Cold and Hot Rolling of Pure Aluminum
Prof. F. Paray
Prof. H. Han
According to the Aluminum Association Aluminum is one of the most abundant elements in the
earth’s crust. To be precise Aluminum makes up roughly 8% of the crust up to 10 miles deep
while Oxygen and Silicon make up about 47% and 28% respectively. Aluminum is widely used
in today’s era due to its mechanical and chemical properties. Aluminum has a very high strength
to weight ratio. This goes along with its low density giving it a lightweight property as a metal.
Aluminum has high thermal/electrical conductivity which is great when working with a current.
Also it is non-toxic and has ideal reflectivity. The high strength to weight ratio of aluminum is
ideal for the transportation industry especially the aerospace industry such as Boeing. This is
because the lightweight material allows for transportation with the expense of less fuel while the
strength provides high safety. The lightweight and non-toxic properties make aluminum ideal for
containers and packaging such as soda cans. The electrical industry benefits greatly from
Aluminums conductivity in high voltage transmission lines and building distribution systems.
Aluminums high reflectivity makes it a great candidate for lighting fixtures and well as the
outline of roofs where it reflects the suns light providing a cooler environment. Since Aluminum
is resistant towards corrosion it can endure harsh environments and can be used to protect a
Metal rolling is one of the most common and reliable manufacturing processes in todays
industries. It includes taking a sample and rolling it between two rollers. An ideal cross section
for manufacturers is obtained due to rolling which forms the metal to the specified cross section.
Also manufacturers use Rolling to change the mechanical properties of the metal creating more
dislocations thus making it harder. Rolling is the first step in creating a raw material because it
decreases the materials thickness while making it easier to work with. Three types of results for rolling of aluminum are plates, foils and sheets. Foils are mainly used for aluminum foils,
electrical wires and drink cans. Aluminum sheets are used for outlines of roofs as well in
transportation vehicles such as aeroplanes and cars. Aluminum plates can be used for bridges,
ships and even military vehicles.
Rolling metals works on both the macro and micro structure levels. By rolling obvious changes
in it macrostructure change. Almost all metals have crystal lattice structures so the atoms are
aligned in a specific order and pattern. When the said metal is then rolled some parts, at the
microstructure level, of the metal pass each other and some overlap creating a defect in the
pattern known as a dislocation. When these dislocations come in contact with other dislocations
they prevent the movement of other dislocations. So the only way for dislocations to move is
through added force so in turn the material becomes harder and stronger. Now after rolling the
metal is able to withstand greater forces and the increasing of crystalline defects like dislocations
is known as work hardening.
It is true that rolling increases the strength of a material but the rate depends on the temperature
which is where the processes of Hot and Cold rolling come in.
Hot rolling occurs at temperatures above the metals recrystallization temperature. This process
reduces the amount of work one puts in rolling by decreasing the crystallization defects. At these
high temperatures the molecules of the material begins moving giving the material greater
energy. This energy will break bonds between atoms. Also at these high temperatures metal are
able to recrystallize creating newly formed crystals different from before. Grain size when hot
rolled is much larger than before because of the little resistance provided in the metal flow. Bigger grains and higher metal flow enable metals to be more ductile but not as strong. Also a
shinier surface occurs when hot rolled.
Cold rolling happens below the recrystallization temperature (room temperature for this
experiment). The crystals are not able to rearrange when cold rolled so more defects occur.
When the defect density increases ductility of the metal decreases but a more rigid, harder and
stronger material is resulted from cold rolling than hot rolling. Smaller grain size occurs in cold
rolling than the large grain sizes in hot rolling.
For the experiment 500 degrees Celsius was chosen for the hot rolling of aluminum because it is
over its recrystallization temperature thus hot rolling can occur. This specific temperature was
chosen for aluminum however if another material was chosen the temperature will vary because
recrystallization temperature is unique to every element due to their properties.
For the cold rolling part of the experiment the hardness of the Aluminum sample is determined
using a Rockwell Machine. Three sample measurements are taken and an average is calculated.
Once the hardness was determined the sample was then measured for its thickness using an
electronic ruler. Now that the initial conditions of the sample are measured the sample can now
be cold rolled. The sample was then placed on the rolling machines belt, then the lever was
rotated and fed through the rollers by a wooden stick to ensure safety. It was rolled in the same
direction each time and flipped after each round of rolling. Flipping is key because it creates similar defects in the sample. Initially the lever is rotated 3 times and the sample is pushed
through, (although it was pushed through in the beginning to ensure a flush rolling surface).
After this measurements for both thickness and hardness were taken three times then an average
was calculated for each respectively. The number of 360 turns the lever did was used to help fine
the percent decrease of thickness with each turn. For the sample it was calculated that :
1.9% reduction required 3 turns
4.96% reduction required 5 turns
9.5% reduction required 8 turns
19.7% reduction required 16 turns
40.3% reduction required 31 turns
*Note that the machine can only handle 3 turns maximum for one pass so the total turns were
broken up into passes of 3, 2 and 1 turns per pass depending on the number of required turns.
After each step thickness and hardness were recorded and averaged out. Once the cold rolling
was complete the lever was rotated back to its original position (Opposite direction with the
same total turns).
A similar approach was taken in the hot rolling process of two samples. Two samples were
heated to around 500 degrees Celsius in a furnace. This process was done quickly because the
heat and temperature of the sample must be preserved in order for the process to be considered
hot rolling because the sample must remain above recrystallization temperature. This operation
required four people to make is as efficient as possible. Two people had on heat resistant gloves
to handle the sample from entering and leaving of the rolling process. Another person used the wooden stick to push the sample through and to ensure the person placing the sample on the belt
flipped it to maintain an equal spread of defects. The fourth person was responsible for the
rotation of the lever to the desired amount of turns.
33% reduction required 49 turns (left sample)
49.8% reduction required 82 turns (right sample)
Like the cold rolling process thickness and hardness were measures after the required amount of
passes however before these were measured the sample was placed in water to cool. The initial
thickness and hardness were measure prior to heating the sample.
Hardness tests were conducted with the aid of the Rockwell Machine using a 1/8” steel ball
penetrator and having 10kg initial/minor and a 60kg final/major load. This is described as the
Rockwell hardness test and the specific scale used was a HRH scale. H is hardness, R is mass
and the last H is the scale. Three points are taken on the sample a reasonable distance away from
each other and the edge to ensure a good reading. The initial values of hardness of the non-
rolled sample were negative indicating the material w