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Using Microstructural Analysis to Solve Practical Problems

Aluminum has a wide range of commercial applications due to its unique combination of good corrosion resistance, light weight, good mechanical properties, ease of fabrication and acceptable cost. Its density of ~2.7 g/cm3 makes it the lightest commercial metal other than magnesium (~1.74 g/cm3) and beryllium (~1.85 g/cm3). But, Al and its alloys have more versatility, fewer problems, and cost less than Mg or Be alloys. The range of melting points for both Al and Mg alloys are fairly similar, ~565 to 640°C (~1050 to 1185°F) for Al casting alloys and ~593 to 648°C (~1100 to 1198°F) for Mg casting alloys.

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Microstructure of Cast and Wrought Aluminum Alloys

ALUMINUM AND ITS ALLOYS

  • Cast vs.Wrought
  • Heat Treatable (PH) vs. Non-heat Treatable
  • Non-heat Treatable – Strengthen by ColdWork
  • Strengthen by Alloying

CAST ALUMINUM ALLOYS

  • Silicon is Major Alloying Element
  • Silicon Improves Fluidity
  • Near-Eutectic Compositions – for Die Casting
  • Refine Eutectic by Adding Sodium (shifts eutectic to about 14%Si)
  • Add Copper to Permit Heat Treatment

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Metallographic Etching of Aluminum and its Alloys

Preparation of Aluminum Specimens

  • Grind with 240-grit SiC water-cooled paper, 240 rpm, 5 lbs. (20N) per specimen, until co-planar
  • Polish with 9-µm Diamond on an Ultra-Pol (silk) cloth, 150 rpm, 5 lbs/specimen, 5 minutes (contra rotation)
  • Polish with 3-µm Diamond on a Trident cloth, 150 rpm, 5 lbs/specimen, 4 minutes (contra rotation)
  • Polish with 1-µm Diamond on a Trident cloth, 150 rpm, 5 lbs/specimen, 3 minutes (contra rotation)
  • Polish with 0.05-µm Colloidal Silica on a Microclothpad, 120 rpm, 5 lbs/specimen, 3 minutes (contra rotation)
  • (Contra: Head and platen rotate in opposite directions)

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