Light Alloys- Introduction Flashcards
What are light elements?
Defined as those with density less than 5g/cm3
Narrowing down the light elements to use
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
Physical properties that are service limiting
Density ρ, Mg lowest
Melting point Tm, Ti highest
Modulus E, Be highest
Deformability and strength of Al
Only one with FCC
Easily drawn, extruded, rolled at low temperatures
Can be strengthened without loss in ductility
Deformability and strength of Ti, Mg, Be
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
Dimensional changes with temperature of Al and Mg
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.
Conductivity of Al
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
Fabrication of light metals
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.
Surface characterisation of light metals
All 4 form stable thin oxide films
High reflectivity and corrosion resistance of Al
Film can be thickened by anodising
Beryllium characteristics and uses
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
Magnesium characteristics
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
Aluminium characteristics and uses
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
Titanium characteristics and uses
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
Properties to consider when selecting light alloy
Cost, strength, density, stiffness,
Ductility, fracture toughness, fatigue resistance,
Creep resistance, corrosion, stress corrosion cracking
What is materials cost made up of?
Cost of purchase
Cost of ownership
How to minimise cost of ownership
Save weight (and fuel cost for transport)
Minimise cost of repair and part replacement (very high for PMSs and low for traditional light alloys)
Order of material cost lowest to highest
Mild steel
Al alloys
Stainless and maraging steels
Ti alloys
Ni based superalloys
Carbon fibre reinforced polymer
Extracting Ti data
TiO2 -> Ti + O2
ΔH0 224900
ΔS0 42.44
Total energy cost 125000 kWh/tonne
Extracting Al data
1/2Al2O3 -> Al + 3/4O2
ΔH0 201630
ΔS0 39.06
Total energy cost 75000 kWh/tonnes (14000 from electrolysis)
Extracting iron data
1/2Fe2O3 -> Fe + 3/4O2
ΔH0 97290
ΔS0 29.96
Total energy cost 15000 kWh/tonne (for steel)
Problem of machining
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
Super plastic forming and diffusion bonding
SPFDB of Ti and Al components is highly material efficient and can produce highly mechanically efficient components with significant weight savings
Specific stiffness
E/ρ
Almost constant between engineering metals
Relevant for bending resistance
Be actually has much higher than rest
Buckling resistance
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
