Heat Treatment of Aluminium Alloys: T6 vs T5 vs T651 Guide

Heat treatment dramatically transforms the performance of aluminium alloys. Unlike steel—where heat treatment increases hardness—aluminium heat treatment improves:

Strength
Toughness
Dimensional stability
Fatigue resistance
Machinability

In 2025, the most widely used heat-treated aluminium tempers are:

T5 — Cooled from extrusion + artificially aged
T6 — Solution-treated + quenched + artificially aged
T651 — T6 + Stress relieved by stretching

Understanding the differences is crucial for aerospace, automotive, machinery, manufacturing, and structural engineering. This guide explains how each temper works, what changes during heat treatment, and when to choose T6, T5, or T651.

What Is Heat Treatment of Aluminium?

Unlike steel, aluminium alloys do not harden by quenching. They strengthen through:

Solution Heat Treatment: Dissolves alloying elements (Mg, Si, Cu, Zn) into aluminium.
Quenching: Rapid cooling creates a supersaturated structure.
Artificial Aging: Controlled heating forms fine precipitates, increasing strength.

The result is dramatic improvement in mechanical properties without increasing weight.

What Do Temper Codes Mean? (T5, T6, T651)

T5 Temper — Cooled From Extrusion & Aged

Material is extruded
Cooled naturally or air-cooled
Artificially aged to increase strength

Used for:

Architectural profiles
Medium-strength parts
Low-stress components

T6 Temper — Full Heat Treatment

Solution heat treated
Quenched
Artificially aged (typically 160–180°C)

Used for:

Aerospace components
Automotive structural parts
Pressure vessels
Industrial machinery

Provides maximum strength for 6000 and 7000 series aluminium.

T651 Temper — T6 + Stress Relieved

T6 heat treatment completed
Material stretched 1–3% to remove internal stresses
Excellent stability during machining

Used for:

CNC machining plates
Die-making plates
Precision components
Aerospace structures

Strength Comparison — T5 vs T6 vs T651

Temper	Strength	Hardness	Machining Stability	Typical Application
T5	Medium	Medium	Medium	Architecture & profiles
T6	High	High	Good	Aerospace, automotive
T651	Highest	High	Excellent	CNC machining & dies

Summary:

Strongest overall → T651
Best strength-to-cost → T6
Most economical → T5

How Heat Treatment Changes Aluminium Microstructure

During heat treatment:

Alloying elements dissolve
Quenching locks them in place
Artificial aging creates nano-sized precipitates
Precipitates block dislocation movement

Results:

Strength increases up to 3×
Hardness increases
Better fatigue resistance
Improved dimensional stability

Which Alloys Respond Best to T5, T6, and T651?

6061 Aluminium

T6: High strength
T651: Best for CNC machining

Applications:

Machinery parts
Frames
Aerospace structures

6082 Aluminium

T6: Structural work
T651: Precision plates

Applications:

Bridges
Cranes
Heavy structures

7075 Aluminium

T6: Maximum strength
T651: Aerospace-grade machining

Applications:

Aircraft parts
High-speed mechanical components
Defence-grade components

2024 Aluminium

Common tempers: T3 / T4 / T6

Applications: Aerospace structural components

Heat Treatment Process Step-by-Step

STEP 1: Solution Treatment

Heat alloy to 500–540°C to dissolve alloying elements.

STEP 2: Quenching

Rapid cooling in:

Water
Oil
Polymer solution

STEP 3: Artificial Aging

Typical temperatures:

160–180°C for T6
115–140°C for some 7000 series

STEP 4: Stress Relief (T651 Only)

Material is stretched or compressed to eliminate internal stresses.

T5 vs T6 vs T651 — Which Should You Choose?

Choose T5 if:

You need economical aluminium
Application does not require high strength
Good machinability is needed
Used for doors, windows, profiles, architectural sections

Choose T6 if:

You need maximum strength for 6000/7000 series
Structural or load-bearing components
Aerospace, automotive, pressure vessels

Choose T651 if:

You need precision machining
Best dimensional stability
Maximum strength without distortion

Best for:

CNC plates
Dies
Mould bases
Aerospace parts

Real-World Industrial Applications

T5 Temper Uses

Architectural extrusions
Curtain walls
Railings
Furniture frames

T6 Temper Uses

Marine fittings
Aircraft parts
Automotive components
Pressure vessels
Structural frames

T651 Temper Uses

CNC machining
High-precision plates
Aerospace tooling
Dies and moulds
High-load mechanical systems

Advantages of Heat-Treated Aluminium Alloys

High strength-to-weight ratio
Excellent fatigue resistance
Improved toughness
Superior machining stability (T651)
High wear resistance
Reduced distortion

Heat-treated aluminium delivers steel-like performance at one-third the weight.

Limitations of Heat Treatment

Distortion risk during quenching — solved by T651
Higher manufacturing cost — T6/T651 more expensive
Weldability issues for 2000 & 7000 series in T6 — may require preheat or post-treatment

Heat treatment transforms aluminium alloys into high-performance materials used in advanced engineering. Whether you need architectural-grade aluminium (T5), structural-grade (T6), or precision machining-grade (T651), aluminium heat treatment unlocks exceptional performance while remaining lightweight and corrosion resistant.

FAQ's

What is the main difference between T6 and T651?

T651 = T6 + stress relieved (better for machining).

Is T651 always better than T6?

Yes, for machining applications. But T6 is cheaper and good enough for many structural uses.

Can T6 aluminium be welded?

Yes, but strength near the weld zone may decrease.

Which temper is best for aerospace components?

T651 and T6 for 6061, 6082, 7075, and 2024.

Which temper is cheaper?

T5 is the least expensive. T6 and T651 cost more due to processing.

Heat Treatment of Aluminium Alloys: T6 vs T5 vs T651 Guide