Cold and Hot Rolling of Pure Aluminum.docx

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Mining & Materials Engineering
MIME 341
Kristian Waters

Cold and Hot Rolling of Pure Aluminum Experiment 3 Austin Fiero 260461686 MIME 261 Prof. F. Paray Prof. H. Han 11-19-2012 Introduction: 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 certain product. 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. Backround: 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. Method: ROLLING: Cold Rolling: 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). Hot Rolling: 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. CHARACTERIZATION: Hardness: 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
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