer 16

Save weight (and fuel cost for transport)
Minimise cost of repair and part replacement (very high for PMSs and low for traditional light alloys)

Question 17

Order of material cost lowest to highest

Answer 17

Mild steel
Al alloys
Stainless and maraging steels
Ti alloys
Ni based superalloys
Carbon fibre reinforced polymer

Question 18

Extracting Ti data

Answer 18

TiO2 -> Ti + O2
ΔH0 224900
ΔS0 42.44
Total energy cost 125000 kWh/tonne

Question 19

Extracting Al data

Answer 19

1/2Al2O3 -> Al + 3/4O2
ΔH0 201630
ΔS0 39.06
Total energy cost 75000 kWh/tonnes (14000 from electrolysis)

Question 20

Extracting iron data

Answer 20

1/2Fe2O3 -> Fe + 3/4O2
ΔH0 97290
ΔS0 29.96
Total energy cost 15000 kWh/tonne (for steel)

Question 21

Problem of machining

Answer 21

Can be very material wasteful
In airbus wing skin panel 80% is machined away
Ti forgings for bulkheads in mid fuselage (not airbus) have about 90% machined away

Question 22

Super plastic forming and diffusion bonding

Answer 22

SPFDB of Ti and Al components is highly material efficient and can produce highly mechanically efficient components with significant weight savings

Question 23

Specific stiffness

Answer 23

E/ρ
Almost constant between engineering metals
Relevant for bending resistance
Be actually has much higher than rest

Question 24

Buckling resistance

Answer 24

E^1/3/ρ
This is specific elastic buckling resistance parameter
Represents performance of long thin panels of material
Low density alloys outperform heavy ones in terms of buckling resistance