From the smartphone in your pocket to the airplanes flying overhead, aluminum is playing a significant role in modern engineering and manufacturing. It provides several properties like light weight, strength, and corrosion resistance. However, not all aluminum is created equal. The metal is typically mixed with other elements to form alloys, each with distinct properties. This guide will discuss the different types of aluminum in the rapid prototyping industry.
Types of Aluminum: Wrought and Cast Aluminum
All aluminum alloys can be divided into 2 primary categories: wrought or cast aluminum. The difference lies in how they are processed into their final form.
Wrought Aluminum
Wrought alloys are mechanically worked, rolled, extruded, or forged to achieve the desired shape and enhanced mechanical properties. This working process aligns the grain structure of the metal, resulting in higher tensile strength and ductility compared to cast aluminum alloys. Wrought aluminum alloys are the most common type, accounting for the most aluminum products.
They are identified by a 4-digit numbering system (e.g., 7075, 6061) established by The Aluminum Association.
- First digit: Indicates the principal alloying element or series.
- Second digit: Signifies a modification of the original alloy.
- Third and fourth digits: Identify the specific alloy within the series.
Cast Aluminum
Cast alloys are produced by melting the aluminum and pouring it into a mold to create a specific shape. This method is efficient for producing complex geometries such as engine blocks or transmission housings. They have lower tensile strength than their wrought counterparts, cast aluminum alloys are cost-effective for high-volume production.
They are identified by a 3-digit system with a decimal point (e.g., 356.0).
- First digit: Indicates the alloy series.
- Second and third digits: Identify the specific alloy.
- Digit after the decimal: Indicates the form (casting or ingot).
| Feature | Wrought aluminum | Cast aluminum |
| Production method | Mechanically worked (rolled, extruded, forged) | Melted and poured into a mold |
| Tensile strength | Higher | Lower |
| Formability | Excellent | Lower |
| Surface finish | Smoother and more consistent | Can be rougher without secondary processing |
| Complexity of shape | Better for simpler, uniform shapes | Excellent for complex, intricate shapes |
| Cost | Can be higher for complex parts | More cost-effective for high-volume, complex parts |
| Typ. applications | Structural parts, sheet metal, extrusions | Engine blocks, pistons, machinery housings |
Grades of Wrought Aluminum Alloys
The wrought alloy series is the most extensive and is broken down by the primary alloying element, which dictates the material’s core characteristics.
1000 Series
This series is comprised of at least 99% pure aluminum, with trace amounts of iron and silicon as the main impurities. These alloys cannot be strengthened by heat treatment; instead, they are strengthened through strain hardening or cold working. Their purity gives them exceptional resistance to chemical attack and weathering, along with superior electrical and thermal conductivity. Aluminum 1100 is a commercially pure grade known for its excellent workability, making it ideal for forming complex shapes. It is widely used in the chemical and food processing industries for storage tanks and containers. Aluminum 1350 is a specific grade with controlled purity to guarantee high electrical conductivity, making it the standard for electrical busbars and wiring.
Advantages:
- Exceptional corrosion resistance: Its high purity makes it highly resistant to chemical attacks and weathering.
- Excellent workability: It’s very soft and ductile, making it easy to form into complex shapes.
- High Conductivity: It has superior electrical and thermal conductivity, making it ideal for related applications.
Disadvantages:
- Very low strength: It has the lowest mechanical strength of all aluminum series, limiting its structural use.
- Non-heat-treatable: Its strength can only be increased through cold working (strain hardening).
- Poor machinability: Its softness can make it “gummy” and difficult to machine cleanly.
2000 Series
Copper is the primary alloying element in this series, allowing these alloys to gain significant strength through solution heat treatment and precipitation hardening. They have one of the best strength-to-weight ratios of any aluminum alloy, rivaling that of mild steel. However, the presence of copper makes them more susceptible to corrosion, often requiring a protective coating or cladding of a purer aluminum alloy (Alclad). They are also notoriously difficult to weld using fusion methods, as this can lead to cracking. Aluminum 2024 is the quintessential aircraft alloy, prized for its high strength and excellent fatigue resistance. It is the material of choice for structures under tension, such as fuselages and wing skins. Aluminum 2011, with additions of lead and bismuth, is known as a “free-machining” alloy, designed for producing intricate parts on automatic lathes.
Advantages:
- Very high strength: After heat treatment, its strength can be comparable to that of mild steel.
- Excellent strength-to-weight ratio: It provides high strength for a very low weight, which is crucial for aerospace.
- High fatigue resistance: It can withstand repeated stress cycles without failing, making it durable.
Disadvantages:
- Poor corrosion resistance: The presence of copper makes it susceptible to corrosion, often requiring protective coatings.
- Poor weldability: It is difficult to weld using fusion methods, as it is prone to cracking.
- Higher cost: These are specialized, high-performance alloys that are more expensive than general-purpose grades.
3000 Series
Manganese is the main alloying element in this series, providing a moderate increase in strength over the 1000 series. These alloys are non-heat-treatable and rely on strain hardening for strengthening. They retain the excellent formability and corrosion resistance of pure aluminum, making them a popular choice for general-purpose applications. Aluminum 3003 is the most widely used of all aluminum alloys. It is about 20% stronger than the 1100 grade, making it a practical and economical material for cookware, storage tanks, and heat exchangers. Aluminum 3004, with the addition of magnesium, is stronger than 3003 and is the primary alloy used to form the bodies of aluminum beverage cans.
Advantages:
- Good formability: It’s highly workable and can be easily formed into various shapes.
- Good corrosion resistance: It retains much of the corrosion resistance of pure aluminum.
- Economical: It offers a good balance of properties at a relatively low cost for general-purpose use.
Disadvantages:
- Moderate strength: Its strength is only slightly higher than the 1xxx series.
- Non-heat-treatable: Like the 1xxx series, it can only be strengthened through cold working.
4000 Series
The addition of silicon significantly lowers the melting point of aluminum and increases its fluidity when molten, without making the metal brittle. This unique characteristic makes the 4000 series ideal for use as a joining material. Most alloys in this series are non-heat-treatable, though some with added copper or magnesium can respond to heat treatment.
Alloy 4043 is one of the most popular filler alloys used for welding and brazing, particularly for joining 6xxx series alloys. Its excellent flow characteristics ensure a clean, strong bond. Alloy 4032, which also contains magnesium and nickel, is heat-treatable and has a low coefficient of thermal expansion, making it a preferred material for high-performance forged engine pistons.
Advantages:
- Excellent for welding/brazing: Its low melting point makes it an ideal filler material for joining other aluminum alloys.
- Good fluidity: It flows very well when molten, ensuring a complete and strong bond.
- Low thermal expansion: Certain alloys in this series (like 4032) are valued for their dimensional stability at high temperatures.
Disadvantages:
- Not for structural use: It is generally not used for primary structural components.
- Low ductility: It is more brittle than many other aluminum series.
- Poor anodizing finish: The high silicon content can cause the surface to turn a dark gray or black after anodizing.
5000 Series
Magnesium is the primary alloying element and provides a powerful combination of strength, ductility, and corrosion resistance. These alloys are strengthened through strain hardening and solid solution strengthening, and they possess the highest strength of all non-heat-treatable aluminum alloys. They are particularly resistant to corrosion in marine environments.
Aluminum 5052 is a highly versatile grade with excellent formability and durability. It is a go-to material for marine applications, fuel tanks, and general sheet metal fabrication. Aluminum 5083 offers even higher strength and is used for more demanding applications such as shipbuilding, rail cars, and pressure vessels.
Advantages:
- Excellent corrosion resistance: It is highly resistant to corrosion, especially in saltwater environments.
- High strength (Non-heat-treatable): It has the highest strength of all the non-heat-treatable aluminum alloys.
- Good weldability: It can be readily welded, retaining much of its strength in the weld zone.
Disadvantages:
- Susceptible to stress corrosion cracking (SCC): Prolonged exposure to temperatures above 150°F (65°C) can make it vulnerable to SCC.
- Non-heat-treatable: Strength can only be increased through strain hardening.
- More Difficult to Extrude: It is less suitable for creating complex extruded shapes compared to the 6xxx series.
6000 Series
This series contains both magnesium and silicon, which together form magnesium silicide. This compound allows the alloys to undergo solution heat treatment and artificial aging (precipitation hardening) to significantly increase their yield strength. These grades are known for their excellent extrudability, good strength, formability, and corrosion resistance. However, they can be sensitive to cracking during welding. Aluminum 6061 is one of the most versatile and widely used heat-treatable alloys. It boasts a great combination of mechanical properties and is used for a vast range of products, including bicycle frames, structural components, and scuba tanks. Aluminum 6063 is known as the “architectural alloy” for its superior surface finish and high corrosion resistance, making it ideal for extrusions like window and door frames.
Advantages:
- Heat-treatable: Its strength can be significantly increased through heat treatment.
- Excellent for Extrusions: It is the easiest alloy series to extrude into complex shapes and has a great surface finish.
Disadvantages:
- Moderate strength: It is not as strong as the 2xxx and 7xxx series.
- Reduced strength after welding: The area around a weld loses a significant amount of its strength until it is heat-treated again.
- Prone to cracking: Certain welding procedures can lead to cracking if not done correctly.
7000 Series
Zinc is the primary alloying element in this series, often combined with magnesium and copper. These additions allow the 7xxx series to be heat-treated to the highest strengths of any aluminum alloy. Their exceptional strength-to-weight ratio makes them indispensable in high-performance applications. However, this high strength comes at the cost of lower fracture toughness and poor resistance to stress corrosion cracking, though newer tempers have improved these properties. They are also difficult to weld. Aluminum 7075 is the benchmark “aircraft grade” alloy, used for highly stressed parts like wing spars and landing gear. Aluminum 7050 is a more advanced alloy that offers an improved balance of strength, fracture toughness, and corrosion resistance, particularly in thick sections, making it ideal for aircraft fuselage frames and bulkheads.
Advantages:
- Highest strength: These are the strongest of all aluminum alloys, with strength levels that can surpass some steels.
- Excellent strength-to-weight ratio: It offers the best performance for applications where weight and strength are critical.
- Good machinability: It can be machined to a very high standard and finish.
Disadvantages:
- Poor weldability: It is very difficult to weld and is not recommended for fusion welding methods.
- Higher cost: It is one of the most expensive aluminum alloys due to its alloying elements and processing requirements.
- Lower corrosion resistance: It is more susceptible to corrosion and stress corrosion cracking than the 5xxx and 6xxx series.
Common Grades for CNC Machining
For Computer Numerical Control (CNC) machining, an alloy’s properties must allow it to be cut cleanly without excessive tool wear.
| Alloy | Relative Strength | Machinability | Corrosion Resistance | Weldability | Typ. Application |
| 6061-T6 | High | Excellent | Excellent | Good | General structural & machined parts |
| 7075-T6 | Highest | Good | Fair | Poor | Aerospace, high-stress components |
| 2024-T4 | Very High | Good | Poor | Poor | Aircraft structures, high-fatigue parts |
| 5052-H32 | Medium | Fair | Excellent | Excellent | Marine components, electronic chassis |
| MIC-6 | Low | Excellent | Good | Fair | High-precision tooling, base plates |
Common Grades for Sheet Metal Fabrication
For sheet metal fabrication, which involves bending and forming, an alloy’s ductility is paramount to prevent cracking.
| Alloy | Relative Strength | Formability | Corrosion Resistance | Weldability | Typ. Application |
| 5052-H32 | Medium | Excellent | Excellent | Excellent | Marine parts, enclosures, tanks |
| 6061-T6 | High | Fair | Excellent | Good | Structural applications requiring bending |
What is the Strongest Type of Aluminum?
Generally, the 7000 series alloys are the strongest commercially available aluminum alloys. Aluminum 7075-T6 (a specific heat treatment) provides a tensile strength that exceeds that of many types of steel, while being only a third of the weight.
However, “strongest” does not always mean “best.” The extreme strength of 7000 series alloys comes with trade-offs:
- Lower corrosion resistance: They are more susceptible to corrosion than 5000 or 6000 alloys.
- Poor weldability: They are difficult to weld using conventional methods.
- Higher cost: They are more expensive to produce.
The choice of alloy is always an engineering decision that balances the need for strength with other critical factors like durability, manufacturability, and cost.
