How is aluminum made, and how is it used in the foundry industry?
The growing popularity of aluminum
Aluminum is the second most popular metal in the world after iron, and its market is growing at twice the rate of the steel market, although this is off a much lower base.
Aluminum is also the most common metal on earth, contributing to more than 8% of the earth’s core mass—but it is difficult to refine compared to iron. For this reason, the use of aluminum has lagged behind other metal products while efficient and cost-effective methods were being developed to overcome these complexities. In the mid-1880s, two distinct methods were invented, used in series to produce aluminum. The Bayer method uses a chemical process to extract aluminum from bauxite (a common aluminum ore). The Hall-Heroult process uses electrolysis to extract aluminum from the alumina or aluminum oxide produced by the Bayer process.
There are many similarities between the aluminum and steel industry. Both rely on the extraction of metals from mineral ores occurring in the earth’s surface. The manufacturing processes of both are energy intensive and involve the pouring of liquid metal into casts or using continuous casting machines. Aluminum and steel even compete in similar markets for the automotive and aerospace industry. However, there are also significant differences in the processing and properties of these metals.
Production and processing
Most bauxite is gathered from open pit mining operations in the form of dirt, rather than rock. Typical aluminum content in bauxite ore ranges from 45% to 60%.
The Bayer process
Bauxite ore is crushed and mixed with caustic soda to produce a slurry containing fine particles of ore. The slurry is held at temperatures between 140°C and 280°C depending on the specific ore being processed. During this time, the aluminum dissolves into the caustic soda solution. All the impurities settle out of the solution into a residue called red mud. The final step in the process is the addition of seed crystals to the caustic soda solution. Dissolved alumina attaches to these seed crystals. The final product from the Bayer process is alumina or aluminum oxide, which has the appearance of a white powder.
The Hall-Heroult process
The reduction unit of an aluminum plant consists of reduction pots or cells which are connected in series. Each pot is made of a steel shell lined with carbon. Molten cryolite (a fluoride mineral) containing aluminum oxide is poured into each pot and carbon electrodes are inserted into the solution from the top. Current passes between the carbon electrodes and the carbon lining of the pot. As the current passes through the cryolite solution, aluminum separates from oxygen, which attaches itself to the carbon of the electrodes forming carbon dioxide gas. Liquid aluminum collects at the bottom of the pot.
The power for the process comes from the direct current through the electrodes. The voltage is kept between 4 and 6 volts and the current generated can be as high as 4 KA. The power supplied from the electric current keeps the cryolite solution at about 950°C .
Liquid alumina is sucked from the reduction pots at regular intervals into vacuum buckets, transferred into a furnace, and then cast into ingots in molds or via a continuous casting machine. Aluminum produced through this process is approximately 99.8% pure.
The electrolytic process for aluminum production is extremely energy demanding, requiring 15Mwh per ton of output. Most smelters are therefore located next to a power generator such as hydroelectric plants.
Aluminum castings are formed by pouring molten metal into molds which have been shaped by a pattern of the desired product. Three common types of molding methods are used to produce aluminum castings:
Die casting uses pressure to force molten aluminum into a steel die (mold). This type of casting is often used for mass production of parts, which require a minimum amount of finishing and machining. Die casting has short cycle times but high costs for tooling. The pressurized casting system creates a high strength skin but weaker interior than permanent mold casting. There are two types of die casting—low-pressure and high-pressure die casting.
Low-Pressure Die Casting
Good strength values
Minimum wall thickness approx. 3 mm (in die)
High dimensional accuracy
Well suited for automation
Less complicated die and machine technology
Slower casting cycles
High-Pressure Die Casting
Lower strength values
Suitable for thin-walled components
High investment and operating costs
Well suited for automation
Complicated and expensive dies
Short casting cycles
Permanent mold casting
Permanent mold casting uses steel (or other metal) molds and cores. Strong castings are formed by pouring aluminum into the mold. Permanent molds are used to create highly repeatable parts with consistency. Their rapid cooling rates generate a more consistent microstructure, which can improve the mechanical properties significantly.
Permanent mold casting is used for creating alloy wheels. Today's alloy wheels are made from an aluminum alloy, known for its durability. Aluminum wheels are also lighter than steel, and require less energy to rotate. They provide greater fuel efficiency, better handling, acceleration, and braking.
Sand castings are created by packing a fine sand mixture around a pattern of the desired product. The pattern is slightly larger than the final product to allow for shrinkage of the aluminum while cooling. Sand casting is economical because the sand is reused multiple times over. It is also effective for creating large moldings or moldings with detailed designs. Upfront tooling costs are low but per part prices are higher, making sand casting more suitable for specialized castings than mass production.
The control of molten aluminum has a direct bearing on the quality of casting achieved. Alloying elements are added to molten aluminum to achieve the aluminum grade and properties desired. Good control over alloy addition and distribution throughout the aluminum will ensure that the product is sound and has the expected mechanical properties.
Aluminum solidifies with a columnar grain structure. These columns grow to the point of contact with another grain—the more grains there are, the finer the molecular structure. Grain refining uses titanium and boron to create grain nucleus sites to achieve this fine structure.
Hydrogen gas is an impurity that can cause defects in the aluminum casting by creating pores as the product solidifies. During casting, degassing and purge gasses are required to keep the environment free of any impurities that may negatively impact the quality of the finished product.
The wide range of casting alloys helps in selecting the most suitable and cost-effective material for each particular application. Each of these casting alloy has its own physical and mechanical properties as well as their own characteristics such as weldability, machinability, corrosion resistance, and heat treatment properties.
Molten aluminum has several characteristics that can be controlled to maximize casting properties. It is prone to picking up hydrogen gas and oxides in the molten state, and may be sensitive to minor trace elements. Although some decorative or commercial castings may have quality requirements that can be met without additional processing, further finishing is often beneficial. Tight melt control and specialized molten metal processing techniques can help provide enhanced mechanical properties when needed.
Finishing and coatings
Certain aluminum alloys are heat treated to enhance their properties for specific applications. The solid cast aluminum is heated to a predetermined temperature, which causes the molecular microstructure to become evenly distributed throughout the material (solid solution). Rapid cooling then causes the microstructure pattern to remain in place and the ideal properties are achieved.
Those alloys which cannot be heat treated are finished by cold working (primarily rolling). The metal strength is greatly enhanced as imperfections in the microstructure are minimized by tightly compacting molecules together.
Aluminum has a high-quality surface finish that is already aesthetically pleasing. However, coating can offer further benefits to the finish. There are a few different types of coating:
Anodizing is used to thicken the oxidized surface layer and enhance the corrosion resistance of the product. The coating is hard and durable and self-repairing, making it a popular choice for architects. The anodizing process itself is carried out using a sequence of dip tanks.
PVDF coatings are solvent based paints, which have very high resistance to weathering. However, they can be scratched. PVDF does not fade from exposure to sunlight and can be made with a metallic appearance.
Liquid paints are much more economical than PVDF coatings but their properties are also less desirable. They have a lower quality finish and weather resistance is not as strong.
Powder coatings have excellent finish appearance. They meet the most stringent durability specifications to the same level as PVDF coatings and are popular in building applications for window and door frames. In addition, they can endure more wear and tear and are often specified in high traffic areas like hotels and stores.
Aluminum is well known for being lightweight. In fact, it is almost three times lighter than iron, with a density of 2,700 kg/m3.
Aluminum sheet production
Aluminum is rolled at the final stage of production to make it ready for commercial sale.
Remarkably, the low density of aluminum does not affect its strength. Aluminum alloys have a wide range of strength characteristics with tensile strengths ranging from 70 to 700 MPa. At low temperatures, the strength of aluminum increases—while at high temperatures, it decreases. This is different to how the properties of steel change with temperature as low temperatures lead to brittleness.
Aluminum can also be easily machined, and the power required is low due to the lower density. The high levels of malleability of aluminum give it the ability to be extruded easily. This enables the product to be bent and rolled and is a key characteristic in the creation of aluminum foils.
One of the aesthetic appeals of aluminum is its high reflectivity. This characteristic has been exploited to make high-end consumer products with a clean and clear surface finish. The clean surface finish of aluminum is enhanced by the natural formation of a thin oxide layer on the surface. The oxide layer can be made thicker by anodizing the product. The presence of the oxide layer effectively seals the aluminum from further oxidation, making it very rare to see aluminum corroding. Corrosion can, however, occur in extremely acidic or basic environments. Unlike steel, aluminum is not magnetic.
Quality and standards
Aluminum products are certified according to the alloying material used in the product. The main alloying elements include the following:
Aluminum is categorized in series with specific properties and compositions:
Excellent corrosion resistance and workability
High thermal and electrical conductivity
Power grid lines
High strength and toughness
Lower corrosion resistance
Lower melting point without brittleness
Moderate to high strength
Resistance to corrosion in marine environment
Building and construction
Silicon and Magnesium
Highly formable and weldable
Moderate high strength Excellent corrosion resistance
Very high strength
Health and safety
During the Hall-Heroult process, large amounts of gasses are emitted. These gasses are captured and treated. Toxic fluoride compounds must be removed from the gas before it is released into the atmosphere. The process of aluminum production generates CO2—therefore, the carbon footprint of aluminum products is high. For this reason, many manufacturers prefer to locate the aluminum smelters alongside renewable energy sources, such as hydropower plants, rather than have the additional environmental burden of generating the electricity from fossil fuels.
In its soluble form, Al3+, aluminum is toxic to plants. This is bad news as acidic soils tend to speed up the release of Al3+ from its minerals and lower the product yields from these fields. As almost half the arable land worldwide is acidic, the negative impact of aluminum on crop yields can be severe.
The human body can only process small amounts of aluminum. Health effects of a buildup of aluminum in the body include an increased risk of Alzheimer’s disease and some cancers, although this is not conclusively proven. At high concentrations, aluminum is a neurotoxin, which acts on the brain and bone structure. Aluminum is found in leaven, emulsifying and coloring agents, as well as some antacid products.
It would not be an exaggeration to say that advances in the aerospace industry have been substantially dependent on the development of aluminum products. Their combination of properties, especially the light weight and strength of the product, have enabled humankind to develop vehicles that could fly and be light enough to escape the earth’s atmosphere. The Wright brothers used aluminum for the engine crankcase of their first wood-frame biplane. Modern commercial transport planes consist of 80% aluminum in the manufacture of their airframes, especially for fuselages and wings. Aluminum is used extensively in the space industry for shuttles and structures at the international space station.
Aluminum beverage cans
Watch as hundreds of aluminum cans move along a conveyor at a beverage production facility.
Beverage cans are another market where aluminum has dominated. The ability to chill quickly, the highly printable surface, and the high levels of recyclability make aluminum an attractive match for this industry. They also protect the flavor and integrity of the beverage sealed inside due to their 100% protection against oxygen, light, and other contaminants.
Construction and architecture
Power grids and electrical transmission lines have transferred to aluminum rather than copper as a base. This is due to the excellent conductivity and light weight for extended lengths of cable runs. Aluminum alloys are also used in construction to provide strong frames that can handle the high weight of large glass panes. Architects use these characteristics extensively in airports and high-rise buildings.
Home appliances have also benefited from the properties of aluminum. Its thermal properties make it ideal for refrigeration applications, while the light weight enables appliances to be easily moved and transported. With the development of the “brushed aluminum” finish, products that are highly aesthetically appealing were created for the high-end market. Apple has led the way in creating aluminum laptops, which are made from a single block of aluminum. Flat screen TVs benefit from the light weight property of aluminum; an equivalent steel product would be too heavy to hang on a wall.
Car makers are under more and more pressure to reduce the carbon footprint of their vehicles. Lightweight aluminum frames, body panels, and engines help this cause by improving the fuel economy. There are also other environmental benefits, as nearly 90% of automotive aluminum scrap is collected for recycling.
Much of the aluminum we use is recycled. Beverage cans and automobile parts are high catchment industries where material is collected and recycled effectively. Once used aluminum is collected, it is taken to a treatment facility, where it is sorted into different grades and cleaned. The metal is then melted down to remove the coatings, inks, and other impurities. While molten, alloys can be added as necessary, after which it is cast into ingots. These ingots can be supplied to foundries where it is used for casting, or moved to other manufacturers for further processing—extruding, and hot or cold working. Recycled aluminum can be returned to the market as new products in as little as six weeks.
Most aluminum products can be kept clean using plain water or a mild soap or detergent. Where stains are more stubborn, turpentine can be used or a non-etching chemical cleaner. Where even more cleaning power is required, wax based polishes, abrasive waxes, or abrasive cleaners can be used. It is important to dry aluminum products after cleaning to avoid streaking, and cleaning residue must be removed from edges and joints.
Both aluminum and aluminium refer to the same element Al—with Americans and Canadians spelling it as "aluminum"—and the British and most other countries using the spelling "aluminium." In recent years, the International Union of Pure and Applied Chemistry (IUPAC) identified "aluminium" as the proper spelling. However, it did not catch on in North America since the American Chemical Society used "aluminum." The IUPAC periodic table presently lists both spellings, with both words being perfectly acceptable.
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