11-10-2016, 09:46 AM
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HISTORY
Forging is one of the oldest known metal working processes. Traditionally, forging was performed by a smith using hammer and anvil, though introducing water power to the production and working of iron in the 12th century drove the hammer and anvil into obsolescence. The smithy or forge has evolved over centuries to become a facility with engineered processes, production equipment, tooling, raw materials and products to meet the demands of modern industry. In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers may have reciprocating weights in the thousands of pounds. Smaller power hammers, 500 lb (230 kg) or less reciprocating weight, and hydraulic presses are common in art smithies as well. Some steam hammers remain in use, but they became obsolete with the availability of the other, more convenient, power sources.
ADVANTAGES AND DISADVANTAGES
Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics. Some metals may be forged cold, but iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forging, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable top precipitation hardening, such as most aluminum alloys and titanium, can be hot forged, followed by hardening.[citation needed]Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high-temperature furnace (sometimes referred to as the forge) is required to heating oats or billets. Owing to the massiveness of large forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations use metal-forming dies, which must be precisely machined and carefully heat-treated to correctly shape the work piece, as well as to withstand the tremendous forces involved.
Advantages:
Enhance in ductile strength significant temperatures will help in removal of homogeneous ingredients on account of accelerated diffusion.
Decline inside the pore size.
Disadvantages:
Lesser tolerance
Warping of substance through the cooling process
Undesirable finish off result on account of response of metal with that of surroundings
A cross-section of a forged connecting rod that has been etched to show the grain flow.
There are many different kinds of forging processes available, however they can be grouped into three main classes:
Drawn out: length increases, cross-section decreases.
Upset: length decreases, cross-section increases.
Squeezed in closed compression dies: produces multidirectional flow Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.
3.1 TEMPERATURE
Main articles : Hot working and Cold working.
All of the following forging processes can be performed at various temperatures, however they are generally classified by whether the metal temperature is above or below the recrystallization temperature. If the temperature is above the material's recrystallization temperature it is deemed hot forging; if the temperature is below the material's recrystallization temperature but above 30% of the recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 30% of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that as the metal is deformed work hardening effects are negated by the recrystallization process. Cold forging typically results in work hardening of the piece.
3.2 DROP FORGING
Drop forging is a forging process where a hammer is raised and then “dropped" onto the work piece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the work piece, while the latter does.
3.2.1 OPEN DIE DROP FORGING
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 work piece) do not enclose the work piece, allowing it to flow except where contacted by the dies. Therefore, the operator needs to orient and position the work piece 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 (including step shafts or with flanges), sleeves, cylinders, flats, hexes, rounds, plate, and some custom shapes. Open-die forging lends itself to short runs and is appropriate for art smiting and custom work. In some cases, open-die forging may be employed to rough-shaping to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction.
Advantages of open-die forging:
*.Reduced chance of voids
*.Better fatigue resistance
*.Improved microstructure
*.Continuous grain flow
*.Finer grain size
*.Greater strength
"Cogging" is the successive deformation of a bar along its length using an open-die drop forge. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via "edging"."Edging" is the process of concentrating material using a concave shaped open-die. The process is called "edging" because it is usually carried out on the ends of the work piece. "Fullering" is a similar process that thins out sections of the forging using a convex shaped die. These processes prepare the work pieces for further forging processes.
3.2.2 IMPRESSION DIE-FORGING
Impression-die forging is also called "closed-die forging". In impression-die forging, the metal is placed in a die resembling a mold, which is attached to an anvil. Usually, the hammer die is shaped as well. The hammer is then dropped on the work piece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the work piece on the scale of milliseconds. Depending on the size and complexity of the part, the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities, forming what are referred to as "flash". The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die, so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging, the flash is removed. In commercial impression-die forging, the work piece is usually moved through a series of cavities in a die to get from an ingot to the final form.
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", "fullering", 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. These stages usually impart the work piece with generous bends and large fillets.
The final shape is forged in a "final" or "finisher" impression cavity. If there is only a short run of parts to be done, then it may be more economical for the die to lack a final impression cavity and instead machine the final features. Impression-die forging has been improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging. One variation of impression-die forging is called "fleshless forging", or "true closed-die forging".
In this type of forging, the die cavities are completely closed, which keeps the work piece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and work piece placement. There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging perform from liquid metal.
The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed, and then the part is quench hardened. Another variation follows the same process as outlined above, except the perform is produced by the spraying deposition of metal droplets into shaped collectors (similar to the spray process).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 industries. Another reason forgings are common in these industrial sectors is that forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.
*.The dies part along a single, flat plane whenever possible. If not, the parting plane follows the contour of the part.
*.The parting surface is a plane through the center of the forging and not near an upper or lower edge.
*.Adequate draft is provided; usually at least 3° for aluminum and 5° to 7° for steel.
*.Generous fillets and radii are used.
*.Ribs is low and wide.
*.The various sections are balanced to avoid extreme difference in metal flow.
*.Full advantage is taken of fiber flow lines.
*.Dimensional tolerances are not closer than necessary.
The dimensional tolerances of a steel part produced using the impression-die forging methods are outlined in the table below. The dimensions across the parting plane are affected by the closure of the dies, and are therefore dependent on die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.
PRESS FORGING
Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the work piece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.
The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete work piece. Drop-hammer forging usually only deforms the surfaces of the work piece in contact with the hammer and anvil; the interior of the work piece will stay relatively unreformed. Another advantage to the process includes the knowledge of the new part's strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled.
There are a few disadvantages to this process, most stemming from the work piece being in contact with the dies for such an extended period of time. The operation is a time-consuming process due to the amount and length of steps. The work piece will cool faster because the dies are in contact with work piece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the work piece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore, heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The work piece may also need to be reheated.
When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer layers of the part, reduced levels of micro cracking occur in the finished part.
Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.
3.4 UPSET FORGING
Upset forging increases the diameter of the work piece by compressing its length. Based on number of pieces produced, this is the most widely used forging process. A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.
Upset forging is usually done in special high-speed machines called crank presses, but upsetting can also be done in a vertical crank press or a hydraulic press. The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of work pieces from one station to the next. The initial work piece is usually wire or rod, but some machines can accept bars up to 25 cm (9.8 in) in diameter and a capacity of over 1000 tons. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the work piece to move from one cavity to the next; the dies then close and the heading tool, or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle, then a finished part will be produced with every cycle, which makes this process advantageous for mass production.
These rules must be followed when designing parts to be upset forged:
*.The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar.
*.Lengths of stock greater than three times the diameter may be upset successfully, provided that the diameter of the upset is not more than 1.5 times the diameter of the stock.
*.In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.
3.5 AUTOMATIC HOT FORGING
The automatic hot forging process involves feeding mill-length steel bars (typically 7 m (23 ft) long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs rapidly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to 6 kg (13 lb), and up to 18 cm (7.1 in) in diameter. The main advantages to this process are its high output rate and ability to accept low-cost materials. Little labor is required to operate the machinery.
There is no flash produced so material savings are between 20 and 30% over conventional forging. The final product is a consistent 1,050 °C (1,920 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±0.3 mm (0.012 in), surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 0.06-second. The downside is that this process is only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.
The process starts by heating the bar to 1,200 to 1,300 °C (2,190 to 2,370 °F) in less than 60 seconds using high-power induction coils. It is then descaled with rollers, sheared into blanks, and transferred through several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be coupled with high-speed cold-forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.
Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for LP gas cylinders. Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.
6 ROLL FORGING
Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated baris inserted into the rolls and when it hits a spot the rolls rotate and the bar is progressively shaped as it is rolled through the machine. The piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the work piece.
Examples of products produced using this method include axles, tapered levers and leaf springs.
3.7 NET-SHAPE AND NEAR NET-SHAPE FORGING
This process is also known as precision forging. It was developed to minimize cost and waste associated with post-forging operations. Therefore, the final product from precision forging needs little or no final machining. Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining. Precision forging also requires less of a draft, 1° to 0°. The downside of this process is its cost therefore it is only implemented if significant cost reduction can be achieved.
3.8 INDUCTION FORGING
Unlike the above processes, induction forging is based on the type of heating style used. Many of the above processes can be used in conjunction with this heating method.
3.9 MMULTIDIRECTIONAL FORGING
Multidirectional Forming is forming of a work piece in a single step in several directions. The multidirectional forming takes place through constructive measures of the tool. The vertical movement of the press ram is redirected using wedges which distributes and redirects the force of the forging press in horizontal directions.
MATERIALS AND APPLICATION
4.1 FORGING OF STEEL
Depending on the forming temperature steel forging can be divided into:
*.Hot forging of steel
*.Forging temperatures above the recrystallization temperature between
950 - 1250 °C
*.Good formability
*.Low forming forces
*.Constant tensile strength of the work pieces
*.Warm forging of steel*.Forging temperatures between 750 – 950 °C
*.Less or no scaling at the work piece surface
*.Narrower tolerances achievable than in hot forging
*.Limited formability and higher forming forces than for hot forging
*.Lower forming forces than in cold forming
*.Cold forging of steel
*.Forging temperatures at room conditions, self-heating up to 150 °C due
to the forming energy
*.Narrowest tolerances achievable
*.No scaling at work piece surface
*.Increase of strength and decrease of ductility due to strain hardening
4.2 FORGING OF ALLUMINIUM
*.Aluminum forging is performed at a temperature range between 350 and 550 °C
*.Forging temperatures above 550 °C are too close to the solidus temperature of the alloys and lead in conjunction with varying effective strains to unfavorable work piece surfaces and potentially to a partial melting as well as fold formation.
*.Forging temperatures below 350 °C reduce formability by increasing the yield stress, which can lead to unfilled dies, cracking at the work piece surface and increased die forces.
Due to the narrow temperature range and high thermal conductivity, aluminum forging can only be realized in a particular process window. To provide good forming conditions a homogeneous temperature distribution in the entire work piece is necessary. Therefore, the control of the tool temperature has a major influence to the process.
4.2.1 APPLICATIONS OF ALLUMINIUM FORGED PARTS
High-strength aluminum alloys have the tensile strength of medium strong steel alloys while providing significant weight advantages. Therefore, aluminum forged parts are mainly used in aerospace, automotive industry and many other fields of engineering especially in those fields, where highest safety standards against failure by abuse, by shocker vibratory stresses are needed. Such parts are for example chassis parts, steering components and brake parts.
FORGING PRESSES
A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks and/or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system, different forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 short tons-force). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages include a slower, larger, and costlier machine to operate.
6. COMPONENTS USED IN PNEUMATIC FORGING MACHINE
Air compressor
Pneumatic cylinder
Solenoid valve
Die
Hammer (punch)
Hoses
6.1 AIR COMPRESSOR
An air compressor is a device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces more and more air into a storage tank, increasing the pressure. When tank pressure reaches its upper limit the air compressor shuts off. The compressed air, then, is held in the tank until called into use. The energy contained in the compressed air can be used for a variety of applications, utilizing the kinetic energy of the air as it is released and the tank depressurizes. When tank pressure reaches its lower limit, the air compressor turns on again and re-pressurizes the tank.
The three basic types of air compressors are:
*.Rotary centrifugal compressor
*.Rotary screw compressor
*.Reciprocating compressor
These types are further specified by:
*.The number of compression stages
*.Cooling method (air, water, oil)
*.Drive method (motor, engine, steam, other)
*.Lubrication
6.1.1 ROTARY CENTRIFUGAL COMPRESSOR
Components of a simple centrifugal compressor:
A simple centrifugal compressor has four components:
a) Inlet
b) Impeller/Rotor
c) Diffuser
d) Collector
1 INLET VALVE
The inlet to a centrifugal compressor is typically a simple pipe. It may include features such as a valve, stationary vanes/airfoils (used to help swirl the flow) and both pressure and temperature instrumentation. All of these additional devices have important uses in the control of the centrifugal compressor.
6.1.1.2 CENTRIFUGAL IMPELLER
The key component that makes a compressor centrifugal is the centrifugal impeller, which contains a rotating set of vanes (or blades) that gradually raises the energy of the working gas. This is identical to an axial compressor with the exception that the gases can reach higher velocities and energy levels through the impeller's increasing radius. In many modern high-efficiency centrifugal compressors the gas exiting the impeller is traveling near the speed of sound.
Impellers are designed in many configurations including "open" (visible blades), "covered or shrouded", "with splitters" (every other inducer removed) and "w/o splitters" (all full blades). Most modern high efficiency impellers use "back sweep" in the blade shape.
6.1.1.3 DIFFUSER
The next key component to the simple centrifugal compressor is the diffuser. Downstream of the impeller in the flow path, it is the diffuser's responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity. Diffusers can be vane less, vane or an alternating combination. High efficiency vaned diffusers are also designed over a wide range of solidities from less than 1 to over Hybrid versions of vaned diffusers include wedge, channel, and pipe diffusers. There are turbocharger applications that benefit by incorporating no diffuser.
6.1.1.4 COLLECTER
The collector of a centrifugal compressor can take many shapes and forms. When the diffuser discharges into a large empty chamber, the collector may be termed a Plenum. When the diffuser discharges into a device that looks somewhat like a snail shell, bull's horn or a French horn, the collector is likely to be termed evolutes or scroll. As the name implies, a collector’s purpose is to gather the flow from the diffuser discharge annulus and deliver this flow to a downstream pipe. Either the collector or the pipe may also contain valves and instrumentation to control the compressor.
6.1.2 ROTARY SCREW COMPRESSOR
A Rotary-screw compressor is a type of gas compressor that uses a rotary-type positive-displacement mechanism. They are commonly used to replace piston compressors where large volumes of high-pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.
HISTORY
Forging is one of the oldest known metal working processes. Traditionally, forging was performed by a smith using hammer and anvil, though introducing water power to the production and working of iron in the 12th century drove the hammer and anvil into obsolescence. The smithy or forge has evolved over centuries to become a facility with engineered processes, production equipment, tooling, raw materials and products to meet the demands of modern industry. In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers may have reciprocating weights in the thousands of pounds. Smaller power hammers, 500 lb (230 kg) or less reciprocating weight, and hydraulic presses are common in art smithies as well. Some steam hammers remain in use, but they became obsolete with the availability of the other, more convenient, power sources.
ADVANTAGES AND DISADVANTAGES
Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics. Some metals may be forged cold, but iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forging, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable top precipitation hardening, such as most aluminum alloys and titanium, can be hot forged, followed by hardening.[citation needed]Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high-temperature furnace (sometimes referred to as the forge) is required to heating oats or billets. Owing to the massiveness of large forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations use metal-forming dies, which must be precisely machined and carefully heat-treated to correctly shape the work piece, as well as to withstand the tremendous forces involved.
Advantages:
Enhance in ductile strength significant temperatures will help in removal of homogeneous ingredients on account of accelerated diffusion.
Decline inside the pore size.
Disadvantages:
Lesser tolerance
Warping of substance through the cooling process
Undesirable finish off result on account of response of metal with that of surroundings
A cross-section of a forged connecting rod that has been etched to show the grain flow.
There are many different kinds of forging processes available, however they can be grouped into three main classes:
Drawn out: length increases, cross-section decreases.
Upset: length decreases, cross-section increases.
Squeezed in closed compression dies: produces multidirectional flow Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.
3.1 TEMPERATURE
Main articles : Hot working and Cold working.
All of the following forging processes can be performed at various temperatures, however they are generally classified by whether the metal temperature is above or below the recrystallization temperature. If the temperature is above the material's recrystallization temperature it is deemed hot forging; if the temperature is below the material's recrystallization temperature but above 30% of the recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 30% of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that as the metal is deformed work hardening effects are negated by the recrystallization process. Cold forging typically results in work hardening of the piece.
3.2 DROP FORGING
Drop forging is a forging process where a hammer is raised and then “dropped" onto the work piece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the work piece, while the latter does.
3.2.1 OPEN DIE DROP FORGING
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 work piece) do not enclose the work piece, allowing it to flow except where contacted by the dies. Therefore, the operator needs to orient and position the work piece 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 (including step shafts or with flanges), sleeves, cylinders, flats, hexes, rounds, plate, and some custom shapes. Open-die forging lends itself to short runs and is appropriate for art smiting and custom work. In some cases, open-die forging may be employed to rough-shaping to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction.
Advantages of open-die forging:
*.Reduced chance of voids
*.Better fatigue resistance
*.Improved microstructure
*.Continuous grain flow
*.Finer grain size
*.Greater strength
"Cogging" is the successive deformation of a bar along its length using an open-die drop forge. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via "edging"."Edging" is the process of concentrating material using a concave shaped open-die. The process is called "edging" because it is usually carried out on the ends of the work piece. "Fullering" is a similar process that thins out sections of the forging using a convex shaped die. These processes prepare the work pieces for further forging processes.
3.2.2 IMPRESSION DIE-FORGING
Impression-die forging is also called "closed-die forging". In impression-die forging, the metal is placed in a die resembling a mold, which is attached to an anvil. Usually, the hammer die is shaped as well. The hammer is then dropped on the work piece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the work piece on the scale of milliseconds. Depending on the size and complexity of the part, the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities, forming what are referred to as "flash". The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die, so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging, the flash is removed. In commercial impression-die forging, the work piece is usually moved through a series of cavities in a die to get from an ingot to the final form.
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", "fullering", 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. These stages usually impart the work piece with generous bends and large fillets.
The final shape is forged in a "final" or "finisher" impression cavity. If there is only a short run of parts to be done, then it may be more economical for the die to lack a final impression cavity and instead machine the final features. Impression-die forging has been improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging. One variation of impression-die forging is called "fleshless forging", or "true closed-die forging".
In this type of forging, the die cavities are completely closed, which keeps the work piece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and work piece placement. There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging perform from liquid metal.
The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed, and then the part is quench hardened. Another variation follows the same process as outlined above, except the perform is produced by the spraying deposition of metal droplets into shaped collectors (similar to the spray process).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 industries. Another reason forgings are common in these industrial sectors is that forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.
*.The dies part along a single, flat plane whenever possible. If not, the parting plane follows the contour of the part.
*.The parting surface is a plane through the center of the forging and not near an upper or lower edge.
*.Adequate draft is provided; usually at least 3° for aluminum and 5° to 7° for steel.
*.Generous fillets and radii are used.
*.Ribs is low and wide.
*.The various sections are balanced to avoid extreme difference in metal flow.
*.Full advantage is taken of fiber flow lines.
*.Dimensional tolerances are not closer than necessary.
The dimensional tolerances of a steel part produced using the impression-die forging methods are outlined in the table below. The dimensions across the parting plane are affected by the closure of the dies, and are therefore dependent on die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.
PRESS FORGING
Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the work piece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.
The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete work piece. Drop-hammer forging usually only deforms the surfaces of the work piece in contact with the hammer and anvil; the interior of the work piece will stay relatively unreformed. Another advantage to the process includes the knowledge of the new part's strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled.
There are a few disadvantages to this process, most stemming from the work piece being in contact with the dies for such an extended period of time. The operation is a time-consuming process due to the amount and length of steps. The work piece will cool faster because the dies are in contact with work piece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the work piece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore, heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The work piece may also need to be reheated.
When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer layers of the part, reduced levels of micro cracking occur in the finished part.
Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.
3.4 UPSET FORGING
Upset forging increases the diameter of the work piece by compressing its length. Based on number of pieces produced, this is the most widely used forging process. A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.
Upset forging is usually done in special high-speed machines called crank presses, but upsetting can also be done in a vertical crank press or a hydraulic press. The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of work pieces from one station to the next. The initial work piece is usually wire or rod, but some machines can accept bars up to 25 cm (9.8 in) in diameter and a capacity of over 1000 tons. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the work piece to move from one cavity to the next; the dies then close and the heading tool, or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle, then a finished part will be produced with every cycle, which makes this process advantageous for mass production.
These rules must be followed when designing parts to be upset forged:
*.The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar.
*.Lengths of stock greater than three times the diameter may be upset successfully, provided that the diameter of the upset is not more than 1.5 times the diameter of the stock.
*.In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.
3.5 AUTOMATIC HOT FORGING
The automatic hot forging process involves feeding mill-length steel bars (typically 7 m (23 ft) long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs rapidly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to 6 kg (13 lb), and up to 18 cm (7.1 in) in diameter. The main advantages to this process are its high output rate and ability to accept low-cost materials. Little labor is required to operate the machinery.
There is no flash produced so material savings are between 20 and 30% over conventional forging. The final product is a consistent 1,050 °C (1,920 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±0.3 mm (0.012 in), surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 0.06-second. The downside is that this process is only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.
The process starts by heating the bar to 1,200 to 1,300 °C (2,190 to 2,370 °F) in less than 60 seconds using high-power induction coils. It is then descaled with rollers, sheared into blanks, and transferred through several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be coupled with high-speed cold-forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.
Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for LP gas cylinders. Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.
6 ROLL FORGING
Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated baris inserted into the rolls and when it hits a spot the rolls rotate and the bar is progressively shaped as it is rolled through the machine. The piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the work piece.
Examples of products produced using this method include axles, tapered levers and leaf springs.
3.7 NET-SHAPE AND NEAR NET-SHAPE FORGING
This process is also known as precision forging. It was developed to minimize cost and waste associated with post-forging operations. Therefore, the final product from precision forging needs little or no final machining. Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining. Precision forging also requires less of a draft, 1° to 0°. The downside of this process is its cost therefore it is only implemented if significant cost reduction can be achieved.
3.8 INDUCTION FORGING
Unlike the above processes, induction forging is based on the type of heating style used. Many of the above processes can be used in conjunction with this heating method.
3.9 MMULTIDIRECTIONAL FORGING
Multidirectional Forming is forming of a work piece in a single step in several directions. The multidirectional forming takes place through constructive measures of the tool. The vertical movement of the press ram is redirected using wedges which distributes and redirects the force of the forging press in horizontal directions.
MATERIALS AND APPLICATION
4.1 FORGING OF STEEL
Depending on the forming temperature steel forging can be divided into:
*.Hot forging of steel
*.Forging temperatures above the recrystallization temperature between
950 - 1250 °C
*.Good formability
*.Low forming forces
*.Constant tensile strength of the work pieces
*.Warm forging of steel*.Forging temperatures between 750 – 950 °C
*.Less or no scaling at the work piece surface
*.Narrower tolerances achievable than in hot forging
*.Limited formability and higher forming forces than for hot forging
*.Lower forming forces than in cold forming
*.Cold forging of steel
*.Forging temperatures at room conditions, self-heating up to 150 °C due
to the forming energy
*.Narrowest tolerances achievable
*.No scaling at work piece surface
*.Increase of strength and decrease of ductility due to strain hardening
4.2 FORGING OF ALLUMINIUM
*.Aluminum forging is performed at a temperature range between 350 and 550 °C
*.Forging temperatures above 550 °C are too close to the solidus temperature of the alloys and lead in conjunction with varying effective strains to unfavorable work piece surfaces and potentially to a partial melting as well as fold formation.
*.Forging temperatures below 350 °C reduce formability by increasing the yield stress, which can lead to unfilled dies, cracking at the work piece surface and increased die forces.
Due to the narrow temperature range and high thermal conductivity, aluminum forging can only be realized in a particular process window. To provide good forming conditions a homogeneous temperature distribution in the entire work piece is necessary. Therefore, the control of the tool temperature has a major influence to the process.
4.2.1 APPLICATIONS OF ALLUMINIUM FORGED PARTS
High-strength aluminum alloys have the tensile strength of medium strong steel alloys while providing significant weight advantages. Therefore, aluminum forged parts are mainly used in aerospace, automotive industry and many other fields of engineering especially in those fields, where highest safety standards against failure by abuse, by shocker vibratory stresses are needed. Such parts are for example chassis parts, steering components and brake parts.
FORGING PRESSES
A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks and/or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system, different forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 short tons-force). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages include a slower, larger, and costlier machine to operate.
6. COMPONENTS USED IN PNEUMATIC FORGING MACHINE
Air compressor
Pneumatic cylinder
Solenoid valve
Die
Hammer (punch)
Hoses
6.1 AIR COMPRESSOR
An air compressor is a device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces more and more air into a storage tank, increasing the pressure. When tank pressure reaches its upper limit the air compressor shuts off. The compressed air, then, is held in the tank until called into use. The energy contained in the compressed air can be used for a variety of applications, utilizing the kinetic energy of the air as it is released and the tank depressurizes. When tank pressure reaches its lower limit, the air compressor turns on again and re-pressurizes the tank.
The three basic types of air compressors are:
*.Rotary centrifugal compressor
*.Rotary screw compressor
*.Reciprocating compressor
These types are further specified by:
*.The number of compression stages
*.Cooling method (air, water, oil)
*.Drive method (motor, engine, steam, other)
*.Lubrication
6.1.1 ROTARY CENTRIFUGAL COMPRESSOR
Components of a simple centrifugal compressor:
A simple centrifugal compressor has four components:
a) Inlet
b) Impeller/Rotor
c) Diffuser
d) Collector
1 INLET VALVE
The inlet to a centrifugal compressor is typically a simple pipe. It may include features such as a valve, stationary vanes/airfoils (used to help swirl the flow) and both pressure and temperature instrumentation. All of these additional devices have important uses in the control of the centrifugal compressor.
6.1.1.2 CENTRIFUGAL IMPELLER
The key component that makes a compressor centrifugal is the centrifugal impeller, which contains a rotating set of vanes (or blades) that gradually raises the energy of the working gas. This is identical to an axial compressor with the exception that the gases can reach higher velocities and energy levels through the impeller's increasing radius. In many modern high-efficiency centrifugal compressors the gas exiting the impeller is traveling near the speed of sound.
Impellers are designed in many configurations including "open" (visible blades), "covered or shrouded", "with splitters" (every other inducer removed) and "w/o splitters" (all full blades). Most modern high efficiency impellers use "back sweep" in the blade shape.
6.1.1.3 DIFFUSER
The next key component to the simple centrifugal compressor is the diffuser. Downstream of the impeller in the flow path, it is the diffuser's responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity. Diffusers can be vane less, vane or an alternating combination. High efficiency vaned diffusers are also designed over a wide range of solidities from less than 1 to over Hybrid versions of vaned diffusers include wedge, channel, and pipe diffusers. There are turbocharger applications that benefit by incorporating no diffuser.
6.1.1.4 COLLECTER
The collector of a centrifugal compressor can take many shapes and forms. When the diffuser discharges into a large empty chamber, the collector may be termed a Plenum. When the diffuser discharges into a device that looks somewhat like a snail shell, bull's horn or a French horn, the collector is likely to be termed evolutes or scroll. As the name implies, a collector’s purpose is to gather the flow from the diffuser discharge annulus and deliver this flow to a downstream pipe. Either the collector or the pipe may also contain valves and instrumentation to control the compressor.
6.1.2 ROTARY SCREW COMPRESSOR
A Rotary-screw compressor is a type of gas compressor that uses a rotary-type positive-displacement mechanism. They are commonly used to replace piston compressors where large volumes of high-pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.