Can suppliers mix materials in production when you import custom metal parts from Vietnam?

I’ve seen how tempting it is for factories to quietly blend scrap or substitute cheaper metals in castings—it can cut costs fast, but it’s risky and unacceptable.
No—suppliers should not mix materials unless you have approved it in writing with strict controls. Mixing or substituting materials without your consent can lead to performance failure, legal issues, and rejected imports.
This article explains what risks to watch for and how to lock down your material specification ¹ in real-world sourcing from Vietnam.

Is using mixed alloys allowed or common?

In most engineered metal parts, the expectation is clear: you specify the exact alloy, and suppliers follow that spec 100%.
Using mixed or substituted materials without your approval is not acceptable, and may violate certifications or contractual terms—even if it’s common in low-grade production.

That said, some mixing of recycled content ² does happen in processes like:

• Die casting
• Sand casting
• Investment casting
• Plastic injection molding

In these cases, factories may mix recycled sprues, gates, or machining scrap back into the raw feedstock to save cost. This can be okay if:

• You approve it in writing;
• They disclose the ratio of new vs. recycled;
• They re-melt, filter, or refine the metal;
• The resulting properties match spec.

Table 1: Where Material Mixing Happens and What to Watch

In short: mixing may happen—but only if you allow it and the factory shows they understand the risks and testing involved.

How to require consistent material specification?

If you want every part to use the same exact alloy—no mixing, no substitutes—you must lock that into your technical and commercial documents.
The most effective way is to combine clear material specification ³, contractual clauses, and documentation requirements.

Here’s what I’ve used successfully:

Step-by-Step Control Framework

1. Precise Specification in Drawing or BOM

   • Use full industry standard: e.g., ASTM A276 Type 316L
   • Include tolerances, temper, heat treatment, etc.
   • Add "no substitution allowed without buyer approval" line

2. Contract Terms

   • Add a “No Substitution” clause in your PO or supply agreement
   • Add right to reject or claim cost for any non-conforming batch ⁴
   • Add penalty clauses if quality control or material trace fails

3. Mill Test Certificates (MTRs)

   • Require factory to provide MTRs from original mill for every heat/batch
   • Each part must trace back to a heat number

4. Third-party testing ⁵

   • Use spectrograph or PMI (Positive Material Identification) on arrival
   • Randomly test shipments—even 1 per batch can reveal issues

Table 2: Supplier Control Requirements You Should Include

When you control both the technical spec and the supply chain process, you ensure consistent metallurgy—and avoid nasty surprises.

What risks of mixing materials?

This is where it gets serious.
Mixing materials—without engineering and approval—can result in failures, safety risks, customs issues, and destroyed shipments.

The Risks Include:

• Mechanical failure ⁶
  – Lower-grade metal might crack, deform, or wear faster
• Corrosion issues
  – Stainless with less chromium? That’s rust waiting to happen
• Thermal expansion mismatch
  – In bimetal parts, unbalanced expansion can break joints
• Warranty claims and liability
  – Failures due to wrong alloy = you’re responsible
• Regulatory noncompliance
  – FDA, aerospace, or military specs don’t allow mixed alloys
• Customs and duty errors
  – HS code must match declared material. If it doesn’t: penalties

I’ve seen importers lose entire shipments over one missing MTR or PMI test failure.

Table 3: Risk Comparison – Mixed vs. Consistent Alloy

Bottom line: even if the factory says, “It’s close enough,” don’t take their word. Test it. Check documents. Reject if it’s wrong.

How to audit material traceability ⁷?

Verifying what goes into your parts isn’t just about trust—it’s about system controls.
Material traceability means being able to follow a part’s material all the way from the mill to the finished shipment.

Here’s how I recommend you do it:

Core Components of a Traceability System ⁸

1. Mill Test Reports (MTRs)
   – Every shipment should include them
   – Shows chemistry, mechanical properties, heat number

2. Heat Number Tracking
   – All parts (or at least batches) must trace to a heat number
   – Your PO, invoice, and packaging list must reference this

3. Labeling and Marking
   – Use batch codes or serials on packaging
   – In some cases, etch the number on the part

4. Pre-shipment Audit
   – Inspect material storage: is it labeled and separated?
   – Ask for raw material inventory sheets

5. Arrival Testing
   – Spot-check with PMI (portable material tester)
   – For high-risk parts, use lab spectrographic testing

Best Practices Summary (Checklist Format)

Keep records of all tests and documents. Customs, insurance, and end-customers may require this proof later.

Conclusion

Yes, some processes in Vietnam involve material mixing—but only when you permit it, control the method, and verify the results. Without those controls, you risk defects, customs issues, and brand damage.

Footnotes

1. Explains how material specifications support sourcing traceability. ↩︎
2. Audit example showing recycled content traceability in manufacturing. ↩︎
3. Comprehensive guide to material traceability standards and processes. ↩︎
4. Foundry supplier audit guide for handling non-conforming batches. ↩︎
5. Article on third-party testing for global supplier verification. ↩︎
6. How improper traceability can cause mechanical failures. ↩︎
7. Guide on implementing effective material traceability programs. ↩︎
8. Explanation of traceability audits in manufacturing operations. ↩︎
9. Framework for supplier approval and monitoring in production. ↩︎
10. Overview of best practices in supplier quality control systems. ↩︎