Aluminum Casting: How is it Done?

Aluminum is one of the most commonly used materials in the manufacturing industry for making parts with complex shapes and geometries.

How is aluminum shaped into such complex geometries?

Aluminum casting is the answer. It is used to produce complex aluminum parts with high accuracy and precision.

So what exactly is aluminum casting? How is it categorized? And which aluminum alloys are suitable for casting applications?

Aluminum casting is a metalworking process in which aluminum or its alloy in molten form is poured into a mold with a predetermined shape and then solidified upon cooling, thereby producing the desired shape. It is generally used to produce complex shapes that otherwise would be difficult to machine.

This article provides a detailed guide on aluminum casting, its types, post-processes, advantages, and limitations.

In the end, I've also discussed some of the best suitable aluminum alloys for casting operations.

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What is Aluminum Casting?

Aluminum casting is the process of melting and shaping aluminum, by using various metal melting tools such as furnaces, crucibles, molds, etc.

The mold needs to be precisely shaped and have a smooth finish, as it will directly affect the shape and surface finish of the final product. 

Depending on the aluminum casting technique, molds are commonly composed of steel or sand.

Initially, a furnace heats the aluminum or its alloy to a molten state, which is then poured into the mold. 

Once the aluminum castings solidify, they are exposed to the environment, resulting in the formation of a corrosion-resistant aluminum oxide layer on the outer surface.

Depending on the requirements, the final product produced by casting may or may not require any further machining process to enhance its surface finish.

Aluminum casting is an excellent DIY metalworking project for hobbyists and you can also perform alloy casting to produce workpieces made of different aluminum alloys.

Types of Aluminum Casting

There are seven types of aluminum casting processes, all of which have broad applications in the automobile, aerospace, defense, and construction industries.

Type of Aluminum Casting	Size Range	Applications
Sand casting	All	Manufacturing engines, metal bars, gears
Pressure die casting	Small	Engine body, wheels, transmission housings, etc.
Shell mold casting	Medium	Engine blocks, cylinder heads, connecting rods
Permanent casting	Medium to large	Gears, suspensions, fuel housings
Investment casting	Small to medium	Weapon, aerospace, automobile components
Centrifugal casting	Small to medium	Jet engine parts and petrochemical furnace tubes
Continuous casting	Medium	Metal bars and pipes

Sand Casting

Sand casting is a technique that uses a mold made of sand mixed with some bonding agent. The mold usually consists of two sections that facilitate easy removal of castings with complex geometries.

The molten aluminum or aluminum alloy, often called charge, is poured through a gating system, which consists of an opening that leads to the mold, where the charge occupies the shape of the mold and solidifies. 

Upon cooling and solidification of the casting, the sand mold can be opened to remove the casting.

This type of casting is cheap and applicable to aluminum and other metals such as cast iron and cast steel.

Pressure die Casting

Pressure die casting of aluminum can be achieved in two ways: high-pressure die casting and low-pressure die casting.

In high-pressure casting, the charge (molten aluminum alloy) is poured into the steel mold, after which a cylinder-piston assembly applies high pressure (around 17,400 lb/in² or 1200 bar) to push the molten aluminum into the mold cavity.

On the contrary, in low-pressure casting, the charge is pumped into the mold through a crucible.

These casting methods result in high-quality parts with tight tolerances and a smooth surface finish that does not require any post-processing.

Generally, high-pressure aluminum casting is used for parts with a maximum wall thickness of around 0.15" (0.4mm), whereas low-pressure aluminum casting is ideal for parts with a wall thickness of around 0.12" (3mm).

Shell mold Casting

Shell molding uses a metallic pattern coated with lubricants. This pattern is heated and placed in a thermoset sand chamber with a resin binder.

This mixture of sand and thermosetting phenolic resin, when comes in contact with the metallic pattern, forms a shell that ultimately constitutes the mold.

Generally, for complex shapes, the shell mold consists of two parts joined together for casting application.

Subsequently, the charge is poured into the mold and solidified, after which the shell is broken to attain the casting.

Shell mold aluminum casting produces parts with a high surface finish and tight tolerances, making it ideal for prototyping.

However, as this type of casting requires a new mold for every casting, it is not recommended for large-scale production applications.

Permanent Mold Casting

Permanent mold aluminum casting, also called metal mold casting, uses metallic molds, similar to pressure die casting. 

The charge is poured into the mold under the action of gravity (gravity casting), due to which the pouring speed is slow. 

As the molds are made of metal, they have a longer service life and lead to higher cooling and solidification rates. 

This casting method, coupled with post-processing and additional surface heat treatment, results in parts with superior surface quality and tight tolerances.

However, this type of casting is not suitable for producing aluminum parts with complex geometry, and the heavy weight of metal molds limits them in terms of size.

Investment Casting

Investment casting consists of a wax pattern. This pattern is coated with layers of refractory ceramic material until it reaches the desired thickness (around 10-15mm).

The pattern is then heated to melt the wax and pour it out, resulting in a ceramic mold with a high surface finish.

After attaining the ceramic mold, the molten aluminum is poured into the mold, solidified, and collected by breaking the mold.

This method results in a smooth and accurate finish and is suitable for the mass production of parts with complex geometries.

Centrifugal Casting

Aluminum centrifugal casting uses a metal, plaster, or graphite mold. 

A centrifugal casting machine rotates the mold at high speed, resulting in an even distribution of molten aluminum along the walls of the mold.

The mold is rotated until the casting takes up the shape of the mold and solidifies under the action of centrifugal force.

This type of casting is generally used to produce hollow workpieces that are symmetric along an axis.

A vertical casting mold rotates about the vertical axis, while a horizontal casting mold rotates about the horizontal axis.

The casting obtained through this technique is highly accurate, with tight tolerances and a smooth surface finish.

Continuous Casting

Aluminum continuous casting is a process that involves pouring charge into a mold surrounded by a water cooling system.

This results in quick solidification of the casting, thereby facilitating a continuous casting process, where the molten metal is poured from the top and solid casting is achieved from the bottom.

Due to higher cooling rates, this technique is suitable for bulk production of identical parts. Moreover, this process can be automated, leading to higher productivity.

The casting is accurate and has a fine granular structure, which means it possesses high strength and ductility.

This casting method is very efficient and commonly used in aluminum foundries.

Post-processes to Enhance the Quality of Cast Aluminum

There are different ways to post-process and enhance the quality of aluminum castings.

Machining

Filing the surface or grinding aluminum castings on a high-speed grinding wheel reduces any unevenness and enhances the surface finish of the product.

Heat Treatment

Heat treatment processes like annealing can increase the ductility and strength of castings by reorienting their grain structures and distributing them evenly.

Moreover, aluminum alloys that cannot be heat treated are cold-worked by either rolling or drawing, which significantly reduces any defects in the microstructure and increases the strength by realigning the molecular structure.

Coatings

Aluminum castings already have an aesthetic reflective surface due to a corrosion-resistant oxide layer. However, additional coatings can further improve the visual appeal and mechanical properties.

PVDF coatings

PVDF (Polyvinylidene Fluoride) coatings are solvent-based paints that have high corrosion resistance but can be scratched easily. 

These coatings are UV resistant and can enhance the aesthetic value of aluminum casting.

Liquid paints

Liquid paints beautify the casting’s surface and prevent the wear caused by environmental factors.

They are the cheapest coatings, making them popular among DIY users.

Powder coatings

Powder coatings provide excellent resistance against weathering and smoothen the surface finish of aluminum castings.

Anodizing

Anodizing is a chemical process that thickens the oxide layer on the surfaces of the castings, further enhancing corrosion resistance.

This additional coating is longer lasting and self-repairing, making it popular for industrial applications.

Advantages of Aluminum Casting

The aluminum casting process has many advantages.

High strength

Aluminum castings possess a fine-grain texture resulting in high ductility and strength, making it ideal for machining applications.

Less Machining and Low Cost

Aluminum casting produces components that would otherwise be difficult to machine, even on CNC machines.

These machining operations are more expensive, time-consuming, and can induce unwanted stress in the component.

Aluminum casting minimizes the risk of unwanted stress development, thereby producing the required part with a high surface finish and low cost.

Tight Tolerance

During the casting process, the molten aluminum solidifies in a closed compact mold, due to which the casting obtained is highly accurate with tight tolerances.

Apart from that, the castings have a fine granular structure and a smooth surface finish, which can be further improved through post-processing.

Ability to Produce Complex Shapes

To obtain the desired complex geometry in the casting, you can shape and prepare the mold accordingly before use.

In this way, you can conveniently obtain complex parts.

Generally, shell mold and investment casting are recommended for producing parts with complex geometries.

Limitations of Aluminum Casting

Need for Post-processing

The aluminum parts produced by casting operations are prone to cracks and high porosity.

As a result, these parts require further post-processing operations like grinding, polishing, coating, etc. to enhance their surface finish.

Moreover, the sprue, which is the large-diameter channel through which the charge enters the mold, also consists of solidified charge, which must be machined and removed.

Voids and Porosity

During the cooling and solidification process, some gases and water vapor get entrapped in the casting, leading to void formation and porosity, which reduce the strength of the material. 

Moreover, fast and non-uniform cooling can further increase these defects. Therefore, it is important to ensure a uniform cooling rate throughout the solidification process.

High Cost of Certain Casting Processes

Although aluminum casting is relatively cheaper when compared to machining operations, it still requires equipment and running expenditures that may not be feasible for DIY users.

Special casting methods such as centrifugal casting and investment casting require special tools and materials to perform the casting operation, which further increases the overall cost of the process.

However, for complex requirements, you can outsource the project to professional aluminum casting services like US Aluminum Castings.

High Risk Factor

The casting process can be dangerous as it requires the handling of pots containing molten metal and hot surfaces.

Negligence and careless approach can cause severe or fatal injuries to the operator, and therefore, it is important to wear safety gloves and follow safety protocols.

Inaccuracies in hollow parts

Castings with hollow parts may possess rough inner surfaces that may not be dimensionally accurate.

Post-processing such parts may not always be possible, especially if the part consists of complex geometry with small internal dimensions.

Aluminum Alloys Suitable for Casting

There are several properties that Aluminum Alloys must possess to be suitable for casting.

Aluminum Alloy Series	Principal Alloying Element
1xyz	99 % pure Aluminum
2xyz	Aluminum + Copper
3xyz	Aluminum + Manganese
4xyz	Aluminum + Silicon
5xyz	Aluminum + Magnesium
6xyz	Aluminum + Silicon + Magnesium
7xyz	Aluminum + Zinc

These aluminum alloys possess adequate corrosion resistance, high strength, good machinability, weldability, and castability. 

Good castability means that the solidifying metal inside the mold does not develop cracks, porosity, or voids and easily flows when in a molten state. 

Al-Si casting alloys (containing more than 4% Si), Al–Cu, and Al–Zn–Mg–Cu alloys are widely common due to their high castability.

The automobile industry widely uses Al-Si alloys, whereas the 2xyz series of aluminum alloys are used in the aerospace industry.

5xyz alloys possess high corrosion resistance properties, making them ideal for the shipbuilding industry.

Frequently Asked Questions (FAQ)

How can the strength of an aluminum casting be enhanced?

The strength of aluminum casting can be enhanced by cold working, alloying molten aluminum metal, and performing heat treatment operations. 

What common defects occur in aluminum casting?

Common defects in aluminum casting include porosity, shrinkage voids, and defects resulting from damaged molds or molten metal not sufficiently heated.

Is cast aluminum prone to rusting?

No, cast aluminum is not prone to rusting because of the protective oxide layer on its surface.