1. 7075 ALUMINIUM T6 ALLOY
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Content
Sl. Title Page No.
1. AluminiumAlloys…………………………………………………….. 1
2. AluminiumAlloys – The 7XXX series ………………………... 1
3. 7075 AluminiumAlloy .………………………………………….... 1
4. ManufacturingMethod …………………………………………… 3
5. Bayer’s Process ………………………………………………………. 7
6. Hall–Héroultprocess ………………………………………………. 7
7. DC Casting ……………………………………………………………….. 10
8. Hot Rolling of Aluminium …………………………................. 12
9. Flowchart for preparationof rolled sheets of 7075 …... 14
10. Comparison of Properties ………………………………………….15
11. The 6061 AluminiumAlloy ………………………………………… 16
12. The 7075 in comparison with the 6061 ……………………….17
13. The 2024 AluminiumAlloy ………………………………………….18
14. The 7075 in comparison with 2024 ……………………………..19
15. 7075 In comparison with Steels …………………………………..19
16. Uses …………………………………………………………………………...21
17. Discussion …………………………………………………………………..24
18. Reference …………………………………………………………………...25
2. 7075 ALUMINIUM T6 ALLOY
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Aluminium Alloys
Aluminiumwhen alloyedwith suitableelements can be used to
produce a selection of different materialsthat can be used in a
wide assortment of structural applications. For example, when
alloyedwith 2-10% copper (2xxx series) the alloy gains substantial
increase in strength enablingit to be used in aerospace , military
applications.The additionof Magnesium and Silicon(6xxx series)
not only increases the strength and improves the strain-hardening
abilitybut also improves the heat treatabilityof the resulting
alloy. Such alloysfind applicationsin handrails,drive shafts,
automobileframe sections and aerospace materials.
Aluminium Alloys – The 7XXX series
In 7xxx series the main alloyingelement is zinc along with
magnesium and/orcopper. The series is characterized by very
good heat treatabilityalong with highest strength. Zinc
substantiallyincreases the strength and also facilitates
precipitationhardening.Common applicationsincludeaerospace
materials, armoured vehicles, bicycle frames, etc.
7075 Aluminium Alloy
It is an aluminiumalloy, with Zn as the primary alloyingelement.
Its compositionis as follows :
3. 7075 ALUMINIUM T6 ALLOY
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Si Fe Cu Mn Mg Cr Zn Al
0.40 0.50 1.70 0.30 2.20 0.23 5.90 Rest
It is produced in many tempers, some of which are 7075-0, 7075-
T6, 7075-T651 .
Some of the key features of 7075 alloyare as follows :-
• High strength-to-weight ratio
• Strength comparable to many steels
• Good fatigue strength
• Average machinability
• But less resistance to corrosion than many other Al alloys
Manufacturing Method
Procedure for Preparationin Laboratory
• First the alloyis melted in a crucible resistance furnace at
760−770 °C
• Then it is incubatedfor 10 min.
• Then the melt is cooled to the chosen pouring temperature .
• Then it is casted into a stainless steel crucible via serpentine
channel.
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• At the same time the outlet temperatures of the slurry are
measured.
• Then the slurry is quenchedin cold water rapidly.
• The T6 temper is usuallyachieved by homogenizing the cast
7075 at 450C for several hours, and then aging at 120C for 24
hours.
• This yieldsthe peak strength of the 7075 alloy.
Serpentine Channel Apparatus
1—K-type thermocouple 7—Collective crucible
2—Serpentine channel 8—Slurry
3—Pouring cup 9—Cold water
4—Serpentine bend
5—Diversion pipe
6—Melting crucible
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The apparatus consists of melting crucible, serpentine channel,
collective crucible and K-type thermocouple. The melting
apparatusis a crucible resistance furnace. The serpentine channel
is made of graphite and it consists of two symmetrical blocks
locked together by screw bolts and nuts. The collectivecrucible,
with an average diameter of 127 mm, is made of stainless steel. A
Ni−Cr/Ni−Sithermocouple is used to measure the temperatures of
the melt, the inner wall of serpentine channeland the slurry. The
temperature displayedwas accurate to ±1 °C.
Industrial Production Procedure
The major raw material required for the alloy productionare
alumina,carbon, power, aluminiumfluoride and Cryolite.
Raw
Materials
Alumina Carbon Cryolite Aluminium
Fluoride
Energy
Theoretical
Consumption
(Kg per ton
of
Aluminium)
1930 415 2 20 13460
KWH/t-
Al
Step 1: Mining bauxite
Four tons of bauxite produce one ton of aluminium—enoughto
make the cans for more than 60,000 soft drinks. Bauxite is formed
over millionsof years by chemical weathering of rocks containing
aluminiumsilicates, producing an ore rich in aluminiumoxide.
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Step 2: Refining alumina
The ore is ground and mixed with lime and caustic soda, then
heated in high-pressure containers. The aluminiumoxide is
dissolved by the caustic soda, precipitated out of the solution,
washed and heated to eliminatewater. The resulting alumina is a
white powder resembling sugar.
Step 3: Smelting into aluminium
An electrolytic reduction process known as smelting dissolves the
aluminain a Cryolite bath inside carbon-linedcells, or pots. A
powerful electric current, which is passed through the bath,
separates aluminiummetal from the chemical solution and the
metal is siphonedoff.
Step 4: Fabricating aluminium products
Aluminiumgoes from the smelting pot into the furnace for mixing
with other metals. These alloyshave specific properties to meet
specific uses. Fluxing purifies the metal, which is then poured into
mouldsor cast into ingots. Fabricationmay includeforging,
casting, rolling, drawing or extruding to create different finished
products from automobilesto aircraft.
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Bayer’s Process
Hall–Héroult process
Temperature within the cell is maintainedvia electrical
resistance. Oxidation of the carbon anode increases the
electrical efficiency at a cost of consuming the carbon
electrodes and producing carbon dioxide.
While solid Cryolite is denser than solid aluminiumat room
temperature, liquidaluminiumis denser than molten Cryolite
at temperatures around 1,000 °C (1,830 °F). The aluminium
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sinks to the bottom of the electrolytic cell, where it is
periodicallycollected.The liquidaluminiumis removed from
the cell via a siphon in order to avoidhaving to use extremely
high temperature valves and pumps. Aluminais addedto the
cells as the aluminiumis removed.
The cell produces gases at the anode. The exhaust is primarily
CO2 produced from the anode consumptionand hydrogen
fluoride (HF) from the Cryolite and flux. The gases are either
treated or vented into the atmosphere; the former involving
neutralizationof the HF to its sodium salt, sodium
fluoride. Particulates are captured using electrostatic or bag
filters. The CO2 is usuallyvented into the atmosphere.
Agitation of the molten material in the cell increases its
productionrate at the expense of an increase in Cryolite
impurities in the product. Properly designed cells can leverage
magneto-hydrodynamicforces inducedby the electrolyzing
current to agitate the electrolyte. In non-agitatingstatic pool
cells the impurities either rise to the top of the metallic
aluminium,or else sink to the bottom, leaving high-purity
aluminiumin the middle area.
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Hall–Héroultapparatus
Variations
Today, there are two primary technologiesusing the Hall–
Héroult process: Söderberg and prebake.
Söderberg uses a continuouslycreated anodemade by
additionof pitch to the top of the anode.The lost heat from the
smelting operationis used to bake the pitch into the carbon form
required for reaction with alumina.
Prebake technology is named after its anodes, which are
baked in very large gas-fired ovens at high temperature before
being lowered by variousheavy industriallifting systems into the
electrolytic solution.
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In both technologies, the anode, attached to a very large
electrical bus, is slowly used up by the process because the oxygen
generated by the electrolytic process can oxidize the carbon
anode. Prebake technology tends to be more efficient, but is more
labour-intensive.Prebake technologyis becoming preferred in the
industry because of the variouspollutantemissionsrelated to the
creation of the anodefrom liquidpitch.
DC Casting
This process involvespouring molten aluminiuminto a
mould;once the metal cools and hardens, it forms a large solid
block called a sheet ingot. The size of the sheet ingot dependson
the size of the DC unit availableand is related to the hot rolling
mill capacity and, to some extent, the alloysbeing cast.
In DC casting, the metal is melted in a holdingfurnace and
then treated in a two-stage process to remove any remaining
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microscopic non-metallicparticles and gases. The holdingfurnace
is tilted and molten metal is poured into a water-cooled casting
unit.
As the metal flows into the moulds, it is chilledby jets of cool
water pumped around and through the base of the mould. The
ingot solidifiesgradually during the casting process, which takes
approximatelythree hours. We continuouslymonitor
temperature, speed and water flow so that each sheet ingot is of
the highest quality.Individual ingotscan weigh up to 20 tons and
can be 500 – 600 mm thick, two meters wide and eight meters
long. A finished 18-ton ingot containsa volume of source
aluminiumequivalentto approximately one millionbeverage
cans.
In this process, critical variables such as temperature, speed
and water flow are carefully managed, and the metal's chemical
compositionand cleanlinesstested on each cast.
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Hot Rolling of Aluminium
Once DC casting is complete, ingots are transferred to a hot
rolling mill. The hot rollingprocess is used to convert sheet ingots
into coils with a gauge suitablefor the cold rolling process.
Precision-controlledhot rolling enables to create the properties
that will result in a finalproduct that meets the standard of
manufacturing excellence. Aluminiumingots are moved from
direct chill (DC) casting and cooled down to room temperature.
Stages in Hot Rolling
Precise milling procedures are adopted to provide the ingots
with the optimum surface for rolling.
First the ingots move to a slab saw, which removes the ends
of the ingots to increase its surface quality.
A scalper then removes the surface layer of the entire ingot
to eliminatesurface oxides, ensuring a uniform, high-quality
surface finish.
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Next, we send the ingot to a pusher furnace, which pre-heats
it to a point above its re-crystallization temperature (around
500°C) in order to prepare it for hot rolling. Thisprocess is
called annealing,and it allows the variousalloyconstituents
in the ingots to distribute themselves, evening out
differences in microstructure and ensuring a homogeneous
structure within the ingot.
Finally,we pass the sheet ingot back and forth between the rolls
of a breakdown hot mill, which reduces the ingot's gauge until it
approaches that of a strip with the desired thickness for cold
rolling.
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Flowchart for preparation of rolled sheets of
7075
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The 6061 Aluminium Alloy
6061 is the least expensive and most versatile of the heat-
treatable aluminiumalloys. It has most of the good qualities
of aluminium. It offers a range of good mechanicalproperties and
good corrosion resistance. It can be fabricated by most of the
commonly used techniques. In the annealedconditionit has good
workability.
The 6061 alloyof aluminiumis primarily composed of
magnesium and silicon. Thisgives 6061 aluminiumalloysuperior
welding abilityover other alloysof aluminium,which are
traditionallydifficult to weld because of theirchemical makeup
and lack of conductivity.Some other elements of 6061 aluminium
alloyinclude small amounts of iron, copper, manganese,
magnesium, chromium, zinc and titanium.
The 6061 compositionof aluminiumis an extensively used
material for the construction of bicycles, airplaneparts,
automotiveparts and aluminiumcans. In many cases, the foil
interiorwrapper on food containersis also made with 6061
aluminiumalloy.Because the material is extremely workable, it is
an idealmaterial for use in these products. Bicycles in particular
benefit greatly from the use of 6061 aluminiumalloybecause of
the ease with which it is welded and the rigid strength of the final
product when compared to its overall weight.
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The 7075 in comparison with the 6061
The 7075 aluminiumalloy,while not being entirely different
from 6061 aluminiumalloy,is both more expensive and more
prone to corrosion than 6061 aluminiumalloy.The 7075 alloyis
primarily composed of zinc as the alloyingagent of the aluminium,
as well as higher levels of magnesium and copper than are found
in 6061 aluminium.Thismakes this composition of aluminium
alloyas hard as many steels while still retaining the lightweight
qualitiesof aluminium.
7075 This is one of the highest strength aluminumalloys
available.Its strength-to weight ratio is excellent. It is ideallyused
for highly stressed parts. It may be formed in the annealed
conditionand subsequently heat treated. In case of 7075 spot or
flash welding can be used, althougharc and gas welding are not
recommended. It is used where highest strength is needed.
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The 2024 Aluminium Alloy
Aluminiumalloy 2024 is an aluminiumalloy,with copper as the
primary alloyingelement. It is used in applicationsrequiring high
strength to weight ratio, as well as good fatigue resistance. It
is weldable only through friction welding, and has
average machinability.Due to poor corrosion resistance
Due to its high strength and fatigue resistance, 2024 is widely
used in aircraft structures, especially wing and fuselage structures
under tension. Additionally,since the material is susceptible to
thermal shock, 2024 is used in qualificationof liquidpenetrant
tests outside of normal temperature ranges.
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The 7075 in comparison with 2024
7075 In comparison with Steels
The 7075 series (especiallythe 7075-T6) is comparable with
most steels with respect to its ultimatetensile strength.
It is also lighter than steel, thereby exhibiting a high strength-
weight ratio. It has an elastic modulus of 71 GPa, which is about
one-third of the elastic modulusof most kinds of steel and steel
alloys. Therefore, for a given load,a component or unit made of
an aluminiumalloywill experience a greater elastic deformation
than a steel part of the identicalsize and shape.
An important structural limitationof aluminiumalloysis their
lower fatigue strength compared to steel. Aluminium alloysdo not
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have this lower fatigue limit and will continueto weaken with
continuedstress cycles.
It also has the disadvantageof being warped at higher
temperatures, a problem not found in steel(they need much
higher temperatures).
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Uses
With the strength of steel and the light weight of aluminium,
7075 aluminumis extensively used in military aircraft construction
and is also used to buildboats and in some automotive parts.
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Other uses include :
Rock climbing equipment
Bicycle components
Inline skating frames
Hang glider airframes
Manufacture of M16 rifles for American military
Shaft for lacrosse stick
