Eutectic Melting Calculator for Metals in Fire Debris

This eutectic melting calculator shows whether two metals found together in fire debris can melt or alloy at an estimated fire temperature. Copper and aluminum can make a liquid where they touch from 548.2 °C (1,018.8 °F), far below the 1,084.6 °C melting point of copper. In the worked example, at 650 °C every copper and aluminum mix from 4.3 to 61.1 wt% copper is fully liquid, although neither pure metal melts.

How to use the calculator

Choose the two metals that are fused or touching, enter your estimate of the local temperature at that spot in °C or °F, and, if a lab has measured the composition of the melted mix, tick the box and enter it in weight percent. The result says whether either pure metal melts at that temperature, the lowest temperature at which the pair makes a liquid where it touches, which mixes are fully liquid, and the liquid fraction of your composition. The diagram shows the simplified phase diagram with your temperature and composition on it, and the full results list every value with the steps.

How to check whether two metals alloyed in a fire

1. Identify the metals. Note which metals are fused together and where the second metal came from, for example an aluminum fixture housing above a copper conductor, a zinc die casting or a solder joint.
2. Estimate the local temperature. Use the fire patterns, other melted materials and the fire’s development to estimate the temperature the metal reached at that spot. A metal part lags the gas or flames around it.
3. Compare with the melting points. Check whether either pure metal could melt at that temperature.
4. Compare with the eutectic. If the temperature is at or above the lowest liquid temperature of the pair, contact alloying can make a liquid that dissolves the other metal, even though neither metal melts on its own.
5. Confirm with the artifact. Alloying is a possibility, not a finding. Confirm it with the location of the damage relative to the aluminum or zinc source, a cross section and an elemental analysis such as SEM/EDS.

Worked example

The cast aluminum housing of a ceiling fixture melts and runs onto a copper branch-circuit conductor. The estimated local temperature is 650 °C (1,202 °F), above the roughly 600 °C usually taken as flashover.

• Pure aluminum melts at 660.3 °C and pure copper at 1,084.6 °C, so neither pure metal melts at 650 °C. Commercial cast aluminum alloys start to melt lower than pure aluminum because of their silicon and copper, which is how molten aluminum reaches the conductor.
• Copper and aluminum make a liquid from 548.2 °C, the eutectic at 32.7 wt% copper, which is 101.8 °C below 650 °C.
• At 650 °C the liquidus lies below the fire temperature from 4.3 wt% copper (between the cited points at 2 wt% and 656 °C and at 5 wt% and 648 °C) to 61.1 wt% copper (between 59.6 wt% at 625 °C and 70.2 wt% at 804 °C). Any mix in that range is fully liquid, so molten aluminum can keep dissolving the copper it touches until the local mix passes about 61 wt% copper.
• A lab finds 20 wt% copper in the solidified mass. Its liquidus is 603 °C and it starts to melt at the 548.2 °C eutectic, so at 650 °C it was fully liquid.

The result is consistent with alloying: a copper conductor can be thinned, notched or beaded where molten aluminum touched it at a temperature well below its own melting point. It does not show how the bead formed. The location relative to the housing and a lab examination of the conductor decide that.

Eutectic and melting points of metals found in fire debris

Copper and aluminum also have a second eutectic on the copper side, at about 1,035 °C and 92 wt% copper. Tin and lead solder is often quoted at 183 °C and 61.9 wt% tin; the table gives the NIST calculated values. Melting points of the pure metals are the ITS-90 fixed points for tin, zinc, aluminum, silver and copper.

Fire temperatures for context

Values from V. Babrauskas, Temperatures in flames and fires (Fire Science and Technology Inc.), which also notes that fire temperatures measured with thermocouples in buildings can be imprecise and that an object next to a flame does not take its temperature at once. Use them to judge whether an estimate is reasonable, not to set the temperature of a particular spot.

What this means for reading a conductor

Alloying can leave beads, notches, thinning or holes on a copper conductor that look like arc damage, at temperatures where copper alone would not melt. Before reading a melted conductor as an arc site, check whether aluminum, zinc or tin was above or against it, look for the second metal on the surface, and section the artifact: SEM/EDS shows whether the melted region holds aluminum or zinc, and X-ray imaging can show the conductor inside a mass of melted metal before it is cut. NFPA 921 discusses alloying among the melting effects an investigator must tell apart from arcing. Our article on the differences between arcing and melting damage covers the features of each.

Frequently asked questions

What is eutectic melting?

Two metals in contact can dissolve into each other and form an alloy that melts at a lower temperature than either metal alone. The lowest such temperature is the eutectic point. For copper and aluminum it is 548.2 °C, about 110 °C below the melting point of aluminum and more than 500 °C below that of copper.

Can a house fire melt copper wire?

A fully developed room fire is typically 900 to 1,000 °C, below copper’s melting point of 1,084.6 °C, but higher local temperatures occur and copper conductors are often found melted. Alloying with aluminum, zinc or tin is one way copper is consumed at much lower temperatures, which is why melted copper alone does not prove a high temperature or an arc.

Why does the calculator use weight percent?

Lab reports of melted debris, such as EDS results, usually give weight percent. Some of the phase diagram sources use atomic percent; those values were converted to weight percent for the calculator.

How accurate is the liquid fraction?

The diagrams are simplified: straight lines between cited points from assessed phase diagrams, so the fraction is approximate between those points, mostly where the true curves bow. Real debris also holds other alloying elements and oxides and may not reach equilibrium in the short time of a fire. Treat the fraction as a screening estimate.

Why are brass and steel not in the list?

Copper and zinc, the metals of brass, form no eutectic: their phase diagram has a series of peritectic reactions instead, and liquid first appears where zinc melts, at 419.5 °C. Steel melts far above typical building fire temperatures. The calculator covers pairs with an assessed two-metal diagram from published sources.

When to bring in an engineer

A phase diagram tells what can melt at a temperature, not what caused a bead or a failure. Dreiym’s licensed Professional Engineers examine conductors and metal debris from fire scenes, document their location, and use cross sections, SEM/EDS and X-ray imaging to tell arc damage from fire melting and alloying. See our fire and explosion investigation and forensic electrical engineering services, the galvanic corrosion chart for metals in contact that do not melt, or contact us for a free quote. We typically respond within one business day.