The welding arc provides the intense heat needed to locally melt the workpiece and the filler metal. In fact, all the electrical energy supplied by the power source is converted into heat (current x voltage). Some energy is lost in the electrical leads, and therefore the energy available for welding is the product of the current (I) and voltage drop between the electrode where the current enters it and the weld pool (V). For example, with 400 A current and 25 V drop from the contact tip to the weld pool, the arc energy is 10,000 Joules/second. This arc energy is partly used up in heating the electrode, melting the consumable electrode or the separately added filler metal in a non consumable electrode process, and heating and locally melting the workpiece. The rest of the heat is lost by conduction, convection, radiation, spatter, etc. The proportion of the energy that is available to melt the electrode/filler metal and the workpiece is termed the arc efficiency.
The arc efficiency for some of the commonly used arc welding processes varies between 20% and 90%.
For a given process, factors like welding in a deep groove, arc length, etc. also influence the arc efficiency. Higher arc efficiency usually means that for a given arc energy, a greater amount of weld metal is deposited and the workpiece cools at a comparatively slower rate.
Voltage Distribution Along the Arc
In any welding set up, there is a continuous drop in voltage from the lower-most point of contact between the contact tip and the wire, to the molten weld pool or the workpiece. Figure 1.5 schematically shows that this voltage drop occurs in four steps.
First, there is a drop in voltage over the electrode extension, that is the length of electrode between the point of electrical contact with the contact tip, and its melting tip, also called cathode spot for the current flow direction shown in the sketch. The magnitude of this voltage drop depends on the electrode extension and the wire diameter as well as the current; a longer electrode extension, a
smaller wire diameter or higher current all increase the voltage drop over the electrode extension length.
The voltage drop over the arc length, that is the distance between the cathode spot and the anode spot (the molten weld pool surface in Figure 1.5) takes place in three steps. Right next to the anode and cathode spots are small, thin, gaseous regions called the anode drop zone and cathode drop zone, respectively, and over these zones there can be a significant drop in voltage, in the range of 1 to 12 V depending on the electrode material.
In between the two drop zones, there is the arc column with a relatively small drop in voltage, of the order of 1 to 2 V per centimetre length of the arc column. There is a jet-like flow of ionized gases in the arc column that gives it some stiffness and force (resistance to deflection). This enables the welder to manipulate the gun and direct the molten metal to be deposited at the desired location in the weld joint. Shorter arcs have greater stiffness than longer arcs.
Arc length is a critical and controllable parameter, which is directly related to the arc voltage. Arc voltage depends on the space between electrodes; electrode composition, diameter and extension; shielding gas composition; metal thickness; joint design; welding position, etc. The voltage measured at the power supply is greater than the arc voltage. Output voltage represents the sum of arc voltage and the voltage drop in the remaining part of the electrical circuit. The longer the electrical cables the greater will be the difference between the voltage read at the power supply gauge and the actual arc voltage.