does not<\/em> get consumed during the welding, and the work-piece. The electrode, therefore, must be made of material that does not get melted due to the intense heat of the arc. The electrode must retain its\u2019 shape and form during welding. Tungsten fits the bill, so it is used in processes such as gas tungsten arc welding and plasma arc welding.<\/p>\n\n\n\nAnother process in which an arc is struck between a non-consumable electrode is the carbon arc welding. In this process, the electrode(s) is made up of plain carbon.<\/p>\n\n\n\n
Function of Welding Arcs<\/h2>\n\n\n\n
The main function of the welding arc is to generate intense concentrated heat. This heat is used to melt the metals being joined together. The generation of heat is inevitably accompanied by a bright glare and noise as well. In some cases, the arc also produces bombardment of metal surface which removes the oxide layer present on the surface.<\/p>\n\n\n\n
The gap between the electrode tip and the metal surface becomes ionized due to the voltage difference between the electrode and metal. This ionized medium acts as a good conductor of electricity. The inner core of the arc is a column of plasma that carries most of the current. The core is surrounded by a envelope of flame that does the job of heating the metal.<\/p>\n\n\n\n
The current carried through the arc can vary over a wide range. It can be as low as 5 amperes, or as high as 1500 amperes. Likewise, the voltage difference across the arc can be as low as 10 volts.<\/p>\n\n\n\n
The shape of the arc resembles somewhat to that of a cone. The electrode tip acts as the apex of the cone. From the electrode tip, the arc spreads out towards the base metal.<\/p>\n\n\n\n
The voltage difference across the arc determines how much arc gap can be sustained. If the arc gap becomes too large, the arc can no longer be sustained, and will get extinguished.<\/p>\n\n\n\n
Likewise, sufficient current is necessary too, to keep the arc ignited. The current should be sufficient to obtain melting of the electrode tip. For thick base metals, the current used is slightly higher than that for low thicknesses, for same electrode diameter. This is because heat gets quickly dissipated to other regions of the metal in the case of thick metals. So, more heat is necessary to keep up the arc. If the current is made too low, the arc will get extinguished.<\/p>\n\n\n\n
The temperature of the arc can be quite high. A temperature of ordinary electric arc can vary between 6500\u00b0F to 10000\u00b0F. For some high power plasma, the temperature can go up to 50000\u00b0F. The temperature of the arc mainly depends on the amount of electrical power to the electrode. High currents mean higher temperatures. The temperature also depends on the type of shielding gas, type of metal, and electrode size.<\/p>\n\n\n\n
The highest temperature in the arc is found at its\u2019 core. The outer flame is relatively cooler.<\/p>\n\n\n\n
Polarity Of The Arc<\/h2>\n\n\n\n
We know that current is a flow of electrons. The rate of flow of electrons determines the amperage. Electrons have a negative charge; hence they are understandably attracted towards the positive pole. So, across the welding arc, electrons flow from cathode to anode. Cathode and anode are electrical terms for negative pole and positive pole respectively.<\/p>\n\n\n\n
Straight Polarity<\/h3>\n\n\n\n
Let us take the case of when the welding circuit in gas tungsten arc welding is connected with straight polarity.<\/p>\n\n\n\n
When the circuit is connected with straight polarity, the electrode is connected to the negative terminal of the power source, while the work-piece is connected to the positive terminal of the power source. This is illustrated in the figure on the left in the figure 1 below. Straight polarity is also called DCEN or direct current electrode negative polarity.<\/p>\n\n\n\n
In this polarity, the electrons are emitted by the red hot electrode in the direction of the work-piece. Since the work-piece is positively charged, the electrons are attracted to the work-piece. So the electrons travel through the arc gap and proceed towards it at a fast speed. The flow of electrons from the power source to the electrode replenishes this supply of electrons continuously. This emission of work-piece bound electrons from the electrode is called thermionic emission.<\/p>\n\n\n\n
The electrons bombard on the surface of base metals with large kinetic energy. This energy gets converted to heat on collision with the base metal. This heat is localized on a small spot; and due to this heat, the base metal gets melted. The penetration of such an arc is deep as shown in the figure on the left in figure 1. This travel of electrons accounts for 99% of the flow of electricity. In addition to the flow of electrons, there exists a flow of positive ions too. <\/p>\n\n\n\n
The electrons discussed above collide with the atoms of the shielding gas (besides hitting the base metal), and ionize the shielding gas atoms. These positively charges ions run towards the negatively charged electrode. This bombardment keeps the electrode tip hot.<\/p>\n\n\n\n