Alternating Current Arc Dynamics: Solving the Magnetic Blow Phenomenon

Shielded Metal Arc Welding (SMAW), or “stick welding,” is the most versatile of all welding processes. However, it is subject to the invisible laws of electromagnetism. One of the most frustrating phenomena for a welder is “Arc Blow.”

Arc Blow occurs when the magnetic field generated by the welding current becomes distorted, usually near the ends of a joint or in corners. This magnetic force literally pushes the electric arc sideways, causing it to wander erratically, spatter violently, and fail to fuse the metal. It is a plague of Direct Current (DC) welding. The solution lies not in skill, but in the physics of the power source: Alternating Current (AC).

The Physics of Arc Blow

An electric current flowing through a conductor (the electrode) creates a magnetic field around it. In DC welding, the current flows in one direction continuously, creating a fixed magnetic field. When welding on ferromagnetic materials (like steel), this field can build up in the workpiece.

When the arc approaches the edge of the plate or a ground clamp, the magnetic lines of force crowd together on one side of the arc. The arc seeks the path of least magnetic resistance and is “blown” away from the concentration of flux. This makes welding into corners almost impossible with DC.

Zero-Crossing Dynamics in 60Hz Cycles

Alternating Current (AC) solves this problem through constant reversal. In a 60Hz AC welder, the current reverses direction 120 times every second. Every time the current reverses, the magnetic field collapses and rebuilds in the opposite direction.

This rapid oscillation prevents the magnetic flux from building up in the workpiece. The “push” of the magnetic field effectively cancels itself out 120 times a second. Consequently, the arc remains stiff and straight, unaffected by the geometry of the joint or the magnetism of the steel. This makes AC welding the superior choice for repairs on magnetized pipelines, machinery, or complex structural corners.

Case Study: The 60Hz Workhorse

The LINCOLN ELECTRIC K1170 AC225 leverages this 60Hz physics to be the ultimate repair tool. Unlike DC inverters that struggle with magnetized metal, the AC225 ignores magnetic interference.

This characteristic is why it is ubiquitous in farm repairs and heavy industry. When fixing a magnetized plow blade or a structural beam, the AC arc drives straight into the root of the joint. The machine’s transformer design produces what is described as a “smooth AC arc,” ensuring that even though the current passes through zero 120 times a second, the arc remains ionized and stable, reducing the likelihood of the rod sticking.

Electrode Chemistry: The 7018AC Solution

Welding with AC requires specific electrode chemistry. Because the current hits “zero” so frequently, the arc tends to extinguish. To combat this, electrode manufacturers developed specific flux coatings containing arc stabilizers (like potassium).

The Lincoln AC225 shines when paired with rods like the E6011 (for deep penetration on rusty metal) and the E7018AC (for high-strength, low-hydrogen welds). The 6011 rod relies on the violent AC arc to blast through rust and paint, while the 7018AC has special stabilizers to keep the puddle fluid and the arc lit during the polarity transitions. Understanding this chemistry is key to unlocking the machine’s potential.

Limitations: The Aluminum Oxide Barrier

While AC is essential for welding aluminum in the TIG process, it is a different story for Stick welding. A common misconception is that because TIG welders use AC for aluminum, an AC stick welder like the K1170 can do the same.

This is incorrect. Aluminum Stick welding is technically possible but notoriously difficult and generally yields poor results. The AC225 lacks the high-frequency start and the adjustable balance required to clean the tough aluminum oxide layer effectively while maintaining a stable arc with an aluminum electrode. The K1170 is a ferrous beast—designed for steel, stainless steel (with correct rods), and cast iron. It is not an aluminum machine.

Conclusion: The Fundamental Skill

Mastering the AC arc is a rite of passage for the welder. It teaches arc length control that DC machines mask. By utilizing the physics of alternating current to neutralize magnetic blow, machines like the Lincoln K1170 provide a reliable, unstoppable force for joining steel, proving that sometimes the “old way” is the only way to defeat the invisible forces of magnetism.