metal forming.ppt

8/14/2019 Metal Forming.ppt

1/80

Metal FormingInstructor: Dr. Syed Amir Iqbal


2/80

Metal Forming

Plastic deformation to change the shape of a metal workpiece

using a tool, called a die, by applying a stress that exceeds the

Yield Strength of the metal

Stresses are typically compressive:

Examples: rolling, forging, extrusion

However, some forming processes

Stretch the metal (tensile stresses)

Others bend the metal (tensile and compressive)

Still others apply shear


3/80

Importance of Bulk Deformation

In hot working, significant shape change can

be accomplished

In cold working, strength can be increased

during shape change

Little or no waste - some operations are near

net shape or net shape processes

The parts require little or no subsequent

machining


4/80

Material Behavior in Metal Forming

Plastic region of stress-strain curve is of primary

interest because material is plastically deformed

In plastic region, metal's behavior is expressed bythe flow curve:

where K= strength coefficient; and n= strain hardening

exponent.

Flow curve based on true stress and true strain

nK


5/80

Flow Stress

For metals at room temperature, strength

increases when deformed due to strain hardening

Flow stress= instantaneous value of stress

required to continuedeforming the material

where Yf = flow stress, that is, the yield strength as a

function of strain

n

f KY


6/80

Temperature in Metal Forming

Kand nin the flow curve depend on temperature

At higher temperatures both K and n are:

At higher temperatures ductility is:

Thus, the force and power required to performdeformation operations at elevated temperatures are:

Three temperature ranges in metal forming:

Cold working

Warm working

Hot working


7/80


8/80

Cold Working

Performed at roomtemperature

Many cold forming processes are important

mass production operations

Minimum or no machining usually required

These operations are near net shapeor net

shapeprocesses


9/80

Advantages of Cold Forming

Better accuracy, closer tolerances

Better surface finish

Strain hardening = strength and hardness

Grain flow during deformation can cause

desirable directional properties in product

No heating of work required


10/80

Disadvantages of Cold Forming

Forces and power required in the deformation

operation are:

Surfaces of starting workpiece must be free of

scale and dirt

Ductility and strain hardening limit the amount of

forming that can be done, metal may not be

ductile enough to be cold worked


11/80

Warm Working

Performed at temperatures above room

temperature i.e. 0.3 Tm- 0.5 Tm

Dividing line between cold working and warm

working often expressed in terms of melting

point:

0.3Tm, where Tm= melting point (absolute

temperature) for metal


12/80

Advantages of Warm Working

Lower forces and power than in cold working

More intricate work geometries possible

Need for annealing may be reduced or

eliminated


13/80

Hot Working

Deformation at temperatures in the range of

0.5Tm0.75 Tm

In practice, hot working usually performedsomewhat above 0.75Tm

Metal continues to soften as temperature increases

above 0.75Tm, enhancing advantage of hot working

above this level


14/80

Advantages of Hot Working

Workpart 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

No strengthening of part occurs from work

hardening Advantageous in cases when part is to besubsequently processed by cold forming


15/80

Disadvantages of Hot Working

Lower dimensional accuracy

Higher total energy required (due to the

thermal energy to heat the workpiece)

Work surface oxidation (scale), poorer surface

finish

Shorter tool life


16/80

What is Strain Rate?

Strain rate in forming is directly related to speedof deformation v

Deformation speed v= velocity of the ram or

other movement of the equipment Strainrateis defined:

where = true strain rate; and h= instantaneousheight of workpiece being deformed

.

h

v

.


17/80

Strain Rate Sensitivity

Theoretically, a metal in hot working behaves like

a perfectly plastic material, with strain hardening

exponent n= 0

The metal should continue to flow at the same flow

stress, once that stress is reached

However, an additional phenomenon occurs duringdeformation, especially at elevated temperatures:

Strain rate sensitivity


18/80

Effect of Strain Rate on Flow Stress

Flow stress is a function of temperature

At hot working temperatures, flow stress also

depends on strain rate

As strain rate increases, resistance to

deformation

This effect is known as strain-rate sensitivity


19/80

Effect of Temperature on Flow Stress

Effect of temperature on flow

stress for a typical metal. The

flow stress decreases with

increase in temperature and

tends to increase with increase

in strain rate.


20/80

Basic Types of Deformation Processes


21/80

Basic Types of Deformation Processes

1. Bulk deformationstarting material has low surface areato volume (e.g. billets & bars)

Rolling

Forging

Extrusion Wire and bar drawing

2. Sheet metalworkingstarting material has high surfacearea to volume (e.g. sheet & coils)

Bending Deep drawing

Cutting


22/80


23/80

Sheet Metalworking

Often calledpress working Parts are often called stampings

Usual tooling:punchand die

Bending Cutting


24/80

Forging

Deformation process in which work is compressedbetween two dies

Oldest of the metal forming operations, dating from

about 5000 B.C. Components: engine crankshafts, connecting rods,

gears, aircraft structural components, jet engineturbine parts

Also, basic metals industries use forging to establishcomponents and parts that are subsequentlymachined to final shape and size


25/80

Types of Forging Dies

Open-die forging - work is compressedbetween two flat dies, allowing metal to flowlaterally with minimum constraint

Impression-die forging - die contains cavity orimpression that is imparted to workpart

Metal flow is constrained so that flash is created

Flashless forging - workpart is completelyconstrained in die

No excess flash is created


26/80

Open-Die Forging

Compression of work-part between two flat dies

Deformation operation reduces height and increases

diameter of work

Common names include Upsetting or Flat die forging.


27/80

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 workpart height and true strain is given by:

h

holn


28/80

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


29/80

Impression-Die Forging 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 dieplates


30/80


31/80

Advantages and Limitations

Advantages of impression-die forging comparedto machining from solid stock:

Higher production rates

Savingof metal Finished part strength

Favorable grain orientation in the metal

Limitations:

Not capable of close tolerances

Machining often required to achieve accuracies andfeatures needed


32/80

Flashless Forging

Starting workpart volumemust equal die cavityvolume within very closetolerance

Process control moredemanding thanimpression-die forging

Accurate control of volumeof material.

Close tolerances can beachieved by proper diedesign.

Good die life.


33/80


34/80

Rolled Products Made of Steel


35/80

Diagram of Flat Rolling

There is a point on the roll where thework velocityequals the roll velocity,

this is the Neutral or no slip point.

Friction is zero on either side of this

point.

The friction is expressed in terms ofcoefficient of friction .

The reduction in thickness of the plate

is called DRAFT, expressed as (totf).

Draft is the function of coefficient of

friction and roll radius R. i.e. thelarger the coefficient of friction and

roll radius, the draft becomes the

maximum and hence the maximum

reduction in thickness is possible.


36/80

Rolling Issues

With Hot rolling, material properties are

improved, but dimensional tolerances are not

as tight, and surface oxidation occurs.

Wavy Edges

Zipper

Edge cracks


37/80

Shape Rolling

Work is deformed into a contoured cross section

rather than flat (rectangular)

Accomplished by passing work through rolls that

have the reverse of desired shape

Products include:

Construction shapes such as I-beams, L-beams, and

U-channels Rails for railroad tracks

Round and square bars and rods


38/80

Thread Rolling

Bulk deformation process used to form threads on cylindrical

parts by rolling them between two dies

Performed by cold working in thread rolling machines

The grain distribution in thread rolling are more improved and

beneficial as compared to thread machining.


39/80

Upsetting and Heading

Forging process used to form heads on nails, bolts,and similar hardware products

More parts produced by upsetting than any otherforging operation

Performed cold, warm, or hot on machines calledheadersorformers

Wire or bar stock is fed into machine, end isheaded, then piece is cut to length

For bolts and screws, thread rolling is then usedto form threads


40/80

Upsetting

An upset forging operation to form a head on a bolt orsimilar hardware item The cycle consists of: (1) wire stockis fed to the stop, (2) gripping dies close on the stock andthe stop is retracted, (3) punch moves forward, (4)bottoms to form the head.


41/80

Heading (Upset Forging)

Examples of heading (upset forging) operations: (a) heading a

nail using open dies, (b) round head formed by punch, (c) and(d) two common head styles for screws formed by die, (e)

carriage bolt head formed by punch and die.


42/80

Swaging

Accomplished by rotating dies that hammer a

workpiece radially inward to taper it as the

piece is fed into the dies Used to reduce diameter of tube or solid rod

stock

Mandrel sometimes required to control shape

and size of internal diameter of tubular parts


43/80

Swaging

Swaging process to reduce solid rod stock; the dies rotate asthey hammer the work In radial forging, the workpiece rotateswhile the dies remain in a fixed orientation as they hammer thework.


44/80

Extrusion

Compression forming process in which work metalis forced to flow through a die opening toproduce a desired cross-sectional shape

Process is similar to squeezing toothpaste out of

a toothpaste tube In general, extrusion is used to produce long

parts of uniform cross sections

Two basic types: Direct extrusion

Indirect extrusion


45/80

Direct Extrusion

Also calledforward extrusion As ram approaches die opening, a small portion

of billet remains that cannot be forced throughdie opening

This extra portion, called the butt, must beseparated from extrudateby cutting it justbeyond the die exit

Starting billet cross section usually round

Final shape of extrudate is determined by dieopening


46/80

Direct Extrusion


47/80

Indirect extrusion

Indirect extrusion to produce (a) a solid cross section and (b) a

hollow cross section.

Limitations of indirect extrusion are imposed by

Lower rigidity of hollow ram Difficulty in supporting extruded product as it exits die


48/80

Hot vs. Cold Extrusion

Hot extrusion - prior heating of billet to above itsrecrystallization temperature

Reduces strength and increases ductility of themetal, permitting more size reductions and morecomplex shapes

Cold extrusion - generally used to producediscrete parts

The term impact extrusion is used to indicate highspeed cold extrusion

Material possess some degree of strain-hardening


49/80

Complex Cross Section

A complex extruded cross section for a heat sink (photo courtesy of

Aluminum Company of America)


50/80

Wire and Bar Drawing

Cross-section of a bar, rod, or wire is reduced by

pulling it through a die opening

Similar to extrusion except work ispulledthrough

die in drawing (it ispushedthrough in extrusion)

Although drawing applies tensile stress,

compression also plays a significant role since

metal is squeezed as it passes through die

opening


51/80

Wire and Bar Drawing

Drawing of bar, rod, or wire


52/80

Area Reduction in Drawing

Change in size of work is usually given by area

reduction:

where r= area reduction in drawing;Ao=

original area of work; andAr= final work

o

fo

A

AA

r


53/80

Wire Drawing

Continuous drawing machines consisting of

multiple draw dies (typically 4 to 12) separated by

accumulating drums

Each drum (capstan) provides proper force to drawwire stock through upstream die

Each die provides a small reduction, so desired total

reduction is achieved by the series

Annealing sometimes required between dies to

relieve work hardening


54/80

Continuous drawing of wire


55/80

Features of a Draw Die

Entry region - funnels lubricant into the die to prevent scoring

of work and die

Approach - cone-shaped region where drawing occurs

Bearing surface - determines final stock size

Back relief - exit zone - provided with a back relief angle

(half-angle) of about 30


56/80

Sheet metal forming

Sheet metal forming is a grouping of many complementary processes that are

used to form sheet metal parts. One or more of these processes is used to take a

flat sheet of ductile metal, and mechanically apply deformation forces that alter

the shape of the material.

Before deciding on the

processes), one should

determine whether a particular

sheet metal can be formed into

the desired shape without

failure. The sheet metal

operations done on a press maybe grouped into two categories,

cutting (shearing) operations

and forming operations.


57/80

Cutting (Shearing) Operations

In this operation, the workpiece is stressed beyond its ultimatestrength. The stresses caused in the metal by the applied forces willbe shearing stresses. The cutting operations include:

Punching (Piercing)

Blanking

Notching

Perforating

Slitting

Lancing

Parting Shaving

Trimming

Fine blanking


58/80

Shearing Operations

Punching (Piercing):It is a cutting operation by which various shapedholes are made in sheet metal. Punching is similar to blanking except thatin punching, the hole is the desired product, the material punched out toform the hole being waste.

Blanking: Blanking is the operation of cutting a flat shape sheet metal. Thearticle punched out is called the blank and is the required product of the

operation. The hole and metal left behind is discarded as waste.


59/80

Notching: This is cutting operation by which metal pieces are cut from the edge of a sheet, strip orblank.

Perforating: This is a process by which multiple holes which are very small and close together are cutin flat work material.

Slitting: It refers to the operation of making incomplete holes in a workpiece.

Lancing: This is a cutting operation in which a hole is partially cut and then one side is bent down toform a sort of tab. Since no metal is actually removed, there will be no scrap.

Parting: Parting involves cutting a sheet metal strip by a punch with two cutting edges that matchthe opposite sides of the blank.

Shearing Operations


60/80

Shaving: The edge of blanked parts is generally rough, uneven andunsquare. Accurate dimensions of the part are obtained byremoving a thin strip of metal along the edges.

Trimming: This operation consists of cutting unwanted excessmaterial from the periphery of previously formed components.

Fine blanking: Fine blanking is a operation used to blank sheetmetal parts with close tolerances and smooth, straight edges in onestep.

Shearing Operations


61/80

Shearing Dies Because the formability of a sheared part can be influenced by the quality of its sheared edges,

clearance control is important. In practice, clearances usually range between 2% and 8% of thesheets thickness; generally, the thicker the sheet, the larger is the clearance (as much as 10%).

However, the smaller the clearance, the better is the quality of the edge. Some common

shearing dies are describe below:

Punch and Die Shapes: As the surfaces of the punch and die are flat; thus, the punch force

builds up rapidly during shearing, because the entire thickness of the sheet is sheared at the

same time. However, the area being sheared at any moment can be controlled be bevelingthe punch and die surfaces, as shown in the following Figure. This geometry is particularly

suitable for shearing thick blanks, because it reduces the total shearing force.


62/80

Compound Dies

Several operations on the same strip may be performed in onestroke with a compound die in one station. These operations are

usually limited to relatively simple shearing because they are

somewhat slow and the dies are more expensive than those for

individual shearing operations.

(a) (b) Schematicillustrations: (a)before and (b) afterblanking a commonwasher in acompound die.

Note the separatemovements of thedie (for blanking)and the punch (forpunching the holein the washer).


63/80

Progressive DiesParts requiring multiple operations, such as punching, blanking and notching are made

at high production rates in progressive dies. The sheet metal is fed through a coil strip

and a different operation is performed at the same station with each stroke of a series

of punches.

(a) Schematic illustration of making a washer in a progressive die. (b) Forming of the toppiece of an aerosol spray can in a progressive die.


64/80

Forming Operations

In this operation, the stresses are below the ultimate strength of the metal. In thisoperation, there is no cutting of the metal but only the contour of the workpiece ischanged to get the desired product. The forming operations include:

Bending: In this operation, the material in the form of flat sheet or strip, isuniformly strained around a linear axis which lies in the neutral plane andperpendicular to the lengthwise direction of the sheet or metal. The bendingoperations include:

Drawing: This is a process of a forming a flat workpiece into a hollow shape bymeans of a punch, which causes the blank to flow into die cavity.

Squeezing: Under this operation, the metal is caused to flow to all portions ofa die cavity under the action of compressive forces.

V-bending

Edge bending

Roll bending

Air bending

Flanging

Dimpling

Press break forming

Beading

Roll forming

Tube forming

Bulging

Stretch forming


65/80

Bending of Flat Sheet and Plate

V-bending Edge bending Roll bending

Bending in 4-slide machine Air bending


66/80

Flanging Flanging is a process of bending the edges of sheet metals to 90o

Shrink flangingsubjected to compressive hoop stress.

Stretch flangingsubjected to tensile stresses


67/80

Dimpling

First hole is punched and expanded into a flange

Flanges can be produced by piercing with shaped punch

When bend angle < 90 degrees as in fitting conical ends its

called flanging


68/80

Press break forming

Sheet metal or plate can be bent easily with simple fixtures using apress. Long and relatively narrow pieces are usually bent in a press

break. This machine utilizes long dies in a mechanical or hydraulic

press and is suitable for small production runs. The tooling is simple

and adaptable to a wide variety of shapes.


69/80

Beading

In beading the edge of the sheet metal is bent into the cavity of adie. The bead gives stiffness to the part by increasing the

moment on inertia of the edges. Also, it improves the

appearance of the part and eliminates exposed sharp edges

(a) Bead forming with a single die. (b) Bead forming with two dies, in a

press brake.


70/80

Roll formingFor bending continuous lengths of sheet metal and for large

production runs, roll forming is used. The metal strip is bent in stages

by passing it through a series of rolls.

Stages in roll forming of a sheet-metal door frame. In Stage 6, the rolls may be

shaped as inAor B.


71/80

Tube BendingBending and forming tubes and other hollow sections require special tooling to avoid

buckling and folding. The oldest method of bending a tube or pipe is to pack the insidewith loose particles, commonly used sand and bend the part in a suitable fixture. This

techniques prevents the tube from buckling. After the tube has been bent, the sand is

shaken out. Tubes can also be plugged with various flexible internal mandrels.

Methods of bending tubes. Internal mandrels,or the filling of tubes with particulate materials such assand,are often necessary to prevent collapse of the tubes during bending .Solid rods and structural shapes

can also be bent by these techniques


72/80

BulgingThe basic forming process of bulging involves placing tabular, conical or curvilinear

part into a split-female die and expanding it with, say, a polyurethane plug. The punch

is then retracted, the plug returns to its original shape and the part is removed by

opening the dies.

(a) Bulging of a tubular part with a flexible plug. Water pitchers can be made by this method. (b) Production

of fittings for plumbing by expanding tubular blanks with internal pressure. The bottom of the piece is thenpunched out to produce a T. (c) Manufacturing of Bellows.


73/80

Stretch FormingIn stretch forming, the sheet metal is clamped around its edges and stretched over a die or

form block, which moves upward, downward or sideways, depending on the particularmachine. Stretch forming is used primarily to make aircraft-wing skin panel, automobile door

panels and window frames.

Schematic illustration of a stretch-forming process. Aluminum skins

for aircraft can be made by this process.

Hydroform (or) Fluid Forming Process


74/80

Hydroform (or) Fluid Forming Process

In hydroforming or fluid forming process, the pressure over the rubber membrane is

controlled throughout the forming cycle, with maximum pressure reaching 100 MPa (15000

psi). This procedure allows close control of the part during forming to prevent wrinkling ortearing. When selected properly, rubber forming and hydroforming processes have the

following advantages:

Low tooling cost

Flexibility and ease of operation

Low die wear No damage to the surface of the sheet and

Capability to form complex shapes.

The hydroform (or fluid forming) process. Note that, in contrast to the ordinary deep-drawing process, the

pressure in the dome forces the cup walls against the punch. The cup travels with the punch; in this way,deep drawability is improved.

Deep Drawing Processes


75/80

Deep Drawing Processes

Drawing operation is the process of forming a flat piece of material (blank) into a hollow

shape by means of a punch, which causes the blank to flow into the die-cavity. Round sheetmetal block is placed over a circular die opening and held in a place with blank holder &

punch forces down into the die cavity. Wrinkling occurs at the edges.

Shallow drawing: depth of formed cup D/2 Deep or moderate drawing: depth of formed cup > D/2

(a) deep-drawing process on a circular sheet-metal blank. The stripper ring facilitates the removal of the formed cup

from the punch. (b) Process variables in deep drawing. Except for the punch force, F, all the parameters indicated inthe figure are independent variables.


76/80

Drawing Operations

Examples of drawing operations: (a) pure drawing and (b) pure stretching. The bead prevents the sheet metalfrom flowing freely into the die cavity. (c) Possibility of wrinkling in the unsupported region of a sheet in drawing.


77/80

Redrawing OperationsContainers or shells that are too difficult to draw in one operation are generally redrawn. In

reverse redrawing, shown in following Figure, the metal is subjected to bending in thedirection opposite to its original bending configuration. This reversal in bending results in

strain softening. This operation requires lower forces than direct redrawing and the material

behaves in a more ductile manner.

Reducing the diameter of drawn cups

by redrawing operations: (a)

conventional redrawing and (b) reverse

redrawing. Small-diameter deep

containers undergo many drawing andredrawing operations.

f


78/80

Top of Aluminum Can

Metal-Forming Process for Beverage Can


79/80

Metal Forming Process for Beverage Can

Steps in Manufacturing an

Aluminum Can

The metal-forming processes involved inmanufacturing a two-piece aluminumbeverage can


80/80

metal forming.ppt

Documents