2: Aluminum Alloys

Introduction to Aluminum Alloys

  • Definition: Aluminum alloys are materials made primarily of aluminum along with other elements to enhance specific properties.
  • Solubility: No elements are completely soluble in aluminum in solid state.
    • Aluminum is not a transition metal, affecting its interaction with other engineering metals.
  • Solid Solubility: Typically only a few percent of alloying elements can dissolve into aluminum. Exceeding this may lead to intermetallic precipitates, useful for creating age-hardened alloys.

Types of Aluminum Alloys

  • Wrought vs. Cast Alloys:
    • Wrought Alloys: Intended to be cold worked or formed from billets via processes like rolling, extruding, and forging.
    • Cast Alloys: Formed by casting methods, typically closer to final shape, generally less corrosion resistant due to alloying elements that alter the aluminum oxide layer.

Alloy Designations

  1. 1,000 Series:

    • Composition: At least 99% aluminum.
    • Properties: Most corrosion-resistant, low yield strength, not heat treatable.
    • Common Alloys: 1100, 1145.
    • Applications: All clad products, combining pure aluminum's corrosion resistance with a high strength core (e.g., 7075, 2024).

  2. 2,000 Series:

    • Composition: Copper as the major alloying element.
    • Properties: High strength, lower corrosion resistance, heat treatable (age-hardening).
    • Common Alloy: 2024.
    • Applications: Aerospace, preferable for high fatigue loadings.

  3. 3,000 Series:

    • Composition: Manganese as the major alloying element.
    • Properties: Improved strength over 1100, good formability, anodizable.
    • Common Alloy: 3003.

  4. 4,000 Series:

    • Composition: Silicon as the major alloying element.
    • Properties: Increased wear resistance, used primarily as welding filler metal.
    • Common Alloy: 4047.

  5. 5,000 Series:

    • Composition: Magnesium as the major alloying element.
    • Properties: High corrosion resistance, not heat treatable, anodizable.
    • Common Alloy: 5052.

  6. 6,000 Series:

    • Composition: Magnesium and silicon.
    • Properties: Moderately high strength, heat treatable, excellent formability and weldability.
    • Common Alloys: 6061, 6063.

  7. 7,000 Series:

    • Composition: Zinc as the major alloying element.
    • Properties: Highest strength alloys, heat treatable, lower fatigue resistance.
    • Common Alloy: 7075.

  8. 8,000 Series:

    • Properties: Miscellaneous alloying elements without common characteristics.

Mechanical Properties Overview

  • Yield Strength Comparisons:
    • 7075-T6: 450 MPa.
    • 2024-T6: 345 MPa.
    • 6061-T6: 255 MPa.
  • Ductility:
    • Higher ductility in 6061 compared to aerospace grades (e.g., 17% elongation).

Corrosion and Weldability

  • Corrosion Resistance:
    • 1,000 Series: Best.
    • 5,000 Series: High, but susceptible to pitting.
    • 2,000 Series: Poor, prone to stress corrosion cracking.
    • 7,000 Series: Lower than 6,000 Series, also susceptible to stress corrosion cracking.
  • Weldability Ratings:
    • A: Readily weldable.
    • B: Special procedures needed.
    • C: Limited weldability.
    • X: Not recommended.

Applications

  • Use of All Clad Products: Combines the properties of pure aluminum for corrosion resistance with the strength of a strong alloy, mainly in aerospace applications.
  • Selection Criteria: Depending on the mechanical properties, corrosion resistance, and welding needs, different series are chosen for specific applications, particularly in aerospace where fatigue resistance and weight are crucial.