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Automobile Manufacturing System

Chapter 1. Forging

1.1 Introduction:

Forging is one of the oldest metal working operations dating back to atleast 4000 B.C. Forging was traditionally done by blacksmiths. The equipments used were heavy hand hammer and simple anvil.Plastic deformation occurs when metal is stretched or compressed beyond elastic limit. During the deformation the metal flows plastically and grain shapes are changed. If the deformation is carried out at higher temperature, total new grains are formed. This process of formation of new grains is known as recrystallisation. The temperature at which this process is complete is known as the recrystallisation temperature. This temperature varies from metal to metal.Plastic deformation of metal below its recrystallization temperature is called as cold working, where as plastic deformation of metal above its recrystallisation temperature, but below melting temperature is called as hot working.1.2 Forging:

Forging can be defined as the controlled plastic deformation of metals at elevated temperatures in to a predetermined size or shape using compressive forces exerted, through some type of die, by a hammer, a press or upsetting machine.Forging enhances the mechanical properties of metals and improves the grain flow, which in turn increases the strength and toughness of the forged components.

1.3 Forgeability:Dhaigude. P. B.


Forgeability can be defined as the tolerance of a metal or alloy for deformation without failure. Thus good forgeability means less resistance to deformation and even adverse effects such as cracking are not there. Forgeability can be evaluated on basis of following tests:

(a)Hot twist test(b)Upset test(c) Hot impact test

(a)Hot Twist Test: In this test, a hot bar is twisted and numbers of twists before failure are counted. A large number of twists before failure indicate better forgeability. (b)Upset Test: In this test the cylindrical billets are upset-forged to various thickness. The limit of upset forging without failure or cracking is considered measure of forgeability. This test is widely used in forging industry.(c) Hot-Impact Tensile Test: A conventional impact-testing machine fitted with a tension test attachment is used. The impact tensile test is taken as measure of forgeability.

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1.4 Forging Materials:

The selection of forging material is made on the basis of certain desirable mechanical properties inherent in the composition of material and some properties can be developed by forging such as strength, resistance to fatigue, good machining characteristics, durability etc.Following is the list of some alloys in ascending order of forgeability

1. Aluminium alloys 5. Low-alloy steels 9. Titanium alloys2. Magnesium Alloys

6.Martensitic stainless Steels

10. Tantalum alloys

3. Copper alloys 7. Austenitic stainless Steels

11. Molybdenum alloys

4. Plain carbon Steels

8. Nickel alloys 12. Tungsten alloys

1.5 Forging Temperatures:

For successful forging, the metal piece must be heated to proper temperature to attain plastic properties before deformation. Excessive temperature may result in the burning of the metal. Insufficient temperature will not induce sufficient plasticity, so it will be difficult to shape by hammering.The finishing temperature is also important to get a fine grained structure. The temperature ranges for some common metals are given in table 1.1

Sr.No. Metal / AlloyForging Temperature oC

Starting Finishing01 Mild steel 1300 80002 Wrought iron 1275 900

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03 Medium carbon steel 1250 75004 High carbon steel 1150 82505 Copper, brass and bronze 950 600

06Aluminium and magnesium alloys

500 350

Table 1.1 Forging Temperatures

1.7 Advantages of forging I.Strength:

Forging reduces the failures. In this process workpiece yields with high strength to weight ratio. Due to this, it can be able to withstand fluctuating stress caused by sudden shock loading.

II. Metal conservation:

Practically there is no waste of metals.

III. Weight saving:

Strong thin-walled parts may be produced without damaging important physical requirements.

IV. Machining time:

Forging can be made to close tolerances, which reduces machining time for finishing operations of the products.

V. Speed of production:

High rate of production is possible.

VI. Incorporation in welded structures: Parts can be welded easily due to fibrous structure.

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VII. It maintains uniform and same quality all over parts.

VIII. Close tolerances.

IX. Smooth surface finish.

1.8 Disadvantages of forging partsI. High tool cost.

II. High tool maintenance.

III. No cord holes.

IV. Limitation in size and shape.

V. Heat treatment process increases cost of the product.

VI. Brittle materials like cast iron can not be forged.

VII. Complex shape can not be produced by forging

1.9 Classification of forging process:Mainly forging process classified into two parts.

I. Open die forging:

a) Hand forging

b) Power forging:

i. Hammer forging

ii. Press forging

II. Close die forging:

a) Drop forging

b) Press forging

c) Machine forging

I. Open die forging:

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It is the simplest forging process. This process also known as smith die forging or flat die forging. A solid workpiece placed between two flat face dies as shown in fig.1.1 (a) and reduces their height by compressing it.

Fig. 1.1 Open Die Forging Processes

The die surfaces have some cavity to produce relatively simple forging. Deformation of workpiece takes place in ideal condition as shown in fig.1.1 (b). Due to constant volume, reduction in height increases the diameter of forged part. But in actual working conditions part gets developed in barrel shape as shown in fig.1.1 (c). This barreling is caused due to friction force between workpiece and die surfaces. Barreling can be reduced easily with help of lubricant.

Smith die forging process sub-divided into two parts

a) Hand process:

According to the name this process purely done by hand. The forging is done on a anvil by hammering the workpiece at suitable temperature.

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This temperature attained by heating the workpiece with tongs.

Fig. 1.2 A Pair Of Fullers And Swages

Hand forging consist of two formers as shown in fig.1.2. These formers are made of high carbon steel in different size to suit various types of workpiece. It has two formers top and bottom formers. Bottom formers also known as fuller. Working edges of the fullers and formers are normally rounded. These are used for making necks by reducing cross-section of a workpiece.

Generally are used in maintenance shops as well as rework area. But now a days, a new forging process is developed known as power forging.

b) Power forging:

Heavy machine parts can not be forged by hand. Low blow of a hand or hammer is not able to produce a great degree of deformation in a workpiece.

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The machine or equipments driven by external source having high power are used in power forging.

Power forging also sub-divided into two parts

i. Hammer forging:

Machine which is work by blow or impact to perform forging process is known as hammer forging

ii. Press forging:

Machine which is work on pressure to perform forging process is known as press forging.

II. Close die forging:

In open die forging Complex shape with great accuracy is very difficult to forged. Close die forging process consist of specially prepared dies to manufacturing forged parts in large quantities. It is also known as impression die forging.

Fig. 1.3 Close Die Forging

It consist of two dies as shown in fig.1.3. the workpiece takes the shape of the die cavities ( impression ) during forging process. Some materials comes outwards and forms a flash. The flash has a very important role in the grain flow of material in close die forging.

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Thin flash cools very easily. Flash have frictional resistance gives high pressure to the material in die cavity. So it helps to filling of die cavity.

Generally this process is used for mass production with high degree of accuracy.

This process sub-divided into two parts

a) Drop forging:

Drop forging is used when mass production of identical parts is required. It is also called as stamping. It consist of special type of hammer known as drop hammer or drop stamp.

Perfectly made steel dies are used for forging. these dies are made into two parts as shown in fig.1.4. One part is connected to the top side and other part is fastened to the anvil block at the bottom side. The top part of the die is raised by mechanical links to a certain height. The heated metal placed accurately in the bottom part of the die. The top part of the die is then allowed to fall suddenly. This gives a high blow and completes the workpiece in a single operation. This

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process done due to squeezing of the metal into the die cavities.

Board being raised Raised board held a proper height

before dropping

Fig. 1.4 Working Principle Of A Drop Hammer

The devices are used for raising the top part of the die is given below

i. Belt

ii. Rope

iii. Friction rolls and board.

Working process:

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At

b) Press forging:

The machine which is work on pressure to perform forging process is known as press forging. This process consist of slow squeezing of metal in closed impression dies, instead of Appling repeated blow by hammers.

Generally two types of presses are used

i. Hydraulic presses:

These presses are used for heavy work. Hydraulic presses are less faster than mechanical presses. But hydraulic presses produce greater squeezing force than mechanical presses.

ii. mechanical presses:

These presses are used for light work. Mechanical presses are faster than hydraulic presses. But mechanical presses not able to produce greater squeezing force than hydraulic presses.

In this presses more complicated shapes and better dimensional accuracy can be easily achieved. The alignment of the die is very easy as compared to drop hammer forging. These presses and dies have longer life than hammer forging. Press forging requires less skill operators. These presses produce less noise and vibration.

c) Machine forging:

This process also known as hot heading or up-setting. This process consist of applying pressure longitudinally on a hot bar clamped or gripped between grooved dies. Forging is done on the

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end of the bar. Forging is done on various shapes of metals. But most commonly used shape is round shape metals. The equipments used for forging is forging machine or up-setter. Generally it gives forging pressure in a horizontal directions.

Fig. 1.5 Machine Forging

These dies are so designed such that complete operation is performed in several stages and gradually final shape is obtained. Operation performed with a die and punch called heading tool. Fig. 1.5 shows step by step operation done on the round bar. Die is either made hallow to receive the round bar through it or in two parts to open out and receive the bar. Heading tool is advanced in to die. Many strokes of the heading tool may be needed to complete the forging. In this process dies does not have draft as well as flash, so it gives better dimensional accuracy.

1.10 Difference between drop forging and press forging:

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Sr. No.

Drop forging Press forging

1) This process involves fast squeezing of metal in dies by applying repeated blows by hammers.

This process involves slow squeezing of metal in dies by applying pressure force.

2) The dies used relatively more draft and therefore more complicated shape can not be forged.

The dies used relatively less draft and therefore more complicated shape can be forged.

3) Alignment of two dies halves is difficult.

Alignment of two dies halves is easy.

4) The life of machines and dies are shorter.

The life of presses and dies are longer.

5) This process requires highly skilled operator.

This process does not requires highly skilled operator.

6) This process have more noise and vibrations.

This process have less noise and vibrations.

7) Production rate is slower. Production rate is faster.8) Less dimensional accuracy. Better dimensional accuracy.

1.11 Difference between open die forging and close die forging:

Sr. No.

Open die forging Close die forging

1) It is the simplest forging process.

It is the complex forging process.

2) This process requires simple and inexpensive dies.

This process requires complex and expensive dies.

3) It is useful only for small scale It is useful for small scale as Dhaigude. P. B.


production. well as large scale production.4) It is very difficult to hold close

tolerances. It is very easy to maintain close tolerances.

5) Relatively poor utilization of materials.

Relatively good utilization of materials.

6) This process requires highly skilled operator.

This process does not requires highly skilled operator.

7) This forging process have less dimensional accuracy.

This forging process have better dimensional accuracy.

8) This forging process have less reproducibility.

This forging process have better reproducibility.

9) This forging process have less die cost.

This forging process have high die cost.

10) In this process machining is necessary to obtain final shape.

In this process machining is not necessary to obtain final shape.

1.12 Forging die design: The design of forging dies requires a knowledge of strength, ductility, sensitivity to deformation rate and melting point of the workpiece material.

Fig. 1.6 Standard Terminology of Forging Die

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The terminology for forging die is as shown in fig. 1.6. Some of these consideration are similar to those for casting. For most of the forging, the parting line is usually at the largest cross-section of the part for symmetrical parts, the parting line at the center of the forging. But for more complex shapes the parting line may not be lie in the center of the forging. The flash materials allowed to flow into a gutter, so that extra flash does not increase the forging load unnecessarily. The length of the land is usually 2 to 5 times the flash thickness. Number of gutter design have been developed throughout the years. To removal of the workpiece from the dies in almost all forging process draft angles are necessary. At the time of cooling, forging shrinks both radialy and longitudinally. To overcome this cause, internal draft angles are made larger than external draft angles. Internal draft angles are about 7° to 10°, and external draft angles are about 3° to 5°. Selection of proper radii for corners and fillets is important to give smooth flow of metal in a die cavity as well as to improve die life. Small radii and fillets are not allowed due to stress concentration and fatigue cracking of the dies. Allowances are provided in forging die design because machining of forging of forging may be necessary to obtain final desired dimensional as well as surface finish. To produce a smooth and accurate cavity with parting plane more care is required. In a die, better and economical results are will be obtained if the following points are considered:

I. The die should part along a single flat plane, if at all possible. If not then parting plane should follow counter of the workpiece.

II. The parting line should be lies in one plane and not close to the upper or lower edge.

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III. Proper draft angles should be provided – at least 3° for aluminum and 5° to 7° for steel. Also 1° to 5° draft is provided in press forging and 3° to 10° in drop forging.

IV. Filets radii should be provided.V. Ribs should be low and wide.VI. The various sections should be balanced to avoid extreme

differences in metal flow.VII. Full advantage should be taken of fiber flow lines.VIII. Dimensional tolerances should not be closer than necessary.IX. Forged component should be easily able to achive a radial flow

grains or fibers.X. Too thin section should be avoided for easy flow of metal.

1.13 Die materials and lubricants: The die material and lubrication is the important aspects of forging.

I. Die materials: In most of the forging operation large parts are forged at elevated temperature. Therefore the general requirements for die materials are:

a) Good strength and toughness at elevated temperatures.b) Good harden ability and ability to harden uniformaly.c) Resistance to mechanical and thermal shocks.d) It should have high wear resistance.

Selection of proper die material depends on following factors:i. Size of the die components.ii. The properties and composition of the workpiece.iii. Complexity of shape.

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iv. Forging temperature.v. Cost of the die material.vi. Number of forging is required.vii. Heat transfer from hot workpiece to die.

Common die materials are tool and die steel containing chromium, nickel, molybdenum, and vanadium. Dies are made from die block , which are forged from casting. These dies are machined and finished to the desired shape and surface finish.

II. Lubrication:

Lubricants generally reduces friction and wear. They also act as thermal conductor between hot workpiece and cool dies. It can slows the cooling rate of workpiece and improving metal flow or grain flow. It also act as a parting agent. Wide variety of lubricants can be used in forging:

a) For hot forging:i. Graphite.ii. Glass.iii. Molybdenum disulfide.

In hot forging lubricants are usually applied directly in to the dies.

b) For cold forging:

i. Mineral oils.ii. Soaps.

In cold forging, Lubricants are generally applied on the workpiece.

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1.14 Defects in forging: All defects in forging are given below with the reasons:

a) Defective structure of metal; Reason – defective original metal.b) Presence of cold shuts or cracks at corners; Reason – improper

forging and faulty die design.c) Incomplete component; Reason – less metal used, faulty die

design and inadequate metal flow.d) Mismatched forging; Reason – die halves not properly aligned.e) Burnt and overheated metal; Reason – improper heating.f) Fiber flow lines or grain flow lines disconnected; Reason – very

rapid plastic flow of metal.g) Scale pits; Reason – formed by squeezing of scale into metal

during forging.h) Oversize components; Reason – worn out dies, incorrect dies.

Also fig. 1.7 shows common examples of defects in forged parts

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Fig. 1.7 Examples Of Defects In Forged Parts

1.15 Forged parts:

I. Gear blank:

It can be made by upsetting the bar stock. The pattern of the grain flow lines in the bar stock is as shown in fig. 1.8 (a). When bar stock is placed in dies such that the flow lines are vertical. The grain pattern produced will be radial and gear blank will give greatest strength to the teeth as

shown in fig. 1.8 (b).

Fig 1.8 Forging Sequence For Gear Blank

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All teeth will be equally strong. This is desired because each tooth considered as a cantilever beam and grain flow lines will be parallel with expected tensile and compressive stress along the face of the gear teeth. If the bar stock is placed in the die in such a manner that its flow lines are horizontal, then the grain pattern produced will be shown in fig. 1.8 (c). the teeth cut on such blank will not be of the same strength . because gear blank have expected stresses are perpendicular to the flow lines of bar stock.

II. Connecting rod: It can be made by forging the bar stock. The blank shape

gradually changes, as shown in fig. 1.9 (a) of connecting rod.

Fig. 1.9 Forging Sequence For Connecting Rod Performing processes, such as fullering and edging [fig. 1.9(b) and fig. 1.9(c)] are used to distribute the material into various regions of the

blank. In fullering, material is distributed away from an area. In Dhaigude. P. B.


edging, it is gathered into a localized area. The part is then formed into the rough shape of a connecting rod by a process called blocking using

blocker dies. The final operation is the finishing of the forging in close die forging that give the forging its final shape. The flash is removed

usually by a trimming operation (fig. 1.9)

Fig 1.10 Practical Forging Process For Connecting Rod

III. Hook :

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Fig. 1.1 Open Die Forging Processes

Fig. 1.2 A Pair Of Fullers And Swages

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Fig. 1.3 Close Die Forging

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