TYPES OF METAL ALLOYS
0
TYPES OF METAL ALLOYS
●
Metals and alloys have many useful
engineering properties and so have wide
application in engineering design.
● Iron and its alloys (principally
steel) account for about 90 percent
of the world’s production of metals
mainly because of their combination
of good strength, toughness, and
ductility at a relatively low cost.
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FERROUS ALLOYS
➢
Iron is the main constituent in ferrous alloys.
➢
They are produced in larger quantities than other metals.
➢
Their widespread use is accounted for by three factors:
●
Iron-containing compounds exist in abundant quantities within the earth’s crust.
●
Ferrous alloys may be produced using relatively economical extraction,
refining, alloying and fabrication techniques.
●
Yükleniyor…
Ferrous alloys have a wide range of mechanical and physical properties.
➢
The ferrous materials are divided into two groups according to the carbon
content:
●
The ferrous materials with carbon content higher than 2% are called as cast
irons, and those with carbon content less than 2% as steels.
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Production of Ferrous Alloys
All ferrous materials begin in a blast furnace where iron ore, limestone and coke (a form of carbon)
react to form PIG IRON.
Iron ores:
•
•
HEMATITE: Fe2O3 contains 70% Fe
The most important iron ore.
MAGNETITE:Fe3O4 contains 72,4% Fe
Coke has a dual role:
➢
It is a fuel for the blast furnace.
➢
It is also a reducing agent.
•
The coke is burned using a blast of air (sometimes
enriched with oxygen).The coke reduces the iron oxide into
Vol.
a molten iron known as pig iron.
~2000 m3
•
Carbon monoxide (CO) and carbon dioxide (CO2) are
produced as gaseous by-products.
•
Limestone (CaCO3) is added as a fluxing agent to help
removing impurities. The limestone decomposes and forms
CaO. The calcium oxide combine with silica and other
~ 1800°C
slag
oxides which exist as impurities in the iron ore to produce
molten slag.
•
Slag is a by-product of the blast furnace process.
•
The slag floats on top of the molten iron, because it is
~ 1000°C ~ 10 m
lighter. The slag are withdrawn off the top of the molten
iron.
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Reactions in the Blast Furnace
•
Heat generation:
C+O2
•
CO2
Reduction of iron ore to pig iron:
CO2+C
CO+Fe2O3
FeO+CO
•
●
2 FeO+CO2
Yükleniyor…
Fe+CO2
Purification:
Decomposition of fluxing agent:
CaCO3
●
2 CO
CaO+CO2
Forming of slag:
CaO+SiO2
CaSiO3 (Molten slag: calcium silicate)
0
•
Pig iron contains about 95% iron, 4% carbon, 0.3 to 0.9% silicon, 0.5% Mn, and 0.025 to
0.05% of sulfur, phosphorus, and titanium, at about 16000C.
•
Pig iron is either used as cast iron or converted into steel in secondary processes.
•
Oxygen is blown into the liquid pig iron in the
converter (BOF: Basic Oxygen Furnace: steel
making furnace) to eliminate the excess carbon
content up to a maximum of 2% (for practical
applications 1.4%).
•
In the converter, in addition to pig iron, scrap and
limestone are added. Limestone is added to
collect impurities such as P, S in slag form.
•
When the refining process is completed, the oxygen
is shut off and the furnace is tilted to remove the
slag.
•
Necessary alloying elements are also added.
•
Steel processing occurs at a very large scale. About
300 tons of pig iron can be refined into molten steel
in about 30 minutes.
•
We can melt pig iron again in another furnace called
cupola to produce cast iron. In cupola, pig irons
are remelted with more coke and limestone and
tap it out into moulds.
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SLAB, BLOOM, BİLLET
• Cast steel ingots are rolled into slabs, blooms, and billets, which are latter further rolled, extruded,
or drawn to different shapes and marketed as engineering materials.
➢A bloom has a square cross
section 150x150mm or larger.
➢ A slab is rolled from a bloom
and has a rectangular cross
section of width 250 mm or
more, thickness 40 mm or
more.
➢ A billet is rolled from a
bloom and is square with
dimensions 40 mm on a side
or larger.
Semi-finished products
Flat products
➢ Finished steel products
obtained upon hot rolling or hot
forging
of
semi-finished
steels (bloom, billet, slab).
➢These cover two broad
categories of products, namely
long products (bars, rods)
and flat products (plate sheet,
strip).
Long products
Some of the steel products made in a rolling mill
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➢
➢
STEELS
Steels are very versatile materials. Steels of low strength and super strength, soft and
hard steels, ferromagnetic and paramagnetic steels, steels to resist temperature extremes,
corrosion, impact, abrasion, and so on.
Steels can be classified based on their composition:
●
Plain carbon steels contain up to about 2% carbon. However, it is generally less than
1.0 wt%. These steels may also contain other elements, such as Mn (up to 1.65%), Si
(max. 0.6%), Cu (up to 0.6%), and residual amounts of S, P (less than 0.05%).
●
Alloy steels contain more than 1.65%Mn, 0.60%Si, or 0.60 Cu. In addition, any steel
to which any other alloying element (such as Ni, Cr, Mo, Ti, etc.) is intentionally added is
considered an alloy steel.
●
However, the term of “alloy steel” is used for steels which contain modest amount of
alloying elements and rely on heat treatment to improve the desired mechanical properties.
●
These steels are used for making tools (hammers, chisels, etc.) and also in making parts
such as axles, shafts, and gears.
●
The total carbon content is up to 1% in alloy steels.
●
For the low alloy steels, the total alloying element content is below 5%.
chise
l
hammer
gea
r
0
➢
➢
➢
➢
➢
➢
➢
LOW CARBON STEELS (< 0.25 wt% C)
Low-carbon steels generally contain less than about 0.25 wt % C.
They are unresponsive to heat treatments intended to form martensite. Strengthening
is accomplished by cold work.
They have relatively low strengths but very high ductilities and toughnesses.
In addition, they are machinable, weldable, of all steels, are the least expensive to
produce.
These steels are used for sheet material for forming applications for fenders and body
panels for automobiles.
Other typical applications include structural shapes (I-beams, channel and angle iron),
and sheets that are used in pipelines, building, bridges, and tin cans.
They have a yield strength of 275 MPa, tensile strengths between 415 and 550 MPa, and
ductility of 25% EL(elongation).
angle iron
fender
I-beams
body panels for automobiles
tin can
channel
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HIGH_STRENGTH, LOW ALLOY (HSLA) Steels
➢
HSLA steels are developed to replace conventional low carbon steels.
➢
Elements such as copper, vanadium, nickel, and molybdenum are added in
low amounts to improve mechanical properties.
➢
Though as a rule the stronger a material the more it costs, these steels
provide substantial savings in weight with only a modest price increase.
➢
Most may be strengthened by heat treatment, giving tensile strengths in
excess of 480 MPa; in addition, they are ductile, formable and machinable.
➢
In normal atmospheres, the HSLA steels are more resistant to corrosion
than the plain carbon steels, which they have replaced in many applications
where structural strength is critical (e.g., bridges, towers, support columns in
high-rise buildings, and pressure vessels).
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MEDIUM CARBON STEELS (0.25-0.60 wt% C)
➢
➢
➢
The medium-carbon steels contain 0.25 to 0.60% carbon.
Their mechanical properties can be improved by heat treatments.
The plain medium-carbon steels have low hardenabilities (Hardenability is a qualitative measure
of the rate at which hardness drops off with distance into the interior of a specimen as a result of diminished martensite
content. Hardenability is not hardness, which is the resistance to indentation).
➢
➢
Additions of chromium, nickel, and molybdenum improve the capacity of these alloy to be
heat treated.
These steels are used in making machinery, tractors, mining equipments, railway wheels
and tracks, gears, crankshafts, and other machine parts.
Yükleniyor…
HIGH CARBON STEELS (0.60-1.4 wt% C)
➢
➢
➢
➢
➢
The high carbon steels normally have carbon contents between 0.6 and 1.4 wt%.
They are the hardest, strongest, and yet least ductile of the carbon steels.
They are almost always used in a hardened and tempered condition.
The tool and carbon die steels, cutters, springs are high alloys.
They usually contain chromium, vanadium, tungsten, and molybdenum. These elements
combine with carbon to form very hard and wear-resistant carbide compounds (e.g.,
Cr23C6, V4C, and WC).
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STAINLESS STEELS
➢ The stainless steels have excellent corrosion (rusting) resistance in
many environments, especially the ambient atmosphere.
➢ The corrosion resistance of stainless steels is due to their high Cr
contents.
➢ In order to make a “stainless steel” stainless, there must be at least
11% Cr in the steel.
➢ Cr permits a thin protective surface layer of chromium oxide to
form when the steel is exposed to oxygen. This surface oxide protects
the underlying Fe-Cr alloy from corroding.
➢ Corrosion resistance may also be enhanced by Ni and Mo
additions. Ni is added for heat resistant applications (Ni≥8%).
➢ Generally stainless steels contain very low carbon (C≤0.15%).
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STAINLESS STEELS
Stainless Steels (related to their microstructure)
Ferritic Stainless
Steels (FSS)
Martensitic Stainless
Steels (MSS)
Austenitic Stainless
Steels (ASS)
➢ Martensitic stainless steels are capable of being heat treated.
● For ASSs , the austenite phase field is extended to room temperature.
● FSSs are composed of α-ferrite (BCC phase).
● ASSs and FSSs are hardened and strengthened by cold work because they
are not heat treatable.
● The ASSs are the most corrosion resistant because of the high chromium
contents and also the nickel additions. So, they are very common.
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100%
100%
DESIGNATION OF STEELS
➢Steels are specified in many ways. TSE (Turkish Standards Institution), AISI
(American Iron and Steel Institute), SAE (Society of Automotive Engineers), ASTM
(The American Society for Testing and Materials), DIN (The German Institute for
Standardization), etc.
➢In the content of this course AISI and SAE specifications will be given. This
designation systems a four-digit number. (If the carbon concentration is higher
than 1%, the number of digits is five.)
➢The first two digits (or numbers) refer to the major alloying elements, and the
last two numbers indicate the weight % carbon concentration multiplied by 100.
Main group
of the steel
XXXX
Carbon content % x 100
Alloy content
(major alloying elements)
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DESIGNATION OF STEELS
➢ For plain carbon steels, the first two digits are 1 and 0.
●An AISI 1040 steel is a plain carbon steel with 0.4% C.
●An SAE 10120 steel is a plain-carbon steel containing 1.2% C.
➢ Alloy steels are designated by other initial two-digit combinations (such as
12, 43, 86).
●An AISI 4340 steel is an alloy steel containing 0.4% C.
➢ One digit in the systems of a five digit sometimes may be an alphabetical
character. Then, only last two characters indicate the carbon content again. The
alphabetical character refers to an additional element.
Free
machining
steel
12L10
Lead addition
0.1% C
●Free machining steels are developed
for fast and economic machining of parts.
●Machinability is improved by addition of
elements such as Pb, S, S-P (these are cheap),
and Te, Se and Bi (expensive, but effective).
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DESIGNATION OF STEELS
AISI/SAE Designation Systems (for plain and low-alloy steels)
AISI number
● 1XXX carbon steels
➢10XX plain carbon steels
➢11XX free machining steels with sulfur (resulfurized)
➢12XX free machining steels with phosphorus (resulfurized and rephosphorized)
● 12LXX free machining steels with P by lead addition (Lead: Insoluble in steel. It is added for
machinability)
➢13XX manganese
● 2XXX Nickel steels***
● 3XXX Nickel-chromium steels***
● 4XXX Molybdenum steels
➢41XX Cr-Mo steels (or shortly chromoly steels)
➢43XX Cr-Ni-Mo steels (≈1.75%Ni)
● 5XXX Chromium steels***
● 6XXX Chromium-Vanadium steels***
● 7XXX Tungsten steels***
● 8XXX Cr-Ni-Mo steels
➢86XX (0.40-0.70%Ni)
●
9XXX Si-Mn steels***
*** PLEASE DO NOT MEMORIZE marked
designations!!!
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AISI/SAE Designations for TOOL STEELS
AISI number
● W1 (Water hardening)
● O1 (Oil hardening)
● D2 (heavy Duty, air)
● A2 (Air hardening)
● M1 (high speed)
● S1 (Shock resisting)
● Applications: Cutting tools, dies, knives, razors, hacksaw blades, springs, high-strength wires etc.
AISI/SAE Designations for STAINLESS STEELS
AISI number
● Martensitic stainless steel
➢ 410 (0.15C, 12.5Cr, 1.0Mn, 0.8Ni, 1.0Si), Applications: Rifle barrels, jet engine parts, etc.
● Austenitic stainless steel
➢ 304 (0.08C, 19Cr, 9Ni, 2.0Mn, 0.75Si), Applications: Chemical and food processing equipment,
cryogenic vessels.
➢ 316 L (0.03C, 17Cr, 12Ni, 2.5Mo,2.0Mn, 0.75Si), Applications: Welding construction
➢ 304L (0.03C, …..) , 316 (0.08C…….), etc. L means LOW CARBON!!!
● Ferritic stainless steel
➢ 430 (0.12 C, 17Cr, 0.75 Ni, 1Mn, 1 Si)
*** PLEASE DO NOT MEMORIZE
compositions!!!
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CAST IRONS
➢ Cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%.
➢ However, most cast irons contain between 3.0 and 4.5 wt% C and, in addition,
other alloying elements, 1-3 wt% Si, Mn, etc.
➢ Cast irons are easily melted because of their lower melting temperatures (115013000C) than for steels, and amenable to casting.
➢ They have a wide range of strength and hardness and in most cases can be
machined easily.
➢ They have relatively low impact resistance and ductility for some applications.
➢ They are common due to their comparatively low cost.
➢ The most common cast iron types:
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Gray Iron
➢ Gray cast irons usually contain 2.5 to 4 wt% C
and 1-3 wt% Si.
➢ For most of gray cast irons (CIs), the graphite
exists in the form of flakes, which are normally
surrounded by an a-ferrite or pearlite (it has the
ferrite-cementite (Fe3C) layered structure) matrix.
Moderate cooling rates favor the formation of a
pearlite matrix, whereas slow cooling rates favor a
ferritic matrix.
➢ Graphite formation is promoted by the presence of Si (graphite stabilizing element)
in concentrations greater than about 1%.
Ferrite
matrix.
(Depending
on heat
treatment, it
may be
pearlite.)
Graphite
flakes
➢ Because of the graphite flakes, a fractured surface takes on a gray appearance, hence its name.
➢ Gray iron is relatively brittle because of graphite flakes. The graphite flakes concentrate stresses
and cause low strength and ductility.
➢ However, gray iron has a number of attractive properties:
●high-compressive strength
●good machinability
●good resistance to wear and thermal fatigue
●good thermal conductivity
●good vibration damping.
Typical applications: Diesel engine castings, cylinders, pistons.
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Ductile (or Nodular) Iron
➢ Nodular CI (Cast Iron) contains spheroidal graphite
particles.
Ferrite
matrix
➢ Adding a small amount of magnesium to the gray iron
before casting produces a distinctly different microstructure
and set of mechanical properties.
➢ The Mg reacts with S and O, so that these elements
cannot interfere with the formation of the sphere-like nodules.
Graphite
nodules
➢ The composition of unalloyed ductile CI is similar to that of gray CI with respect to C and Si contents.
➢ Ductile cast iron has
●good castability,
●excellent machinability,
●good wear resistance.
➢ In addition, it has a number of properties similar to those of steel such as high strength,
toughness, ductility, hot workability, and hardenability.
Applications: Valves, pump bodies, crank-shafts, gears, and other automotive & machine
components.
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White Iron
Pearlite
➢ It is produced by chilling (sudden cooling).
➢ For low-Si cast irons (Si<1 wt%) and rapid cooling
rates, most of the carbon exists as cementite (Fe3C)
instead of graphite.
Cementite
No graphite formation!!!
➢A fractured surface of a white iron appears white, hence the name.
Properties:
● White cast iron has extremely hard and very brittle.
● They are unmachinable because of large amounts of the cementite phase.
● They have excellent resistance to wear.
● They also serves as the row material for malleable cast irons.
Applications: Limited. For example: Rollers.
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Malleable Iron
➢ Malleable iron formed by their heat treatment of white cast
Ferrite
iron, produces rounded clumps of graphite.
Properties:
● Very machinable
● It exhibits better ductility than gray or white cast irons.
Graphite
rosettes
(temper carbon)
Typical applications: Connecting rods, transmission gears, flanges, and so on.
Heat treatment of white irons to produce malleable CIs
a) Graphitization: The white iron is heated about 9000C and held for 3 to 20 hour (in a neutral
atmosphere to prevent oxidation) depending on the size of the castings. In this stage, the iron
carbide of the white iron is transformed to temper carbon( graphite rosettes) and austenite.
b) Cooling: The casting, after the first stage heating, is fast cooled to 7500C and then slowly
cooled at a rate of about 3 to 110C per hour (for ferritic malleable iron).
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Compacted Graphite Iron
Ferrite
➢ A relatively recent addition to the family of cast irons.
➢ Carbon exits as graphite.
➢ Si content ranges between 1.7 and 3%, whereas C
concentration is normally between 3.1 and 4%.
➢ The graphite has a worm-like (or vermicular) shape. The
microstructure is intermediate between that of gray iron and
ductile iron.
Worm-like
graphite
➢ The presence of sharp edges of graphite flakes leads to a
reduction in fracture and fatigue resistance of the material.
Properties:
Typical applications:
Diesel engine blocks,
exhaust manifolds, etc.
● higher thermal conductivity
● better thermal shock resistivity
●Gray
lowerand
oxidation
at CIs
elevated
temperatures
ductile
are produced
in approximately same amounts; however
white and malleable CIs are produced in smaller quantities.
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NONFERROUS METALS
➢ Ferrous metals have some limitations, chiefly:
● a relatively high density,
● a comparatively low electrical conductivity, and
● an inherent susceptibility to corrosion in some common environments.
➢ Although ferrous alloys are specified for more engineering applications, large
family of nonferrous metals offers a wider variety of properties. For example:
● The lightest metal is lithium, 0.53 g/cm3 (but it is not a structural metal. The
lightest structural metal is Mg, 1,7 g/cm3).
●The heaviest metal is osmium, 22,5 g/cm3.
●Mercury melts at -400C, while tungsten liquefies at 34100C.
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NONFERROUS METALS: a)
ALUMINUM and its alloys
(ρ=2.7 g/cm3,
Tm=6600C)
➢Aluminum currently is probably the most important of the nonferrous metals.
➢Aluminum has light weight (2.7 g/cm3 as compared to 7.9 g/cm3 for steel),
high thermal and electrical conductivities, corrosion resistance in some
common environments, including the ambient atmosphere, good ductility even at
low temperatures. However, its melting temperature is low (6600C).
➢It is suitable for casting, all machining and forming operations.
➢It has low strength (90 MPa), but the mechanical strength of Al may be
enhanced by cold working and by alloying, but processes tend to diminish
resistance to corrosion.
➢The principal alloying elements for Al are Cu, Mg, Si, Mn, and Zn.
➢Aluminum alloys number hundreds, such as 2024, 7075, 6061 and so on. These
are Aluminum Association Numbers.
0
➢
Al alloys are classified as either cast or wrought.
Al alloys
Wrought alloys (shaped by plastic deformation)
(Designation: 1XXX-9XXX)
Cast alloys: (Designation: 1XX.X-9XX.X)
Last digit indicates product form. 0 is for
casting. 1 or 2 is ingot (depend upon purity)
Heat-treatable
2024-T4:
Nonheat-treatable
Heat-treatable
Nonheat-treatable
1100-0
356.0-T6***
443.0-F***
4.4 Cu, 1.5 Mg, 0.6 Mn.
elongation: 20%, Tensile strength: 470 MPa
Aircraft structures, rivets.
6061-T4:
>99%Al, elongation: 45%
Tensile strength: 90 MPa
Food/chemical
handing
storage
equipment,
exchangers
5052-H32 ***
1.0 Mg, 0.6 Si, 0,3 Cu, 0,2 Cr.
elongation: 25%, Tensile strength: 240 MPa
Trucks, furniture, pipelines
2.5 Mg, 0.25 Cr
elongation: 18%
Tensile strength: 230 MPa
7.0 Si, 0.3 Mg.
5.2 Si,
elongation: 3%
elongation: 8%
and Tensile strength: 228 MPa
heat
7075-T6: 5.6 Zn, 2.5 Mg, 1.6 Cu, 0.23 Cr.
elongation: 11%, Tensile strength: 570 MPa
Aircraft structures and other high stressed
applications.
➢ 2024 and 7075 are as strong as 1040 steel. Alloy 2024-T4 is the most widely used aircraft alloy.
➢ T (Temper designation): Age hardened (changes T1 to T10). For example: T4 (T6) Solution treated and naturally (artifically)
aged.
H: Cold worked. F: As fabricated (hot worked, forged, cast, etc.)
➢ There are also prepcipitation-hardened Al-Li alloys. They are new generation alloys and they have low density (2.5
g/cm3) and high elastic modulus.
➢*** PLEASE DO NOT MEMORIZE marked AISI codes!!!
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NONFERROUS METALS: b)
Copper and its alloys
(ρ=8.9 g/cm3,
Tm=10830C)
➢Copper has very well thermal and electrical conductivity. Pure copper is red.
➢Unalloyed copper is so soft and ductile, that is difficult to machine; also it has an almost unlimited
capacity to be cold worked.
➢Its corrosion resistance in many environments including ambient atmosphere, seawater and some
chemicals is highly good.
➢The mechanical and corrosion-resistance properties might be developed by alloying.
Cu alloys
BRASS
BRONZE
➢ Brass is Cu-Zn alloy of ➢ Bronzes are alloys of Cu and
5-40% Zn.
➢Zinc additions produce a
yellow color.
➢Brasses are the most common
copper alloys.
➢ They are soft, ductile and
easily cold worked.
➢ Some of the common uses for
brass alloys include jewelry,
cartridge casings, radiators and
coins.
several other elements, such as Sn,
Al, Si, and Ni.
➢ Tin bronze (Cu-Sn), (10%Sn,
2%Zn)
➢ Phosphorous bronze (Cu-P)
➢ Aluminum bronze (Cu-Al)
➢ Bronzes are very good wear
resistant with low coefficient
friction.
➢They are utilized when in addition
to corrosion resistance, good tensile
properties are required.
BERYLIUM
➢ 1.0-2.5% Be
COPPER
COPPER➢ 30% Ni
NICKELS
➢ Berylium coppers are the ➢ Nickel produces a
most common heat treatable silver color.
Cu alloys.
➢
Applications:
➢ Their tensile strengths are Condenser and heat
as high as 1400 MPa.
exchanger
➢ Applications of these components, saltwater
coppers include jet landing piping.
gear
bearing,
springs,
surgical
and
dental
instruments.
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NONFERROUS METALS: c)
Magnesium and its alloys
(ρ=1.7 g/cm3,
Tm=6510C)
➢Magnesium is the lightest of all the structural engineering materials. Therefore, its alloys
are used in aircraft and missile applications.
➢In many environments, the corrosion resistance of Mg approaches that of aluminum;
however, exposure to salts, such as that near a marine environment, causes rapid
deterioration.
➢Its alloys are difficult to deform by cold working at room temperature. Consequently,
most fabrication is by casting or hot working at temperatures between 200 and 3500C.
Yükleniyor…
➢Its alloys are also classified as either cast or wrought, and some of them
treatable.
are heat
➢Some applications: in automobiles (steering wheels, columns, seat frames, transmission
cases), in TV-computers equipments (laptop computers, camcorders, TV sets).
➢Very good for die casting, can be welded and riveted.
➢Generally used in alloys with Al.
➢Fine Mg powders ignite easily when heated in air. In other words, they have very low flash
point. Therefore, they should be handed with care.
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NONFERROUS METALS: d)
Titanium and its alloys
(ρ=4.5 g/cm3,
Tm=16680C)
➢ Titanium is a relatively light metal.
➢ Ti alloys are extremely strong; room temperature tensile strengths as high as 1400 MPa.
➢ They are highly ductile (up to 25% elongation), easy forged and machined.
➢ The major limitation of Ti is its chemical reactivity with other materials, such as O,N,
and C, at elevated temperatures, and so special techniques must be used to cast and work
the metal. Therefore, Ti alloys are expensive.
➢ In spite of their high temperature reactivity, the corrosion resistance of Ti alloys at normal
temperatures (below 5000C) is excellent.
➢ It is a good alloying element in steel. Protective TiO2 film provides excellent resistance
to corrosion and contamination below 5350C.
➢ They are commonly utilized in airplane structures, space vehicles, biomedical implants,
and in chemical and petroleum industries.
➢ The Ti-6Al-4V alloy is the most-extensively used Ti alloy, since it combines high strength
with good workability (about 1200 MPa, heat treated and aged).
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NONFERROUS METALS: e)
Refractory Metals
➢They have extremely high melting temperatures.
➢They have high strength, hardness, and corrosion resistance at ambient
and as well as elevated temperatures.
➢Melting temperatures range between 24680C for niobium (Nb) and 34100C
for tungsten (W). Molibdenum (Mo) melts at 26250C.
➢Tungsten has the highest melting point, 34100C.
(Note: With today’s technologies, it
is impossible to melt in vast quantities. So, it is produced by direct reduction of its oxides to form
tungsten powder. These powders are hot consolidated and then extruded for production wire used as
filament in lamp bulbs.)
➢It is very good carbide former, used in steel alloys and cermets.
➢The application of these metals are varied. For example, tantalum and
molybdenum are alloyed with stainless steel to improve its corrosion resistance.
Molybdenum can be also used as filament.
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NONFERROUS METALS: e)
Precious Metals
➢ The family of metals called precious metals can be divided into three groups:
● Gold (Au) and alloys
● Silver (Ag) and alloys
● Platinum (Pt) group
Pt group
•Pt
•Rh (rhodium)
•Ru (ruthenium)
•Pd (palladium)
•Os (osmium)
•Ir (iridium)
➢ All of them have very good corrosion resistance.
➢ They are expensive.
➢ They have very good thermal and electrical conductivities.
➢ Gold is very soft and ductile.
➢ Au and Ag are used as reflecting surfaces and in integrated circuits as conductor in
computer technology.
➢ Platinum and others are very good erosion resistant.
➢ Almost non of the acids attack them.
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(1)
(2)
(3)
MATERIAL SELECTION
One of the most important areas of design thinking is the
selection of the material from which a part will be
produced. Selection of right materials from the many
thousands that are available is very difficult. It is not
possible to select a material for one property. There are
several criteria to make the final decision:
A material that must develop the desired physical
and mechanical properties
A material that can be processed or manufactured
into the desired shape
A material and process that are economical.
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As one example, we may decide to
produce
a
commercial
gas
cylinder, that is a container and
must be capable of storing gases at
some pressures. There some
choices to make a final selection of
appropriate type of material (such as
metals, ceramics, and plastics) for
the gas cylinder.
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➢ Plastics must be firstly rejected because of typically low strength.
➢ Although some ceramics can withstand the service load, they generally have low ductility. The use
of such a brittle material in a pressure-containing design can be extremely dangerous.
➢ Several common metals provide sufficient strength and ductility to serve as excellent candidates.
➢ Many fiber-reinforced composites can satisfy the design requirements. However, the third criterion,
cost, eliminates the composite materials from competition. The added cost of fabricating make them
expensive. Thus, for the gas cylinder , metal is the practical material selection. In selecting the
material for the gas cylinder, cost determined the choice of metals over composites.
➢ For many aerospace applications, on the other hand, weight reduction can be a critical design
factor. For pressure vessels on certain aircraft and rockets, low density, rather than cost, leads to the
final selection. Many advanced aerospace vehicles use composite materials instead of metal
materials.
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Example 1: You wish to select the materials needed to carry a current between the
components inside an electrical black box. What materials would you select?
Answer:
➢ The material that carries the current must have a high electrical conductivity. Thus,
we need to select a metal wire. Copper and aluminium might serve.
➢ However, the metal wire must be insulated from the rest of the black box to prevent
short circuits.
➢ Although a ceramic coating would be an excellent insulator, ceramics are brittle; the
wire could not be bent without the ceramic coating breaking off.
➢ Instead we would select a plastic coating with good insulating characteristics yet
good ductility.
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Example 2: What materials are used to make coffee cups? What particular
property makes these materials suitable?
Answer:
➢ Coffee cups are normally made of ceramic or plastic materials. Both ceramics
and plastics have excellent thermal insulation due to their low thermal conductivity.
➢ Disposable expanded polystyrene cups are particularly effective, since they
contain many gas bubbles which further improve insulation.
➢ Metal cups, however, are seldom used because the high thermal conductivity
permits the heat to be transferred, burning our hands.
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Example 3: Select suitable material for bicycle frame and fork.
Answer:
➢ The selected material must be strong enough to support the load without
yielding (permanent deformation) or fracture.
➢ The selected material must be stiff to resist excessive elastic deformation and
fatigue failure (due to repeated loading).
➢ The corrosion resistance of the material may be a consideration over the life of
the bicycle.
➢ Also, the weight of frame is important if the bicycle is used for racing: It must be
lightweight.
➢ A number of materials may satisfy the strength, stiffness, and weight
considerations including some aluminum alloys, titanium alloys, magnesium
alloys, steel, carbon fiber reinforced plastic (CFRP), and even wood.
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Example 3: (Cont.)
● Wood has excellent properties for our application but it cannot be easily shaped to form a
frame and the forks.
● Further analysis shows CFRP is the best choice; it offers a strong, stiff, and lightweight frame
that is the both fatigue and corrosion resistant. However, the fabrication process is costly.
● If cost is a major issue, steel emerges as the most suitable choice.
● On the other hand, if lower bicycle weight is important, the aluminum alloy emerge as the
most suitable material.
● Titanium and magnesium alloys are more expensive than both aluminum and steel alloys
and are lighter than steel; they, however, do not offer significant advantages over aluminum.
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Relationships Between
Processing-Structure-Composition-Properties
Properties
cost
•What is the strength –to-density
ratio?
•What is the formability? How does
this relate to the crashworthiness of
the vehicle?
•What is the cost of fabrication?
A: Compositions
•Iron-based?
Composition
•Aluminum-based?
•What alloying elements
should be used?
•What quantities?
B: Synthesis and processing
Structure
Processing
•How can the steel making be controlled so as
to provide a high level of thoughness and
formability?
•How can aerodynamic car chassis be
formed?
C: Microstructure
•What features of the structure limit
the strength and formability?
•What controls the strength?
Figure. Application of tettrahedron of materials science and
engineering to sheet steels for automotive chassis. Note that the
microstructure-synthesis and processing-composition are all
interconnected and affect the properties-to-cost ratio.
Materials science and engineering tetrahedron shows how
the properties –to cost ratio of materials depends on the
compositon, microstructure and processing.
➢Properties depend on structure (ex: hardness vs.
structure of steel), composition and processing.
➢Processing can change structure
(ex: structure vs. cooling rate of steel)
Let’s examine “sheet steels” used in the manufacture of car
chassis (or body):
➢In the manufacture of automobile chassis, a material is
needed the possesses extremely high strength but is easily
formed . Another consideration is fuel-efficiency, so the
sheet steel must also be thin and lightweight. The sheet
steels should also be able to absorb significant amounts of
energy in the event of a crash, thereby increasing vehicle
safety. These are somewhat contradictory (opposite)
requirements. Thus, in this case, materials scientists are
concerned (interested in) with the sheet steel’s
-Composition,
-Strength, -Weight, -Energy
absorption properties, -Malleability
(formability)
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