3 WELDING RE 1 - P1

Department of Mechanical & Manufacturing Engineering, MIT, Manipal 1 of 73

MANUFACTURING TECHNOLOGY

CHAPTER 3

WELDING


Welding Defined

It is a metallurgical process of joining two metal pieces

together to produce essentially a single piece of metal by

heating them to suitable temperatures with or without the

application of pressure and with or without the use of filler

material.

Department of Mechanical & Manufacturing Engineering, MIT, Manipal


PRESSURE PROCESS FUSION PROCESS

FORGE WELDING RESISTANCE

WELDING THERMIT

WELDING ARC

WELDING

GAS

WELDING

SPOT

WELDING

BUTT

WELDING

SEAM

WELDING

PERCUSSION

WELDING

METAL ARC

WELDING

ATOMIC HYDROGEN

WELDING

CARBON ARC

WELDING

WELDING

TUNGSTEN INERT

GAS WELDING


Department of Mechanical & Manufacturing Engineering, MIT, Manipal 4 of 73


Principle of arc welding:


Principle of arc welding:

In this process contact is made first between the

electrode and the work piece to create an electric circuit

and then, by separating the conductors, an arc is

formed.

The electric energy is converted into intense heat in the

form of an arc which attains a temperature of around

2500°C.

Heat produced by the arc is utilized to melt and join the

pieces to be welded.



• Additional filler rod is used sometimes.

• The electrode may be coated with flux or it may be bare.

• When the arc is produced, the intense heat quickly melts

the work piece metal which is directly under the arc,

forming a small molten metal pool.

• At the same time the tip of the electrode at the arc also

melts, and this molten metal of the electrode is carried

over by the arc to the molten metal pool of the work

piece.

• A solid joint will be formed when the molten metal cools

and solidifies.



Polarity



Electrodes of arc welding:

Electrodes are commonly divided into 2 types.

• Bare electrodes

• Coated electrodes

Bare electrodes:

• These are cheaper but the welds produced through these are

of poor quality and their use requires very high degree of skill

on the part of welder if satisfactory results are to be expected.



Coated electrodes:

• This consists of a bare metallic wire provided with a

coating on the outside surface.

• Mild steel is the most commonly used material for core

wire.



Factors influencing the selection of electrode

• Availability of current – A.C or D.C

• Composition of the base metal

• Thickness of the base metal

• Welding position – flat, horizontal, vertical, or overhead.

• Expected physical properties of welded joints – i.e.,

strength, ductility, soundness, appearance etc.

• Amount of penetration required in welding

• Skill of the welders in using particular type of electrodes.

• Economic considerations.



Consumable electrode:

• It is consumed during welding process.

• It melts in to weld zone, thus electrode must be moved

continuously towards the work piece to maintain constant

arc length.

• It will act as a filler metal.

• It can be made of steel, cast iron, copper, brass, bronze,

etc.



Non – consumable electrodes:

• Tungsten and its alloys, carbon, graphite are used as

non consumable electrodes.

• When consumable electrodes are used for arc welding,

then welding process is termed as Metal arc welding.

• If non-consumable electrodes are used then welding

process is termed by the electrode material used.

• The filler metal required has to be deposited through a

separate filler rod.



– To provide a protective atmosphere

– To add alloy elements.

– To add the additional filler metal.

– To provide a protective slag to accumulate impurities,

prevent oxidation and minimize the cooling rate of the

weld.

– Minimize the thermal losses to the atmosphere.

– To remove the impurities from the molten metal.

Functions of flux coated electrodes:



– Electrode classification provides information on,

• constituents of flux coating

• nature of slag

• current & polarity

• welding position

• appearance of weld deposit

• quality of weld etc.

– These are required to select the most suitable electrode.

Classification and Coding of electrodes:





Example: E307411 means

• It is a solid extruded electrode

• Its covering contain appreciable amount of titania

• It is all position electrode

• It can be operated on DC or AC with a power source having,

open circuit voltage 50 volts

• Weld metal tensile strength ranges between 410 and 510 N/mm2

and minimum yield stress is 330N/mm2

• Minimum % elongation of weld metal is 20%



Tungsten Inert Gas welding: (TIG)/ (GTAW)



• This arc welding process uses the intense heat of an

electric arc between a non-consumable tungsten electrode

and the material to be welded.

• The shielding is obtained from the inert gas such as

helium or argon or a mixture of two.

• Filler metal may or may not be used. When filler metal rod

is used, it is usually fed manually into the weld pool.

• Electrodes used in this process are made of tungsten or

tungsten alloys.

• The tungsten electrode is used only to generate an arc.

• The arc doesn’t melt the tungsten, which has a melting

point of over 3300C .



Advantages:

• No flux is required, hence no special cleaning is

required.

• It produces high quality welds in nonferrous metals.

• TIG weld joints are stronger, more ductile, more

corrosion resistance.

• Dissimilar metals can be welded.



Disadvantages:

• Process is comparatively slow

• There exists possibility of electrode contamination

• Inert gases are expensive

• High equipment cost

• Skilled operators are required

• Welder manually feeds the filler metal into the weld area

with one hand while manipulating the welding torch in the

other.



Metal Inert gas welding (MIG)/ (GMAW)



• Gas metal arc welding is a gas shielded, metal arc

welding process which uses a high heat of an electric arc

between a continuously fed, consumable electrode wire

and the material to be welded.

• In this process the wire is fed continuously from a reel,

such that feeding rate of the electrode to the arc is equal

to the melting and deposition rate of it on the base metal.

• The continuously fed bare electrode melts and acts as

filler rod.




• No pressure is applied for welding purpose.

• Arc is shielded by an inert gas. Shielding gases may be

carbon dioxide or Helium or Argon, generally CO2 is

used as it is cheap and is used for welding steels.

• A reverse polarity DC arc is generally used because of

its deeper penetration, good spray transfer, and smooth

welds with good profile.




Advantages

• No flux is required.

• High welding speed.

• Increased corrosion resistance.

• Easily automated welding

• Applicable to both ferrous and nonferrous metals




Disadvantages

• High equipment cost

Applications: shipbuilding, car body, pressure vessels,

etc.



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3 WELDING RE 1 - P1

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