Understanding Arc Blow in Welding

Arc blow is a phenomenon encountered in arc welding that causes the arc to wander or deflect from its intended path. This deflection affects the quality of the weld, making it difficult to control the deposition of filler metal and potentially leading to defects like incomplete fusion, excessive spatter, and uneven bead shape.

Why Arc Blow Occurs

The primary cause of arc blow is the magnetic field generated by the flow of welding current. This magnetic field interacts with the main magnetic field of the earth or magnetic fields present in the workpiece itself (residual magnetism or induced magnetism due to current flow). When these magnetic fields interact, they exert a force on the welding arc, pushing it away from dense magnetic areas or towards areas with less magnetic resistance.

Arc Blow and DC Welding Current

Arc blow is most commonly encountered when using Direct Current (DC) for arc welding, specifically DC Electrode Positive (DCEP) or DC Electrode Negative (DCEN). With DC current, the magnetic field generated around the conductor (the electrode and the arc) is constant in direction. This constant magnetic field creates a steady force that can deflect the arc, particularly at the beginning or end of a weld joint, or when welding near the edge or corners of a magnetized workpiece.

Comparison with AC Welding Current

In contrast, when using Alternating Current (AC) for arc welding, the direction of the current and thus the magnetic field reverses polarity rapidly (typically 50 or 60 times per second). This rapid reversal causes the magnetic forces on the arc to cancel out over time, significantly reducing or eliminating the effect of arc blow. Therefore, arc blow is generally not a problem in AC arc welding.

Arc Blow in Other Welding Processes

Gas welding and thermit welding are different processes that do not rely on an electric arc to generate heat. Gas welding uses the combustion of fuel gases (like acetylene and oxygen) to melt the base metal and filler material. Thermit welding uses an exothermic chemical reaction between a metal oxide and a reducing agent (like aluminum) to produce molten metal. Since neither of these processes involves the flow of a large electric current through an arc, they do not generate the magnetic fields that cause arc blow. Hence, arc blow is not encountered in gas welding or thermit welding.

Conclusion

Based on the nature of the welding processes and the effect of current types on magnetic fields, arc blow is a defect specifically associated with arc welding, and it is most prominent when using DC current due to the constant direction of the magnetic field.