How to Specify Copper Tungsten Contacts on a Drawing: Density, Hardness, Conductivity and Infiltration Notes
Most disputes between a switchgear OEM and a contact supplier trace back to a drawing that says "Material: CuW80" and nothing else. The supplier delivers a part that is 80 percent tungsten. The OEM's incoming inspection rejects it for low density, or the wrong hardness unit, or a braze joint with no acceptance limit to check against. Both sides are right, and the drawing is wrong.
This article lists what a copper tungsten contact drawing needs to carry so that any supplier can quote it, any inspector can check it, and the first-article meeting is short.
1. Material Grade and Production Route
Grade by nominal tungsten weight percent: CuW70, CuW75, CuW80. Reference a public standard, GB/T 8320 or ASTM B702, or state "or equivalent" if the drawing uses a supplier's trade name. See GB/T 8320 vs ASTM B702 vs RWMA for the mapping.
Production route. State "infiltrated". Copper tungsten can be made two ways: a pressed and sintered tungsten skeleton infiltrated with molten copper, or a mixed powder pressed and sintered in one step. Infiltrated material has higher density, a continuous copper network and far better arc erosion behaviour. Pressed-and-sintered mixed powder is cheaper and adequate for some resistance welding electrodes, and it is what you get if the drawing does not say otherwise and the supplier is optimising for price. For an arcing contact, always specify infiltrated.
2. Density
The most useful single number on the drawing. Density confirms that the infiltration filled the skeleton. Low density means porosity, and porosity means erratic erosion, poor conductivity, and in a sealed SF6 interrupter a potential leak path.
State a minimum, not a nominal. Industry-typical minimums per GB/T 8320:
| Grade | Density min (g/cm³) |
|---|---|
| CuW70 | 13.80 |
| CuW75 | 14.50 |
| CuW80 | 15.20 |
Many OEM specifications set their own floor a little above the standard, for example 15.4 for CuW80. That is reasonable for HV arcing contacts.
Test method: ASTM B311 (Archimedes, water immersion) or the equivalent clause in GB/T 8320. State it. Density measured by dimensions and weight on a machined part is not acceptable for a composite with possible internal porosity.
3. Hardness
State the unit. GB/T 8320 uses Brinell (HB); North American drawings often use Rockwell B (HRB). Conversion between the two on a tungsten-copper composite is approximate, so pick one and test in it.
| Grade | Hardness min (HB) | Approximate HRB |
|---|---|---|
| CuW70 | 175 | 88 to 90 |
| CuW75 | 195 | 92 to 94 |
| CuW80 | 220 | 97 to 99 |
Test method: ASTM E10 for Brinell, ASTM E18 for Rockwell. Specify the load and ball for Brinell (typically HBW 2.5/62.5 or HBW 10/3000 depending on part size) or results will not be comparable between supplier and OEM.
Hardness alone is a weak specification. It is easy to raise by adding tungsten. Always pair it with density and conductivity.
4. Electrical Conductivity
State a minimum in % IACS. Industry-typical minimums: CuW70 42, CuW75 38, CuW80 34.
Test method: ASTM E1004 (eddy current) is the practical per-lot method. Four-point resistivity measurement is more accurate and slower. Either is fine; state which, and state the test temperature (20 °C is the IACS reference).
Conductivity confirms the copper network is continuous. A lot with correct density and hardness but low conductivity usually has oxide films or contamination in the copper phase.
5. Microstructure
For HV arcing contacts, add a metallographic requirement:
- Uniform distribution of tungsten, with no copper-rich zone ("copper lake") larger than a stated dimension, typically 0.3 to 0.5 mm.
- No cracks, laminations or unfilled porosity visible at a stated magnification (50× or 100×).
- Sample: one section per lot, or per first article, taken from a location the drawing identifies.
Without this, a part can pass density on average and still have a copper-rich band at the arcing face.
6. Composition Tolerance and Impurities
Tungsten content ± 1.5 to 2 percent of nominal is normal. If the application is sensitive to specific impurities (iron, nickel, oxygen), state limits. Most switchgear drawings do not need to; the standards' default limits are adequate.
7. Backing Material and Joint
Where the CuW is joined to a copper or CuCr shank, the drawing has to specify three things:
Backing material. Cu-OF (C10200) or Cu-ETP (C11000) for pure copper; CuCr1 (C18200) or CuCr1Zr (C18150) for age-hardened shanks. State the temper or the required hardness after joining, because the joining process affects it.
Joining method. Brazed (with filler designation, for example BAg-8 or a specific AWS A5.8 classification), cast-on (integral infiltration), friction welded, or EB welded. If more than one is acceptable, list them. See Cast-On vs Brazing vs Friction Welding for how to choose.
Joint acceptance. This is what most drawings miss. State:
- Ultrasonic inspection, with maximum unbonded area as a percentage (5 to 10 percent is common) and maximum single defect size.
- Or a bond strength minimum, tensile or shear, on a test coupon or sample per lot.
- Metallographic section at first article showing the interface.
"Brazed" with no acceptance criterion is not inspectable.
8. Dimensions, Tolerances and Surface Finish
Copper tungsten is machined to final size after infiltration. Tolerances typical of turned and ground parts apply, but note:
- Edges. CuW80 chips at sharp edges. Specify a minimum edge radius or chamfer on the arcing face unless the design needs a sharp edge, and if it does, say so.
- Concentricity between the CuW tip and a friction-welded or brazed shank is a common tolerance; state the datum.
- Surface finish on the arcing face is usually not critical (the first arc reshapes it) but the mating faces and plating surfaces are. State Ra where it matters.
- Grinding. If a feature needs grinding rather than turning, state it. Ground CuW has a different surface condition and tool cost.
9. Plating
Silver plating is common on the current-carrying faces of contact assemblies and rare on the arcing face itself. Where specified:
- Plating material and thickness range (industry-typical for switchgear contact faces: 5 to 20 µm silver, thicker for high-cycle sliding contacts).
- Underplate if any (nickel strike under silver on CuW is common for adhesion; on pure copper faces it may be omitted).
- Masked areas: the arcing face is normally unplated, and the drawing should show it.
- Test: thickness by XRF or cross-section; adhesion by bend or tape test per the referenced standard.
10. Marking, Cleanliness and Packaging
Small items that avoid returns:
- Marking: lot number by laser or ink on a non-functional surface, position shown.
- Cleanliness: for parts going into a sealed SF6 or vacuum assembly, specify a cleaning requirement and no-fingerprint handling. Residual machining oil in the porosity of a poorly infiltrated part is a real outgassing problem.
- Packaging: individually bagged, with desiccant if silver plated. Silver tarnishes in transit.
11. Certification
State what comes with the lot:
- Material certificate per lot: composition, density, hardness, conductivity. EN 10204 type 3.1 is the common format for European OEMs.
- First-article inspection report: full dimensional, plus the metallographic section and joint inspection.
- Plating certificate where applicable.
- Certificate of conformance referencing the drawing revision.
A Complete Example Callout
Material: Copper tungsten, 80 % W nominal, infiltrated. Per GB/T 8320 CuW80 or ASTM B702 equivalent.
Properties (min): density 15.2 g/cm³ (ASTM B311); hardness 220 HB (ASTM E10, HBW 2.5/62.5); conductivity 34 % IACS at 20 °C (ASTM E1004).
Microstructure: uniform W distribution, no Cu-rich zone > 0.5 mm, no cracks or porosity at 50×. One section per lot.
Backing: CuCr1Zr (C18150), ≥ 120 HB after joining.
Joint: friction welded or EB welded. UT: unbonded area ≤ 5 %, no single defect > 2 mm. Section at first article.
Plating: silver 8 to 12 µm on faces marked; arcing face unplated. Thickness by XRF.
Certification: EN 10204 3.1 per lot; FAIR at first delivery.
That is eight lines. It replaces months of email.
Why This Matters to the Supplier Too
A complete drawing is not only protection for the buyer. It is what lets a supplier quote accurately. A drawing that says only "CuW80, brazed" forces the supplier to guess whether the OEM will accept 15.2 or wants 15.4, whether the joint will be checked by UT, whether the shank hardness is tested after brazing. Every guess is either padding in the price or risk in the first article.
When we receive a drawing with gaps, we list the open points in the quotation and propose values. That is not a delay; it is the shortest route to a part that passes incoming inspection the first time.
Related Reading
- GB/T 8320 vs ASTM B702 vs RWMA — Grade designations across standards
- Cast-On vs Brazing vs Friction Welding — Joint methods and acceptance criteria
- Copper Tungsten Properties Guide — What the property numbers mean in service
- Copper Tungsten Component Selection Guide — Component types and typical specs
- CuW Alloy Component — Made-to-drawing CuW parts