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Metallic materials


1. Metallic materials. EF420 lecture 1. John Taylor. 2. Metal properties. Strength. Ductility ... Sand is packed round a pattern made of wood or metal in a flask ... – PowerPoint PPT presentation

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Title: Metallic materials

Metallic materials
  • EF420 lecture 1
  • John Taylor

Metal properties
  • Strength
  • Ductility
  • Resistance to fracture
  • Colour and sheen
  • Chemical reactivity
  • Corrosion resistance
  • Thermal conductivity
  • Electrical conductivity
  • Electrical resistance
  • Magnetic properties
  • Reflectivity
  • Radioactivity
  • Neutron capture
  • Neutron scattering

Commercially available forms
  • Flat products sheet (coiled), plate
  • Long products bar, sections (T, I or angle)
  • Rod, wire
  • Castings, forgings
  • Fasteners, welding consumables
  • Components, weldments, fabrications
  • Machinery, structures

Designations and specifications
  • Patented commercial alloys
  • May or may not be to a national standard
  • Inconel, Monel, FV520, 254SMO
  • Standard specifications
  • Varies with country or region AS, ASTM, EN, ISO
  • Alloy designations UNS
  • Number of alloys depends on base metal
  • Irons and steels 500,000 alloys
  • Ti alloys 50 alloys

Pure metals
  • Commercial examples Al, Cu, Ti, Mg, Ni
  • Strength - relatively low
  • Ductility - relatively high
  • Electrical conductivity - high
  • Thermal conductivity - high
  • Corrosion resistance - high

Properties influenced by
  • Lattice type
  • Face-centred cubic, body-centred cubic, close
    packed hexagonal
  • Grain size
  • eg, yield strength sy s0 kd-1/2
  • d mean grain diameter, s0 and k are material
  • Degree of cold work
  • Dislocation density, entanglement
  • Solid phase changes

Solid solution alloys
  • Single phase alloys
  • Examples 70-30 brass, Inconel 625
  • Strength - solid solution strengthening
  • Ductility - some reduction
  • Electrical properties - conductivity reduced
  • Corrosion resistance often not affected or is
    substantially improved if alloy creates
    protective coating

Factors affecting properties
  • As for pure metals
  • Lattice structure
  • Grain size
  • Degree of cold work
  • Solid phase changes
  • Solute effects
  • Higher concentration has stronger effect
  • Interstitial solutes more powerful than
    substitutional solutes
  • Uniformity of distribution (segregation)

Substitutional solute
Interstitial solute
Multiphase alloys
  • Examples Al 5083, Cu-Be
  • Strength - often substantially increased
  • Ductility - usually reduced
  • Thermal and electrical conductivity often little
  • Corrosion resistance - substantially reduced

Properties influenced by
  • Properties of matrix
  • Solubility of secondary phases
  • Properties of secondary phases
  • Often hard and brittle
  • Distribution of secondary phases
  • Continuous films versus fine particles
  • Particle shape (needles, plates, spheres,
    eutectic mixtures)
  • Particle size

Extraction and purification
  • Reduction of ore often accompanied by melting
  • Fused material is therefore usually cast
  • First casting is rarely to final metal product
  • Casting as ingots or continuos cast strands, then
    forming into wrought product

Further processing
  • Secondary metallurgy
  • Alloying and secondary casting
  • Metal working (forming, cutting, fabrication)
  • Wrought products
  • Heat treatment
  • Joining (consolidation)
  • Welding, brazing, soldering

  • Solid material is melted, heated to the proper
    temperature. Its composition may be modified. It
    is poured into a mould of the desired shape, and
    allowed to solidify
  • Produces complex shapes in one quick process
  • Fluidity of liquid, shape of mould
  • Suits large range of materials
  • Iron, steel, aluminium, copper, nickel, plastics
  • Suits materials which may have low formability

Types of casting
  • Huge size range zipper teeth to 10m ships
  • Expendable mould (Sand, sodium silicate-CO2,
    shell, plaster, ceramic)
  • Multiple use patterns
  • Single use patterns (investment, lost foam)
  • Multiple mould casting (die casting)
  • Die casting (pressure die casting)
  • Permanent mould casting (gravity die casting)

Casting terminology
Core print
Pouring cup
Mould box or flask
Mould cavity
Gating system
Parting line
  • Low melting temperature (energy cost, mould life)
  • Low viscosity and surface tension allows fine
    detail and complex shapes
  • Low solidification contraction (avoids cracks)
  • Low thermal capacity and high conductivity
    promotes high production rates (die casting)
  • Low solubility for gases to avoid porosity
  • Not contaminated by air
  • Adequate strength

Sand casting
  • Mould material is sand (SiO2) bound with clay and
    water or other additives
  • Sand is packed round a pattern made of wood or
    metal in a flask
  • Pattern is removed and mould is reassembled.
    Molten metal is poured into the cavity left by
    the pattern.
  • After solidification the mould is broken and the
    casting removed

Features of sand casting
  • Wide range of metals can be cast
  • Steel, cast iron, stainless steel, aluminium,
    copper, nickel
  • Almost no limit to size and shape of casting.
    Thickness range from 2.5 mm to unlimited
  • Poorer tolerance than other processes, coarse
    surface texture
  • Clean-up is dirty may be expensive
  • Economic for a low number of castings

Investment casting
  • Make a master pattern of some easily worked
    material (wood, plastic, metal)
  • Make a master die from low melting point metal,
    steel, rubber or wood
  • If master die can be machined from steel, a
    master pattern is not needed
  • Produce wax pattern by casting wax into master
  • Coat wax pattern with investment material (eg a
    slurry of refractory oxides) pack into sand box
    while slurry dries
  • Heat mould to melt out wax, bake preheat mould,
    prior to pouring casting metal

Features of investment casting
  • Almost limitless intricacy
  • Excellent surface finish and precision
  • Used for wide range of metals Au, Ag, steels,
    Ni, Cu, Mg
  • A high cost process used mostly for complex
    shapes, such as art works, jewellery, gas turbine

Permanent mould casting
  • Also called gravity die casting
  • Mould cavity is machined into mating metal
    blocks. Molten material is poured into mould
  • Variations use low positive pressure or vacuum to
    add molten metal
  • Mould material is cast iron, steel, bronze,
    graphite etc
  • Mould must disassemble without locking to casting
    (2 to 3 draft)

  • Mould is expensive (high volume production
    necessary), but can be reused many times (up to
  • Mould life is limited when casting high melting
    point metals (eg steels, which are cast in
    graphite moulds)
  • Good surface finish dimensional accuracy
  • Cooling is rapid, allowing high production rates
  • Examples truck car pistons

Die casting
  • Molten metal is injected into closed metal dies
    at pressures from 10 MPa to 170 MPa. Pressure is
    maintained during solidification.
  • After solidification the die parts are removed by
    movements in linear or circular paths. Cores are
    simple metal removable segments.
  • Dies must be able to withstand the high pressure.
    0.1 mm slits at parting lines provide for escape
    of air. No risers are used.

  • Dies are made of expensive tool steels, and a
    high volume of production is necessary to pay for
    their cost
  • Has been limited to high fluidity (low melting
    point) non-ferrous metals
  • Al, Zn, Mg, Pb
  • High surface finish, precision castings with
    thickness ranging from 0.75mm to 12mm

Properties of metal castings
  • Wrought products often have better strength and
    ductility than castings
  • Grain size and directionality is dependent on
    cooling rate
  • Section thickness, mould type
  • Inoculation - deliberately introduced impurities
    nucleate more grains and therefore lead to finer
    grain size
  • Higher strength and ductility

Cast metal microstructure
Chill zone (fine equiaxed grains)
Columnar zone Anisotropic (directional properties)
Equiaxed zone (low pouring temperature, Alloy
additions, inoculation) Isotropic
Casting defects
  • Cold shuts and miss-runs
  • Penetrations, wash-outs and sand inclusions
  • Porosity
  • Shrinkage cavities or cracks
  • Shrinkage during freezing
  • Wide freezing temperature range
  • High coefficient of expansion

Design of castings
  • Distribute casting evenly around parting plane(s)
  • Pattern must be removable from mould or mould
    from casting
  • No re-entrant angles
  • Cope angle between surfaces
  • Allow for shrinkage
  • An allowance needs to be made on final dimensions
    for contraction after solidification (patterns
    made using shrink rules)

Parting line
Flow and solidification
  • Thinner areas cool first and have higher strength
  • Hot spots at thick regions will solidify last
  • Keep thickness uniform
  • Ideal casting shape is a cylinder
  • Use chills to increase cooling rate
  • Gating system designed to control flow rates

Casting shape
  • There is a minimum thickness which depends on
    material casting process
  • Sand cast steel - 6.5mm
  • Die cast zinc - 0.8mm
  • Use fillets instead of sharp corners to minimise
    stress concentration
  • Avoid large unsupported areas, which will warp
    during cooling (add ribs or stiffeners)
  • Avoid constraint as at cruciform stiffeners,
    which may crack on cooling