Heat Treatment of Aluminum Alloys: T5, T6, and T7 — Process, Properties, and Applications

Introduction to Aluminum Heat Treatment

Heat treatment is the process by which the mechanical properties of an aluminum alloy are modified through controlled cycles of heating, holding, and cooling. Unlike steel, where hardening is based on martensitic transformations, aluminum hardens by precipitation: the formation of nanometric intermetallic phase particles that obstruct dislocation movement.

Not all aluminum alloys respond to heat treatment. Only those containing alloying elements with temperature-dependent solubility — primarily Mg₂Si in the 6xxx series and Al-Si-Mg casting alloys, and Al₂Cu in the 2xxx series — can be precipitation hardened. This guide focuses on T5, T6, and T7 tempers, the most relevant for the aluminum casting and extrusion industry.

Temper Designation System

The Aluminum Association defines heat treatment conditions using letters and numbers. The most important designations are:

T5: Direct Artificial Aging

The T5 temper consists of cooling the part from hot working temperature (extrusion, forging, or casting) and directly applying an artificial aging cycle, without solution heat treatment or rapid quenching. This treatment is more economical and faster than T6 but produces lower mechanical properties.

When Is T5 Used?

• 6063-T5 extrusion profiles: the most common application. The profile is cooled upon exiting the press (by fan or forced air) and aged at 185-205 °C for 1-4 hours. Ideal for architectural applications where mechanical tolerances are moderate.
• Low-demand castings: in die casting where rapid cooling from the metal mold provides sufficient supersaturated solid solution.
• Cost reduction: eliminates the solution heat treatment furnace and quench system, significantly reducing processing cost.

T6: Solution Heat Treatment + Quench + Artificial Aging

T6 is the most widely used heat treatment for heat-treatable aluminum alloys and produces the maximum combination of strength and hardness. It consists of three critical stages:

Stage 1: Solution Heat Treatment (Solutionizing)

The part is heated to a temperature near the alloy solidus to dissolve precipitates and alloying elements into solid solution. Temperature and time depend on the alloy:

During solutionizing, beneficial microstructural changes also occur in casting alloys: eutectic silicon spheroidizes (rounds) and fragments, improving the ductility of the treated part.

Stage 2: Quenching

Immediately after solutionizing, the part is rapidly cooled to retain alloying elements in supersaturated solid solution. Transfer time from furnace to quench medium is critical and should not exceed 10-15 seconds for thin sections.

The minimum required quench rate varies by alloy. Cu-containing alloys (2xxx series, 319) are more sensitive and require rates > 100 °C/s. Al-Si-Mg alloys (A356, 6063) tolerate lower rates of 50-100 °C/s without significant property loss.

Stage 3: Artificial Aging

The quenched part is heated to an intermediate temperature to promote controlled precipitation of hardening phases. The precipitation sequence in Al-Mg-Si alloys is:

Supersaturated solid solution → GP zones → β" (needle) → β' (rod) → β (equilibrium Mg₂Si)

Maximum hardening (peak aging, T6) is achieved when nanometric β" precipitates predominate (4-5 nm diameter, 20-50 nm long). These precipitates are coherent with the aluminum matrix and create the maximum obstacle to dislocation movement.

T7: Overaging for Stability

The T7 temper follows the same solutionizing and quench process as T6, but aging is carried beyond the hardening peak (overaging). Precipitates grow until they lose coherence with the matrix, reducing strength but gaining important benefits:

• Dimensional stability: incoherent precipitates do not change over time or with moderate temperature exposure, eliminating post-treatment dimensional growth.
• Stress corrosion cracking (SCC) resistance: more uniform precipitate distribution reduces susceptibility to intergranular corrosion and SCC.
• Lower residual stress: overaging partially relieves quench residual stresses.
• Thermal stability: T7 parts maintain their properties up to 200-250 °C, while T6 parts begin losing hardness above 150 °C.

Typical T7 conditions involve aging temperatures 20-50 °C higher than T6 or significantly longer times. For example, A356-T7 is aged at 200-220 °C for 4-6 hours, compared with 155 °C / 5 hours for T6.

Property Changes: Before and After Heat Treatment

The impact of heat treatment on mechanical properties is dramatic. The following tables show representative data for the most common alloys:

6063-T5 vs 6063-T6: The Extrusion Decision

The choice between T5 and T6 for 6063 profiles is one of the most frequent decisions in the extrusion industry. The difference lies in processing cost and resulting properties:

Common Heat Treatment Defects

Incipient Melting

Occurs when solution temperature exceeds the solidus. Low melting point eutectic phases (Al-Si eutectic at ~577 °C, Al₂Cu eutectic at ~548 °C) partially melt, creating voids at grain boundaries. The defect is irreversible and the part must be scrapped. Prevention: frequent furnace thermocouple calibration, uniformity verification per AMS 2750, use of thermal control specimens.

Quench Cracking

Thermal stresses generated during rapid quenching can exceed the material strength at elevated temperature, causing cracks. More common in parts with abrupt section changes, sharp corners, or holes. Prevention: use warm water or polymer quench, design parts with generous radii, correct immersion orientation.

Incomplete Precipitation

If quench rate is insufficient, solutes precipitate coarsely during cooling, leaving less solute available for subsequent aging. The result is T6 hardness below specification. This is particularly common in thick sections (> 10 mm) quenched in air or warm water. Solution: verify quench rate with embedded thermocouples, adjust quench medium.

Over/Under-aging

Incorrect aging time or temperature produces out-of-specification properties. Underaging results in low hardness and insufficient strength. Overaging reduces strength but increases ductility, effectively resulting in an unintentional T7 condition. Control: use furnaces with continuous temperature recording, perform hardness tests on witness specimens processed with the load.

Practical Process Considerations

• Furnace loading: space parts to allow uniform air circulation. Avoid stacking parts that may deform under their own weight at solutionizing temperature.
• Furnace atmosphere: air furnaces are standard. Controlled atmosphere furnaces (nitrogen) are optional but reduce surface oxidation.
• Process recording: document temperature, time, quench medium, and hardness results for each batch. Automotive standards (CQI-9, IATF 16949) require complete traceability.
• Control specimens: include test bars from the same alloy lot in each furnace load for destructive property verification.
• Metal quality effect: parts with high hydrogen porosity or inclusions will not respond well to T6. Liquid metal quality is a prerequisite.

Conclusion

Heat treatment transforms aluminum alloys from modest-property materials into high-performance engineering materials. Correct selection between T5, T6, and T7 depends on mechanical requirements, dimensional stability needs, and process economics. Rigorous control of each stage — solutionizing, quenching, and aging — is essential for consistent results.

At Transformación Puebla we supply aluminum alloys optimized for heat treatment, both extrusion billets (6063, 6061, 6005, 6082) and casting ingots (A356, A357). Contact us for advice on the alloy and temper best suited to your application.