Diffusion bonding · Copper to steel · Fixture & tooling plates
Copper-to-Steel Diffusion Bonded Plates for Welding Fixtures and Heat-Sink Tooling
A thick steel backbone for stiffness, a thin copper face to pull heat out of the weld zone, joined solid-state through a nickel interlayer — then machined flat. No drawing needed to get a ballpark price.
Typical build: 12–50 mm steel + 2–6 mm copper, plates up to 800 × 400 mm, supplied in matched sets so you can rotate fixtures while one heat-soaks.
Is this your requirement?
Most enquiries for this part look alike: a steel fixture or backing bar that needs a copper contact face for thermal conductivity, and a joint that has to survive milling and repeated heating without delaminating. If your spec sits inside the envelope below, we can quote from a sketch.
What we build
- ConfigurationFlat bimetal plate, copper bonded over the full steel face
- Steel thickness12 – 50 mm
- Copper thickness2 – 6 mm as bonded; finish-milled to 1.5–3 mm if required
- Plate sizeUp to 800 × 400 mm
- QuantityPrototype 1 pc to rotated sets of 3–8 pcs; batch production on request
- InterlayerNickel, 50–100 µm, applied by us
- Post-bondFace milling, drilling, tapping, dowel holes, surface finish to your CAD
Typical duty
- Use caseWelding fixture, backing plate, chill bar, hot-tooling heat sink
- Service temperatureUp to 600 °C at the copper face (bond formed at ~900 °C)
- Thermal cyclingFrom once per day to once per hour; rotated sets recommended
- LoadCompressive clamping loads of 5–50 kN are not joint-critical (see Joint performance)
- FlatnessPlate bows slightly on cooling; we mill the copper face flat afterwards
- MaterialsSupplied by us, or free-issue your own steel and copper
How the joint is made
Diffusion bonding is a solid-state process: no filler, no liquid phase. Under vacuum, temperature and pressure, atoms migrate across the interface until the two metals share a continuous metallurgical bond. Copper and iron barely dissolve in each other, so a nickel interlayer — soluble in both — is used to build a sound diffusion zone and suppress Kirkendall voids.
Surface preparation
Steel and copper faces are ground flat and cleaned. Surface flatness and cleanliness decide bond coverage, so this step is measured, not assumed.
Nickel interlayer
A 50–100 µm nickel layer (foil or plating) is placed between the two faces. It bonds to both metals and keeps brittle Fe–Cu interfaces out of the joint.
Vacuum hot press
The stack is held at roughly 850–950 °C under a uniform compressive load in vacuum for about one hour, then cooled under control.
Machining to CAD
The copper face is milled flat and the plate is drilled, tapped and finished. The bonded blank is treated as one solid part from here on.
Choosing the steel and the copper
Every steel passes through the bonding temperature, so a hardened or tempered grade arrives in roughly normalised condition. For fixture stiffness that costs nothing: elastic modulus is about 200 GPa for all steels, so a plain non-heat-treated grade gives the same stiffness as an alloy steel at a fraction of the price and with far better thermal conductivity than stainless.
Steel
| Grade | Stiffness (E) | Thermal cond. | After bonding | Recommendation |
|---|---|---|---|---|
| 1045 / S45C / C45 | ~200 GPa | ~50 W/m·K | Normalised, ~180–210 HB | Default choice: stiff, machinable, low cost |
| S355 / A572 / 1020 | ~200 GPa | ~50 W/m·K | Normalised, soft | Fine where surface wear is not a concern |
| 304 / 316 stainless | ~193 GPa | ~16 W/m·K | Annealed | Only if corrosion demands it; higher CTE mismatch stress, one-third the conductivity |
| Hardened tool steel | ~200 GPa | ~25–40 W/m·K | Hardness lost | Not recommended — the bond cycle undoes the heat treatment |
Copper
| Grade | Thermal cond. | Notes |
|---|---|---|
| C11000 (ETP) | ~390 W/m·K | Standard for fixture faces; lowest cost |
| C10100 / C10200 (OFHC) | ~390 W/m·K | Oxygen-free; preferred for repeated high-temperature cycles in air |
| Nickel-plated copper face | — | Optional 5–10 µm Ni on the working face to slow oxidation at high temperature |
Stock thickness tip: if the face will be milled flat after bonding, start with 4 mm copper to finish at 2–3 mm. Copper’s expansion (~17 µm/m·K) is higher than steel’s (~12 µm/m·K), so a long plate bows slightly on cooling; thin copper on thick steel keeps that bow small and the milling allowance covers it.
Joint performance: what the numbers mean for a fixture
Shear strength reference
Published solid-state bonds of copper to 410 stainless steel through a 100 µm nickel foil (900 °C, 12 MPa, 60 min, vacuum) reach a maximum shear strength of about 145 MPa, with failure in the copper rather than at the interface.
Source: Yildiz et al., Materials Characterization 61 (2010), via OSTI.GOV. Bond strength depends on the exact steel, copper and cycle; we confirm each combination with a shear coupon from the same run before releasing production plates.
Why the fixture load is not the limit
A 20 kN clamping load acting straight down on a 400 × 200 mm face is a compressive stress of only 0.25 MPa — pressing the layers together, not apart. Even a conservative 100 MPa-class bond is two to three orders of magnitude above anything the fixture sees in service.
The real demands on the joint are post-bond face milling (interrupted cutting on the copper) and thermal cycling. A continuous, void-free bond handles both; a partial bond does not. That is why bond coverage is checked, not just bond strength.
Service up to 600 °C
The bond is formed at ~900 °C, so 600 °C service does not weaken it. What changes at temperature is the copper face: it oxidises in air and softens. Rotating a set of plates, or specifying a nickel-plated face, is the practical answer, and a periodic light re-skim restores the surface.
Diffusion bonding, brazing or explosion-clad plate?
We also build vacuum-brazed copper–steel assemblies, so the recommendation follows the job rather than the process.
| Process | Joint | Max service temp. | Copper thickness | Best for |
|---|---|---|---|---|
| Diffusion bonding (Ni interlayer) | Solid-state, no filler, no liquid phase | Limited by copper, not the joint | 0.5 – 5 mm, custom | Thick steel + thin copper, high-temperature tooling, machined fixture blanks |
| Vacuum brazing | Filler alloy (Ag or Cu-based) | Below filler melting point | Any | Complex shapes, internal channels, liquid cold plates |
| Explosion-clad sheet | Wavy mechanical-metallurgical bond | High | Fixed mill sizes | Large stock sheets cut to size; not for thick steel with a custom copper layer |
What drives the price
Each bonded plate is priced from five inputs. Send them with a sketch and we return a ballpark within 48 hours; a firm quote follows once the drawing is fixed.
1. Plate area
Sets the vacuum-press cycle size and the nickel interlayer area.
2. Steel and copper stock
Grade, thickness and whether you free-issue material.
3. Cycle count
Several plates of one size share a press cycle; a rotated set of 4–6 is usually one or two cycles.
4. Machining
Face milling, holes, tapping, dowels and finish per your CAD.
5. Verification
Shear coupon from the same run, ultrasonic bond-coverage check, flatness report.
What to send for a ballpark quote
No drawing required. A CAD screenshot or hand sketch plus these six points is enough. Details stay under NDA on request.
- Plate length × width, and quantity per set
- Steel thickness and preferred grade (or ask us to recommend)
- Copper thickness as finished, and whether the face will be milled after bonding
- Peak service temperature and cycles per day
- Any holes, slots or dowels to be machined
- Who supplies material: ToneCooling or free-issue
Send your enquiry
Email info@tonecooling.com with the subject “Copper-steel bonded plate”. Ballpark reply within two business days.
ToneCooling: custom liquid cold plates and thermal management products since 2004. ISO 9001 facility, 30,000 m², 46 patents, 200+ staff. Regional entities in Texas, Europe and Australia.
Frequently asked questions
Can copper be diffusion bonded to steel?
Yes. Copper and iron have very low mutual solubility, so a direct bond is weak. With a nickel interlayer, which dissolves in both, a continuous solid-state bond forms at around 850–950 °C under vacuum and pressure.
Do you need a nickel interlayer to bond copper to steel?
For a reliable, void-free joint, yes. A 50–100 µm nickel layer builds sound diffusion zones on both sides and suppresses Kirkendall voids at the interface. We apply it as part of the process.
What is the shear strength of a diffusion bonded copper–steel joint?
Published work on copper to 410 stainless steel with a nickel foil reports a maximum of about 145 MPa, failing in the copper. Treat 100–150 MPa as the reference class; the exact value for your steel and copper is confirmed by a shear coupon from your production run.
What steel should I use for a copper–steel bonded fixture plate?
A plain non-heat-treated carbon steel such as 1045 or S355. All steels have about the same stiffness, and the bonding cycle would anneal a hardened grade anyway. Stainless is only worth its lower conductivity if corrosion requires it.
How thick should the copper layer be on a welding fixture?
2–3 mm finished is typical for a chill or backing face on thick steel. If the plate will be milled flat after bonding, start with about 4 mm copper stock and finish to 2–3 mm.
Can a copper–steel bonded plate survive 600 °C?
The bond is formed at around 900 °C and is stable at 600 °C. The copper face oxidises and softens in air at that temperature, so plates are usually run as a rotated set, optionally with a nickel-plated face, and re-skimmed when needed.
How flat is a diffusion bonded plate, and can it be machined afterwards?
The plate bows slightly on cooling because copper contracts more than steel. It is fully machinable afterwards: we mill the copper face flat and machine holes and features to your CAD, treating the bonded blank as one solid part.
How much does copper-to-steel diffusion bonding cost?
Price is driven by plate area, material, number of press cycles, machining and verification. Send a sketch with size, thicknesses, quantity and service temperature and we return a ballpark within 48 hours.
