WELDING 2015-16

Rohan Desai, Automobile Engg. Dept., NPK Page 1

Topic and Contents Hours Marks

3.1 Introduction 04 Marks Classification and selection of welding process. Working principle of Gas welding and types of flames. 3.2 Arc welding process 08 Marks Metal arc, TIG and MI G 3.3 Resistance welding, Brazing and soldering 04 Marks

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3. WELDING PROCESSES

3.1 INTRODUCTION

Welding is a process for joining two similar or dissimilar metals by fusion. It

joins different metals/alloys, with or without the application of pressure and

with or without the use of filler metal. The fusion of metal takes place by

means of heat. The heat may be generated either from combustion of gases,

electric arc, electric resistance or by chemical reaction. During some type of

welding processes, pressure may also be employed, but this is not an

essential requirement for all welding processes.

Definition: Welding is a process of joining similar metals by application of

heat with or without application of pressure and addition of filler metal.

Q. What are types of weld joints?

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Q. what are types of welds?

� ADVANTAGES & DISADVANTAGES OF WELDING

ADVANTAGES:

1. Welding is more economical and is much faster process as compared

to other processes (riveting, bolting, casting etc.)

2. Welding, if properly controlled results permanent joints having strength

equal or sometimes more than base metal.

3. Large number of metals and alloys both similar and dissimilar can be

joined by welding.

4. General welding equipment is not very costly.

DISADVANTAGES:

1. It results in residual stresses and distortion of the workpieces.

2. Welded joint needs stress relieving and heat treatment.

3. Welding gives out harmful radiations (light), fumes and spatter.

4. Jigs and fixtures may also be needed to hold and position the parts to be

welded.

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� CLASSIFICATION OF WELDING PROCESS

1. OXY-FUEL GAS WELDING PROCESSES

1. Air-Acetylene Welding

2. Oxy-Acetylene Welding

2. ARC WELDING PROCESSES

1. Shielded Metal Arc Welding

2. Gas Tungsten Arc Welding

3. Gas Metal Arc Welding

4. Plasma Arc Welding

3. RESISTANCE WELDING

1. Spot Welding

2. Seam Welding

3. Projection Welding

4. SOLID-STATE WELDING PROCESSES

1. Forge Welding

2. Friction Welding

� WORKING PRINCIPLE OF GAS WELDING

Gas welding is done by burning a combustible gas with air or oxygen in a

concentrated flame of high temperature. As with other welding methods, the

purpose of the flame is to heat and melt the parent metal and filler rod of a

joint.

Q. Write short note on Oxy-Acetylene welding.

Oxy-acetylene gas welding is accomplished by melting the edges or surface

to be joined by gas flame and allowing the molten metal to flow together,

thus forming a solid continuous joint upon cooling. With material thicker

than 15 mm, additional metal called filler metal is added to the weld in the

form of welding rod. The composition of the filler rod is usually the same or

nearly the same as that of the part being welded. To remove the impurities

and oxides present on the surfaces of metal to be joined and to obtain a

satisfactory bond a flux is always employed.

Common mixtures of gases are oxygen and acetylene, oxygen and

hydrogen, oxygen and other fuel gas, and air and acetylene. The oxygen-

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acetylene mixture is used to a much greater extent than the other and has a

prominent place in the welding industry. The temperature of the oxy-

acetylene flame in its hottest region is about 3,200°C.

OXY-ACETYLENE FLAME SETTINGS:

In an oxyacetylene flame, O2 and C2H2 are mixed burnt to release heat. The

complete combustion of acetylene in an atmosphere of oxygen represented

by the following reaction:

C2H2 + 2.5O2 = 2CO2 + H2O (vapour) + 1284.57 k J/mol

Q. What are the types of flames used in Gas welding?

Neutral flame:

A neutral flame is obtained when equal amounts of oxygen and acetylene

are mixed and burnt in a torch. The flame is recognized by two sharply

defined zones, the inner white cone flame and the outer blue flame

envelope. Applications: Steel, stainless steel, cast iron and aluminium.

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Carburizing or Reducing Flame:

It is obtained when percentage of acetylene is more than that of oxygen.

Three distinct zones are observed. a) Well defined inner cone.

b) Whitish intermediate cone which indicates the excess of acetylene.

c) Outer cone of bluish colour.

This flame is suitable for materials that oxidize rapidly like steel, Al.

Oxidising Flame:

The oxidizing flame has an excess of oxygen over the acetylene. An oxidizing

flame can be recognized by the small cone, which is shorter, much bluer in

colour and more pointed than that of the neutral flame. The outer flame

envelope is much shorter and tends to fan out at the end.

Applications: Brass and Bronze.

3.2 ARC WELDING PROCESS

Q. Write short note on metal arc welding.

In the metal-arc welding a metal rod is used as one electrode, while the work

being welded is used as another electrode. The temperature produced is

about 2,400°C and 2,600°C on the negative and positive electrode

respectively. During the welding operation, this metal electrode is melted by

the heat of the arc, and is fused with the base metal, thus forming a solid

union after the metal has been cooled. Both ac and dc may be used. A metal-

arc-welding circuit is illustrated.

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Q. Write short note on M.I.G. (Gas metal arc welding- GMAW)

Gas-metal-arc welding is a gas shielded; metal arc welding process which

uses the high heat of an electric arc between a continuously fed, consumable

electrode wire and the material to be welded. Metal is transferred through

protected arc column to the work.

In this process, the wire is fed continuously from a reel through a gun to

constant surface which imparts a current upon the wire.

Advantages:

1. Suitable for ferrous as well as nonferrous metals

2. Very high quality of weld

3. Less operator skill is required.

Applications: Aluminium, Copper, Nickel & their alloys, Variety of

Steels.

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Q. Write short note on T.I.G. (Gas tungsten arc welding-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 an inert gas such as helium or argon or a

mixture of the two. The shielding gas displaces the air surrounding the arc

and weld pool. This prevents the contamination of the weld metal by the

oxygen and nitrogen in the air. Filler metal may or may not be used.

Advantages:

1. Produces high quality of welds in nonferrous metals.

2. No weld cleaning is necessary.

3. The arc and weld pool is clearly visible to the welder.

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Applications: Aluminium, Copper, Nickel & their alloys, Variety of

Steels.

Q. What are the types of Resistance welding?

(1) Resistance spot welding, (2) Resistance seam welding, (3) Projection

welding and (4) Resistance butt welding (Upset and Flash butt).

Q. Explain working of Spot welding with applications.

It is used to join overlapping strips, sheets or plates of metals. The pieces are

assembled and squeezed between two electrodes, which must possess high

electrical & thermal conductivity. 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 metal to be

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

by hydraulic cylinders.

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Applications: This may be applied to all types of boxes, cans, enclosing

cases, etc.

Q. Explain with sketch Seam welding process.

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 place 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.

Advantages:

• It can produce gas tight or liquid tight joints.

• Overlap can be less than that for spot and projection weld.

Disadvantages:

• Welding can be done only along a straight or uniformly curved line.

• It is difficult to weld thickness greater than 3 mm.

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Applications: Seam welding is used on many types of pressure tight or leak

proof tanks for various purposes, silencer bodies, transformers, refrigerators,

and aircrafts.

Q. Explain Projection welding process.

Projection welding is a modification of spot welding. The current and pressure

are localised at the weld section by the use of embossed, machined or coined

projections on one or both pieces of the Work. The flattening out of these

projections under pressure results in good welds at all points of contact.

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Advantages:

• More than one weld at a time hence more output.

• Low current density and low pressure increases electrode life.

Disadvantages:

• It is limited to combinations of metal thickness and compositions.

• For proper welding, all projections must be of same height.

Applications: It is used for joining nuts, bolts and studs to steel plates in car

bodies.

Q. Give comparison between resistances welding arc welding.

RESISTANCE WELDING ARC WELDING

1) It is a form of a plastic welding. 1) It is a form of fusion welding.

2) External pressure is required. 2) No external pressure is required

hence the equipment is simpler.

3) No filler is required. 3) Suitable filler metal electrodes are

necessary to get proper strength.

4) Supply is AC only. 4) Supply can be AC or DC.

5) Voltage required is very low. 5) The striking voltage is high so

requires voltage control.

Q. Write short note on Brazing or what are the steps in Brazing?

It is the process to join two metal pieces heated to suitable temperatures by

using a filler metal. Filler metals should have melting point lower than the

base metals. During brazing, the base metals are not melted.

1. The surface to be joined are cleaned and fitted closely together.

2. A flux is applied to all surfaces where the filler metal is to flow.

3. After that joint is heated to the proper brazing temperature. Solid filler

metal may be pre placed on the metal pieces and thus melted as the

metal pieces are heated, or it may be applied to the metal pieces after

the brazing temperature is reached.

Butt joint, seam joint, lap joint, pipe joint and edge joint are obtained from

the brazing process.

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Fluxes: Compositions of borates, fluorides, chlorides, borax, and boric acid are

the flux materials. These are in the form of powder, paste or slurry.

Filler metals: Brass, Bronze, Copper, Silicon and Silver alloys. These are in the

form of wire, strips, performs, powder or paste.

Applications:

Assembly of pipe fittings, Carbide tips to tools, Radiators, heat exchangers,

electrical parts, repair of castings and joining of special materials like

stainless steels.

Q. Give short note on Soldering.

Soft soldering: It is used extensively in sheet-metal work for joining parts

that are not exposed to the action of high temperatures and are not

subjected to excessive loads and forces. Soft soldering is also employed for

joining wires and small parts. The solder, which is mostly composed of lead

and tin, has a melting range of 150 to 350°C. A suitable flux is always used

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in soft soldering. Its function is to prevent oxidation of the surfaces to be

soldered or to dissolve oxides that settle on the metal surfaces during the

heating process. Although corrosive, zinc chloride is the most common

soldering flux. Rosin is non-corrosive, but it does not have the cleaning

properties of zinc chloride. A blow torch or soldering iron constitutes the

equipment for heating the base metals and melting the solder and the flux.

Hard soldering: It employs solders which melt at higher temperatures

and are stronger than those used in soft soldering. Silver soldering is a

hard soldering method, and silver alloyed with tin is used as solder. The

temperatures of the various hard solders vary from about 600 to 900°C.

The fluxes are mostly in paste form and are applied to the joint with a

brush before heating. In hard soldering, a blow torch constitutes the

equipment.

Q. What are the steps in hand soldering?

A. Parts to be soldered must be thoroughly cleaned.

B. The flux is applied on the surfaces to be joined.

C. The joint is then heated to the melting temperature of the solder by

soldering iron.

D. Solder is applied which melts and flows into the joint.