7.2 Non-ferrous Metal Alloy

Properties of pure aluminium and aluminium alloy

low density

low MP (limited high temp capabilities)

high electrical and thermal conductivity

corrosion resistant →produce thin layer of alumina

only can weld in inert atmosphere (have to remove oxide layer for metal to weld together)

difficult to extract as only exist in mineral form

low strength(pure), comparable strength to iron(alloy)

but high ductility (pure), lower ductility (alloy)

Two main type of aluminium alloy

1. cast (eutectic) → casted then straight to service

2. wrought (formed)

→ further categorised into heat treatable or not

Advantages of having a eutectic composition for casting processes

no intermediate process between liquid and solid (convert fully from liquid to solid in one temeprature)

→ easier for no defects to form during casting

Main alloying addition to aluminium

Cu & Mg→ solid solution strengthening + precipitation hardening

Si→ for eutectic composition(~10%Si)

Changes in property with addition of Cu to Al alloy

solid solution strengthening →minor improvement in strength

Increasing Cu promote intermetallic CuAl2 formation (θ) under equilibrium cooling → hard but poor tensile strength

Process of age hardening (form of precipitation hardening)

1. solution treat alloy to be within single phase super saturated solid solution region(α= low temp phase)

2. Quench to prevent formation of 2nd phase

2. reheat/age at 2 phase region for a certain period of time so that hard theta forms in small precipitate

How does precipitate hardening increase strength

precipitate acts as obstacles which require more stress for dislocation line to pass through precipitate

Relationship of precipitate spacing and strength of material

σ inversely propertional to spacing btw precipitate λ

Requirement for an effective strengthening effect

1. precipitate or second phase is harder than base material

2. require suitable phase transition (can cool to a single phase first before aging it to two phases)

3. Quenchable alloy (able to avoid 2 phase transformation initally)

Relationship of mechanical property with aging time

reach peak mechanical property

over aging: longer time for aging increase size of precipitate -. become less effective in preventing dislocation

Relationship between composition of alloying addition and mechanical properties

higher Cu→ better mechanical properties + reach peak strength quicker in aging time

Relationship of temperature with peak hardness and aging time

Higher temp → lower time to reach peak mechanical property

higher temp→ lower peak hardness value

Condition for alloy to go through Guinier-Preston (GP) zones

• at lower aging temperature for higher composition

• Al alloy only

Process of aging for materials going through GP phase

1. Before aging

2. in GP zone, copper gather to form large needle shape during intermediate phase

3. As more copper joins, transition to more intermediate phase denoted as θ’,θ’’,…

4. Reaches equilibrium and precipitate size increase to critical

Limitations of aged Al alloy

1. Limited temperature capabilites

• precipitate dissolve at high temperatures

• OR lead to overage which deprove mechanical properties

2. Welding difficulties

• when locally heating metal, lose precipitate due to overheating, over aging occur as well

Alloying addition of Al for aerospace

Li and Be

→ reduce weight

Properties of pure copper and copper alloy

• good ductility/toughness as they are FCC

• good heat and electric conductivity

• good corrosion resistance (Cu oxide/sulphide forms on surface)

Main copper alloys types

1. Bronze (Cu-Sn (12%)): used for marinetime, musical instruments, low friction components

2. Brass (Cu-Zn)

properties improve by solid solution strengthening

Categories of brass

1. α brass (max 40% Zn)

FCC single phase for solid solution strengthening

2. α-β brass (40-45% Zn)

HCP (hexagonal closed packed) + FCC phase → can be heat treated to form various mechanical properties

Generally: ductility and strength is high in α-β copper composition

Property of Nickel alloy

1. Good ductility/toughness (FCC)

2. excellent corrosion resistance (Ni-Cr alloy)

3. very good oxidation resistance (from Cr and Al)

3 types of Ni alloy

1. Nichrome (20%Cr): high temp application and harsh chemical condition

2. Monel (28-34%): expensive, marintine ,chemical industry

3. Ni-based superalloy