When copper is the wrong material for a part

Copper is the wrong material for a part when the requirement is structure, enclosure or wear rather than conduction. A copper bracket specified for corrosion resistance weighs 8.96 g/cm³ against 2.70 for aluminium, yields at 60–90 MPa in the annealed condition, and puts a cathodic metal against an anodic one at every fastener. The drawing passes review and the joint is where it fails.

Deciding when copper is the wrong material for a part is a cost question before it is a metallurgy question. Conductivity above 43% IACS, thermal conductivity above 200 W/m·K, a solderable surface, or a specific chemical duty will justify it. A cover, a frame, a spacer or a bracket will not.

What follows is the crossover in numbers, the failure modes that come with the material, and the alloy to put in its place for each job. The wider set of grade decisions sits in the materials and grades hub.

When copper is the wrong material for a part

Copper is the wrong material when the section can grow and the part does not carry current. Three questions settle most cases:

• Does the part need to carry more than 43% IACS through a section that cannot be enlarged?
• Does it need to move heat at more than 200 W/m·K, which is roughly the thermal conductivity of 6061 aluminium?
• Does it need to be soldered or brazed without a nickel or tin barrier?

If the answer to all three is no, copper is a cost with no matching function. That covers most brackets, covers, panels, frames and spacers that arrive at quotation described as copper for corrosion resistance. Copper does resist atmospheric attack, but 304 stainless resists it at 215 MPa proof stress, and a coated CRCA part resists it at a fraction of the metal cost.

There is a second, quieter case: copper specified because a prototype was made from it. A prototype busbar becomes a production bracket because nobody revisited the drawing. The drawing note should name the function, not the metal.

Conductivity per unit mass decides most copper calls

Copper is the reference conductor at 100% IACS, which is 58 MS/m, and C11000 electrolytic tough pitch copper delivers about 101% annealed. Aluminium 1350 delivers 61%, so an aluminium conductor needs 1.66 times the cross-section for the same resistance. Aluminium is 0.30 times the density of copper, so that larger bar still weighs about half as much.

That trade is the whole decision. Copper wins when the cross-section cannot grow: a terminal block, a busbar in a crowded panel, a 12 mm slot. Aluminium wins when there is room and mass or metal cost matters. Steel is not in the conversation as a conductor, which is why the mass index in the table runs to 9.8 for CRCA.

Read the mass index as the weight of metal needed to match one kilogram of copper at the same resistance. It is not a price index, because the price per kilogram differs between the metals, but it sets the floor: a steel conductor is never a cost saving, and an aluminium one is a mass saving that costs volume.

Thermal duty follows a similar ratio. Copper conducts heat at about 390 W/m·K, 6061 aluminium at 167 W/m·K and CRCA at roughly 50 W/m·K. A copper spreader needs about 2.3 times less section than an aluminium one for the same temperature rise, which is the other legitimate reason to pay for copper. It is also why a copper heat sink bolted to an aluminium enclosure is a thermal joint problem as well as a galvanic one.

Worked example: a busbar that has to fit a 12 mm slot

A 200 mm × 10 mm copper bar has a section of 2,000 mm². At a design current density of 1.6 A/mm² it carries 3,200 A. The aluminium bar of equal resistance needs 2,000 × 1.66 = 3,320 mm², which at the same 200 mm width is 16.6 mm thick.

The copper bar weighs 17.9 kg per metre. The aluminium bar weighs 8.97 kg per metre, half as much, and is 66% thicker. In a 12 mm slot the aluminium does not fit, so copper is the right answer and the reason is the slot, not the conductivity.

Move the slot to 25 mm and the answer reverses. The aluminium bar fits, halves the mass on a 2 m run from 35.8 kg to 17.9 kg, and removes the bimetallic joint at the terminal if the terminal is also aluminium. That is the decision rule in one line: copper for a fixed section, aluminium for a fixed mass or cost, and the slot dimension decides which constraint you are under.

The failure modes copper brings with it

Galvanic corrosion at the aluminium joint

The conductivity that makes copper useful also makes it noble. At a copper-to-aluminium interface the aluminium loses metal, and the loss concentrates at the fastener where the clearance is smallest. A joint that measures a few milliohms when built can read several times that after a humid season, and the heat at the terminal is the first symptom. The fix is at the design stage: a transition plate, a tin-plated contact face, or a joint that never sees moisture.

Creep and relaxation under load

Annealed copper yields at 60–90 MPa and creeps under sustained load. A copper spacer in a bolted stack is effectively a soft washer: it conforms, then relaxes, and the preload drops. Copper sealing washers work for exactly that reason. A bracket that depends on bolt preload to hold position should be steel, with a hardened washer under the head, and copper should be kept to the conductor.

Put a number on the strength gap. A 20 mm × 3 mm copper strap has a 60 mm² section and carries 5.4 kN at its 90 MPa upper yield. The same strap in CRCA at 210 MPa carries 12.6 kN, and does not creep at room temperature under that load. At 150 °C the copper relaxes faster still, so a copper clamp that holds preload on a hot component is the worst of both cases: soft, and soft at the temperature where it matters.

Differential expansion against a steel frame

Copper expands at about 17 µm/m·K and steel at about 12 µm/m·K. On a 1,000 mm panel bolted to a steel frame, a 60 K rise gives 1,000 × 5 × 10⁻⁶ × 60 = 0.30 mm of differential movement. Fixed at both ends, that movement goes into buckling or into bolt slip. Slot one end, or use a slotted hole with a Belleville washer, and the panel stays flat.

Coating and handling

Copper oxide forms between cleaning and coating, and the oxide is what the powder or paint actually bonds to unless the surface is prepared for copper rather than for steel. Copper also scratches visibly, and a bare copper part tarnishes in handling before it reaches the customer. If the part is coated, say whether the coating is decorative, whether the edges are coated, and whether the contact faces stay bare. The degreasing sequence for copper parts covers the chemistry difference.

Joining

Resistance spot welding copper is unreliable because the conductivity that carries the current also shunts the welding current away from the interface. Copper is brazed, soldered or mechanically fastened. A copper-to-copper structural weld is a specialist job, not a sheet metal station operation, and it should not appear on a drawing that expects a standard MIG route.

Brazing brings its own constraint. The braze temperature anneals the copper either side of the joint, so a bend placed 10 mm from a brazed lug is forming soft metal even if the drawing called for half-hard strip. Set the bend radius for annealed temper, or move the bend away from the heat.

The cheaper alternative for each job

The alternative is decided by the function, not by the metal.

• Structural bracket or frame: CRCA DC01 at 140–210 MPa proof stress, powder coated, or 304 where the environment demands it. Both are stiffer than copper and both weld on a standard line.
• Enclosure or panel: CRCA or 5052 aluminium. Copper belongs only on the earth strap or the EMI contact strip, not on the panel itself.
• Spring contact or clip: phosphor bronze C51000 at 450–600 MPa, or beryllium copper C17200 at about 1,000 MPa aged. A soft copper contact takes a set after a few deflections.
• Machined terminal or connector body: brass C26000 or C36000. Brass machines freely, takes a thread, and carries 28% IACS, which is enough for a terminal that is not the main conductor.
• Conductor with room to grow: 1350 aluminium, or 6101 busbar alloy for a formed bar. Link the joint with a transition plate.
• Conductor in a fixed slot: C11000 copper. This is the case copper was made for, and the section is the reason.
• Marine heat exchanger or salt-water contact: cupronickel 90/10, not ETP copper. ETP copper in seawater is a corrosion experiment.

Two of those substitutions are worth a number. Brass at 28% IACS and a mass index of 3.4 is a poor conductor and a good machined part. Phosphor bronze at 15% IACS and a mass index of 6.7 is a poor conductor and an excellent spring. Neither is a copper substitute for current; both are copper substitutes for shape and strength.

The pressed copper and brass components guide covers the die clearance and burr side for the grades in this table, and the laser cutting constraints for copper and brass covers what changes when the part is cut rather than pressed.

What Ganesh runs, and where copper stops

Copper sheet is cut on the 12 kW fibre laser with a 40 ft × 16 ft bed, alongside two 6 kW and two 3 kW machines, and formed on the 200 T, 5-axis brake with a 4,450 mm bed. The material range covers mild steel, CRCA, stainless, aluminium, brass and copper from 0.8 mm to 40 mm, with a house tolerance of ±0.5 mm on formed parts and tighter features agreed on the drawing.

Copper runs as its own cutting job rather than sharing a nest with steel, because mixed offcuts are priced as mixed scrap and because copper needs its own cutting parameters. Repeat parts run with first article inspection and PPAP where the customer asks for it, under ISO 9001:2015 and ISO 14001:2015.

What we do not do matters as much. Copper bar, machined terminals and thick copper weldments are outside the sheet metal route. A drawing that needs a machined copper block is a different supplier, and saying so at quotation is cheaper than discovering it at first article. The copper, brass and aluminium fabrication capability sets out the cut and form envelope.

What to put on the drawing

• Alloy and temper by UNS number, for example C11000 H04, not the word copper. Temper decides yield, springback and bend radius.
• Conductivity requirement in % IACS minimum, and whether it is measured on the finished part or accepted from the mill certificate.
• Thickness and its tolerance from ASTM B152 or the purchase order, because the mill band is wider than most forming tolerances assume.
• Joint detail: which faces are tinned or plated, the fastener torque, and whether the joint is isolated from steel or aluminium.
• Bend radii by temper, with the grain direction if the part is formed across the rolling direction.
• Coating and masking: whether the copper is coated, left bare, or masked at the contact faces.

When copper is not the wrong answer

Copper remains the correct choice for busbars in a fixed envelope, earth straps, RF shielding, heat sinks, soldered terminals, cupronickel marine parts and architectural surfaces where the patina is the point. The test is whether the property that copper is paid for appears in the function, not in the finish note.

If the drawing says copper and the function says bracket, change the material before the tooling is made. If the function says conductor, keep the copper and spend the effort on the joint. Send the drawing and the joint detail to our engineering team and we will confirm which case you are in before quoting.