In this article we will discuss about:- 1. Definition of Arc Welding 2. Power Supply for Arc Welding 3. Arc Welding with DC and AC.
Definition of Arc Welding:
By definition, arc welding is that process in which the pieces of metal to be welded are brought to the proper welding temperature at point of contact by the heat liberated at the arc terminals and in the arc stream so that the metals are completely fused into each other, forming a single solid homogeneous mass, after it solidifies.
In this process an electric arc is produced by bringing two conductors (electrode and metal piece) connected to a suitable source of electric current, momentarily in contact and then separating by a small distance. The current continues to flow across the small gap and gives intense heat. The heat developed is utilised to melt the part of the workpiece, and the filler metal and thus forms the joint. So arc welded joint is a union of metal parts made by localised heating without any pressure. That is why sometimes this type of welding is known as the non- pressure welding. The heat developed by arc is also used for cutting of metal.
The voltage necessary to strike the arc is higher than that required to maintain it. The temperature is of the order of 3,600°C, at which mechanical pressure is not required for jointing. According to BSS 638 maximum voltage specified for welding is about 100 volts for the safety of the operator. The arc voltage varies from 20 to 40 volts and current from 50 A in sheet metalwork to 1,000 A in heavy automatic welding.
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The welding circuit for an electric arc welding process is shown in Fig. 6.9. The welding circuit consists of a welding machine, two leads, an electrode holder, an electrode and the work itself.
The electric arc welding is widely used for joining of metal parts, the repair of fractured castings and the fillings by the deposition of new metal on worn-out parts.
Arc welding is again subdivided into five groups namely carbon arc welding, metal arc welding, atomic hydrogen arc welding, inert gas metal arc welding and submerged arc welding.
Power Supply for Arc Welding:
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Both dc and ac are used for electric arc welding, each having its particular applications; in some cases either is suitable.
The electric properties of the arc determine the requirements that should be met by source of power supply for arc welding. To initiate an arc, a higher voltage is required than to maintain it under normal conditions. For this reason the open- circuit voltage of the power supply source (when there is no arc drawn between the electrode and the work) must be higher than the arc or closed-circuit voltage (when there is an arc drawn between the electrode and the work).
With dc the open-circuit voltage must be at least 30 or 35 volts, while with ac it should be not lower than 50 or 55 V. An open arc will be sustained at 18 to 25 volts. Open-circuit voltage usually ranges from 50 to 90 volts.
To initiate an arc, the electrode is touched to the work and is then withdrawn a short distance. On contact, the electrode shorts the welding circuit and an inadvertent short circuit may give rise to an excessive current in the circuit, which may dangerously heat and even burn the insulation. A power supply source for welding should, therefore, be able to limit the surge of current, when the arc is initiated, to a small percentage above the desired value.
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In metal arc welding the globules of molten electrode metal are carried across the arc, thus changing the resistance of the arc and consequently the arc voltage and current. In fact the arc resistance is continually varying within broad limits in very short periods of time. The point is that the molten globules of metal pass from the electrode to the plate at the rate of 30 or more per second, continually causing approximate short circuits from the electrode to the work. If the arc is not to go out, the power supply source must be able to quickly vary its voltage with the resistance of the arc.
Of special importance is the ability of a power supply source to restore the voltage quickly after a globule of molten metal has broken away from the electrode. The arc will only restrike at not less than 25 volts. The voltage should, therefore, recover to 25 volts within 0.05 second.
For manual welding with coated electrodes the static characteristic (curve giving relation between the source voltage and the welding current) is represented by curve I in Fig. 6.25. This is a drooping or negative characteristic. OA represents the open-circuit voltage, OB represents the arc or closed circuit voltage, OD represents the arc current and OE represents the short-circuit current. The point C on the curve I shows the instant of striking the arc. With this type of volt- ampere characteristic the short-circuit current in the welding circuit is reduced to a limit safe enough for the windings of the welding generator or transformer.
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If the arc has a flat (curve III) or a rising volt-ampere characteristic (curve II) such as in shielded arc welding, automatic welding with large currents the source of power supply should also have a flat or a rising characteristic respectively.
To sum up, the power supply for manual arc welding should meet the following requirements:
1. The open-circuit voltage must be sufficient for an arc to strike without difficulty, but safe for the operator (below 80 volts).
2. The short-circuit current must be within limits of safety for generator or transformer windings.
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3. The voltage of the source of power supply should vary rapidly with changes in arc length.
4. The wattage of the source of power supply should be sufficient to give the desired arc current.
Arc Welding with DC and AC:
AC and DC both can be used in arc welding, each having its distinctive merits and applications.
The advantages of arc welding with dc lie in the higher arc stability and the degree to which the work is heated. It is best suited for thinner sheet metal (below 6 mm) and also for welding of non-ferrous metals.
AC welding, due to the absence of ‘arc blow’, is considered superior for production welding involving large size electrodes.
AC welding has a number of economical advantages such as given below:
1. As power supply in factories is usually ac so a rotating dc generator or a rectifier is required for dc welding whilst for ac welding only transformer is required which is relatively cheaper in initial cost.
2. Maintenance of a transformer is less difficult and expensive than that of a dc generator.
3. Operating costs of ac equipment are also lower than those of dc equipment.
4. The electric energy consumption per kg of deposited metal in ac welding ranges from 3 to 4 kWh while for dc welding it is as high as 6 to 10 kWh.
One disadvantage of ac welding is the comparatively low power factor (about 0.4 lagging) which can be corrected by the use of capacitors.