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Basic Knowledge The Role of Aluminum in Light Metal Casting

Author / Editor: Simon Morrison / Nicole Kareta

Aluminum is one of the most valuable raw industrial materials due to its versatility, low weight, and high durability. It is used to produce castings for a huge range of products and components. In this article, we’ll take a closer look at the history of aluminum production, the properties of aluminum, and why aluminum plays a crucial role in light metal casting.

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The versatility and strength of aluminum and its alloys allow it to be cast using all current metal casting processes.
The versatility and strength of aluminum and its alloys allow it to be cast using all current metal casting processes.
(Source: gemeinfrei / Unsplash)

The Aluminum Industry Worldwide

Currently, the main players in the worldwide aluminum industry include Russia’s UC RUSAL, the Aluminum Corporation of China (CHALCO), the UK’s Rio Tinto, and the US-based Alcoa Corporation. China is far and away the biggest global market for aluminum. With the huge amounts of urbanization and industrial development being undertaken by China, they are expected to remain both the world’s biggest consumer and producer of aluminum for years to come.
As well as the increased demand for aluminum from the Chinese market, worldwide factors such as the rise of electric vehicles, the increased production of solar panels, and the demand for packaged foods have seen the consumption of aluminum skyrocket.

However, recent industry growth has been hampered because of the impact of the COVID-19 pandemic. Many manufacturing sectors have slowed output due to shortages of workers and decreased demand from the automotive and aerospace industries. Future growth is also expected to be restrained by the introduction of more stringent environmental regulations and tighter emissions standards.

Nevertheless, demand for aluminum by packaging companies and the automotive sector looks set to remain strong. According to recent industry reports, by 2027, the value of aluminum per metric ton is expected to climb to USD1894.00 and the worldwide aluminum industry is projected to reach an overall value of USD242.44 billion.

Aluminum Production

Although human beings were making use of clays containing aluminum oxides in ancient times, pure aluminum was not produced in an industrial sense until the late 1800s. The first successful act of separating pure aluminum from ore was invented by the Danish chemist Hans Christian Ørsted in 1825. This process was then further developed by German chemist Friedrich Wöhler. However, these methods could only produce small amounts of aluminum, which by 1852 was priced at more than twice the cost of gold.

In 1886, American student Charles Hall and French engineer Paul Heroult independently devised a process of extracting aluminum from aluminum oxide via electrolysis. This method could produce large quantities of aluminum, but production was limited due to the vast amounts of electricity required.

The next breakthrough in the industrialization of aluminum production occurred when Austrian engineer Karl Josef Bayer created a chemical method of extracting naturally occurring alumina from bauxite. Bayer’s chemical process and the electrolysis method devised by Hall and Herout form the basis of modern aluminum production.

Most of the aluminum produced today is derived from bauxite ore. The ore is first processed using Bayer’s chemical method to produce alumina (aluminum oxide). The alumina then undergoes a smelting process using the electrolysis method developed by Hall and Heroult. The end product is pure aluminum. Typically, it takes approximately five tons of bauxite to produce two tons of alumina which can then be smelted into one ton of aluminum.

Aluminum production is incredibly resource intensive. Mining bauxite and refining it into aluminum requires large amounts of both electricity and water. To produce one ton of aluminum takes approximately 14,000 kWh. Without the use of renewable energy sources, aluminum production can have a significant negative impact on the environment. Aluminum is, however, extremely recyclable, which does offset the adverse effects of production somewhat.

Global aluminum production has been steadily increasing throughout the last decade. Recent data shows that China remains the world’s top aluminum producer. India and Russia take second place, followed by Canada, the United Arab Emirates, and Australia as top aluminum-producing countries. Although the United States has listed aluminum as a critical metal, it produces less than half the amount of aluminum that it consumes.

The Properties of Aluminum

Aluminum is one of the most abundant metals on the planet, forming up to 8 % of the earth’s crust. Over 300 compounds and minerals contain aluminum. After oxygen and silicon, aluminum is the third most common chemical element. Despite its overall abundance, pure aluminum rarely occurs in nature.

It is the unique properties of aluminum that make it such a useful and constantly in-demand metal. Aluminum is the second most malleable metal and the sixth most ductile. It is incredibly lightweight, just one-third of the weight of copper or steel. The density of aluminum is 2.70 g/cm3 when measured by gravity in comparison to water. Using the same comparison, iron has a density of 7.87 g/cm3.

On its own, pure aluminum is not particularly strong. However, increased tensile strength can be achieved by adding alloying elements such as manganese, copper, or silicon. Pure aluminum has a tensile strength of 90 MPa. The addition of alloys can increase this to 690 MPa. Aluminum’s tensile strength increases in lower temperatures, unlike steel which tends to become brittle when exposed to cold weather for long periods.

When aluminum is exposed to air, a layer of aluminum oxide forms. This oxidization makes aluminum highly resistant to corrosion. Aluminum also has excellent resistance to most acids but is less resistant to many alkalis. The corrosion-resistant properties of aluminum can be enhanced by painting or anodizing the surface.

Pure, unalloyed aluminum has a melting point of 1220 °F and a boiling point of 4,478 °F. Compared to other metals, aluminum has an advantageous thermal conductivity level, almost three times the amount of steel.

Aluminum is also non-toxic and completely odorless. Its high malleability and ductility mean it can be pressed incredibly thin and is easily bent into any shape.

The high electrical conductivity of aluminum makes it an ideal conductor. Although aluminum’s conductivity rate is not as high as copper, its low density means that it can conduct double the amount of electricity per weight as copper.

Aluminum has excellent reflective properties. It can reflect up to 80 % of visible light and can also be used to reflect radiation.

Aluminum Properties at a Glance

  • Up to 8% of the earth’s crust
  • Found in over 300 compounds and minerals
  • The third most common chemical element
  • The second most malleable metal
  • The sixth most ductile metal
  • Density of 2.70 g/cm3
  • Tensile strength of 90 MPa, up to 690 MPa when alloyed
  • Melting point of 1220°F
  • Boiling point of 4,478°F
  • Highly corrosion-resistant
  • Good thermal conductivity
  • High electrical conductivity
  • Excellent reflective properties
  • Non-toxic
  • Odorless
  • 100% recyclable

The Advantages of Aluminum for Metal Casting

Aluminum alloy castings are often seen as having a range of advantages. The versatility of aluminum allows it to be used to produce a wide range of products and components.

The high malleability of aluminum makes it possible to produce aluminum castings that are near net shape (NNS). Manufacturers can forge complex castings with high dimensional accuracy in a wide range of geometric shapes.

Aluminum castings are much lighter than castings made from other metals. Despite their light weight, the alloying process ensures that an aluminum casting will remain hard and retain a good strength to weight ratio. Many aluminum alloys are stronger than steel.

In comparison to any other alloy, aluminum alloy castings can withstand the highest operating temperatures. Aluminum dissipates heat quickly, which increases safety and speeds up production times.

Castings made using aluminum alloys are particularly resistant to corrosion. They provide excellent shielding against EMI (Electromagnetic Interference) and Radio Frequency Interference (RFI). Aluminum castings also have high electrical conductivity.

The reflective properties of aluminum mean that aluminum alloy castings can be used to create high-end products that have clean surface finishes. The pleasing aesthetic characteristics of aluminum alloy castings can be further enhanced by anodizing the product or by adding other coatings or finishes.

The Disadvantages of Aluminum for Metal Casting

While there are many advantages to using aluminum alloy castings, there are also several drawbacks. Aluminum does have some typical internal defects that may cause an engineer or manufacturer to think twice about using it for their casting processes.

The first drawback of using aluminum as a casting material is its cost. Although it is a cheap metal overall, aluminum is available in a wide range of alloys, each of them having its own price point. Aluminum alloys can often be more expensive than carbon steel, for example.

Aluminum dissipates heat quickly and has a low melting point. While these attributes can be seen as plus points, they may also be viewed as negatives. The low melting point means that the aluminum casting process can result in spills that can damage equipment. Because aluminum shrinks when it cools, cracks and breakages can occur.

Another property of aluminum that can be seen as both an advantage and disadvantage, is its light weight. If an aluminum cast is to be used as a weight-bearing product, it may have to be further engineered post-production to ensure it meets requirements.

Although solid aluminum is not porous, liquid aluminum is able to hold large quantities of gas. This can result in bubbles forming in the metal during the cooling process. These gas bubbles can weaken the overall strength and reliability of a component or product.

Types of Aluminum Alloys

When compared to other metals, aluminum in its purest form lacks hardness and strength. Even so, pure aluminum does have a range of applications, but it is usually alloyed to increase its overall strength.

Aluminum alloys are created by adding other elements to pure aluminum. The most common elements used to create aluminum alloys are copper, silicon, manganese, magnesium, tin, and zinc. By weight, these elements can make up to 15 % of the alloy.

Other elements that are less commonly added to aluminum include lithium (used by the aerospace industry), as well as smaller amounts of zirconium, chromium, titanium, boron, vanadium, bismuth, and lead. Iron may also be found in aluminum alloys, often as an impurity.

Aluminum alloys are designated with a four-digit number. The first number represents the class of alloy to which they belong. In total, there are nine categories of aluminum alloy with the ninth class being unused at present.

The designation series of aluminum alloys are as follows:

  • 1xxx – The 1 series is used to designate the purest types of aluminum, containing 99.99% percent aluminum.
  • 2xxx – The 2 series has copper as the principal alloying element. Although extremely strong, these alloys are not as resistant to corrosion as others.
  • 3xxx – 3 series alloys contain manganese combined with a smaller amount of magnesium. They are very malleable and often used to make cans or cooking utensils.
  • 4xxx – Silicon is the main element in the 4 series. These alloys have a lower melting point and are used to produce welding wire and filler material.
  • 5xxx – With high tensile strength and good corrosion resistance, 5 series alloys are often used to make bridges and ships. The main element in 5 series alloys is magnesium.
  • 6xxx – The 6 series contains silicon and magnesium, which combine to form magnesium silicide. These alloys possess good corrosion resistance and moderate strength. They are often used to make frames for the automotive and construction industries.
  • 7xxx – The main alloying element in the 7 series is zinc. This creates an aluminum alloy that is exceptionally strong and is often found in sporting equipment and used by the aerospace industry.
  • 8xxx – The 8 series consists of combinations of other less common elements, such as iron and lithium. These alloys are often used to make wire or aluminum foil.
  • 9xxx – Not currently in use as a designation.

Typically, the aluminum alloys used in industrial casting are the 3xxx, 5xxx, and 6xxx series.

Aluminum Casting Technologies and Processes

The versatility and strength of aluminum and its alloys allow it to be cast using all current metal casting processes.

Mold casting is the most common form of casting used by foundries. It is the oldest form of casting metal and is often said to be the easiest. There are three common types of mold casting methods: Sand casting, die casting, and permanent mold casting.

Sand casting uses re-useable patterns to create sand molds. Green sand molds consist of sand, clay, and moisture, while dry sand molds are made up of sand and synthetic binders. Liquid aluminum is poured into the sand mold, which is then broken away once the aluminum cools and hardens. Sand casting is inexpensive and is usually used for smaller production runs.

In die casting, molds are made from steel or cast iron. They are permanent, so are not destroyed once the aluminum cools. Liquid aluminum is injected into the mold using pressure. There are many different die casting methods, including low pressure die casting, high pressure die casting, gravity die casting, vacuum die casting, and squeeze casting. High pressure die casting accounts for approximately 50 % of all aluminum alloy production. Low-pressure casting is used in roughly only 20 % of aluminum casting, but its use is on the increase. Because die casting uses a pressurized injection process, it is the fastest casting method.

Permanent mold casting, like die casting, does not involve breaking the mold away at the end of the process. Instead, reusable metal molds are used. These molds are typically larger than the molds used in die casting. The molds are filled with liquid aluminum by gravity injection. This process produces the highest-strength products.

To enhance the production speeds and improve product quality, new casting technologies are continuously being developed. While the basic premise of the process remains the same, engineers are creating new methods of injecting aluminum and improving on existing casting techniques. Molds that allow for computerized devices to be implanted during the forging process are now extensively used. Hybrid casting and fiber integration processes now allow for fiber composites to be combined with aluminum alloy products during manufacturing.

Other expected advances in foundry and casting technology include the increased use of autonomous systems and digitalized machinery. 3D metal printing techniques like Direct Metal Laser Sintering (DMLS) have yet to claim a significant market share but are also becoming popular. Markings on metal castings have traditionally been made using barcodes or data matrix codes (DMC). There is now a movement towards contact free, electronic Radio Frequency ID (RFID) technology. More environmentally conscious methods of operation are also being developed, including using batteries to store power and more efficient molds that reduce waste materials.

Post-Processing Methods of Aluminum Castings

There are a variety of post-processing treatments that are used to improve the properties of aluminum castings. These treatments can be employed to remove defects, enhance the surface of the product, or create internal properties that the untreated product does not have. There are four main post-processing categories:

  • Deburring
  • Heat treatment
  • Vacuum treatment
  • Surface treatment

Deburring

Burrs are small imperfections caused by the casting process. If burrs are not removed during post-processing, then the finished product will not have a smooth surface. There are many methods used to deburr aluminum. Manual or artificial deburring uses handheld tools to remove burrs. Grinding deburring uses vibrations or sandpaper. Burrs can be removed using extreme heat or by freezing the metal. There are also chemical deburring methods, high-pressure water methods, magnetic deburring, ultrasonic deburring, and the use of electrolysis. The most common deburring method for aluminum is die deburring, where a specialized production die is used in conjunction with a punch tool.

Heat treatment

Heat treatment is used to harden aluminum alloys or to make them more malleable for further machining. Annealing is a process where a product is heated almost to melting point, then slowly cooled to make it more pliable and less brittle. Similarly, solution heat treatment uses a rapid cooling process to make aluminum more pliable. To harden aluminum products, engineers use techniques such as quenching and natural or artificial age hardening. To avoid stress cracks in the finished structure, engineers will often use the stress relieving method of heating a product at lower temperatures, then allowing it to cool slowly.

Vacuum treatment

Vacuum treatment is used to avoid gas buildups in an aluminum cast. The cast product is placed in a vacuum impregnation chamber where air is removed via a deep vacuum. The path formed by the vacuum is then sealed, any unwanted sealant is removed, and the finished product is left to harden.

Surface treatment

Surface finishing can improve the corrosion resistant properties of aluminum and is also used to enhance its aesthetic appeal, reflectivity, and wear resistance. Common surface treatments include electrochemical methods such as anodizing and chemical treatments like degreasing or etching. Paint or powder-based coatings are also commonly added to provide protection from the elements or to enhance the look of a finished product.

Aluminum Casting Applications

Aluminum is used by almost every industry across the globe. Since the 1940’s there has been a continuous high demand for aluminum from diverse end user sectors such as the construction industry, pharmaceuticals, defense contractors, the automotive industry, component manufacturers, packaging manufacturers as well as power producers, and the shipbuilding, engineering, and aerospace industries.

More than half of the components used in today’s automobiles are made from aluminum casting. Aluminum is also widely used to build parts for trains, aircraft, and ships. Automobiles and aircraft can reduce their fuel consumption and their carbon emissions by using aluminum components. Because aluminum is lightweight, reflects radiation, and has high tensile strength under pressure, NASA incorporates aluminum components throughout their spacecraft.

The construction industry is a major consumer of aluminum casting products. The building industry uses aluminum in framing for homes, offices, industrial buildings, sports facilities, and skyscrapers. Aluminum is also used to manufacture windows frames, doors, piping, and fittings.

Aluminum is widely used to produce a broad range of consumer products and industrial machinery, from cookware to utensils, refrigerators, dryers, hand tools, lawnmowers, and manufacturing equipment.

Because of its high electrical conductivity, light weight, and its shielding capabilities, aluminum is heavily used by the computing sector and by electrical power producers and suppliers. Aluminum is used for high-voltage power lines, telecom wiring, transformers, underground cabling, as well as computer components and casings. The renewable power industry relies on aluminum for solar cell components.

Aluminum Recycling

Aluminum is one of the most environmentally friendly materials used by industry today. It is 100 % recyclable and retains its inherent properties forever. The aluminum sector proudly states that more than 75 % of the aluminum produced in the US is still in use today. In fact, over 90 % of aluminum used in the construction and automotive industries is recycled.

A closed-loop recycling process is where a product is recycled to make the same product again, thus retaining the original properties of the material. Open-loop recycling turns recycled materials into raw materials for use in products other than what the product was originally created for. Open-loop recycling involves more processing than closed-loop recycling. Aluminum can be recycled in both open-loop and closed-loop processes.

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