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Arc Structure and Mechanism |
Arc Structure and Mechanism -
The arc constitutes a mechanism by which electrons are emitted from the cathode, they pass through the hot ionized arc column and are transferred to the anode where they get condensed and absorbed.
It is presently well accepted that there are three distinct regions of a welding are, namely cathode drop region or cathode fall space, arc column or arc plasma region and anode drop region or anode fall space.
The arc column is situated in between anode and cathode drop regions which are spread over an approximate distances of 10-2 mm and 10-1 to 10-2 mm respectively. The cathode is negative, anode is positive and arc column is electrically neutral as it contains equal number of ions and electrons.
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The conditions in the arc column are quite different from the regions where the arc comes in contact with the electrode, (i.e. cathode drop region in DCSP) and the workpiece (i.e. anode drop region in DCSP). In the immediate vicinity of the electrode or the job, the plasma can no longer maintain its high temperature because it comes in contact with comparatively much colder workpiece and electrode.
High temperature gradients exist on both the ends of the arc column and naturally the arc gets divided into three distinct zones. Out of these zones, the most concentrated source of heat is the cathode spot, hottest region is the arc column and the largest quantity of heat is produced at the anode.
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In a welding arc, the electrons are emitted from the cathode, get accelerated in the cathode drop region and gain energy. As they enter arc column, they lose their energy by colliding with gas atoms and molecules which in turn get ionized, i.e. electrons and positive ions are separated.
The ions and electrons then move towards cathodic and anode respectively and get concentrated over there. Because of this concentration of charge carriers (i.e., electrons and ions) in the anode and cathode drop zones, a nonlinear voltage distribution is prevalent along the arc length, and high electric field strengths arc found in cathode and anode drop zones.
Approximate potential drop in the cathode and anode drop regions is of the order of 10 volts and 12 to 1 volt respectively. In such an are, magnetic field strength, current density and pressure all decrease from cathode drop region towards the arc column, because arc crosssection increases rapidly in the arc column.
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Depending upon certain conditions, the electric current flows, along the axis of the arc or both along the axis and transversely. The current flow through the arc gives rise to self induced magnetic field which compresses the arc plasma resulting in appreciable axial and radial pressure gradients in the arc.
The radial pressure gradient constricts (pinch effect) the arc and raises the temperature of the arc discharge whereas axial pressure gradient gives rise to plasma streaming which transports material (metal and slag particles) and heat, from the electrode to the workpiece.
Plasma streams stabilize the arc and exert a pressure on the molten pool which helps increase penetration. Both these effects are proportional to the square of the arc current.
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The three regions of the welding arc will be discussed briefly as follows:
1. Cathode drop zone. It is contained within two imaginary planes, one just at the end of cathode spot and the other at the beginning of arc plasma column. Cathodic tip appears darker as compared to arc column.
Cathode drop region is important because electrons are produced here, and the arc stability depends upon a regular supply of electrons. The significance of cathode drop zone still increases when, in AC welding, arc is to be reignited every half cycle (as the current passes through zero in AC cycle)
The cathode drop mechanism for electrodes made up of high melting point and low work function materials like tungsten, thoriated or Zirconiated tungsten and carbon graphite is as follows:
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At high temperature, the electrons are emitted from the cathode by THERMIONIC EMISSION, get accelerated in cathode drop zone, gain kinetic energy, which is lost in the arc column when the electrons collide with the gas atoms and molecules. These gas atoms get ionized. The ions so produced travel towards the cathode (being attracted by it), strike it and give up their kinetic energy.
This produces high heat and even with conduction and radiation losses from the cathode, it is maintained at high temperature necessary for further emission of electrons. In this case cathode spot is not well defined.
Another cathode mechanism which can be explained through FIELD EMISSIONS is associated with low melting point electrodes. There is a relatively larger cathode area containing many active well defined small cathode spots which move around with a velocity of about 104 cm/sec.
These cathode spots, constantly form, are vanished and get reformed elsewhere. Perhaps, vaporisation of the electrode material accompanies this mechanism. A third mechanism, PLASMA EMISSION, is related to high pressure and low current (though high current density) arcs, In this case the cathode is stationary and well marked.
Arc Plasma Column
Arc column is that portion of the welding arc which is situated between anode and cathode drop regions. Arc column consists of a radiating mixture of electrons, ions (+) and highly excited neutral atoms and molecules. In order to keep current flowing between (the electrode and the job, or) cathode and anode (in DCSP) arc column provides and maintains, a regular supply of ions and electrons.
The current carried by the ions because of their heavy mass and thus less mobility is much lesser and can normally be neglected in comparison to the current carried by electrons or the total current. Thus it is concluded that mainly the density of electrons is responsible for maintaining current between electrode and job. Arc column may have temperatures ranging from 5,000 to 50,000°K.
Energy to reach this temperature is achieved from the catholically emitted electrons which collide with gas atoms (in the arc column) raise their temperature and ionize them to single, double or triple degrees of ionization, and in turn producing more electrons which again collide with neutral atoms and thus the degree of ionization increases.The approximate temperatures of electrons and ions are of the order of 15,000-50,000°K and 6,000°K respectively.
Anode Drop Zone
It is situated in between the anode spot and the place where the arc column finishes. This region forms the electrical connection between the arc plasma column and the anode. The potential drop in the anode drop region exists because of the concentration of electrons which enter in this zone from arc column.
Three phenomena occur in the anode drop zone, i.e., temperature falls (from that of arc column), ions are produced, and accelerated towards arc column. The formation of positive ions is influenced by temperature and anode plasma composition.
The chances of ion formation increase as the anode plasma temperature rises. Depending upon anode material, the metal vapours formed exert a substantial effect on the anode drop, because due to collisions with the electrons, vapours get ionized and supply positive ions.
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