Fundamentals of Physical Metallurgy (ENME2PM)
Metal Shaping Processes and Mechanical Properties of
Metals
Lecturer: Mr Anthony Govender
Room 237, School of Mechanical Engineering, (Howard College)
University of KwaZulu-Natal
2021
Metal Shaping Processes
• Metals are typically shaped by casting, rolling, extrusion and forging.
• During casting, metal is melted in a furnace, and if required alloying elements
are added to the melt. The addition of alloying elements improves the
properties of the material for specific applications.
• The metal melt is thereafter cast into a mould/poured into a mould.
• Sheet ingots are produced by casting molten metal with a direct chill
semicontinous casting unit. An aluminium ingot is illustrated in the figure on
the right. Ingots are generally converted into sheet (a useable form) by rolling
operations.
• Some ingots are cast with round cross section, as illustrated in the figure
below. This allows them to be used in extrusion processes to create shapes
such as channels. Two extruded forms of aluminium are illustrated below.
• The products that are formed from working ingots into final shapes, are
collectively called wrought alloy products.
Metal castings
• The process of melting metal and casting is also applied to smaller products. Such as
casting of pistons, that are used in car engines. A piston is illustrated in the figure on
the right.
• Molten metal is poured into the mould of the piston. Once cooled, and removed from
the mould a minimal amount of machining is thereafter required. Heat treatment
may also be necessary to enhance the properties of the piston specific to it’s
application.
• A wheel which cast from copper, and a dog ornament which is cast from bronze are
illustrated on the right.
• A commonly used method, to cast complex shapes is Investment casting. Initially a
shape of the object is created from wax (wax is easily shaped, as it is soft).
• Thereafter a ceramic mixture coats the wax pattern. Once the ceramic coating
solidifies around the wax, the wax is melted out of the mould, with the application of
heat. Investment casting is thus also called the lost wax process.
• The molten metal is then poured into the ceramic shell that remains. Investment
casting is used to cast products such as knee and hip prosthesis, golf cub heads etc.
• Two images on the right, illustrate a wax model of a component, and molten metal
been poured into a ceramic mould during investment casting.
Casting processes
• The lost foam process
• During the lost foam process, the desired shape of the component is initially made
from polystyrene foam, as it is easily shaped. Thereafter the foam shape is placed in
a container of sand, and the sand is compacted around the foam shape.
• The polystyrene foam melts as the molten metal is poured into the mould, and the
metal is cast into the shape of the polystyrene. This process of using sand to cast the
object is also known as sand casting.
• The 2nd figure on the right illustrates, molten metal been poured into a sand mould.
• Permanent mould castings, make use of metallic moulds. This makes the mould
reusable, unlike the lost foam and lost wax process.
• Due to the metallic mould, good surface finish and dimensional accuracy are
obtained from the cast components.
• However, permanent moulds are expensive and complex shaped are not easily
achievable.
• Pressure die casting uses high-pressure to force the melt into the die cavity. The
pressure is held until the molten metal solidifies. The pressure die casting process is
illustrated in the figure on the right.
• Pressure die cast moulds are required to withstand high-pressures and are thus
expensive, as they are manufactured from high strength materials.
• Metals such as zinc, aluminium and magnesium are cast using pressure die casting.
Hot and cold rolling
• Rolling is a process that is used to create sheet and plate from metal ingots.
For example aluminium sheet, is produced by rolling of aluminium ingots
• Prior to the rolling process, the ingots are heated in a furnace. Heating reduces
the strength of the ingot (softens), which allows greater reductions of the
ingot/slab to be achieved from a single roll pass.
• Typically ingots are heated to temperatures in the range of 1200 C.
• It is also more economical to roll the ingot directly after the ingot is removed
from the continuous caster, provided the ingot is at an acceptable
temperature.
• Reversing rolling mills, are generally used to roll the heated ingots. Reversing
mills roll the ingot in the forward direction, to reduced the ingot thickness, and
thereafter roll the ingot in the reverse direction, to further reduce the ingot
thickness. Thus large reductions of the ingot thickness can be achieved on a
single reversing mill.
Cold rolling
• When the temperature of the slab reduces (due to heat loss) to a level that is
insufficient to effectively roll the ingot. The temperature of the slab is raised by
reheating, which allows the slab hot rolling process to continue.
• The hot rolling process continues and stops when the sheet reaches a thickness
such that it can be rolled into a coil. Aluminium sheet that has been coiled is
illustrated in the figure below.
• Cold rolling process
• The hot rolled coils are initially annealed (heat treated), to soften the sheet.
• The sheet is then cold rolled at room temperature, to the desired sheet
thickness.
• If the rolling process is performed at a temperature below the recrystallization
temperature of the material, it is referred to as cold rolling.
• Generally rolling at below 0.3 x (melting temperature) of the metal is considered
to be cold rolling
• And rolling above 0.6 x (melting temperature) of the metal is considered to be
hot rolling.
Factors affecting recrystallized grain size
• Recrystallization of the metal occurs during annealing (a heat treatment)
• As mentioned in previous lectures, the grain size of a material significantly effects material
properties. In most instances grain size is controlled, to be finer rather than coarse.
• Grain size can be controlled by the following factors:
• Annealing temperature – lower temperatures reduce grain growth.
• Annealing time – shorter annealing durations reduce grain growth.
• The amount of cold work – increasing the amount of cold work increases the number of
dislocations and thus creates finer grains, during annealing.
• A common parameter that is used to quantify the amount of cold work done on a sheet is the
percentage cold work (% CW) .
% 𝐶𝑊 =
𝑡𝑓 −𝑡𝑖
𝑡𝑓
× 100 %
Where,
𝑡𝑓 = 𝑓𝑖𝑛𝑎𝑙 𝑠𝑡𝑟𝑖𝑝 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠, 𝑚𝑚
𝑡𝑖 = 𝐼𝑛𝑡𝑖𝑎𝑙 𝑠𝑡𝑟𝑖𝑝 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠, 𝑚𝑚
Extrusion of metals
• The process of forcing a material under high pressure, through a die, is
known as extrusion. As the material passes through the die, it’s cross
sectional area reduces, and length increases.
• The extrusion process is illustrated in the diagram below.
• Commonly extruded shapes include hollow tubes, however irregular
extruded shapes are also produced from metals such as aluminium
and copper.
• Extrusion processes are normally conducted at elevated temperatures,
to reduced the resistance of the metal when passing through the die.
• There are two types of extrusion, direct and indirect extrusion.
• The extrusion of material through a die, in the same direction as the
ram, is known as direct extrusion.
• During indirect extrusion the direction of the extruded material is
opposite to the motion of the ram, and the die is driven by the ram.
• With advancements in technology, some stainless steels can also be
extruded.
Forging
• The process of pressing or hammering a metal into a desired shape is
known as forging. Forging is normally performed on metal that is hot,
however some forging processes may also be performed in the cold
condition.
• Hammer forging is the process of dropping a ‘hammer’ on the surface of a
metal to shape it. This process is repeated until the metal reaches the
desired shape.
• Press forging is the process of exerting a slow moving force on the metal, to
press it into the desired shape.
• Two types of dies are commonly used in forging, i.e. Open dies and closed
dies.
• An open die generally consists of simple shapes, e.g. are flat dies or have
simple shapes such as Vee-shape or semi-circular halves. The lower image
illustrates an open die forging, with dies that have a semi-circular shape.
• A forging press is illustrated in the upper image. A heated ingot is been
pressed into a shape, by flat dies. Open dies are used for producing large
parts such as steel shafts.
Forging and wire drawing
• Closed die forging – the metal that is to be shaped is completely covered by the die
halves.
• Examples of products that are made by closed die forging are engine connecting rods. An
example of a connecting rod is illustrated in the figure on the right.
• Forging is useful in creating irregular shaped components. Such as wrenches, garden
tools, hammers etc., which are illustrated in the images provided below.
• Forging also produces components with less porosity, than some manufacturing
processes. E.g. a component that has been cast will have higher porosity than a
component that has been forged, thus making the forged component stronger than the
cast component.
• Wire drawing is a process that is used to create wire products.
• During wire drawing, a metal rod is drawn through a tapered die, which reduces the cross
section area of the wire. The wire drawing process is illustrated in the figure below.
• Hard materials such as tungsten carbide are used as the die, as it is resistant to wear
during drawing.
• If the drawing process work hardens the wire, often a heat treatment is used to soften
the wire.
Deep drawing
• Deep drawing is a process that produces cup shaped articles from sheet metal.
• A sheet of metal is placed over a die which is then clamped into position.
• Thereafter a punch presses the sheet metal into a die. The Deep drawing process is
illustrated below.
• Examples of products that are produced by deep drawing are kitchen sinks, aluminium
cans etc.
Tensile properties of metals
• Tensile properties of metals such as the strength and ductility (elongation),
are important in engineering designs.
• These properties can be determined from a tensile test. During a tensile test,
a metal sample is loaded uniaxially, and the force applied to the specimen
and the corresponding elongation of the sample is measured. A tensile
tester is illustrated in the figure alongside, note that a metal tensile sample
is clamped at both ends by the two grippers. Examples of tensile specimens
are illustrated below.
• During the application of a load the metal sample is extended/elongated.
The initial elongation of the material is known as the elastic
elongation/deformation. When a force is applied to a sample and removed,
and if the sample returns to it’s original dimensions, the elongation that
occurred is known as elastic deformation.
• However if a material is elongated to an extent that when the force applied
is removed, the material does not return to it’s original dimensions, the
material has undergone plastic deformation.
• During plastic deformation the atoms in the material are permanently
displaced.
Stress and strain
The engineering stress experienced by the tensile sample is calculated by the following equation.
𝐹
(𝐸𝑛𝑔𝑖𝑛𝑒𝑒𝑟𝑖𝑛𝑔 𝑠𝑡𝑟𝑒𝑠𝑠) 𝜎 =
𝐴𝑜
Where,
𝐹 = 𝑈𝑛𝑖𝑎𝑥𝑖𝑎𝑙 𝑡𝑒𝑛𝑠𝑖𝑙𝑒 𝑓𝑜𝑟𝑐𝑒, 𝑁
𝐴𝑜 = 𝑂𝑟𝑔𝑖𝑛𝑎𝑙 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛, 𝑁
The engineering strain of the tensile sample is calculated by the change in length of the sample, divided by the
original length of the sample.
𝑙 − 𝑙𝑜
(𝐸𝑛𝑔𝑖𝑛𝑒𝑒𝑟𝑖𝑛𝑔 𝑠𝑡𝑟𝑎𝑖𝑛) 𝜀 =
𝑙
Where
𝑙 = 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑒𝑑 𝑙𝑒𝑛𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 , 𝑚𝑚
𝑙𝑜 = 𝑂𝑟𝑔𝑖𝑛𝑎𝑙 𝑙𝑒𝑛𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛, 𝑚𝑚