Abdul Durrani

[Date]

Emirates Aviation University

Aircraft materials and

hardware

Ferrous and non-ferrous alloys and their properties

Introduction
This assignment will cover the methods of producing aluminum and steel, as well as
treating ferrous and non - ferrous metals and alloys. Also explaining the effect of
microstructure on the macroscopic properties of these metals. It will also explain cold
working and alloying of metals. Comparing the mechanical properties of steel, titanium
and magnesium.

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The steel production process starts with iron ore. The iron ore is dug out of the ground
from open cast mines or it can be mined from deep underground. This iron ore is then
crushed into a fine powder which is then mixed together with water, creating type of
slurry. Once this has been done, clay is added to the slurry and the mixture is then
shaped into small pellets using the help of a hydraulic press. It is then baked which will
make it form a hard shell. These pellets are then sent to a steel mill in order to extract
the iron which is normally converted into steel.
The next stage is the blast furnace. The iron ore, coke, limestone and sinter are
charged into the top of the blast furnace. The blast furnace which usually has a height of
32 meters, contains thick steel sides that are lined with refractory bricks. This is to
ensure that the heat inside the furnace is not lost. The blast furnaces are used
continuously and are only shut down when their brick lining needs replacing.
As the mixture of iron ore, coke and limestone heats, the hot waste gasses are collected
and cleansed. They are then used to help heat the air blast required if the blast furnace
is to reach the high temperatures needed to produce molten iron. The stock level is
constantly topped up and the molten iron ore is tapped at the bottom of the blast
furnace. It is then poured into the iron ladle and removed for use. The slag is removed
at the tap hole. This slag can actually be used in roadmaking.
Inside the blast furnace, the iron ore, limestone and coke is the “stock”. This is fed into
the furnace at its top. Many different process take place inside the blast furnace as the
stock slowly moves downwards and temperatures increase.
The removal of oxygen is crucial. This is done by the process of reduction, this is where
oxygen combines with carbon monoxide which will then form carbon dioxide gas. As the
stock travels down the blast furnace, various impurities and gasses are removed until
only molten iron and slag are left.
The molten iron is then transported from the blast furnace to a steel furnace. This can
also be called a converter. At this converter, molten iron is added to scrap iron or steel,
which lowers the temperature as it acts as an impurity. A high pressure stream of
oxygen and powdered lime is blown through the mixture, causing chemical reactions to
occur, this will remove some of the carbon from the iron. The amount carbon removed
from the iron will determine the quality or the grade of the steel. The molten steel is then
poured into molds forming blooms. From here it can be formed into other structures.

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Heat treatment is the process used for heating and cooling the metals to change their
microstructure as well as increasing their hardness. It also will increase their strength,
toughness and ductility as well as corrosion resistance.
One of the processes that can be done to achieve this is called annealing. Annealing is
a form of heat treatment that will bring the metal closer to its equilibrium state. It will
soften the metal which will make it provide ductility. In this process the metal should be
heated above its upper critical temperature to change its microstructure. After this is
done, the metal is slowly cooled.
A less expensive alternative to annealing is quenching. This heat treatment method
quickly returns the metal to room temperature after it has been heated above its upper
critical temperature. The quenching process stops the cooling process from altering or
change the metals microstructure. This method can be done with water, oil and other
mediums, it hardens steel at the same temperature that full annealing does.

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4130 is a low alloy steel that contains molybdenum and chromium used as
strengthening agents. It contains a carbon content of about 0.30% and because of this
low carbon content the alloy is very good for fusion and weldability. It also can be
hardened by heat treatments.
This alloy can be used for manufacturing ball and roller bearings. It is also used for
manufacturing gears due to its heat treatment property. This alloy was used to construct
the aircraft fuselages. It is also used for the tubing of the aircraft. This is mainly because
of its resistance to corrosion.

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The first step to the production of aluminum is the mining of bauxite ore. Layers of
bauxite are generally found near the surface so it is extracted through open cast mining.
The bauxite should be refined into alumina using the Bayer process. This is achieved by
crushing the bauxite ore and mixing it with caustic soda and processing it in the grinding
mill to produce a slurry. The slurry is then dumped into a digester, this is a tank that’s
functions similarly to a pressure cooker. The slurry is then heated to around 110 to 270
degrees Celsius. The condition will be maintained for up to several hours. After this has
been done, the slurry is pumped into a settling tank, where the impurities that will not
dissolve in the caustic soda will settle to the bottom of the tank. The residue that this
creates is called red mud and it accumulates in the bottom of the tank. It will consist of
iron oxide and fine sand.
When the impurities have settled out, the remaining liquid id pumped through a series of
cloth filters. This will trap any fine impurities that remain. Then the filtered liquid will be
pumped through a series of precipitation tanks. Alumina can bond with water molecules.
This creates seed crystals of alumina hydrate and they are added through the top of
each tank. The seed crystals grow as they settle through the liquid and dissolved
alumina attaches itself to them.
The crystals will precipitate and settle to the bottom of the tank, then they are removed.
After they are washed, they are transferred for them to be calcining. This is heating to
release the water molecules that are chemically bonded to the alumina molecules.
Alumina is then smelted into aluminum using the Hall-Heroult process. It takes two tons
of alumina to produce one ton of aluminum. Alumina is poured into special reduction
cells called pots that have an electrolytic bath of molten salt called cryolite which is at
temperatures around 960 degrees Celsius. An electrical current is then projected into
the mixture at 400KA or above. This current then breaks the bonds between aluminum
and oxygen atoms in alumina, this will result in liquid aluminum settling at the bottom of
the reduction cell. The aluminum is then transferred to the casting stage, where it is
made into products using many different methods.

A heat treatment process to return aluminum to its malleable state is annealing.

Annealing will provide softening for the metal. It will be carried out in a range of 300 to

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410 degrees depending on the alloy. The heating can be done from half an hour up to
three hours. The rate of cooling after annealing is not critical. Aluminum alloys can be
subjected to strain hardening. This occurs when the aluminum alloy is being molded by
plastic deformation. This will cause the grain structures in the aluminum to slide against
each other. This will make the metal malleable. Where parts have been solution heat-
treated a maximum cooling rate of 20 degrees Celsius per hour must be maintained
until the temperature is reduced to 290 degrees Celsius. Below this temperature, the
rate of cooling is not important.

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7075 aluminum alloy:
This type of aluminum alloy is in the group series of 7xxx and it has zinc as its main
alloy element. The category for this alloy is heat-treatable. Aircraft manufactures use
high strength alloys to help in strengthening the aluminum aircraft structures. Aluminum
alloy 7075 contains 1.6% copper, 2.5% magnesium and 5.6% zinc added for ultimate
strength. However the copper content can make it difficult to wield. It is often used in
construction of the aircraft and commonly used as aluminum bars, pipes and rods. It is
widely used for the construction of aircraft structure such as the wings and fuselages. Its
strength and its light weight can be applied in other sectors as well.

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Aluminum is quite similar to steel, physically, chemically and mechanically. It can be
melted and casted to be machined and it also conducts electric currents.
Aluminum is extremely light weight and it is a third of the weight of steel. This cuts the
costs of manufacturing with aluminum. Its use in the aircraft will reduce dead weight and
energy consumption, while it can increase the load capacity. This will also reduce noise
and provide better comfort.
Aluminum will naturally generate its own protective layer of thin oxide coating which will
keep the metal from making further contact with the atmosphere, a corrosion resistance.
Aluminum alloys are less corrosion resistant than pure aluminum, exception of marine
magnesium alloy. Different types of surface treatments such as anodizing and painting
can improve its resistance to corrosion.
Aluminum is an excellent heat and electricity conductor and in for its weight it is almost
twice as good a conductor as copper. This also makes it usable in computer
motherboards and LED lights.
It is also quite ductile and has a low melting point and density. Aluminums ductility can
allow aluminum products to be formed close to the end of a product’s design. Unlike
steel which rapidly becomes brittle at low temperatures, aluminum will show an
increased tensile strength as the temperatures drop. This can be useful for the aircraft.
Aluminum is also an excellent shock absorber as well as sound absorber. It produces
no sparks when it comes into contact with itself or non-ferrous metals, therefore making
it non-sparking.
It can be severely deformed without a failure occurring. So it can be cast to a high
tolerance. The tensile strength of aluminum is about 90 MPa however this can be
increased to over 690MPa for some heat treatable alloys.

Titanium’s
properties are a combination of high strength, toughness, stiffness, low density and
good corrosion resistance. The atomic weight of titanium is 47.88. Meaning it is
extremely light weight but strong at the same time. Making it very suitable for use in the
aircraft structures. It possess tensile strengths from 30,000 psi to 200,000 psi, this is
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equivalent to what is found in most steels, but for titanium it is significantly lighter
compared to other metals.
Titanium is a low-density element that can be strengthened by alloying and deformation
processing. Titanium is non-magnetic and has very good heat-transfer properties. Its
coefficient of thermal expansion is relatively lower than that of steels and less than half
that of aluminum. Titanium also has a high melting point of up to about 1725 degrees
Celsius.
Titanium can exhibit a high degree of immunity to attack by most types of corrosion by
acids and chlorides. It is non-toxic as well. Titanium is not a good conductor of
electricity. If the conductivity of copper is considered to be 100%, titanium would have a
conductivity of 3.1%. From this it is understood that titanium would not be used where
good conductivity is a prime factor. For comparing it to steel, stainless steel has a
conductivity of 3.5% and aluminum has a conductivity of 30%.
Electrical resistance is the opposition a material presents to the flow of electrons. Since
titanium is a poor conductor, it follows that it is a fairly good resistor.

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Body centered cubic structure (BCC):
This specific type of structure contains atoms at each of the eight corners of its cubic
structure, also having a single atom in the center of the cube structure. The corner
atoms are shared between eight cell units. The bcc structure consists of two atoms as
its net total, which includes one in the center and the eight eights from the corners. This
type of arrangement will not allow the atoms to pack together as closely as other
arrangements. It is usually the high temperature form of metals that are closely packed
at lower temperatures. The packing factor is the total volume of the cell to the volume of
the atoms in that cell, and the BCC structure has a packing factor of 0.68. Metals that
have this structure are generally much harder and less malleable compared to metals
that are closely packed together. When a metal goes through deformation, the planes of
atoms will slip and slide over each other, this is more difficult in a BCC structured metal.

Face centered cubic structure (FCC):

In this cubic structure the atoms are located at each of the corners and at the center of
all the cubic faces. Each of the atoms located in the corners are the corners of another
cube and so they are shared among eight unit cells. Each of the six face centered
atoms are shared between an adjacent atom. Twelve of the atoms are shared therefore
it has a coordination number of twelve. The FFC structure unit cell consists of a four
atom net total. The atoms in this structure can pack together very close to each other,
this is closer than in the BCC structure. The atoms from one layer will fill space into the

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empty spaces that are present between the adjacent layers. Some metals that have this
structure are aluminum, silver copper and gold. The packing factor is 0.74 for FCC
crystals.

Close-packed hexagonal structure (CPH):

This structure has three layers of atoms. In each the top and bottom layer, there are six
atoms that arrange themselves in the shape resembling a hexagon, with a seventh
atom that will sit in the middle of the hexagon. The layer located in the middle of the
structure, contains three atoms in triangular shapes. There are six of these surrounding
each atom in the hexagonal plane, however only three can be filled by atoms. There are
six atoms in the CPH unit cell. The coordination number of the atoms in this structure is
twelve. There are six nearest atoms in the same close packed layer, three in the layer
above and then three in the layer below. The packing factor is 0.74 which is the exact
same as FCC unit cell. The CPH structure is commonly seen in metals such as titanium,
magnesium, zinc and cadmium.

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A slip plane id the plane of the greatest atomic density. Slip direction is the close
packed direction that is within the slip plane, the slip system is the slip plane and slip
direction put together. Five independent slip systems are required to create a
polycrystalline material ductile.
For the CPH structure, it contains three slip systems, one plane and three directions.
This means it has a total of three slip systems and minimum five independent systems
are necessary to make a polycrystalline material ductile. Therfore CPH structure is
known to be brittle.
The FCC system has twelve slip systems. Three of these are a family of planes and the
remainig four are a family of directions, meaning it has a twelve slip systems. This is
clearly more than the minimum five that is required leading to FCC structures bein
ductile.
For the BBC structure, it has 48 slip systems and this would lead to expect it to be
ductile, however it is actually brittle. It has six planes and 2 family of directions, which
means 12 slip systems. It contains more of these same combinations. The BCC
structure has too much slips sytems as it contains 48. This means the slip systems will
interfere or mutually obstruct each other so the slip movement in BCC is made
extremley difficult. Which is why it ends up being brittle.
Crystalline structure contributes to the properties of a material and therfore it is very
important. It is easier for planes of atoms to slide by each other if those planes are
closely packed together. Therefore, lattice structures that have closely packed planes
aloow more plastic deformation than those that are not closely packed. Also, cubic
lattice structures allow slippage to occur more easily than non cubic lattices. This is
because their symmetry will provide closely packed planes in severla directions. A face
centerd cubic crystal structure will exhibit more ductility than a body centered cubic
structure. CPH lattices are closely packed ut not cubic, they are hexagonal shaped.
CPH metals such as zinc are not as ductile as FCC metals.

Sae 316 steel, ti 6al4v, 5553 titanium, az31b magnesium.

Get the mechanical properties of each of the chosen alloys.( fatigue, tensile strength,
yield, ductility, impact)

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 Properties

Density

Ultimate tensile
strength

Elongation at bre

Yield strength

Modulus of
elasticity

Fatigue strength

Hardness

Different aircraft require different building materials. Aircraft can be constructed from
wood, fabric, many various types of metal and also composite materials like carbon
fiber. Most aircraft today are made of aluminum because of it being strong but
lightweight at the same time.
However, other materials can be used such as titanium, steel and magnesium alloys.
Steel can be up to three times stronger and stiffer than aluminum, however it is also
three times its density which is 0.289 lb/in³. This means it is not ideal because of its
weight which needs to be much lighter for the aircraft to fly. It is however used in certain
areas of the aircraft such as landing gear primarily due to its strength and hardness. It
has also been used for the skin of the high speed aircrafts, this is because it holds its
strength at higher temperatures better than aluminum. It is still not recommended for
use on most types of aircraft.

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Magnesium is much softer than both steel and titanium and it also has the lowest
fatigue strength of 70MPa. However they are not ideal for aircraft structures as there is
a high concern over their flammability. Magnesium has a reputation of being highly
flammable and so there is a huge risk to use it. Magnesium alloys are also not strong
enough to be built for aircraft purposes as it only has an ultimate tensile strength of
260MPa and a yield strength of 200MPa. The only aspect it can offer is its extremely
light weight which is only 0.0639 lb/in³ and this is significantly lighter than other
materials.
Titanium is about as strong as steel if not stronger than it, as well as it being several
time lighter with a density of 0.160 lb/in³ compared to steel alloy density of 0.289 lb/in³.
It holds high strength at higher temperatures and it resists corrosion better than steel or
aluminum. The only drawback to it is the fact that it can be quite expensive compared to
other materials. However it is still the best option between these three materials.

Use of steel in aircrafts:

The uses of steel alloys in aircraft include the landing gear of the aircraft, because of
steels hardness and strength. Fuel tanks are also made of steel due to its high
corrosion resistance and also being able to withstand high temperatures and protect
from structural damage. This similar principle extends to exhaust components, engine
parts, and other crucial systems related to the aircrafts power source.

Uses of titanium in aircrafts:

Due to its high tensile strength to density ratio, as well as high corrosion resistance,
titanium alloys are used for aircraft skin and many structural applications. They can be
used for the main fuselage as well as the wings of the aircraft.

Uses for magnesium in aircrafts:

Magnesium alloys can be used for aircraft gear box castings. And forged magnesium
parts are also used in aircraft engine applications. They can also be used for areas that
high temperatures are present.

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Passive material:
A passive metal is a metal on which an oxide film is readily formed. An oxide film will
protect and also prevent the metal from further attack. A passive layer is usually formed
of metal oxides and hydroxides which can form when in contact with the electrolyte or
only when the current is flowing through the system. This process will make the metal
resistant to corrosion. In stainless steel, passivation is removing the free iron from the
surface of the metal using an acid solution to prevent rust. Passivation is necessary and
needed to remove embedded contaminants and return the part to its original corrosion
specifications. It can improve the corrosion resistance of certain stainless steel alloys,
however it will not get rid of imperfections such as micro cracks and heat tints.
Aerospace parts that have been machined or formed with steel tools need to undergo a
passivation process to remove this iron contamination.

Passivation can be used on metallic surfaces on the aircrafts as well. It is done

especially on parts that are made from steel tools need to undergo this process so that

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it can remove the iron contamination. The passivation will remove the free iron and
other surface contaminants from stainless steel parts. This will eliminate the
contamination left behind by metal forming processes like machining. By restoring the
stainless steel parts to mill condition, the passivation improves their corrosion
resistance. This can be needed for the metal pipes that are installed inside the aircrafts
interior structure, as it can be easily corroded and therefore needs to be protected from
this. It can also be done for the aircraft fuel tanks. Both these parts of the aircraft are
vulnerable to corrosion and therefore the passivation process will prevent this corrosion
and also protect the parts from corroding quickly. This same principle applies to the
aircrafts landing gear and also the cooling vents, which are also exposed to corrosion.
The process of passivation will prolong the lifetime of the parts of the aircraft that are
subjected to corrosion.

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Cold working effect on microstructure:
Cold working is the process of strengthening the metal by changing its shape without
the use of heat. It will subject the metal to its mechanical stress causing a permanent
change to the metals crystalline structure, which will then cause an increase in strength.
The metal is rolled in between two rollers or pushed through smaller holes. As the metal
is being compressed, the grain size can be reduced which will increase the strength.
The process is conducted and taken place at temperatures below the specific metals
recrystallization point. Instead of heat, mechanical stress is used to affect change. This
application is most commonly used for steel, aluminum and copper. When these metals
are cold worked, permanent defects will change their crystalline structure. The defects
will reduce the ability of crystals to move within the metal structure and the metal
becomes more resistant to further deformation. This will lead to the resulting metal
product to have increased tensile strength and hardness, but less ductility.
One of the effects of cold working is that the grain structure will be distorted and
resistance to working will continue increasing because of the lattice distortion. Another
effect is that residual stresses are set up in the metal which remains unless they are
removed by subsequent heat treatment. When reheating is done below the
crystallization temperature, the residual stresses are then removed without appreciable
change in the physical properties of grain structure. Further heating into the recrystallize
range eliminates the effect of cold working and restores the metal to its original
condition.
The degree of plastic deformation can be represented as a percent cold work rather
than as strain. Ao is the original area of the cross
section that experinces deformation and Ad is the area
after the deformation. Increseing the percent cold work
will also increase the yeild steghths and the tensile
streghth. However increasing it will also decrease the
ductilty of the metal. A schematic tensile stress to strain graph can show the elastic
strain recovery and strain hardening. The designated yeild strength is σ0, σi is the yeild
strength after releasing the load at the point shown as D and then upon reloding. This is
because the dislocation density in the metal will increas with deformation because of the
dislocation multiplication or the formation of new dislocations. The dislocation strain
reactions are repulsive and the result of it is that the motion of a dislocation is hindered
by the prescence of other dislocations. When the dislocation density increses, the
resistance to dislocation motion by the other dislocations becomes more prominent.
Therfore the imposed stress that is required to deform a metal will increse with
increseing the cold work.

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Alloying effect on microstructure:
Almost all metals are used as alloys which is a mixture of several elements, because
these have properties that are superior to pure metals. Alloying is mainly done to
increase the strength and increase the corrosion resistance. Generally, alloys are mixed

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from commercially pure elements. Mixing is relatively easy in the liquid state but slow
and much more difficult in the solid state, so most alloys are made by melting the base
metal, for example, iron and aluminum and then adding the alloying agents. Alloying
can be done with impurity atoms that go into either substitutional or interstitial solid
solution. This is also known as solid solution strengthening. The alloys are stronger than
pure metals because impurity atoms that will enter the solid solution will usually impose
lattice strains on the surrounding host atoms.

When impurity atoms are present, the resistance to slip is greater because the overall
lattice strain must increase if a dislocation is torn away from them. The same lattice
strain interaction is between the impurity atoms and dislocations in motion during plastic
deformation. Therefore a greater stress is needed and required to first initiate and then
continue on the plastic deformation for solid solution alloys, opposing to pure metals.
This results in increase in tensile strength, hardness, toughness and resistance to
corrosion.

References

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 https://www.thomasnet.com/articles/pumps-valves-accessories/stainless-steel-
pipe-types/

 https://en.wikipedia.org/wiki/Bayer_process

 https://www.gabrian.com/7075-aluminum-properties/

 https://www.britannica.com/science/aluminum

 https://www.britannica.com/science/titanium

 https://www.e-education.psu.edu/matse81/node/2132

 https://www.thefabricator.com/thefabricator/article/testingmeasuring/passivation-
basics-will-this-stainless-steel-rust-

 https://engineering.stackexchange.com/questions/8045/does-unloading-beyond-
yield-point-also-affect-tensile-strength

 http://www.matweb.com/

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