Brazing Copper Tungsten to Copper: Process Notes for Switchgear OEMs

In MV switchgear contact assemblies, CuW rarely stands alone. The arc-resistant CuW handles the contact face; a copper or copper-alloy carrier handles the continuous current path and mechanical mounting. Joining the two, typically by brazing, is one of the most common sub-assembly operations in switchgear OEM manufacturing.

A brazed CuW-to-copper joint that's done well lasts the full service life of the contact. Done poorly, the joint becomes the weakest point of the assembly, failing under thermal cycling, mechanical impact, or arc-induced stress. This article covers what a good braze looks like, what filler materials work, and where the common failure modes are.

Why Braze Rather Than Other Joining Methods

The alternatives to brazing for CuW-to-copper joints are:

• Welding (TIG, electron beam): Possible but rare. Tungsten's high melting point relative to copper makes welding awkward; the heat affected zone has different properties from either parent material.
• Mechanical attachment (riveting, pressing): Used in some LV contact assemblies (typical for AgW contact buttons in LV contactors), but for CuW contacts in MV duty, mechanical attachment gives higher contact resistance at the joint and lower mechanical strength.
• Diffusion bonding: Used for high-end specialty applications. Higher cost; not standard for general switchgear production.

Brazing fills the practical sweet spot: high joint strength, low contact resistance, repeatable process, mature manufacturing infrastructure. The trade-off is the brazing operation itself, a controlled-atmosphere furnace step that adds time and cost to the assembly.

Filler Material Selection

The choice of braze filler depends on the joint geometry, the operating temperature range, and the joint strength requirement.

Silver-Based Fillers (BAg series)

The most common choice for CuW-to-copper brazing.

• BAg-7 (Ag 56% / Cu 22% / Zn 17% / Sn 5%): Lower-temperature filler, ~620–650°C melting range. Used where heat input during brazing needs to be limited.
• BAg-8 (Ag 72% / Cu 28%): Higher-temperature filler, ~780°C melting. Higher joint strength than BAg-7; used in mechanically loaded applications.
• BAg-1 (Ag 45% / Cu 15% / Zn 16% / Cd 24%): Mature filler with broad use, but cadmium content limits its use in many regions due to RoHS / REACH concerns. Use BAg-7 or BAg-8 in markets requiring cadmium-free.

Copper-Phosphor Fillers (BCuP series)

• BCuP-2 (Cu 92.7% / P 7.25%): Self-fluxing on copper alloys. Lower cost than silver-based. Brittle joint behavior at low temperatures; used where mechanical impact is moderate.

For most MV switchgear contact applications, BAg-7 or BAg-8 are the right choice. BCuP fillers are common in LV apparatus and lower-cost assemblies.

Joint Geometry

Brazing joint design follows standard practice:

• Lap joint: CuW disc / button overlapping the copper carrier. Provides large surface area for the braze and a mechanically strong joint. Most common geometry.
• Butt joint: CuW and copper ends butted together. Smaller braze surface; lower joint strength. Used where geometry constraints prevent lap.
• Stepped joint: CuW with a small shoulder that nests into a matching pocket in the copper. Combines lap surface area with mechanical alignment. Common for higher-end designs.

The braze gap (the distance between the CuW and copper surfaces at the joint) matters for capillary flow of the filler. Industry-standard practice is 0.05–0.15 mm gap for silver-based fillers; outside this range, the filler either won't flow properly (gap too small) or will leave voids (gap too large).

The Process Sequence

A typical CuW-to-copper braze sequence in a switchgear OEM facility:

1. Surface preparation. Both surfaces cleaned of oxide, oil, and contamination. Mechanical brushing or acid cleaning is standard for copper; CuW is usually cleaned with a different acid (or sometimes mechanical only) because the tungsten phase resists most copper-cleaning acids.

1. Filler placement. Filler shim, paste, or ring positioned at the joint. Quantity matters, too little leaves voids; too much creates fillets that need machining off or that interfere with the assembly.

1. Flux application (for some fillers). Most silver-based fillers used in inert-atmosphere furnaces don't need flux. Air-atmosphere brazing requires flux for surface protection during heating.

1. Furnace cycle. Heating in controlled atmosphere (typically hydrogen / argon mix or pure hydrogen) to a temperature 20–50°C above the filler's liquidus. Hold time long enough for filler to flow and form the joint (typically 5–15 minutes at temperature).

1. Cooling. Controlled cooling to room temperature. Rapid cooling can introduce thermal stress at the joint; gradual cooling reduces stress.

1. Post-braze inspection. Visual check for filler flow, joint integrity. Penetrant testing for safety-critical joints. Tensile or shear strength testing on samples per lot for production qualification.

Joint Strength

Per GB/T 8320-2025 (the current Chinese national standard for CuW contacts), the bonded joint between CuW and the conductive end must meet minimum tensile strength:

• ≥185 MPa for joints to pure copper (Cu) conductive end
• ≥226 MPa for joints to copper alloy (CuCrZr or similar) conductive end

These are minimums; well-designed braze joints with BAg-8 filler regularly exceed these values. The standard also requires the contact to be inspected for cracks at the joint interface, penetrant testing is the standard method for 40.5 kV-and-above applications.

For your own brazing operation, the joint strength target should match or exceed these minimums. Test pieces from each production lot should be tensile-tested to verify.

Common Failure Modes

When a brazed CuW-to-copper joint fails in service, the usual culprits are:

Insufficient Filler Flow

Voids or porosity in the braze layer reduce the effective joint area, weakening the joint. Caused by:

• Joint gap too small or too large
• Inadequate surface cleaning
• Wrong furnace temperature or hold time
• Filler quantity too low

CuW-Side Adhesion Failure

The braze filler flows but doesn't form a metallurgical bond with the CuW surface. Caused by:

• Inadequate CuW surface preparation (oxide layer remaining)
• Wrong filler choice (some fillers don't wet CuW well)
• Furnace atmosphere problem (insufficient hydrogen partial pressure)

Thermal Cycling Failure

The joint develops cracks during repeated thermal cycling in service. Caused by:

• CTE mismatch between CuW and copper at the joint (different thermal expansion rates create stress)
• Brittle filler choice (BCuP fillers can be susceptible)
• Joint design that concentrates stress (sharp corners, no fillet)

Arc-Induced Failure

The joint fails specifically under arc events, not under normal thermal cycling. Less common but particularly serious in service. Caused by:

• Joint location too close to the arc zone (arc heat reaches the joint)
• Filler choice that becomes brittle at elevated temperature

Sourcing Considerations

When sourcing CuW contacts pre-brazed to copper carriers (a common ordering pattern for switchgear OEMs), the specification should cover:

• Filler material (BAg-7, BAg-8, or other per requirement)
• Joint geometry (lap, butt, stepped, per drawing)
• Joint strength target (typically the GB/T 8320-2025 minimum or higher)
• Inspection requirements (visual, penetrant, tensile testing per lot)
• Allowable filler quantity (for assembly-fit requirements)

For sub-supply where we braze the assembly in our supply network, the assembly goes through standard QC including joint strength sampling. Material certification and joint strength documentation are available per lot on request.

For customers brazing in-house, we supply CuW components with surfaces prepared for brazing, clean, dimensionally stable, with consistent geometry from piece to piece.

Summary

Brazing CuW to copper is a mature manufacturing operation with established filler choices, joint geometries, and process parameters. The key to a reliable joint is careful attention to surface preparation, joint gap, filler quantity, and furnace cycle. Per GB/T 8320-2025, minimum bonded joint strength is 185 MPa for Cu and 226 MPa for CuCrZr conductive ends; well-designed joints with silver-based fillers regularly meet or exceed these values.

For specific joint design or process consultation, send the assembly drawing and intended duty cycle. We can advise on filler choice and brazing parameters based on the application.

Related Reading

• CuW Static Arc Contact: Standard CuW contacts, brazed or as standalone
• CuW Arc Contact Pair (Pre-Brazed): Matched pair with pre-brazed CuW + copper sub-assembly
• Moving Contact Conductor: Copper conductor with optional pre-brazed CuW tip
• Copper Tungsten Properties Guide: CuW material reference