Fabrication Process (Welding)

Introduction:

Welding is a process of joining similar or dissimilar metal by fusion with or without the application of pressure and with or without use of filler rod. Fusion is obtained by heat. The heat may be obtained from electric arc, electric resistance, chemical reaction, friction or radiant energy. The result is continuity of the homogenous material of the composition and the characteristics of two parts which are being joined together.

Welding process has the wide application. The future of any new metal may depend upon how far it would lend itself to fabrication by welding. Two metals parts of same type can be joined together and the joint is strong as well. We can see different application around us like the grills, corridor barriers, gates, etc.

Objectives:

The main objectives of the welding process are as follows:

i) To know the various aspects of welding.

ii) To know the different tools and raw materials and different kinds of welding.

iii) To know how to weld and to know the different ways of welding.

Applications:

a.       Automobile constructions

b.      Railroad equipments

c.       Construction of ships

d.      Aircraft construction

e.       Building construction

f.       Bridge construction

g.      Pressure vessels

h.      Storage tanks

i.        Piping and pipe lines

j.        Fabrication of jigs, fixture and machine tools

k.      Repair of broken and damaged parts

l.        Household materials

m.    Material handing equipments etc..

CLASSIFICATION OF WELDING

In general, various welding and allied process are classified as follows:

1. Gas welding

v  Air acetylene welding

v  Oxy-acetylene welding

v  Oxy-hydrogen welding

v  Pressure gas welding

2. Arc welding

v  Carbon arc welding

v  Metal arc welding

v  Flux cored arc welding

v  Submerged arc welding

v  Gas tungsten arc welding(TIG)

v  Gas metal arc welding(MIG)

v  Plasma arc welding

v  Electro slag arc welding

v  Electro gas arc welding

v  Stud arc welding

3. Resistance  welding

v  Spot welding

v  Seam welding

v  Projection welding

v  Resistance butt welding

v  Flash butt welding

v  Percussion welding

v  High frequency resistance welding

4. Solid state welding

v  Cold welding

v  Diffusion welding

v  Explosive welding

v  Forge welding

v  Friction welding

v  Hot pressure welding

v  Roll welding

v  Ultra sonic welding

5. Thermo chemical welding process

v  Thermit welding

v  Atomic hydrogen welding

6. Radiant energy welding process

v  Electron beam welding

v  Laser beam welding

        i.            ARC WELDING

Arc welding is the most extensively employed method of joining metal part. Here the source of heat is an electric arc.

The arc column is generated between an anode, which is the positive pole of DC power supply, and the cathode, the negative pole when this two conductor of an electric are brought together and separated for a small distance ( 2 to 4mm) such that current continue to flow through the part of ionized particle(gaseous medium) called plasma, an electric arc is formed. The ionized gas column act as high resistance conductors that enable more ions to flow from anode to cathode. Heat is generated as the ions strike the cathode.

1.      ARC WELDING EQUIPMENT

The most commonly used equipment for arc welding consist of the following:

                                i.            AC or DC machine

                              ii.            electrode holder

                            iii.            cables, cables connectors

                            iv.            cables lug

                              v.            chipping hammer

                            vi.            earthing clamps

                          vii.            wire brush

                        viii.            helmet

                            ix.            safety goggles

                              x.            hand gloves

                            xi.            aprons, sleeves

                          xii.            electrode

a. FLUX SHIELDED ARC WELDING

In this method, a rod of carbon is used as a negative electrode and the work being welded as a positive. The arc produced between these two electrodes heats the metal to the melting temperature. The reason to use carbon rod as negative electrode is that less heat will be generated at the electrode tip than that at the workpiece, and carbon from the electrode will not fuse and mixed up with the job.         

TOOL USED:

        i.            Arc welding Machine

      ii.            Electrode for welding

    iii.            Earthing Clamp

    iv.            Chipping Hammer

      v.            Goggles

    vi.            Welding Shield

  vii.            Gloves

viii.            Metal Brush

2.      PRECAUTIONS

        i.            Cotton and leather aprons, helmet, gloves, goggles should be wear while working.

      ii.            Red hot metal should be carefully taken out with the aid of tong.

    iii.            Brushing should be done carefully.

a.      MIG (METAL INERT GAS WELDING)

Welding heat is produced from continuously fed metal electrode and the (parent metal) job. Argon, Helium, Carbon dioxide or mixture gas is used for shielding the arc and molten metal. The current from the welding machine is changed by changing the rate of feeding of the electrode wire:

• if arc length decreases voltage decreases and so current increases. Increased current melts the electrode at the faster rate normalizing ac length.
• Electrode feeding motor is controlled by the arc voltage; change in arc length is thus adjusted by change in feed rate.

      ii.            GAS WELDING

Gas welding is a fusion welding process in which gas flame is used to raised the temperature of the ends of the pieces to be joined to a heat sufficient to melt them. The metal thus melted start flowing along a strong weld. A filler material may be added to the flowing molten metal to fill up the cavity made during the end preparation.

 

a.      OXY-ACETYLENE WELDING

This process is used for welding almost all metal and alloys used in engineering practice. It produces a higher temperature then any other gas welding and also a inert gas envelope, consisting of carbon dioxide and water vapours, which prevent the molten metal from oxidation.

Types of oxy-acetylene welding:

i)                    high pressure system

ii)                  low pressure system

In first system both oxygen and acetylene are derived for use from high pressure cylinders. In second types of system oxygen is taken as usual from high pressure cylinder but acetylene is generated by the action of water on carbide(usually calcium carbide) in low pressure acetylene generator.

Gas Welding Equipment

i.        Welding torch

It is a tool for mixing oxygen and acetylene in correct proportion and burning the mixture at the end of the tip. Comercially there available in two types a. equal pressure and injector type.

ii.      Welding tip

It is the portion of the welding apparatus through which the gas passes just prior to their ignition and burning.

iii.    Pressure regulator

Its function is to reduce the cylinder pressure to the required working pressure and also to produce a steady flow of gas regardless of the pressure variation of the source.

iv.    Hose and hose fitting

v.      Goggles, gloves and spark lighter

vi.    Gas cylinder 

Tools Used:

The different tools in the arc welding, as we have performed in the workshops, are as follows:

i) Arc Welding Machine

An arc current is used for electric arc welding. In some cases DC current is also used depending upon the application. For arc welding supply transformers are predominantly used for almost all arc welding where mains electricity supply is available. A 100 to 200-ampire machine is small but portable and satisfactory for light manual welding. It consists of rectangular tank mounted on three-tired wheels, the front wheel swiveling and steer able by means of draw bar. An oil cooled double-wounded step down transformer reduces the supply mains voltage to a welding voltage of 80. All the welding is totally enclosed in the steel tank.

ii) Electrode for arc welding

For arc welding both consumable and non-consumable electrodes are used. We have used the coated electrode which is useful in many ways that are to facilitate the establishment and maintenance of arc, to protect the molten metal from oxygen and nitrogen of air by producing a shield of gas around the arc and weld pool and to provide the formation of slag so as to protect the welding ream from rapid cooling.

iii) Earthing clamp

It is connected with the welding machine and it should be kept in touch of the job piece in order to weld.

iv) Chipping hammer

Chipping hammer is used to remove the chips remained in the job piece during welding process, thus, cleaning the welded area.

v) Goggles

Goggles are used to protect eyes from direct light as well as from the gas that comes from the welding process.

vi) Welding Shield

Welding shields are used to protect the eyes from the direct light that comes from the welding process.

vii) Gloves

Hand gloves are used to protect our hand from direct heat that evolves during the process as well as from the materials that may spread out while welding. It also protects our hand from the electric shock if incidentally we touch non-insulated cables.

Arc Welding Method:

There are various types of arc welding methods. Some of them are:

i) Carbon Arc: 

In carbon arc welding a rod of carbon is used as negative electrode and the work being welded as positive. The arc produced between these two electrodes heats the metals to the melting temperature. The reason to use the carbon rod, as negative electrode is that less heat will be generated at the electrode tip than at the work piece and carbon will not fuse and mix up with the job.

ii) Metal arc welding

In this process a metal rod is used as one electrode while the work being welded is used as another electrode. During the welding process the metal electrode is melted by the head of the arc and is fused with the base metal.

iii) Gas metal arc welding

Gas metal arc welding is a gas shielded metal arc welding process which uses the heat of an electric arc between a continuously fed, consumable electrode wire and the material to be welded.

iv) Gas Tungsten arc

The shielding is obtained from air inert gas such as helium or argon or a mixture of the two argon is more widely used than helium because it is a heavier gas, producing better shielding at lower flow rate. The shielding gas displaces the air surrounding the arc and weld pool.

v.) Atomic Hydrogen Welding:

As the molecules of hydrogen pass through the electric arc they are changed into the atomic state and thus absorb a considerable amount of energy and when they recombine into molecules just outside the arc. Large amount of heat is liberated. This provides the welding temperature.

CONCLUSION

By the welding process we came to know about use of various tools in the welding, such as use of electrode, gloves, chipping hammer etc .As we need to join the various metals in the near future, so by these welding practices we have reach to the stage that could weld the metal pieces with strong and smooth joints. So such practices are very much important for us.

 Safety Precautions:

1. The means of safety such as goggles, gloves, and apron should be worn.
2. Right tools should be used for the right purpose.
3. The two electrodes should not be touched at once with naked hands.
4. We should stay vigilant towards the excessive heat and light that evolve during the process.