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Metal Forging
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Metal Forging
Hot Vs. Cold Forging
Process Classification
Open Die Forging
Impression Die Forging
Precision Forging
Flashless Forging
Metal Forgeability
Metal Forging Defects
Forging Die Material
Forging Die Design
Formation Of Flash
Ribs And Webs
Fillet Radius
Draft Angle In Forging Die
Parting Line Location
Metal Forging Process Design
Metal forging is a metal forming process that involves applying
compressive forces to a work piece to deform it, and create a desired
geometric change to the material. The forging process is very important
in industrial metal manufacture, particularly in the extensive iron and
steel manufacturing industry. A steel forge is often a source of great
output and productivity. Work stock is input to the forge, it may be rolled,
it may also come directly from cast ingots or continuous castings. The
forge will then manufacture steel forgings of desired geometry and
specific material properties. These material properties are often greatly
Metal forging is known to produce some of the strongest manufactured
parts compared to other metal manufacturing processes, and obviously, is
not just limited to iron and steel forging but to other metals as well.
Different types of metals will have a different factors involved when
forging them, some will be easier to forge than others. Various tests are
described latter to determine forging process factors for different
materials. Aluminum, magnesium, copper, titanium, and nickel alloys are
also commonly forged metals. It is important to understand the principles
of manufacturing forged products, including different techniques and
basic metal forging design. The following will provide a comprehensive
overview of the metal forging process.
Metal forging, specifically, can strengthen the material by sealing cracks
and closing empty spaces within the metal. The hot forging process will
highly reduce or eliminate inclusions in the forged part by breaking up
impurities and redistributing their material throughout the metal work.
However, controlling the bulk of impurities in the metal should be a
consideration of the earlier casting process. Inclusions can cause stress
points in the manufactured product, something to be avoided. Forging a
metal will also alter the metal's grain structure with respect to the flow of
the material during its deformation, and like other forming processes, can
be used to create favorable grain structure in a material greatly
increasing the strength of forged parts. For these reasons, metal forging
manufacture gives distinct advantages in the mechanical properties of
work produced, over that of parts manufactured by other processes such
as only casting or machining.
Metal forgings can be small parts, or weigh as much as 700,000 lbs.
Products manufactured by forging in modern industry include critical
aircraft parts such as landing gear, shafts for jet engines and turbines,
structural components for transportation equipment such as automobiles
and railroads, crankshafts, levers, gears, connecting rods, hand tools such
as chisels, rivets, screws, and bolts to name a few. The manufacture of
forging die and the other high costs of setting up an operation make the
production of small quantities of forged parts expensive on a price per
unit basis. Once set up, however, operation costs for forging manufacture
can be relatively low, and many parts of the process may be automated.
These factors make manufacturing large quantities of metal forgings
economically beneficial.
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Hot Die Vs. Cold Die Forging
Most metal forging operations are carried out hot, due to the need to
produce large amounts of plastic deformation in the part, and the
advantage of an increased ductility and reduced strength of the work
material. Hot die forging also eliminates the problem of strain hardening
the metal. In cases where it is desirable to create a favorable strain
hardening of the part, cold die forging may be employed. Cold die forging
manufacture, while requiring higher forces, will also produce greater
surface finish and dimensional accuracy than hot die forging. Some
specific metal forging processes are always performed cold, such as
Classification Of Metal Forging
Metal forging processes can be classified by the degree to which the flow
of material is constrained during the process. There are three major
classifications in metal forging manufacture. First, open die forging, in
which the work is compressed between two die that do not constrain the
metal during the process. Secondly, impression die forging, in which
cavities within the die restrict metal flow during the compression of the
part, causing the material to deform into a desired geometric shape.
Some material in impression die forging is not constrained by the cavities
and flows outward from the die, this metal is called flash. In industrial
metal forging, a subsequent trimming operation will be performed to
remove the flash. The third type of metal forging is flashless forging. In
flashless forging manufacture the entire work piece is contained within
the die in such a way that no metal can flow out of the die cavity during
the compression of the part, hence no flash is produced.
Open Die Forging
The manufacturing process of metal forging has been performed for at
least 7,000 years, perhaps even 10,000 years. The most basic type of
forging would have been shaping some metal by striking it with a rock.
Latter the employment of different materials, such as bronze then iron
and steel, and the need for forged metal products such as swords and
armor, led way to the art of blacksmithing or blacksmith forging.
Blacksmithing is an open die forging process where the hammer and anvil
surfaces serve as opposing flat die. Bronze forgings, followed by iron and
steel forgings, mark some of man's earlier manufacturing prowess.
A simple type of open die forging is called upsetting. In an upsetting
process the work is placed between two flat die and its height is
decreased by compressive forces exerted between the two die. Since the
volume of a metal will remain constant throughout its deformation, a
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reduction in height will be accompanied by an increase in width. Figure
155 shows a flat die upsetting process, under ideal conditions.
In real conditions during industrial manufacturing, friction plays a part in
the process. Friction forces at the die-work interface oppose the
spreading of the material near the surfaces, while the material in the
center can expand more easily. The result is to create a barrel shape to
the part. This effect is called barreling in in metal forging terms.
Barreling is generally undesirable and can be controlled by the use of
effective lubrication. Another consideration, during hot forging
manufacture, that would act to increase the barreling effect would be the
heat transfer between the hot metal and the cooler die. The metal nearer
to the die surfaces will cool faster than the metal towards the center of
the part. The cooler material is more resistant to deformation and will
expand less than the hotter material in the center, also causing a
barreling effect.
Another common open die forging process performed in industrial metal
forging manufacture, involves using flat die to round an ingot. With the
use of mechanical manipulators, a work piece is compressed and rotated
in a series of steps eventually forming the metal into a cylindrical part.
The compressions affect the material of the forging, closing up holes and
gaps, breaking down and reforming weak grain boundaries, and creating
a wrought grain structure. As this open die forging process progresses
the material of the part will be altered from the outside first, progressing
inward. It is important that when manufacturing a metal forging by this
process, the part is worked significantly enough to change the structure
of the material in the center of the work piece. Large shafts for motors
and turbines are forged this way from cast ingots.
Cogging, or drawing out, is often used in manufacturing industry.
Cogging is an open die forging process in which flat or slightly contoured
die are employed to compress a work piece, reducing its thickness and
increasing its length. In a cogging operation, the forging is large relative
to the size of the die. The part is forged in a series of steps. After each
compression of the material, the open die advance along the length of the
work piece and perform another forging compression. The distance the
die travel forward on the work piece between each forging step is called
the bite, and is usually about 40 to 75 percent of the width of the die, in
industrial practice. A greater reduction in the thickness of the forged part
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can be accomplished by decreasing the width of the bite. Cogging allows
for smaller machinery with less power and forces to form work of great
length. Often in commercial manufacture of metal products, cogging may
be just one metal forging process in a series of metal forging processes
required to form a desired part. Sometimes formed products such as
metal fences may be produced directly from cogging.
A typical open die forging process performed in metal forging
manufacture is fullering. Fullering is mostly used as an earlier step to
help distribute the material of the work in preparation for further metal
forging operations. This often occurs when a manufacturing process
requires several forging operations to complete. The metal forging
process design section will discuss this concept later. In fullering, open
die with convex surfaces are used to deform the work piece. The result is
to cause metal to flow out of one area and to both sides.
Edging is also an open die forging process often used in manufacturing
practice, to prepare a work for sequential metal forging processes. In
edging, open die with concave surfaces plastically deform the work
material. Edging acts to cause metal to flow into an area from both sides.
Edging and fullering both are used to redistribute bulk quantities of the
metal forging's material.
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Impression Die Forging
Impression die forging manufacture involves compression of a work piece
by the use of impression die, (a mold), that contain cavities that act to
restrict the flow of metal within the die during the deformation of the
work. The metal will fill the space within the die cavity as it is plastically
compressed into the mold. Closing of the mold completes the
deformation, hence impression die forging is also referred to as closed die
forging. The forged metal part will now have the geometric dimensions of
the mold, provided a complete filling of the die cavity occurred during the
process. The operation of forcing metal to flow into and fill the
impressions in the die will also alter the grain structure of the metal. The
creation of favorable grain structure through controlled material
deformation should always be a consideration in the design of an
impression die forging process.
One characteristic of impression die forging manufacture is the formation
of flash or fin around the forged part. During the design of the metal
forging operation, the volume of the starting work piece is made slightly
higher than that of the closed die cavity. As the die close, and the work
metal flows into and fills the contours of the impression, some excess
material will flow out of the die and into the area between the two die.
This will form a thin plane of metal all around the work at the parting
line, (where the two die meet when they close), of the forged product.
Flash is trimmed from the forging in a latter process.
Precision Forging
Modern technological advances in the metal forging process and in the
design of die, have allowed for the development of precision forging.
Precision forging may produce some or no flash and the forged metal part
will be at or near its final dimensions, requiring little or no finishing. The
number of manufacturing operations is reduced as well as the material
wasted. In addition, precision forging can manufacture more complex
parts with thinner sections, reduced draft angles, and closer tolerances.
The disadvantages of these advanced forging methods are that special
machinery and die are needed, also more careful control of the
manufacturing process is required. In precision forging, the amount of
material in the work, as well as the flow of that material through the mold
must be accurately determined. Other factors in the process such as the
positioning of the work piece in the cavity must also be performed
Flashless Forging
Flashless forging is a type of precision forging process in which the entire
volume of the work metal is contained within the die and no material is
allowed to escape during the operation. Since no material can leave the
mold as the part is forged, no flash is formed. Like other precision forging
processes, flashless forging has rigorous process control demands,
particularly in the amount of material to be used in the work piece. Too
little material and the die will not fill completely, too much material will
cause a dangerous build up of forces.
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Metal Forgeability
Metal selection must be considered carefully in forging manufacture. The
ability of a metal to experience deformation without failure or cracking is
an important characteristic to consider in its selection as a material for a
forging process. In metal forging industry, several tests have been
developed to try and quantify this ability. The amount of deformation a
particular metal can tolerate without failure is directly related to that
metal's forgeability. The higher the amount of deformation, the higher the
One popular test involves compressing a cylindrical work stock between
two flat die. This is called upsetting the work, thus this test is called the
upsetting test. In an upsetting test, the work stock is compressed by flat
open die, reducing the work in height until cracks form. The amount of
reduction can be considered a measurement of forgeability. Upsetting
tests can be performed at different temperatures and different
compression speeds. Testing various temperatures and strain rates will
help determine the best conditions for the forging of a particular metal.
Another common test used in modern industry is called the hot twist
test. In a hot twist test, a round bar is twisted in one direction until
material failure occurs. The amount of rotation is taken as a quantitative
measurement of metal forgeability. This test is often conducted on a
material at several different temperatures. Other tests are also used in
industrial metal forging manufacture. Impact testing is sometimes used to
gauge the forgeability of a material. Cracks in the metal are the common
criteria for failure for most tests, however, forgeability tests can also
determine other negative effects that a material may exhibit under
different conditions of stress, strain rate, and temperature.
Defects In Metal Forging
Inspection is an important aspect of metal forging manufacture. All parts
should be checked for defects after the manufacturing process is
complete. Defects of metal forged product include exterior cracking,
interior cracking, laps, cold shuts, warping of the part, improperly formed
sections and dead zones. Cracking both interior and exterior is caused by
excessive stress, or improper stress distribution as the part is being
formed. Cracking of a forging can be the result of poorly designed forging
die or excess material in the work piece. Cracks can also be caused by
disproportionate temperature distributions during the manufacturing
operation. High thermal gradients can cause cracks in a forged part.
Laps or folds in a metal forging are caused by a buckling of the part, laps
can be a result of too little material in the work piece. Cold shuts occur
when metal flows of different temperatures meet, they do not combine
smoothly, a boundary layer, (cold shut), forms at their intersection. Cold
shuts indicate that there is a problem with metal flow in the mold as the
part is being formed. Warping of a forged part can happen when thinner
sections cool faster than the rest of the forging. Improperly formed
sections and dead zones can be a result of too little metal in the work
piece or flawed forging die design resulting in incorrect material
distribution during the process.
In general, defects in parts manufactured by metal forging can be
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controlled first by careful consideration of work stock volume, and by
good design of both the forging die, (mold), and the process. The main
principle is to enact the right material distributions, and the right
material flow to accomplish these distributions. Die cavity geometry and
corner radius play a large roll in the action of the metal. Forging die
design, and forging process design will be discussed in later sections.
Lubrication In Industrial Metal
Forging Manufacture
Frictional forces within the mold, between the work and the surfaces of
the die cavity, have a large influence over the flow of material in a metal
forging operation. Lubricants are used in industrial metal forging
production in order to lower frictional forces, and enact a smoother flow
of metal through the mold. In addition, they are used to slow the cooling
of the work and reduce temperature gradients, in hot forging
manufacture, serving as a thermal barrier between the metal work and
the die. Lubricants also help keep the metal and die surfaces from
sticking together and assist in the removal of the metal forging from the
die. Common lubricants used in modern forging industry include, water,
mineral oil, soap, saw dust, graphite, molybdenum disulfide, and liquid
Forging Die Material
The exact material used to make a forging die,(mold), is dependant upon
all the details of that particular metal forging process. In general, a
forging die must be tough, possess high strength and hardness at
elevated temperatures, good shock resistance, resistance to thermal
gradients, hardenability and ability to withstand abrasive wear. During
the manufacture of a hot forged part, the forging die is usually preheated
before the operation begins. Preheating forging die reduces thermal
cycling that can cause cracks in the die.
Metal forging die are hardened and tempered. Forging die dimensions
must account for shrinkage of the work, as well as extra material
allowances for the finishing of the part. The abrasive wear present in hot
forging operations is due largely to the scale on the work stock. Much of
the scale can be removed from the blank immediately after heating in the
furnace, prior to the forging of the part. Adequate lubrication can also
greatly mitigate wear. Sometimes a forging die may be assembled using
different die sections. These sections, called die inserts, are manufactured
separately and may be of different materials. Complex cavities can be
produced easier with die inserts, also different sections of the forging die
can be individually replaced.
Some factors to consider when determining the material composition of a
forging die are, type of operation, number of die forgings, size of forged
parts, complexity of forged parts, type of machinery to be used,
temperature that the metal will be forged at, and the cost of materials.
Forging die are made from tool steels that, depending upon process
criteria, are alloyed with various levels of one or more of these materials,
chromium, molybdenum, vanadium, and nickel. Die blocks are cast from
the alloy, forged themselves, then machined, and finished.
Forging Die Design
Forging die design will always depend on the factors and requirements of
the manufacturing process. However, there are some general principles
to consider for good forging die design. During the forging process metal
is flowing under pressure to fill the impression within the die cavity,
(mold). Similar to the metal casting process of die casting, in metal
forging, an increase in pressure on the metal within the die will increase
the ability to fill the die cavity completely. One main difference between
the processes is that in die casting the metal is liquid, while in forging the
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work is a solid metal above or below its recrystallization temperature.
Smaller, thinner, longer, and more complex sections can be produced with
more pressure, but too much pressure within the die cavity is bad
because it can damage the die and machinery.
The formation of metal flash is an important part of impression die
forging manufacture. First, flash provides a way for excess material from
the work stock to exit the forging die. If this material could not escape
during the compression the build up of pressure, as the volume of work
metal exceeded the volume of the die cavity, could easily crack the die.
Flash, while allowing material to escape, does increase the pressure
within the die cavity. Flash must travel through a narrow passage, called
land, before it opens up into a gutter.
As it flows through land, the friction between the metal flash and the
mating surfaces resists further flow of material out of the die cavity,
increasing pressure within the forging die. In addition, the cooling of the
flash from the mating surfaces increases resistance to flow of material out
of the die, thus also increasing pressure within the die cavity. A longer
land will cause the metal flash to have to flow further under resistance,
increasing the die pressure. Decreasing the width of land will also
increase pressure within the forging die by increasing the cooling rate of
the flash, as the temperature goes down the metal's resistance to flow
goes up. More resistance to metal flow will cause a thinner land to create
higher die pressure. The pressure within the forging's die cavity is often
controlled by varying the width of land.
One of the main principles to remember when designing a forging die for
a specific manufacturing process is that while deformation of the metal is
occurring, the material will tend to flow in the direction of least
resistance. Proper metal flow within the die is important in ensuring a
complete filling of the die cavity, preventing defects, and in controlling
the grain structure of the forged part. Friction in the die is an important
consideration in metal forging manufacture. Friction will act to resist the
movement of the material and increase the forces required to fill the die
cavity during the process. More forces, in turn, mean more stress and
wear on the mold and equipment.
Another critical factor in the movement of material within the die cavity
during the forming of the part, is the interior geometry of the die cavity.
The size of the forged part, work material, complexity of the forged part,
size and thickness of different part features, and distance different areas
are from the parting line, are some of the important factors concerning
the structure of the forging die. Basically thinner more complex features
will be more difficult to fill completely, as would areas further from the
parting line or out of the way of the predominant flow of metal.
Thin portions of a metal forging are called ribs and webs. A rib is a
section that runs perpendicular to the forging plane as determined by the
parting line. Long narrow ribs are harder to fill and require more forces,
increasing the width of a long rib will better facilitate the filling of the rib
with material during the process. A web is a portion of the metal forging
that runs parallel to the forging plane. The thickness of webs can be
minimized as much as practical. When designing a forging die, web
thickness should not be too small or else there may be trouble completely
filling the web with metal. Webs that are too thin may also cool faster
than the rest of the metal forging, the resulting shrinkage could cause
tears or warping of the part.
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As the work material fills the die cavity, the flow of metal will have to
change directions depending upon the part's geometry. Smooth, large
filleted turns will allow the metal flow to change directions while adhering
to the die's dimensions. If corners within the metal forging are too sharp
then the material may not completely follow the path of those corners,
resulting in vacancies, laps, or cold shuts. Sharp corners will also act as
stress raisers within the die cavity. Good forging die design should
provide adequate enough fillet and corner radius to allow for easy metal
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Draft angle, in metal manufacturing processes, is the taper around the
internal and external sides of a part. Draft angle is necessary to include in
the forging die design in order to allow the removal of the work from the
die after the part has been forged. The larger the draft angle, the better it
will facilitate the metal forging's removal. As the metal forging cools, it
shrinks away from the outer surfaces of the die cavity, therefore exterior
draft angles are usually made smaller than interior angles.
In general, easier to forge metals, such as aluminum and magnesium,
require less draft angles than harder to forge materials, such as steel,
nickel, and titanium alloys. Often in metal forging operations, there is an
ejector to help push the part from the die cavity. However, ejectors are
not used in drop forging. Draft angle effects the complexity of the forging
that may be produced. The greater the draft angle, the more it limits
metal forging complexity. Some precision forging operations produce a
forged part with no draft angle. Common draft angles used in
manufacturing industry are 3, 5, 7, and 10 degrees.
Similar to the pattern in metal casting, the size of the die cavity in metal
forging manufacture should account for the size of the part, shrinkage of
the part during cooling, and allowances for machining and other finishing
operations that may follow the metal forging process.
Location of the parting line is of primary importance in metal forging die
design. The parting line, which defines the forging plane of the operation,
is a large determinant in how metal flows through the die during the
forging's compression. The parting line dictates where flash will be
formed, and effects the grain structure of the manufactured part. It is
easier to fill sections closer to the parting line than further away. In
determining a parting line the maximum periphery of the metal forging
should be considered.
Figure 167 shows a metal forging with three possible locations for a
parting line. The location of the parting line of C will better facilitate the
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flow of metal through the die cavity, since unlike A or B, location C makes
use of the maximum periphery of the forging. It is easier to fill material
near the forging plane than in the further recesses of the die cavity. In
addition to being a major factor in the flow of metal during the forging
process, the location of the parting line is also critical in the formation of
the grain structure of the forged work. The parting line acts to disrupt the
metal's grain structure.
Figure 168 also shows three possible parting line locations for a metal
forging. The placement of the parting line in A and B acts to disrupt the
grain structure of the metal at the plane through which it passes.
Locating the parting line at the top of the forging as in C eliminates the
rupture of the forging's grain structure. Also this particular location of
the parting line will allow for the entire impression to be formed in one
die, while the other die can be flat. Design of the die as in C is both more
economical and provides superior grain structure of the metal forging.
Forging Process Design
In modern manufacturing industry, metal parts of complex geometry are
often forged completely with the need for only minor finishing operations.
These parts can not be manufactured with a single forging. The work
stock is taken through a series of metal forging operations that, in steps,
alter the geometric shape of the material until the final process creates
the desired forging. In these types of design sequences each operation
must be planned in such a way as to prepare the work piece for the next
forging process. Together the series of metal forging operations that are
required to create a part, make a larger single process and each
individual forging operation has its place within the larger process.
When designing a complex metal forging process, great consideration
should be taken with each step and how it relates to the final product.
Also, design the chosen path for the redistribution of the work material
from the start of the process to the end of the last step, concentrating on
smooth metal flow. Forging design, in general, should first accomplish a
rough redistribution of the material, then the more detailed impression
die forging operations, and finally finishing operations. In addition to
providing a smooth transition of material the forging processes, as a
whole, should be designed to produce a controlled grain structure in the
final product. When choosing a path for material redistribution, a metal
forging design should consider how this particular method of metal
deformation will effect and change the grain structure of the part. It is
desirable that the final product contain a favorable grain orientation
throughout the structure of its material. Such a grain structure should
strengthen the part, particularly with respect to that part's application.
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Open die forging often plays a roll in the early stages, providing a general
mass redistribution of the work metal. Before the more detailed
impression forgings can shape the work, metal must be formed in such a
way as to place higher concentrations of material in regions that will
require more material. Fullering and edging of the metal, discussed in the
open die forging section, are very important open die forging processes
used to accomplish a rough transfer of material. Fullering and edging will
squeeze more metal into some areas of the work, while causing other
areas to have less depending on the needs of the process. Figure 170
shows two rough forms, one was subject to fullering the other to edging,
the nature of the different processes should be apparent.
Impression die forging occurs after the rough form has been shaped. This
closed die forging process will create the geometric features of the part
on the work. The flow of metal must be carefully designed both before
and during this phase. Finishing processes, such as sizing, create less but
very accurate geometric change to the forging in the final stages of part
manufacture. Figure 171 shows the different steps in the metal forging
process used to manufacture a complex part.
Most industrial metal forging products will be processed by further
manufacturing operations that will impart higher tolerances and
dimensional accuracy than forging manufacture alone. These operations,
(such as machining), although more accurate than forging, do not
produce the stronger material of forged metal work. By combining
different types of processes such as machining and metal forging, a
manufacturer can utilize the benefits of both processes, creating very
accurate parts, good surface finish and superior mechanical properties.
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