Aluminum Alloy Classification: Wrought, Cast, and Forged Aluminum

Aluminum alloys can generally be classified into three major categories based on their manufacturing method and material characteristics: wrought aluminum alloys, cast aluminum alloys, and forged aluminum alloys.

Each category includes multiple alloy series with different mechanical properties, machinability, corrosion resistance, surface treatment performance, and typical applications.

Wrought Aluminum Alloys

Wrought aluminum alloys are aluminum alloys that are mechanically processed into products such as sheets, plates, bars, tubes, and extrusions through rolling, extrusion, drawing, or forging.

According to their primary alloying elements, wrought aluminum alloys are commonly divided into the 1xxx through 7xxx series.

1xxx Series: Commercially Pure Aluminum

The 1xxx series consists primarily of commercially pure aluminum. These alloys typically contain a very high percentage of aluminum and are known for excellent corrosion resistance, high electrical conductivity, high thermal conductivity, relatively low mechanical strength, and excellent ductility and formability.

However, their turning and milling performance is relatively poor because the material is soft and may tend to stick to cutting tools during machining. Their strength can be moderately increased through strain hardening.

Typical applications include chemical processing equipment, reflectors, heat exchangers, electrical conductors, capacitors, packaging foil, architectural components, and decorative trim.

Surface Etching and Finishing Characteristics

Commercially pure aluminum generally has excellent texture-etching performance. Good surface textures can be achieved using either acidic or alkaline etching systems.

For applications requiring a higher surface roughness, a two-step acid-alkali etching process can produce a highly uniform surface texture.

Among commonly used materials, Korean aluminum generally produces a fine texture, while materials from Taiwan, China often provide a fine and uniform brushed appearance. High-gloss pure aluminum supplied by Sumitomo in Japan can develop a clearly visible brushed texture after etching. Certain German pure aluminum materials can also achieve similar brushed finishes.

Commercially pure aluminum also responds well to chemical and electrolytic polishing and can achieve a highly bright surface after anodizing.

2xxx Series: Aluminum-Copper Alloys

Copper is the primary alloying element in 2xxx-series aluminum alloys. Magnesium and manganese are also commonly added as secondary alloying elements.

Typical alloy systems include aluminum-copper-magnesium, aluminum-copper-magnesium-manganese, aluminum-copper-magnesium-iron-nickel, and aluminum-copper-manganese.

These alloys are heat-treatable and are known for their high strength, which is why they are often referred to as high-strength aluminum alloys or duralumin-type alloys.

They offer good heat resistance and excellent machinability. However, their corrosion resistance is relatively poor, and some grades may be susceptible to intergranular corrosion.

For this reason, 2xxx-series aluminum sheet is often clad with high-purity aluminum or 6xxx-series aluminum-magnesium-silicon alloy to improve corrosion resistance.

Typical Applications

2xxx-series aluminum alloys are especially suitable for components requiring a high strength-to-weight ratio.

Typical applications include truck and aircraft wheel components, truck suspension components, aircraft fuselage skins, and structural components that must retain good strength at temperatures up to approximately 150°C (302°F).

Except for alloy 2219, most alloys in this series have limited weldability. However, many 2xxx-series alloys provide excellent machinability.

2024 Aluminum and Surface Treatment

One of the most widely used grades in this series is 2024 aluminum, which corresponds approximately to Chinese grade 2A12.

After single-step alkaline texture etching, 2024 aluminum sheet can achieve a surface roughness greater than Ra 1.5 while maintaining excellent brightness and surface uniformity.

The anodizing performance of 2xxx-series aluminum is relatively poor. When the anodic oxide film becomes thick, the surface may develop a yellowish tone, making these alloys less suitable for light-colored decorative dyeing.

For aerospace applications, alkaline etching is generally not recommended after degreasing. Acid treatment is preferred for removing the passive surface layer.

Boric-sulfuric acid anodizing or chromic acid anodizing is commonly used, followed by sealing with dilute chromic acid. When light-colored dyeing is required, a modified boric-sulfuric acid anodizing process may be used.

3xxx Series: Aluminum-Manganese Alloys

Manganese is the primary alloying element in the 3xxx series.

These alloys are generally non-heat-treatable, but their strength is approximately 20% higher than that of 1xxx-series aluminum.

Because only a limited amount of manganese, typically around 1.5%, can be effectively added to aluminum, manganese is used as the principal alloying element in only a relatively small number of commercial alloys.

The three most widely used general-purpose alloys in this series are 3003, 3004, and 3105. Chinese grade 3A21 is approximately equivalent to 3003.

3xxx-series aluminum alloys provide higher mechanical strength than 1xxx-series alloys while maintaining good ductility and weldability. Their corrosion resistance is only slightly lower than that of commercially pure aluminum.

Typical Applications

Typical applications include beverage cans, cookware, heat exchangers, storage tanks, awnings, furniture, highway signs, roofing panels, and curtain wall panels.

Texture Etching Characteristics

After texture etching, 3xxx-series aluminum often displays a combination of matte, sand-like texture and brushed linear texture.

Single-step alkaline texture etching provides good decorative results and is commonly used as a pretreatment for nameplates and panels before screen printing.

For applications requiring higher surface roughness, a two-step acid-alkali etching process usually produces both sand-like and brushed textures.

However, due to differences in processing methods among manufacturers, some similar aluminum alloys may show mainly a sand-textured appearance after acid-alkali etching, with very little visible brushed texture.

4xxx Series: Aluminum-Silicon Alloys

Silicon is the primary alloying element in the 4xxx series and can be added to aluminum in relatively high amounts, sometimes reaching approximately 12%.

These alloys are generally non-heat-treatable and are commonly used as welding wire and brazing filler material for aluminum products.

During anodizing, 4xxx-series alloys typically develop a dark gray to charcoal-black surface appearance.

Alloy 4032 has a low coefficient of thermal expansion and high wear resistance, making it particularly suitable for forged engine pistons and other components requiring dimensional stability at elevated temperatures.

5xxx Series: Aluminum-Magnesium Alloys

Magnesium is the primary alloying element in the 5xxx series.

When magnesium is used as the main alloying element, either alone or together with manganese, it produces strain-hardenable alloys with medium to high strength.

These alloys have good weldability, excellent fatigue resistance, and strong corrosion resistance, particularly in marine environments.

5xxx-series aluminum alloys are non-heat-treatable but generally have higher strength than 1xxx- and 3xxx-series alloys.

Typical Applications

Typical applications include construction materials, architectural and decorative trim, cans and can ends, household appliances, streetlight components, marine structures, cryogenic fuel tanks, crane components, and automotive structural parts.

Surface Treatment Performance

The texture-etching characteristics of 5xxx-series aluminum are similar to those of 3xxx-series alloys.

They also provide good anodizing performance. Certain grades can achieve a bright, mirror-like finish after appropriate surface treatment, making them suitable for automotive decorative components.

6xxx Series: Aluminum-Magnesium-Silicon Alloys

6xxx-series aluminum alloys contain magnesium and silicon as their primary alloying elements and are heat-treatable.

These alloys offer medium strength, although their strength is generally lower than that of most 2xxx- and 7xxx-series alloys.

One of the main advantages of the 6xxx series is its balanced combination of formability, weldability, machinability, corrosion resistance, and mechanical strength.

Typical Applications

Typical applications include construction materials, bicycle frames, transportation equipment, bridge railings, welded structures, machinery components, and extruded aluminum profiles.

6061, 6063, and 6463 Aluminum

The most widely used alloys in this series are 6061 and 6063.

6061 aluminum is commonly selected for mechanical components and structural parts because of its good strength, machinability, corrosion resistance, and general-purpose performance.

6063 aluminum is widely used for extruded profiles and architectural components because of its excellent extrusion performance and attractive anodized appearance.

Alloy 6463 was developed for applications such as extruded automotive trim, digital-device frames, and television bezels. After anodizing, 6463 can achieve a bright, mirror-like surface finish.

However, the iron impurity content in the aluminum should be kept as low as possible. Excessive iron content can significantly reduce surface brightness after anodizing.

7xxx Series: Aluminum-Zinc Alloys

Zinc is the primary alloying element in 7xxx-series aluminum alloys, generally at levels ranging from approximately 1% to 8%.

These alloys are heat-treatable and can provide strength levels ranging from medium to extremely high.

Because of their excellent strength-to-weight ratio, 7xxx-series aluminum alloys are widely used in highly stressed structural components.

Typical applications include aircraft structural components, mobile equipment, aerospace parts, high-strength mechanical components, and other applications subjected to high loads and stresses.

Cast Aluminum Alloys

Cast aluminum alloys are specifically designed for manufacturing components through casting processes.

According to their primary alloying elements, cast aluminum alloys can generally be divided into four major categories:

• Aluminum-silicon alloys
• Aluminum-copper alloys
• Aluminum-magnesium alloys
• Aluminum-zinc alloys

Cast Aluminum Alloy Designations

Different countries use different alloy designation systems.

In China, cast aluminum alloys are commonly designated using the letters ZL followed by three digits.

The first digit identifies the main alloy system:

• 1 – Aluminum-silicon alloys
• 2 – Aluminum-copper alloys
• 3 – Aluminum-magnesium alloys
• 4 – Aluminum-zinc alloys

The second and third digits identify the specific alloy within that series.

Key Characteristics of Cast Aluminum Alloys

To produce high-quality precision castings in a wide range of shapes and sizes, aluminum casting alloys generally have several important characteristics.

• Good fluidity, allowing molten aluminum to fill narrow slots, thin walls, and complex mold cavities.
• A relatively low melting point compared with many other metals while still meeting the requirements of most industrial applications.
• Good thermal conductivity, allowing heat from molten aluminum to transfer rapidly into the mold and resulting in relatively short casting cycles.
• Hydrogen and other harmful gases in the molten metal can be effectively controlled through proper melt treatment.
• Low tendency toward hot cracking and hot tearing during casting.
• Good chemical stability and strong corrosion resistance.
• Low tendency to develop surface defects, with castings generally providing relatively low surface roughness, good appearance, and good suitability for subsequent surface finishing.

Advantages and Applications of Cast Aluminum

Cast aluminum alloys provide excellent casting performance and can be used to produce components with complex geometries.

They also offer advantages such as material savings, lower manufacturing costs, shorter production times, and reduced requirements for additional manufacturing equipment.

As a result, cast aluminum alloys are widely used in aerospace, automotive, machinery, and general industrial applications.

Typical products include beams, gas turbine blades, pump housings, brackets, wheels, inlet lips, and engine casings.

They are also commonly used to manufacture automotive cylinder heads, transmission housings, pistons, instrument housings, and supercharger pump housings.

Forged Aluminum Alloys

Forged aluminum alloys are designed to provide good plasticity and formability at elevated temperatures, making them suitable for forging processes.

Many forging aluminum alloys are based on aluminum-magnesium-silicon alloy systems and may also contain copper, magnesium, and silicon as important alloying elements.

Their primary advantage is excellent hot formability, which makes it possible to manufacture large, complex, and high-strength forged components.

Advantages of Aluminum Forgings

Replacing certain steel components with aluminum alloys can significantly reduce the overall weight of mechanical products while maintaining good structural performance.

Because of their excellent strength-to-weight ratio, aluminum and aluminum alloys have become essential materials in aerospace, aviation, automotive, and defense-related industries.

Aircraft, for example, use a large number of forged aluminum components.

Aluminum is also widely used in construction, automotive manufacturing, and packaging, which have become three of the most important application areas for aluminum in modern industry.

Aluminum Forgings in the Automotive Industry

In recent years, automotive aluminum forgings have developed rapidly.

Global warming, energy shortages, stricter fuel-efficiency requirements, and increasing consumer expectations for ride comfort and vehicle handling have accelerated the development of lightweight automotive structures.

Reducing vehicle weight by approximately 10% can reduce fuel consumption by about 6% to 8%.

According to available industry data, aluminum alloy usage in forged and stamped automotive components in the United States has reached approximately 36.3%, while aluminum usage in vehicles produced in Europe and Japan can account for more than 15% of total vehicle weight.

Historically, the amount of aluminum alloy used in vehicles produced in China has been relatively lower.

Automotive industry specialists expect that an increasing number of aluminum alloy forgings will replace traditional steel forgings as vehicle lightweighting continues to develop.

Conclusion

Different aluminum alloy series offer very different combinations of strength, corrosion resistance, machinability, weldability, surface treatment performance, and manufacturing characteristics.

Commercially pure 1xxx-series aluminum is ideal for applications requiring high conductivity and corrosion resistance, while 2xxx- and 7xxx-series alloys are preferred for high-strength structural applications.

3xxx- and 5xxx-series alloys offer good corrosion resistance and formability, while 6xxx-series aluminum provides one of the best overall combinations of strength, machinability, weldability, anodizing performance, and cost.

Cast aluminum alloys are suitable for producing complex shapes efficiently, while forged aluminum alloys are particularly useful for highly loaded structural components where strength and weight reduction are critical.

Selecting the correct aluminum alloy should always be based on the required mechanical properties, manufacturing process, surface finish, corrosion resistance, operating environment, and overall cost of the finished component.