Most metals and alloys conduct electricity at room temperature due to the presence of free electrons. A considerable amount of heat can be produced from the flow of the current in a circuit. Typical examples of this heating effect, also called resistance heating, are tungsten filament bulbs and heating coils in ovens. In comparison, gases like oxygen, nitrogen, carbon dioxide, etc. do not conduct any electricity at room temperature. However, if sufficient energy is applied to a gas it also can become conductive. When sufficient voltage is applied to a gas it can be ionized – changed into positively charged ions and negatively charged electrons. The electrons move in response to the applied voltage to produce a current flow and this movement of electrons allows the initiation of an arc. The current flow causes resistance heating in the gas which promotes further ionization and increased current flow. As long as the voltage source is able to supply the necessary voltage and the current needed by the arc, it can be sustained in a stable manner and used for welding applications.
Based on the above principle, a conventional arc is formed between two non-consumable electrodes in a gas or vapour medium when an appropriate voltage, depending on the electrode material and gas phase, is applied to the electrodes. As seen in Figure 1.1, one of the two electrodes forms a positive terminal of the electrical circuit and is called the anode; the negative terminal of the circuit is called the cathode. When an arc is created, electrons are evaporated from the cathode and transferred to the anode through the ionized gas in between. Flow of electrons is the same thing as flow of current or electricity.
A welding arc is formed when a fairly high current (10 to 2000 A) is forced to flow across a gap between two electrodes at relatively low voltage (10 to 50 V). A welding arc is intensely hot with temperatures exceeding 3000°C (see Figure 1.2) and forms a concentrated heat source suitable for melting most metals rapidly. The intense heat of the welding arc causes the filler metal to melt and when added to the locally hot melted workpiece, it forms the weld fusion zone. Its subsequent freezing (solidification) produces the bond (weld) between the workpieces. Arc welding processes do not require application of pressure to cause fusion.
In welding, the arc may be established between an electrode and the workpiece, or between two electrodes.
When the workpiece is one of the electrodes of the electrical circuit, the other electrode may be consumable or nonconsumable. A consumable electrode is designed to melt and add filler material to the welding joint.
The electrical current for welding is provided by a “power source” that draws high-voltage electric power from the main transformer and converts it into higher current and lower voltage suitable for welding (Figure 1.3). Power sources are broadly classified as constant current or constant voltage type, and the static volt/ampere output characteristics for these two types of power sources are shown in Figure 1.4.