Final answer:
In GMAW welding, alternating current (AC) is not used; instead, a direct current (DC) power supply is employed with the electrode being either the positive or negative terminal to ensure a stable arc and consistent welding performance.
Step-by-step explanation:
The type of current not used when GMAW welding is alternating current (AC). Gas Metal Arc Welding (GMAW), also known as Metal Inert Gas (MIG) welding, typically uses a continuous and consumable wire electrode and a shielding gas. This process employs a direct current (DC) power supply, with the electrode being the positive terminal, known as Direct Current Electrode Positive (DCEP), or sometimes the negative terminal, known as Direct Current Electrode Negative (DCEN). AC is not used in GMAW because it can lead to an unstable arc and inconsistent welding performance.