Module 5

V

High velocity forming of metals:-effects of high speeds on the stress strain

relationship steel, aluminum, Copper – comparison of conventional and high

velocity forming methods- deformation velocity, material behavior, strain

distribution.

Stress waves and deformation in solids – types of elastic body waves- relation at

free boundaries- relative particle velocity.

Sheet metal forming: - explosive forming:-process variable, properties of

explosively formed parts, etc.

Electro hydraulic forming: - theory, process variables, etc., comparison with

explosive forming.

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Introduction

Metal forming is a very important manufacturing operation. It enjoys industrial importance

among various production operations due to its advantages such as cost effectiveness, enhanced

mechanical properties, flexible operations, higher productivity, considerable material saving. Of

these manufacturing processes, forming is a widely used process which finds applications in

automotive, aerospace, defense and other industries. Forming is the process of obtaining the

required shape and size on the raw material by subjecting the material to plastic deformation

through the application of tensile force, compressive force, bending or shear force or

combinations of these forces.

A typical automobile uses formed parts such as wheel rims, car body, valves, rolled shapes for

chassis, stamped oil pan, etc. In our daily life we use innumerable formed products e.g. cooking

vessels, tooth paste containers, bicycle body, chains, tube fitting, fan blades etc.

Metal forming: Large set of manufacturing processes in which the material is deformed

plastically to take the shape of the die geometry. The tools used for such deformation are called

die, punch etc. depending on the type of process. Stresses beyond yield strength of the work

piece material are required for the plastic deformation.

General classification of metal forming processes

Typically, metal forming processes can be classified into two broad groups. One is bulk forming

and the other is sheet metal forming.

Bulk forming: It is a severe deformation process resulting in massive shape change. The surface

area-to-volume of the work is relatively small. Mostly done in hot working conditions

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Rolling: Work piece (slab/plate) is compressed between two rotating rolls, thickness is reduced.

Forging: The work piece is compressed between two dies containing shaped contours.

Extrusion: The work piece is compressed or pushed into the die opening to take the shape

Wire or rod drawing: similar to extrusion, except that the work piece is pulled through the die

Sheet forming: Forming and cutting operations performed on metal sheets, strips, and coils.

Bending: The sheet material is strained by punch to give a bend shape

Deep (or cup) drawing: Forming of a flat metal sheet into a hollow shape like a cup

Shearing: This is nothing but cutting of sheets by shearing action

Cold working process: Generally done at room temperature or slightly below RT

Advantages compared to hot forming

1. Closer tolerances can be achieved;

2. Good surface finish;

3. Because of strain hardening, higher strength product

4. Grain flow during deformation provides the desirable directional properties;

5. Since no heating of the work is involved, furnace, fuel, electricity costs are minimized,

6. Machining requirements are minimum since near net shaped forming is obtained.

Disadvantages:

1. Higher forces and power are required;

2. Strain hardening of the work metal limit the amount of forming that can be done,

3. Sometimes cold forming-annealing-cold forming cycle should be followed

Hot working: Involves deformation above recrystallization temperature, (0.5Tm to 0.75Tm)

Advantages

1. Significant change in work piece shape,

2. Lower forces are required,

3. Materials with premature failure can be hot formed,

4. Absence of strengthening due to work hardening.

Disadvantages:

1. Shorter tool life,

2. Poor surface finish,

3. Lower dimensional accuracy,

4. Surface oxidation

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High energy rate forming

Conventional forming processes typically accomplish plastic deformation by applying forming

stresses at relatively low velocities. While, high velocity forming gives large amount of kinetic

energy to the work piece and form it as the kinetic energy dissipate as plastic deformation.

In conventional forming conditions, inertia is neglected, as the velocity of forming is typically

less than 5 m/s, while typical high velocity forming operations are carried out at work-piece

velocities of about 100 m/s

High Energy Rate Forming (HERF) involves a very hard order of the rate of energy flow for a

very short interval of time. HERF is based on the principle that the kinetic energy of a moving

body is proportional to the square of its velocity, and therefore, a significant amount of energy

can be supplied by a relatively smaller body moving at high speed. The forming processes are

affected by the rates of strain used.

Important features of HERF processes

1. The energy of deformation is delivered at a much higher rate than in conventional practice.

2. Larger energy is applied for a very short interval of time.

3. Many metals tend to deform more readily under extra fast application of force.

4. Large parts can be easily formed by this technique.

5. The strain rate dependence of strength increases with increasing temperature.

6. The yield stress and flow stress at lower plastic strains are more dependent on strain rate

than the tensile strength.

Effects of high strain rates during forming

1. The flow stress increases with strain rates

2. The temperature of work is increases due to adiabatic heating.

3. Many difficult to form materials like Titanium and Tungsten alloys, can be deformed

under high strain rates.

4. No necking

5. No spring back effect

6. No wrinkling effect

7. High strength product with good mechanical properties

Advantages of high energy rate forming (HERF) Processes

1. The mechanical properties of the product are better than for conventional processes

2. Production rates are higher, as parts are made at a rapid rate.

3. Tolerances can be easily maintained. Product shape accuracy is enhanced,

4. Versatility of the process – it is possible to form most metals including difficult to

form metals.

5. No or minimum spring back effect on the material after the process.

6. Production cost is low as power hammer (or press) is eliminated in the process.

7. Complex shapes / profiles can be made much easily, as compared to conventional

forming.

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Limitations:

1. Highly skilled personnel are required from design to execution.

2. Not suitable to highly brittle materials

3. Source of energy (chemical explosive or electrical) must be handled carefully.

4. Governmental regulations/ procedures / safety norms must be followed.

5. Dies need to be much bigger to withstand high energy rates and shocks

6. Controlling the application of energy is critical as it may crack the die or work.

7. It is essential to know the behavior performance of the work metal initially.

There are a number of methods for high velocity forming (HVF) mainly based on the source of

energy used for obtaining high velocities. The common methods are explosive forming, electro-

hydraulic forming (EHF) and electromagnetic forming (EMF).

Formability of materials

There is significant interest in the automobile industry to make body components and panels with

aluminum or high strength thin steel sheets, with the main advantages of weight savings and the

accompanied fuel efficiency in mind. However, the propensity of these metals to neck and tear at

relatively low strain levels makes it difficult to use them for making geometrically complex

parts, with conventional forming processes. Requirements such as the introduction of super

tough alloys for space vehicles., supersonic aircraft and the need for shaping incredibly small,

thin and brittle materials for electronic components helped in the growth of high velocity

forming processes. In high velocity forming of metals, the metal is shaped in micro-seconds with

pressures generated by the sudden application of large amounts of energy.

As a result, there is renewed interest in High Velocity Forming (HVF) techniques, which have

been proven to address many issues of conventional metal forming processes through dramatic

improvements in formability of metals. High velocity forming processes began to make their

mark and grow in application in 1960.

One of the more important factors in high energy rate forming processes is the ductility of the

material under the conditions of the operation. It is apparent that the ductility of a material at

high rates of strain is influenced not only by the flow behavior of the material but also by the

mechanics of deformation in the particular piece being formed. In other words, identical pieces

of material might fail in a ductile manner at quite different levels of strain if the plastic stress

wave patterns involved in the forming operation are quite different.

The application of high strain rate techniques to the forming of metal parts has received considerable

attention in recent years. Steel alloys traditionally comprise the majority of automotive components as

well as many other manufactured parts. This is mainly due to steel’s low cost, good strength, high

formability, the possibility of tight dimensional tolerance and good strength. However, steel has a

relatively high density, leading to relatively heavy components. This weight may dramatically increase

the lifecycle cost of a component.

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Aluminum alloys offers the possibility to reduce weight and good strength properties.

Unfortunately, higher strength materials have invariably lower formability, which when

combined with its relatively high cost, put them in disadvantage relatively to steel.

Advantages of using Al alloys vehicles

The use to aluminum alloys over some heavy steels, in the automobile sector can lead to various

advantages:

Aluminum parts can be twice as thick as steel but still 40% lighter and 60% stiffer. Their

lower mass leads to improved fuel economy, acceleration, and braking performance.

Aluminum’s unique combination of lightweight, high-strength and corrosion resistance

characteristics make it the ideal alloy for developing marine applications like high-speed

aluminum ferries, Bicycle frames, baseball bats, golf clubs etc.

Aluminum alloys also have superior recycling ability which becomes increasingly

important in terms of the total life cost of vehicles

Aluminum parts have excellent collision energy management characteristics and can be

designed to absorb the same energy as steel at only 55% of the weight thereby leading to

safety in automobiles in the instance of a crash

Problems associated with using Al alloys with conventional forming

Al alloys have low formability (approximately 2/3rd) in comparison to most steels. The press

forming of aluminum alloys has problems in comparison to steel principally due material

parameters like low strain rate hardening, normal anisotropy, strain rate sensitivity.

They have a tendency to neck and tear at relatively low strain levels, making it difficult to

use them to make geometrically complex parts conventionally.

Al alloys have high spring back due to a low elastic modulus (approximately 1/3rd of

steels). The elastic modulus for aluminum is one-third that of steel. Therefore, aluminum

spring back will be three times that of steel.

Due to all these factors, virtually all Al vehicle construction so far has been relatively low

volume use. One way of overcoming aluminums’ poor formability has been through the use of

high velocity forming (HVF) techniques.

Spring back

The origin of spring back stress lies in the differential elastic strains through the thickness of the

sheet while forming. Spring back is reduced in high velocity forming due to through-thickness

compressive stresses that act in the sheet, at impact with the die that cause the residual elastic

strains in the sheet to be minimized. If adequate energy is provided to the work piece, it impacts

the die in all areas while still possessing sufficient kinetic energy and experiences reduced spring

back. The elastic modulus for aluminum is one-third that of steel. Therefore, aluminum spring

back will be three times that of steel.

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Stress strain relation of materials

Flow stress = instantaneous value of stress required to continue deforming the material. In

forming processes, such as forging, the instantaneous flow stress can be found from the flow

curve, as the stress required to cause a given strain or deformation.

The flow stress considerably changes during the forming process as the material gets work

hardened considerably. In such cases, an average flow stress is determined from the flow curve.

The average flow stress is given as:

ε is maximum strain during deformation process and n is strain hardening exponent.

To be successfully formed, a metal must possess certain properties. Low yield strength and high

ductility are the properties required for forming process. Ductility increases with temperature and

yield strength decreases with temperature increases.

The modulus of resilience is then the quantity of energy the material can absorb without

suffering damage. Similarly, the modulus of toughness is the energy needed to completely

fracture the material. Materials showing good impact resistance are generally those with high

modulus of toughness

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High velocity/speed/energy rate forming process stress strain relation

High-speed forming is widely investigated to overcome the lower formability of high strength

material. The mechanical properties and the formability of the work piece material can be

improved at the high strain rates. In these forming processes large amount of energy is applied

for a very short interval of time. Many metals tend to deform more readily under extra – fast

application of load which make these processes useful to form large size parts out of most metals

including those which are otherwise difficult – to – form.

The most widely used compression method; whereby round or tubular work pieces are

compressed radially inward onto mating work pieces;

The expansion method; whereby round or tubular work pieces are expanded into a mold

or mating work pieces;

Flat sheet metal forming; whereby electromagnetic pressure is applied to a flat sheet

metal which then accelerates the material into a die or mold

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Deformation velocity in HERF

In conventional forming conditions, inertia is neglected, as the velocity of forming is typically

less than 0.03 to 5 m/s, while typical high velocity forming operations are carried out at work-

piece velocities of about 100 to 300 m/s.

Strain distribution

As compared to conventional forming technique, the strain distribution is much more uniform in

a single operation of HERF. In high velocity forming process, the strain required to failure are

much higher as compared to conventional process. The ductility obtained is more.

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Elastic stress waves

In classical mechanics, when a free body is acted upon by an external force, the whole body is

assumed to be affected by the force immediately, and moves with an acceleration, given by

Newton's Second Law (F = ma). Clearly, this is a simplification, since it is impossible for the

that a load has been applied to travel throughout the body instantaneously. When a structure is

subjected to a rapidly changing load, such as that caused by an impact or explosion, part of the

structure close to the point of application of the load can be highly stressed, while a more distant

area is still unaware of any loading having occurred. The loading has occurred is carried from the

point of application to other parts of the structure by stress waves.

Elastic Waves

Elastic waves act as the propagation of a disturbance through a material medium due to the

repeated periodic motion of the particles of the medium about their mean positions, the

disturbance being handed over from one particle to the next. Energy and momentum propagates

by motion of particles of medium. But medium remains at previous position. The Propagation is

possible due to property of medium like elasticity and inertia. For an elastic wave the particles

always tend to come back to their original positions when set in wave motion. The

propagation velocity of the waves depends on density and elasticity of the medium.

Types of elastic body waves: Body waves travel through the interior of the body.

Longitudinal wave

P-Waves (P stands for primary or pressure or push-pull). These waves are also

called longitudinal waves or compressional waves due to particle compression during their

transport. P-wave is transmitted by particle movement back and forth along the direction of

propagation of the wave.

Transverse Wave

S-Waves (S stands for secondary or shear). Also known as transverse waves, because particle

motions are transverse to the direction of movement of the wave front, or perpendicular to the

ray. These waves involve shearing of the material as the wave passes through it, but not volume

change

Surface Waves are waves that are guided along the surface of the Earth and the layers near the

surface.

Plastic Waves

Plastic (inelastic) deformation takes place in a ductile metal when the stress in the material

exceeds the elastic limit. Under dynamic loading conditions the resulting wave propagation can

be decomposed into elastic and plastic regions.

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Elastic Stress Wave equation (Longitudinal waves in an infinitely long rod)

We have expressed the displacement, velocity, and acceleration of a point of the material in

terms of the time and the position of the point in the reference state. The velocity is expressed in

terms of the time derivative of the material description of the displacement. Elastic wave theory

is concerned with the propagation of small stresses and strains through deformable media.

Initially, the force is applied to the particles at the very end of the bar. These displace by a tiny

amount and hence disturb the next particles, etc., propagating a stress wave along the bar. Each

particle moves only a tiny amount, as the pulse passes, the stress wave itself travels from particle

to particle at very high speed.

An elastic bar of length L and cross section A. the material has density ρ and modulus of elascity

E. the right end of the bar, at length x=L is subjected to an axial impact force F(t). As the stress

wave passes, the element is displaced axially, changes in length and becomes stressed. The

normal stresses over the cross section at the end of the bar, x=L, will propagate (x,t) through

the rod towards the other end where x=0.

The axial strain/deformation/displacement in the element is given by the new length, minus the

original length/ all divided by the original length. The particle displacements caused by a

longitudinal wave must be parallel to the axis of the rod.

The displacement in the axial direction u= u (x, t)

The normal stress over the cross section stress = (x, t)

The face A of the element is displaced by a distance u

The face B of the element is displaced by a distance dxx

uu

)1(..................................... equationx

uE

E

x

xx

As a stress wave travels along the rod and passes through the small element shown in, the axial

stress at the left end of the element ( x) is x.

The other end of small element (x = x+dx) is dxx

x x

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BFaceontermndneglectingnote

t

uastakenisdxofonacceleratiAverage

BandAatnaceleratiot

dxx

u

t

u

t

u

BFaceandAFaceonactingforceAdxx

A

BFaceandAfaceonstressdxx

BFaceandAFaceatntdisplacemedxx

uuu

xxx

xxx

2

......

.......

......

.......

2

2

2

2

2

2

2

2

The unbalanced external forces acting on the ends of the element (the left side) must equal the

inertial force induced by acceleration of the mass of the element (the right side).

The net axial force on “dx” is the force acting on face B, minus that acting on A

Adxx

AAdxx

x

x

x

x

)(

The mass of the element is ( Adx ) while the axial acceleration

The acceleration of particle 2

2

t

u

By Newton’s Second Law (F=ma),

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Now we have

t

uAdxAdx

x

x

2

2

Simplifying

t

u

x

x

2

2

Substitution from equation 1 in the above equation

2

2

2

2

x

uE

t

u

We have wave equation, and it can be shown that it describes the motion of a wave passing through a

body with velocity C,

EC

Substituting in the above equation, we get elastic longitudinal wave equation for a rod of infinite

length

2

22

2

2

x

uC

t

u

….. velocity of an elastic wave travelling along the rod under investigation.

Relation between Particle Velocity and Stress

A stress wave may move through a body at a tremendous velocity, the actual velocity imparted

to the particles in the body by the stress wave is typically very small.

A direct correlation can be obtained, between the velocity of the displaced particles as a stress

pulse passes, and the magnitude of the stress, as shown below:

velocitynpropagatiotvx

relationstrainstressE

relationntdisplacenestrainx

u

x

x

x

CC

C

E

C

tE

tC

t

x

t

uvvelocityparticle xxxxx

2,

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Types of high velocity forming processes

1. Explosive forming

2. Electro-hydraulic forming.

3. Magnetic forming

4. Pneumatic- mechanical high velocity forging.

Explosive forming

Explosive forming is a high velocity forming technique that utilizes the chemical energy of

explosives to generate shock waves through a medium and use them to change the shape of a

metal blank or perform work piece.

shows the schematic phases of an unconfined explosive forming: (1) setup, (2) explosive is

detonated, and (3) the shock wave forms the work piece and plume escapes from the water

surface.

Explosive forming was one of the most widely used high rate forming technique for large and

bulky components, typically for military operations. It was mostly used for low-volume

production of complex parts of tough metals. It is one of the only affordable methods of

fabricating large sections from thick plates like sections of ships, large nuclear reactor

components and heat exchangers. Explosive Forming can be utilized to form a wide variety of

metals, from Aluminum to high strength alloys

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Types of explosive forming (Process variations)

Depending on the position of the explosive charge relative to the work piece, this technique is

divided into two categories

1. Unconfined type or Standoff technique

2. Confined type or Contact technique

Standoff Technique: In a Standoff operation, energy is released at some distance from the work

piece and is propagated through an intervening medium (typically water), in the form of a

pressure pulse.

The sheet metal blank is clamped over a die and the assembly is lowered into a tank filled with

water. The air in the die is pumped out. The explosive charge is placed at some predetermined

distance from the work piece. On detonation of the explosive, a pressure pulse of very high

intensity is produced. A gas bubble is also produced, which expands spherically and then

collapses. When the pressure pulse impinges against the work piece, the metal is deformed into

the die with a velocity as high as 120 m/s of 25 mm thick and 400 mm length and to bulge steel

tubes with thicker as high as 25 mm.

The process is versatile – a large variety of shape can be formed, there is virtually no limit to the

size of the work piece, and it is suitable for low – quantity production. The process has been

successfully used to form steel plates. In general, large standoff distances produce greater

amounts of stretch forming while lower standoff distances increase the amount of draw.

Contact Technique

In a contact operation the explosive charge is detonated while it is in contact with the work piece.

Thus there are differences in energy requirements and the mechanical behavior of the work

piece. The explosive charge in the form of a cartridge is held in direct contact with the work

piece while the detonation is initiated. The detonation builds up extremely high pressures (up to

30,000MPa) on the surface of the work piece, resulting in metal deformation, and possible

fracture. The process is used for bulging tubes locally.

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Advantages of Explosive Forming over Conventional forming

1. Component is generally formed in one shot only.

2. Only one die either male or female is needed. For this reason, tooling costs are low

3. the ultimate strength and yield strength are improved by high explosive forming.

4. Less probability of damage to work.

5. Large and thick parts can be easily formed (expensive presses are not needed)

6. Economical, when compared to a hydraulic press (Low capital investment)

Limitations:/disadvantages

1. Optimum SOD is essential for proper forming operation.

2. Vacuum is essential and hence it adds to the cost.

3. Dies must be larger and thicker to withstand shocks.

4. Not suitable for small and thin works.

5. Explosives must be carefully handled according to the regulations of the government.

Applications of explosive forming

1. Sheet metal panels and tubing

2. Jet engine parts

3. Missile nose cones

4. Ship building,

5. Radar dish,

6. Elliptical domes in space applications

Materials for explosive forming

1. Both ferrous and nonferrous metals including steel, aluminum, magnesium, and their

alloys.

2. Some metal matrix composites like aluminum matrix, copper matrix and lead matrix

composites

Role of water:

1. Acts as energy transfer medium

2. Ensures uniform transmission of energy

3. Muffles the sound of explosion

4. Cushioning/ smooth application of energy on the work without direct contact.

Process Variables

1. Type and amount of explosive: wide range of explosive sis available.

2. Standoff distance – SOD- (Distance between work piece and explosive): Optimum SOD

must be maintained.

3. The medium used to transmit energy: water is most widely used.

4. Work size:

5. Work material properties vi) Vacuum in the die

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Electro-hydraulic forming (EHF)/ electro spark forming

A sudden electrical discharge in the form of sparks is produced between electrodes and this

discharge produces a shock wave in the water medium. This shock wave deforms the work plate

and collapses it into the die. Electro hydraulic forming involves the conversion of electrical

energy to mechanical energy in a liquid medium. As the energy produced is less than

that produced in explosive forming, it is usually necessary to repeat the operation several times

to achieve the desired results

A large amount of energy stored in a capacitor bank is discharged across a spark gap or through

wire bridging an electrode gap, submerged in a liquid (usually water) bath, over a very short

time. This vaporizes the surrounding fluid and creates a high intensity shock wave in it, which

provides transient pressures forcing the work piece in contact with the fluid, into the die cavities.

Voltages of 10,000 to 30,000 volts are generally used when the spark discharge method is

utilized. This potential difference will jump the air gap present between two electrodes,

submerged in the liquid.

Advantages of EHF over conventional forming

1. When EHF is used, the cost of tooling will almost always be less than that for conventional

equipment.

2. Large amounts of energy can be directed into isolated areas as required in some piercing

operations.

3. Reproducibility is another main advantage

4. Better control of the pressure pulse as source of energy is electrical- which can be

easily controlled.

5. Safer in handling than the explosive materials.

6. More suitable if the work size is small to medium.

7. Thin plates can be formed with smaller amounts of energy.

8. The process does not depend on the electrical properties of the work material

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Disadvantages of EHF

1. One of the most significant limitations of the process is the energy rating of the

capacitor bank itself, and the amount of energy which can be dumped by the triggering

device is another.

2. Materials having critical impact velocities below 30 meters per second are not practical

for electrohydraulic forming.

3. Neither is EHF of parts from materials having low ductility, such as the titanium alloys,

likely to be successful

4. Suitable only for smaller works

5. Need for vacuum makes the equipment more complicated.

6. Proper SOD is necessary for effective process.

The characteristics of this process are similar to those of explosive forming. The major

difference, however, is that a chemical explosive is replaced by a capacitor bank, which stores

the electrical energy. The capacitor is charged through a charging circuit. When the switch is

closed, a spark is produced between electrodes and a shock wave or pressure pulse is created.

The energy released is much lesser than that released in explosive forming.

Process Characteristics:

1. Standoff distance: It must be optimum.

2. Capacitor used: The energy of the pressure pulse depends on the size of capacitor.

3. Transfer medium: Usually water is used.

4. Vacuum: the die cavity must be evacuated to prevent adiabatic heating of the work due to

a sudden compression of air.

5. Material properties with regard to the application of high rates of strain.

Applications

They include smaller radar dish, cone and other shapes in thinner and small works. The process is

widely accepted in aerospace industries to accomplish bulging, forming, beading, drawing, blanking and

piercing.

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Electro Magnetic Forming (not in syllabus)

Electromagnetic forming is a type of high velocity, cold forming process for electrically

conductive metals, most commonly copper and aluminum. The work piece is placed into or

enveloping a coil. A high charging voltage is supplied for a short time to a bank of capacitors

connected in parallel. The amount of electrical energy stored in the bank can be increased either

by adding capacitors to the bank or by increasing the voltage. When the charging is complete,

which takes very little time, a high voltage switch triggers the stored electrical energy through

the coil. A high – intensity magnetic field is established which induces eddy currents into the

conductive work piece, resulting in the establishment of another magnetic field. The forces

produced by the two magnetic fields oppose each other with the consequence, that there is a

repelling force between the coil and the tubular work piece that causes permanent deformation of

the work piece.

Two types of deformations can be obtained generally in electromagnetic forming system: (i)

compression (shrinking) and (ii) expansion (bulging) of hollow circular cylindrical work pieces.

When the work piece is placed inside the forming coil, it is subjected to compression (shrinking)

and its diameter decreases during the deformation process.

The electromagnetic forming technology has unique advantages in the forming, joining and

assembly of light weight metals such as aluminum because of the improved formability and

mechanical properties, strain distribution, reduction in wrinkling, active control of spring back,

minimization of distortions at local features, local coining and simple die The applications of

electromagnetic tube compression include, shape joints between a metallic tube and an internal

metallic mandrel for axial or torsional loading, friction joints between a metallic tube and a wire

rope or a non-metallic internal mandrel, solid state welding between a tube and an internal

mandrel of dissimilar metallic

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1. Metals with poor conductivity can only be formed effectively if an auxiliary driver sheet

with high conductivity is used to push the metal sheet

2. The discharge time of electromagnetic forming is very short, generally very short in the

order of 10 microseconds

Electromagnetic forming usually is used to accelerate the sheet metal at velocities up to a few

hundred meters per second, which are 100 to 1000 times greater than the deformation rates of

conventional quasi-static forming such as the sheet metal stamping (~0.1m/s to ~100m/s). It is

well known that high deformation velocity (over about 50m/s) can significantly increase the

formability of metals by several times, compared with those obtained in conventional quasi-static

forming

Some of these advantages are common to all the high rate processes while some are unique to

electromagnetic forming.

Advantages

1. Improved formability.

2. Wrinkling can be greatly eliminated.

3. Forming process can be combined with joining and assembling even with the dissimilar

components including glass, plastic, composites and other metals.

4. Close dimensional tolerances are possible as spring back can be significantly reduced.

5. Use of single sided dies reduces the tooling costs.

6. Applications of lubricants are greatly reduced so, forming can be used in clean room

conditions.

7. High production rates are possible.

Limitations:

1. Applicable only for electrically conducting materials.

2. Not suitable for large work pieces.

3. Rigid clamping of primary coil is critical.

4. Shorter life of the coil due to large forces acting on it.

Applications:

1. Crimping of coils, tubes, wires

2. Bending of tubes into complex shapes

3. Bulging of thin tubes

Electromagnetic forming (EMF) is a high velocity forming technique that uses electromagnetic

forces to shape metallic work pieces. The process starts when a capacitor bank is discharged

through a coil. The transient electric current which flows through the coil generates a time-

varying magnetic field around it. By Faraday’s law of induction, the time-varying magnetic field

induces electric currents in any nearby conductive material. According to Lenz’s law, these

induced currents flow in the opposite direction to the primary currents in the coil. Then, by

Ampere’s force law, a repulsive force arises between the coil and the conductive material. If this

repulsive force is strong enough to stress the work piece beyond its yield point, then it can shape

it with the help of a die or a mandrel.

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