Understanding the Main Types of Copper for Casting

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  Pure Copper: Conductivity With a Low Yield Strength
• 2
  Brass: The Easiest Copper Alloy to Cast and Machine
• 3
  Bronze: Wear and Corrosion Without the Zinc Problem
• 4
  Copper-Nickel: Built for Moving Seawater
• 5
  Copper-Beryllium and Specialty Alloys: Strength Where Conductivity Still Matters
• 6
  How Casting Method Changes the Alloy Choice
• 7
  Six Types of Copper for Casting at a Glance

Copper casting usually starts with electrolytic tough pitch copper, C101 or C110. It runs about 100% IACS conductivity when the oxygen content is kept under control, which is why busbars, contact pads and induction coils are still cast rather than welded from bar. The catch is mechanical. Annealed C110 sits near 70 MPa yield, so the alloy carries current well and load badly.

Oxygen is the reason. Tough pitch copper holds roughly 0.02–0.04% oxygen as Cu2O particles. That is fine for conductivity and fine for machining, but heat it in a reducing atmosphere and the oxygen reacts with hydrogen. The result is steam inside the grain boundaries, a defect usually described as gassing. Wall sections above 12 mm thick are where this shows up most in a foundry.

For castings that must be welded or brazed after machining, oxygen-free C10200 is the safer choice. Conductivity stays in the same range, and there is no hydrogen embrittlement risk. It costs more per kilogram and is harder to source in small heats, so it rarely makes sense for a bracket that only needs to carry current.

One more boundary. Pure copper is sticky. It work-hardens fast at the cutting edge, so a cast blank that needs ±0.005 mm on a bore should be machined with sharp carbide, high rake and generous coolant. Leave 0.3–0.5 mm for the finishing pass and expect to change inserts more often than on brass.

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  Best forBusbars, contact pads, induction coils, RF shielding parts
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  Watch outWall sections over 12 mm in reducing atmospheres
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  Machining noteSticky chip, fast edge wear; leave 0.3–0.5 mm for finishing

Brass is copper with zinc, and the zinc content decides everything. Below about 35% zinc the alloy is single phase, alpha. It is ductile, casts cleanly and machines with short chips. Above that, a harder beta phase appears, and the alloy gets stronger and less ductile. C27400 and C28000 sit in the middle of that range and are the workhorses for plumbing bodies and valve housings.

C36000 is the free-machining grade, with lead added to break the chip. It machines faster than any other copper alloy, which is why it dominates fittings and small turned parts. Two limits matter. Leaded brass is restricted in potable water systems under most current regulations, and it is not the right base for parts that see sustained seawater.

Zinc also brings a casting risk. Zinc boils at 907 °C, so if the melt overheats the zinc leaves as vapor, the chemistry drifts and the surface finish goes rough. Foundries hold pouring temperature close to the minimum that still fills the mold. For sand casting, that usually means roughly 980–1,050 °C depending on section thickness.

Machinability is where brass pays for itself. Chips break, tool life is long, and surfaces come off the cutter smooth. If a part is mostly features and small volume, the lower melting point and easy machining often beat the cheaper cost per kilogram of a bronze.

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  Alpha brassUnder 35% zinc; ductile, good for cold work after casting
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  Alpha-beta brassHigher strength, lower ductility, harder to finish
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  Leaded brassFastest machining, restricted in potable water
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  Melt controlKeep pour temperature low or zinc vapor changes the alloy

Bronze covers a wide field. The classic tin bronzes run from about 5% to 12% tin, and above roughly 8% tin the alloy can form hard, brittle intermetallic phases if cooling is slow. Those phases are excellent for wear resistance in bearings and bushings, and disastrous for a part that has to take a bend or a shock load. Cooling rate is a process decision, not just an alloy decision.

The engineering bronzes add aluminium, silicon or manganese instead of tin. Aluminium bronze is the strongest common casting bronze, with yield strength often three to four times that of a tin bronze, plus good resistance to seawater and dilute acids. Silicon bronze is the easier one to cast and weld, with lower strength and better ductility.

Aluminium bronze has its own trap. A thin aluminium oxide skin forms almost instantly on the melt, and if that film folds into the casting you get a cold shut that looks like a crack after machining. Gating and turbulent fill control matter more here than on most other copper alloys.

Where does bronze stop making sense? High-volume parts with moderate loads and no corrosion demand. At that point a brass or a die-cast aluminium part will be cheaper. Bronze earns its price when the part runs against another metal, sees seawater, or has to survive a hundred thousand load cycles.

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  Tin bronzeBearings, bushings, gears, pump wear rings
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  Aluminium bronzeSeawater valves, propellers, high-load sleeves
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  Silicon bronzeCast-and-weld assemblies, architectural hardware

Copper-nickel castings are the 90/10 and 70/30 families, sometimes with iron and manganese added. The 90/10 grade is the standard choice for seawater piping, valve bodies and heat exchanger water boxes. The 70/30 grade costs more and resists higher flow velocities, which matters in condenser tubes and pump casings where erosion, not general corrosion, is the failure mode.

The mechanism is a protective film. In clean seawater, copper-nickel forms a thin, adherent oxide layer that slows further attack. That film needs a few weeks of exposure to stabilize, and it does not form in stagnant, sulfide-rich or polluted water. If a system sits idle for months, copper-nickel can pit worse than a simpler alloy.

Casting copper-nickel is not a beginner's job. The alloys absorb gas easily, they need clean, well-degassed melts, and they are sensitive to hot cracking in heavy sections. Yield is lower than brass, and scrap cannot simply be remelted without chemistry checks.

Machining is another factor. Copper-nickel work-hardens quickly and cuts with a long, stringy chip. Surface speeds around 60–90 m/min with sharp positive-rake carbide and constant feed keep the tool in the cut instead of rubbing. A tool that dwells will harden the surface and shorten insert life.

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  90/10 CuNiPiping, valve bodies, water boxes; moderate velocity
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  70/30 CuNiCondensers, pump casings; higher velocity and temperature
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  Weak spotStagnant or sulfide-rich water breaks down the protective film

Copper-beryllium, C17200 and C17500, is the answer when a part has to carry current and take a spring load at the same time. After age hardening, C17200 reaches roughly 1,100–1,300 MPa tensile strength while holding 22–28% IACS. No other copper alloy covers that combination. It shows up in switch contacts, connector springs, mould inserts and aerospace bushings.

The trade is process and safety. Beryllium-bearing dust is a health hazard, so machining, grinding and polishing need controlled coolant and dust collection. Foundries that pour it keep the melt covered and handle scrap under a written procedure. Beyond that, beryllium copper is expensive, often several times the cost of a brass, so it belongs only where conductivity and spring strength are both hard requirements.

Specialty alloys cover the rest of the field. Chromium copper, C18200, keeps about 80% IACS with better softening resistance than pure copper, which suits resistance welding electrodes. Tellurium copper, C14500, machines far better than C110 with only a modest conductivity loss, so it is common in turned electrical parts.

The honest boundary: if the part needs conductivity above 80% IACS with no strength requirement, buy pure copper. If it needs strength with no conductivity requirement, buy a bronze. Copper-beryllium and chromium copper only make sense in the overlap.

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  C172001,100–1,300 MPa tensile, 22–28% IACS after aging
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  C18200About 80% IACS, resists softening at weld temperatures
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  C14500Free-machining with lower conductivity than C110
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  SafetyBeryllium dust needs coolant control and dust collection

The alloy and the process are one decision, not two. Sand casting handles nearly every copper alloy and is the only practical route for parts over roughly 200 kg, but cooling is slow. Slow cooling promotes the brittle phases in high-tin bronze and coarse grain in copper-nickel, so a sand-cast part often needs a different alloy than the same part in investment.

Investment casting gives thin walls and fine detail, commonly down to 1.5–2 mm, with much faster cooling. That is why investment is the default for copper-beryllium springs and small aluminium bronze valve parts. The wax pattern cost is real, but it disappears on runs above a few hundred pieces.

Die casting and permanent mould work for brass and some bronzes, driven by the higher melting point of copper. Tooling life is shorter than in aluminium die casting, and the alloys used are restricted to those that flow well and do not attack the die. For high volume brass fittings this is still the cheapest route per part.

Centrifugal casting is a special case worth knowing. Spinning the mould throws denser metal outward and pushes gas and inclusions to the bore, which is then machined away. For bearing sleeves and pump rings, that gives a denser wear surface than static casting at similar cost.

After casting, most copper parts still go to the machine shop. Cast tolerances rarely beat ±0.5 mm, and critical bores, faces and threads need CNC work. Casting and machining should be quoted together, because the machining allowance and the parting line location decide how much material the cutter has to remove.

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  SandLarge parts, most alloys, slow cooling, coarse structure
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  InvestmentThin walls, fine detail, fast cooling, higher per-piece tooling
• 3
  Die and permanent mouldHigh-volume brass, restricted alloy list
• 4
  CentrifugalSleeves and rings, dense outer surface, gas pushed inward