ALUMINUM CASTING PROCESS

INTRODUCTION TO ALUMINUM

To understand what the aluminum casting process is we must first look at the properties of aluminum. Aluminum is the most abundant metallic element in the earth's crust, of which it forms nearly 8%. It always occurs as a compound, some of its minerals being bauxite, cryolite, corundum, alunite, diaspore, turquoise, spinel, and such silicates as kaolin, feldspar, and mica. Bauxite a ferruginous aluminum hydroxide, is the basic raw material from which the metal aluminum is produced.

Impurities are removed from the bauxite by chemical processing to make alumina (aluminum oxide). The bauxite is crushed, mixed in a caustic soda (sodium hydroxide) solution, and then heated by steam under pressure to dissolve the alumina. Settling and filtration remove impurities. After cooling, the solution is agitated and crystals of hydrated alumina are formed. These crystals are washed and then dried in long rotating kilns. The final product, alumina or aluminum oxide is a very hard crystalline substance having the appearance of refined sugar. Four pounds of bauxite yields approximately two pounds of alumina.

Electrolyzing a solution of alumina in molten fluorides makes aluminum; this electrolysis takes place in reduction pots or cells at a reduction plant. Aluminum reduction plants produce pure, high-quality primary aluminum. The reduction process removes the oxygen from the alumina, which consists of almost equal parts of aluminum and oxygen, and leaves pure aluminum. Two pounds of alumina yields one pound of aluminum.

CHARACTERISTICS OF ALUMINUM

Aluminum is lightweight, has excellent strength, high thermal and electrical conductivity, high reflectivity, good corrosion resistance, excellent workability, and an attractive appearance. It can be given almost any finish. It is nonmagnetic, nontoxic, and non- sparking.

Aluminum weighs 1.175 lb/in3, approximately 1/3 the density of steel, copper, and brass. Some of the stronger aluminum alloys exceed the strength of mild steel. The melting point of aluminum is 1215 degrees Fahrenheit.

The high thermal conductivity of aluminum is a marked advantage in any application where it is desirable to conduct or dissipate heat quickly and uniformly. On a weight basis, aluminum is the most efficient heat conductor of the common metals.

ALUMINUM ALLOYS

An alloy is a substance having metallic properties, composed of two or more chemical elements of which at least one is a metal.

The 6000 series principal alloying constituent is Magnesium silicide. This series has good formability and corrosion resistance, with medium strength. These alloys are the most popular aluminum extrusion alloy class. They have good strength, corrosion resistance, machinabilty, weldability, formability, and are heat treatable. 6063, 6005 and 6061 are produced in Bonnell Aluminum's Carthage, Tennessee and Newnan, Georgia casting facilities. 6463 is only manufactured in Newnan.

1. 6063 is the most popular of the aluminum extrusion alloys. This alloy makes a good surface finish, is corrosion resistant, and can be heat-treated for strength. This alloy is used in fabricated parts such as: storm windows, storm doors, storefronts, and commercial buildings. In other words, it is primarily used in nonstructural applications, but occasionally it is used in structural applications.

2. 6005 is used in structural applications where more strength is needed. 6005 alloy is also less corrosion resistant compared to the other alloys in the 6000 series. An example of a product manufactured with this alloy is bicycle rims.

3. 6061 has good corrosion resistance and is used in transportation and structural applications. This alloy has the highest content of magnesium and silicon of all the alloys in the 6000 series. Examples of products that are manufactured from this alloy are car parts, electrical housings, and material handling systems (i.e. load bearing applications).

4. 6463 is manufactured Bonnell Aluminum's Newnan, Georgia casting facility. This alloy is used for making bright extrusions. Bright dip is a very shiny finish created by anodizing the extruded aluminum. Anodizing is a finishing process which puts a protective oxide coating on the aluminum. This alloy has low iron and high copper content to aid in the brightness of the material. It is mainly used in the production of shower and tub enclosures, running boards, and decorative trim.

5. 6060 is an alloy also known as easy squeeze. It is primarily used in the aluminum clad window market for products with thin walls and high tongue ratios. This alloy was created to extrude with 20% less pressure.