Aluminium

Aluminium

1. General properties

   • Low density-2700 Kg/m3
   • High specific strength(sigma y/rho)
   • High specific strength (E/rho)
   • Good ductility due to FCC
   • High electrical conductivity
   • good corrosion resistance due to AL2O3
   • bad point- low service temperature due to low Tmelt

2. third most abundant element, 8wt%

3. extraction from ore=expensive

   • pure AL203
     • Pure Al, low yield strength, less than ten MPa
       • several ways of increasing yield strength
         • solid solution strengthening(alloying)
           • dissolve another element 'solute' in Al solvent, atoms adopt random sites in Al lattice (FCC)
             • solute atoms make dislocation slip more difficult
           • need a large atomic misfit (0.1-0.15)
             • need high solute concentration (limited at RT)
               • C>solvus, then more stable phases form precipitate. (Mg5Al8 forms when C>1.8%)
           • super saturated solid solution(SSS)
             • solubility higher at higher T, cool quickly, quench, freeze in (SSS)for less than a min=Al-5.5%Mg
               • Mg effective yield stress up to 170 MPa
         • precipitation hardening,(age hardening)
           • precipitation hardening
             • three stages
               • solution treatment at high T =dissolve alloying elements
               • quench to RT=obtain super saturated solid solution (SSS)
               • ageing at controlled T and t to decompose SSS to form finely dispersed precipitates (ppts)
             • slow transformation
               • Cu atoms diffuse through Al lattice and several forms of ppt arise over time
         • work-hardening
           • metal working process(rolling, forging, drawing-large amount of plastic deformation
             • more plastic def=greater stress, metal is strengthened
           • great number of dislocations develop
           • strength up, decrease ductility
   • two processes
     • Bayer process
     • Hall-Heroult process

4. 15-20% of Al products are cast.

   • 80-85% are wrought products, (rolled, plate, sheet, foil)

5. melting-casting-homogenisation-fabrication-thermal treatment.

   • melting
     • Al+scrap+alloying additions=uniform composition
     • degassing-H gas formed by water vapour in furnace, cause porosity, removed by blowing N,Cl or Ar gas through melt
     • filtering
       • melt=poured through glass cloth on porous ceramic foam to remove oxide skin (Al203) and particulates
   • casting
     • Direct-chill (semi-continuous) process.
       • solidifies under water spray cooling
       • ingots up to several tonnes
     • continuous casting
       • lower strength alloys.
       • produces near final form
       • lower cost than post-forming DC product
     • cast metal = polycrystalline-consists of many grains.
       • small grain size=desirable(Hall-Petch)
         • add grain refiners (small ingots of Al-Ti-B) before casting=nucleate grains
   • homogenisation
     • 450-600 degrees C-uniformity
       • diffusion of alloying elements from grain boundaries and other solute rich regions
   • fabrication
     • DC-ingots=semi-fab form
       • Hot working
         • break down cast structure-uniform grains
       • anneling
         • 350-420 degrees for 5 mins-3 hours
           • removes residual stress, softens work-hardened metal
       • cold working
         • increases yield stress
   • Age hardening- heat treatable alloys.
     • Types of alloys
       • Non-heat treatable:cold work hardening
         • either annealed or cold worked
           • Al(1xxx):low yield stress, elec conductors, chemical and decorative strucutres
           • Al-Mn, Al-Mn-Mg(3xxx)=moderate yield stress and fracture strain, excellent corrosion resistance, drinks cans, sheet, foil.
           • Al-Mg(5xxx)
             • good corrosion resistance, welded application, fuel tanks etc.
       • heat treatable
         • Age-hardening, yield stress between 200 and 500 Mpa, fracture strain 10-20%
         • Al-Cu (2xxx) high strength at RT, good creep strength at high T, good Kc at cryogenic T, fuel tanks
         • Al-Cu-Mg(2xxx), high strength, aircraft structures, screw fittings, riverts
         • Al-Mg-Si(6xxx) medium strength, good weldability,good corrosion resistance, automotive applications, pipes etc.
         • Al-Zn-Mg (7xxx):medium strength, good weldability, railway carriages
         • Al-Zn-Mg-Cu(7xxx), high strength, good resistance to stress corrosion cracking, good Kc
       • heat treatable cast alloys
         • Al-Cu-Si(2xxx) vehicel wheels
         • Al-Si-Mg(6xxx) vehicle transmission cases, engine blocks, pump parts)
   • cast alloys-similar to wrought.
     • low Tm, low solubility for gases, high shrinkage on solidification (bad) some alloys=heat treat, Si=most common alloying element to assist casting (1-1.3%),
       • Si, gives high fluidity, reduced shrinkage, good corrosion resistance, good weldabilty