Cast-On vs Brazing vs Friction Welding: How CuW Arcing Contacts Are Joined to Copper
An arcing contact in an SF6 or air-blast circuit breaker is two materials doing two jobs. The copper tungsten tip takes the arc. The copper or chromium-copper shank behind it carries current, takes the mechanical load, and mounts to the mechanism. The joint between them is where most arcing contact failures start, and it is the line on the drawing that decides which supplier can quote.
OEM drawings specify the joint one of four ways: brazed, cast-on (integral infiltration), friction welded, or electron beam welded. This article covers the first three. EB welding gets its own article because the question it raises is different: Can Copper Tungsten Be Electron Beam Welded?
Why the Joint Is the Hard Part
Copper tungsten is not an alloy. It is a tungsten skeleton with copper infiltrated into the pores. Tungsten melts at about 3,400 °C, copper at 1,085 °C, and the two are practically insoluble in each other. Any joining process therefore works on the copper phase only: you are bonding copper in the CuW to copper in the shank, with tungsten particles sitting in the joint zone.
Two numbers explain most joint problems:
- Thermal expansion mismatch. Tungsten expands about 4.5 µm/m·K, copper about 17. CuW80 lands around 7 to 8. Every heating and cooling cycle, including the arc itself, puts shear stress on the interface.
- Shank material behaviour. Pure copper (Cu-OF, Cu-ETP) tolerates any joining temperature. Chromium copper (CuCr1, CuCr1Zr) gets its strength from age hardening at roughly 450 to 500 °C. Hold it at brazing temperature and it over-ages; hardness drops and the shank can deform under contact spring load.
The joining method has to survive the first and respect the second.
Brazing
Brazing is the default for MV and many 145 kV class arcing contacts. A silver-based filler (BAg series, or a copper-phosphorus filler in some low-duty applications) is melted between the CuW tip and the copper backing, usually in a vacuum or controlled-atmosphere furnace, sometimes by induction.
Where it fits. Flat and stepped geometries: contact plates, tips brazed onto fingers, segmented arcing rings. Any joint face that is not rotationally symmetric. Small batch sizes, because tooling is minimal.
What it costs you. The joint has a filler layer with lower conductivity than either parent material. Under repeated arc heating the filler can creep or re-melt locally if the tip runs hot. Voids in the braze layer are the classic hidden defect; a 20 percent unbonded area may pass visual inspection and fail after a few hundred operations.
Shank compatibility. Fine with pure copper. With CuCr the brazing cycle over-ages the shank, so either the OEM accepts a softer shank, or a low-temperature filler and short cycle is specified, or the joint moves to friction welding.
We covered filler selection, joint gap and process sequence in Brazing Copper Tungsten to Copper. This article is about when to choose brazing over the alternatives.
Cast-On (Integral Infiltration)
In a cast-on joint there is no filler and no second operation. The tungsten skeleton is pressed and sintered, then placed in the furnace with a copper slug and, in the same infiltration cycle, the molten copper fills the skeleton and solidifies as an integral copper back. The copper shank is literally cast onto the tungsten.
Drawings call this "cast-on copper", "integral back", "infiltration bonded" or, in Chinese-origin drawings, 整体烧结 (integral sintering). Same process.
Where it fits. Round and simple stepped parts where the copper back is pure copper: arcing contact tips with a short copper stub for subsequent brazing or machining, moving arcing contact heads, nozzle-side contact rings. High-current designs where the OEM wants no braze layer in the current path.
What it gives you. The best conductivity across the joint of any method, because the interface is a diffusion zone rather than a filler layer. Bond strength is typically at or above the strength of the copper itself; in a pull test the copper back yields before the interface lets go.
What it cannot do. The back is pure copper, and soft. If the drawing calls for CuCr or CuCrZr behind the tip, cast-on is not an option. Part size is limited by the infiltration furnace, and the copper-to-tungsten volume ratio is limited by what the skeleton can hold and what the furnace fixture allows. Very long copper shanks are made by cast-on to a short stub and then friction welding or brazing the stub to the full-length shank.
Friction Welding
Rotary friction welding spins one part against the other under axial load. Frictional heat softens the copper phase at the interface, the rotation stops, forge pressure is applied, and the parts bond in the solid state. No melting, no filler. Flash is removed by machining.
Where it fits. Anything round. Moving arcing contact pins, fixed arcing contact rods, contact fingers with round stems. This is the standard route for HV breaker arcing contact rods from 145 kV upward, where the shank is often CuCr and the part is long.
What it gives you. A narrow heat-affected zone, so a CuCr shank keeps most of its age-hardened strength. Joint strength close to the copper parent. Excellent repeatability once parameters are set, which matters for OEM lot qualification.
What it needs. Rotational symmetry at the joint face. A machine sized for the part: a 60 mm diameter CuW-to-CuCr weld needs a different machine than a 16 mm pin. Parameter development per part number, so it suits repeat production more than one-offs. Flash removal adds a machining step, and the drawing should say whether flash may remain on non-functional surfaces.
Choosing Between Them
| Criterion | Brazing | Cast-on | Friction welding |
|---|---|---|---|
| Joint geometry | Any, incl. flat and segmented | Round or simple stepped | Round only |
| Shank material | Cu; CuCr with compromise | Cu only | Cu or CuCr |
| Filler in current path | Yes | No | No |
| Joint conductivity | Lowest of the three | Highest | High |
| CuCr shank keeps hardness | Partly | n/a | Mostly |
| Batch economics | Good at low volume | Good at all volumes | Best at repeat volume |
| Typical voltage class | MV to 145 kV | MV to HV | 145 kV and up, also MV pins |
| Hidden defect | Braze voids | Infiltration porosity | Incomplete bond at centre |
A few rules of thumb from OEM drawings we see:
- Flat tip on a finger or plate: brazing. There is no other practical route.
- Round tip, pure copper back, moderate length: cast-on. Cheapest joint with the best conductivity.
- Round tip, CuCr shank, or long rod: friction welding. Cast-on cannot make CuCr, brazing softens it.
- Cast-on stub plus friction weld to shank: common on 245 kV and above, where the OEM wants the integral tip and a hardened long shank.
How the Joint Is Verified
Whatever the method, the OEM's acceptance test is usually one or more of:
- Ultrasonic inspection of the bond area, with an allowed unbonded percentage stated on the drawing (often 5 to 10 percent maximum, with no single void above a set diameter).
- Metallographic section on a sample per lot, showing the interface free of cracks and, for brazed joints, continuous filler.
- Bond strength test: tensile or shear on a sample or a test coupon made in the same cycle. Some drawings reference the joint strength minimums in GB/T 8320; others state a value directly.
- Dye penetrant on the machined joint line for surface-breaking defects.
If the drawing does not state a joint acceptance criterion, ask before quoting. "Brazed" without an unbonded-area limit is a specification gap that becomes an argument at first-article inspection.
What This Means for Sourcing
The joining method on the drawing is not a suggestion. Substituting brazing for a specified friction weld changes the shank hardness, the joint conductivity and the failure mode, and no switchgear OEM will accept that on a part inside a type-tested assembly.
Our practice is straightforward. When a drawing specifies cast-on, friction welding or brazing, we confirm at the quotation stage which route the part will be produced on and whether the specified process is available for that size. If a drawing calls for a process we cannot run for that geometry, we say so in the quote rather than substituting. Joint acceptance criteria, sample sectioning and ultrasonic reports are agreed before the first lot, not after.
If you are writing a new drawing rather than sourcing to an existing one, How to Specify Copper Tungsten Contacts on a Drawing covers the joint callout alongside grade, density and hardness.
Related Reading
- Brazing Copper Tungsten to Copper — Filler selection, joint gap, process sequence
- Can Copper Tungsten Be Electron Beam Welded? — When EB is specified and what the alternatives are
- CuW70 vs CuW80 for 145 kV, 245 kV and 420 kV — Grade selection by voltage class
- CuW Moving Arc Contact — Made-to-drawing arcing contacts
- Copper Tungsten Series — Full CuW product range