ARC AND ITS CHARACTERISTICS
Definition :- Electric arc is a sustained electrical discharge through an ionised gas between the two terminals of an electrical circuit .
In other words , an electric arc is an electric discharge in gases , accompanied by very bright glow and tremendously high heat .
Creation of an Arc :- An arc is formed by bringing the two currents carrying conductors close together to make an electrical contact and then by seperating them through a suitable distance or gap .
This way considerable resistance is created in the flow of current and thus the tips of the conductors . cor electrode are raised to a very high temperature . This causes the electrons to emerge from negative electrode . These electrons collide with the molecules and atoms of air ( that is between the gap of two electrodes ) , breaking them up into free electrons and ions and there by rendering the air gap to become good conductor of electricity due to ionisation .
This way the flow of current continues through the air gap and the arc is maintained . In the beginning of striking the arc comparatively higher voltage is needed because some of the valtage is required to ionise the gap . Once the arc is formed , one can maintain the are for welding at lower voltage than that required for striking the arc .
Characteristics of Arc :-
An electric arc has two characteristics.
1. Electrical characteristics
2. Welding characteristics
(i) Electrical Characteristics of Arc :-
The electrical characteristics of an arc are those which can be readily effected by the electrical influence of current , voltage or other negative phenomenon .
Although the temperature of the are may be fairly constant the total amount of heat in the arc depends upon the current and voltage across the arc . More heat is obtained by increasing the current .
The current is always selected according to the type of electrode and its size , the metal and thickness of the workpiece and the nature of operation ( ie . welding or cutting ) .
The voltage across the are depends upon the flux coating on the electrode e.g. cellulose type electrodes give higher are voltage and faster melting rate . The arc heat can be however increased by increasing the arc length which causes the electrode to melt at faster rate .
For faster output welders often use long length of arc but it causes more spattering of metal and thus should be avoided .
An electric arc is a very flexible type of conductor and can be deflected by various factors :
The space around the arc and in the adjacent metal is always effected by magnetic fields which tend to deflect the are . This phenomenon of arc deflection is known as arc blow . Are blow causes great hinderance especially when large currents are used because the intensityof magnetic field is proportional to the square of the current .
The magnetic field deflects the arc when it is not uniformly distributed relative to the arc and the distribution of the field depends upon position and shape of work piece and air gap etc. Arc blow happens more with direct current .
The arc of DC and AC also differ in several characteristics . For example , DC arc is more stable whereas an AC are is not , because alternating current keeps changing from Zero to maximum and vice - versa .
For this reason every type of electrode can be used in DC whereas only certain types of electrodes are used with AC . The non - ferrous electrodes do not work well with AC but work satisfactorily with DC .
However , manufacturers of electrodes provide a special coating on the electrodes so that during welding when electrode melts down , the arc stabilizing elements mixed in the coating on flux come into play and keeps the arc stabilized .
(ii) Welding Characteristics of Arc :-
The welding characteristics of the arc are those which are effected by So many factors that come into play during the process of welding . The characteristics often known by welding characteristics of an arc are crater , arc length and penetration etc. which are discussed below .
Arc Length =
Arc length is the distance between the tip of the electrode and the work surface upon which the molten globules are deposited duning welding .
The length of arc is an important variable in welding . The globules of molten metal should not be allowed to come in contact with air for longer period because atmospheric oxygen and nitrogen have adverse efflects upon the later developed mechanical properties of the weld .
If the arc is longer ... than normal , the gaseous shield can no longer protects the arc stream . The metal will form oxides and nitrides , resulting into a very weak and brittle weld .
This way a long arc renders a wide shallow bead and enough heat is wasted into air . The penetration is poor , and more spattering is produced . Although electrode may be speedily consumed with long arc but the effective weld production is low and insufficient .
A short arc gives little chance for air to come into contact with molten metal . The short are will effectively deposit more and will have following advantages :
• Maximum Penetration
• Maximum Strength of Weld
• Slight overlap of weld beads
• Minimum porosity in the weld
• Maximum ductility in the weld
• Alloy electrode work better with short arc
Crater = Welding arc forces molten metal out of pool and the metal piles up around the edges of a small depression in which the metal is in a motten state . This depression is referred as the arc crater . The crater depth depends upon the thickness of the metal welded .
Penetration = Penetration is the depth from the surface of the work to the bottom of the molten metal.
Angularity of Electrode = The angular position of the electnode to the work has a remarkable effect on the under cutting and inclusion of slag . The work angle is the angle between the electrode and the work taken in a plane normal to the Joint plane .
The lead angle of the electrode is the angle between the joint and the electrode taken in a longitudinal plane . The lead angle is usually kept about 20 ° . A large angle results in building up a larger bead whereas too lange a work angle results in undercutting .
