Metallurgy
Metallurgy is a domain of
materials science and of
materials engineering that studies the physical and chemical behavior of metallic
elements, their
intermetallic compounds, and their mixtures, which are called
alloys.
Extractive metallurgy is the practice of separating metals from their
ore, and refining them into a pure
metal. In order to convert a metal
oxide or
sulfide to a metal, the metal oxide must be
reduced either
chemically or
electrolytically.
In
production engineering, metallurgy is concerned with the production of metallic components for use in consumer or
engineering products. This involves the production of alloys, the shaping, the heat treatment and the surface treatment of the product. The task of the metallurgist is to achieve design criteria specified by the
mechanical engineer, such as cost,
weight,
strength,
toughness,
hardness,
corrosion resistance and performance in extremes of
temperature.
Common engineering
metals are
aluminium,
chromium,
copper,
iron,
magnesium,
nickel,
titanium and
zinc. These are most often used as alloys. Much effort has been placed on understanding one very important alloy system, that of purified iron, which has
carbon dissolved in it, better known as
steel. Normal steel is used in low cost, high strength applications where weight and
corrosion are not a problem. Cast irons, including
ductile iron are also part of this system.
Stainless steel or
galvanized steel are used where resistance to corrosion is important. Aluminium alloys and magnesium alloys are used for applications where strength and lightness are required.
Most engineering metals are stronger than most
plastics and are tougher than most
ceramics. Composites of plastics and materials such as
glass fibre and
carbon fibre rival metals in applications requiring high tensile strength with little weight.
Concrete rivals metals in applications requiring high compressive strength and resistance to the effects of water.
Wood rivals metal in applications requiring low cost and availability of materials and low cost of construction, as well as in applications requiring certain aesthetics.
The operating environment of the product is very important; a well-designed material will resist expected failure modes such as
corrosion,
stress concentration,
metal fatigue,
creep and
environmental stress fracture. Ferrous metals and some aluminium alloys in water and especially in an electrolytic solution such as seawater, corrode quickly. Metals in cold or
cryogenic conditions tend to lose their toughness becoming more brittle and prone to cracking. Metals under continual cyclic loading can suffer from metal fatigue. Metals under constant
stress in hot conditions can
creep.
Metals are shaped by processes such as
casting,
forging,
rolling,
extrusion,
sintering,
metalworking,
machining and
fabrication. With casting, molten metal is poured into a shaped
mould. With forging, a red-hot
billet is hammered into shape. With rolling, a billet is passed through successively narrower rollers to create a sheet. With extrusion, a hot and malleable metal is forced under pressure through a
die, which shapes it before it cools. With sintering, a
powdered metal is compressed into a die at high temperature. With machining,
lathes,
milling machines,
planing machines and
drills are used to cut the cold metal to shape. With fabrication, sheets of metal are cut with
guillotines or
gas cutters and bent into shape.
"
Cold working" processes, such as rolling and fabrication, where the product's shape is altered while the product is cold, can increase the strength of the product by a process called
work hardening. Work hardening creates
microscopic defects in the metal, which resist further changes of shape.
Various forms of
casting exist in industry and academia. These include
sand casting,
investment casting (also called the "
lost wax process"),
die casting and
continuous casting.
Welding is a technique for joining certain ferrous metals and certain aluminium alloys. The metals in the weld and on both sides of the join are generally similar alloys.
Brazing is a technique for joining copper-based metals.
Metals can be
heat-treated by
annealing,
quenching and tempering]} and [[case hardening to alter properties of toughness, hardness or resistance to corrosion. Annealing is used to make a shaped product tougher by reducing the effects of work hardening, it also softens the metal. Quenching and case hardening are used to make a shaped product harder. Quenching by it self makes the metal very hard and very brittle, tempering after quenching is used to reduce the brittleness and improve overall properties.
Electroplating is the main surface treatment technique and involves bonding a thin layer of another protective metal such as
gold,
silver,
chromium or
zinc to the surface of the product to reduce corrosion.
Metallurgy is also applied to electrical and electronic materials where as metals such as
aluminium,
copper,
tin and
gold are used in power lines, wires,
printed circuit boards and
integrated circuits.
Soldering is a method of joining metallic electrical conductors where high strength is not required.
Metallurgists study the microscopic and macroscopic mechanisms that cause a metal or
alloy to behave in the way that it does, i.e. the changes that occur on the atomic level that affect the metal's (or alloy's) macroscopic properties. Examples of tools used for microscopic examination of metals are optical and electron
microscopes and
mass spectrometers.
Metallurgists study
crystallography, the effects of temperature and heat treatment on the component phases of
alloys, such as the
eutectic and the properties of those alloy phases.
The macroscopic properties of metals are tested using machines and devices that measure tensile
strength, compressive strength and
hardness.
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Timeline of materials technology*
metal working*
Pyrometallurgy
