Manufacturing Process - I · Metal forming processes •Large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces

Manufacturing Process - I

UNIT –II

Metal Forming Processes

Prepared By

Prof. Shinde Vishal Vasant

Assistant Professor

Dept. of Mechanical Engg.

NDMVP’S Karmaveer Baburao Thakare

College of Engg. Nashik

Contact No- 8928461713

E mail:- [email protected]

Website:- www.vishalshindeblog.wordpress.com

PROF. V.V.SHINDE NDMVP'S KBT COE NASHIK

28/07/2016

mailto:[email protected]





Introduction • Practically all metals, which are not used in cast form are

reduced to some standard shapes for subsequent processing.

• Manufacturing companies producing metals supply metals in form of ingots which are obtained by casting liquid metal into a square cross section.

• Slab (500-1800 mm wide and 50-300 mm thick)

• Billets (40 to 150 sq mm)

• Blooms (150 to 400 sq mm)

• Sometimes continuous casting methods are also used to cast the liquid metal into slabs, billets or blooms.

• These shapes are further processed through hot rolling, forging or extrusion, to produce materials in standard form such as plates, sheets, rods, tubes and structural sections.

28/07/2016 PROF. V.V.SHINDE NDMVP'S KBT COE

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Sequence of operations for obtaining

different shapes


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Metal forming processes

• Large group of manufacturing processes in which

plastic deformation is used to change the shape of

metal work pieces

• The tool, usually called a die, applies stresses that

exceed the yield strength of the metal

– The metal takes a shape determined by the

geometry of the die


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Basic Types of

Metal Forming Processes

1. Bulk deformation – Rolling processes

– Forging processes

– Extrusion processes

– Wire and bar drawing

2. Sheet metalworking – Bending operations

– Deep or cup drawing

– Shearing processes


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Temperature in Metal Forming

• Any deformation operation can be accomplished with

lower forces and power at elevated temperature

• Three temperature ranges in metal forming:

– Cold working

– Warm working

– Hot working


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Hot and cold working processes


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Cold Working

• Performed at room temperature or slightly

above

• Many cold forming processes are important

mass production operations

• Minimum or no machining usually required

–These operations are near net shape or net

shape processes


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Advantages of Cold Forming

• Better accuracy, closer tolerances

• Better surface finish

• Strain hardening increases strength and

hardness

• Grain flow during deformation can cause

desirable directional properties in product

• No heating of work required


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Disadvantages of Cold Forming

• Higher forces and power required for deformation

• Surfaces of starting work must be free of scale and dirt

• Ductility and strain hardening limit the amount of forming that can be done – In some cases, metal must be annealed before further

deformation can be accomplished

– In other cases, metal is simply not ductile enough to be cold worked


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Hot Working • Deformation at temperatures above the re crystallization

temperature.

• The process of formation of new grains is called

recrystallisation process and corresponding temperature is

called recrystallisation temperature

– Recrystallization temperature = about one-half of melting

point on absolute scale

• In practice, hot working usually performed somewhat

above 0.5Tm

• Metal continues to soften as temperature increases

above 0.5Tm, enhancing advantage of hot working

above this level


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Advantages of Hot Working

• Work piece shape can be significantly altered

• Lower forces and power required

• Metals that usually fracture in cold working can be hot formed

• Strength properties of product are generally isotropic

• No strengthening of part occurs from work hardening – Advantageous in cases when part is to be subsequently

processed by cold forming


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Disadvantages of Hot Working

• Lower dimensional accuracy

• Higher total energy required, which is the sum of

– The thermal energy needed to heat the workpiece

– Energy to deform the metal

• Work surface oxidation (scale)

– Thus, poorer surface finish

• Shorter tool life

– Dies and rolls in bulk deformation


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Hot working operations

• Forging

• Rolling

• Welding

• Extrusion

• Spinning

• Hot piercing and rolling


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Cold working operations

• Cold rolling

• Extrusion

• Pressing

• Deep drawing

• Squeezing

• Bending

• Shearing


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Bulk Deformation Processes

Rolling: Compressive deformation process in which the thickness of a

plate is reduced by squeezing it through two rotating cylindrical

rolls.

Forging: The work piece is compressed between two opposing dies so

that the die shapes are imparted to the work.

Extrusion: The work material is forced to flow through a die opening

taking its shape

Drawing: The diameter of a wire or bar is reduced by pulling it through

a die opening (bar drawing) or a series of die openings (wire

drawing)


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Basic bulk deformation processes: (a) rolling (b) Forging


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Basic bulk deformation processes: (c) extrusion (d) Drawing


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Sheet Metal working

• Forming and related operations performed on metal sheets, strips, and coils

• High surface area-to-volume ratio of starting metal, which distinguishes these from bulk deformation

• Often called press working because presses perform these operations

– Parts are called stampings

– Usual tooling: punch and die


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Basic sheet metalworking operations: (a) bending (b) Drawing


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Basic sheet metalworking operations: (c) shearing


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Comparison of Hot and Cold Working


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Sr.

No

Hot Working Cold Working

1 Working above

recrystallization temperature

Working below recrystallization

temperature

2 Formation of new crystals (Grains) No crystal formation (Grains)

3 Surface finish not good Surface finish is good

4 No stress formation Internal Stress developed

5 Improved Mechanical Property due

to grain formation

Leads to distortion of metal

grains

6 Closed dimensional tolerances

cannot be maintained

Superior dimensions can be

obtained

7 Improves some mechanical

properties like impact strength and

elongation

During process, strength and

elongation decreases

Friction in Metal Forming

• In most metal forming processes, friction is

undesirable:

– Metal flow is retarded

– Forces and power are increased

– Wears tooling faster

• Friction and tool wear are more severe in hot working

• if the co efficient of friction is very high, that

condition is called as sticking or sticking friction


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Lubrication in Metal Forming • Metal working lubricants are applied to tool-work

interface in many forming operations to reduce harmful effects of friction

• Lubricant for cold working processes are mineral oil, fats and fatty oils, soaps

• Lubricants for hot working processes are mineral oils, graphite, moltan glass etc

• Benefits:

– Reduced sticking, forces, power, tool wear

– Better surface finish

– Removes heat from the tooling

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Hot rolling

• Deformation process in which work thickness is

reduced by compressive forces exerted by two

opposing rolls


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• The cross section of the work piece is reduced by the process. The material gets squeezed between a pair of rolls, as a result of which the thickness gets reduced and the length gets increased.

• Mostly, rolling is done at high temperature, called hot rolling because of requirement of large deformations. Hot rolling results in residual stress-free product.

• Bloom is has a square cross section, with area more than 230 cm2. A slab, also from ingot, has rectangular cross-section, with area of at least 100 cm2 and width at least three times the thickness. A billet is rolled out of bloom, has at least 40 mm X 40 mm cross-section.

• Blooms are used for rolling structural products such as I-sections, channels, rails etc. Billets are rolled into bars, rods. Bars and rods are raw materials for extrusion, drawing, forging, machining etc. Slabs are meant for rolling sheets, strips, plates etc.


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Types of rolling mills

• Rolling Mills

• Equipment is massive and expensive

• Rolling mill configurations:

Two-high – two opposing rolls

Three-high – work passes through rolls in both

directions

Four-high – backing rolls support smaller work rolls

Cluster mill – multiple backing rolls on smaller rolls

Tandem rolling mill – sequence of two-high mills


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ARRANGEMENTS OF ROLLERS

USED IN ROLLING MILLS

TWO HIGH MILL

THREE HIGH MILL

The stock is

returned to the

entrance for

further reduction.

Consist of upper and

lower driven rolls and

a middle roll, which

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CLUSTER ROLLING MILLS

FOUR HIGH MILL

Small-diameter rolls

(less strength &

rigidity) are

supported by

larger-diameter

backup rolls

Each of the work

rolls is supported

by two backing

rolls.


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• Tandem rolling mill – sequence of two-high

mills

• A series of rolling stands in sequence.


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Forging process

• Forging is a process in which the work piece is shaped by compressive forces applied through various dies and tools. It is one of the oldest metalworking operations. Most forgings require a set of dies and a press or a forging hammer.

• Unlike rolling operations, which generally produce continuous plates, sheets, strip, or various structural cross-sections, forging operations produce discrete parts.

• Typical forged products are bolts and rivets, connecting rods, shafts for turbines, gears, hand tools, and structural components for machinery, aircraft, railroads and a variety of other transportation equipment.


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• Depending upon complexity of the part forging is carried out as open

die forging and closed die forging.

• In open die forging, the metal is compressed by repeated blows by a

mechanical hammer and shape is manipulated manually.

• In closed die forging, the desired configuration is obtained by

squeezing the work piece between two shaped and closed dies.

• On squeezing the die cavity gets completely filled and excess material

comes out around the periphery of the die as flash which is later

trimmed.

• Press forging and drop forging are two popular methods in closed die

forging.

• In press forging the metal is squeezed slowly by a hydraulic or

mechanical press and component is produced in a single closing of die,

hence the dimensional accuracy is much better than drop forging.

• Both open and closed die forging processes are carried out in hot as

well as in cold state.


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Open-Die Forging

• Compression of work piece between two flat dies

• Deformation operation reduces height and increases

diameter of work

• Common names include upsetting or upset forging or

cogging


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Open-Die Forging with No Friction

• If no friction occurs between work and die surfaces,

then homogeneous deformation occurs, so that radial

flow is uniform throughout work piece height


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Fig:- Homogeneous deformation of a cylindrical work part

1. start of process with work piece at its original length and

diameter, 2. partial compression, 3. final size.

Open-Die Forging with Friction

• Friction between work and die surfaces constrains

lateral flow of work, resulting in barreling effect.

• In hot open-die forging, effect is even more pronounced

due to heat transfer at and near die surfaces, which

cools the metal and increases its resistance to

deformation


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Impression-Die Forging or closed die forging

• Compression of work part by dies with inverse of desired part

shape

• Flash is formed by metal that flows beyond die cavity into small

gap between die plates

• Flash serves an important function:

As flash forms, friction resists continued metal flow into gap,

constraining material to fill die cavity

In hot forging, metal flow is further restricted by cooling against

die plates


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Trimming operation (shearing process) to remove the flash after

impression-die forging.


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Sr.

No

Open Die forging Closed die forging

1 In this method, the work piece is

compressed between two flat dies

Work piece is compressed

between two impressed dies

2 Cost of dies are low Higher cost

3 The process is simple process is complex

4 No stress formation Residual stress formation takes

place

5 During process, poor utilization of

metals

Better utilization

6 After the process, machining of

component is required

Machining of component is not

required

7 Dimensional accuracy is not good Dimensional accuracy is good

8 Suitable only for production of

simple component

Simple and complex component

Extrusion • A plastic deformation process in which metal is forced under

pressure to flow through a single, or series of dies until the

desired shape is produced.

• Process is similar to squeezing toothpaste out of a toothpaste tube

• In general, extrusion is used to produce long parts of uniform

cross sections

• Typical products made by extrusion are railings for sliding doors,

tubing having carious cross-sections, structural and architectural

shapes, door and windows frames.


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Types

(1)Direct Extrusion (Forward Extrusion)

(2)Indirect Extrusion (Backward Extrusion)

(3)Cold Extrusion

(4)Hot Extrusion

• Direct Extrusion

• Billet is placed in a chamber and forced through a die opening

by a hydraulically-driven ram or pressing stem.

• Dies are machined to the desired cross-section


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• Indirect Extrusion

• Metal is forced to flow through the die in an opposite

direction to the ram’s motion.

• Lower extrusion force as the work billet metal is not

moving relative to the container wall.

• Limitations

• Lower rigidity of hollow ram

• Difficulty in supporting extruded product as it exits die


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Drawing • Commonly used to make wires from round bars

• Large quantities of wires, rods, tubes and other sections are

produced by drawing process which is basically a cold

working process.

• In this process the material is pulled through a die in order to

reduce it to the desired shape and size.

• In a typical wire drawing operation, once end of the wire is

reduced and passed through the opening of the die, gripped

and pulled to reduce its diameter.


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• Drawing is an operation in which the cross-section of

solid rod, wire or tubing is reduced or changed in shape

by pulling it through a die. Drawn rods are used for

shafts, spindles, and small pistons and as the raw material

for fasteners such as rivets, bolts, screws.

• Drawing also improves strength and hardness when these

properties are to be developed by cold work and not by

subsequent heat treatment.


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• Wire drawing machines consisting of multiple draw

dies (typically 4 to 12) separated by accumulating

drums

Each drum (capstan) provides proper force to draw

wire stock through upstream die

Each die provides a small reduction, so desired

total reduction is achieved by the series


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• Die Materials

Commonly used materials are Tool Steels and

Carbides

Diamond dies are used for fine wire.

For improved wear resistance, steel dies may be

chromium plated, and carbide dies may be coated

with titanium nitride

For Hot drawing, cast-steel dies are used


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Tube drawing

• Tube drawing is also similar to wire drawing, except that a

mandrel of appropriate diameter is required to form the

internal hole.

• The process reduces the diameter and thickness of the tube.

• Tube drawing is very similar to bar drawing, except the

beginning stock is a tube. It is used to decrease the diameter,

improve surface finish and improve dimensional accuracy.

• A mandrel may or may not be used depending on the

specific process used.

• Here two arrangements are shown in figure (a) with a

floating plug and (b) with a moving mandrel


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Tube Drawing


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Documents

Manufacturing Process - I · Metal forming processes •Large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces