MIG/MAG (Gas Metal Arc) welding feeds a continuous consumable wire electrode through a torch while a shielding gas protects the molten pool. The power supply is almost always direct current (DC), most commonly in DCEP / reverse polarity (electrode positive).

Why DC is preferred:

• DC maintains a constant direction of current flow, so the arc burns steadily without the zero-crossing interruptions that occur twice per cycle in AC. In AC the arc tries to extinguish and re-ignite at each zero crossing, making it flicker and become unstable.
• A stable arc gives smooth, consistent metal transfer — droplets detach from the wire and cross to the weld pool uniformly — producing an even bead, good fusion and low spatter.
• With DCEP the wire tip runs hot, giving good penetration and reliable droplet transfer, which is ideal for the continuously fed MIG electrode.

Hence DC is used because it offers a stable arc and consistent metal transfer, the correct answer.

DC does not limit the range of weldable metals — MIG welds steels, stainless and aluminium alloys across a wide range. It does not exist to reduce penetration on thick material; good penetration is actually a benefit of DCEP. And it does not increase spatter — an unstable, spatter-heavy arc is exactly what DC helps to avoid, so more spatter would be a drawback, not a purpose.