When an electric current passes through a conductor, a magnetic field is created that surrounds the conductor (Figure 1.6). Unless this magnetic field is balanced in all directions, the welding arc will tend to be deflected from its normal axial orientation in line with the electrode. This phenomenon is called arc blow. It is more likely to be present during welding of magnetic materials (steels) and can cause incomplete fusion types of flaws in welds.
Some degree of imbalance in the magnetic field is always present. The path of the magnetic flux in the workpiece is continuous behind the arc and discontinuous ahead, due to the change in the direction of the current as it goes from workpiece to electrode (Figure 1.7). Since a shorter arc is stiffer, it is also less susceptible to arc blow.
The magnetic field introduced by the current flowing in the electrode also plays a role in metal transfer. When the tip of the electrode melts, there are several forces that act at the molten tip. These include surface tension, gravity, plasma jet and electromagnetic pinch force. Surface tensions tends to prevent the detachment of the liquid drop at the electrode tip, irrespective of the welding position. Gravity supports droplet detachment when welding in the flat (downhand) position and attempts to prevent it in the overhead position. The plasma jet in most situations tries to detach and propel the molten drop across the arc column to the workpiece. The electromagnetic pinch force helps in the process of detaching the molten metal drops from the electrode tip. Generally, when there is some necking between the molten tip and the unmelted electrode, the magnetic field introduces a pinch force acting in both directions away from the neck (Figure 1.8). This helps to separate the drop from the electrode. Since this pinch force increases as the square of the current, smaller and smaller drops are detached as the current increases.