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 non-
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.
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:
- Pre-heat oven: used to pre-heat
prime (prime is 99.90% pure
aluminum) before it is added to the furnace. The heat comes from the
recuperator and does not require the use of additional gas.
Pre-heating is done to remove water from the prime. The presence of
water when the metal is placed in the furnace will cause an explosion
as the water rapidly vaporizes.
- Recuperator: Hot air from
furnace flows over a series of tubes where the air is heated which in
turn feeds the burners for the furnace and in turn reduces gas
- Front-end Loader/ Fork Truck:
the front-end loader is a lift with a large bucket that is used to
add scrap to the furnace when preparing a charge. The fork truck is
used to add prime ingot into the side doors of the furnaces as well
as remove dross and stir the mix.
- Furnaces: used to melt down
aluminum and cast logs. The furnaces are open well reverb. Open well
means there is a well opening in the front of the furnace where is
scrap added. The definition of reverb is when the flame does not melt
the aluminum but the heat from the walls and ceiling of the furnace.
The following diagram will illustrate the above definitions.
The furnaces are powered by natural gas 95-98% of the time. When
there is a gas curtailment propane gas is used. The furnaces are made
and lined with 18"- 24" refractory, which is a heat resistant
material that comes in two forms: brick and a castable mix.
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
Prime: 99.80% pure aluminum. Prime comes
in three forms: T-bars, tub sows, and pig.
- T-Bar and Tub Sows weigh more than fifty pounds
- Pig is prime that weighs less than 50 pounds
- Prime is also categorized by the iron content for example,
10/20 prime tells us there is a maximum of .10% silicon and .20%
Scrap: material purchased from outside
sources and that which is generated from within the plant.
Hardeners: Elements, which are added to a
bath of aluminum to increase strength and give the final product the
characteristics, desired such as finish, strength, and grain
refinement. The elements are as follows: Silicon (Si), Iron (Fe),
Copper (Cu), Manganese (Mn), Magnesium (Mg), Chromium (Cr), Zinc (Zn),
Titanium (Ti), and Boron (Bo). Silicon, iron, copper, manganese,
magnesium, chromium, and zinc are used to increase strength and to
improve finish. Titanium and Boron are used for grain refinement which
is a reduction in the size of the grains, creates a more consistent
grain, and better extrudability.
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.
- 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, stadium seats, and commercial buildings. In other words,
it is primarily used in nonstructural applications, but occasionally
it is used in structural applications.
- 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.
- 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, sun
rooms, and material handling systems (i.e. load bearing
- 6463 is manufactured Bonnell
Aluminum's Newnan, Georgia casting facility. This alloy is used for
making brite dip extrusions. Brite 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.
- 6060 is an alloy also known as
easy squeeze. It is primarily used in the wood clad window market for
products with thin walls and high tongue ratios. This alloy was
created to extrude with 20% less pressure.
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
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:
= [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.
Degassing 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.
Dross is a mixture of aluminum
oxides and non-metallic material, which float to the surface of molten
aluminum. Dross is produced whenever aluminum is added to the furnace
that has been painted, anodized, or dirty. Dross is skimmed off of the
top of the molten aluminum into dross pots. Dross is cooled with argon
gas to eliminates the oxygen in the mixture and prevent thermiting. Thermiting
is the temperature at which aluminum will burn up. The dross is
recycled to recover the aluminum from within it.
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.