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METAL WORKING I1. INTRODUCTION

Materials have to be processed in to a great variety of shapes in order to make component parts of every

type. The shapes required vary enormously, both in size & complexity, ranging microelectronic

components to large casing and forging having mass of 100s of tones. Engineer must be aware range of

manufacturing processes available and of advantages and limitation of the various processes. The

properties of materials in the finished component are also influenced to a considerable extend by the

type of shaping process employed, and by the condition existing during process. The whole ranging of

shaping process is classified into four categories:

1) Casing – pouring of liquid metal into prepared moulds.2) Manipulative processes involving plastic deformation (metal forming) of materials.3) Powder metallurgy - producing shapes by compacting powder and sintering.4) Machining and grinding – most metals are initially produced in the liquid phase and are then cast

in to shapes, either to give a casting or into ingots which can be further processed by manipulative techniques such as rolling, forging, extrusion, wire drawing, etc.

The engineering stress-strain curve does not give a true indication of the deformation characteristics of a

metal because it is based entirely on the original dimensions of the specimen, and these dimensions

change continuously during the test. Also, ductile metal which is pulled in tension becomes unstable and

necks down during the course of the test. Because the cross-sectional area of the specimen is decreasing

rapidly at this stage in the test, the load required continuing deformation falls off. The average stress

based on original area like wise decreases, and this produces the fall-off in the stress-strain curve beyond

the point of maximum load.

METAL FORMING

Metal forming is defined as solid state deforming a material by application of stress and heat beyond

yield point to obtain desired shape and size permanently without changing its mass and composition.

Forming involves deformation of body by application of force, heat or any other cause or combination

of these. Metal forming is a manufacturing through plastic modification of a shape while retaining its

mass and cohesion. Two ways of classifying metal forming processes are given below.

Classification of metal forming process

Bulk forming processes Metal sheet forming processes

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Conventional Non Conventional

Forging Rolling Extrusion Drawing HERF HVF

1. Explosive 1. Dynapak

Forming

2. EMF 2. Pneumo

3. EHF mechanical Petroforge

Forging Rolling Extrusion Drawing

1. Closed die forging

without flash

1. Sheet rolling 1. Lubricated direct

hot extrusion

1. Drawing

2. Closed die forging

with flash

2. Shape rolling 2. Non lubricated

extrusion

2. Ironing

3. Coining 3. Tube rolling 3. Hydrostatic

extrusion

3. Tube sinking

4. Upsetting 4. Ring rolling

5. Forward extrusion 5. Rotary rolling

Sheet Metal Forming

Bending and

straight flanging

Surface contouring

of sheet

Linear

contouring

Shallow

recessing

Deep

recessing

Roll bending Stretch forming Dimpling Spinning

Brake

bending

Bulging Expo

forming

Deep

Drawing

Vacuum

forming

Rubber pad

forming

Age forming Mar forming

Hydro forming

Bulk forming Sheet forming

1. In bulk forming, the input material is

in billet, rod or slab form and surface

1. In sheet forming, a piece of sheet

metal is plastically deformed by

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to volume ratio in the formed part

increases considerably under the

action of largely compressive loads.

tensile loads into a three dimensional

shape often without significant

change in thickness or surface

characteristics.

2. Generic bulk forming processes are

forming, rolling, extrusion, drawing

2. Sheet forming processes include

cutting operation (punching,

blanking, notching etc.), forming,

bending, drawing, etc.

3. The deforming material or workpiece

undergoes large plastic deformation

resulting in an appreciable change in

shape or cross section.

3. The workpiece is a sheet or a part

fabricated from a sheet. The

deformation usually causes

significant change in the shape but

not in the cross sectional area of the

sheet.

4. The portion of the workpiece

undergoing plastic deformation is

generally much larger than the portion

undergoing elastic deformation;

therefore elastic recovery after

deformation is negligible.

4. In some cases, the magnitudes of

plastic and elastic deformation are

comparable and therefore spring back

is present and significant.

5. Predominant mean stress of

deformation is compression

5. Predominant mean stress of

deformation is compression

6. Bulk forming parts have lower surface

area to thickness (volume) ratio.

6. Sheet metals by their nature have a

high ratio surface area to (thickness)

volume.

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Fig. 1

Metal Working Theory: The main objective of this theory is to predict the stresses acting during metal deformation and consequently the forces which must be employed. The working load determines the power requirement and the size of the equipments necessary to perform a particular operation. Measurement of forces required to perform the operation can be compared with the one computed for ideal condition which will give an indication of the efficiency of the process.

Comparison of metal forming processes with other manufacturing processes:

1. Parts produced using metal forming processes are stronger than the produced using other

manufacturing processes due to

(i) Presence of strain hardening which makes them stronger and harder than the initial raw material

and make possible to use of low cost raw material .Strain hardening that occurs during warm and

cold forming results on the one hand in higher levels of flow stress, but also in higher ultimate and

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fatigue strength. Therefore, it is possible to use lower cost steel grades with lower initial

characteristics in order to achieve the same mechanical properties obtainable in machined parts.

The work hardening behaviour is interesting from economic point of view, as cheaper materials

can be extruded or drawn to obtain higher mechanical strength generally obtainable in higher grade

costlier materials. This aspect is all the more important in case of certain materials like pure

aluminium and pure copper which cannot be strengthened except by work hardening.

(ii) Preferred orientation of fiber flow lines in the direction of load makes the parts to take up more

load or safer for given loads.

Since the grains are elongated in the direction of flow, they would be able to offer more

resistance to stresses along them. As a result, the mechanically worked metals also called wrought

products achieve better mechanical strength in specific orientation that is along the flow direction.

Since it is possible to control these flow lines in any specific direction by careful manipulation of

the applied forces, it is possible to obtain maximum mechanical properties. The metal, of course

would be weak across the flow lines.

(a)

Direction of grain flow in a gear blank; (i) bar stock and (ii) forged

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(b)

(c)

Fig. 2: Fiber flow lines.

(a) (b)

Crane Hook

(a) (b)

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Gear BlankFig. 3: Grain flow directions obtained in forging.

Forging produces parts with unbroken grain flow following the contour of the part, making a part

stronger than one that has been cast or machined from the solid.

In crane hook, the desired fiber flow lines are obtained by bending after drawing out. As a result

the grain flow is also bent along the hook and thus provides the necessary strength for lifting loads.

(iii) Hot working processes such as rolling, extrusion and forging are typically used as first in the

conversion of cast ingot into a wrought (mechanically worked) part. The blowholes and porosity is

eliminated by welding of these cavities and the coarse columnar grain structure of cast ingot is

broken down and refined into a small equiaxed recrystallized grains. In fact, a cast ingot is always

rolled so that ductile matrix is made to flow into spaces between various brittle phases and bind

(weld) them perfectly into a sound material. It results in improved ductility and toughness of the

material over the cast structure.

It improves homogeneity of the material non uniform grains are broken down. Segregation of

elements are broken down and distributed evenly.

In sum, plastic deformation creates fiber structure, strain hardening and integrity in structure. It is

carried out:

i. To produce desired shape, size, accuracy and surface finish with minimum wastage.

ii. To improve properties through redistribution of grains and impurities and imparting strain

hardening.

2. Reduction /elimination of subsequent machining

Subsequent metal machining is necessary only in the case of geometries which present a particular

problem for forming processes, for example recesses, undercuts or threads. By using cold forging,

substantial savings can be achieved by reducing investment in machine tools for metalcutting and in

metal operating staff.

Parts produced using metal forming processes have superior quality in terms improved strength,

accuracy and surface finish. Surface finish obtained in cold working processes is of high order and

seldom requires further machining.

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3. Lower Material Input

Almost all initial volume of the billet is processed into the finished part. Comparing with machining,

saving up to 75 % of material is possible. When high alloyed and costly materials are used, the benefit

of low material input becomes increasingly significant in relation to overall manufacturing cost.

The wastage of material in metal working processes is either negligible or very small. Compare

a manufacturing of a sparkplug using hexagonal bar stock. Wastage of material in machining

was 74 % and hardly 6 % in metal forming as metal is moved in metal forming rather than

removed and thus attain economy in material usage.

Fig. 4: Comparison of the input weight and achievable geometry for machining and forming process.

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Fig. 5

4. Generally High Productivity

The production rates are very high. Small workpieces produced from wire on horizontal forging

machines can be manufactured at stroking rates of up to 200/min. For large parts produced by forging

from billet in vertical presses from billet, production speeds of up to 50 parts/min are possible. A screw

manufacturer shifted from screw machining to screw cold forming and reduced material wastage by 2/3.

It also improved the production rate from 8 per minutes to 40 per minutes.

Cold forming was earlier associated with non ferrous metals and alloys but now it is extensively

used for steel. Shapes are generally axis-symmetric or those having smaller departure from

symmetry. All cold forming processes have high production rates, close dimensional accuracy and

tolerances and excellent surface finish.

5. Indespensability

Very large reductions are possible in the metal forming processes and therefore metal forming

(rolling)is necessary to convert large cast ingot into usable size intermediate products such as

merchant sections ,structurals, rails, plates, sheets etc.

Many items that simply cannot be produced by any alternative means. Extra thin foils, wire, metal

sheets and other products which are indispensable in modern civilization are originated with the

advent and development of plastic deformation of metals.

Limitations

1. Since metal flow takes place in semi solid state, it poses severe constraints in forming intricate

details as obtained in casting especially in die casting.

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2. Metal forming processes required heavy and sturdy tooling, equipments and presses to withstand

heavy forces arising due to

i. Flow stress of metal is high especially in cold forming ranging from 5 to 250 Kgf/mm2.

ii. Entire or at least major part of the workpiece is deformed at one and same time. e.g. in closed

die forging, whole part is deformed at a time.

iii. Many a times the size of the part itself is very large especially in processes like rolling and

extrusion.

3. All these means heavy investment required compared to other manufacturing processes. For

example, to set up a steel plant having capacity of 1 million ton requires investment of about

Rs.1800 cr. To recover such a huge investment, large volume of production is a necessary

condition. It provides (i) high productivity and production rate (ii) minimum wastage of

material. (iii) Improved mechanical properties.

4. Difficulties encountered in metal forming

i. During metal forming virgin surfaces are generated and continuously exposed to the

atmosphere. This situation calls for continuous lubrication to protect such virgin surfaces.

Maintaining lubrication at high temperature and pressure is very difficult.

ii. Manufacturing of metal forming tools calls for well equipped tool room and skilled manpower

to accomplish close tolerances required on the tooling.

iii. Very high tonnage requirement.

iv. Certain minimum production volume requirement to make plants economically viable.

Important areas of application

i. Components for automobiles and machine tools as well as for industrial plants and equipments.

Here metal forming is vital link in the development of modern design in light alloys.

ii. Hand tools, such as hammers, pliers, screw drivers and surgical instrument.

iii. Fasteners such as screws, nuts, bolds and rivets.

iv. Containers such as metal boxes, cans and canisters.

v. Fitting used in the building industry such as for doors and windows.

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