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Posted on May 5, 2021
The aluminum and steel industries share several similarities. They both rely on extracting metals from Earth's surface and the processes are energy-intensive. Extracting metals from minerals is a process of pouring liquid metal into castings or molds which involves the use of casting equipment. The use of aluminum and steel is primarily found in the automotive and aerospace industries. Though with several similarities, there are still defining differences between the processing and properties of the metals. In this article, we will place the main focus on the processing of aluminum in an aluminum foundry.
Aluminum is the third most abundant element on Earth and the most abundant metal in Earth's crust. It accounts for more than 8% of Earth's core mass. However, it is difficult to refine compared to other metals such as iron. As a result, the use of aluminum lags behind other metal products. That said, effective and cost-effective methods have been developed to overcome these complexities.
The aluminum and steel industries share several similarities. To begin with, they both rely on extracting metals from Earth’s surface and the processes are energy-intensive. Extracting metals from minerals is a process of pouring liquid metal into castings or molds which involves the use of casting equipment. Further, the use of aluminum and steel is primarily found in the automotive and aerospace industries. Though with all these similarities, there are still defining differences between the processing and properties of the metals. In this article, we will place the main focus on the processing of aluminum in an aluminum foundry.
:: Read More: What Is an Iron Foundry?
Bauxite is a sedimentary rock with high aluminum content, usually about 46-60%. It is normally covered by a thick layer of rock and clay, which must be removed before bauxite is recovered. The bauxite ore then passes through a crushing or washing plant before it can be transported for processing.
In the mid-1880s, two completely different methods were invented and used in series to produce aluminum. The Bayer process uses chemical methods to extract aluminum from bauxite. The Hall-Heroult process uses electrolysis to extract aluminum from the alumina or alumina produced by the Bayer process.
The bauxite ore is crushed and mixed with caustic soda to produce a slurry containing fine particles of the ore. Depending on the specific ore being processed, the slurry is maintained at a temperature between 140°C and 280°C. During this time, aluminum dissolves in the caustic soda solution. All impurities precipitate out of the solution, forming a residue called red mud. The last step of the process is to add seed crystals to the caustic soda solution. The dissolved alumina adheres to these seed crystals. The final product from the Bayer process is alumina, which is a white powder in appearance.
The reduction device of the aluminum plant consists of reduction tanks or electrolytic cells connected in series. Each tank is made of a steel shell lined with carbon. Pour molten cryolite (fluoride mineral) containing alumina into each tank and insert the carbon electrode into the solution from the top. When electricity passes through the cryolite solution, aluminum separates from oxygen, forming carbon dioxide gas. Liquid aluminum accumulates at the bottom of the pot.
Then the liquid alumina is sucked from the reduction tank into the vacuum barrel at regular intervals. It is transferred to a furnace and then cast into an ingot in a mold by a continuous casting machine. The purity of aluminum produced by this process is about 99.8%. The electrolysis process of aluminum production is very energy-intensive, requiring 15 MWh per ton of output. Therefore, most smelters are located next to generators such as hydroelectric power plants.
After the aluminum is extracted and processed, the next step is to cast it into product form. Aluminum castings are formed by pouring molten metal into a mold that has been shaped through the desired final product pattern. Three common forming methods are used to produce castings: die casting, permanent molds, and sand casting. We will break down these three methods in the following sections.
Die casting uses pressure to force molten aluminum into a steel mold. This type of casting is usually used for the mass production of parts that require a minimum amount of finishing and machining. The cycle time of die casting is short, but the mold cost is high. The pressure casting system can produce high-strength skin, but the internal structure is weaker than a permanent mold. Die casting can be further divided into low pressure and high pressure die casting.
Permanent molds use steel or other metal molds and cores. A strong casting is formed by pouring aluminum into a mold. Permanent molds are used to manufacture highly repeatable parts with consistency. Their rapid cooling rate produces a more consistent microstructure, which can significantly improve mechanical properties.
Permanent molds are used to make alloy wheels. Aluminum wheels are also lighter than steel wheels and require less energy to spin. They provide higher fuel efficiency, as well as better handling, acceleration, and braking. However, for heavy industrial rail applications, steel wheels are more commonly used. Their durability makes them almost impossible to bend or break. When used on the track, the steel wheel can tolerate uneven track and increase safety.
Sand castings are created by stacking a mixture of fine sand around the pattern of the desired product. The pattern is slightly larger than the final product to allow the aluminum to shrink as it cools. Sand casting is economical because the sand can be reused many times. This is also effective for creating large molded products or molded products with detailed designs. The early mold cost is lower, but the parts prices are higher, which makes sand casting suitable for special castings in mass production.
The control of molten aluminum directly affects the quality of castings. Add alloying elements to molten aluminum to achieve the required aluminum grade and performance. The controlled alloy addition and distribution throughout the aluminum will ensure that the product is intact and has the expected mechanical properties.
Aluminum solidifies in a columnar grain structure. These columns grow to the point of contact with another type of particle-the more particles, the finer the molecular structure. Grain refinement uses titanium and boron to create nucleation sites to achieve this fine structure of any impurities that may negatively impact the final product.
Aluminum foundries provide a variety of casting alloys to suit the end application as well. These casting alloys all have their own characteristics, such as weldability, workability, corrosion resistance, and heat treatment properties.
Molten aluminum has several properties that can be controlled to maximize casting performance. It easily absorbs molten hydrogen and oxides and may be sensitive to trace elements. Though certain decorative or commercial castings do not require additional processing, further finishing is usually beneficial. Strict melt control and specialized molten metal processing technology can provide enhanced mechanical properties.
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