Metallurgy
Heat treatment of aluminium castings: T5, T6 and T7 explained
Tempers, the T6 cycle step by step, why conventional HPDC parts blister, which alloys respond, and the controls that make heat treatment repeatable.
Temper designations
| Aluminum Association | BS 1490 | Meaning |
|---|---|---|
| F | M | As cast, no heat treatment |
| T4 | TB | Solution treated and naturally aged |
| T5 | TE | Cooled from casting and artificially aged (precipitation only) |
| T6 | TF | Solution treated, quenched and artificially aged, for maximum strength |
| T7 | TF7 type | Solution treated, quenched and over aged, for ductility and dimensional stability |
| Stress relief | TS | Heated to relieve casting or machining stresses, little change in strength |
The T6 cycle, step by step
| Step | Typical range for A356 and LM25 type alloys | Control point |
|---|---|---|
| Solution treatment | About 535 to 545 °C, held about 4 to 12 hours | Furnace uniformity within a few degrees; never exceed the solidus or the part will melt at grain boundaries |
| Quench | Into water at about 60 to 80 °C | Short quench delay, typically under 10 to 15 seconds; warm water reduces distortion |
| Artificial ageing | About 150 to 170 °C, held about 3 to 6 hours | Time and temperature set the final strength and elongation |
| Verification | Hardness on every lot, tensile bars where specified | Records kept per load with furnace charts |
Why most HPDC parts are not solution treated
Conventional HPDC traps small amounts of air and die release agent vapour under pressure inside the casting. At solution temperatures that gas expands and raises blisters on the surface. HPDC parts are therefore usually supplied as cast, stress relieved or T5 aged. Full T6 or T7 is possible when the part is made with high vacuum die casting and an alloy designed for it, such as the AlSi10MnMg family.
Which alloys respond to heat treatment
- Heat treatable: alloys with magnesium, such as LM25, A356, LM9 and LM13, and copper bearing gravity alloys such as LM4.
- Not usually heat treated: copper bearing HPDC alloys such as ADC12, A380 and LM24, and eutectic LM6, which are used as cast or stress relieved.
Common heat treatment problems
| Problem | Likely cause | Prevention |
|---|---|---|
| Blisters | Entrapped gas in HPDC parts | Avoid solution treatment for conventional HPDC; use T5 or vacuum HPDC |
| Distortion | Fast quench, poor racking, thin sections | Warm water quench, fixtures, straightening allowance |
| Low strength | Low solution temperature or long quench delay | Survey the furnace, shorten the transfer |
| Incipient melting | Solution temperature above the solidus | Calibrated controls and load thermocouples |
Values are typical published ranges for guidance. Every part is different: process windows are set and proven for each component through trials, and the governing standard and your drawing take precedence. Reviewed by the Alcast engineering team, November 2026.
Questions engineers ask
What is the T6 heat treatment for A356?
Typically solution treatment at about 535 to 545 °C for 4 to 12 hours, a quench into warm water, then artificial ageing at about 150 to 170 °C for 3 to 6 hours. Exact times follow the part and the specification.
What is the difference between T5 and T6?
T5 is artificial ageing only, straight from the casting process. T6 adds solution treatment and a quench before ageing, which gives much higher strength but can blister HPDC parts.
Can ADC12 be heat treated?
ADC12 is normally used as cast. A stress relief or T5 age is possible, but solution treatment blisters conventional HPDC parts.
What does TF mean on a BS 1490 drawing?
TF is the British equivalent of T6: solution treated and precipitation hardened.