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Aluminium and its alloys

EF420 Lecture 11John 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 designationsAlloy Group Designation

Pure aluminium (99.00% min) 1xxx

Al-Cu 2xxx

Al-Mn 3xxx

Al-Si 4xxx

Al-Mg 5xxx

Al-Mg-Si 6xxx

Al-Zn 7xxx

Al + other element 8xxx

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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 tempersCode Description-O Annealed-F As fabricated (no mechanical

property limits)-H1x Strain hardened-H2x Strain hardened and partially

annealed-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 20Distance from fusion line mm

6061-T4, AW

60

70

80

90

100

110HV6061-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)