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.
Characteristics of Aluminum
Aluminum is lightweight, has excellent strength, high thermal and
electrical conductivity, high reflectivity, good corrosion
resistance, excellent workability, and attractive appearance. It
can be given almost any finish. It is nonmagnetic, nontoxic, and
Charging the Cast Furnace
Aluminum Casting Equipment
Charging the furnace is the first step in the casting process. The cast house is equipped with the following in order to prepare a charge, which is a mixture of raw materials that are melted down to make an alloy. The equipment is as follows:
Aluminum Casting Materials
There are two types of aluminum logs, primary and secondary. Primary consists of pure aluminum and secondary consists of prime and scrap. There are three components used when charging a furnace to make secondary aluminum: prime, scrap, and hardeners. Bonnell Aluminum's Carthage, Tennessee and Newnan, Georgia casting facilities manufacture secondary logs. However, all billets/logs are cast within the Aluminum Association's specifications.
Prime: 99.80% pure aluminum. Prime comes in three forms: T-bars, tub sows, and pig.
material purchased from outside sources and that which is
generated from within the plant.
An alloy is a substance having metallic properties, composed of
two or more chemical elements of which at least one is a metal.
Preparing the Bath
The first step in charging the furnace is adding the appropriate amount of alloying agents: scrap, prime, and hardeners. A charge is a mixture of raw materials that are melted down and mixed to make an alloy. Prime and scrap are first added to the furnace. The alloy being cast determines the quantity of scrap and prime to be used to charge the furnace. Prime and scrap are preheated to remove moisture, which would cause an explosion.
To calculate how much prime needs to be added to the furnace you must first know how much of a heal was left in the furnace from the previous drop. A heal is the remaining metal left in the furnace after one heat has been completed. A drop is the act of pulling the pin on the furnace and allowing the aluminum to flow into the pan until the formation of logs is complete. The formula for calculating how many pounds of prime to add is:
Prime (lbs) = [Charge (lbs) - Heal (lbs)] * [Target % of Prime]
Once the prime has melted, scrap is put into the furnace until the furnace is full and allowed to melt. The melting point of aluminum is 1215 degrees Fahrenheit.
is the removal of hydrogen from molten metal by bubbling
a mixture of gasses up through the melt. Flux
is a substance that promotes fusion, especially of metals
or minerals. Fluxing
causes impurities, such as alkaline, sodium, and lithium,
(which cause the material to have a bad finish), to rise to the
surface of the bath. Once degassing is complete a sample is
taken and analyzed for proper chemical content.
Samples are taken and analyzed using a spectrometer. A spectrometer determines the concentration of elements in an alloy by comparing the relative intensity of the wavelengths of radiation produced when a sample is struck with an electric spark. Hardening agents, such as silicone and magnesium, are added to bring the concentration of the alloying agents up to the specification of the alloy being cast.
Making a Cast
The temperature of the metal must be between 1300-1340 degrees Fahrenheit, the analysis must be within the ranges specified, and degassing must be complete to begin a drop. The casting process used at Bonnell Aluminum is called direct chill casting because the cooling of the log is taking place in the mold itself where water is running through the mold. After all criteria are met the drop can begin. Before tapping the furnace safety equipment must be put on. Tapping is the process of pulling the plug pin out of the furnace so the aluminum can flow into the trough and pan.