Metallic materials - PowerPoint PPT Presentation

Loading...

PPT – Metallic materials PowerPoint presentation | free to download - id: 1e68ba-ZDc1Z



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Metallic materials

Description:

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

Number of Views:217
Avg rating:3.0/5.0
Slides: 32
Provided by: john1297
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Metallic materials


1
Metallic materials
  • EF420 lecture 1
  • John Taylor

2
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

3
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

4
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

5
Pure metals
  • Commercial examples Al, Cu, Ti, Mg, Ni
  • Strength - relatively low
  • Ductility - relatively high
  • Electrical conductivity - high
  • Thermal conductivity - high
  • Corrosion resistance - high

6
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
    constants
  • Degree of cold work
  • Dislocation density, entanglement
  • Solid phase changes

7
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

8
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
9
Multiphase alloys
  • Examples Al 5083, Cu-Be
  • Strength - often substantially increased
  • Ductility - usually reduced
  • Thermal and electrical conductivity often little
    affected
  • Corrosion resistance - substantially reduced

10
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

11
Extraction and purification
  • Reduction of ore often accompanied by melting
  • Fused material is therefore usually cast
  • First casting is rarely to final metal product
    form
  • Casting as ingots or continuos cast strands, then
    forming into wrought product

12
Further processing
  • Secondary metallurgy
  • Alloying and secondary casting
  • Metal working (forming, cutting, fabrication)
  • Wrought products
  • Heat treatment
  • Joining (consolidation)
  • Welding, brazing, soldering

13
Casting
14
Casting
  • 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
    etc
  • Suits materials which may have low formability

15
Types of casting
  • Huge size range zipper teeth to 10m ships
    propeller
  • 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)

16
Casting terminology
Core print
Pattern
Core
Casting
Pouring cup
Vent
Mould box or flask
Cope
Sprue
Riser
Core
Mould cavity
Gating system
Parting line
Drag
17
Castability
  • 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

18
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

19
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

20
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
    die
  • 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

21
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
    blades

22
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)

23
Features
  • Mould is expensive (high volume production
    necessary), but can be reused many times (up to
    25,000)
  • 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

24
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.

25
Features
  • 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

26
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

27
Cast metal microstructure
Chill zone (fine equiaxed grains)
Columnar zone Anisotropic (directional properties)
Equiaxed zone (low pouring temperature, Alloy
additions, inoculation) Isotropic
28
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

29
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)

Cope
Parting line
30
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

31
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

Bad
Better
About PowerShow.com