Aluminum Alloys: Properties and Classification

• Most students confuse steel and aluminum
• Steel and aluminum have different alloy designations, heat treatment processes, phase diagrams, and phases

Learning Objectives

• State 2-3 properties of pure aluminium
• State two types of aluminium alloys.
• Know how to strengthen/harden heat-treatable & non-heat-treatable aluminium alloys
• List the alloying elements in the aluminium series
• State how to increase aluminium's corrosion resistance
• Draw the Al-Cu phase diagram, and identify Duralumin
• Identify the phases in Al-Cu alloy/phase diagram
• Describe the two types of heat treatment processes for aluminium

Properties of Pure Aluminum

• Low density
• Corrosion resistance
• High electrical and thermal conductivity, better than copper per unit mass
• Non-magnetic
• Soft and ductile
• Malleable
• Reflective

Classification of Aluminum Alloys

• Aluminum alloys are classified into wrought and cast alloys

Wrought Alloys

• Ductile
• Suitable for cold working
• Designation series includes:
  • 1xxx indicates pure aluminum
  • 2xxx indicates copper
  • 3xxx indicates manganese

Cast Alloys

• High fluidity
• Low shrinkage
• Short freezing range
• Designation series includes:
  • 2xx.x indicates copper
  • 3xx.x indicates manganese

Strengthening/Hardening Methods

• Heat-treatable alloys can be strengthened/hardened via heat treatment processes
• Wrought non-heat-treatable alloys are strengthened/hardened via cold working
• Cast non-heat-treatable alloys are strengthened/hardened via adjusting the alloy composition

Alloying Elements in Wrought Aluminum Series

• 1xxx indicates aluminium 99% or more
• 2xxx indicates copper as the major alloying element
• 3xxx indicates manganese as the major alloying element
• 4xxx indicates silicon as the major alloying element
• 5xxx indicates magnesium as the major alloying element
• 6xxx indicates magnesium and silicon as major alloying elements
• 7xxx indicates zinc as the major alloying element

Mnemonic

• Pure Cop Manages Silly Magazine Stand Zealously
  • The cop has an airport badge 2xxx
  • Managing the corrugated roof 3xxx
  • Silly decorations like a piston on the roof of the magazine stand 4xxx
  • There are train & bus magazines 5xxx
  • Stand has scaffoldings 6xxx
  • Zealously in the Air wing department 7xxx

Ways to Increase Aluminum Corrosion Resistance

• Cladding adds a layer of pure aluminum to improve corrosion resistance
• Anodizing thickens the layer of aluminum oxide film on the surface
  • Improves corrosion resistance
  • Improves surface abrasion resistance
  • Improves aesthetic appearance

Aluminum-Copper (Al-Cu) Phase Diagram

• The point of maximum solubility of Alpha Aluminum is ~5.65% at ~550°C
• Duralumin (2024 Al series) is heat treatable and contains about 4% copper, and has a high strength-to-weight ratio

Phases in Al-Cu Alloy

• Alpha Al (α Al): Pure Aluminum
• Copper Compound (CuAl2)
• Liquid: Molten Aluminum + Copper Compound

Types of Heat Treatment Process for Aluminum

• Solution Heat Treatment makes the aluminum alloy soft and ductile for cold working
  • This happens at 550°C, with sufficient time (12 hours), and quenching
• Precipitation Hardening Heat Treatment strengthens and hardens to high strength properties
  • This happens at 165°C for 10 hours, with air cooling

Heat Treatment of Duralumin

• Duralumin is heat-treated to alter its microstructure and properties
• Solution Treatment involves heating to 550°C for a long time (12 hours) to produce Brittle & weak properties
• Rapid cooling/quenching after solution treatment retains a soft, ductile state
• Precipitation Hardening involves heating to 165°C for 10 hours (optimal) for Hard & strong properties
• Over "aging" at 165°C for 20 additional hours, results in Hard & strong properties changing to Brittle & weak properties