All About Aluminum Casting

Casting is the oldest metallurgical technique. The first functional metal components were accidentally refined copper, from chalcopyrite and chalcocite rocks in a fire. The metal could be cast into ingots in the ashes and hammered into a useful shape. Aluminum casting is a critically important group of methods that allow the manufacture of complex parts. The shaping is usually achieved in a single-stage process from liquid metal to finished or near-finished parts. This article will discuss what aluminum casting is, how it works, its importance, types, and applications.

What Is Aluminum Casting?

Aluminum casting is the process of introducing molten aluminum into a cavity to create a part. Aluminum and many of its alloys have relatively low melting points and low viscosity when molten, yet cool to form strong, rigid solids. A variety of casting processes make use of these properties by forming a heat-proof cavity (in one or two parts), into which molten aluminum is poured. The metal then cools and solidifies, taking on the shape of the cavity it has filled. The cavities used for this vary in material and construction, and the processes have various names.

Aluminum casting is perhaps the most important cluster of technologies and methods in the aluminum product supply chain. Human technology development has gone through seismic shifts over millennia. The bronze age societies were eventually supplanted by those utilizing iron. In turn, iron gave way to steel in the 19th century and steel began to give ground to aluminum in the 1940s. Aluminum is the third most common element in the earth's crust. It’s easy to find its salts in high concentrations—and with energy available, it is straightforward to refine. Aluminum is the key to most areas of technology and aluminum casting is one of the key processes that bring the material into mainstream use and appreciation. The ability to produce net shape-finished parts of high precision, low weight, and moderate strength empowers every manufacturing sector.

Properties of Cast Aluminum

The aluminum used in casting generally refers to its alloys rather than pure aluminum. The properties of which are as follows: 

1. Aluminum alloys offer a range of ductilities, generally lower than pure aluminum metal which is very soft.
2. Ultimate tensile strengths (UTS) vary considerably with the process. Where low porosity is achieved by aggressive degassing of the melt and high-pressure casting, the UTS approaches the optimum for the alloy. Some alloys are developed for greater resilience and others for greater rigidity, implying more tendency to brittleness. Tensile strength is dependent on alloy type, with pure aluminum offering the lowest value. For example, pure aluminum has a tensile strength of 90 MPa while casting alloy A07130 has a value of 221 MPa.
3. Higher porosity castings are severely weakened by gas inclusions/cavities that can be common in less controlled approaches. Porosity is a chronic problem for low-pressure aluminum casting.
4. Aluminum castings offer an exceptional stiffness-to-weight ratio, only beaten by more costly magnesium or titanium parts.
5. Great corrosion resistance is normal in cast aluminum parts.
6. Excellent electrical and thermal conductivity.
7. Generally heat-resistant. Cast aluminum is resistant to heat within the limits of the material, which varies considerably by alloy. For example, AlSi alloys are easily cast, resist heat cracking, and can serve up to 180 °C. For higher temperature resilience, AlCu alloys can handle 350 °C but require simpler shapes and thicker sections to cast well. AlMg alloys, on the other hand, also offer good temperature performance and better castability than AlCu but lower strength.

Durability of Cast Aluminum Parts

Aluminum castings are very tough when designed to exploit the strengths and offset the weaknesses of the process. Cast aluminum is of no use as a bearing surface, or under impact loads and must be carefully designed to handle high and cyclic loads. Aluminum castings suffer from stress cracking, creep, abrasion, and shock loading, all of which are factors to be managed in the product design process. 

Different Types of Aluminum Casting Processes

Aluminum casting processes can be broken down into different types as listed below:

1. Die Casting

Die casting uses a hardened steel, two-part cavity tool, in which molten aluminum is poured or forced. These tools generally have 100,000 to 150,000 cycles of shot life and produce high-accuracy and high-quality parts. Die casting ranges from using simple tools with hand-poured fill-up to completely automated systems for very high volumes.

2. Investment Casting

This process uses a sacrificial positive model of the required part. Before molten aluminum is poured into the plaster form, the wax is vaporized or driven out by baking the plaster tool. The cavity retains its shape and is then filled with molten metal that cools quickly.

In a more up-to-date approach, the positive model used to cast the cavity can be cut from expanded polystyrene, or 3D printed in wax and potentially polymers such as PET and PLA. In the case of plastic “positives”, they must also be burned out of the cavity to leave no residues. This process tends to be reserved for high-value and intricate parts. The tool is destroyed to extract the finished part.

3. Sand Casting

Sand casting uses a stronger pattern part, generally from wood, and then packs this into two box halves, using sand with a binding agent, and talc as a release and separating agent. Cavities are usually filled by hand pouring, though automation is possible. This is usually referred to as sand casting and is widely employed in the manufacture of lower-volume and/or larger shot-weight castings. While good precision is possible, the destruction of the cavity tool makes this a one-shot process.

4. Permanent Mold Casting

This process is analogous to die casting in that two permanent (often cast steel) mold halves are clamped together and the cavity filled, generally by hand pouring. 

5. Lost Foam Casting

Lost foam casting is identical to investment casting. The pattern, however, is made from low-density foamed polymer rather than wax. 

6. Shell Mold Casting

A refinement of basic sand casting uses resin-bonded sand cast around the pattern to form a strong shell. Two shells are fixed together to form a sacrificial (single-use) cavity that is filled with molten aluminum. This process makes casting cavities faster and at a lower cost than ordinary sand casting and is used for medium to large casts needing higher precision.

7. Vacuum Die Casting

Vacuum die casting differs from pressure or gravity die casting in that it uses a reservoir below the casting tool and pulls the molten material up by a vacuum applied to the tool. This results in lower porosity and higher-quality parts and is slowly displacing older die-casting methods.

8. Ceramic Mold Casting

This process casts a silicate slurry around a pattern and then either burns the volatiles from the slurry (Osborn-Shaw process) or bakes them off (Unicast process), leaving a ceramic cavity into which molten metal can be poured. In many regards, this process is analogous to shell mold casting.

9. Plaster Mold Casting

This process is similar to shell mold and ceramic mold casting, but gypsum plaster is used to form the cavity tool.

10. Centrifugal Casting

This process uses a fast-spinning steel cavity tool to cast aluminum. Larger parts with rotational symmetry lend themselves to this process. The tool is spun once filled and centripetal force fills the extremities very cleanly, with no inclusions of porosity. Centrifugally cast aluminum is fine-grained, and defect-free, with improved hardness and strength, compared with other processes. It's used for asymmetrical parts that require the best possible properties and are not cost-sensitive. 

11. Open Mold Casting

Some more primitive casting in aluminum is performed using open cavities made from brick, stone, or steel, or pressed into sand. This is usually a method for producing simple billets for later forge/machining. It can be referred to as a branch of permanent mold casting.

Different Applications of Cast Aluminum

Some examples of the different applications of cast aluminum are:

1. Automotive Parts: Transmission housings, pistons, and steering knuckles.
2. Small Appliances and Home Items: Hand tools, lawnmowers, cookware, and patio furniture types.
3. Medical Devices: Surgical tools, monitor components, hospital bed gearboxes.

Aluminum die-cast parts can be expected to serve indefinitely under optimum conditions. It is common for automotive die-cast aluminum parts, for example, to remain in a functional state after 50+ years in service.

Benefits of Aluminum Casting

Aluminum die casting offers a range of benefits, including:

1. High strength-to-weight ratio.
2. High strength.
3. Low weight.
4. Durability under variable and cyclic loads. 
5. High precision.
6. Easy machining. 
7. High-volume and automated production.
8. High thermal and electrical conductivity.
9. Low corrosion, even in marine environments.

Limitations of Aluminum Casting

Aluminum die casting has limitations, including:

1. Moderate to poor fatigue resistance.
2. Very poor abrasion resistance. 
3. Zero capacity as a bearing surface.
4. Moderate temperature vulnerability. 

Cast Aluminum vs. Aluminum

There is no difference between cast aluminum and aluminum. Cast aluminum is the same base material (aluminum) that has undergone a process. However, due to the various casting methods which exist, it is possible that the cast material will have inclusions and porosity that were not present in the raw feedstock.

Cast Aluminum vs. Forged Aluminum

The primary difference between forged and cast aluminum is the crystalline structure. Casting imposes very little control on the cooling and resultant crystal growth, making crystal structure within cast parts highly variable and disorientated. This makes thicker sections lower strength, as they cool more slowly, forming larger crystals. 

Forging is often performed on cast billets. The upset process disrupts the microstructure by breaking up crystals into smaller units, without creating disjoints between them. This causes the material to become both harder and stronger, to a considerable degree. It also causes a “flow” of material around obstructions and corners which gives the crystal microstructure a “grain” that enhances strength and stress distribution.

Inclusions (non-solution contaminants such as oxide slag) are generally aggregated to some degree in cast aluminum parts, often creating severe weakness. In forged parts, any inclusions (from the billet casting process) are smashed and distributed, reducing their influence. Aluminum castings are capable of great detail and fine sections, whereas forging is a brute-force process best suited to simple shapes.

Summary

This article presented aluminum casting, explained what it is, and discussed its various applications and benefits. To learn more about aluminum casting, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including casting and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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