Guide
Can ADC12 die castings be heat treated to T6?
ADC12 is the Japanese Industrial Standard name for an aluminum-silicon-copper alloy made for high-pressure die casting, and it is normally supplied and used in the as-cast condition. A full T6 treatment is possible on paper, because the alloy contains copper that can precipitation-harden, but on a conventional die casting the solution-treatment step usually raises blisters and distortion before it delivers the strength you wanted.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design engineers who have been asked to “make the casting stronger” and for sourcing managers who have received a drawing with “ADC12-T6” in the material block. It covers the temper letters, why trapped gas makes solution treatment a scrap risk, what a low-temperature age can do, and how to write the temper on the drawing.
A note on sources. JIS H 5302, ANSI H35.1 and ASTM B917 are sold by their publishers, and this page does not reproduce text from any standard it has not read in full. Where a figure appears, it is taken from a free primary source linked in the same sentence, or from the published sample pages of EN 1706.
On this page
Can ADC12 be heat treated to T6 at all?
Yes in a laboratory or on a specially made casting, but not as a routine specification for an ordinary high-pressure die casting. For a standard ADC12 part, the safe callouts are as-cast (F) or a low-temperature artificial age (T5), not T6.
The North American Die Casting Association states plainly that aluminum alloy die castings are not usually solution heat treated, and that low-temperature aging treatments may be used for stress relief or dimensional stability (NADCA, aluminum alloys). The same page gives the reason the omission costs less than most engineers expect: the fast chill in a steel die and the very fine grain leave the as-cast structure close to a solution-treated one, so a T4 or T5 temper gives properties quite similar to a full T6.
University research agrees on the risk. A review from the Future Liquid Metal Engineering Hub at Brunel University London says die castings cannot normally undergo solution treatment and quenching because of the high risk of surface blistering and geometrical distortion (Lordan et al., Metals, 2022). A separate paper on semi-solid casting opens with the same point: conventional high-pressure die castings are typically not heat treated because entrapped air and lubricant cause blisters during solution heat treatment (Hu et al., Acta Materialia, 2017).
Why do die castings blister during solution treatment?
Air and burned die lubricant get trapped just under the skin while the die fills, then the intensification pressure squeezes those pockets small. When the part is reheated close to its solidus, the trapped gas expands and pushes the softened surface up into a blister.
The mechanism is described step by step in the Acta Materialia paper cited above. First, the turbulent, non-planar metal front in conventional die casting folds gas under the surface. Second, the intensification pressure applied during solidification compresses each pocket, while the die wall supports the surface so nothing shows. Third, once the casting is out of the die the pocket keeps its small volume. Solution treatment is carried out only slightly below the solidus, about 10 to 20 K below it per the same study, and at that temperature the gas pressure rises while the strength of the metal falls. When the pressure wins, the metal over the pocket deforms plastically and a blister forms.
Two consequences matter on a drawing. Blisters come from pores that sit close to the surface, which is exactly where a cosmetic face, a sealing face or a coating lives. And because gas entrapment varies shot to shot, a trial batch that comes through clean does not prove the next batch will. That is why the EN 1706 temper list carries a note that solution heat treatment of castings involves quenching from elevated temperatures and that distortion can occur, even before blistering is considered.
The research also shows why suppliers sometimes offer a “modified T6”. In a study of alloy A384, a high-silicon grade in the same family as ADC12, samples solution treated at 520 and 530 °C blistered, while a shorter cycle of 30 min at 510 °C left polished sections free of blisters (Demirtaş, Karakulak and Hari Babu, 2023). Those were degassed, cast-to-shape test bars, not production housings, so treat the result as evidence that a narrow window exists, not as a recipe for your part.
What do F, T5 and T6 mean on a casting drawing?
F means as cast, with no heat treatment. T5 means the casting is cooled from the casting process and then artificially aged at a low temperature, while T6 means it is solution heat treated, quenched and then fully artificially aged.
US die-casting specifications take their temper letters from ANSI H35.1: ASTM B85, the specification for aluminum-alloy die castings, states that its alloy and temper designations follow that standard (ASTM B85/B85M scope). H35.1 itself is a paid document. The European casting standard publishes an equivalent list in its free sample pages, and the definitions below are copied from that list. The letters F, T5 and T6 describe the same broad conditions in both systems, but check the wording in H35.1 if your drawing cites it.
| Temper | Definition in EN 1706 | Practical meaning for an ADC12 die casting |
|---|---|---|
| F | As cast | The normal delivery condition; properties come from the chill of the die |
| T4 | Solution heat treated and naturally aged where applicable | Needs the high-temperature step, so it carries the blister risk |
| T5 | Controlled cooling from casting and artificially aged or over-aged | Low-temperature age only; the usual option when some heat treatment is required |
| T6 | Solution heat treated and fully artificially aged | Blister and distortion risk on conventional die castings |
| T7 | Solution heat treated and artificially over-aged (stabilized) | Same solution step as T6; used for stability on structural castings |
EN 1706 also asks that the full designation go on the drawing: the standard number, the alloy, a letter for the casting process (D for high-pressure die casting) and the temper. Its own worked example is a chill-cast alloy in T6: it is the die-casting process, not the aluminum, that makes T6 difficult.
Is T5 or a stress relief a useful alternative for ADC12?
Often, yes: a low-temperature artificial age stays well below the range where trapped gas expands enough to blister, and it can raise strength and improve dimensional stability. It will not match what a full T6 does on a sound, blister-free casting.
NADCA lists T2 and T5 tempers as ways to improve die-casting properties and low-temperature aging as a stress-relief or stability step (NADCA, aluminum alloys). The reason it works is that the fast cooling in the die already traps some alloying elements in solution, so direct aging has something to precipitate. The Brunel review reports that a T5 cycle raised the average yield strength of a die-cast LM24 alloy, a British grade in the same aluminum-silicon-copper family, from 147 to 222 MPa (Lordan et al., 2022).
Three cautions before you write T5 on an ADC12 drawing. First, the result depends on the lot chemistry, especially copper and magnesium, so ask for the lot analysis with the parts. Second, aging changes dimensions slightly, so any tight bore or datum face should be machined after the age, not before. Third, a T5 that no one tests is a label, not a property: name the check you will accept, such as hardness on a stated location or tensile bars cast with the lot.
What are the options if the part needs T6-level strength?
Change the design, the process or the alloy, rather than forcing a solution treatment onto a conventional ADC12 casting. The main routes are more section where the load is, a high-integrity casting process with a heat-treatable alloy, or a machined wrought part.
Start with geometry. Most die-cast housings fail at a boss, a rib root or a thin wall, and adding a fillet or a rib is cheaper than any heat treatment. NADCA’s design pages on walls and ribs are the right first read. If the section really cannot grow, the next route is a process that traps less gas: NADCA notes that heat-treatable specialty alloys, such as the AA 365 family, are available for structural applications, and the Acta Materialia paper lists high-vacuum die casting, squeeze casting and semi-solid die casting as the processes developed to overcome gas entrapment.
| Element | A380.0 | 383.0 | 365.0 |
|---|---|---|---|
| Silicon (Si) | 7.5–9.5 | 9.5–11.5 | 9.5–11.5 |
| Iron (Fe) | 1.3 | 1.3 | 0.15 |
| Copper (Cu) | 3.0–4.0 | 2.0–3.0 | 0.03 |
| Manganese (Mn) | 0.50 | 0.50 | 0.50–0.8 |
| Magnesium (Mg) | 0.10 | 0.10 | 0.10–0.50 |
| Zinc (Zn) | 3.0 | 3.0 | 0.07 |
| Others, total | 0.50 | 0.50 | 0.10 |
The table shows why a structural die-casting alloy is a different material, not a better batch of the same one. The 365.0 row has almost no copper or zinc, a very low iron ceiling, and a controlled magnesium range that gives it a heat-treatment response without the copper phases. ADC12 sits on the other side of the table, with the high copper and high iron of the general-purpose grades. Switching alloy or process means new process settings and new qualification, so decide it before the die is built.
The last route is to drop casting for the loaded feature. A turned or milled part from wrought bar in a heat-treated temper carries its strength through the whole section, and the CNC milling page describes how machined parts are handled. Many programs split the difference: a die-cast body in ADC12 as cast, with a machined insert where the load concentrates.
How should an ADC12 drawing call out the temper?
Write the governing alloy standard, the casting process and the temper in one line, and for most parts make that temper F. If you need any heat treatment, name it, name the test that proves it, and say how blisters on finished faces will be judged.
A drawing that says only “ADC12” is read as as-cast by most die casters. A drawing that says “ADC12-T6” is read three ways: some suppliers will quote a full solution treatment and accept the scrap, some will quietly deliver a T5, and some will decline. Remove the ambiguity with a line such as “ADC12 per JIS H 5302, high-pressure die cast, as cast (F)”, or, where you accept a low-temperature age, “T5, hardness verified at the location shown”. If the drawing is written in A380 or an EN grade instead, the ADC12 vs A380 guide explains why the substitution must be agreed in writing.
This matters most when the casting is quoted in Asia. Die casters in Vietnam, China and Japan commonly quote JIS grades, and the quote often says nothing about temper at all. Ask the supplier to state the temper it will deliver, whether any heat treatment is done on site or by a subcontractor, and which report comes with the lot. The RFQ preparation guide lists the other items to put in the package. The two alloys Saola Manufacturing sources as die castings from partner foundries are ADC12 aluminum and Zamak 3 zinc, and a drawing with a temper callout can be sent through the quote form as drawn, so the temper is discussed before tooling.
Frequently asked
Does heat treatment change which finish suits ADC12?
No. Heat treatment does not change the high silicon and copper content, so the alloy is still not a good anodizing alloy, and powder coat or plating is the usual finish. The finish comparison guide covers the choice.
Will a T5 age change the dimensions of an ADC12 part?
Slightly, which is why stress relief is used for stability in the first place. Machine critical bores and datum faces after the age, and hold the casting tolerances to the standard named on the drawing, as described in the tolerance guide.
Can blisters be seen before a part is heat treated?
Not reliably. The gas pockets that blister sit under a sound-looking skin. X-ray or a heat-treated sample from each lot is a better check than visual inspection of as-cast parts.
Does impregnation stop blistering?
No. Impregnation seals porosity that is connected to the surface so the part holds pressure. The pockets that blister are closed pores under the skin, which the sealant does not reach.
Is JIS ADC12 ever supplied in a heat-treated temper?
It is normally supplied as cast. If a supplier offers a heat-treated ADC12, ask which temper, which cycle and which test result will come with the parts, and get it in the purchase order.
Sources
- Aluminum Alloys, section Heat Treatment & Welding, NADCA Design, North American Die Casting Association, accessed September 2026
- EN 1706:2020+A1:2021, castings: chemical composition and mechanical properties, clause 5.3 temper designations (published sample pages), CEN via SIST, 2021
- Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot (Pink Sheets), The Aluminum Association, revised August 2025
- ASTM B85/B85M, Standard Specification for Aluminum-Alloy Die Castings, scope, ASTM International, 2018
- ASTM B917/B917M, Standard Practice for Heat Treatment of Aluminum-Alloy Castings From All Processes, scope, ASTM International, 2020
- X.G. Hu et al., Blistering in semi-solid die casting and its avoidance, Acta Materialia 124, University of Leicester and partners, open access, 2017
- E. Lordan et al., High-Pressure Die Casting: A Review of Progress from the EPSRC Future LiME Hub, Metals 12(10), Brunel University London, 2022
- H. Demirtaş, E. Karakulak and N. Hari Babu, Heat Treatment Conditions for High-Performance High-Pressure Die-Cast Al–Si–Cu–Mg Alloy, Advanced Engineering Materials, Karabük University and Brunel University London, 2023