Bismuth Bronze is a specialized copper-based alloy in which bismuth is added as a non-toxic alternative to lead.
It offers excellent machinability, strong corrosion resistance, high strength, and superior wear characteristics.
Because bismuth is environmentally safe and compliant with global regulations (such as RoHS), Bismuth Bronze is widely used as a lead-free alloy in industries requiring safe and reliable metal components.
Bismuth Bronze maintains the advantages of traditional bronze while improving safety and machinability, making it ideal for precision parts, bearings, valves, and components used in potable water systems, food processing, and electrical industries.
Composition & Alloy Variants
| Element | Typical Range | Role |
|---|---|---|
| Copper (Cu) | ~83-91% | Base metal; conductivity & corrosion resistance. |
| Tin (Sn) | ~3-7.5% | Increases strength and corrosion resistance. |
| Bismuth (Bi) | ~1-4% (can go up to ~6% in some grades) | Machinability enhancer, lead substitute. |
| Zinc (Zn) | ~2-10% optional | Improves strength & castability. |
| Nickel (Ni) | up to ~1% | Adds corrosion resistance. |
| Trace elements | Fe, P, Sb | Minor adjustments to mechanical behavior. |
Common Standard Alloys:
UNS C89835 – a widely used lead-free bismuth tin bronz
Microstructure & Metallurgical Behavior
Bismuth doesn’t dissolve in the copper matrix at normal bronze temperatures, so it exists as fine, insoluble particles along grain boundaries
These particles act as internal solid lubricants — they improve machinability similar to lead inclusions but without toxicity.
During casting, bismuth’s expansion on solidification helps offset shrinkage of the copper matrix, reducing porosity and improving cast integrity.
Mechanical & Physical Properties
| Property | Typical Range |
|---|---|
| Tensile Strength | ~100–270 MPa (varies with grade and temp.) |
| Yield Strength | ~97–140 MPa |
| Hardness (Brinell) | ~50–70 HB (or ~65 HB typical in C89835) |
| Elongation | ~6–30% depending on heat treatment |
Physical Properties
| Property | Typical |
|---|---|
| Density | ~8.4–8.9 g/cm³ |
| Melting Range | ~785–1012 °C (solidus to liquidus for C89835) |
| Electrical Conductivity | Lower than pure copper due to alloying |
| Thermal Conductivity | Moderate (e.g., ~65.8 W/m·K in C89835) |
Key Takeaway:
Compared to traditional leaded bronzes, bismuth bronze tends to be slightly softer but significantly more machinable and lead-free.
Strengthening & Metallurgical Behavior
Since bismuth isn’t soluble in copper:
Solid solution strengthening is mainly due to tin and other alloy additions (like Zn and Ni).
Dispersion strengthening occurs through fine Bi particles acting as obstacles to dislocation motion — improving machinability but not dramatically increasing strength.
Heat treatment (like annealing) can improve ductility and relieve casting stress, but gains in strength are limited due to the alloy’s nature.
Processing & Refinement
Casting
Bismuth bronze’s lower melting point (~830–1020°C) and good fluidity make it excellent for casting complex shapes.
Common techniques:
Sand casting
Permanent mold casting
Continuous casting
Machining
The presence of Bi enhances chip breaking and lubricity, resulting in free-machining behavior — reduced tool wear and cleaner odds.
Heat Treatment
Annealing improves ductility and reduces internal stresses but doesn’t drastically alter fundamental strength
Powder Metallurgy
Bismuth bronze powders can be produced (e.g., via atomization) for additive manufacturing and sintered components — maintaining machinability benefits.
Available Forms
Castings (complex shapes)
Bars & rods
Forgings
Powders (for additive and powder metallurgy)
Custom profiles based on design needs
Manufacturers often supply material coded to UNS standards (e.g., C89835, C89320) for engineering consistency.
Key Characteristics
| Feature | Benefit |
|---|---|
| Lead-free / non-toxic | Suitable for potable water systems, medical parts, and consumer goods. |
| Excellent machinability | Improves production efficiency and tool life. |
| Good corrosion resistance | Ideal for marine and wet environments. |
| Low melting point & great fluidity | Enables intricate castings. |
| Moderate strength & ductility | Balances machinability with mechanical performance. |
Applications
✔ Bearings & Bushings – free-machining plus low friction makes it ideal.
✔ Plumbing fittings & valves – especially where lead-free compliance is necessary.
✔ Fasteners & small precision parts – easy machining and corrosion resistance.
✔ Electrical & Hardware components – conductive and corrosion resistant.
✔ Art & sculptures – good casting qualities and finish
Advantages & “Why Choose Bismuth Bronze?”
Environmental & Health
Lead-free: sustainable and compliant for drinking water and food contact.
Manufacturing Efficiency
Easy casting at lower temps = lower energy costs.
Excellent machinability = reduced machining time and tool wear.
Performance
Solid corrosion resistance and wear properties suitable for many industrial uses.
Limitations & Considerations
Lower tensile strength compared to some high-strength bronzes, especially leaded ones.
Cost of bismuth (rare element) can make the alloy more expensive.
Not suitable for extremely high load structural components where very high strength is mandatory.
Bismuth bronze is a lead-free, machinable, corrosion-resistant copper alloy optimized for casting and precision machining.
It strikes a balance between mechanical performance and manufacturing efficiency, making it ideal for bearings, fittings, and parts where health/safety regulations or machining efficiency are priorities