Key points
- A temper letter on the drawing is a requirement, not evidence that a lot met it.
- The route is qualified once; each production heat-treat batch is verified again.
- Link each batch record to part number, drawing revision, lot and melt.
- Hardness after T6 at an agreed location is a quick lot check, not a design value.
- Tensile tests run on the specimen basis named in the order, at the stated frequency.
- Check final dimensions after heat treatment, because quench can move the part.
Why the temper letter is not proof
T5 and T6 are different thermal routes. A T5 casting is cooled from the casting and then artificially aged, with no solution treatment; a T6 casting is solution treated, quenched and artificially aged, and the solution treatment step blisters gas-bearing castings. Parameters come from the alloy and the governing specification, and a cycle cannot be transferred from another material. A temper designation alone is not evidence of how a lot was processed, and T6 is not a quality grade.
This is also why conventional die castings are normally used as cast: high-pressure die casting traps gas, and that gas expands during solution heat treatment. Squeeze casting fills at low velocity and solidifies under pressure, which is why it is CEX's route for A356-T6 (company-reported). Alloy choice decides whether T6 applies at all: AlSi9Cu3(Fe), EN AC-46000 in EN 1706 with A380 as its usual North American counterpart, is used as cast and not heat treated to T6; AlSi10Mg only on a qualified route.
Qualification connects alloy, process, thermal cycle and specimen location in one record; see the T5 and T6 qualification page. Verification is the batch-by-batch routine that follows.
Checks per heat-treat batch: record, hardness, tensile
| Check | What it shows | What it cannot show | Frequency to agree |
|---|---|---|---|
| Batch and cycle record | What the furnace was set to do and recorded, the condition, and the controlling document. | Which parts were in the load, unless tied to part and lot IDs. | Every batch. |
| Hardness after T6 at an agreed location | Whether aging response is in the expected range for that batch. | Tensile or elongation; hardness is not a design value. | Per heat-treat batch, if specified. |
| Tensile test on the named specimen | Properties reached on the specimen basis named in the order. | Another batch; a separately cast bar does not represent a thick boss. | First samples, then as agreed, for example one test set per heat-treat batch. |
| Dimensions after heat treatment | Final dimensions in the delivered condition, after any movement in quench. | Properties or internal soundness. | First article, then at the agreed frequency. |
| Visual check for blisters (T6) | Whether solution treatment blistered the parts inspected. | Tensile or elongation; it covers only the parts inspected. | As agreed. |
What the batch record should contain
Each heat-treat batch needs its own record set. The batch ID links to the part number, drawing revision, lot and melt. The record names the controlling document and carries the cycle record for that load, and it records the heat-treatment condition. Specimens that represent the parts are processed in the same load, because a test set from one batch says nothing about another batch. Where heat treatment is subcontracted, the processor is part of the approved route. Qualification records stay separate from lot records.
The qualified thermal route belongs in the inspection plan and the change-notification list: alloy source, tooling, casting route, heat-treatment processor and cycle. Lot definition and release are covered in lot release and sampling.
Writing hardness and tensile into the drawing
An A356-T6 note is a grade name, not a purchase specification; the requirement should name the governing material standard, the heat-treatment condition and how properties are verified. Usual US references are ASTM B26/B26M for sand castings and ASTM B108/B108M for permanent-mold castings; a squeeze casting is often ordered to one of them by agreement.
The drawing should state the property and its minimum, minimums not typicals; the condition (as-cast, T5, T6, T61); the specimen basis (separately cast, cast-on, or cut from a named zone, with orientation); the method (ASTM B557 or ISO 6892-1, with gauge length for elongation); the sampling (per heat-treat batch, per lot, or first article); and the retest rules. The guide to specifying mechanical properties on casting drawings covers each of these fields.
ASTM B557 notes that specimens from selected portions may not represent the whole product, and a test method does not supply acceptance limits.
Published A356-T6 values are typical test-bar values, not a guarantee for the part; guaranteed minimums come from the named standard on its defined specimen, or from part minimums agreed with a defined specimen location. CEX publishes a reference range of 260–320 MPa tensile and 220–260 MPa yield for squeeze-cast A356-T6 (company-reported, reference values, not acceptance criteria). The A356 and A356-T6 specification page covers standard, specimen and drawing note.
Drawing note example: tensile, yield and elongation minimums [ ] on specimens cut from zone [ ] or separately cast, per ASTM B557; hardness [ ] HB at location [ ] after T6; one test set per heat-treat batch; dimensions verified after heat treatment and machining. Bracketed values come from the buyer's design.
When a batch fails: retest, re-treat or reject
Hold and identify the batch first. Agree in advance how affected parts are held, who is informed and who approves the disposition; rework must not silently change the requirement.
Then work through the checks in order: specimen location (a thick, porous zone the requirement did not intend?), fracture surface (porosity or oxide films explain low elongation), condition (was the batch record complete?), chemistry (magnesium and iron against the limits), then the test method and machine calibration.
Retest only under a pre-agreed rule, so a failing result is not retested until it passes. Re-treatment follows the approval path and is recorded against the batch. Changing an acceptance criterion after a failure needs the design authority. A coupon result does not replace component testing for a loaded or safety-relevant part.
Changes that reopen heat-treatment approval
| Change | Why it matters for heat treatment | What to ask for |
|---|---|---|
| Heat-treatment processor or cycle | The processor and cycle are part of the qualified thermal route and belong on the change-notification list. | Updated cycle record and processor name; re-verification per the approved change process. |
| Alloy source or ingot | An alloy or heat-treatment change is a change to an approved part. | Melt composition, plus mechanical results with specimen type, location and heat-treatment condition. |
| Alloy substitution, for example AlSi9Cu3 / EN AC-46000 or AlSi10Mg offered for A356 | AlSi9Cu3(Fe) is used as cast and not heat treated to T6; AlSi10Mg only on a qualified route. | A substitution review comparing condition and process records, and mechanical properties with method, specimen origin, location, condition and results; see the alloy substitution approval checklist. |
| Casting route change, for example die casting to squeeze casting | Gas trapped in high-pressure die casting expands during solution treatment, so the route decides whether T6 is practical. | Re-verification of the thermal route and a repeated first article; see moving a die casting that blisters or leaks to squeeze casting. |
| Tooling or cavity repair | Cavity repair is one of the agreed events after which first article is repeated. | First-article results including hardness after heat treatment and tensile on the named specimen. |
How CEX verifies heat treatment (company-reported)
CEX performs heat treatment, including T6, in-house (company-reported). T6 is scheduled before finish machining of critical bores and faces, so final dimensions are checked in the delivered condition (company-reported). The in-house lab includes a tensile testing machine, a hardness tester, an optical emission spectrometer and a fully automatic CMM (company-reported). Tensile specimens and hardness after T6 are run where the drawing requires them (company-reported). The A356-T6 squeeze casting page lists the published capability range.
The factory entity is Qingdao Dechengxin Metal Products Co., Ltd., which holds ISO 9001:2015 and IATF 16949:2016 in its own name; the IATF 16949 scope is manufacturing of aluminium alloy die-cast parts, with clause 8.3 product design excluded (company-reported). CEX is a combined factory and trading company, and the trading company Qingdao Chengenxin has the same owner. Acceptance limits come from your drawing and specification; design authority stays with the buyer.
Put the condition, the governing standard, the specimen basis, the hardness location and the test set per heat-treat batch in the inquiry. The casting RFQ checklist lists the inputs that let a foundry quote and plan verification.