Engineering Aluminium Alloys

Explore the elements of metallurgy and engineering alloys. Explore the elements of metallurgy and learn about solidification and grain refinement in engineering aluminium alloys. Gain insights into solidification, grain refinement, and hardening in aluminium alloys.

In the book “Elements of Metallurgy and Engineering Alloys”

edited by F.C. Campbell

Selected Contents

Part 1: Physical and Mechanical Metallurgy

Metallic Structure

Crystalline Imperfection and Plastic Deformation

Solid Solutions

• Interstitial Solid Solutions
• Substitutional Solid Solutions
• Ordered Structures
• Intermediate Phases
• Dislocation Atmospheres and Strain Aging

Introduction to Phase Transformation

Diffusion

Phase Diagrams

Solidification and Casting

• The Liquid State
• Solidification Interfaces
• Solidification Structure
• Segregation
• Grain Refinement and Secondary Dendrite Arm Spacing
• Porosity and Shrinkage
• Casting Processes
  • Sand Casting
  • Plaster and Shell Molding
  • Evaporative Pattern Casting
  • Investment Casting
  • Permanent Mold Casting
  • Die Casting

Recovery, Recrystallization, and Grain Growth

• Recovery
• Recrystallization
• Recrystallization – Temperature and Time
• Recrystallization – Purity of Metal
• Recrystallization – Original Grain Size
• Recrystallization – Temperature of Deformation
• Grain Growth
• Normal Grain Growth
• Abnormal Grain Growth

Precipitation Hardening

• Particle Hardening
• Theory of Precipitation Hardening
• Precipitation Hardening of Aluminum Alloys
• Solution Heat Treating
• Quenching
• Aging
• Dispersion Hardening

Mechanical Behavior

Fracture

Fatigue

Creep

Deformation Processing

Physical Properties of Metals

Corrosion

Part II: Engineering Alloys

Aluminum

Aluminum has many outstanding properties, leading it to be used for a wide range of applications. It offers excellent strength-to-weight ratio, good corrosion and oxidation resistance, high electrical and thermal conductivity, exceptional formability, and relatively low cost. This chapter examines the metallurgy, composition, processing, and mechanical properties of aluminum and its alloys, both cast and wrought forms. It also covers heat treating and basic temper designations, including annealed, work hardened, solution heat treated, and solution heated treated and aged. The chapter concludes with information on corrosion and oxidation resistance.

• Aluminum Metallurgy
• Aluminum Alloys Designation
• Aluminum Alloys
  • Wrought Non-Heat-Treatable Alloys
  • Wrought Heat-Treatable Alloys
• Melting and Primary Fabrication
  • Rolling Plate and Sheet
  • Extrusion
• Casting
  • Aluminum Casting Alloys
  • Aluminum Casting Control
• Heat Treating
• Annealing
• Fabrication
• Corrosion

Magnesium and Zinc

Titanium

 

 

 

 

 

 

 

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The notes

Chapter 9 contains the best explanation of the essence of precipitation hardening that I have come across.
Here it is.

Precipitation hardening

Precipitation hardening is used extensively to strengthen not only

• aluminum alloys

but also

• magnesium alloys
• nickel-base superalloys
• beryllium-copper alloys and
• precipitation-hardening stainless steels.

In precipitation hardening:

• An alloy is heated to a high enough temperature to take a significant amount of an alloying element into solid solution.
• It is then rapidly cooled (quenched) to room temperature, trapping the alloying elements in solid solution.
• On reheating to an intermediate temperature, the host metal rejects the alloying element in the form of fine precipitates that create matrix strains in the lattice.
• These the fine precipitate particles act as barriers to the motion of dislocations and provide resistance to slip, thereby increasing the strength and hardness.

Particle Hardening

Particle, or dispersion, hardening occurs when extremely small particles are dispersed throughout the matrix. When a dislocation encounters a fine particle, it must either cut through the particle or bow (loop) around it, as shown in Fig. 9.1.

There are two types of particle strengthening:

• Precipitation hardening. Takes place during heat treatment.
• True dispersion hardening. Can be achieved by mechanical alloying and powder metallurgy consolidation.

For effective particle strengthening, the matrix should be soft and ductile, while the particles should be hard and discontinuous (Fig. 9.2). A ductile matrix is better in resisting catastrophic crack propagation. Smaller and more numerous particles are more effective at interfering with dislocations motion then lager and more widely spaced particles.

Fig. 9.1 – Paricle-strengthening

Fig. 9.2

Precipitation Hardening of Aluminium Alloys

Aluminium alloys are one of the most series of alloys that can be precipitation hardened, including

• 2xxx (Al-Cu) alloys
• 6xxx (Al-Mg-Si) alloys
• 7xxx (Al-Zn) alloys
• some of the 8xxx (Al-Li) alloys.

Precipitation hardening consists of three steps:

• Solution heat treating
• Rapid quenching
• Ageing

In solution heat treating, the alloy is heated to a temperature that is high enough to put the soluble alloying elements in solution. Then, after holding at the solution-treating temperature long enough for diffusion of solute atoms into the solvent matrix in occur, it is quenched to a lower temperature (e.g., room temperature) to keep the alloying elements trapped in solution. During ageing, the alloying elements trapped in solution precipitate to form a uniform distribution of very fine particles. Some aluminium alloys will harden after a few days at room temperature, a process called natural ageing, while others are artificially aged by heating to an intermediate temperature (Fig. 3).

Fig. 9.3 – Typical age hardening for aluminium:
T₁ – natural ageing (room temperature) artificial  underageing (low temperature);
T₂ – artificial underageing; T₃ – artificial top ageing; T₄ – artificial overageing