Heat treating of aluminum and aluminum alloys

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Introduction

Heat treating processes for aluminum demand precision and must be carried out in properly designed furnaces that provide required thermal conditions. These furnaces must be equipped with adequate control instruments to ensure continuity and uniformity of temperature-time cycles. Process details must be carefully established and controlled for each product type.

Major Heat Treatment Categories

1. Preheating or homogenizing: Reduces chemical segregation of cast structures and improves workability
2. Annealing: Softens strain-hardened and heat-treated alloy structures, relieves stresses, stabilizes properties and dimensions
3. Solution heat treatments: Effects solid solution of alloying constituents and improves mechanical properties
4. Precipitation heat treatments: Provides hardening by precipitation of constituents from solid solution

Ingot Preheating Treatments (Homogenizing)

The initial thermal operation applied to ingots prior to hot working serves multiple purposes depending on alloy, product, and fabricating process. Primary objective is improved workability, particularly important because most alloys' as-cast microstructure is heterogeneous, even in relatively dilute alloys.

Annealing

Cold-worked aluminum's distorted, dislocated structure is less stable than the strain-free, annealed state. Lower-purity and commercial aluminum alloys undergo structural changes only at elevated temperatures. These changes occur in several stages:

(a) Recovery:

• Dislocation reduction greatest at grain fragment centers
• Produces subgrain structure with dislocation networks at boundaries
• Increased time and temperature lead to more perfect polygonization
• Subgrain size gradually increases
• Many subgrains develop boundaries free of dislocation tangles

(b) Recrystallization:

Characterized by gradual formation of microscopically resolvable grain structure, largely strain-free with few dislocations within grains or at boundaries.

(c) Grain Growth:

Post-recrystallization heating may produce several forms of grain coarsening.

Precipitation Hardening

General Principles:

Heat treatable alloys contain soluble alloying elements exceeding equilibrium solid solubility at room and moderately higher temperatures.

Nature of Precipitates:

Forty years of research has revealed complex time-dependent and temperature-dependent changes in supersaturated solid solutions during precipitation.

Kinetics:

Solution and precipitation reaction rates depend on diffusion rates, solubilities, and alloy contents, determined through various experimental methods.

Nucleation:

• Zones form through homogeneous nucleation in continuous crystal lattice
• Requires critical vacancy concentration
• Vacancy-solute atom clusters consistent with solution temperature and quenching rate effects
• Discontinuities provide preferred nucleation sites

Quenching

Quenching is the most critical step in heat-treating operations, aimed at preserving the solid solution by rapidly cooling the material to near room temperature. This process must occur quickly enough to avoid precipitation or phase transformations within the critical temperature range, which is influenced by nucleation theory and depends on supersaturation levels and diffusion rates. Water is the most used quenching medium due to its effectiveness, with its cooling rate adjustable by modifying its temperature and surface tension.

Aging Processes

Natural Aging:

• Occurs at room temperature after quenching
• Varies by alloy
• No visible microstructural changes
• Attributed to zone structure formation
• Materials softer immediately after quenching

Artificial Aging:

• Accelerated by reheating to 100-200°C
• Changes differ fundamentally from room temperature aging
• Increases yield strength more than tensile strength
• Decreases ductility
• T6 temper stronger but less ductile than T4

Specialized Applications

Direct Precipitation Treatment

Certain alloys respond to precipitation heat treatment without prior solution heat treatment, particularly those insensitive to cooling rates during quenching.

Cast Product Treatment

Permanent mold, sand, and plaster castings benefit from complete heat treatment cycles, following practices similar to wrought products.