PART - D - mcehassan.ac.inmcehassan.ac.in/department/auto/files/AU303_Manufacturing_Process... · increases the strength and toughness of the forged components. ... Smith’s hammers

AU306 MANUFACTURING PROCESS - I: PART – D

M.K.Ravishankar, Associate Professor, AED, MCE, Hassan – 573 201. 81

9. Forging

Forging practice, forgeability of metals and alloys, forging equipments, spring power hammer, pneumatic power hammer, automatic forging machines, drop forging process, upset forging.

Introduction:

Forging: is the process of shaping heated metal by the application of sudden blows (i.e.,

hammer forging) or steady pressure (i.e., press forging) and makes use of the characteristic of

plasticity of the material.

A metal such as steel can be shaped in a cold state but the application of heat lowers the yield

point and makes permanent deformation easier.

Forging may be done by hand or machine.

Forging by machine involves the use of dies and is generally used in mass-production.

Hand forging or Black-smithy is employed for small quantity production and for special work. It

is essentially a manually controlled process even though some machinery such as power

hammers are sometimes used.

Black-smithy is, therefore, a process by which metal may be heated and shaped to its

requirements by the use of blacksmithy tools either by hand or power hammer.

In smithy small parts are shaped by heating them in an open fire or hearth. Shaping is done

under hand control using hand tools. This work is done in a smithy shop.

Forging refers to the production of medium size and heavy parts in large scale using closed

heating furnaces and heavy hammers, forging presses and machines.

Advantages:

Although the metal piece has to be heated to the correct forging temperature before shaping,

less metal will be used than if the shape were machined from a solid block of metal.

It enhances the mechanical properties of metals and improves the grain flow which in turn

increases the strength and toughness of the forged components.

Forgeable materials:

To be a forgeable metal, it should possess the required ductility.

Ductility refers to the capacity of a material to undergo deformation under tension without

rupture.

The selection of a forging material is made on the basis of certain desirable mechanical

properties inherent in the composition and/or for those which can be developed by forging



such as strength, resistance to fatigue, shock or bending, good machining characteristics,

durability etc.

Listed below are some of forgeable materials, in order of increasing forging difficulty:

1. Aluminium alloys

2. Magnesium alloys

3. Copper alloys

4. Carbon and low alloy steels

5. Martensitic stainless steels

6. Maraging steels

7. Austenitic stainless steels

8. Nickel alloys

9. Titanium alloys

10. Columbium alloys

11. Tantalum alloys

12. Molybdenum alloys

13. Tungsten alloys

14. Berylium

Forging temperatures:

Before forging, the metal workpiece is heated to a proper temperature so that it gains required

plastic properties before deformation, which are essential for satisfactory forging.

Excessive temperatures may result in the burning of the metal, which destroys the cohesion of

the metal.

Insufficient temperatures will not introduce sufficient plasticity in the metal to shape it properly

by hammering etc. Moreover under these conditions, the cold working defects (e.g., hardening,

cracking etc.) are liable to come into product.

The finishing temperature of the workpiece should be that at which no grain growth takes

place, so that the workpiece possesses a fine grained structure.

Given below are the forging temperatures for different metals and alloys:

Metal / Alloy Forging Temperatures, 0C (Approx.)

Starting Finishing

1. Mild steel

2. Medium carbon steel

3. High carbon steel

4. Wrought iron

5. Aluminium and Manesium alloys

6. Copper, Brass and Bronze

1300

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1150

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800

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Usually the hot chisels are thinner and therefore can not be substituted with the cold chisel.

Tongs:

Tongs are used by the blacksmith for holding hot metals securely. The mouths are made in various

shapes to suit the gripping of various shapes of metal.

• Tong type 1,2,3,4,5,6,8,9,10, II, 12, 13 and 14 are used for holding workpieces as shown in Figure itself.

• Type 7 is used for holding round bars or links cross-wise.

• Type 15 is known as a pick-up tong and is used for picking up fallen workpieces.

Types of Blacksmith’s tongs

Fuller:

Fullers are forming tools of different shapes used in making grooves or hollows.

They are often used in pairs, the bottom fuller has a square shank which fits into the hardy

hole in the anvil while the top fuller has a handle.

The work is placed on the bottom fuller and the top is placed on the work and struck with a

hammer.

The top fuller is also used for finishing round corners and for stretching or spreading metal.



Types of hammers:

Smith’s hammers

Hardy:

The hardy is a cutting tool similar to the chisel.

It is used as a chisel or hammer for cutting both hot and cold metals.

It has a square shank that fits into the hardy hole in the anvil, with the cutting edge facing

upwards.

The metal to be cut is placed on the cutting edge and struck with a hammer.

Swage block:

Top view of a swage block showing various sized holes and sections

Right side view of the above swage block

A swage block is a large, heavy block of cast iron or steel used in smithing, with variously-

sized holes in its face and usually with forms on the sides.

The through-holes are of various shapes and sizes and are used to hold, support or back up a

hot bar of metal for further shaping.



Operations performed on a swage block include but are not limited to bending, cutting,

punching and forming.

The sides are scalloped to present formed shapes for forging operations.

Shapes are for example the curve of a wheel, which could be used to finish a wheel rim, using

a suitable hammer.

Other shapes, such as the half hexagon, can be used with a matching fuller (die) to form a

hexagonal cross-section on a bar.

The image shows a 15-inch, square swage block with various semi-circular, hexagonal, and

square shapes around its perimeter along with a selection of through-holes.

There are many different kinds of forging processes available; however they can be grouped into three

main classes:

• Drawn out: length increases, cross-section decreases

• Upset: Length decreases, cross-section increases

• Squeezed in closed compression dies: produces multidirectional flow

Common forging processes include:

Roll forging,

Swaging,

Cogging,

Open-die forging,

Impression-die forging,

Press forging,

Automatic hot forging and

Upsetting.

Upsetting:

Upsetting or heading is the process of increasing the thickness of a bar at the expense of its

length and is bought about by end pressure.

The pressure may be obtained by driving the end of the bar against the anvil, by supporting on

the anvil and hitting with the hammer, by placing in swage block hole and hitting with the

hammer or by clamping in a vice and then hammering.



Figure (a) shows the effect of heavy hammer blows on a uniformly heated bar; (b) shows the

effect of comparatively light hammer blows; (c) and (d) are local upsets obtained by heating

only the end or the middle of the bar.

Upset forging operations

Drop forging process:

Commonly known as die forging or drop forging, impression-die forging makes use of cavities in

specially prepared dies to produce forged shapes in quantities, the finished forging being

commercial negatives or duplicates of each other.

The drop forging denotes forging conducted upon a hammer.

This process generally contains preliminary shaping steps to permit the change from the original

forging block to the finished forging without mechanical defects.

1. Edger: serves to proportion the cross-sectional area along the length by flowing metal from a

section being reduced to a section being enlarged

2. Fullering step or fuller: reduces the cross-sectional area between the ends of the forging block

without appreciable change to the end sections.

3. Bending step or bender: forms the length of the forging stock to a shape for the finishing

impression.

4. The preform may be further shaped to bring it closer to the final configuration in a so called

blocker die which assures proper distribution of material but not the final shape.

5. Excess material is allowed to run out between the flat die surfaces and this flash is sometimes

removed or trimmed prior to forging in finishing die.

6. The excess material is again allowed to escape into a flash, which must now be thin to assure

die filling close tolerances.

7. The flash is reduced to its minimum thickness over only a small width called flash land, and the

rest is allowed to flow into a flash gutter.



Spring power hammer:

A spring hammer has a simple design to regulate the speed and force of its blows.

It is used for drawing out operations.

As shown in figure, the frame 5 supports the bearing 4 in which oscillates the leaf spring 3, as

the connecting rod 1 moves up and down when the sheave 2 which is connected to the crank

gear commences to rotate.

As the spring oscillates, the ram 7 moves up and down between the guides 8, and strikes the

job placed on the anvil.

Spring hammer



Pneumatic power hammers:

• The hammer has two cylinders: compressor cylinder and ram cylinder.

• The piston of the compressor cylinder compresses air, and delivers it to the ram cylinder where

it actuates the piston which is integral with ram delivering the blows to the work.

• The reciprocation of the compressor piston is obtained from a crank drive which is powered

from a motor through a reducing gear.

• The air distribution device between the two cylinders consists of rotary valves with ports

through which air passes into the ram cylinder, below and above the piston alternatively.

• This drives the ram up and down respectively.

• The size of a pneumatic hammer may vary in a range from 50 to 1000 kg. Hammers operate at

70 to 190 blows per minute.

A pneumatic hammer (HC)

1. Compressor cylinder, 2. Ram cylinder, 3. Piston, 4. Ram, 5. Crank driver, 6. Motor, 7. Reducing gear,

8 & 9. Rotary valves, 10. Foot treadle, 11. Ram die, 12. Anvil die, 13. Cap



10. Welding

Introduction: Classification, preparation of base metal and joint, fluxes- need and types.

Arc Welding: Principle, classification, TIG, MIG, Atomic hydrogen welding.

Welding:

Welding is defined as the metallurgical joining together of two metal pieces to produce essentially a

single piece of metal, achieved by coalescence, induced by a combination of temperature, pressure

and metallurgical conditions.

This is a list of welding processes, separated into their respective categories.

Arc welding:

Name Characteristics Applications

Atomic hydrogen welding Two metal electrodes in hydrogen atmosphere Historical

Bare metal arc welding Consumable electrode, no flux or shielding gas Historical

Carbon arc welding Carbon electrode, historical Copper, repair (limited)

Electro-gas welding Continuous consumable electrode, vertical positioning, steel only Storage tanks, shipbuilding

Electroslag welding Continuous consumable electrode, vertical positioning, steel only Storage tanks, shipbuilding

Flux cored arc welding Continuous consumable electrode filled with flux Industry, construction

Gas metal arc welding ¹ Continuous consumable electrode and shielding gas Industry

Gas tungsten arc welding ² Nonconsumable electrode, slow, high quality welds Aerospace

Plasma arc welding Nonconsumable electrode, constricted arc Tubing, instrumentation

Shielded metal arc welding ³ Consumable electrode covered in flux, steel only Construction, outdoors

Stud arc welding Welds studs to base material with heat and pressure Construction, shipbuilding

Submerged arc welding Automatic, arc submerged in granular flux Ship industry

1. Also known as metal inert gas (MIG) welding or metal active gas (MAG) welding

2. Also known as tungsten inert gas (TIG) welding

3. Also known as manual metal arc (MMA) welding or stick welding



Oxyfuel gas welding:


Air acetylene welding Chemical welding process, not popular Limited

Oxyacetylene welding Combustion of acetylene with oxygen produces high-temperature flame, inexpensive equipment Maintenance, repair

Oxyhydrogen welding Combustion of hydrogen with oxygen produces flame Limited

Pressure gas welding Gas flames heat surfaces and pressure produces the weld Pipe, railroad rails (limited)

Resistance welding:


Flash welding

Pressure-controlled resistance welding

Projection welding

Resistance seam welding Two wheel-shaped electrodes roll along workpieces, applying pressure and current

Resistance spot welding Two pointed electrodes apply pressure and current to two or more thin workpieces Automobile industry

Upset welding Butt joint surfaces heated and brought together by force

Solid-state welding:


Coextrusion welding

Cold welding

Diffusion welding

Explosion welding

Forge welding



Friction welding Thin heat affected zone, need sufficient pressure Aerospace industry, railway, land transport

Friction stir welding

Hot pressure welding

Roll welding Dissimilar materials.

Ultrasonic welding

Other welding:


Electron beam welding Deep penetration, fast, high equipment cost

Electroslag welding Welds thick workpieces quickly, vertical position, steel only

Heavy plate fabrication, construction

Flow welding

Induction welding

Laser beam welding Deep penetration, fast, high equipment cost Automotive industry

Laser-hybrid welding Combines LBW with GMAW in the same welding head, able to bridge gaps up to 2mm (between plates), previously not possible with LBW alone.

Automotive, Shipbuilding, Steelwork industries

Percussion welding

Thermite welding Railway tracks

Preparation of base metal:

It always necessary to prepare the surfaces before welding, so that welding carried out will be

effective. This involves,

i) Cleaning the base metal surface:

It is always desirable that the surfaces to be joined by welding should be clean.

The base metal should be free of oil, grease, paint, moisture and other materials which may

interfere with the welding processes; as a result, the surfaces should be thoroughly cleaned.



Also, the oxides present or formed on the metal surface during welding may interfere

considerably with the welding, hence should be cleaned.

Many oxides, for example Al2O3 have higher melting temperatures than their metals namely

Aluminium, resulting in improper fusion between base metal and weld metal.

ii) Edge preparation:

To obtain sound welds it is necessary to prepare the edge properly.

This involves, leveling the edges properly and then to clean the surfaces to remove duct, oil, grease

etc.

While welding butt joints, the edge preparations recommended are as follows:

Types of edge preparations:

a) Square: can be used for thicknesses from 3 to 5 mm. Further, the edges of the plate are

spaced about 3 mm apart before welding.

b) Single V: recommended for plates having thicknesses from 8 to 16 mm. The edges are

beveled to form an included angle of 700 to 900.

c) Double B: is recommended when plates are over 16 mm thick and facilitates welding on both

sides of the plate.

d) Single and double U: are generally recommended when metal thickness is beyond 20 mm.

Types of joints:

In industrial practice various types of joints are used depending on the relative positions

of the two pieces being joined.

The important joints include,

Butt joint

Lap joint

T joint, Corner joint and

Edge joint



All other possible joints are variations of these basic types.

Types of welded joints

Fluxes:

Fluxes are chemical compounds used to prevent oxidation and other chemical reactions.

Use of flux ensures a good, sound weld.

Fluxes react with oxides to produce slag with adequately low melting temperatures.

The resulting slag is more a fluid and readily floats to the surface of the molten metal puddle.

Moreover, slag covers the molten metal puddle and help to keep out atmospheric oxygen,

nitrogen and other gases.

Thus, fluxes are used to reduce oxidation, to remove any oxide formed and other impurities.

Use of flux also makes the welding operation simple and gives a stronger and more ductile

joint.

Types of flux:

Different fluxes are used for different welding processes.

i) Oxy-acetylene process:

Flux used in oxyacetylene welding acts in the same way as other welding processes.

It removes all oxides and grease from metal surface.

It makes the operation much easier, helps in achieving better bond with the parent metal.

Fluxes used for bronze, brass and copper are essentially one of the borax types.

For aluminum alloys, the flux used consists of Lithium chloride, Potassium fluoride, potassium

chloride and sodium chloride.

After welding, the surfaces should be treated with 5% nitric acid in water to remove traces of

flux.

For cast iron, fluxes contain sodium, potassium borates, carbonates and bicarbonates etc.



ii) Arc welding:

In arc welding electrode itself is covered with flux.

These fluxes enable the arc to be struck and maintained easily.

In addition it provides a shield of gases such as hydrogen and carbon dioxide which prevents

the molten metal and parent metal from reacting with oxygen and nitrogen of the atmosphere

to form oxides and nitrides.

Rutile, which is a mineral consisting of upto 88 to 90% TiO2 is one of the most widely used

material for electrode coating.

Other materials used are, Ilmenite (FeTiO3), calcium carbonate or limestone, fluorspar,

solkafloc (cellulose acetone), feldspar (anhydrous aluminium silicate), Ball clay, sodium

aginate (extracted from sea weed), etc.

The flux coatings on electrodes may be applied by solid extrusion or dipping.

In solid extrusion, the flux in the form of a plate is forced under pressure around the wire core,

and this is the most widely used process while for certain a special application, dipping is used.

Arc Welding:

Arc welding is a group of welding processes wherein coalescence is produced by heating with an

electric arc or arcs, mostly without the application of pressure and with or without the use of filler metal

depending upon the base plate thickness.

Classification:

1. Carbon-arc welding

2. Shielded-metal arc welding

3. Submerged arc welding

4. TIG (or GTAW) welding

5. MIG (or GMAW) welding

6. Electroslag welding

7. Electrogas welding

8. Plasma arc welding

9. Arc spot welding

10. Stud (Arc) welding

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Manual gas tungsten arc welding is often considered the most difficult of all the welding

processes commonly used in industry. Because the welder must maintain a short arc length,

great care and skill are required to prevent contact between the electrode and the workpiece.

Unlike most other welding processes, GTAW normally requires two hands, since most

applications require that the welder manually feed a filler metal into the weld area with one

hand while manipulating the welding torch in the other. However, some welds combining thin

materials (known as autogenous or fusion welds) can be accomplished without filler metal;

most notably edge, corner, and butt joints.

To strike the welding arc, a high frequency generator provides a path for the welding current

through the shielding gas, allowing the arc to be struck when the separation between the

electrode and the workpiece is approximately 1.5–3 mm (0.06–0.12 in).

Bringing the two into contact in a "touch start" ("scratch start") also serves to strike an arc. This

technique can cause contamination of the weld and electrode.

Once the arc is struck, the welder moves the torch in a small circle to create a welding pool,

the size of which depends on the size of the electrode and the amount of current. While

maintaining a constant separation between the electrode and the workpiece, the operator then

moves the torch back slightly and tilts it backward about 10–15 degrees from vertical.

Filler metal is added manually to the front end of the weld pool as it is needed.

Welders often develop a technique of rapidly alternating between moving the torch forward (to

advance the weld pool) and adding filler metal.

The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never

removed from the gas shield to prevent oxidation of its surface and contamination of the weld.

Filler rods composed of metals with low melting temperature, such as aluminum, require that

the operator maintain some distance from the arc while staying inside the gas shield. If held

too close to the arc, the filler rod can melt before it makes contact with the weld puddle.

As the weld nears completion, the arc current is often gradually reduced to allow the weld

crater to solidify and prevent the formation of crater cracks at the end of the weld.

Operation Modes:

GTAW can use a positive direct current, negative direct current or an alternating current,

depending on the power supply set up.

A negative direct current from the electrode causes a stream of electrons to collide with the

surface, generating large amounts of heat at the weld region. This creates a deep, narrow

weld.



In the opposite process where the electrode is connected to the positive power supply

terminal, positively charged ions flow from the tip of the electrode instead, so the heating

action of the electrons is mostly on the electrode.

This mode also helps to remove oxide layers from the surface of the region to be welded,

which is good for metals such as Aluminium or Magnesium.

A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current

gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot

of heat as well.

Applications:

• The aerospace industry

• For welding thin workpieces, especially nonferrous metals

• In the manufacture of space vehicles, and is also frequently employed to weld small-diameter,

thin-wall tubing such as those used in the bicycle industry

• Used to make root or first pass welds for piping of various sizes

• Used to repair tools and dies, especially components made of aluminum and magnesium

• Welding process permits the welding of so many alloys in so many product configurations

Materials:

Gas tungsten arc welding is most commonly used to weld stainless steel and nonferrous

materials, such as aluminum and magnesium, but it can be applied to nearly all metals, with

notable exceptions being lead and zinc.

Its applications involving carbon steels are limited not because of process restrictions, but

because of the existence of more economical steel welding techniques, such as gas metal arc

welding and shielded metal arc welding.

Furthermore, GTAW can be performed in a variety of other-than-flat positions, depending on

the skill of the welder and the materials being welded.

Advantages:

• Concentrated Arc - Permits pin point control of heat input to the workpiece resulting in a

narrow heat-affected zone

• No Slag - No requirement for flux, therefore no slag to obscure the welder’s vision of the

molten weld pool

• No Sparks or Spatter - No transfer of metal across the arc. No molten globules of spatter to

contend with and no sparks produced if material being welded is free of contaminants



• Little Smoke or Fumes - Compared to other arc-welding processes like stick or flux-cored

welding, few fumes are produced. The base metals being welded may contain coatings or

elements such as lead, zinc, copper, nickel that may produce hazardous fumes, however.

• Welds more metals and metal alloys than any other process

• Good for welding thin material.

Disadvantages:

• Slower travel speeds than other processes

• Lower filler metal deposition rates

• Hand-eye coordination is a required skill

• Brighter UV rays than other processes

• Equipment costs can be higher than other processes

• Concentrations of shielding gas may build up and displace oxygen when welding in confined

areas

Gas metal arc welding:

Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc

welding process in which a continuous and consumable wire electrode and a shielding gas are

fed through a welding gun.

A constant voltage, direct current power source is most commonly used with GMAW, but

constant current systems, as well as alternating current, can be used.

There are four primary methods of metal transfer in GMAW, each of which has distinct

properties and corresponding advantages and limitations, namely

Globular,

Short-circuiting,

Spray, (well suited for aluminium and stainless steel) and

Pulsed-spray (for thin workpieces and non-ferrous materials),

Originally developed for welding aluminium and other non-ferrous materials in the 1940s,

GMAW was soon applied to steels because it allowed for lower welding time compared to

other welding processes.

The cost of inert gas limited its use in steels until several years later, when the use of semi-

inert gases such as carbon dioxide became common.



Further developments during the 1950s and 1960s gave the process more versatility and as a

result, it became a highly used industrial process.

Today, GMAW is the most common industrial welding process, preferred for its versatility,

speed and the relative ease of adapting the process to robotic automation.

The automobile industry in particular uses GMAW welding almost exclusively.

Unlike welding processes that do not employ a shielding gas, such as shielded metal arc

welding, it is rarely used outdoors or in other areas of air volatility.

A related process, flux cored arc welding, often does not utilize a shielding gas, instead

employing a hollow electrode wire that is filled with flux on the inside.

Inert gas:

Carbon dioxide is used for working with steel.

Argon or argon-helium mixtures are used for welding aluminium or copper.

For stainless-steel welding either argon-oxygen or helium-argon gas mixtures are used.

Titanium requires pure argon gas shielding and

High-nickel alloys use argon helium mixture.

Equipment:

To perform gas metal arc welding, the basic necessary equipment is

A welding gun,

A wire feed unit,

A welding power supply,

An electrode wire, and

A shielding gas supply.

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Keeping a consistent contact tip-to-work distance (the stickout distance) is important, because

a long stickout distance can cause the electrode to overheat and will also waste shielding gas.

Stickout distance varies for different GMAW weld processes and applications.

For short-circuit transfer, the stickout is generally 1/4 inch to 1/2 inch, for spray transfer the

stickout is generally 1/2 inch.

The position of the end of the contact tip to the gas nozzle is related to the stickout distance

and also varies with transfer type and application.

The orientation of the gun is also important—it should be held so as to bisect the angle

between the workpieces; that is, at 450 for a fillet weld and 900 for welding a flat surface.

The travel angle or lead angle is the angle of the torch with respect to the direction of travel,

and it should generally remain approximately vertical.

However, the desirable angle changes somewhat depending on the type of shielding gas

used—with pure inert gases; the bottom of the torch is out often slightly in front of the upper

section, while the opposite is true when the welding atmosphere is carbon dioxide.

Advantages:

1. Since consumable metal electrode is continuously fed, this process is much faster compared to

TIG welding.

2. Flux is not used and hence joint essentially smooth, clean and sputter free.

3. Deeper penetrations can be achieved.

4. The process can be easily mechanised.

5. Welds all metals including aluminium and stainless steel

6. High economy

Disadvantages:

1. The process parameters are more, such as electrode, feed rate etc., which need closer

control.

2. The equipment is much costlier and less portable.

3. Because there is no slag, weld metal cooling rate is high.

Atomic hydrogen welding:

Atomic hydrogen welding (AHW) is an arc welding process that uses an arc between two

metal tungsten electrodes in a shielding atmosphere of hydrogen.



The process was invented by Irving Langmuir in the course of his studies of atomic hydrogen.

The electric arc efficiently breaks up the hydrogen molecules, which later recombine with

tremendous release of heat, reaching temperatures from 3400 to 4000 °C.

An acetylene torch merely reaches 3300 °C. This is the third hottest flame after cyanogen at

4525 °C and dicyanoacetylene at 4987 °C.

This device may be called an atomic hydrogen torch, nascent hydrogen torch or Langmuir torch.

The process was also known as Arc-Atom welding.

The heat produced by this torch is sufficient to melt and weld tungsten (3422 °C), the most refractory

metal. The presence of hydrogen acts as a gas shield and protects metals from contamination by

carbon, nitrogen, or oxygen, which can severely damage the properties of many metals. It eliminates

the need of flux for this purpose.

The arc is maintained independently of the workpiece or parts being welded. The hydrogen gas is

normally diatomic (H2), but where the temperatures are over 600 °C (1100 °F) near the arc, the

hydrogen breaks down into its atomic form, simultaneously absorbing a large amount of heat from the

arc.

When the hydrogen strikes a relatively cold surface (i.e., the weld zone), it recombines into its

diatomic form and rapidly releases the stored heat. The energy in AHW can be varied easily by

changing the distance between the arc stream and the workpiece surface. This process is being

replaced by shielded metal-arc welding, mainly because of the availability of inexpensive inert gases.

In atomic hydrogen welding, filler metal may or may not be used. In this process, the arc is maintained

entirely independent of the work or parts being welded. The work is a part of the electrical circuit only

to the extent that a portion of the arc comes in contact with the work, at which time a voltage exists

between the work and each electrode.

Atomic hydrogen welding



Advantages:

1. A close control over rate of heat input is possible just by varying the distance of the electrode

from the workpiece.

2. The rate of deposition of filler metal can be easily controlled.

3. Rate of deposition is quite high and therefore welds can be quickly made.

4. It produces practically defect free joints.

Disadvantages:

1. Welding is limited to flat position only.

2. The process is quite expensive.

3. A close control over the process is essential.

Applications:

1. Used for welding thin pieces of aluminium, stainless steel, etc.

2. Used for building up of a metal surface in the repair of a worn out steel molds and dies.



11. Resistance welding

Resistance welding: Principle, Resistance Spot welding, Resistance Seam welding, Projection welding, upset welding.

Principle:

In resistance welding, the heat is obtained at the desired location of the weld by the electrical

resistance through the metal pieces to high amperage, low voltage current passed for a

relatively short duration.

As the metal reaches its fusion point at that location due to resistance heating, the metal

pieces are pressed together between the electrodes until that weld becomes solid.

Spot welding:

A Miller spot welder Spot Welding Robot

Spot welding is employed to join overlapping strips, sheets or plates of metal at small areas.

The pieces are assembled and placed between two electrodes, which must possess high

electrical and thermal conductivity and retain the required strength at high temperatures.

The electrodes are made of pure copper for a limited amount of service, and of alloys of

copper or tungsten, or copper and chromium for continuous working.

When the current is turned on, the pieces are heated at their areas of contact to a welding

temperature, and with the aid of mechanical pressure the electrodes are forced against the

metals to be welded.

The pressure may be developed by a foot lever or by air pressure or by hydraulic cylinders.

This may be used to weld steel and other metal parts up to a total thickness of 12 mm.

Practically all combinations of ductile metals and alloys can be spot welded.

This is used for fabricating structures for mechanical strength rather than for water or air

tightness.



To obtain good welds, the sheet metal should be free of foreign matter and scale.

Films of any type have a tendency to cause variations in surface resistance and also increase

the heating effect of the metal in contact with the electrodes.

The attractive feature of spot welding is a lot of energy can be delivered to the spot in a very

short time (ten to one hundred milliseconds). That permits the welding to occur without

excessive heating to the rest of the sheet.

The amount of heat (energy) delivered to the spot is determined by the resistance between the

electrodes and the amplitude and duration of the current.

The amount of energy is chosen to match the sheet's material properties, its thickness, and

type of electrodes.

Applying too little energy won't melt the metal or will make a poor weld.

Applying too much energy will melt too much metal and make a hole rather than a weld.

Another attractive feature of spot welding is the energy delivered to the spot can be controlled

to produce reliable welds.

Most spot welded technologies use water to cool the electrode assemblies due to the intense

heat generated.

Applications:

Spot welding is typically used when welding particular types of sheet metal.

Thicker stock is more difficult to spot weld because the heat flows into the surrounding metal

more easily.

Spot welding can be easily identified on many sheet metal goods, such as metal buckets.

Aluminum alloys can also be spot welded. However, their much higher thermal conductivity

and electrical conductivity mean that up to three times higher welding currents are needed.

This requires larger, more powerful, and more expensive welding transformers.

KUKA industrial robots welding a car body in the white section of a production line Spot welding:



Perhaps the most common application of spot welding is in the automobile manufacturing

industry, where it is used almost universally to weld the sheet metal to form a car.

Spot welders can also be completely automated, and many of the industrial robots found on

assembly lines are spot welders (the other major use for robots being painting).

Spot welding is also used is in the orthodontist's clinic, where small scale spot welding

equipment is used when resizing metal "molar bands" used in orthodontics.

Another application is spot welding straps to nickel-cadmium or nickel-metal-hydride cells in

order to make batteries. The cells are joined by spot welding thin nickel straps to the battery

terminals. Spot welding can keep the battery from getting too hot, as might happen if

conventional soldering were done.

1. Electrode

2. Workpiece

3. Clamp

Spot welding (HC)



Seam welding:

Seam welding is a method of making a continuous joint between two overlapping pieces of

sheet metal.

The normal procedure for making a seam weld is to pace the work between the wheels

which serve as conductors for producing continuous welds.

As pressure is applied, the drive is started and the welding current switched on.

Then, at the same time, the over-lapping surfaces of the metal are forced together as fast as

they are heated.

A coolant is applied to conserve the electrodes and cool the work rapidly to speed the

operation.

The materials that may be seam welded include most of those that may be spot welded.

Steel plates of 10 mm thick have been seam welded to hold about 20000 kN/m2 pressure.

It is used on many types of pressure tight or leakproof tanks for various purposes, and

numerous other products.

Instead of using two cylindrical electrodes as in case of spot welding, here two circular disks

are used as electrodes.

The work piece is passed through the space between the two discs, and under pressure

applied by the discs and current flowing through them, a continuous weld is formed.

With seam welding the material passes between two rotating wheels or welding rollers. The

welding rollers perform three tasks:

Weld current transmission

Welding pressure

Feed motion transmission

Most seam welded technologies use water cooling through the weld roller assemblies due to

the intense heat generated.



1. Wheel

2. Workpiece

Seam welding

Projection welding:

In this case, one of the metal pieces to be welded has projections at the locations where weld

are to be made.

By using embossing technique in punch presses, such projections can be easily made.

These projections should preferably extend about 0.8 mm from the surface to be welded.

When the two pieces are assembled, the contacts between the surfaces to be welded are

established only at these projections.

The electrodes used are of flat type as shown in figure.

When the electrodes are connected to the power supply, the contact between the plates is

established only at these projections which gets heated up and melts.

Immediately, force is applied to the electrodes so that the two pieces get spot welded together

at these projections.

The number of projections used varies from 3 to 6.

Projection welding



Advantages:

1. At a time welding is achieved at more than one spot.

2. As the area of contact between electrode and workpiece is more, the life of the electrodes will

be much larger.

3. This process helps in assembly of metal pieces

4. The spacing of the welds can be closer than is possible in spot welding.

Upset welding:

Upset welding is a special way of welding, in which two pieces of material are forged together

at elevated temperatures.

There are different ways to reach this temperature, but a current through the interface, friction

or sometimes an external source is used.

No extra weld material is added to the weld.

During the forging, some material will be expelled from the weld, this material is called the

upset.

This removal is necessary to ensure that surface contaminants (such as oxides or oil) are

removed from the weld metal.

When these contaminants remain in the weld they can become initiation points for corrosion or

crack forming.

Documents

PART - D - mcehassan.ac.inmcehassan.ac.in/department/auto/files/AU303_Manufacturing_Process... · increases the strength and toughness of the forged components. ... Smith’s hammers