When beginning any project, material selection is one of the most fundamental choices that can dictate its success. Airplanes, computers, buildings, and other modern technologies all use specialized materials that allow them to complete amazing tasks, and one of the most important materials in this regard is the metal aluminum. Aluminum is the most abundant metal on Earth, making it an attractive, cost-effective option for builders when considering metal for their project. Along with its abundance, aluminum has the ability to be alloyed – a process that improves a base metal’s properties by adding trace amounts of other metallic “alloying” elements into it. This alloying process has allowed many grades of aluminum alloys to be produced, and there are so many grades that the Aluminum Association has classified these types of aluminum into categories based on alloying elements and material properties. This article will give a brief introduction to the different types of aluminum, how they differ, and which alloys are best suited for certain applications.
Aluminum Association Naming Scheme
The Aluminum Association Inc. is the foremost authority on aluminum metal and its derivatives in North America. They have organized the hundreds of aluminum alloys into grades, which are given four-digit identifiers that contain information about their composition and processing. Many of these alloys have been divided into classes, which are denoted by the first digit in their names (ex. 4xxx, 6xx.x, and 2xxx, are all different grades of aluminum). The following three digits describe specific alloys, hardening processes, and other information that could be useful to manufacturers, but will not be explored in this article, as they are more pertinent to alloy makers and not buyers.
Cast vs. Wrought Aluminum
Aluminum alloys can be broadly separated into two categories: cast aluminum alloys and wrought aluminum alloys. Cast alloys of aluminum are those which contain > 22% alloying elements by composition, whereas wrought aluminum alloys contain ≤4%. This may seem like a simple difference, but the percentage of alloying elements has a huge impact on material properties. Aluminum loses its ductility as more alloying elements are added, making most cast alloys susceptible to brittle fracture. Conversely, wrought alloys have allowed designers to increase aluminum’s strength, corrosion resistance, conductivity, etc. while still retaining ductility and other beneficial qualities.
Cast aluminum alloys typically have low melting points and tensile strength when compared to wrought aluminum; the most commonly used aluminum alloy is aluminum-silicon, which features high levels of silicon that enable the alloy to be easily cast. Wrought aluminum accounts for the majority of aluminum products, such as those manufactured from extrusion or rolling. Elements such as copper, manganese, silicon, magnesium, magnesium silicon combinations, zinc, and lithium define the individual wrought aluminum alloy categories.
Cast Alloys
Cast alloys of aluminum are named using four numbers, with a decimal between the third and fourth digit. The first three numbers indicate the alloy, and the fourth number indicates the form the product is in. Below, in Table 1, is shown the different types of cast aluminum, their common alloying elements, and their basic material properties. Note that the properties (cracking, corrosion, finishing, joining) are given ratings of 1 to 5, 5 being the worst and 1 being the best, and are generalized quantifications of their capabilities:
Table 1: Different cast aluminum grades, with their general information shown.
Note: Cells with no number indicate that the value is not often specified, or is too difficult to generalize. A rating of 1 is considered exceptional, a rating of 5 is considered very poor, and 2-4 fall within this range.
[b]Aluminum grade[/b]
[b]Alloying elements[/b]
[b]Strengthening Process[/b]
[b]Cracking[/b]
[b]Corrosion Resistance[/b]
[b]Finishing[/b]
[b]Joining[/b]
1xx.x
unalloyed
Non-heat-treatable
-
1
1
1
2xx.x
Copper
Heat-treatable
4
4
1-3
2-4
3xx.x
Silicon, Magnesium, Copper
Heat-treatable
1-2
2-3
3-4
1-3
4xx.x
Silicon
Heat-treatable
1
2-3
4-5
1
5xx.x
Magnesium
Non-heat-treatable
4
2
1-2
3
6xx.x
NOT USED
NOT USED
-
-
-
-
7xx.x
Zinc
Heat-treatable
4
4
1-2
4
8xx.x
Tin, Copper, Nickel
Heat-treatable
5
5
3
5
1xx.x alloys
1xx.x cast alloys are commercially pure, unalloyed aluminum, which has exceptional corrosion resistance, finishing qualities, and welding characteristics. 1xx.x alloys are often used in manufacturing rotors or cladding corrosion-prone alloys.
2xx.x alloys
2xx.x cast alloys use primarily copper as their alloying element, though magnesium, manganese, and chromium are often included. They are heat-treatable, meaning they can gain additional strength via the heat-treatment process (find our explanation on heat-treatment in our article all about 2024 aluminum alloy). They sport the highest strength and hardness among all casting alloys, especially at higher temperatures. The copper in its composition leaves it susceptible to corrosion, and it is less ductile and susceptible to cracks when heated. Common applications for 2xx.x alloys include automotive cylinder heads, exhaust system parts, and aircraft engine parts.
3xx.x alloys
3xx.x cast alloys use silicon, copper, and magnesium as the main alloying elements, oftentimes with supplemental nickel and beryllium. They are heat treatable, have high strength, good resistance to cracking and wear, and have good machinability. Common applications for 3xx.x alloys include automotive cylinder blocks/heads, car wheels, compressor/pump parts, and aircraft fittings.
4xx.x alloys
4xx.x cast alloys use solely silicon as their alloying element. 4xx.x alloys are non-heat treatable and have great casting qualities, along with good welding characteristics, strength, corrosion resistance, and wear resistance. Common applications for 4xx.x alloys include pump casings, cookware, and bridge railing support casings.
5xx.x alloys
5xx.x cast alloys use magnesium as their primary alloying element and are not heat-treatable. They resist corrosion well, have good machinability, and have a great surface aesthetic when anodized. Common applications for 5xx.x alloys include sand casted parts.
7xx.x alloys
7xx.x cast alloys contain zinc as their main alloying element and are heat-treatable. They do not cast well but have good dimensional stability, machinability, finishing qualities, and fair corrosion resistance.
8xx.x
8xx.x cast alloys use primarily tin, as well as small amounts of copper and nickel in its composition, and are not heat-treatable. These alloys have low strength, but great machinability and wear resistance. They were developed for bearing applications, such as bi-metal slide bearings for internal combustion engines.
Wrought Alloys
Wrought aluminum alloys are named using a four-digit indicator just as with cast alloys, but they do not contain any decimal places. It is therefore easy to differentiate a cast aluminum alloy from a wrought alloy by simply looking at the structure of its name. The first digit denotes the class of aluminum alloys that share alloying elements, where each alloy within a class contains different percentages of trace elements specific to each blend. These alloys tend to be more versatile than cast alloys thanks to their increased material properties, and Table 2 shows the different wrought alloy classes, their strengthening processes, as well as their improved characteristics (strength, corrosion resistance, workability, joining/welding). These wrought alloys use the same ratings as shown in Table 1 (1 being the best and 5 being the worst):
Table 2: Different cast aluminum grades, with their general information shown.
[b]Aluminum grade[/b]
[b]Alloying elements[/b]
[b]Strengthening Process[/b]
[b]Strength[/b]
[b]Corrosion Resistance[/b]
[b]Workability/Formability[/b]
[b]Joining/Welding[/b]
1xxx
Unalloyed (99% Al)
Strain-hardening
5
1
1
3
2xxx
Copper
Heat-treatable
1
4
4
5
3xxx
Manganese
Strain-hardening
3
2
1
1
4xxx
Silicon
Depends on alloy
3
4
1
1
5xxx
Magnesium
Strain-hardening
2
1
1
1
6xxx
Magnesium, Silicon
Heat-treatable
2
3
2
2
7xxx
Zinc
Heat-treatable
1
1
4
3
8xxx
Other elements
Limited
-
-
-
-
1xxx alloys
1xxx alloys are not true alloys, as they are 99% pure commercial aluminum. They are very useful as chemical/electrical materials and have exceptional corrosion resistance and workability. These alloys can be strain hardened or given increased strength by mechanical deformation (more information on strain hardening can be found in our article all about 5052 aluminum alloy).
A popular alloy of this class is 1100 aluminum alloy, which is commercially pure aluminum. The material is soft and ductile and has excellent workability, making it suitable for hard-forming applications. It can be welded with any method, but it cannot be heat-treated. It has excellent corrosion resistance and is widely used in the chemical and food processing sectors.
2xxx alloys
2xxx alloys are wrought alloys that principally use copper, and often small amounts of magnesium as their alloying elements. They gain exceptional strength when heat-treated, rivaling low carbon steels, but are prone to corrosion due to its copper content.
2024 aluminum alloy is one of the most frequently used aluminum alloys of high strength. It is frequently used where an excellent strength-to-weight ratio is desired with its mixture of high strength and outstanding fatigue resistance. This grade can be machined to a high finish, and if necessary, it can be formed with subsequent heat treatment in the annealed condition. The corrosion resistance of this grade is comparatively low. When this is a problem, 2024 is frequently used in an anodized finish or in clad form (thin high purity aluminum surface layer) known as Alclad. Learn more by reading our article all about 2024 aluminum alloy.
3xxx alloys
3xxx alloys use manganese as their main alloying element, which improves its strength over other non-heat treatable alloys such as the 1xxx series. They are moderate-strength alloys with great working and finishing characteristics, and this grade contains one of the best general-purpose alloys available today, 3003 aluminum. This is the most widely used of all aluminum alloys, and is made from commercially pure aluminum with added manganese (20% stronger than the 1100 grade) to increase its strength. It has excellent resistance to corrosion and workability. This grade can be deep drawn or spun, welded, or brazed. Learn more about this invaluable alloy in our article on 3003 aluminum alloy.
4xxx alloys
4xxx alloys use silicon as its alloying element to lower their melting points without compromising ductility. They are commonly used as welding wire and brazing alloy to join other grades of aluminum. Certain 4xxx alloys can be heat treated to a limited extent but are generally not responsive to heat treatment. Oxide finishes of 4xxx alloys are aesthetically pleasing and often used in architectural applications. Aluminum alloy 4047 is a popular type of this alloy grade which offers good thermal and electrical conductivity, corrosion resistance, and a higher melting point.
5xxx alloys
The major alloying element in 5xxx aluminum alloys is magnesium, with trace amounts of manganese in certain alloys. These alloys are strain-hardenable, readily weldable, and resist corrosion exceptionally well, especially in marine environments. Common applications of 5xxx alloys are boat hulls, gangplanks, and other marine equipment.
5052 aluminum is the highest strength alloy of the more non-heat-treatable grades. Its resistance to fatigue is better than most grades of aluminum. Alloy 5052 has a good marine atmosphere corrosion resistance of saltwater and excellent workability. It can be drawn or formed easily into intricate forms. More information can be found in our article on 5052 aluminum alloy.
6xxx alloys
6xxx alloys implement magnesium with silicon as their principal alloying elements. Their strength is improved with heat treatment, and while not as strong as 2xxx and 7xxx alloys, they couple good strength with good formability, weldability, machinability, and fair corrosion resistance. They are commonly used in architectural, marine, and general-purpose applications.
6061 aluminum alloy is the most flexible of the heat-treatable aluminum alloys while maintaining most of the excellent aluminum characteristics. This grade has a wide range of mechanical properties and resistance to corrosion. It can be manufactured using common methods and it has excellent workability in the annealed condition. It is welded with all techniques and can be brazed with a furnace. More information can be found in our article on 6061 aluminum alloy.
7xxx alloys
7xxx alloys are the strongest of all wrought alloys, boasting strengths exceeding some steels, which is due to using zinc as its primary alloying element. The inclusion of zinc also decreases its workability and machinability, but its exceptional strength justifies these downsides.
7075 aluminum is a commonly used 7xxx alloy for aircraft applications, mobile equipment, and other highly stressed parts, as it is one of the highest strength aluminum alloys available. It has an excellent weight-to-strength ratio and is ideal for highly stressed parts. In the annealed condition, this grade can be formed and heat-treated if necessary. It can also be welded in place or flash (not recommended for arc and gas). Learn more in our article on 7075 aluminum alloy.
8xxx alloys
8xxx alloys use many different kinds of alloying elements and are reserved for specific requirements such as elevated temperature performance, lower densities, higher stiffness, and other unique properties. They are commonly used in helicopter components, and other aerospace applications, and are experimental in design.
Aluminum Grade Specification & Selection Criteria
The odds are that, given a certain set of needs, there is an aluminum alloy that will fit the situation at hand. Specifying the material properties needed for a project is the first step in choosing the right type of aluminum for the job. Designers must calculate the desired strength, resilience, and manufacturing characteristics of their project first, and then decide which alloy is most suited towards that application. When choosing an aluminum grade, the following are essential factors to be considered:
- Formability or Workability
- Weldability
- Machining
- Corrosion Resistance
- Heat Treating
- Strength
- Typical end-use applications
A good starting place is a general-purpose alloy such as 6061, 3003, or 5052 aluminum, but of course, specific needed properties will call for a more specialized alloy. When in doubt, go with aluminum that is used in similar applications, and/or use the information contained in this article to inform your material selection. Feel free to use our supplemental articles to provide more information on specific alloys, and don’t be afraid to ask an aluminum supplier for guidance; they will most likely know best.