Melting point is a crucial physical property. The temperature at which a substance changes from a solid to a liquid state directly impacts how that substance is handled and applied. Aluminum has a melting point of approximately 1220°F. Relative to other metals, this is about double the melting point of zinc, and half the melting temperature of stainless steel. Notably, the melting point of aluminum changes depending on the alloy composition. This is a vital piece of information when it comes to manufacturing aluminum.
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Aluminum Spec Sheet
Manufacturing processes often require metals to be heated to their melting temperature. Smelting, fusion welding, and casting all require metals to be liquids in order to be performed. Aluminum and aluminum alloys can be melted and remelted as needed. This includes melting for the casting of ingots or billets, and also for subsequent fabrication processes like rolling, extruding, drawing, forging, and recycling.
First, Understanding How Prime Aluminum is Produced
Aluminum always occurs as a compound, never as a pure form. Pure forms of the metal must first be chemically refined into alumina (aluminum oxide) and then smelted into aluminum through the Hall–Héroult electrolytic reduction process, commonly called the primary production process. According to the Aluminum Association, the primary production process has grown more efficient, improving by 20% over the past 20 years.
Getting aluminum starts with mining bauxite, the basic raw material from which the metal aluminum is produced. Impurities from the bauxite are then removed through settling and filtration. This chemical processing makes alumina. The bauxite is crushed, mixed in a sodium hydroxide solution, and then heated by steam under pressure to dissolve the alumina.
The melting point of aluminum oxide is higher than aluminum, at about 3,762°F. This is why the extraction of aluminum from aluminum oxide is done through electrolysis before the metal is melted. Electrolysis, the technique by which elements can be separated, takes place in reduction pots or cells in a reduction plant. This reduction process removes the oxygen from the alumina, which consists of almost equal parts of aluminum and oxygen. Only after all of these steps, do we have pure, prime aluminum.
Factors Affecting Aluminum Melting Point
Melting temperatures range slightly relative to alloy composition. Melting ranges for a few common alloys are outlined by ASM International:
1100: 1190 – 1215°F
2024: 935 – 1180°F
3003: 1190 – 1210°F
5052: 1125 -1200°F
6061: 1080 – 1205°F
7075: 890 – 1175°F
Regular aluminum and its alloys inevitably contain impurities. An impurity can be described as any chemical elements not intentionally introduced into the alloy composition.
Chemical impurities will change a the melting point temperature. This is called a melting point depression. If the melting point varies by more than 41 degrees Fahrenheit that’s proof that there are impurities.
Impurities can occur at a number of different points throughout the melting process. Contamination of charge during the during melting and casting, or interaction with the metal lining and fluxes. Dissolution of elements of the casting equipment and the casting tool.
Impurities may enter during remelting of aluminum waste in secondary production.
Composition of the Molecules
Molecules with strong bonds require more energy to break. Aluminum has covalent bonds leading to a higher melting point.
Force of Attraction
Aluminum has strong polar bonds leading to low volatility, high melting and boiling points, and high density.
Applying pressure during the heating process can alter melting points. For aluminum, pressures around 3,500 gigapascals would induce re-entrant heating, or when the melting point actually starts to decrease above a certain critical pressure.
Melting Metals: The Basics of Casting
Casting processes have been known for thousands of years. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools.
Highly engineered castings are found in 90% of durable goods, including cars, trucks, aerospace, trains, mining and construction equipment, oil wells, appliances, pipes, hydrants, wind turbines, nuclear plants, medical devices, defense products, toys, and more.
There are many different casting techniques that fall under either expendable or non-expendable mold casting techniques. Expendable casting involves the use of temporary, non-reusable molds. In non-expendable mold casting, molds are reused without needing to be reformed after each production cycle. For more information on each of the casting processes, visit the Total Materia metals database.
The Process of Casting Aluminum
Millions of pounds of melting aluminum are cast safely every day, making it one of the most commonly casted metals around. This is likely due to the fact that aluminum is one of the few metals that can be cast by all of the processes used for casting metals. But, there are differences in the type of aluminum that is cast.
Difference Between Prime Aluminum, Scrap, and Hardeners
When asking how to melt aluminum, understanding the difference between prime aluminum, scrap, and hardeners is essential. Each play a key role in the casting process.
Prime aluminum is 99.9% pure aluminum. It shows up in three forms: T-bars, tub sows, and pig. Prime aluminum gets added to the furnace first.
Scrap is recycled aluminum. Scrap can show up directly from either the production process, or from products collected after consumer use and disposal. After all the prime aluminum has melted, scrap is added to remove moisture.
Hardeners are added elements intended for hardening the aluminum. The purpose of hardeners is to increase strength and give the final product desired features. This includes finish, strength, and grain refinement. Silicone and magnesium are two staple hardening agents in the aluminum casting process. Typically, they are added last to a bath of aluminum in order to bring the concentration of the alloying agents up to the specification of the alloy being cast.
Casting Furnaces to Melt Aluminum
Aluminum furnaces are used to melt down aluminum and cast logs. In general, furnaces might use different technologies to melt material, but every furnace aspires to minimize heat and fuel consumption. Energy efficient furnaces help control melting conditions, which increases the metal yield.
Aluminum furnaces are designed with an open well. This design accounts for the steps in the aluminum casting process outlined below, since, once the prime aluminum melts, scrap gets added through this opening.
What Are the Steps in the Casting Process?
First a charge is prepared where a mixture of raw materials is melted down to make an alloy. Prime aluminum is added, and once all the prime aluminum has melted, scrap is added to remove moisture.
Methods and techniques for treatment of molten aluminum increase and verify its purity. This includes fluxes processing, blowing inert gases, filtration, and settling.
Once purity is verified, hardening agents are added. Again, which elements are added to the mixture and in what amounts depends entirely on the specification of the alloy being cast.
Extruding aluminum begins by heating a solid piece of aluminum plate, tube, sheet or bar until it softens. The softened aluminum is then forced by compression through a smaller die opening. Aluminum extrusions can be solid, semi-hollow, or hollow and serve to make some consumer products lighter, and others more durable.
Melting Aluminum Safely
Any molten material bears risk. Failure to use proper procedures in melting and casting aluminum can be dangerous. Molten Aluminum is typically handled at 1300-1450°F to avoid premature solidification. Contact with molten aluminum can cause severe burns and create a serious fire hazard.
Risk of Explosions
Explosions can be caused by mixing water or other contaminants with molten aluminum. Explosions can also occur in the aluminum scrap re-melting process due to moisture and contamination in scrap.
These explosions can result in injury or death as well as destruction of equipment and plant facilities. Where there is possibility of splash or other direct exposure, personnel working with molten aluminum must wear eye and face protection as well as protective clothing.
The Aluminum Association believes that these hazards can be minimized or eliminated entirely by careful attention to safe handling practices and the sharing of best practices and safety programs including:
The development and distribution of publications on the safe production and handling of aluminum
Annual Casthouse Safety Workshops to educate plant workers and managers on workplace safety
The release of an annual molten metal incident report to share information regarding explosion and related events at facilities that melt aluminum
Ongoing testing and reporting with member companies for continued safety improvements
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Sara Montijo is a writer for Kloeckner Metals. She graduated with honors from NYU and has previously facilitated multimedia programming and worked alongside renowned chefs. Her friends call her a time warp.