What is CUPAL? Copper-aluminium bimetal

CUPAL definition {#cupal-definition}

CUPAL is a cold-roll-bonded copper-aluminum bimetal composite material: 70% aluminum (AL 99.5 / Werkstoff-Nr. 3.0255) and 30% copper (E1-Cu58 / Werkstoff-Nr. 2.0065), diffusion-welded into one inseparable layer. Not an alloy — each metal layer retains its own physical and chemical properties. Density 4.6 g/cm³, electrical conductivity 41.9 m/(Ω·mm²), bond strength 70–110 N/cm band-width. CUPAL eliminates galvanic corrosion at bolted joints where aluminum and copper conductors meet.

The name CUPAL comes from CU (copper) and AL (aluminum). The material is supplied through our German source partner, WMH Westdeutscher Metall-Handel GmbH (ISO 9001:2015 certified). The two metal layers are inseparably cold-roll-bonded and diffusion-welded, yet each retains its own physical and chemical properties.

Why it matters

When copper and aluminum conductors are joined directly, the electrochemical potential difference between the two metals causes galvanic corrosion. This corrosion loosens the joint over time, raises contact resistance, can cause temperature rise, and in the extreme can become a fire risk.

CUPAL solves this by placing the dissimilar-metal interface inside the composite, where it is protected from environmental exposure. Each outer face contacts its own metal — copper face to copper conductor, aluminum face to aluminum conductor.

Composition and structure

CUPAL consists of 70% aluminum (carrier layer) and 30% copper (clad layer). The aluminum component is AL 99.5 (Werkstoff-Nr. 3.0255, Alloy 1050A); the copper component is E1-Cu58 (Werkstoff-Nr. 2.0065, Cu 99.9% purity).

The two metal layers cannot be separated — the bond strength achieved during manufacturing is 70–110 N/cm band-width, an exceptionally strong, permanent connection.

Manufacturing process: cold-roll-bonding

CUPAL production proceeds in multiple stages:

Surface preparation: both the aluminum and copper surfaces are conditioned for optimal adhesion.
Cold-roll-bonding (Kaltwalzplattieren): the prepared sheets pass through a rolling stand under extreme pressure, bonding at room temperature. This cold process distinguishes CUPAL from hot-rolled bimetal products.
Stress relief: high-pressure rolling causes significant work-hardening in both metals; controlled heat treatment restores the desired material properties and formability.
Diffusion welding: during heat treatment, a diffusion-welding process occurs at the Al–Cu interface, substantially improving bond strength.
Controlled intermetallic layer: annealing time and temperature are precisely limited to prevent uncontrolled formation of an alloy layer (Legierungsschichtbildung). The resulting intermetallic layer is only sub-microscopic in thickness, preventing layer separation during cold forming.

Technical properties

Property	Value	Unit
Density	4.6	g/cm³
Electrical conductivity	41.9	m/(Ohm·mm²)
Specific electrical resistance	0.0239	Ohm·mm²/m
Cross-section factor vs. Cu	1.41	(multiplier)
Cross-section factor vs. Al	0.906	(multiplier)
Thermal conductivity	265	W/(m·K)
Linear thermal expansion	21.8	10⁻⁶/K
Modulus of elasticity	81	kN/mm²
Tensile strength	130–180	N/mm²
Bond strength	70–110	N/cm (band-width)

Surface hardness

Side	Vickers	Brinell
Aluminum side	30–40 HV 0.2	30–40 HB 2.5/62.5
Copper side	50–70 HV 0.2	50–70 HB 2.5/62.5

Key benefits

Weight savings

At 4.6 g/cm³, CUPAL is approximately 49% lighter than pure copper (8.96 g/cm³). This is a significant advantage wherever weight matters — switchgear, EV fast-charger Cu/Al transitions, overhead-line applications.

Galvanic corrosion prevention

CUPAL solves the galvanic-corrosion problem of direct copper–aluminum contact. The dissimilar-metal interface lives inside the composite, protected from atmospheric exposure.

Excellent conductivity

At 41.9 m/(Ohm·mm²), CUPAL needs only a 1.41× cross-section relative to pure copper to deliver equivalent ampacity. At high frequencies the difference shrinks further due to skin effect.

Broadly machinable

CUPAL accepts almost every cold-forming process: bending, deep drawing, riveting, stamping, profiling. The copper side is soft-solderable and accepts standard galvanic coatings.

Application areas

Switchgear and distribution boards: washers and transition plates between Al busbars and Cu cable lugs.
Cable lugs and connectors: bimetallic Al/Cu cable lugs and CUPAL washers M6–M20.
Lightning protection: CUPAL strips for transitions between dissimilar metals.
Renewable energy: Cu/Al transition points in PV inverters, wind-turbine generator cabinets, BESS systems.
EV infrastructure: Cu/Al transition elements in EV fast-charger cabinets (not cell-level use).
High-frequency engineering: components in capacitors and transmitter/receiver assemblies.
Machinery and equipment manufacturing: enclosures, vessels, piping.

Available product forms

CUPAL sheet is available in 0.5, 1.0, 1.5, and 2.0 mm thicknesses, in 500 × 500 mm and 500 × 2000 mm sizes. Pre-cut washers (M6–M20), strips (500 × 40 mm) and custom shapes (discs, strips, rectangles, arbitrary contours) are also available.

The same copper-aluminum pairing is also supplied as finished pressed parts: Al/Cu cable lugs — tubular eye-lugs in 10–625 mm² with M8–M20 bolt holes, Al/Cu butt-splices and pin lugs — and bimetallic washers in M6–M20. That is 109 lug sizes and 11 washer sizes, all from stock. Which of them a given joint needs — a crimped lug, a butt-splice, a washer or a cut plate — is worked through joint by joint in connecting copper to aluminium.

Anything those two size ladders do not cover — an odd bore, a larger outside diameter, a busbar transition plate — is cut from sheet to your drawing. Send DXF, PDF or JPG: we quote within 24 hours, the minimum order is one 500 × 500 mm board, and production takes 5–10 working days.

CUPAL material is supplied by our German partner, WMH Westdeutscher Metall-Handel GmbH — ISO 9001:2015 certified supplier quality management, consistent material quality across shipments.

Further questions? Request a quote or write to us via chat — we respond within minutes on business days.

Frequently asked questions about CUPAL

No. CUPAL is a bimetal composite, not an alloy. The copper and aluminum layers are cold-roll-bonded and diffusion-welded together, yet each metal retains its own physical and chemical properties.

CUPAL is 70% aluminum (the carrier layer, AL 99.5 / 3.0255) and 30% copper (the clad layer, E1-Cu58 / 2.0065). The aluminum forms the body while the copper provides a conductive, solderable face.

CUPAL has an electrical conductivity of 41.9 m/(Ω·mm²). To match the ampacity of pure copper it needs only a 1.41× cross-section, and at high frequencies the gap narrows further due to skin effect.

At a density of 4.6 g/cm³, CUPAL is roughly 49% lighter than pure copper (8.96 g/cm³). This makes it advantageous wherever weight matters, such as switchgear or overhead-line applications.

CUPAL moves the dissimilar-metal interface inside the composite, where it is shielded from atmospheric exposure. The copper face contacts the copper conductor and the aluminum face contacts the aluminum conductor, so no galvanic corrosion forms at the bolted joint.

CUPAL sheet comes in 0.5, 1.0, 1.5, and 2.0 mm thicknesses, in 500 × 500 mm and 500 × 2000 mm sizes. Pre-cut washers (M6–M20), strips (500 × 40 mm), and custom shapes are also available.

It is the trade name for a connector with a different metal on each face, so that in a bolted joint aluminum meets aluminum and copper meets copper. In CUPAL that means Al/Cu cable lugs in 10–625 mm² with M8–M20 bolt holes, washers in M6–M20, and plates or discs cut to your drawing.

The vocabulary around CUPAL — bimetal, galvanic corrosion, Werkstoff number, EN 10204 material certificate — is collected with one-line definitions in the CUPAL glossary. If you already know the size you need, the washer size guide and the CUPAL sheet page will help you choose.

Several names describe copper-aluminium bimetal, but they are not all interchangeable, and a buyer working from a drawing needs the distinction.

The same idea, made a different way: Cuponal. Cuponal is the brand of Hydrostatic Extrusions Ltd. of Perth, Scotland (part of the Bruker Corporation); VDM Engineered Solutions is its exclusive sales partner for the German-speaking countries, the Benelux region, France and Taiwan. It is produced by hydrostatic extrusion rather than roll bonding, and it is supplied as busbars, bars, profiles and wire — long products. Cuponal is not available as sheet or as washers. So if a specification calls for a Cuponal busbar profile, that is a different supply chain from ours; if it calls for a copper-aluminium transition in sheet, disc, strip or washer form, that is what we cut. CoppAl is another busbar brand in the same long-product family.

Genuinely interchangeable descriptions: copper-clad aluminium, Cu-Al bimetal, cupro aluminium and bimetallic copper-aluminium all describe the composite in general terms. If a drawing or bill of materials uses one of those for a sheet, disc, strip or washer, send us the spec and we will confirm the match.

The finished parts: bimetallic connectors. Once the material is formed into a part, catalogues name the part rather than the composite — bimetallic cable lug, bimetallic washer, transition plate between an aluminium busbar and a copper lug. Tell us the conductor cross-section, the bolt size and what it bolts to, and we will say which one you need.

Two different products share one name: transition washer. One is a profiled copper-alloy washer whose ridges pierce the aluminium oxide film under bolt pressure; the other — ours — is a roll-bonded copper-aluminium bimetal disc, where the two metals are joined permanently in the material and each face only ever touches its own metal. Both are sold for the same job, so check which mechanism a specification means before you order.

Not the same thing at all: CCA. In cable and wire catalogues, CCA means copper-clad aluminium wire, where a thin copper skin surrounds an aluminium conductor for cost reasons. That is a conductor material, not a transition element, and it does not solve a bolted copper-to-aluminium joint. The difference is set out in CUPAL vs. copper–aluminium transition methods.

What is CUPAL?