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Файл:Materials Processing Technology Part 2. Coursebook
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Unit 1 Materials science
1.1 Structure of materials and general material classifications
Metals behave differently than ceramics, and ceramics behave differently than
polymers. The properties of matter depend on which atoms are used and how they
are bonded together.
The atomic structure primarily affects the chemical, physical, thermal,
electrical, magnetic, and optical properties. The microstructure and
macrostructure can also affect these properties, but they generally have a larger
effect on mechanical properties and on the rate of chemical reaction. The strength
of metals suggests that these atoms are held together by strong bonds. However,
these bonds must also allow atoms to move since metals are also usually
formable.
There are two kinds of materials used in engineering - metals and nonmetals. We can divide metals into ferrous and non-ferrous metals. The former
contain iron and the latter do not contain iron.
Most materials fall into one of three classes that are based on the atomic
bonding forces of a particular material. These three classifications are metallic,
ceramic and polymeric. Additionally, different materials can be combined to
create a composite material. Within each of these classifications, materials are
often further organized into groups based on their chemical composition or
certain physical or mechanical properties. Composite materials are often grouped
by the types of materials combined or the way the materials are arranged together
Metals include ferrous metals and alloys (irons, carbon steels, alloy steels,
stainless steels, tool and die steels). Nonferrous metals and alloys include
aluminum, copper, magnesium, nickel, titanium, precious metals, refractory
metals, and superalloys.
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Polymeric materials include thermoplastics plastics, thermoset plastics,
and elastomers.
Ceramic materials include glasses, glass ceramics, and graphite diamond.
Composites include reinforced plastics, metal-matrix composites,
ceramic-matrix composites, and sandwich structures.
A metal is a material that is typically hard, opaque, shiny, and has good
electrical and thermal conductivity. Metals are generally malleable - that is, they
can be hammered or pressed permanently out of shape without breaking or
cracking. They are also fusible (able to be fused or melted) and ductile (able to be
drawn out into a thin wire). Metals have useful properties including strength,
ductility, high melting points, thermal and electrical conductivity, and toughness.
1.2 New steels meet changing needs
As structural material, steel has two drawbacks: its weight and its
susceptibility to rust. Yet steel has long been used in structural applications from
bridges and buildings to ships, automobiles and household appliances. This is
because of many advantages of steel. It is superior to other structural materials in
strength, toughness, workability and other properties that are critical for such
applications and it is mass-produced with uniform, reliable quality and at low
cost.
Since steel is the most popular structural material available, steel-makers
make every effort to meet the changing needs of these markets. New, more
sophisticated processes for steel-making and treatment have led to steel products
of higher grade and greater variety.
Yet, it can no longer be said that a steel product is satisfactory if it is
simply a good structural material. Today's market needs can be classified broadly
as:
1) The need for lighter weight;
2) The need for new properties;
3) The need for maximum performance;
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4) The need for cost reduction.
The need for lighter weight is really a requirement for materials having higher
specific, strength. Materials offering new properties not found in conventional
materials will include new breeds of steel, hybrid materials and truly novel materials
such as amorphous metal. The need for maximum performance calls for materials
approaching the limits of durability, toughness and the like. Finally, the need to
reduce costs is leading to materials diversification in which steel materials precisely
suited to a specific application are developed.
The stainless steels are highly resistant to corrosion in a variety of
environments, especially the ambient atmosphere. Their predominant alloying
element is chromium. Corrosion resistance may also be enhanced by nickel and
molybdenum additions.
A wide range of mechanical properties combined with excellent resistance
to corrosion makes stainless steels very versatile in their applicability. The
austenitic stainless steels are the most corrosion resistant because of the high
chromium contents and also the nickel additions; they are produced in the largest
quantities. Some stainless steels are frequently used at elevated temperatures and
in severe environments because they resist oxidation and maintain their
mechanical integrity under such conditions; the upper temperature limit in
oxidizing atmospheres is about 1000 °C. Equipment employing these steels
includes gas turbines, high-temperature steam boilers heat-treating furnaces,
aircraft, missiles, and nuclear-power-generating units.
1.3 Methods of steel heat treatment
Quenching is a heat treatment when metal at a high temperature is rapidly
cooled by immersion in water or oil. Quenching makes steel harder and more
brittle, with small grains structure.
Tempering is a heat treatment applied to steel and certain alloys. Hardened
steel after quenching from a high temperature is too hard and brittle for many
applications. Tempering, that is re-heating to an intermediate temperature and
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cooling slowly, reduces this hardness and brittleness. Tempering temperatures
depend on the composition of the steel but are frequently between 100 and
650 °C. Higher temperatures usually give a softer, tougher product. The color of
the oxide film produced on the surface of the heated metal often serves as the
indicator of its temperature.
Annealing is a heat treatment in which a material at high temperature is
cooled slowly. After cooling, the metal again becomes malleable and ductile
(capable of being bent many times without cracking).
All these methods of steel heat treatment are used to obtain steels with
certain mechanical properties for certain needs.
A number of phenomena occur in metals and alloys at elevated
temperatures – for example, recrystallization and the decomposition of austenite.
These are effective in altering the mechanical characteristics when appropriate
heat treatments or thermal processes are employed. In fact, the use of heat
treatments on commercial alloys is an exceedingly common practice.
Annealing refers to a heat treatment in which a material is exposed to an
elevated temperature for an extended time period, and then slowly cooled down.
Typically, annealing is carried out to 1) relieve stresses; 2) increase softness,
ductility, and toughness; and 3) produce a specific microstructure.
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Unit 2
Metalworking and Welding
2.1 Drawing and forging
Drawing consists of pulling metal through a die. One type is wire drawing.
The diameter reduction that can be achieved in one die is limited, but several dies
in series can be used to get the desired reduction.
Forging is a manufacturing process involving the shaping of metal using
localized compressive forces. The blows are delivered with a hammer (often a
power hammer) or a die. Forging is often classified according to the temperature
at which it is performed: cold forging (a type of cold working), warm forging, or
hot forging (a type of hot working). For the latter two, the metal is heated, usually
in a forge.
Forged parts can range in weight from less than a kilogram to hundreds of
metric tons.
Open-die forging is also known as smith forging. In open-die forging, a
hammer strikes and deforms the work-piece, which is placed on a stationary
anvil. Open-die forging gets its name from the fact that the dies (the surfaces that
are in contact with the workpiece) do not enclose the workpiece, allowing it to
flow except where contacted by the dies. Therefore the operator, or a robot, needs
to orient and position the workpiece to get the desired shape.
The dies are usually flat in shape, but some have a specially shaped surface
for specialized operations. For example, a die may have a round, concave, or
convex surface or be a tool to form holes or be a cut-off tool. Open die forgings
can be worked into shapes which include discs, hubs, blocks, shafts, sleeves,
cylinders, flats, hexes, rounds, plate, and some custom shapes.
Closed-die forging is the shaping of hot metal within the walls of two dies
that come together to enclose the workpiece on all sides. The hammer is dropped
on the workpiece, causing the metal to flow and fill the die cavities. Depending
on the size and complexity of the part, the hammer may be dropped multiple
times in quick succession.
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Impression-die forging is also called closed-die forging. The first
impression is used to distribute the metal into the rough shape in accordance to
the needs of later cavities; this impression is called an edging, or bending
impression.
The following cavities are called blocking cavities, in which the piece is
working into a shape that more closely resembles the final product. The final
shape is forged in a final or finisher impression cavity.
Closed-die forging has a high initial cost due to the creation of dies and
required design work to make working die cavities.
However, it has low recurring costs for each part, thus forgings become
more economical with more volume. This is one of the major reasons closed-die
forgings are often used in the automotive and tool industry.
2.2 Welding
Welding is one of the most important operations that are used in industry.
Many parts of machines, automobiles, airplanes, ships, bridges and buildings are
welded.
In order to join two metal pieces it is necessary to soften them with heat
and then to press, hammer or fuse them together. The most widely used method
of welding is electric arc welding where the workpieces are joined by means of
electricity at the temperature of about 7,000 °C. This is the hottest heat that can
be obtained for engineering purposes.
In electric arc welding two workpieces are welded by an electric arc. In
order to create the arc a powerful electric current should be provided.
To supply the current it is necessary to use a transformer. The latter must
be switched on to strike the arc. To join the workpieces the electrode holder
should contain an electrode rod. When the arc is struck the electrode must brush
against the workpiece at 80° to its surface. As the current flows between the
electrode and the workpiece the tip of the electrode melts and falls onto the
workpiece. Thus a joint is created.
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The electrode must be moved across the joint continuously backwards in a
straight line. However, if it is moved too quickly neither the electrode nor the
workpiece will melt.
And it is important to remember that to weld plates by an electric arc is
quite dangerous. In order to protect yourself you should always follow certain
safety rules. For example, it is absolutely necessary to wear overalls with long
sleeves, gloves, an apron, a cap, and rubber boots. A mask or helmet is used to
protect the face and especially eyes from sparks.
2.3 Gas welding, arc welding, electric resistance welding
Gas welding is a welding process that melts and joins metals by heating
them with a flame caused by the reaction between a fuel gas and oxygen.
Oxyacetylene welding is the most commonly used gas welding process because
of its high flame temperature. A flux may be used to deoxidize and cleanse the
weld metal. The flux melts, solidifies, and forms a slag skin on the resultant weld
metal.
Arc welding is a type of welding that uses a welding power supply to create
an electric arc between an electrode and the base material to melt the metals at
the welding point . They can use either direct or alternating current, and
consumable or non-consumable electrodes .
In electric resistance welding heat to form the weld is generated by the
electrical resistance of material combined with the time and the force used to hold
the materials together during welding.
2.4 Laser beam welding
The unique properties of lasers account for their widespread application in
manufacturing industry. Laser beam welding is currently used in order to weld
steels, aluminum alloys and dissimilar materials. This high power density
welding process has unique advantages of cost effectiveness, deep penetration
and narrow bead in comparison with conventional welding processes. As the
thermal cycles of laser beam welding are generally much faster than those of arc
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welding it is possible to form a rather small weld zone that exhibits locally high
hardness.
However, the metallurgical and mechanical properties of laser welds and
the response of conventional materials to this new process have not been fully
established yet. It is currently difficult to determine the tensile properties of the
laser welded joint area owing to the small size (2-3 mm) of the fusion zone.
Therefore an experimental investigation of the mechanical properties of laserwelded joints was carried out.
The welding process may lead to drastic changes in the micro-structure
with accompanying effects on the mechanical properties and, hence, on the
performance of the joint. Laser welded joints, like all other welded joints, may
contain defects in the form of cracks in the narrow weld area. The size and
location of such cracks directly affect the joint performance and the lifetime of a
structure. Nevertheless, it is essential to remember that laser beam welding has a
number of advantages over conventional processes. Despite the high investment
cost of laser welding equipment, it is expected that laser beam welding will have
a great impact on fabrication and manufacturing industries within the next
decade.
Unit 3
Plasma Spraying
3.1 Introduction in plasma spraying
When the temperature of a gas is raised to about 2000 °C, the gas
molecules become dissociated into separate atoms. At higher temperatures
(30,000 °C) these atoms become ionized. The gas in this stage is termed plasma.
Machining by plasma was adopted in the early 1950’s as an alternative method
for oxy-gas flame cutting of stainless steel, aluminum, and other nonferrous
metals.
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In plasma machining a continuous arc is generated between a hot tungsten
cathode and the water-cooled copper anode. A gas is introduced around the
cathode and flows through the anode. The temperature, in the narrow orifice
around the cathode, reaches 28,000 °C, which is enough to produce a hightemperature plasma arc. Under these conditions, the metal being machined is very
rapidly melted and vaporized. The stream of ionized gases flushes away the
machining debris as a fine spray creating flow lines on the machined surface. The
removal rates by this method are substantially higher than those of conventional
single-point turning operation. Plasma machining systems are divided into
plasma arc, plasma jet, shielded plasma, and air plasma.
3.2 Air plasma
Compressed air is used as the machining gas instead of nitrogen or argon.
When air is subjected to the high temperature of the electric arc, it breaks down
into its constituent gases. Since the oxygen, in the resulting plasma, is very
reactive especially with ferrous metals, machining rates are raised by 25%. The
main drawback of this method is the heavily oxidized surface, which is frequently
obtained in case of stainless steel and aluminum. Because tungsten is reactive
with oxygen, hafnium copper or hafnium-zirconium alloys also replace tungsten
electrodes. However, the life of these electrodes becomes shorter. Electrically
conductive materials, such as stainless and chrome nickel alloy steels, aluminum,
and copper can be machined by air plasma. Since air is used for machining and
shielding purposes, the machining cost is about half that of gas- or water-shielded
plasma. When a gas mixture of 80% nitrogen and 20% oxygen is used, the
machining rate of mild steel is increased by about 25 percent.
Plasma machining rate is 10 times the rate of grinding and chipping
methods. Lower machining rates are obtainable when these grooves are cut in
nonconductive materials. The groove dimension however depends on the traverse
speed, arc power, and the angle and height of the plasma arc.
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3.3 Other forms of spraying
Wire arc spray is a form of thermal spraying where two consumable metal
wires are fed independently into the spray gun. These wires are then charged and
an arc is generated between them. The heat from this arc melts the incoming
wire, which is then entrained in air jet from the gun. This entrained molten
feedstock is then deposited onto a substrate. This process is commonly used for
metallic, heavy coatings.
High velocity oxygen fuel spraying is performed when a mixture of gaseous
or liquid fuel and oxygen is fed into a combustion chamber, where they are
ignited and combusted continuously. The resultant, hot gas emanates through a
converging-diverging nozzle and travels through a straight section. The fuels can
be gases (hydrogen, methane, propane, propylene, acetylene, natural gas, etc.) or
liquids (kerosene, etc.).
In cold spraying, particles are accelerated to very high speeds by the carrier
gas forced through a converging-diverging de Laval type nozzle. Upon impact,
solid particles with sufficient kinetic energy deform plastically and bond
mechanically to the substrate to form a coating. The critical velocity needed to
form bonding depends on the materials properties, powder size and temperature.
To accelerate powders to higher velocity, finer powders are used. It is possible to
accelerate powder particles to much higher velocity using a processing gas
having high speed of sound (helium instead of nitrogen). However, helium is
costly and its flow rate, and thus consumption. To improve acceleration
capability, nitrogen gas is heated up to about 900 °C. As a result, deposition
efficiency and tensile strength of deposits increase.
Regarding the safety issues, equipment should be operated automatically,
in enclosures specially designed to extract fumes, reduce noise levels, and
prevent direct viewing of the spraying head. Metal spraying equipment uses
compressed gases, which create noise. Combustion spraying equipment produces
an intense flame, which may have a peak temperature more than 3,100 °C, and is
very bright. Electric arc spraying produces ultra-violet light, which may damage
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