Metals

• Categories of metals


  • Ferrous, iron, magnetic
  • Non-ferrous, no iron, non magnetic, no rust
  • Alloy - Two or more metals, one must be metal
• Ore to extraction


  1. Metal mined from ground

  2. Bauxite is aluminium ore, extracted by electrolysis
• Ferrous metals


  • Mild steel - Low carbon steel, malleable, ductile, relatively inexpensive, good tensile strength, nuts, bolts, easy to weld
  • Medium carbon steel - Less ductile than carbon steel, hard, malleable, tough, good tensile strength, springs, gears, rail tracks
  • Grey cast iron - Distributes heat evenly, withstands compression, resisting deformation, very hard, good resistance to oxidation, brittle, has low impact resistance, radiators, cookwares, stores
  • White cast iron - Excellent resistance to abrasion, hard, brittle, great tensile strength, cast with little shrinkage, extrusion nozzles, dies, stone crusher, industrial roller
• Non-ferrous metals


  • Aluminium - Abundant metal, excellent strength to weight ratio, soft, malleable, natural oxide layers help prevent corrosion, window frames, phone casings, aeroplane parts and bicycles
  • Copper - Used for around 10,000 years, malleable, extremely ductile, excellent conductor of electricity/heat, electrical wire, motors, hot water pipes, pans
  • Zinc - Very versatile, normally alloyed, ductile, easily worked, layer of oxide, corrosion resistance, galvanising, roofing material, batteries, sacrificial anode on boats/ships to reduce corrosion
  • Tin - Mined in Cornwall for over 4000 years, soft, malleable, good corrosion resistance, tin plate coating, solder for electronics
• Ferrous Alloys


  • Stainless steel - Contain iron, carbon, chromium, nickel, very hard, good impact resistance, very good corrosion, medical and catering equipment, construction
• Non-ferrous Alloys


  • Brass - 65% copper and 35% zinc, low melting point (900 C), low friction, self lubricating metal, relatively corrosion resistant, good conductor of heat and electricity, valves, taps, ornaments, musical instruments
• Stock forms


  • Sheet - Large panels up to 3mm thick, ideal for pressing, punching, bending, spinning, riveting, tin cans, car body parts, pots, pans, roofing and panels
  • Plate - Thicknesses greater than 3mm but is usually narrower than sheet form, plasma cutting, laser cutting, construction/fabrication, architectural features and military machines
  • Bar - Available in flat, round, square, hexagonal and other forms, flat/square bar more easily milled, drilled, welded, turned using four jaw chuck, round/hexagonal sections can be more easily turned into lathe, tool making, gym equipment, construction
  • Tube - Same stock as bar, rectangular called box section, tubular forms are structurally strong, bike frames, heating/plumbing, musical instruments, some medical applications
  • Structural beams - I beam, L beam, H beam, T bar, channel, I beams spread vertical loading, reducing material/weight, L beam, ideal shape, for easy connections in construction, not as strong as tube, construction, bridges, skyscrapers, RSJ term given to rolled steel joists used in construction
• Atomic structure of metals

  • Regular lattice structure
  • Positive metal ions surrounded by sea of free-moving negative electrons
  • Good electrical conductors
  • Tight bond
• Mechanical properties (types of durability)

  • Hardness
  • Toughness
  • Malleability
  • Ductility
  • Elasticity
  • Tensile strength
• Hardness


  Ability to withstand wear and abrasion. Strong metallic bond. Alloys usually harder than pure metals. Less able slide over one another making hard.
• Toughness


  Ability to absorb energy without cracking. Atoms in tough metals move past each other, deform not break. Superalloys - above average levels of toughness, c300 maraging steel.
• Malleability


  Ability to be bent, pressed and manipulated without fracturing.
• Ductility


  Ability to be drawn or stretched without breaking.
• Elasticity


  Material resistance to distortion and ability to return to its original shape. Steel more elastic than rubber-requires greater deforming force. Nitinol (nickel titanium) known as super-elastic alloy.
• Tensile strength


  Ability of metal to withstand pulling forces. Tensile strength is the point at which material goes from elastic to plastic deformation. Limit of tensile stress before either ductile failure or brittle failure.
• Physical properties


  • Density
  • Thermal conductivity
  • Melting point
  • Electrical conductivity
  • Resistance to corrosion
  • Ability to be joined with heat
  • Ability to take finishes
• Density


  Most metals have high density as atomic lattice structure closely packed together. Platinum and tungsten very high densities. Aluminium + titanium low density.
• Thermal conductivity


  Copper good at conducting heat. Steel poor conductor of heat, high temp environment.
• Melting point


  Tungsten highest melting point. Light bulb filaments - tungsten. Mercury, tin, lead much lower melting points.
• Electrical conductivity


  Silver best electrical conductor. Copper good conductivity far less expensive. Gold better corrosion resistance.
• Resistance to corrosion


  Metals have variable resistance to damage. Silver, bronze, gold thousands of years old.
• Ability to be joined with heat

  Metals' ability to be joined with heat relies on melting point of metal and conductivity. Soldering, brazing, welding are heat processes used to join metal together. Copper, brass and silver solder easily. Mild steel requires greater heat and may be brazed and welded.
• Ability to take finishes

  Metals require coatings and finishes in order improve durability and aesthetics. Steel requires rust. Anodising used to increase aluminium oxide layer prevents corrosion. Stainless steel, no finish.
• Defects in metals are imperfections in arrangement of atoms. Dislocations are line defects where the atoms of crystal lattice are misaligned.
• Tensile strength testing


  1. Clamp samples end of bench

  2. Apply load

  3. Measure deflection

  4. Industry standard with tensometer
• Tensile strength


  Pulling force the metal can resist
• Toughness testing


  1. Moving load hit metal sample

  2. Izod impact test used industry level

  3. Pivoted arm raised to height, released

  4. Arm swings and hits notched sample, breaking it

  5. Amount of swing after breaking indicates amount of energy absorbed
• Toughness


  Ability of a material to absorb energy and plastically deform without fracturing
• Hardness testing


  1. Rockwell - Diamond indenter break surface

  2. Brinell - Steel ball indent surface

  3. Vickers - Diamond square based pyramid indent surface

  4. Depth indentation or diameter of mark is measured
• Hardness


  Resistance of a material to plastic deformation, usually by penetration
• Malleability testing


  1. Metal placed in one continuous bend

  2. Amount cracking or fracturing on inside of bend is measured
• Malleability


  Ability of a material to be hammered or rolled into a thin sheet without fracturing
• Corrosion testing


  1. Salt spray testing, accelerates corrosive attack used to test finishes

  2. Humidity test used in bridges and construction works
• Corrosion


  Gradual deterioration of a material by chemical or electrochemical reaction with its environment
• Electrical conductivity testing


  1. Workshop - Multimeter measure resistance

  2. Industry - Four point probe test, outer lead measure current, inner lead measure volts
• Electrical conductivity


  Ability of a material to allow the flow of electric current
• Thermal conductivity testing


  1. Industry - Heat flow metre test with temp control and flow sensors

  2. Workshop - Time how long heat moves through metal sample
• Thermal conductivity


  Ability of a material to conduct heat
• Non-destructive testing methods


  • Visual inspection
  • X-rays
  • Ultrasonic tests
  • Liquid penetration
• Lattice structure


  Regular arrangement of atoms in a metal