Set 8 MME2203 Aluminium and its alloys 2022 PDF

Ferrous and non-ferrous materials Prof Inġ. Bertram Mallia Room 222, Faculty of Engineering [email protected] 1 Introduction to Aluminium Main source of aluminium is the bauxite ore. The ore is refined to obtain alumina (Bayer p...

Ferrous and non-ferrous materials Prof Inġ. Bertram Mallia Room 222, Faculty of Engineering [email protected] 1 Introduction to Aluminium Main source of aluminium is the bauxite ore. The ore is refined to obtain alumina (Bayer process) followed by smelting via electrolytic dissociation into metallic aluminium (Hall-Heroult process) Aluminium consists of about 8% of the mass of the earth’s crust making it the most abundant metal in the crust. Characteristics of Aluminium FCC structure Low melting temperature (660 °C) Non-magnetic; Non-sparking; Reflective; non toxic Low Yield strength. Alloys can increase YS up to 600 MPa. High ductility; Good fabricability Good Corrosion resistance in various environments Easy to machine Weight and a stiffness around a third of steel Excellent thermal conductivity (237W/mK ) Low electric resistivity (27n.ohms.m at 20°C) 2 Image source : https://i.pinimg.com/originals/b9/79/a5/b979a5beccf7b589857e0128ba7b371e.jpg Introduction to Aluminium Low Yield strength: Super purity Al is ~10MPa, commercial purity annealed Al ~30MPa annealed and ~165MPa when fully cold worked. High performance Al alloys can exceed yield strengths of 550 MPa! Stiffness about 70 GPa 3 Introduction to Aluminium 1909 Duralumin was patented by Alfred Wilm – first age hardened alloy (3.5-4.5%Cu, 0.4 -0.8 %Mg, 0.4-1%Mn) which more than doubled the strength. Al-8%Cu used by Wright brothers in first successful aircraft for engine parts. Probably hardened inadvertently. 4 Aluminium alloys Aluminium alloys are very versatile in that a large variety of physical, chemical and mechanical properties can be derived through alloying aluminium. The low strength, hardness, resistance to wear, creep and fatigue of Aluminium limits its commercial usefulness which makes research towards the improvement in the mechanical properties a major Tracked armoured vehicle objective. How? This can be done through changes in composition, mechanical working and/or heat treatment. 5 Aluminium alloying elements Aluminium Elements most commonly present in commercial alloys to provide increased strength – Cu Mn Si Mg Zn particularly when coupled with strain hardening and/or heat treatment These elements all have significant solid solubility in aluminum. Their solubility increases with increasing temperature. The decrease in solubility of elements with temperatures is one fundamental characteristic that provides the basis for substantially increasing the hardness and strength of aluminum alloys by solution heat treatment and subsequent precipitation aging operations. Equilibrium binary solid solubility as a function of temperature for alloying elements most frequently added to aluminum 6 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Solid solubility of elements in Aluminium Element Temperature Maximum solid , C solubility, at% Cu 548 2.4at% (5.7wt%) Cr 661 0.4 Fe 655 0.025 Li 600 16.3 Mg 450 18.5 Mn 658 0.9 Ni 640 0.02 Si 577 1.59 Ag 566 13.8 Sn 228 0.01 Section of Al-Cu eutectic phase diagram Ti 665 0.74 GP zones are extremely fine-scaled (on the order of 3–10 nm in size) solute enriched regions of the material. They provide Zn 443 66.4 hindrance to the motion of dislocations, above that of the solid- Zr 661 0.08 solution strengthening of the solute components. 7 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Aluminium Alloy Classification Al alloys can be divided into 2 main categories: 1. Wrought alloys 2. Cast alloys A further classification then depends on mechanism to develop properties: 1. Heat treatable alloys; where HT can consist of solution heat treatment, quenching and precipitation hardening. 2. Non-heat treatable/ Work hardening alloys; wrought alloys which rely on work hardening through mechanical reduction used in combination with annealing procedures. Some casting alloys are essentially not heat treatable and are used only in as-cast or in thermally modified conditions unrelated to solution or precipitation effects. 8 Wrought and Cast Aluminium and Aluminium Alloy Designation System Designation system is based on the classification used for many years by the Aluminum Association of the United States. Aluminum, ≥99.00% 1xxx Aluminum, ≥99.00% 1xx.x Aluminum alloys grouped by major alloying elements: Aluminum alloys grouped by major alloying elements: Copper 2xxx Copper 2xx.x Manganese 3xxx Silicon, with added copper and or magnesium 3xx.x Silicon 4xxx Silicon 4xx.x Magnesium 5xxx Magnesium 5xx.x Magnesium and silicon 6xxx Zinc 7xx.x Zinc 7xxx Tin 8xx.x Other elements 8xxx Other elements 9xx.x Unused series 9xxx Unused series 6xx.x A four-digit numerical designation system for wrought aluminium A four-digit numerical designations incorporating a decimal point and aluminium alloys. for cast aluminium and aluminium alloys. - The first digit indicate group - The first digit indicates the alloy group - For 1xxx; 10xx is used for unalloyed composition. Last two digits - For 2xx.x – 8xx.x alloys, the second two digits identify the indicate the min Al % specific aluminium alloy. Last digit indicates product form (0 - - For 2xxx- 8xxx, second digit indicated modification. If 0 indicates casting or 1- ingot) original alloy. Last two digits have no significance but serve to - A serial letter preceding the numerical designation ex A identify alloy indicates modification to original alloys or of impurity limits for 9 unalloyed aluminium The principal aluminum alloys 10 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Main Strengthening mechanisms in Al alloys Crystal lattice distorts (increase with increased difference in atomic radii) Solute Increase stress required for dislocation movement. hardening Depends on type and amount of the alloying element. Can increase YS 6 fold Mn and Mg are examples of elements used with Aluminium. Strain Applicable for wrought alloys. hardening Strength increases due to increase in no of dislocations. by Cold Some alloys ex Al-Mg (5xxx) alloys respond very well to strain hardening. working Strengthening mechanisms Precipitates as result of a series of heat treating processes (Solution heat treatment then quenching followed by aging) and produce the strongest alloys. Precipitation/ Can increase YS of aluminium up to 15 fold depending on alloy and treatment. age hardening Only certain alloys can be precipitation strengthened (2xxx, 6xxx, 7xxx; 2xx.x,3xx.x,7xx.x) Intermetallic compounds (ex Al12 (Fe, Mn)3Si; Al20Cu2Mn3, Al12Mg2Cr) and Second- elemental Si formed during solidification contribute to strengthening. phase constituents Sometimes intermetallic are made to produce fine incoherent precipitates that Grain size prevent recrystallisation (control grain structure) and inhibit grain growth. Grain size reduction to increase strength 11 Hall-Petch relationship Strengthening Mechanisms: Strain Hardening Strain-hardening curves for aluminium (1100),and for Al-Mn (3003) and Al-Mg (5050 and 5052)alloys 12 ASM Handbook, Vol2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials. ASM International 1990 Strengthening Mechanisms: Solid Solution Strengthening For solid solution hardening to be possible, the solute must have: Appreciable solid solubility at the annealing temperature Able to remain in solution after a slow cool; and Not react with other elements to form insoluble phases Strengthening effect increases with increasing difference in the atomic radii between the solute and solvent. Zn, Mg, Cu and Si have significant solid solubilities. 13 Strengthening Mechanisms: Solid Solution Strengthening (a) Listed in order of increasing percent difference in atomic radii. Solid solution strengthening of high purity binary aluminium alloys 14 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Strengthening Mechanisms: Ageing/ Precipitation Hardening Heat treatment involves: Solution heat treatment at a high temperature to maximise solubility (above equilibrium solvus) Rapid cooling or quenching to a low temperature to obtain a super saturated solid solution with solute elements and vacancies. Heat treatment (ageing/precipitation hardening) may then be applied. (below metastable miscibility gap called Guinier-Preston (GP) zone solvus line) Solution heat treatments designed to maximise solubility of elements that participate in subsequent aging treatments. They are most effective near the solidus or eutectic temperature (maximum solubility and rapid diffusion rate). 15 Strengthening Mechanisms: Ageing/ Precipitation Hardening Natural aging curves for three solution heat-treated wrought aluminium alloys EN AW-6061 (AlMg1SiCu) Al-Mg-Si wrought alloy (6061) hardened through precipitation. Note changes in yield strength with increasing time and temperature. 16 ASM Handbook, Vol2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials. ASM International 1990 Strengthening Mechanisms: Ageing/ Precipitation Hardening Al-Cu alloys (2xxx series) Al-Mg-Si alloys (6xxx series) Al-Zn-Mg alloys (7xxx series) 17 Strengthening Mechanisms: Ageing/ Precipitation Hardening Aluminium 7xxx series Cu Si Mg Zn GP zones (spherical) → η hexagonal MgZn2 → T’ semi- coherent hexagonal Mg32 (Al,Zn)49 2XXX 7XXX → T incoherent cubic Mg32(Al,Zn)49 2xxx series GP zones (thin Cu plates on 6XXX {100} planes) → θ’’ coherent, 2 6xxx series layers of Cu separated by 3 Al GP zones (needles along directions layers → θ’ semi-coherent plates in alpha phase)→ β’ semi-coherent rods of of tetragonal CuAl2 on {100} cubic or hexagonal Mg2Si → β fcc Mg2Si planes → θ incoherent forming as plates on {100} planes 18 equilibrium phase of bct CuAl Aluminium Alloy Temper Designation (cast and wrought) (H tempers for wrought products only) W, Solution Heat-Treated (unstable temper) H1, Strain-Hardened Only. H2, Strain-Hardened and Partially Annealed. H3, Strain-Hardened and Stabilized. T1, Cooled From an Elevated-Temperature Shaping Process and Naturally Aged to Substantially Stable Condition. T2, Cooled From an Elevated-Temperature Shaping Process, Cold Worked, and Naturally Aged to a Substantially Stable Condition. T3, Solution Heat Treated, Cold Worked, and Naturally Aged to a Substantially Stable Condition. T4, Solution Heat Treated and Naturally Aged to a Substantially Stable Condition. T5, Cooled From an Elevated-Temperature Shaping Process and Artificially Aged. T6, Solution Heat Treated and Artificially Aged. T7, Solution Heat Treated and Overaged or Stabilized. T8, Solution Heat Treated, Cold Worked, and Artificially Aged. T9, Solution Heat Treated, Artificially Aged, and Cold Worked. T10, Cooled From an Elevated-Temperature Shaping Process, Cold Worked, 19 and Artificially Aged. (T10 - designated in ANSIH35.1 but not in EN515 or ISO2107) Tensile properties of Wrought Al-Cu for different tempers Tensile properties of high-purity, wrought aluminium-copper alloys. O, annealed; W, tested immediately after water quenching from a solution heat treatment; T4, as in W, but aged at room temperature; T6, as in T4, followed by precipitation treatment at elevated temperature 20 ASM Handbook, Vol2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials. ASM International 1990 Aluminium Alloys: Wrought Around 85% of total aluminium is used in the wrought condition. What does this mean? Hot or cold working applied (e.g. rolling, extrusion, drawing) so as to transform cast ingot to the desired product form. Types of Mill Products: Flat rolled (plate, sheet, foil) Rods, bar, wires Tubular Shapes Forgings 21 Strength ranges of various wrought aluminium alloys 22 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Wrought super purity, and CP-Al (1xxx series) 1xxx group includes super pure Al and commercial pure Al having up to 1% of impurities. Properties: Excellent corrosion resistance (resistant to SCC, intergranular corrosion and exfoliation), high thermal and electrical conductivities, low mechanical properties, excellent workability and weldability. Pitting is the principal type of corrosion encountered. Strain hardening may be used to moderately increase strength. Ex CP Al YS30 -> 165MPa Applications: chemical equipment, reflectors, heat exchangers, electrical conductors, capacitors and packaging foil, decorative trim. e.g. 1100 (99.00min Al-0.12Cu) used for food packing trays, chemical processing equipment e.g. 1350 (99.50min Al) used for electrical conductors 23 Non-heat treatable Al-Mn alloys (3xxx series) Mn as major alloying element. Not-heat treatable, but stronger than 1xxx series. A limited % of Mn can be added to Al (1.5wt%) – hence used as a major element in only few alloys. Often small amount of Mg may be added as well. Mn present as submicroscopic particles of precipitate, and in larger particles of Al6(Mn,Fe) or Al12(Mn,Fe)3Si phases have similar solution potentials almost the same as the solid-solution matrix- not significant sites for corrosion initiation. Pitting maybe encountered but not the more drastic forms of corrosion (SCC, Intergranular) Properties: Moderate strength, high ductility, excellent corrosion resistance, Good formability and weldability. Applications: General purpose alloy, beverage cans, cooking equipment, heat exchangers, storage tanks, furniture and roofing, garage doors, and other architectural applications. e.g. 3003 (Al-1.2Mn- 0.12Cu) – (Annealed 35MPa, 20%El; H18 YS165MPa El 2%) most popular general purpose alloy; fuel tanks, chemical equipment, trim e.g. 3004 (Al-1.2Mn-1Mg) – (Annealed 70MPa, 20%El, H18 YS 250MPa El 1%) for Al beverage cans; storage tanks; building products 24 Non-heat treatable Al-Si alloys (4xxx series) Major alloying element is Si (up to 12%). Si is present as second-phase constituent particles. Si is cathodic to Aluminium solid solution by several 100mVs – still minimal effect on corrosion resistance Most alloys are non-heat treatable. Si lowers melting temperature and weight. Low melting temperature makes it useful for joining applications where a lower melting point than the base material is needed. When used in welding HT alloys, it will pick some of the elements are respond to HT to a limited extent. Applications: filler wire for welding ex 4043 (5.2Si) and brazing for structural and automotive applications, architecture with dark gray/charcoal anodic oxide finish (alloys with appreciable amounts of Si), Alloy 4032 (12.2Si-0.9Cu-1.0Mg-0.9Ni) has a low coefficient of thermal expansion and high wear resistance. Well suited for forged engine pistons. YS (T6 temper) 320MPa and El of 7% (strongest 25 of 4xxx) Non-heat treatable Al-Mg alloys (5xxx series) 5xxx group has Mg as major alloying element. Can contain from 0.8% to 5% of Mg. Affects strength very effectively (solid solution) Alloy 5005 Al-0.8Mg annealed YS:40 MPa, UTS: 125 MPa Alloy 5456 (Al-5.1Mg-0.8Mn-0.12Cr) annealed YS: 160 MPa, UTS 310 MPa. High Elongation ~ 25% Work hardening rate increases rapidly with increasing Mg content. Ex 5456 H343: YS 300 MPa, UTS 385 MPa, El 7% Solid solution strengthening of Mg in Al alloys 26 ASM Handbook, Vol2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials. ASM International 1990 Non-heat treatable Al-Mg alloys (5xxx series) Limited on the amount of cold work and safe operating temperatures permissible for higher-magnesium alloys (over ~3.5% for operating temperatures above ~65 °C) to avoid susceptibility to stress corrosion cracking. This occurs due to Mg5Al8 precipitating at grain boundaries and slip bands. These work hardened alloys may also soften over time (age-softening) due to localised recovery within the deformed grains. (Use H3 tempers) 5052 Aluminium alloy fuel tank Main Characteristics: moderate to high strength; Good resistance against salt water corrosion; good weldability Applications: welded applications ex large fuel/milk/grain transport tanks, pressure vessels. Good corrosion resistance - hulls for small boats; super structures of ocean going vessels; chemical plants; good polishability – automotive trim, architecture components. 27 Misc. Alloys (8xxx series) Alloys with wide range of compositions: Eg 8001 (Al-1.1Ni-0.6Fe) – used on nuclear energy installations where resistance to corrosive attack by water at high temperature and pressure e.g. 8011 (Al-0.75Fe-0.7Si) is used for bottle caps due to good deep drawing qualities. Also as electrical conductor e.g. 8280 based on Al-Sn used as bearing alloy for diesel engineed motor cars and trucks. e.g. 8090 (Al-2.4Li-1.9Cu-0.9Zn-0.12Zr) - heat treatable - based on Li to reduce density; increase stiffness. Replaced medium-to-high strength 2xxx and 7xx in some aircraft applications. 28 Heat treatable Wrought Al alloys Heat-treatable alloys depend on age hardening to develop an enhanced strength (2xxx, 6xxx, 7xxx). Divided into 2 groups:  Medium strength and weldable e.g. Al-Mg-Si (6xxx) and Al-Zn-Mg (7xxx)  High strength alloys developed primarily for aircraft construction (most have very limited weldability) e.g. Al-Cu (2xxx), Al-Cu-Mg (2xxx), Al-Zn-Mg-Cu (7xxx) 29 Heat treatable Al-Cu alloys (2xxx) Based on Cu, often Mg as a secondary addition. Solution treatment applied to obtain optimum properties after which properties are comparable to/exceed those of low- carbon steel. Ageing sometimes employed to further increase yield strength (reduction in ductility). Inferior corrosion resistance to other alloys. Can be subject to intergranular corrosion therefore when used as sheets they are usually clad with a high-purity Al or Mg-Si alloy (6xxx) to provide galvanic protection (Alclad). Usually have limited weldability except alloy 2219 (Al-6.3%Cu- 0.3Mn). Temperature limit of 150 oC. Applications: Parts structures requiring high strength to weight ratio. Ex truck and aircraft wheels, truck suspension parts, aircraft fuselage and wing skins, and structural parts 30 Heat treatable Al-Cu alloys (2xxx) Alloy Si Fe Cu Mn Mg Cr Zn Other design. 2024 0.5 0.5 3.8-4.9 0.3-0.9 1.2- 0.1 0.25 0.15Ti 1.8 2014 0.5-1.2 0.7 3.9-5.0 0.4-1.2 0.2- 0.1 0.25 0.15Ti 0.8 2017 0.2-0.8 0.7 3.5-4.5 0.4-1.0 0.4- 0.1 0.25 0.15Ti 0.8 2090 0.1 0.12 2.4-3.0 0.05 0.25 0.1 / 0.15Ti 0.08-0.15Zr 1.1-2.6Li 2219 0.2 0.3 5.8-6.8 0.2- 0.02 / 0.1 0.02-0.1Ti 0.40 0.05-0.15V 0.1-0.25Zr 2618 0.1- 0.9- 1.9-2.7 / 1.3- / 0.1 0.9-1.2Ni 0.25 1.3 1.8 2048 0.15 0.2 2.8-3.8 0.2-0.6 1.2- / 0.25 0.1Ti 1.8 2024-T4 TS 470MPa YS 325MPa El 20% 2014-T4 TS 430MPa YS 290MPa El 20% 2014-T6 TS 480MPa YS 410 MPa El 13% 2219-T62 TS 410MPa YS 290MPa El 10% 2219-T87 TS 480MPa YS 390MPa El%10% 31 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Heat treatable Al-Mg-Si alloys (6xxx) Based on Mg and Si (up to 1.5wt% each in the ratio to form Mg2Si i.e. 1.7:1 by weight) Strengthened during heat treatment (age hardening). Alloy first solution treated at about 530 oC to uniformly distribute Mg and Si. Followed by quenching and ageing ex 165 oC for 10 h. Precipitation upon age hardening occurs by formation of Guinier-Preston (G-P) zones and a very fine precipitate. Both restrict movement of dislocations increasing strength. May be formed (e.g. extruded) in solution treated condition (T4) and strengthened after forming to full T6 properties by precipitation heat treatment. Main characteristics: Medium strength, good corrosion resistance, immune to stress corrosion cracking, good weldability Applications: Architecture extrusions; pipe railings; heavy duty structures required good corrosion resistance; truck and marine; pipeline; automobile body sheet; high strength electric conductor wire/bus conductor; extrusions and forgings for welded structures 32 Heat treatable Al-Mg-Si alloys (6xxx) 380TS 350YS Alloy Si Fe Cu Mn Mg Cr Zn Ti design. 310TS 280YS 6061 0.4- 0.7 0.15- 0.15 0.8- 0.04- 0.25 0.15 0.8 0.4 1.2 0.35 345TS 310YS 6063 0.2- 0.35 0.1 0.1 0.45- 0.1 0.1 0.1 0.6 0.9 290TS 250YS 360TS 325YS 6003 0.35-1 0.6 0.1 0.8 0.8- 0.35 0.2 0.1 1.5 6201 0.5- 0.5 0.1 0.03 0.6- 0.03 0.1 0Ti 0.9 0.9 0.06B 6010 0.8- 0.5 0.15- 0.2- 0.6-1 0.1 0.25 0.1 1.2 0.6 0.8 6013 0.6-1 0.5 0.6-1.1 0.2-0.8 0.8- 0.1 0.25 0.1 300TS 255YS 1.2 Al-Mg2Si groups: (1) Mg+Si < 1.5% ex 6063 -> extruded – low quench sensitivity – air-quenched at press and artificially aged ex T6 – YS 215MPa TS245MPa 260TS 240YS (2) Mg+Si > 1.5% with other additions ex Cu to increase strength at T6, Mn, Cr, Zr used for controlling grain structure ex 6061 -> more quench sensitive (require separate Relationships among commonly used alloys in the 6xxx series solid treatment followed by rapid quenching) – general purpose structure materials. (Al-Mg-Si). Strength values in MPa Ex 6061 -T6 YS-250MPa Ductility ~11% (3) Mg+Si overlapping prior groups but with substantial excess Si -> excess Si increases strength - better response to age hardening; but may reduce ductility Ex 6009; 6010 Ex 6009-T4 TS 230MPa YS130MPa El 24% 33 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 6009-T6 TS345MPa YS325MPa El 12% Heat treatable Al-Zn alloys (7xxx) Zinc, in amounts of 1 to 8% is the major alloying element in 7xxx series alloys. When coupled with a smaller percentage of magnesium results in heat-treatable alloys of moderate to high strength. Increasing Mg+Zn conc increases strength but decrease the overall corrosion resistance. The copper-free alloys of the series have many desirable characteristics: moderate-to-high strength; excellent toughness; and good workability, formability, good resistance to general corrosion, and weldability Addition of Cu to the Al-Zn-Mg system together with small amounts of Cr and Mn results in the highest strength aluminium based commercially available alloys – lower resistance to general corrosion but benefit SCC resistance Control over microstructure, composition and heat treatment are often necessary to maintain adequate resistance to SCC and exfoliation. Example controlled by: Overaging ex T73 Cooling rate after solution treatment Maintaining a nonrecrystallized structure through the use of additions such as zirconium Copper or chromium additions Adjusting the zinc-magnesium ratio closer to 3:1 7xxx series alloys are used in airframe structures, mobile equipment and other highly stressed parts. Higher strength 7xxx alloys exhibit reduced resistance to stress corrosion cracking and are often utilized in a slightly over- aged temper (ex T76) to provide better combinations of strength, corrosion resistance, and fracture toughness. 7050 and 7150 for upper wing skin due to good fatigue resistance, fracture toughness and compressive strength 34 Heat treatable Al-Zn alloys (7xxx) Alloy Zn Si Fe Cu Mn Mg Cr Ti/other 675TS design. 630YS 7075 5.1- 0.4 0.5 1.2-2.0 0.3 2.1- 0.18- 0.2 6.1 2.9 0.28 600TS 540YS 7150 5.9- 0.12 0.15 1.9-2.5 0.1 2.0- 0.04 0.06Ti 6.9 2.7 0.08-0.15Zr 7049 7.2- 0.25 0.35 1.2-1.9 0.20 2.0- 0.1- 0.1 8.2 2.9 0.22 7001 6.8- 0.35 0.4 1.6-2.6 0.2 2.6- 0.18- 0.2 580TS 540YS 8.0 3.4 0.35 7005 4.0- 0.35 0.40 0.1 0.2- 1.0- 0.06- 0.01-0.06Ti 5.0 0.7 1.8 0.2 0.08-0.2Zr 7008 4.5- 0.1 0.1 0.05 0.05 0.7- 0.12- 0.05 5.5 1.4 0.25 7075-O TS 230MPa YS 100MPa El 17% 7075-T6 TS 570MPa YS 500MPa El 11% 7075-T73 TS 500MPa YS 430MPa 35 Relationships among commonly used alloys in the 7xxx series (Al-Zn-Cu-Mg-Cr). Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Cast Aluminium alloys Aluminium is one of the most versatile of the common foundry metals. Characteristics: - Good fluidity for filling thin sections - Low melting point - Rapid heat transfer to mould (short cycle times) - Hydrogen is the only gas with a appreciable solubility in Al but can be readily controlled by processing methods. - Chemically stable - Many alloys are free from hot short cracking and hot tearing. - Good as cast surface finish with lustrous surface and little to no blemishes. 36 Strength ranges of various cast aluminium alloys Aluminum-silicon phase diagram and cast microstructures of pure components and of alloys of various compositions Cast alloys are strengthened by same mechanisms as wrought (exception strain hardening) and classified as heat treatable and non-heat treatable. A major difference is that alloys used in greatest volumes contain much higher Si content. Casting alloys need to have sufficient amount of eutectic-forming elements (usually Si) in addition to the strengthening elements. 37 Alloying: Understanding the Basics, Edited by J.R. Davis, ASM International 2001 Cast Aluminium alloys: designations Aluminium 1XX.X Cu Si (Mg, Cu) Si Mg Zn Sn 2XX.X 3XX.X 4XX.X 5XX.X 7XX.X 8XX.X Non-heat Alloying Heat treatable element treatable 38 Aluminum - Copper alloys (2xx.0) Capable of developing the highest strengths among all casting alloys - used where strength is a predominant requirement These alloys (ex A201.0, 202.0, 204.0, A206.0) contain 4 to 6% Cu and 0.25 to 0.35% Mg, with highly restrictive impurity (iron and silicon) limits, and in some cases also contain 0.25 to 0.35% Mn or Cr and sometimes Ag - fair castability (no second fluid phase) The 2xx.x alloys also have the highest strengths and hardness of all casting alloys at elevated temperatures (to 300 °C). Alloys 222.0, 224.0, 238.0, 240.0, 242.0, and 243.0, some with higher copper contents and up to 2% Mg (6% in alloy 240.0) and additions of manganese, nickel, vanadium, and/or zirconium, are used primarily at elevated temperatures. Heat treatment is required with the 2xx.x alloys for development of highest strength and ductility and must be properly applied to ensure high resistance to SCC. General corrosion resistance of these alloys is lower than those of other types of casting alloys, and protection by surface coatings is required in critical applications. Applications: diesel and aircraft Engine pistons; aircraft cylinder heads; electric hand irons; aircraft wheels 39 Cast alloys Al-Si (Mg, Cu) (3xx.0) 3xx.x group which in addition to Si (5 to 22%) contain Mg (0.3 to 1%), Cu (0 to 4.5%) or both is the most high-volume used group. (Al-Si-Mg, Al-Si-Cu, Al-Si-Cu-Mg) Cu, Mg contributes to strength & hardness ( as cast F – increased solid solution; artificial aging (T5 or solution + aging (T6, T7) – result of precipitates based on Mg2Si, Al2Cu, Al2CuMg or combinations of). Alloys containing both Cu and Mg have higher strengths at elevated temperatures. Si improves castability and reduces hot shortness - Therefore for more complex castings and permanent mould processes higher Si alloys are used. Increase Si and Ni products a low coe – benefits applications for pistons and cylinder blocks. If Si exceeds 12% (ex 390.0) primary Si phases are present imparting a good wear resistance. e.g. 390.0 (17Si-4.5Cu-1.3Fe-0.6Mg) and 380.0 (8Si-2Fe-3.5Cu-0.5Mn-0.1Mg-0.5Ni-3Zn) both used for die casting. Applications: motor frames and housings (380.0); automotive cylinder block (390.0) 40 Cast alloys Al-Si (4xx.0) Si is the major alloying element in 4xx.0 alloys which contain from 5 to 12% Si. These alloys find many applications where combinations of moderate strength and high ductility and impact resistance are required ex Bridge railing support; utensils, marine fittings 5.2%Si; thin walled intricate, pressure tight castings (12%Si) The Al-Si eutectic phase impart high fluidity and makes possible the commercial viability of most high volume Aluminium casting. When no copper is added these alloys have good castability and good corrosion resistance. The microstructure comprise aluminum containing about 1% Si in solid solution as the continuous phase, with particles of essentially pure silicon. Alloys with less than 12% Si are referred to as hypoeutectic, those with close to 12% Si as eutectic, and those with over 12% Si as hypereutectic. Strength and ductility of these alloys, can be substantially improved by modification of the Al-Si eutectic. Modification of hypoeutectic alloys (