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Aluminium and its alloys

EF420 Lecture 11

John Taylor

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Features

Large growth in use since 1950 (6 times) Abundant metal - 8% of earth’s crust Light weight SG = 2.7 Moderate to high strength (depending on alloy) Conductivity high (pure metal & low alloys) Corrosion resistant (Al2O3 coating)

Reflectivity high Non-magnetic

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Extraction

Al2O3 obtained from bauxite by the Bayer process

Al2O3 reduced electrolytically by the Hall-Heroult process to make aluminium

The large energy requirement for this process is the major proportion of the cost of aluminium, even with cheap energy sources.

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Fabrication

Ductile metal easily fabricated by rolling and extrusion

Commercially pure metal can undertake a cold reduction of 80 - 90% without annealing

Anneal at 350˚C

Machineability is good, but limited by the tendency to gall

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Aluminium products

Cast alloys Wrought products

Sheet, plate, foil Rod, bar, wire, tube Standard and special extruded shapes Forgings, impacts (combined extrusion and

forging)

Powder metallurgy (dispersion strengthened) products

Structural applications

Static building structures AS1664 series Scaffolding and ladders Transportation

Aerospace, road (trucks, buses, trailers), railway

Machinery and industrial equipment Non-sparking tools, roofs to tanks, chemical process

vessels, jigs, patterns, instruments

Consumer durables Structure of appliances: refrigerators, furniture, cooking

utensils

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Thermal and electrical

Electrical Pure Al has 200% of the conductivity of copper

weight for weight Conductors, heat sinks, capacitors, wave guides,

antennas

Reflectors Mirrors, search lights, loft insulation

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Miscellaneous applications

Packaging Drink cans, foil, hermetically sealed packs

Powders and pastes Reflective paint, printing inks, pyrotechnics,

thermit welding

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Limits of use

Temperature range of -240˚C to +200˚C for normal alloys

Up to 350˚C for special alloys Up to 480˚C for short periods for dispersion

strengthened alloys Low modulus of elasticity, requires stiffening Inferior wear, creep, & fatigue properties to

steel

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Aluminium alloy designations

Aluminum Association (USA) and adopted in Australian standards

Alloy designation systems for wrought products and cast alloys

UNS numbers - ‘A’ followed by AA number Temper designation system for wrought

products Some proprietary alloys

Wrought alloy designations

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Alloy types and properties

Strain hardened alloys (plus solid solution hardening) Precipitation (age) hardened alloys Dispersion strengthened alloys Yield strength from 28 MPa for 1050-O to 455 MPa

for 2024-T815 Strength increases at low temperature No ductile-brittle transition

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Strain hardened alloy tempers

Code Description

-O Annealed

-F As fabricated (no mechanicalproperty limits)

-H1x Strain hardened

-H2x Strain hardened and partiallyannealed

-H3x Strain hardened and stabilised

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Second and third digits

‘x’ usually is a digit between 0 and 9, but may be 2 digits

If x = 8, there has been the equivalent of a 75% cold thickness reduction after anneal

Other ‘x’ numbers represent a proportional amount of strain

A 3rd digit indicates a special variation eg 5083-H116 indicates this material has had a special heat

treatment to reduce exfoliation attack

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Precipitation (age) hardening

Solution treatment - alloy is heated into temperature range to dissolve all B. Quenching retains B in solution (supersaturated)

Soft & ductile condition

Aging - holding solution treated alloy at a temperature at which fine precipitates of are formed

Strengthened condition

T (deg C)

Percentage BPure A

Liquid

L +

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Composition effects

Alloys with low levels of B will only display a weak age hardening effect

Alloys with a high level of , which cannot be dissolved on solution treatment only display a weak hardening effect

Alloys with with intermediate levels of B have the highest strengthening effect

These alloys only display a narrow temperature range for the single phase () phase field

These alloys also have a wide solidification range, are likely to crack during welding, and therefore have low weldability and castability

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Age hardening alloy tempers

-O and -F tempers as above -W: solution treated, but naturally aged -T1 to -T10: indicates a combination of hot

work, cold work, solution treatment, and aging

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Temper designations

T1 Hot work, then naturally age

T2 Hot work, cold work, then naturallyage

T3 Solution treat, cold work, thennaturally age

T4 Solution treat, then naturally age

T5 Hot work, then artificially age

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Temper designationsT6 Solution treat and artificially age

T7 Solution treat and stabilise (overage)

T8 Solution treat, cold work, thenartificially age

T9 Solution treat, artificially age, thencold work

T10 Hot work, cold work, then artificiallyage

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1000 Series alloys

Pure Al, can be work hardened Corrosion performance excellent Electrical and thermal conductivity excellent

eg 1060: 99.6%Al min. 62 IACS

Yield strength up to 145 MPa (1050-H18) Food, chemical, heat exchangers, electrical wiring,

capacitor foil Weldable

2000 Series alloys

Up to 6.3% Cu Eg 2014: 4.4Cu-0.8Si-0.8Mn-0.5Mg Precipitation hardened

130 to 230˚C aging temperature - T6 RP0.2 410 MPa typical

Aircraft structure and mechanical components, vehicle body panels

Weldability poor to fair depending on alloy

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3000 Series

Up to 1.2Mn + Fe in some alloys Eg 3004: 1.2Mn-1.0Mg

Strength from fine particles (Mn,Fe)Al6 which pin grain boundaries

Excellent formability and weldability, very high corrosion resistance

Work hardened up to 250 MPa yield typical 3004-H38

Cans, chemical vessels, industrial roofing, culvert pipe

5000 Series alloys

Mg up to 5.1% in solution increases work hardening rate

Eg 5083: 4.4Mg-0.7Mn-0.15Cr

Work hardened up to 260 MPa yield 5083-H116: RP0.2 228 MPa typical

Excellent weldability, moderate strength, Good corrosion resistance

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5000 series alloy applications

Very popular alloys Marine, auto and aircraft applications Pressure vessels, cryogenics Communication towers Armour plate Some alloys prone to exfoliation or stress corrosion if

Al8Mg5 forms in grain boundaries Avoid high Mg over 65˚C

6000 series

Combination of Mg and Si allows precipitation hardening with Mg2Si precipitates

Eg 6061: 1Mg-0.6Si-0.3Cu-0.2Cr 6061-T6: RP0.2 276 MPa typical

Easily workable alloy with excellent strength, corrosion resistance

Limited to excellent weldability (alloy dependant) Available as sheet, plate, extrusions Applications as 5000 series

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Alclad

Many multiphase alloys have inferior corrosion resistance

These alloys are available as Alclad sheet or plate

This material has a thin layer of pure aluminium roll bonded to one or both surfaces to provide corrosion resistance

Fabrication must be undertaken so as to maintain the integrity of this coating

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Dispersion strengthened

Precipitates resist dissolution at high temperatures (up to 340˚C)

Powder metallurgy (P/M) products SAP - sintered aluminium product (Al2O3)

Metal matrix composites Rapid solidification (RS-P/M) hypereutectoid iron

alloys (plus other systems) Mechanical alloying

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Dispersion strengthening

Proprietary and standard alloys Can be welded but with some loss of properties Aerospace applications

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Aluminium lithium

1 to 4% Lithium raises strength, raises elastic modulus by up to 6%, lowers density by up to 4%

Strengthened by Al3Li, Al2CuMg, AL2CuLi precipitates on aging (depending on composition)

Up to 585 MPa yield typical

Good weldability as well as high strength Inferior toughness, ductility & stress corrosion

performance Aerospace applications

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Fabrication

Machineability better than steel Cold and hot workability excellent Complex extrusion forms common Joined by fusion and non-fusion welding, brazing,

soldering, adhesive bonding and mechanical methods

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Cutting and machining

Plasma cutting - edges require further dressing Shearing Sawing

Extra clearance for chip release

Planing and milling Power plane or rotary tungsten burrs

Filing and scraping

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Joining processes

GTAW and GMAW are the most common processes OFW and MMAW require powerful fluxes Resistance and pressure processes can be used for

many alloys Brazing and soldering some application Adhesives and mechanical fastening widely

applicable

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Weldability

Depends on welding process and alloy composition

Varies from readily weldable (eg wrought alloys by GTAW or GMAW) to not recommended (eg brazing of 2014)

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Fusion welding

Tenacious oxide film Prone to contamination by O2, N2, H2

High conductivity, low melting temperature Slags are tenacious, corrosive Welds tend to under match strength Solidification cracking, particularly of many heat

treatable alloys

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GTAW technique

Power type AC sine or square wave for cleaning action

Zirconiated tungsten electrode Shielding gas

Argon or argon helium

High current for thicker materials

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GMAW technique

Fine electrode wire tends to suffer feeding problems

Feeders and guns Push type limited to thicker wires and short cables Push-pull Spool guns

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Metal transfer

Spray transfer High current works in all positions

Pulsed GMAW

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Technique

Porosity Cleanliness

Preheat Overcome high conductivity

Weld craters Avoid solidification cracking

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Weld filler selection

Avoidance of cracking Tensile strength of weld Weld ductility Service temperature Corrosion resistance Colour match after anodising

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Solidification cracking

Use matching filler for strain-hardened grades Don’t mix 4000 series and 5000 series alloys (Mg2Si

eutectics formed) Don’t mix 5000 series and 2000 series alloys

Many heat treatable grades are hot short No autogenous welding Avoid high dilution Medium copper grades are unweldable

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Crack sensitivity

0 1 2 3 4 5 6 7Percentage Copper

0 1 2 3 4 5 6 7Percentage Magnesium

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HAZ cracking

Liquation cracking can occur in the HAZ of age hardenable grades

Keep arc energy low Choose fillers with a low solidification temperature

Weld metal properties

Matching weld to base material is easy for strain hardened grades, but less easy for heat treatable grades

Careful selection gives optimum properties 4346 filler for 6061-T6 heavy sections for highest strength

when solution treated and aged 1000 and 5000 series fillers for high ductility Avoid filler with more than 3% Mg (5183, 5356, 5556, and

5654) for temperatures over 65˚C to avoid sensitisation to SCC

HAZ properties

Heat of fusion welding causes softening of work hardened grades

Effect is most pronounced where work hardening is highest (H6, 7, 8 and 9 tempers)

Softening of heat treated grades can also occur Age hardened grades (T6) have the most pronounced effect If possible use the solution treated grade (6061-T4) and age

after welding

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Effect of temper

0 5 10 15 20

Distance from fusion line mm

6061-T4, AW

60

70

80

90

100

110HV

6061-T4, PWA

6061-T6, AW6061-T6, PWA

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Solid phase welding

Cold welding - 75% thickness reduction at lap joint Ultrasonic welding - 1.5mm lap joints Explosion welding - cladding and lap joints Friction welding - Low weldability alloys and dissimilar

combinations, Friction-stir welding

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Brazing

Restricted range of alloys 1000, 3000, 5000 with <2% Mg, 6000

Al-Si filler materials Torch, dip, furnace or vacuum processes Clean parts by etching Fluxes are fluoride and chloride salts

Residues MUST be removed

Temperature control is critical

References

AS/NZS 1734: Sheet and plate AS/NZS 1865: Wire, rod, strip AS/NZS 1866: Extrusions AS/NZS 1664: Design rules AS/NZS 1665: Welding WTIA Technical Note 2 AWS Handbook Volume 3 ASM Handbook Volumes 3 and 6 Metals and Alloys in the Unified Numbering System: Society of

Automotive Engineers (USA)