Melting Point of Aluminum: From Pure Al to Alloys

1. Introduction

Aluminum ranks among the most versatile and abundant metals used today, underpinning industries from aerospace to consumer electronics.

Its combination of light weight, good conductivity, and corrosion resistance makes it indispensable.

To manufacture, recycle, or join aluminum effectively, engineers must know exactly when it transitions from solid to liquid.

In this article, we delve into aluminum’s melting point—its precise value, influencing factors, measurement techniques, and industrial implications.

By clarifying these details, we aim to equip materials scientists and production engineers with actionable insights for optimizing processes that rely on aluminum’s melting behavior.

2. What is the Melting Point?

In thermodynamics, the melting point marks the temperature at which a solid and its liquid phase coexist in equilibrium.

At this precise temperature, the solid absorbs enough heat to break the crystal lattice,

transforming into a liquid while maintaining constant temperature until melting completes.

Several factors influence the equilibrium temperature:

• Purity: Pure substances have sharp, well‐defined melting points. Even trace impurities can broaden the melting range and reduce the onset temperature.
• Pressure: As pressure rises, melting points typically increase according to the Clapeyron relation,
  which links changes in pressure and temperature at phase boundaries via the volume and entropy differences.
• Alloying: Mixing aluminum with elements like silicon or copper creates liquidus and solidus lines on the phase diagram.
  The liquidus represents the temperature above which the alloy is fully liquid,
  while the solidus denotes the temperature below which it is fully solid. Between these two lines, solid and liquid coexist.

3. The Melting Point of Pure Aluminum

Standard Value: 660.32 °C (1220.58 °F)

Under standard atmospheric pressure (0.1 MPa), pure aluminum melts at 660.32 °C (1,220.58 °F).

Laboratories confirm this value using high‐precision fixed‐point cells and comparison with certified reference materials.

Industrial thermocouples often read 5–10 °C higher than true melt temperature due to superheating and measurement error,
so operators typically set furnace setpoints around 680–700 °C before pouring.

Factors Influencing Aluminum’s Melting Point

Effect of Alloying Elements

When alloying aluminum, elements such as silicon (Si), magnesium (Mg), copper (Cu), and zinc (Zn) alter its melting behavior:

• Silicon (Al–Si) alloys (e.g., A356, A319) exhibit eutectic compositions around 12.6 wt % Si. Their eutectic mixture melts at 577 °C, whereas the liquidus lies near 615 °C.
• Magnesium (Al–Mg) additions (e.g., 6061 alloy) push the liquidus to approximately 650 °C and the solidus to 582 °C, creating a melting range of roughly 68 °C.
• Copper (Al–Cu) and Zinc (Al–Zn) shift melting ranges further: for instance, 7075 (Al–Zn–Mg–Cu) has a liquidus near 635 °C and a solidus around 475 °C, a spread of ~160 °C.
• Each alloy’s melting range appears on its phase diagram, and manufacturers must target casting
  or extrusion temperatures well above the liquidus to ensure complete fluidity and proper feeding of thin sections.

Impurities and Liquidus/Solidus Depression

Even small amounts of iron (Fe), nickel (Ni), or chromium (Cr) act as impurities,

often forming intermetallic compounds (e.g., Al₃Fe) and depressing the liquidus temperature by several degrees.

For example, just 0.1 wt % Fe can lower the liquidus by ~2–3 °C.

Foundries mitigate this by employing fluxes (chloride‐ or fluoride‐based) and degassing to remove oxides and hydrogen,

thus sharpening the melting plateau and reducing the gap between solidus and liquidus.

Pressure Dependence of Melting (Clapeyron Relation)

Under elevated pressures, aluminum’s melting point rises at a rate of approximately 6 K/GPa.

For most industrial processes operating at or near 1 atm, this effect proves negligible.

However, high‐pressure research (e.g., diamond‐anvil cell experiments) reveals that at 1 GPa, aluminum’s melting point climbs to around 666 °C.

Although not directly applicable to standard casting, this information underscores how pressure influences solid–liquid equilibrium.

4. Alloy Systems and Melting Ranges

Below is a non-exhaustive but extensive listing of common aluminum alloys and their approximate solidus/liquidus (melting) temperatures.

In many cases, each alloy exhibits a range between the solidus (onset of melting) and liquidus (fully liquid) due to alloying and eutectic reactions.

Notes and Observations:

• Pure aluminum (1100) melts exactly at 660.3 °C (1 220.5 °F); commercial 1100 may show a slight ± 1 °C (± 1.8 °F) variation due to trace impurities.
• Al–Si casting alloys (A356, A380, ADC12, A413) feature solidus values from 546 °C (1 015 °F) to ~ 577 °C (1 071 °F), with liquidus near 595–615 °C (1 103–1 139 °F).
  The relatively narrow freezing intervals in some (e.g., A356) yield fine microstructures and good mechanical properties.
• Mg-bearing wrought alloys (5052, 5083, 6061, 6082, 6063) show solidus temperatures between 545 °C (1 013 °F) and 582 °C (1 080 °F),
  while liquidus lies between 640 °C (1 184 °F) and 655 °C (1 211 °F).
  As Mg content climbs, the solidus drops lower, broadening the melting range.
• High-strength 7000 series (7050, 7075) exhibit very wide freezing ranges,
  eutectics near 470–475 °C (878–887 °F) and liquidus around 635–640 °C (1 175–1 184 °F).
  Careful process control (vacuum casting, HPDC) is essential to prevent hot cracking.
• Copper-rich aluminum alloys (2024, 2014) have solidus values near 490–502 °C (914–935 °F)
  and liquidus near 640–642 °C (1 184–1 188 °F)—a very large interval of ~140 °C (≈ 252 °F), demanding precise temperature management to avoid defects.
• Emerging alloys (Al–Li, ScAl, AlBeMet, nano-alloys) tweak melting behavior by only a few degrees but offer unique mechanical or processing advantages.

5. Measurement and Determination Methods

Accurately pinpointing aluminum’s melting point requires controlled laboratory methods. Engineers and researchers rely on:

Differential Scanning Calorimetry (DSC)

DSC measures heat flow into a small aluminum sample (5–10 mg) as temperature ramps at a known rate (e.g., 10 °C/min).

The endothermic peak at 660.3 °C corresponds to the latent heat of fusion (roughly 10.71 kJ/mol, or 394 J/g).

High‐precision DSC instruments achieve ±0.5 °C accuracy by calibrating with primary references such as indium (melting point 156.6 °C) and zinc (419.5 °C).

Differential Thermal Analysis (DTA)

In DTA, a reference (inert material) and the aluminum sample share the same heating program. The temperature difference between them reveals a melting onset.

Although less precise than DSC, DTA provides ±1 °C resolution, making it useful for characterizing alloy ranges when paired with cooling curves.

Thermocouple‐Based Furnace Tests

Industrial foundries often rely on Type K (NiCr–NiAl) or Type N (NiCrSi–NiSi) thermocouples inserted into molten aluminum.

As the sample reaches 660 °C, operators note a temporary plateau (ice‐point furnace style) indicating latent heat absorption.

However, superheating can push the apparent temperature to 680–700 °C before it drops to the true liquidus.

Repeated calibration against reference metals helps correct for systematic errors but cannot fully eliminate oxidation‐related biases.

Challenges in Precision (Oxidation, Superheating)

Molten aluminum quickly forms an alumina (Al₂O₃) film on its surface, insulating inner liquid and skewing temperature readings.

Simultaneously, bulk aluminum often superheats by 20–30 °C above its liquidus because nucleation barriers delay the onset of melting.

To overcome these issues, laboratories stir samples under inert gas (argon) or apply fluxes to break oxide films before taking measurements.

They also mount fixed‐point cells to calibrate thermocouples against certified standards.

6. Industrial Melting and Casting Practices

In industrial settings, aluminum rarely melts in isolation; operators grist through a sequence of specialized practices to produce quality castings:

Typical Furnace Types

• Induction Furnaces: Electromagnetic coils rapidly heat scrap or ingots.
  Because induction concentrates heat within the metal, these furnaces melt aluminum efficiently at 700–750 °C.
• Reverberatory Furnaces: Gas‐fired hearths allow large batches (up to several tons) to melt at 700–720 °C. Operators skim off dross while maintaining minimal temperature overshoot.
• Rotary Furnaces: Tilted drums rotate to combine heating and stirring, maintaining uniform temperature around 700–750 °C and offering good mixing for alloy homogeneity.
• Crucible Furnaces: Smaller capacity units (50–200 kg) heat aluminum via electrical elements or propane, holding metal near 680–700 °C until pouring.

Fluxing and Degassing

Molten aluminum readily traps hydrogen (solubility up to 0.7 cm³ H₂/100 g Al at 700 °C).

To minimize shrinkage porosity, foundries bubble inert gases (argon, nitrogen) through the melt, encouraging hydrogen to escape.

They also introduce fluxes—typically a blend of chlorides or fluorides—that dissolve and float alumina, rendering it easier to skim.

Effective fluxing reduces oxide inclusion by more than 80 %, directly improving final casting integrity.

Energy Consumption and Efficiency Considerations

Melting primary aluminum consumes about 13–15 kWh per kilogram of metal produced.

In contrast, secondary (recycled) aluminum requires only 1.8–2.2 kWh per kilogram—a roughly 85 % energy saving.

Modern furnaces leverage ceramic fiber linings, regenerative burners, and waste‐heat recovery to cut energy use by an additional 15–20 %.

Foundries track energy cost per ton of melt closely, as heating accounts for up to 60 % of total casting cost.

Melt Treatment and Temperature Control for Quality

To ensure consistent alloy composition and minimize macro‐segregation, operators stir molten aluminum using mechanical impellers or electromagnetic stirring.

They hold melt at 700–720 °C for a brief soak (5–10 minutes) before transfer to holding furnaces.

Temperature controllers—often linked to infrared pyrometers—maintain ±5 °C stability, preventing excessive superheating while ensuring fluidity for thin‐section castings.

7. Industrial and Practical Implications

Metallurgy: Melting and Casting Processes

Foundries calibrate furnaces to 20–40 °C above the alloy’s liquidus to ensure complete filling of molds.

Too low a temperature (e.g., less than 50 °C above liquidus) causes cold shuts and misruns,

while excessive superheat (e.g., > 150 °C above liquidus) accelerates oxidation and dross formation.

Melt quality directly influences mechanical properties: well‐controlled melts yield elongations

above 12 % in A356 castings, while poor control can reduce ductility to below 5 %.

Aerospace, Automotive, and Construction Uses

• Aerospace: Precision investment casting of Al–Li alloys (liquidus ~ 640 °C, solidus ~ 510 °C) demands melt cleanliness to avoid porosity in critical jet engine components.
• Automotive: High‐pressure die casting of A380 (liquidus ~ 595 °C) for transmission cases requires mold heating to 240–260 °C to avoid chills.
• Construction: Extrusion of 6061 for window frames happens at 500–520 °C, well below liquidus, balancing formability with dimensional stability.

Welding and Additive Manufacturing Considerations

• Fusion Welding: Gas tungsten arc welding (GTAW) of 6061-T6 runs at DC electrode negative with heat input tailored to keep weld pool at 650–700 °C.
  However, the heat‐affected zone (HAZ) may drop below 500 °C, causing softening if not re‐aged.
• Additive Manufacturing (SLM/EBM): Fine aluminum powders (particle size 15–45 µm) in
  powder bed fusion require lasers or electron beams generating local temperatures of 1,000 °C+ to compensate for high reflectivity and conductivity.
  Process parameters must minimize keyholing and spatter, despite aluminum’s lower melting point than steel.

Designing Heat Treatment & Hot Working

Forging or extrusion schedules stay well below solidus—typically 350–550 °C (662–1 022 °F)—to avoid incipient melting.

After forming, alloys often undergo solutionizing near 515–535 °C (959–995 °F) and quenching to establish T6 or other tempers.

Recycling Efficiency

Secondary aluminum smelters melt most alloys at 700–720 °C (1 292–1 328 °F),

achieving 90–95 % recovery at ~ 0.5–0.8 kWh/kg—far lower energy than re-melting steel (1,400–1,600 °C / 2–4 kWh/kg).

8. Comparisons with Other Metals

9. Conclusion

The melting point of Aluminum, 660.32 °C, anchors countless industrial operations, from primary smelting to advanced additive manufacturing.

Its relatively low melting threshold reduces energy consumption, accelerates recycling,

and simplifies casting compared to higher‐melting metals like copper and steel.

As industries continue pushing for lighter, stronger, and more complex aluminum components,

understanding and managing aluminum’s melting behavior will remain crucial.

Further research into nano-alloying, extreme pressure melting, and energy-efficient heating methods promises

to deepen our understanding of this foundational transition—solid to liquid—that defines aluminum’s role in modern metallurgy.