What buyers should verify first
A mill certificate is useful only when its issuer, specification, heat number and reported values can be tied to the stock received. Buyers should verify the document and the physical lot rather than accepting a PDF by appearance.
| Review area | What good control looks like | Buyer action |
|---|---|---|
| Purchase requirement | Exact grade, temper/condition and governing specification | PO and drawing |
| Certificate | Issuer, heat/lot and reported results | MTC or CoC |
| Receiving | Marking matches certificate and quantity | Incoming record |
| Production | Cut blanks retain lot identity | Traveler or label |
| Verification | Independent testing based on risk | PMI, hardness or lab test |
1. Write an unambiguous material callout
“Aluminum” or “stainless” is insufficient. State alloy, temper or condition, product form and specification revision where necessary. A callout such as 6061-T6 plate must also identify the relevant product specification when certified properties are required. Do not substitute a bar specification for plate simply because the alloy number matches. Record any approved alternative before stock is purchased.
2. Inspect the certificate fields
Check that units, test values, heat number, producer and specification make sense. A polished PDF with no lot connection is not traceability. Match issuer identity, heat or lot number, product form, specification and test units to the ordered stock. Compare each required chemical and mechanical value with the applicable limits, including thickness-dependent limits. A certificate describing another heat cannot release a visually identical bar.
3. Protect identity after cutting
Bars and plates lose mill markings when sawn. Require a traveler or durable lot label that follows blanks, work-in-process, inspection and packing. For a saw-cut batch of 80 blanks, label each controlled container and transfer the lot identity when parts move to a new tray. Do not mix the remaining blanks with material from a second heat during cleanup. Reconcile issued, scrapped and finished quantities so missing identity cannot be hidden by a correct total.
4. Use risk-based verification
For critical parts, define positive material identification, hardness, conductivity or third-party chemistry testing as appropriate to the alloy and risk. Handheld chemistry testing can screen alloy composition but does not by itself prove an aluminum temper. Hardness, conductivity or documented heat treatment may be needed, with methods suitable for that alloy and thickness. Agree who interprets conflicting results and whether stock remains quarantined during investigation.
5. Control substitutions
Require written approval before any equivalent grade, alternate temper, remelt source or material change is used. An equivalent national designation may have different composition or property limits. Require the supplier to compare the actual specifications and explain the functional impact. Retain the engineering disposition with the lot record, including the limited quantity or order to which approval applies.
What to check on a material certificate
- Mill or authorized issuer identity and certificate number
- Heat, melt or lot number that matches the stock marking
- Product form, dimensions and governing material specification
- Alloy, grade, condition or temper and the stated test units
- Chemical values compared with the specification limits
- Mechanical values, test direction and applicable acceptance limits
- Quantity or mass covered and any processing or heat-treatment record
- Authorized signature, date and evidence of later amendments
Example: certificate and bar stock do not match
A certificate stated 7075-T6 plate while the incoming material was round bar with a different heat marking. The chemistry could still resemble 7075, but the certificate did not cover the received product form or lot. The correct response was to quarantine the stock, request a matching certificate and verify the marking before cutting production parts. Positive material identification can support grade verification, but aluminum temper normally needs hardness, conductivity or documented heat-treatment evidence in addition to chemistry.
Practical buyer scenario
For 7075-T6 structural brackets, the PO should identify alloy and temper, and the receiving record should tie the mill heat to the production lot. A generic “aluminum certificate” is not enough.
Topic-specific review checklist
- Issuer and certificate number
- Heat or lot number matching the stock
- Product form and material specification
- Grade, condition or temper
- Chemical and mechanical values with units
- Received-stock marking and segregation
- Required independent verification method
Frequently asked questions
Is an MTC always required?
It depends on the contract and risk. Traceability should be proportional to safety, regulation and cost of failure.
Can PMI identify every aluminum temper?
Not by chemistry alone; temper may require hardness, conductivity or documented heat-treatment evidence.
What if a supplier combines material lots?
Require separate identification and reporting, or prohibit mixed lots for the order.
Who should keep records?
Define retention period and retrieval time in the purchase agreement.
Sources and further reading
Discuss this drawing requirement with WEKO before releasing material or committing the production route.
