Heat Treatment of Aluminum Casting Alloys
Introduction
Heat treatment of aluminum casting alloys is a complex set of heating, holding, and cooling operations that change the structure and properties of the metal without changing its chemical composition. Properly selected heat treatment can increase strength by 1.5-2 times compared to the as-cast state, while providing the necessary level of ductility and other operational characteristics.
Types of Heat Treatment
1. Homogenization (Homogenizing Anneal)
This is a preliminary high-temperature treatment performed at temperatures of 500-540°C for 10-36 hours. The main goals of homogenization are:
- Leveling the chemical composition of the solid solution throughout the metal volume
- Elimination of dendritic segregation formed during crystallization
- Dissolution of non-equilibrium phases
- Improving ductility and processability
- Reducing the tendency to form hot cracks during subsequent processing
2. Annealing
Full Annealing is conducted at temperatures of 350-430°C with a hold of 2-4 hours, followed by slow cooling in the furnace or in air. It provides:
- Maximum softness and ductility of the metal
- Complete removal of internal stresses
- Formation of an equilibrium structure
- Preparation for pressure processing or assembly
Partial Annealing is performed at slightly lower temperatures (240-360°C) and is intended for:
- Partial stress relief without significantly changing strength
- Stabilization of the structure and properties
- Improved machinability by cutting
3. Quenching
Quenching is the primary strengthening operation and consists of several sequential stages:
Heating to a temperature of 515-540°C (depending on the alloy's chemical composition) for:
- Complete dissolution of strengthening phases in the aluminum solid solution
- Formation of a homogeneous solid solution
- Holding time is 4-12 hours
Cooling in water with a temperature of 20-80°C for:
- Fixing the supersaturated solid solution
- Preventing the precipitation of strengthening phases
- Cooling rate is critically important — it must be high enough (50-300°C/s)
After quenching, the alloy is in a metastable state with a supersaturated solid solution but does not yet have high strength. Maximum strengthening is achieved in the next stage — aging.
4. Aging (Precipitation Hardening)
Aging is the final strengthening operation, during which controlled precipitation of fine strengthening phases from the supersaturated solid solution occurs.
Natural Aging
Occurs at room temperature (15-25°C) for 4-5 days. Provides a moderate increase in strength while maintaining high ductility.
Artificial Aging
Maintained at elevated temperatures (150-200°C) for 2-20 hours. Provides higher strength and reduces processing time.
Multi-stage aging includes several holds at different temperatures:
- First stage: 100-120°C for 4-6 hours
- Second stage: 160-180°C for 6-10 hours
- Provides an optimal combination of strength and corrosion resistance
Overaging — aging at higher temperatures (200-240°C) or for a long time:
- Slightly lower strength
- Increased corrosion resistance and structural stability
- Reduced tendency to stress corrosion cracking
5. Modified Quenching
Interrupted Quenching: cooling in cold water to 200-300°C followed by transfer to a hot medium (60-100°C). Reduces residual stresses.
Step Quenching: cooling to an intermediate temperature (250-300°C), holding, and final cooling. Reduces thermal stresses in complex castings.
Temper Designation System
Basic Designations
As-cast state without heat treatment. Properties are not controlled or guaranteed. For non-critical parts.
Annealed state. Maximum ductility, minimum strength. Used before pressure processing.
Unstable state — freshly quenched. Temporary designation as properties change due to aging.
Thermally treated state with numeric indices to indicate the specific mode.
Heat Treatment Modes (T-indices)
Т1 — cooling from casting temperature and natural aging to a stable state
Т2 — cooling from casting temperature, cold work, and natural aging
Т3 — solution treatment, cold work, and natural aging
Т4 — solution treatment and natural aging (quenching at 515-540°C + 4-5 days of holding)
Т5 — cooling from casting temperature and artificial aging (without preliminary solution treatment)
Т6 — solution treatment and artificial aging (full cycle, maximum strength)
Т7 — solution treatment and overaging (stabilization, corrosion resistance)
Т8 — solution treatment, cold work, and artificial aging
Т9 — solution treatment, artificial aging, and cold work
Т10 — cooling from casting temperature, cold work, and artificial aging
Modified Modes
Т51, Т52, Т54 — variations of T5 mode with different ways of stress relief (stretching, compression, or combined).
Т61, Т62 — special variations of T6 mode for composite products.
Т71, Т72, Т73, Т74 — variations of T7 mode with different degrees of overaging (T73 — maximum corrosion resistance).
Т81-Т89 — variations of T8 mode with different deformation conditions.
Physicochemical Processes
During Quenching:
- Diffusion of alloying element atoms into the aluminum matrix
- Dissolution of intermetallic phases
- Formation of a homogeneous solid solution
- Fixation of the supersaturated state upon rapid cooling
During Aging:
- Formation of Guinier-Preston zones (atom clusters)
- Precipitation of metastable coherent phases
- Formation of stable incoherent phases (during overaging)
- Creation of elastic strain fields around precipitates
Influence on Properties
Strength Characteristics:
Tensile strength can increase 1.5-2 times, hardness increases by 40-80%, and yield strength increases 2-3 times.
Ductility:
Elongation usually decreases with strengthening. Annealing maximizes ductility, while T4 mode provides the best balance.
Other Properties:
Corrosion resistance improves with overaging, residual stresses are reduced, and dimensional stability is improved.
Technological Features
Equipment: Chamber and shaft furnaces for heating, quenching baths with circulation systems, aging furnaces with precise temperature control.
Control Modes: Temperature must be controlled to within ±5°C for quenching and ±3°C for aging. The time between quenching and the start of artificial aging should not exceed 1-2 hours.
Defects: Underquenching (low temperature/time), overheating (melting), distortions, and cracks due to uneven cooling.
Practical Application
Typical Temper Applications:
- F Temper: Housing parts without special requirements, intermediate workpieces.
- O Temper: Parts for subsequent deformation, thin-walled products.
- T4-T6 Tempers: Critical loaded parts, aerospace and automotive components.
- T7 Temper: Parts for aggressive environments, marine equipment.
Conclusion
Heat treatment is a key technological process in the production of aluminum casting products, allowing for the control of their mechanical and operational properties. The correct choice of mode and strict adherence to technological parameters ensure the reliability of products in operation. The international temper designation system allows for unambiguous identification of the treatment mode and guarantees the corresponding material properties.