25'1' Uses of cast Al alloys

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25.1. Uses of cast Al alloys

• 2xx.x Al-Cu cast alloy
• For permanent and sand mold castings
• Age hardenable
• High strength at room and elevated temperatures
  (to 200 C, 140-400 MPa)
• Engine parts for automotive and aerospace
  applications,engine blocks and piston heads,

2

• 3xx.x Al-Si-Cu cast alloy
• For permanent and sand mold castings
• High fluidity of molten metal
• Age hardenable
• High strength at room temperature (200-410 MPa)
• Low temperature engine parts for automotive and
  aerospace applications, crank shaft, transmission
  parts, alloy wheels

3

• 4xx.x Al-Si cast alloy
• Best fluidity of Al alloys
• Non age-hardenable
• Medium strength (120-170 MPa)
• Resistant to corrosion
• Casting intricate shapes.

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25.2 Basic Al alloys

• Wrought alloy
• Hardenable medium ?y 2024 (Al-4.4Cu-0.6Mn-1.5Mg)
• Hardenable high ?y 7075 (Al-6Zn-2.5Mg-1.5Cu-0.5Fe
  )
• Non-hardenable 6061 (Al-0.6Si-0.3Cu-1Mg)
• Cast alloy
• Hardenable 204.0 (Al-4.6Cu-0.25Mg-0.35Mn)
• Non-hardenable 443.0 (Al-5Si)

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25.3 Basic heat treatments of Al alloys

• T6 solution treat WQ AA
• T8 solution treat WQcold work AA

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25.4 Corrosion resistance of Al alloys

• Unprotected Al is anodic to most metals.
• Protection by naturally formed dense oxide layer.
• Resistant to natural atmospheres, water and sea
  water, soils and most foods.
• Corrosion resistance is best for pure Al and
  decreases with addition of alloying elements, esp
  those that form precipitates

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What will attack Al?

• Alkali (e.g. NaOH) water solutions, where Al2O3
  is soluble.
• Pitting and discoloring in sea water which
  affects appearance but not durability.

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Stress corrosion cracking

• SCC occurs in all grades of Al alloys
• Even in mild corrosion environments (e.g.
  airplanes in marine condition).
• Particular strong in age-hardened materials (2xxx
  and 7xxx alloys)

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25.5 Coatings of Al alloys

• Paint
• Anodizing treatment (anodized Al)
• Converting a thick surface layer into Al2O3.
• Oxidation by an electrochemical method in an
  oxidizing acid (e.g. H2SO4).
• The oxide layer provides both corrosion
  protection and an abrasion-resistant surface.
• Not protective in alkali solutions.

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25.6Titanium alloys basic characteristics

• Resistance to corrosion, acids and bases
• Non-magnetic
• Low density (57 of Fe)
• High stiffness (57 of Fe)
• Pure Ti is biocompatible
• Low thermal expansion (70 of Fe)
• High melting temperature (1670 C)
• Resistance to fire

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25.7 Types of Ti alloys

• Pure Ti ? (HCP) below 900 C
• ? (BCC) above 900 C

• Ti alloys
• ? or near ? alloys
• ? alloys or near ? alloys
• ?? two-phase (duplex) alloys
• No numerical designation

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?? alloys (Pure Ti and alloys with Al, Ni, Sn)

• Low to medium yield strength (300-400 MPa)
• Ductile and tough (40 for pure Ti)
• Weldable
• Not age hardenable
• Not quench hardenable
• No brittle-ductile transition usable at low T
• Good resistance to creep useful at high T

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?? alloys (with V, Nb, Cr, Fe, Mo)

• Medium to high strength (400-1100MPa)
• Ductile in fully annealed condition.
• Age hardenable
• Brittle-ductile transition brittle at low T.
• Resistance to creep poorer than ? alloys

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?? alloys (Ti-6Al-4V, 50 all Ti alloys)

• Typically 50 ?, 50 ? actual fraction can be
  altered with heat-treatment
• Medium to high ?y (900-1150 MPa)
• Ductile in fully annealed condition
• Age hardenable.
• Quench hardenable.
• Limited ductility at low T (lt14 annealed)
• Poorer resistance to creep than ? alloys

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• ? stabilizers
• V, Cr, Fe, Mo
• Enlarges the ? domain of the phase diagram
• Promote proportion of ? phase in the final
  structure
• Use to improve low temperature strength

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Cr a ? stabilizer.
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• Non-? stabilizers
• Al, Sn, Zr. Hf, Si
• Narrows or has no effect on the ? domain of the
  phase diagram
• Used to improve alloy properties without causing
  ? phase formation
• Used to improve creep resistance of ? alloys.
• Used to improve oxidation resistance of ? and/or
  ? alloys

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Al an ? enhancer
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25.8 Uses of Ti alloys

• Corrosion resistance
• Mass efficient high strength (high ?y/?)
• No improvement in mass efficiency of
  stiffness-limited application
• Biocompatibility
• High temperature strength and fire resistance.