Aluminum oxide on the surface is non-conductive, so it blocks current and prevents proper fusion between the base metal and filler. If you weld over this oxide, you’ll get lack of fusion, porosity, and weak welds. The fix is to remove the oxide before welding. You clean the surface by mechanical means—scrubbing or grinding with a stainless-steel brush or abrasive tool—to physically remove the oxide. Chemical cleaners or etchants can also be used to dissolve or loosen the layer, often followed by rinsing and drying to prevent re-oxidation. Simply wiping with water won’t reliably remove the oxide and won’t prepare the surface for true fusion. Removing the oxide is essential even when shielding gas is used.

Shielding gas purity directly affects arc stability in GMAW by maintaining a clean protective environment around the arc and the molten weld pool. When the gas is impure, contaminants such as oxygen, moisture, or other impurities can enter the arc and melt pool. This changes the arc characteristics, promotes oxidation, increases porosity, and causes erratic arc behavior or wandering, all of which make the weld less stable and less consistent. A pure shielding gas creates a stable, uniform atmosphere that protects the arc and metal from atmospheric contamination, leading to a steadier arc, better bead shape, and fewer defects. The other options misstate the effect: purity doesn’t have no effect, and it certainly isn’t primarily about color or about increasing amperage.

Cracks in the weld signal a fundamental defect that codes treat as unacceptable for structural integrity. The ability of a fillet weld to carry load depends on a sound fusion without cracks; even small cracks can propagate under service stresses and lead to failure, so the presence of cracks is a primary reject criterion. Other statements don’t reflect universal acceptance rules: welds aren’t required to be perfectly flat or have a finishless surface, since fillet welds commonly have some convexity and finishing requirements vary by project. The idea that the weld size must be kept well above the specified throat isn’t a standard acceptance rule, as welds must meet prescribed throat dimensions rather than exceed them by a large margin. And complete melting of the base metal isn’t a defining criterion for fillet welds; they are designed to achieve fusion at the joint without needing to melt all the base metal.

The main idea here is how the distance from the contact tip to the workpiece (CTWD) affects the shielding gas protection and arc stability in MIG welding. When CTWD isn’t kept consistent, the shielding gas may not blanket the molten weld properly, allowing air and moisture to contaminate the pool. That contamination shows up as porosity in the solidified weld, and an unstable or poorly heated arc can also lead to incomplete bonding or poor fusion. So porosity or poor fusion is the most likely defect when CTWD isn’t maintained.