What Is the Melting Point of Bronze？

1. Introduction

The melting point of bronze is a key concept in metallurgy, manufacturing, and design.

Unlike pure metals, bronze is an alloy — primarily of copper and tin, though many modern bronzes include aluminum, silicon, nickel, or phosphorus.

As a result, bronze does not melt sharply at one temperature but instead passes through a mushy zone between solidus (start of melting) and liquidus (completely molten).

This distinction is critical for foundry engineers, welders, and materials designers who rely on precise temperature control to ensure sound, defect-free components.

2. What Is Bronze?

Bronze is a copper-based alloy in which copper (Cu) is the principal component and tin (Sn) is traditionally the primary alloying element.

Unlike pure metals, bronze is an engineered material—its mechanical, thermal, and chemical properties can be tailored by adjusting composition and processing.

Modern bronzes may also contain aluminum, silicon, phosphorus, nickel, zinc, or lead to achieve specific performance characteristics.

Historical Perspective

Bronze is one of the earliest alloys developed by humans, dating back to the Bronze Age (circa 3300 BCE).

The introduction of tin into copper created a harder, more durable material than pure copper, enabling advancements in tools, weapons, art, and architecture.

Today, bronze remains essential in both traditional artistic applications (sculpture, bells) and advanced engineering (aerospace, marine, and energy systems).

Classification of Bronze Alloys

Bronze is not a single alloy but a family of copper alloys categorized by their secondary elements:

• Tin Bronzes – Cu–Sn alloys (typically 5–20% Sn), valued for strength, wear resistance, and bearing properties.
• Phosphor Bronzes – Tin bronzes with small additions of phosphorus (0.01–0.5%), improving fatigue resistance and corrosion resistance.
• Aluminum Bronzes – Cu–Al alloys (5–12% Al, often with Fe or Ni), offering excellent strength and marine corrosion resistance.
• Silicon Bronzes – Cu–Si alloys (2–4% Si), combining corrosion resistance with good castability and weldability.
• Leaded Bronzes – Cu–Sn–Pb alloys, where lead improves machinability and bearing properties.
• Nickel-Aluminum Bronzes – Cu–Al–Ni alloys with superior seawater resistance, often used in shipbuilding.

Key Properties of Bronze

• Mechanical: Higher strength and hardness than copper, with good wear resistance.
• Thermal: High thermal conductivity, but lower than pure copper due to alloying.
• Chemical: Excellent corrosion resistance, especially against seawater, making bronze indispensable in marine and chemical industries.
• Acoustic: Distinct resonance properties, used in musical instruments, bells, and gongs.

3. Melting Behavior of Alloys — Solidus and Liquidus

For alloys, melting occurs across a temperature interval:

• Solidus Temperature: The lowest temperature at which melting begins.
• Liquidus Temperature: The temperature at which the alloy becomes fully liquid.
• Freezing Range (Mushy Zone): The interval between solidus and liquidus where both solid and liquid coexist.

4. Typical Melting Ranges by Bronze Family

Because bronze is not a single alloy but a family of copper-based alloys, its melting behavior varies widely depending on alloying elements and their proportions.

Instead of a sharp melting point (as seen in pure metals), bronze exhibits a melting range, defined by the solidus (where melting begins) and the liquidus (where it becomes fully molten).

The table below summarizes typical melting ranges for major bronze families:

5. How Composition and Alloying Elements Influence Melting Range

The melting range of bronze is fundamentally controlled by its chemical composition.

Pure copper melts at 1,085 °C (1,985 °F), but when alloying elements such as tin, aluminum, silicon, phosphorus, nickel, or lead are introduced, the melting behavior shifts significantly.

These elements either lower or raise the solidus and liquidus temperatures depending on their interaction with copper.

Effect of Major Alloying Elements

Alloying Interactions and Microstructural Effects

• Eutectic Formation (Cu–Sn, Cu–Pb): Lowers melting point significantly, resulting in broader melting ranges.
• Intermetallic Compounds (Cu–Al, Cu–Ni): Increase melting temperatures and create stronger, more stable alloys.
• Solid Solution Strengthening (Cu–Si, Cu–Ni): Retains relatively high melting range while improving ductility and corrosion resistance.

6. Microstructure and Processing Effects

While chemical composition is the dominant factor in determining bronze’s melting behavior, microstructural state and processing history also play a subtle yet important role.

These factors influence how uniformly the alloy transitions from solid to liquid and can shift effective solidus or liquidus points by tens of degrees.

Microstructural State: Grain Size and Phase Distribution

• Grain Size: Fine-grained bronze (grain diameter <10 μm) generally exhibits a solidus temperature ~5–10 °C lower than coarse-grained bronze (>50 μm).
  This is because fine grains introduce more grain boundary area, where atomic diffusion accelerates local melting.
• Phase Segregation: In multiphase alloys (e.g., α+β bronze such as C61400), non-uniform phase distribution creates localized melting behavior.
  β-phase regions may begin melting at ~1,050 °C, while α-phase regions persist until ~1,130 °C. This widens the effective melting range by 10–20 °C.
• Practical Example: Cold-worked phosphor bronze (C52100) typically develops finer grains than its as-cast counterpart.
  During annealing, cold-worked C52100 shows a solidus near 930 °C, compared to ~950 °C for cast material—requiring tighter temperature control to avoid incipient melting.

Processing History: Thermal Cycles and Alloy Degradation

• Tin Vaporization (Welding/Casting): Prolonged exposure above ~1,100 °C can gradually vaporize tin, despite its high boiling point (2,270 °C).
  For instance, heating C92200 bronze (10% Sn) at 1,200 °C for one hour can reduce Sn content by 1–2%, shifting its liquidus upward from ~1,020 °C to ~1,030 °C.
• Heat Treatment (Annealing/Homogenization): Annealing bronze at 600–800 °C (below solidus) promotes diffusion and reduces microsegregation.
  This narrows the melting interval by 5–15 °C. For example, C92700 (15% Sn) annealed at 700 °C shows a melting range of 880–1,030 °C, compared to 880–1,050 °C in the as-cast state.
• Casting Rate: Rapid solidification (e.g., chill casting) produces finer dendrites and more uniform phase distribution, reducing the likelihood of premature local melting.
  Slow cooling enhances segregation, widening the melting interval.

7. Industrial Manufacturing Implications of the Melting Point of Bronze

Precise control of bronze’s melting range is non-negotiable in manufacturing.

Even a 10 °C deviation from the target processing temperature can reduce yield by half, either through incomplete mold filling, vaporization of alloying elements, or microstructural damage.

The three most sensitive operations—casting, welding, and heat treatment—rely heavily on accurate knowledge of the solidus–liquidus window.

Casting: Balancing Fluidity and Alloy Integrity

In casting, bronze must be heated above its liquidus by 50–100 °C to achieve sufficient fluidity for mold filling, while avoiding excessive overheating that accelerates oxidation (dross formation) or vaporization of volatile alloying elements such as lead and tin.

Critical Control: For leaded bronze C90700, pouring below 950 °C results in misruns (unfilled cavities), while above 1,050 °C lead vaporization exceeds 1%, degrading machinability and producing gas porosity.

Welding: Avoiding Melting and Alloy Degradation

Bronze welding requires temperatures below the liquidus to prevent base metal melting, using filler metals with lower melting ranges than the base alloy.

• TIG Welding (Marine Propellers): Use C92200 base metal (10% Sn, 920–1020°C melting range) with a C93200 filler (5% Sn, 880–980°C melting range).
  Preheat to 200–300°C and maintain weld pool temperature at 900–950°C (between filler liquidus and base solidus) to avoid fusion defects.
• Brazing (Electrical Connectors): Use a copper-phosphorus filler (Cu-5% P, melting at 714–800°C) with C51000 phosphor bronze (970–1070°C melting range).
  Heat to 750–800°C—filler melts while base metal remains solid, preventing distortion.

Failure Mode: Overheating C92200 during TIG welding (temperature >1020°C) causes tin vaporization (2% Sn loss), reducing tensile strength by 25% and increasing corrosion susceptibility in seawater.

Heat Treatment: Strengthening Without Melting

Heat treatment temperatures are strictly limited to below the solidus to avoid partial melting and microstructural damage:

• Solution Annealing (Aluminum Bronze): C63000 (15% Al, 1080–1200°C melting range) is annealed at 800–900°C to dissolve β-phase into α-phase, improving ductility (elongation increases from 10% to 30%).
• Aging (Phosphor Bronze): C52100 (0.3% P) is aged at 400–500°C (well below its 930°C solidus) to precipitate Cu₃P, increasing tensile strength from 450 MPa to 550 MPa.

8. Testing Methods for Bronze’s Melting Range

Accurate measurement of bronze’s melting range requires laboratory or industrial techniques tailored to precision and sample size.

Differential Scanning Calorimetry (DSC)

• Principle: Measures heat flow into/out of a 5–10 mg bronze sample as it is heated at 10°C/min.
  The solidus is detected as the start of endothermic heat absorption; the liquidus is the end of the endotherm.
• Precision: ±1–2°C for solidus/liquidus; ideal for characterizing new bronze alloys (e.g., low-lead grades for drinking water fixtures) to verify compliance with ASTM B505.
• Example: DSC analysis of C61400 (10% Al) confirms a solidus of 1050°C and liquidus of 1130°C—critical for setting die casting temperatures.

High-Temperature Melting Apparatus

• Principle: A 1–5 g bronze sample is heated in a graphite crucible with a thermocouple inserted directly into the sample.
  The solidus is the temperature when the first liquid forms; the liquidus is when the sample is fully molten.
• Precision: ±5–10°C; suitable for industrial quality control (e.g., verifying batch consistency of leaded bronze for bearings).
• Advantage: Simulates real casting conditions, accounting for impurity effects that DSC may miss.

Thermal Gravimetric Analysis (TGA)

• Principle: Measures mass loss of a bronze sample during heating.
  Tin or lead vaporization causes mass loss above their boiling points, but the onset of melting is indicated by a subtle mass change (due to surface oxidation) coinciding with the solidus.
• Precision: ±3–5°C for solidus; often used with DSC to cross-validate melting range data.
• Application: Studying tin vaporization in high-tin bronze (C92700) to optimize casting hold times (minimizing Sn loss to <0.5%).

9. Common Misconceptions About Bronze’s Melting Point

Despite its industrial importance, bronze’s melting behavior is often misunderstood. Below are key clarifications:

“Bronze has a fixed melting point like pure copper.”

False: Pure copper melts at 1083°C (fixed), but bronze—an alloy—has a melting range.

For example, C92200 tin bronze melts between 920°C and 1020°C, not at a single temperature.

“Adding more tin always lowers bronze’s melting range.”

Partially True: Tin content up to 15% lowers the melting range (from 1083°C for pure Cu to 880–1050°C for 15% Sn), but above 15% Sn, brittle δ-phase (Cu₃Sn) forms, widening the melting range and slightly raising the liquidus.

“Lead is always beneficial for lowering bronze’s melting range.”

False: Lead lowers the melting range but causes hot shortness (brittleness at high temps) if >5% Pb.

High-lead bronze (C90700, 5% Pb) cannot be used in high-heat applications (e.g., furnace parts) due to cracking risk.

“All bronzes are weldable if heated to their melting range.”

False: Welding bronze above its liquidus causes base metal melting and alloying element loss (tin vaporization).

Bronze requires filler metals with lower melting ranges than the base alloy to avoid fusion defects.

10. Quality, Defects, and Mitigation

The melting behavior of bronze is a critical determinant of product quality.

Even small deviations from its defined solidus–liquidus window can trigger metallurgical defects that compromise mechanical performance, corrosion resistance, and dimensional stability.

Common Defects Related to Melting Range

Segregation and Microstructural Inhomogeneity

• Cause: Slow cooling or wide melting ranges (e.g., high-Sn bronzes) lead to segregation of tin or lead at grain boundaries.
• Impact: Reduced toughness, intergranular corrosion susceptibility.
• Example: In C92700 (15% Sn), excessive β-phase segregation lowers impact resistance by ~30%.

Gas Porosity and Shrinkage Cavities

• Cause: Pouring above recommended superheat (> liquidus + 100 °C) increases oxidation and gas absorption.
• Impact: Porosity reduces fatigue life by up to 40%.
• Example: Leaded bronze C90700 develops voids if poured >1,080 °C due to lead vaporization.

Hot Cracking (Solidification Cracking)

• Cause: Narrow solidification ranges in some alloys (e.g., Cu–Al bronzes) make them prone to thermal stresses during cooling.
• Impact: Cracks initiate at grain boundaries, compromising structural integrity.

Overheating and Alloying Element Loss

• Cause: Extended exposure >1,100 °C causes tin vaporization (~1–2% per hour) and lead loss in leaded bronzes.
• Impact: Lower strength, poor machinability, and increased brittleness.

Key Takeaway:

Most quality failures in bronze manufacturing arise not from alloy selection but from improper temperature control during melting and pouring.

By combining strict thermal management, alloy optimization, and advanced inspection techniques, defect rates can be reduced by more than 70%.

11. Future Trends: Low-Lead and Additive Manufacturing

Bronze technology is evolving to meet environmental regulations and advanced manufacturing needs, with melting range considerations at the forefront:

Low-Lead and Lead-Free Bronze

• Driver: Environmental regulations (e.g., California Proposition 65, EU RoHS) limiting lead in drinking water fixtures and food-contact surfaces.
• Melting Range Challenge: Replacing lead with bismuth (Bi) or silicon (Si) requires reoptimizing melting ranges—bismuth lowers the liquidus by ~10°C per 1% Bi, but excess Bi causes brittleness.
• Solution: C90800 (Cu-10% Sn-2% Bi) has a 920–1000°C melting range, matching leaded bronze’s castability while meeting lead-free standards.

Additive Manufacturing (3D Printing)

• Driver: Complex geometries (e.g., custom bearings) that traditional casting cannot achieve.
• Melting Range Challenge: Powder bed fusion (PBF) requires precise control of laser temperature (above the liquidus for full melting, below for sintering).
• Solution: For C52100 phosphor bronze PBF, use a laser temperature of 1050–1100°C (liquidus + 20–70°C) to ensure layer bonding without tin vaporization.

12. Conclusion

The melting point of bronze is best understood as a melting range defined by solidus and liquidus temperatures.

This range is influenced by alloy composition, microstructure, and impurities, and directly governs how bronze is cast, welded, and heat-treated.

Careful control of melting and pouring temperatures ensures defect-free components, extends service life, and reduces costs.

By integrating phase diagram knowledge with practical foundry experience, engineers and manufacturers can fully exploit the versatility of bronze while minimizing risks in production.

FAQs

What is the melting range of bronze used in marine propellers?

Marine propellers typically use C92200 naval tin bronze (10% Sn) or C61400 medium-aluminum bronze (10% Al).

C92200 melts at 920–1020°C, while C61400 melts at 1050–1130°C. Aluminum bronze is preferred for larger propellers due to its higher strength at high temperatures.

How does lead content affect the melting range of bronze?

Lead acts as a melting point depressant—each 1% increase in lead lowers the liquidus by ~15°C.

For example, C90300 (2% Pb) has a liquidus of 1030°C, while C90700 (5% Pb) has a liquidus of 980°C.

However, lead >5% causes hot shortness, making the bronze brittle at high temperatures.

Can I weld bronze with the same temperature as steel?

No. Steel (e.g., A36) melts at 1425–1538°C, far higher than bronze.

Welding C92200 tin bronze requires a maximum temperature of 950°C (below its 1020°C liquidus) to avoid tin vaporization and base metal melting.

Using steel welding temperatures would destroy the bronze.

How do I measure the melting range of bronze in a foundry?

Use a high-temperature melting apparatus with a graphite crucible and K-type thermocouple.

Heat a 5 g bronze sample at 5°C/min, recording the temperature when the first liquid forms (solidus) and when the sample is fully molten (liquidus).

This method has ±5–10°C precision, sufficient for batch quality control.

Why does aluminum bronze have a higher melting range than tin bronze?

Aluminum forms high-melting intermetallic compounds (e.g., Cu₃Al, melting at 1037°C) with copper, which raise the solidus and liquidus.

Tin, by contrast, forms a more ductile solid solution with copper, disrupting atomic bonds and lowering the melting range. For example, 10% Al in bronze raises the liquidus by ~100°C vs. 10% Sn.