Metal Casting

Ask a Metallurgist: A Brief Intro to Aluminum Grades

A plane sits in a hanger being cleaned and maintained

Choosing your Alloy

A plane sits in a hanger being cleaned and maintained
The 7000 series of aluminum alloyed with zinc is often used in airplane manufacture.

At Reliance, we primarily use two main aluminum grades for our bollards and benches. They have slightly different naming conventions, which can lead to confusion: are we buying from different countries with different international metal standards?

Aluminum vs. aluminium

When it comes to aluminum, various countries around the world don’t even agree on a spelling. The North American spelling, “aluminum” was the second choice of the original namer, British chemist Sir Humphry Davy. He started with “alumium,” and then coined “aluminum.” Other chemists, however, pointed out that an -ium ending would be preferred, to match other chemicals like sodium and potassium. North American usage stayed with Sir Davy’s second choice; some American chemists and common usage elsewhere chose aluminium as a more consistent lexical construction.

The two main types of aluminum grades

It is neither etymology nor international standard that produce two types of aluminum grade systems. The reason for the mismatch is that cast metal and worked (wrought) metal are graded differently. The needs and uses of cast and worked metals are not identical, so the same specification is not used for both.

Iron is widely known to have cast iron and wrought iron forms—though the meaning of these words has gotten lost enough that many misspeak wrought iron as “rod iron.” Aluminum, like iron, is found in both cast and wrought forms. Cast aluminum is poured or injected as a molten, liquid metal into a mold. When the metal cools, the solid object that emerges is near-net shape. It may be machined to smooth or finish it, or be used as a piece in a larger build, but the point of casting is to create something very close to final shape. Wrought aluminum, on the other hand, is produced in billets, slabs, and blooms. These are rolled and flattened into sheets, rods, and tubes. It can be forged, machined, joined, or worked in other ways to become the final shape it needs to be.

We cast aluminum bollards, just as we cast our ductile iron bollards. A good casting alloy catches lovely details like fluting and soft curves in the mold. It is good for complex, curving shapes made in a single piece. Our benches are made from aluminum sheets that are formed, joined, and laser cut; these pieces are more weldable and workable, so they can be cut and joined into complex designs with many components built together.

These different manufacturing processes require different grades of aluminum.

A silver bench sits on the grass beside a seawall, decorated with laser cut organic designs
Our Newport and Austin benches are made with 6061, a builder’s grade aluminum.

Aluminum Grades for Wrought Alloys

Many different international organizations have come together to sign an agreement that sets forth an International Alloy Designation System for wrought aluminum. These alloys are grouped into families by their first digit, which tells an experienced metallurgist what the major alloying metals are.

1xxx series: 99%+ aluminum content

Aluminum in its purest form shows up in the 1000 series. These metals highlight the material properties of the element, and 1100 aluminum is the most common “pure” aluminum grade, with less than 1% added elements like zinc, copper, or silicon for strength. Even with those additions, this family is not as strong as other families of aluminum, but the alloys are easy to work and resistant to corrosion. This group is good at transmitting heat and electricity. 1000 series aluminum alloys are often used in chemical, food, and electrical applications.

2xxx series: Aluminum and Copper

2011, 2014, and 2024 are the most common grades in the 2000 series of aluminum alloys, which are prized for their strength and machineability. This series has copper as its main alloying element. The 2000 series is tough, strong, and hard, but it is also prone to corrosion, so it needs to be sealed if exposed to oxidizing elements. This may be done with coatings like paint, powder, or plastic; it is also often done with pure aluminum cladding. 2000 series aluminum is found in aerospace applications, fasteners, and industrial applications.

A photo showing an old milk jug and two used pots with scorch on the sides
Pots, pans, signs, and roofing all are likely made with 3000 series aluminum.

3xxx series: Aluminum and Manganese

Manganese, an element incredibly important to metallurgists and glassmakers, is little known to most people. It provides strength and deoxidizes alloys. Manganese is everywhere and is used in common stainless-steel grades and in the general-purpose aluminum we use every day. 3003 is the aluminum most often used because it can be worked, welded, and finished. From sheet metal to cooking utensils, the aluminum you see most is probably 3000 series.

The sides of aluminum cans are made from 3004 series aluminum. Surprisingly, this is a different grade than that which is used on the top and bottom. Top and bottom are not just thicker: they’re made with grade 5182. 

4xxx series: Aluminum and Silicon

Silicon helps these alloys be more heat tolerant: it lowers the melting point of the aluminum. Aluminum with silicon is also more ductile, which means the resulting metal is more flexible and less brittle. For these reasons, the 4000 series is often used in automotive or electrical applications; it can handle the heat and the electricity.

5xxx series: Aluminum and Magnesium

Boaters are familiar with the 5000 series of aluminum. When aluminum and magnesium are melted together, they produce a corrosion resistant alloy. Marine applications, tanks, valves—anything dealing with chemicals or weather—often come in these alloys. Helpfully, these alloys also are good with welding and forming, making it easy to create hulls, valves, and cryo-tanks from the material. This is the series featuring the 5182 grade aluminum used for the top and bottom of soda cans! 

6xxx series: Aluminum, Magnesium and Silicon

The 6000 series alloys are builders’ grade alloys, combining the features from the 4000 and 5000 series: corrosion resistance and strength. Our aluminum benches are made from 6061, for corrosion resistance when exposed to water, salt, snow, sun, and rain; the alloy also provides the strength and stability needed for a rugged piece of site furniture.

An astronaut floats in a space suit during an EVA with the lights of Eurasia below
7000 series is light-and-strong: 8000 series alloys are created for specific purposes.

7xxx series: Aluminum and Zinc

Zinc added to aluminum creates steel-like strength. If aluminum is doing the work of steel, in a place where steel’s weight is too heavy, a 7000 series aluminum may be used instead. Aeronautics and aerospace, cars, and sports equipment often use this light but strong series.

8xxx series: The Weird Ones

Metals often have uncommon alloys that are made for specific applications. They often end up in naming conventions in a grab-bag series, and for wrought aluminum, the oddballs are in the 8xxx series. In most cases, if you’re buying an uncommon alloy, you know exactly why and for what purpose; for example, NASA’s metallurgists are looking for custom alloys for very, very specific environments.

Understanding grades for Aluminum Cast Alloys

A black pawn-like bollard with straight sides and a ball cap

Height: 35-3/4″
Max Interior Pipe
Diameter: 4-1/2″

Material: Aluminum
Grade: A356

A black pawn-like bollard with a widening base, fluted sides, and a ball cap

Height: 35″
Max Interior Pipe
Diameter: 4-1/2″

Material: Aluminum
Grade: A356

A black bollard with a widening base and a cloud-like cap

Height: 32″
Max Interior Pipe
Diameter: 4-1/2″

Material: Aluminum
Grade: A356

A black pawn-like bollard with straight sides and a ball cap

Height: 39″
Max Interior Pipe
Diameter: 3-1/2″

Material: Aluminum
Grade: A356

Cast aluminum has more than one commonly used naming standard. For cast products there can be different specifications depending on where you are.

The main naming standards for cast aluminum come from:

The Aluminum Association’s naming standard is at first glance similar to the wrought aluminum standard. Series numbers reflect the major alloying element… but due to the difference in casting alloys, the leading number does not indicate the same star player.

As with wrought aluminum, the 1xx.x series are near pure aluminium and the 2xx.x series are copper. After that, the major alloying element shifts. 3xx.x and 4xx.x series both have silicon. As with wrought aluminums, the silicon in the alloy lowers the melting point and maintains ductility. Ductility is useful in casting metals for capturing cleaner, smaller details in the mold. 3xx.x relies on other elements in the mix for other properties like strength. 5xx.x indicates magnesium, 7xx.x zinc, 8xx.x tin, and in the cast aluminum category the 9xx.x block contains “the weird ones”—alloys made for specific purposes. 6xx.x is unused in this naming convention.

Cast alloys will sometimes come with letters denoting heat treatment. These are:

  • F: as fabricated
  • O: annealed
  • H: work-hardened
  • W: solution heat-treated
  • T(1-10): Other heat treatments, designated by the digits following.

In our case, the A in front of 356 has nothing to do with heat treatment. Instead, for this alloy the A suggests higher chemical purity: the range or tolerance for each chemical in the mix is constrained.

A sapphire gemstone hangs in a platinum setting on a platinum chain
Aluminum oxide is a mineral called corundum; rubies and sapphires are both versions of this mineral.

Aluminum: once a luxury, now a staple

Aluminum is the third most plentiful element in the earth’s crust, after oxygen and silicon—making it more plentiful than iron. Unlike iron, the metal cannot be extracted through simple smelting. Rubies, sapphires, and less precious stones like gibbsite are minerals in which aluminum is stored.

It wasn’t until the 1820s when chemists figured out how to extract elemental aluminum. At first the process was complicated enough that the “new” metal was more precious than platinum, gold, or silver.

It is an often-told story that Napoleon III had a set of aluminum cutlery that was reserved for his most honored guests. A well-sourced historical forum questions the accuracy of this claim, but notes that in a French book about aluminum talks about the emperor’s interest in the metal in military applications, from decorative uniform elements to weaponry, and that he had an aluminum rattle made for his son. It is certainly true that before 1855, aluminum was expensive enough that bars of the pure metal were stored at Fort Knox.

What changed? In 1855 a process was invented to extract aluminum from water using a current of electricity. The process was relatively easy, and industrially scalable, and aluminum is plentiful. By 1859 aluminum was only worth slightly more than silver. These days, of course, aluminum is sold by the pound at a twentieth of the cost of an ounce of silver (although supply chain issues, 2020-2022, created a rising aluminum cost).

Why is aluminum foil called tin foil?

Before aluminum was available commercially, foil made from tin was used. Like aluminum foil, tin foil was used to wrap food to preserve it. However, it was thicker, less ductile, and left an aftertaste in the food. Tin foil was also used in other applications. Dentists used tin foil to fill cavities; tin foil was also used in the first phonograph devices.

Foil made of element 50, tin, still exists, mostly used by chemists and other scientists. It is more expensive than aluminum foil.

Human technological advances often depend on metallurgical advances. The Bronze Age and the Iron Age are obvious examples, but the Industrial Revolution was dependant on Bessemer steel processes that could support mechanization and large-scale construction. Aluminum is the metal of the Space Age, as silicon is the metalloid of the Information Age. Cast or wrought, from new ore or recycled, aluminum alloys offer a wide range of material properties for almost any application.

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